Static failure of hybrid bolted-bonded composite joints in hygrothermal environment

Abstract

Hygrothermal environment has a significant impact on the safety and reliability of hybrid bolted-bonded composite joints. This study aims to investigate the effects of hygrothermal environment and geometrical parameters on the mechanical properties of hybrid bolted-bonded joints. In this paper, through the static tensile test and SEM scanning electron microscopy of hybrid bolted-bonded joints with different end-to-diameter ratios (E/D) and width-to-diameter ratios (W/D) under room temperature dry (RTD) and elevated temperature wet (ETW) environments, the change rule of the static strength of the joint and the macro-micro failure form are obtained. A finite element model of the hybrid bolted-bonded joint was established to obtain the initial stress distribution states of different layups and adhesive layers of laminates. The results indicate that the ultimate load of the hybrid bolted-bonded joint increases with the increase of E/D and W/D within a certain range. The ETW environment significantly reduces the ultimate load of hybrid joints. SEM scanning electron microscopy revealed that the compression damage of laminates in the ETW environment was aggravated. The hole edges exhibited an obvious state of matrix fracture and collapse. In the thickness direction, there was an increase in fiber breakage and debonding in the 90° fiber orientation, accompanied by more resin debris.

Keywords

Composite laminates hygrothermal environment hybrid bolted-bonded joints static tensile single-bolt single-lap

Introduction

Hybrid bolted-bonded composite joints exhibit superior mechanical properties compared to bonded joints and bolted joints, attracting increasing attention from researchers in recent years. These joints have been studied for their resistance to environmental aging, parameter design, structural load-bearing, and overall structural safety and reliability.^1–3 In the actual civil aircraft operation process and engineering applications, a variety of complex environments will affect the efficiency of hybrid bolted-bonded composite joints, where the hygrothermal environment will lead to premature degradation of the performance of the hybrid bolted-bonded joint, affecting the mechanical properties and durability of the structure, thereby adversely affect the continued airworthiness of civil aircraft. Different design parameters also affect the design, repair, and safety of their joints to varying degrees. Currently, domestic and international scholars are conducting research through experimental studies, theoretical analysis, and simulation methods to investigate the effects of different environments and geometric parameters on the mechanical properties, damage forms, and progressive damage of laminates in hybrid bolted-bonded joints.

In terms of the influence of environmental factors on the performance of hybrid bolted-bonded joints. F. Delzendehrooy et al.⁴ studied the influence of geometric parameters on the mechanical properties of hybrid bolted-bonded joints under different hygroscopic conditions. The results showed that with the increase of aging time, the degradation of the adhesive and the decrease of mechanical properties would be accelerated. Hasan Ulus⁵ studied the effect of different temperatures on the mechanical properties of bonded joints, bolted joints, and hybrid bolted-bonded single lap joints. The results showed that the hybrid bolted-bonded joint had better durability than the bonded or bolted joints at different temperatures. The increase in temperature increased the final failure displacement of the bolted and hybrid bolted-bonded joints, while this phenomenon did not occur in bonded joints. Sajjad Karimi et al.^6–8 studied the effects of hygrothermal aging and thermal aging on the fatigue and static strength of hybrid bolted-bonded joints. The results showed that the hybrid bolted-bonded joint has the highest flexural strength, and thermal aging and hygrothermal aging have an impact on the static bending of the joint. Performance has a significant impact. Hanyu Zhang et al.^9–11 studied the fracture mechanism of CFRP-aluminum alloy hybrid bolted-bonded joints under different hygrothermal environments and fatigue loading conditions by combining experiments with simulations. The results showed that complex fatigue failure modes were formed under different hygrothermal aging and fatigue loading conditions, and its failure mode changed with the changes in the hygrothermal environment and fatigue load. Yanfeng Zhang et al.,^12,13 conducted compression shear tests on hybrid bolted-bonded single-lap composite joints at 800°C. The failure process of the joint was obtained through finite element analysis, and the failure mode of the joint, stress distribution of the bonding layer, and progressive damage were determined. The variation law of the proportion of the load shared by the bolt to the total load with displacement was obtained.

At present, researchers have conducted analysis and research at different levels on the failure laws of mechanical properties, macro-micro failure forms, and progressive damage analysis of hybrid bolted-bonded composite joints under different external environments through experimental methods and finite element methods. They have summarized the adverse effects of hygrothermal environments on material integrity, adhesive layer life, structural strength, and macro-micro failure forms, and analyzed their failure laws and possible preventive measures. However, existing studies have not combined specific geometric parameters in a hygrothermal environment to further explore the static strength performance of the hybrid bolted-bonded joint and the macro-micro failure forms of these joints.

In terms of the influence of geometric parameters on the structural performance of hybrid bolted-bonded joints and the damage of laminates. Valentin S. Romanov et al.^14–17 studied the influence of geometric parameters on the static properties of hybrid bolted-bonded single-lap joints. The study found that the overlap length of the joint has a significant effect on the strength and stiffness of the joint, and reducing the overlap length to a certain extent is beneficial to the performance of the overall joint. Xiaoqi Li et al.^18–21 discussed the influence of design parameters on tensile properties, such as joint strength, stiffness, and energy absorption, through experiments and simulations. The results showed that except for the modulus of the adhesive, the free part length ratio (S/D) and the bolt pre-torque, increasing other parameters within a certain range is beneficial to the static strength performance of the hybrid bolted-bonded joint. Kelly G et al.^22–24 studied the quasi-static strength of hybrid bolted-bonded composite joints through experiments and simulations and found that the strength of hybrid bolted-bonded joints was increased by 11% to 22% compared with bolted joints. Through a combination of experiments and simulations, Chao-Yu Zhang²⁵ found that when the overlap width is 30 mm, the bearing capacity of the triple-nail hybrid bolted-bonded joint is 43.2% higher than that of the double-nail hybrid bolted-bonded joint. Increasing the end-to-diameter ratio (E/D) of this joint can improve its maximum bearing capacity. Through experiments and simulations, Yong-jian Xiong²⁶ proved that within a reasonable range, the maximum bearing capacity of the hybrid bolted-bonded composite joint can be increased by improving the W/D, E/D and aperture ratio, and the optimal ratios of these three are 4.5, 1.5, and 4.5, respectively.

At present, researchers have conducted detailed studies on the effects of other important geometric parameters such as E/D, W/D, and S/D on the static strength performance and macro-micro failure modes of hybrid bolted-bonded composite joints, analyzed the effects of different parameters on the mechanical properties of hybrid bolted-bonded joints, and explored the reasonable parameter design range, and at the same time, they compared and studied the reasonable design process of hybrid bolted-bonded composite joints, and proposed various reasonable design steps and processes. However, most of the studies focused on room temperature dry environments or single temperature-and-humidity environments and did not conduct in-depth analysis of the static strength performance and joint failure modes of hybrid bolted-bonded composite joints in a hygrothermal environment combined with specific geometric parameters.

In summary, researchers have conducted certain experimental research, simulation analysis and theoretical analysis on the effects of complex environmental factors, geometric parameters, process parameters, adhesive selection and molding process on the static strength performance, fatigue performance, and macro-micro failure modes of hybrid bolted-bonded composite joints. Although most studies have investigated and analyzed the strength and fatigue properties of hybrid bolted-bonded joints with different temperatures, humidity, and geometrical parameters, the static strength properties of hybrid bolted-bonded joints with different geometrical parameters of composites in hygrothermal environments, as well as the macro and microscopic failure patterns of joints, have not been analyzed and investigated in detail. Most studies have explored and analyzed the strength and fatigue performance of hybrid bolted-bonded composite joints under different temperatures and humidities and different geometric parameters. However, no detailed analytical research has been conducted on the static strength performance and macroscopic and microscopic failure laws of hybrid bolted-bonded composite joints with different geometric parameters in hygrothermal environments. Therefore, this paper uses experimental and simulation methods, combined with different geometric parameters, to explore the static strength performance of hybrid bolted-bonded composite joints under hygrothermal environments, as well as the macro and micro failure modes of the joints, and the initial damage state of the laminates. The content studied in this paper can provide a certain reference for the static strength failure laws and joint damage research of hybrid bolted-bonded composite joints in hygrothermal environments, and can also provide a certain reference for the application of hybrid bolted-bonded composite joints in actual engineering environments.

Experiment

Materials and equipment for experiment

As shown in Figure 1, the test specimen is hybrid bolted-bonded composite joints, which means, a hybrid joint form of bonded joint and bolted joint is adopted in the overlap area. The specimen is made of T300 carbon fiber/epoxy resin composite laminate with a plate length of L = 135 mm, a plate thickness of t = 3.6 mm, a ply method of ${[0 / 45 / 90 / - 45]}_{3 s}$ , (a) thickness of each single layer of 0.15 mm, and a bolt hole diameter of D = 5 mm. The specific geometric parameters design of the experiment is shown in Table 1. The bolts are made of 6Al-4V titanium alloy with an elastic modulus of 110 GPa and a Poisson’s ratio of 0.34. The film used was FM73 M film produced by Cytec Corporation (USA), and the film thickness was t = 0.1 mm. Table 2 shows some material parameters of FM73 M film. Table 3 presents the specific mechanical parameters of the laminate under RTD environment.

Table 1.

Material parameters of adhesive film.

Parameter	Value
E/D	1	1.5	2	3	3.5	4
W/D	3	4	5	6	7	8

Table 2.

Material parameters of adhesive film.

Elastic constant	Value	Elastic constant	Value
$E_{11} = E_{22} = E_{33} /$ GPa	1.092	$μ_{12} = μ_{13} = μ_{23}$	0.33
$G_{12} = G_{13} = G_{23} /$ GPa	0.42	$ρ / (K g / m^{3})$	1105.26
Curing temperature /°C	120	Curing time at 120°C /min	90

Table 3.

Mechanical properties of T300 composites in RTD.

Elastic constant	Value	Elastic constant	Value
$E_{11} /$ GPa	154	$X_{t} /$ GPa	2.347
$E_{22} = E_{33} /$ GPa	9.5	$X_{c} /$ GPa	1.209
$G_{12} = G_{13} /$ GPa	4.278	$Y_{t} /$ MPa	50
$G_{23} /$ GPa	3.4	$Y_{c} /$ MPa	169
$v_{12} = v_{13}$	0.33	$S_{12} = S_{13} /$ MPa	121
$v_{23}$	0.4	$S_{23} /$ MPa	78

The manufacturing process of the hybrid bolted-bonded joint selects the mechanical cutting hole-making method and the connection method of first making holes and then gluing to ensure that the hole diameter error is kept within 0.04 mm. The adhesive film curing process involves vacuum bags and sealing strips to vacuum-pack the hybrid bolted-bonded joint specimens, and then utilizing a thermal patching instrument to cure the specimens. As shown in Figure 2, this is a typical curing process curve. First, the temperature is gradually raised to 120°C within 90 minutes according to the setting. Then, it is cured at a constant temperature of 120°C for 90 minutes. Finally, the test ends and the temperature is naturally cooled to room temperature.

In this paper, the static tensile tests of all hybrid bolted-bonded composite joint specimens under two environments were carried out using an MTS tensile-fatigue tester, as shown in Figure 3(a). According to the test standard ASTM-D5229-04, a triple-electric electric constant temperature water bath was used to conduct a moisture absorption and temperature rise test on the hybrid bolted-bonded joint at a high temperature of 70°C until the moisture absorption saturation state was reached, as shown in Figure 3(b). The microscopic damage modes of hybrid bolted-bonded joints were analyzed by scanning electron microscopy (SEM), as shown in Figure 3(c). The adhesive film curing of the hybrid bolted-bonded joint specimens was carried out using a hot patching instrument, as shown in Figure 3(d).

Experimental process

Hygroscopic process

Before the static tensile test, a digital torque wrench was used to apply a tightening torque of 2.1 Nm to the hybrid bolted-bonded joint specimen. Then, the surface of the specimen was treated to remove stains and impurities in the RTD environment. According to ASTM D5229M-14²⁷ standard, the hybrid bolted-bonded joint specimen was placed in a 70°C constant temperature water bath for moisture absorption and temperature rise testing until the specimen reached the moisture absorption saturation state. During the test, the mass of the test specimens with different width-to-diameter ratios was measured regularly by an electronic balance. When the mass change rate of the specimen measured before and after two consecutive measurements was less than 0.02% (calculated using equation (1)), the specimen was considered to have reached the moisture absorption saturation state. As shown in Figure 4, this is the change in the moisture absorption curve of the test specimens with different width-to-diameter ratios in this article. After 30 days of moisture absorption, the specimens basically reach the moisture absorption saturation state, and the test can be terminated at this point. The final average moisture absorption of all the specimens in this paper is $Δ C = 0.76 %$

\frac{W_{i} - W_{i - 1}}{W_{b}} < 0.0002

(1)

where

W_{i}

is the current specimen test mass,

W_{i - 1}

is the previous specimen test mass, and

W_{b}

is the specimen test mass in the initial RTD environment.

Static tensile testing

According to the ASTM-D5766 and ASTM-D3039 test standards, static tensile tests were carried out on the tensile-fatigue tester on hybrid bolted-bonded composite joint specimens with different end-to-diameter ratios, width-to-diameter ratios, and different layup methods under RTD and ETW environments. Under the RTD environment, the displacement loading rate was set at 2 mm/min, and static tensile tests were directly carried out on hybrid bolted-bonded composite joint specimens with six end-to-diameter ratios, six width-to-diameter ratios, and three layup methods. The test was terminated, and the data were recorded after the joint specimens reached the failure state. Under the ETW environment, after the end of the hygroscopic heating test, and after the specimen reaches the high temperature hygroscopic saturation state, under the condition of heat preservation and moisture preservation, set a 2 mm/min displacement loading rate, perform a static tensile test on the specimen with different design parameters, and similarly wait until the connected specimen reaches the failure state after the end of the test and record the data. Under the ETW environment, after the moisture absorption and heating test is completed and the specimen reaches the high-temperature moisture absorption saturation state, a displacement loading rate of 2 mm/min is set under the condition of thermal insulation and moisture retention, and static tensile tests are carried out on specimens with different design parameters. Similarly, the test is terminated and the data are recorded after the connected specimen reaches the failure state. The test results of different design parameters under the two environments were taken as the average of three tests, totaling of 90 groups of tests.

Damage modeling of hybrid bolted-bonded composite joints in hygrothermal environment

Stress–strain relationships in hygrothermal environment

The strain of composites in a hygrothermal environment consists of the strain due to heat and humidity, as well as the mechanical strain due to stress, as shown in equation (2)

ε = S σ + ε^{ht}

(2)

where

ε_{i j}

is the actual strain in the composite.

ε^{ht}

is the hygrothermal strain component (caused by uniformly distributed temperature and humidity).

σ_{i j}

is the stress component in the composite. And S is the flexibility matrix in the principal material coordinate system, where

ε^{ht}

can be expressed as:

ε^{ht} = \partial Δ T + β c

(3)

where

\partial_{i j}

is the thermal expansion coefficient of the composite.

Δ T

is the temperature change.

β_{i j}

is the moisture expansion coefficient of the composite. And c is the moisture concentration. The stress-strain relationship of the composite under ETW environment can be obtained according to equaation (2) and (3):

σ = C (ε - \partial Δ T - β c)

(4)

where C is the conversion stiffness matrix, and the expansion coefficient relationship can be expressed as:

\partial_{12} = \partial_{13} = \partial_{23} = 0

and

β_{12} = β_{13} = β_{23} = 0

²⁸. In addition, it also conforms to the following relationship:

\partial_{22} = \partial_{33}

and

β_{22} = β_{33}

Among them, $\partial_{i j}$ and $β_{i j}$ can be calculated by equation (5)–(8)

α_{11} = \frac{V_{f} E_{f} α_{f} + V_{m} E_{m} α_{m}}{V_{f} E_{f} + V_{m} E_{m}} \approx α_{f}

(5)

α_{22} = α_{33} = V_{f} (1 + ν_{f}) α_{f} + V_{m} (1 + ν_{m}) α_{m} - (V_{f} ν_{f} + V_{m} ν_{m}) α_{11}

(6)

β_{11} = \frac{E_{m} ρ}{(V_{f} E_{f} + V_{m} E_{m}) ρ_{m}} β_{m} = \frac{E_{m} ρ}{E_{11} ρ_{m}} β_{m}

(7)

β_{22} = β_{33} = \frac{(1 + ν_{m}) ρ}{ρ_{m}} β_{m} - (V_{f} ν_{f} + V_{m} ν_{m}) β_{11}

(8)

where

α_{f}

is the thermal expansion coefficient of the composite fiber.

α_{m}

is the thermal expansion coefficient of the composite matrix.

V_{m}

is the volume fraction of the composite matrix.

V_{f}

is the volume fraction of the composite fiber, which meets

V_{m}

V_{f}

= 1.

E_{m}

is the Young’s modulus of the matrix under RTD environment.

E_{f}

is the Young’s modulus of the fiber under RTD environment.

β_{m}

is the coefficient of wet expansion of the composite matrix.

ν_{m}

is the Poisson’s ratio of the matrix under RTD environment.

ν_{f}

is the Poisson’s ratio of the fiber under RTD environment.

ρ

is the density of composite.

ρ_{m}

is the density of composite matrix.

Failure guidelines for composites in hygrothermal environment

In this study, the improved Hashin failure criterion²⁹ and the improved Tserpes abrupt degradation model by Zhang et al.³⁰ are considered in the model, as shown in Tables 4 and 5, respectively. In order to more accurately simulate the stiffness degradation process of this study, the abrupt degradation coefficient in the material degradation model was changed from 0 to 0.1.

Table 4.

Failure criteria of composites.

Failure mode	Failure criterion
Matrix tension (MT) $(σ_{22} > 0)$	${(\frac{σ_{22}}{Y_{t}^{ht}})}^{2} + {(\frac{σ_{12}}{S_{12}^{ht}})}^{2} + {(\frac{σ_{23}}{S_{23}^{ht}})}^{2} \geq 1$
Matrix compression (MC) $(σ_{22} < 0)$	${(\frac{σ_{22}}{Y_{c}^{ht}})}^{2} + {(\frac{σ_{12}}{S_{12}^{ht}})}^{2} + {(\frac{σ_{23}}{S_{23}^{ht}})}^{2} \geq 1$
Fiber tension (FT) $(σ_{11} > 0)$	${(\frac{σ_{11}}{X_{t}^{ht}})}^{2} + {(\frac{σ_{12}}{S_{12}^{ht}})}^{2} + {(\frac{σ_{13}}{S_{13}^{ht}})}^{2} \geq 1$
Fiber compression (FC) $(σ_{11} < 0)$	${(\frac{σ_{11}}{X_{c}^{ht}})}^{2} \geq 1$
Fiber-matrix shear-out (FMS) $(σ_{11} < 0)$	${(\frac{σ_{11}}{X_{t}^{ht}})}^{2} + {(\frac{σ_{12}}{S_{12}^{ht}})}^{2} + {(\frac{σ_{13}}{S_{13}^{ht}})}^{2} \geq 1$
Interlaminar tension (IT) $(σ_{33} > 0)$	${(\frac{σ_{33}}{Z_{t}^{ht}})}^{2} + {(\frac{σ_{13}}{S_{13}^{ht}})}^{2} + {(\frac{σ_{23}}{S_{23}^{ht}})}^{2} \geq 1$
Interlaminar compression (IC) $(σ_{33} < 0)$	${(\frac{σ_{33}}{Z_{c}^{ht}})}^{2} + {(\frac{σ_{13}}{S_{13}^{ht}})}^{2} + {(\frac{σ_{23}}{S_{23}^{ht}})}^{2} \geq 1$

Table 5.

Material degradation model.

Failure mode	Material degradation criteria
MT	$E_{22}^{d} = d_{mt} E_{22}^{ht}$ , $G_{12}^{d} = d_{mt} G_{12}^{ht}$ , $G_{23}^{d} = d_{mt} G_{23}^{ht}$ , $v_{12}^{d} = d_{mt} v_{12}^{ht}$ , $v_{23}^{d} = d_{mt} v_{23}^{ht}$
MC	$E_{22}^{d} = d_{mc} E_{22}^{ht}$ , $G_{12}^{d} = d_{mc} G_{12}^{ht}$ , $G_{23}^{d} = d_{mc} G_{23}^{ht}$ , $v_{12}^{d} = d_{mc} v_{12}^{ht}$ , $v_{23}^{d} = d_{mc} v_{23}^{ht}$
FT	$E_{11}^{d} = d_{ft} E_{11}^{ht}$
FC	$E_{11}^{d} = d_{fc} E_{11}^{ht}$
FMS	$G_{12}^{d} = d_{fm 1} G_{12}^{ht}$ , $G_{13}^{d} = d_{fm 1} G_{13}^{ht}$ , $G_{23}^{d} = d_{fm 2} G_{23}^{ht}$ , $v_{12}^{d} = d_{fm 1} v_{12}^{ht}$ , $v_{13}^{d} = d_{fm 1} v_{13}^{ht}$ , $v_{23}^{d} = d_{fm 2} v_{23}^{ht}$
IT	$E_{33}^{d} = d_{dt} E_{33}^{ht}$ , $G_{13}^{d} = d_{dt} G_{13}^{ht}$ , $G_{23}^{d} = d_{dt} G_{23}^{ht}$ , $v_{13}^{d} = d_{dt} v_{13}^{ht}$ , $v_{23}^{d} = d_{dt} v_{23}^{ht}$
IC	$E_{33}^{d} = d_{dc} E_{23}^{ht}$ , $G_{13}^{d} = d_{dc} G_{13}^{ht}$ , $G_{23}^{d} = d_{dc} G_{23}^{ht}$ , $v_{13}^{d} = d_{dc} v_{13}^{ht}$ , $v_{23}^{d} = d_{dc} v_{23}^{ht}$

where $σ_{i j}$ is the actual stress of the composite structure in the hygrothermal environment. $X_{t}^{ht}$ and $Y_{t}^{ht}$ are the axial and transverse tensile strength of the composite in the hygrothermal environment. $X_{c}^{ht}$ and $Y_{c}^{ht}$ are the axial and transverse compressive strength of the composite in the hygrothermal environment. $S_{12}^{ht}$ is the internal shear strength of the composite in the hygrothermal environment. $Z_{t}^{ht}$ and $Z_{c}^{ht}$ are the vertical tensile and compressive strengths of the composite in the hygrothermal environment. $S_{13}^{ht}$ and $S_{23}^{ht}$ are the vertical shear strengths of the composite in the hygrothermal environment. The superscript “ht” indicates that the parameter includes the effect of hygrothermal. $d_{ft}$ , $d_{fc}$ , $d_{mt}$ , $d_{dt}$ , $d_{mc}$ , $d_{dc}$ , $d_{fm 1}$ , $d_{fm 2}$ are the abrupt degradation coefficients of the composites, respectively.³¹

In this case, “d” is used for the post-damage parameter, and the original stiffness parameter needs to be reduced by using the degradation factor of the composite properties in the ETW environment due to the influence of the ETW environment

d_{ft} = \frac{V_{m} E_{m}^{ht}}{E_{11}^{ht}}, d_{fc} \approx \frac{E_{22}^{ht}}{E_{11}^{ht}}, d_{mt} = d_{dt} = 0.1

(9)

d_{mc} = d_{dc} \approx 0.1, d_{fm 1} = (\frac{1 - \sqrt{V_{f}}}{1 - \sqrt{V_{f}} + V_{f}}) \frac{G_{m}^{ht}}{G_{23}^{ht}}

(10)

d_{fm 2} = (\frac{1 - \sqrt{V_{f}}}{1 - \sqrt{V_{f}} + V_{f}}) \frac{G_{m}^{ht}}{G_{23}^{ht}}

(11)

where

E_{m}^{ht} = E_{m}^{0} {(T^{*})}^{i}

and

G_{m}^{ht} = G_{m}^{0} {(T^{*})}^{j}

denote the matrix axial modulus and matrix tangential modulus, respectively.

V_{f}

and

V_{m}

denote the fiber content and matrix content, respectively. And

E_{11}^{ht}

E_{22}^{ht}

G_{12}^{ht}

, and

G_{23}^{ht}

denote the elastic constants under hygrothermal environment.

Adhesive yield guidelines for hygrothermal environment

The degradation model of the adhesive layer in the hybrid bolted-bonded composite joint adopts a bilinear cohesive model, and the cohesive unit with a bilinear constitutive relationship is used to simulate the failure of the adhesive layer. The constitutive relationship of the cohesive model is as follows

{\begin{cases} σ_{n} \\ σ_{s} \\ σ_{t} \end{cases}} = [\begin{array}{l} (1 - d) K_{n n} \\ (1 - d) K_{s s} \\ (1 - d) K_{t t} \end{array}] {\begin{cases} σ_{n} \\ σ_{s} \\ σ_{t} \end{cases}}

(12)

where

K_{nn} = \frac{E_{nn}}{T_{0}}

K_{ss} = \frac{E_{ss}}{T_{0}}

, and

K_{tt} = \frac{E_{tt}}{T_{0}}

T_{0}

is the initial thickness of the Cohesive cell.

σ_{n}

is the Cohesive unit node normal relative displacement.

σ_{s}

and

σ_{t}

are the two tangential relative displacements of the Cohesive unit node.

The quadratic nominal stress criterion is used as the damage initiation criterion of the adhesive layer,³² as shown in equation (13). When the interaction function is equal to 1, it indicates the onset of adhesive layer damage

{\frac{〈 σ_{n} 〉}{σ_{0, n}}}^{2} + {\frac{σ_{s}}{σ_{0, s}}}^{2} + {\frac{σ_{t}}{σ_{0, t}}}^{2} = 1

(13)

The damage extension criterion of the adhesive layer is the B-K criterion, as shown in equation (14)

G_{c} = G_{c, n} + (G_{c, s} - G_{c, n}) {(\frac{G_{s} + G_{t}}{G_{n} + G_{s} + G_{t}})}^{η}

(14)

where

G_{c, n}

is the fracture toughness of mode I crack.

G_{c, s}

is the fracture toughness of mode II crack.

G_{c}

is the equivalent fracture toughness.

G_{n}

is the energy release rate of mode I.

G_{s}

and

G_{t}

are the energy release rates of mode II. Damage factor

η

= 2.³³

The damage evolution variable d of the adhesive layer from initial damage to overall failure is shown in equation (15)–(17)

d = \frac{δ_{m}^{f} (δ_{m}^{\max} - δ_{m}^{0})}{δ_{m}^{\max} (δ_{m}^{f} - δ_{m}^{0})}

(15)

δ_{m}^{f} = \frac{2 G_{C}}{T_{eff}^{0}}

(16)

T_{eff}^{0} = {\sqrt{{〈 σ_{n} 〉}^{2} + {(σ_{s})}^{2} + {(σ_{t})}^{2}} |}_{{\frac{〈 σ_{n} 〉}{σ_{0, n}}}^{2} + {\frac{σ_{s}}{σ_{0, s}}}^{2} + {\frac{σ_{t}}{σ_{0, t}}}^{2} = 1}

(17)

where

δ_{m}^{\max}

is the maximum value of the equivalent displacement during the damage extension process.

T_{eff}^{0}

is the equivalent stress corresponding to the onset of damage.

δ_{m}^{0}

is the equivalent displacement corresponding to the onset of damage.

δ_{m}^{f}

is the equivalent displacement corresponding to complete failure.

Finite element modeling

In the commercial ABAQUS software, a hybrid bolted-bonded single lap composite joint model was established, and the specific design parameters were E/D = 3, W/D = 6 and layup method 1, as shown in Figure 5. The laminate, bolts, and films are all modeled as solids. The laminate is divided into 24 layers in the thickness direction using positive column segmentation. The meshing of the laminate employs the C3D8R three-dimensional integral solid element. The tangential behavior between each instance and the laminate is simulated by the penalty function, with the friction coefficient f = 0.1 is set. The normal behavior utilizes the “penalty” hard contact method. When setting displacement loading, one end of the hybrid bolted-bonded joint is fixed in all directions of X, Y, and Z. The other end is only fixed in the Y and Z directions, and displacement loading is applied in the X direction. Before the displacement loading analysis step in both RTD and ETW environments, it is necessary to set an analysis step for applying bolt preload. At the same time, the improved UMAT subroutine is used to perform finite element simulation calculations in the two environments. The simulation in the ETW environment adds hygrothermal conditions in the subroutine. Where the preload force in the finite element is converted to the torque of the test bolt by the formula in equation (18)³⁴

P = \frac{T}{K D}

(18)

where p is the bolt preload force. T is the bolt torque. K is the torque coefficient (taken as 0.2 in this paper).

Finite element model validation

In order to verify the accuracy of the prediction results of the finite element model, taking the hybrid bolted-bonded joint with E/D = 3 and W/D = 6 as an example, the load-displacement curve of the test under the ETW environment was compared with the curve results of the finite element simulation.

As shown in Figure 6, both the test and simulation show that the adhesive layer fails in the initial load-bearing stage. The load increases in a nearly linear trend as the displacement increases, and a sudden drop failure occurs when the limit load is reached. At this time, the hybrid joint is loaded by bolts. When the bolts just start to load, the load-displacement curve increases nearly linearly with a steep slope. The slope of the curve decreases with the continuous application of displacement load, and then the first load drop occurs. As subsequent loads continue to be applied, the hybrid bolted-bonded joint will experience sudden load drops until the entire structure fails completely. The growth trends of the load-displacement curves of the test and simulation, as shown in Figure 6, are basically similar. The ultimate load of the finite element simulation of the hybrid bolted-bonded joint is 8.81 kN, and the test ultimate load is 9.76 kN. The relative error between the two is 9.7%, which is less than 10%. Therefore, the accuracy of the prediction results of the finite element model can be verified.

Analysis of results

Effect of hygrothermal environment on the static failure of hybrid bolted-bonded composite joints

Effect of end-to-diameter ratio on the static failure of hybrid bolted-bonded joints in hygrothermal environment

The change of the end-to-diameter ratio (E/D) directly affects the lap length and the bonding area of the hybrid bolted-bonded joint, and further affects the static strength performance of the hybrid joint. Fixed width-to-diameter ratio W/D = 7.2, bolt preload p = 2.1 Nm, film thickness t = 0.15 mm. Six hybrid bolted-bonded composite joints with different end-to-diameter ratios (1, 1.5, 2, 3, 3.5, and 4) were designed. Static tensile tests were carried out under RTD and ETW environments to explore the effect of the change in end-to-diameter ratio on the static strength performance of the hybrid bolted-bonded composite joint under hygrothermal environments.

As shown in Figure 7, the load-displacement curves of hybrid bolted-bonded composite joints with different end-to-diameter ratios under RTD and ETW environments are presented. Bonded joints exhibit approximately linear behavior under static tensile loading. Bolted joints exhibit approximately linear elastic behavior at the initial load and exhibit a nonlinear phase after continued loading.

For typical hybrid bolted-bonded composite joints, their load-displacement curve has the characteristic curve behavior of both bonded and bolted joints. In the initial load-bearing stage, when the adhesive layer is intact, the adhesive layer and the bolts bear the external load together, so the hybrid bolted-bonded composite joint shows near-linear behavior similar to that of the bonded joint in the initial load-bearing stage. The first load peak drop point indicates the complete damage failure of the adhesive layer. When the adhesive fails, there will be a clear sound of the adhesive film cracking, and the load-displacement curve will show nearly linear drop. After the adhesive layer is damaged and loses its bearing capacity, the overall load of the hybrid bolted-bonded composite joint is mainly borne by the bolts. The load-displacement curve of a typical bolt joint shows approximately linear elastic growth in the initial load-bearing stage. After the load continues to be applied to a certain extent, the load-displacement curve undergoes the first sudden drop. The specimen was accompanied by a crisp sound of fiber breaking, and the hybrid joint material produced initial damage. The load continues to be borne until the curve drops suddenly for the second time, and more frequent and severe fiber breakage sounds occur. At this time, the damage to the composite quickly expands to serious damage, until the entire hybrid joint completely loses its load-bearing capacity. It can be seen from Figure 7 that the curve variation rules of the hybrid bolted-bonded joint specimen under RTD and ETW environments are roughly similar.

The variation rules of the ultimate loads of hybrid bolted-bonded composite joints with six end-to-diameter ratios under RTD and ETW conditions are shown in Figure 8. The ultimate load of the hybrid bolted-bonded joint will increase with the increase in the end-to-diameter ratio within a certain range, but when the end-to-diameter ratio is too large, its ultimate load begins to show a downward trend.

In RTD environment, when the end-to-diameter ratio increases from 1 to 3, the ultimate load increases from 8.40 kN to 12.74 kN, an increase of 51.67%, and the increasing trend is obvious. At this time, as the end-to-diameter ratio increases, the area of the adhesive layer gradually increases, and the load-bearing ratio of the adhesive layer also gradually increases. This will effectively alleviate the stress concentration phenomenon of the hybrid joint and delay the initial damage of the hybrid bolted-bonded joint. When the end-to-diameter ratio continues to increase from 3 to 4, the ultimate load changes from 12.74 kN to 11.38 kN, a decrease of 10.68%, showing a decreasing trend. At this time, the load-bearing ratio of the adhesive layer begins to gradually decrease, and the degree of delaying the initial damage of the hybrid bolted-bonded joint is also significantly reduced.

In ETW environment, when the end-to-diameter ratio increases from 1 to 3, the ultimate load increases from 8.02 kN to 11.53 kN, an increase of 43.77%. When the end-to-diameter ratio continues to increase from 3 to 4, the ultimate load changes from 11.53 kN to 11.18 kN, a decrease of 3.04%. Compared with the RTD environment, the ultimate load of the hybrid bolted-bonded joint in the ETW environment was reduced to a certain extent. The ultimate loads of the hybrid bolted-bonded joint with six different end-to-diameter ratios (1, 1.5, 2, 3, 3.5, and 4) decreased by 4.52%, 5.71%, 9.70%, 9.50%, 10.92%, and 1.76%, respectively. As shown in Figure 8, the optimal end-to-diameter ratio of the hybrid bolted-bonded joint in this paper is 3.

Effect of width-to-diameter ratio on the static failure of hybrid bolted-bonded joints in hygrothermal environment

The change of the width-to-diameter ratio (W/D) directly affects the overlap width and the area of the bonding region of the hybrid bolted-bonded joint, which in turn affects the static strength performance of the hybrid joint. The fixed end diameter ratio E/D = 3, the bolt preload force p = 2.1 N·m, and the film thickness t = 0.15 mm. Six types of hybrid bolted-bonded composite joints with width-to-diameter ratios (3, 4, 5, 6, 7, and 8) were designed, and static tensile tests were carried out under RTD and ETW environments to explore the influence of changes in width-to-diameter ratio on the static strength characteristics of hybrid bolted-bonded composite joint. As shown in Figure 9, the change law of the load-displacement curves of six hybrid bolted-bonded composite joints with different width-to-diameter ratios under two environments is presented. The variation pattern of the curve is similar to that of the hybrid bolted-bonded joint with width-to-diameter ratios, and is consistent with the variation pattern of the load-displacement curve of the typical hybrid bolted-bonded composite joints.

Figure 10 illustrates the variation in the ultimate load of the hybrid bolted-bonded joint with different width-to-diameter ratios under two environments. Within a certain range, the ultimate load of the hybrid bolted-bonded joint increases with increasing width-to-diameter ratio. Beyond a certain range, the ultimate load begins to gradually decrease.

In RTD environment, when the width-to-diameter ratio increases from 3 to 6, the ultimate load changes from 7.33 kN to 11.83 kN, an increase of 61.39%. There is an obvious increasing trend. At this time, the area of the adhesive layer gradually increases, and the load-bearing ratio of the adhesive layer also gradually increases, from 24% to 45%. When the width-to-diameter ratio increases from 6 to 7 and 8, the ultimate load changes from 11.83 kN to 10.90 kN and 11.80 kN, which decreases by 7.86% and 0.25%. Both show a decreasing trend. At this time, although the area of the adhesive layer is further increased and the load-bearing proportion of the adhesive layer increases from 45% to 96%, the ultimate load-bearing capacity of the hybrid joint does not increases significantly.

In ETW environment, when the width-to-diameter ratio increases from 3 to 6, the ultimate load changes from 7.07 kN to 9.76 kN, an increase of 38.05%. When the width-to-diameter ratio increases from 6 to 7 and 8, the ultimate load changes from 9.76 kN to 9.23 kN and 9.37 kN, which is a decrease of 5.43% and 3.99%. In comparison to the RTD environment, the ultimate load of the hybrid bolted-bonded joint in the ETW environment exhibits varying degrees of reduction. The ultimate loads of the hybrid joints with six different width-to-diameter ratios (3, 4, 5, 6, 7, and 8) decreased by 3.55%, 17.53%, 17.91%, 17.50%, 15.32%, and 20.59%, respectively. As shown in Figure 10, the optimal width-to-diameter ratio of the hybrid bolted-bonded joint in this paper is 6.

Compared with the RTD environment, the static strength performance of hybrid bolted-bonded joints with different parameters in the ETW environment has decreased to a certain extent. This is because the mechanical properties of the composite will decrease significantly in the ETW environment, and the mechanical properties of the film will also decrease significantly. This will adversely affect the mechanical properties of the hybrid joint. Under hygroscopic aging with distilled water, the smaller water molecule size (compared to the free volume of microcracks and voids) will lead to easier diffusion of water through the polymer network, thus filling the free space before reaching saturation. The inherent brittleness of the adhesive film causes the bonded joint to break suddenly after reaching the maximum load, resulting in a sudden drop in load.

On the other hand, temperature also has a significant impact on the bearing behavior of hybrid joints. As the temperature increases, the relaxation of the adhesive layer causes a decrease in both the stiffness and the ultimate load of the adhesive layer. Likewise, polymer matrix relaxation is also a major factor in reducing bolted joint performance. In bolted and hybrid bolted-bonded joints, the force gradually decreases after reaching a peak value in tests conducted at room temperature. However, as the temperature increases, this gradually decreasing behavior begins to disappear. In addition, with the increase in temperature, the second ultimate load peak of the hybrid bolted-bonded joint is getting closer and closer to the ultimate load of the bolt connection structure. As the temperature increases, the molecules are arranged in a free molecular packing, the molecular bonding force weakens, and the performance of the hybrid joint decreases. Therefore, in the nonlinear softening region, the adhesive film in the hybrid bolted-bonded joint cannot effectively bear the load. In addition, the different thermal expansion coefficients of materials are another reason for the degradation of mechanical properties.

Initial damage stress state of hybrid bolted-bonded joints in hygrothermal environment

Figure 11 shows the overall stress level (MPa) analysis of four different layups of hybrid bolted-bonded single lap joints in the ETW environment. It can be seen from the figure that the stress level distribution of each layup is related to the layup direction. Since the load of the 0° layer is mainly borne by the fibers, its overall stress level is higher. The 90° layer only bears the load of the matrix, and there are no fibers in the tensile direction to support the load. Its stress level is the lowest. Compared with the 90° layer, the ±45° layer has a slightly higher stress level. This is because it is subjected to the joint load-bearing effect of the fibers and the matrix, but its stress level is slightly lower than that of the 0° layer.

As shown in Figure 12, as the load gradually increases, the damage initiation of the adhesive layer first appears at the two ends of the film along the tensile direction and gradually expands inward. At the same time, damage will also occur at the edge of the film near the hole, and as the displacement load is applied, the damage range gradually expands. It spreads inward from the edge of the film and outward from the edge of the film hole at the same time, eventually reaching a state where the entire film is damaged.

Analysis of damage modes of hybrid bolted-bonded joints in hygrothermal environment

Hygrothermal environments can adversely affect the microscopic failure modes and damage characterization of composite laminates and adhesive films. It will damage the matrix and fiber resin of the laminate, affect the bonding performance of the adhesive film, and further have an adverse effect on the integrity and safety of the overall hybrid joint. In order to understand the failure modes of hybrid bolted-bonded joints in more detail, it is necessary to analyze each characteristic damage mode of the joint and the failure surface of the joint to determine the failure modes. The failure modes of bolted joints include tensile failure, shear failure, pull-off failure, tearing failure, and load-bearing failure. When net tensile failure occurs, the joint breaks along the smallest cross-section perpendicular to the tensile direction due to the destruction of material integrity. Shear failure means that during loading, the bolt neck exerts pressure on the part in contact with the composite material. If it cannot withstand the shear force, it will break. Cracking damage occurs when the composite component in contact with the bolt neck cannot withstand the load. Extrusion failure is caused by local compressive pressure, contact force and friction force acting on the hole surface. The failure modes of bonded joints include cohesive failure, adhesive failure of the adhesive layer and failure of the adherend. When the bonding interface strength is greater than the adhesive film strength, bonding failure occurs along the cleavage surface of the adhesive. The bonding strength of the adhesive interface is low, and adhesive failure is generally caused by the weak lateral strength of the composite material. In addition, fiber tearing failure mode may also occur in hybrid joints, where the crack propagates as close to the cleavage plane as possible on the adhesion surface. In hybrid bolted-bonded joints, the failure mode is a mixture of the failure mode of bolted joints and the failure mode of bonded joint.

Analysis of damage modes of hybrid bolted-bonded joints with different end-to-diameter ratios in hygrothermal environment

The macroscopic failure morphologies of hybrid bolted-bonded joints with different end-to-end ratios in RTD and ETW environments are shown in Figure 13, 14 and 15. The main macroscopic failure forms in static tension are debonding and shear failure of the 730000 adhesive layer, cracking and pulling out of the bolt holes, severe compression around the holes, fiber tearing and bolt breaking, and delamination of the laminate.

As shown in Figure 14, when E/D is small, pullout of the bolt holes occurs due to the small end spacing, and fiber tearing around the holes is more pronounced. One of the destruction modes of fiber tearing, which is a good performance of the adhesive film, when the adhesive layer exhibits cohesion damage and debonding damage. As E/D increases, the glued area gradually increased and the adhesive layer appeared to be completely debonded. The adhesive layer is accompanied by some fiber fragmentation in the region near the hole perimeter, and the outer bolt hole perimeter also shows varying degrees of shear damage, while the delamination of the laminate was not obvious.

Comparing Figures 14(a) and (b) and Figures 15(a) and (b), the macroscopic failure mode of the joint in the ETW environment is generally similar to that in the RTD environment. In the ETW environment, the shear damage of the joint and the compression damage around the holes are more pronounced, and the cohesive damage of the adhesive layer is also more serious. This is because in the ETW aging state, the properties of the adhesive film deteriorate, causing cracks inside the film to expand. After aging in the ETW environment, the interlaminar properties of the composite laminates, except for the adhesive film, are affected. The damage starts from the film and spreads to the inner layer of the film. Hygrothermal aging also alters the failure mode of crack initiation and crack propagation within the adhesive film.

Taking the specimen with the optimal end-to-diameter ratio of 3 as an example, the hole circumference of the two surfaces of the laminate on the nut side, the laminate thickness direction, and the adhesive surface are selected. The microscopic damage morphology of the joint in RTD and ETW environments was investigated by SEM scanning electron microscopy.

In the vicinity of the bolt hole, compare Figure 16(a) with Figure 17(a) and Figure 16(b) with Figure 17(b), respectively. It can be found that the composite laminates suffered more serious damage in the ETW environment. The compression of the laminate is more severe, and the debonding of the fibers and the matrix is also intensified. There is also a more obvious matrix fracture and collapse state at the hole edge. It has an increased number of fiber breaks and increased fiber bundle pull-up. At the same time, more obvious delamination appeared around the holes, and the cracking around the holes of the laminate also increased. The reasons for these phenomena are mainly attributed to the influence of the ETW environment. The ETW environment will expand the scope of damage to a certain extent and enhance the water absorption capacity of the composite matrix and fibers. However, since water molecules have different diffusion speeds inside the two materials and diffuse asynchronously, it will aggravate the expansion of cracks, collapse of the matrix, debonding of resin and fiber, and increase in fiber breakage.

As shown in Figure 18, there is no serious delamination in the thickness direction similar to that of bolted joints of the laminate due to the adhesive layer. In the ETW environment, more dense holes appeared in the 0° direction, which is also caused by the diffusion of water molecules. In the 90° direction, the interfacial bonding between the fiber and the matrix is weakened due to the high temperature environment. The fiber breakage and debonding phenomena increase, and more resin debris also appears. As shown in Figure 19, in the ETW environment in the bonding area, it can be seen that the adhesive exhibits greater deformation than that in the RTD environment. The debonding phenomenon of the adhesive layer is more obvious, and some residual adhesive can be observed. The bonding cracking damage is also more serious, and the number of holes in the adhesive film has increased significantly. This phenomenon indicates that the fiber and adhesive together show significant deformation before the fiber breaks and pulls out. In summary, as water molecules penetrate the adhesive from the edges of the laminate joints and around the bolt holes, the effect of moisture diffusion is amplified, intensifying the chemical changes and hydrolysis process around the bonding area around the holes. This will further significantly reduce film performance.

Analysis of damage modes of hybrid bolted-bonded joints with different width-to-diameter ratios in hygrothermal environment

The macroscopic failure morphologies of hybrid bolted-bonded joints with different width-to-diameter ratios in RTD and ETW environments are shown in Figures 20–22. The main macroscopic failure forms in static tension are debonding and shear failure of the adhesive layer, cracking and pulling out of the bolt holes, severe compression around the holes, fiber tearing and bolt breaking, and delamination of the laminate.

As shown in Figures 21(a) and (b), when the width-to-diameter ratio is small (W/D = 3, 4), the bolts show serious tilt deformation, and part of the nut is embedded in the laminate due to tension. Perpendicular to the tensile direction, obvious cracking and breaking of the laminate material occurred around bolt holes in the laminate. This is because the distance of the hole edge from the edge of the laminate is too small when the width-to-diameter ratio is small. At the same time, the laminate was accompanied by obvious shear failure, and serious delamination occurred in the thickness direction of the laminate. In the ETW environment, this phenomenon exacerbated to varying degrees. When the width-to-diameter ratio reaches 6, the delamination of the laminate is significantly reduced. This may be due to the increase in the adhesive layer area, which has mitigated this phenomenon to a certain extent. As the width-to-diameter ratio increases, the bonding area further increases, and more obvious debonding of the adhesive layer occurs. As shown in Figure 22(a) and (b), obvious film debonding and cohesive failure occur in the bonding area. Compared with the RTD environment, the laminate has less residual adhesive in the ETW environment. This indicates that the high temperature and high humidity environment will weaken the interface performance to a certain extent.

Taking the specimen with the optimal width-to-diameter ratio of 6 as an example, the hole circumference of the two surfaces of the laminate on the nut side, the thickness direction of the laminate, and the adhesive surface are selected. The microscopic damage morphology of the joint in RTD and ETW environments was investigated by SEM scanning electron microscopy.

Near the bolt hole, compare Figure 23(a) with Figure 24(a), and Figure 23(b) with Figure 24(b). It can be found that its microscopic damage morphology characteristics are similar to those of the specimen with the optimal width-to-diameter ratio of 3. The compression damage of composite laminates in ETW environment is more severe than that in RTD environment. The fiber fragmentation, fiber bundle breakage, fiber-matrix crossing and embedding phenomena are obvious, accompanied by matrix cracking and delamination. Moreover, the fiber fracture is relatively neat in the ETW environment because the material expands and becomes more ductile in the high temperature environment.

As shown in Figure 25, under the ETW environment in the thickness direction, there was obvious debonding between the fiber and the resin in the 0° direction, accompanied by fiber pullout and fragmentation. This is also due to the decrease in molecular bonding caused by the diffusion of water molecules, which in turn leads to a decrease in interface performance. Obvious fiber breakage and less disordered resin distribution appeared in the 90° direction. This is because the thermal stress generated by high temperature reduces the brittleness of the matrix and weakens the fiber-matrix interface adhesion. As shown in Figure 26, in the bonding area under ETW environment, there is less residual adhesive on the surface of the composite laminate. Due to the diffusion of water molecules and the effect of high-temperature thermal stress, the microscopic morphology of the adhesive surface shows more holes and is accompanied by a certain degree of cracks.

Compared with the RTD environment, the macro- and micro-damage forms of the hole circumference, thickness direction, and adhesive layer of the composite laminates in the ETW environment have changed and intensified to a certain extent. In the area near the hole periphery, the adhesive layer undergoes plastic deformation. Under the combined effect of shear stress and plastic deformation of the bolt, the bolt fails and breaks on the mid-axis surface. This indicates the stress concentration process in the contact area between the bolt and the bolt hole surface. In the case of hygroscopic distilled water, the smaller water molecule size will result in water diffusing more easily through the polymer network, thus filling the free space before reaching the saturated state. The inherent brittleness of the adhesive film causes the hybrid joint to suddenly drop in load after reaching the maximum load. Secondly, moist heat aging will affect the adhesive and the substrate nearby, resulting in composite bonding and delamination failure of the substrate. Fiber pullout is the main failure mechanism caused by weakened fiber-matrix interface bonding at higher temperatures. The damage of the film is mainly manifested as cohesive fracture near the interface.

Conclusion

In this paper, the static strength properties, joint macroscopic and microscopic damage modes, and damage forms of laminates of hybrid bolted-bonded joints with different geometrical and material parameters in RTD and ETW environments are investigated using a combination of experimental and simulation methods. The static strength failure laws under different end-to-diameter ratios, width-to-diameter ratios, and lay-up methods both at room temperature and in wet heat environments are summarized. The macroscopic and microscopic damage morphology of hybrid joints in RTD and ETW environments is investigated, and the damage forms of composite laminates are analyzed using the limiting element model. The main conclusions are as follows:

(1) A damage model of hybrid bolted-bonded composite joints under a hygrothermal environment was established. The error between its ultimate failure load and the test ultimate load is controlled within 10%. This model can provide a certain reference for predicting the strength degradation of hybrid bolted-bonded composite joints under hygrothermal environments.

(2) Different design parameters have different effects on the static strength performance of the hybrid bolted-bonded joint. Within a certain range, the bearing capacity of the hybrid bolted-bonded joint increases with the increase in the end-to-diameter ratio and the width-to-diameter ratio. In this paper, the hybrid bolted-bonded joint with the end-to-diameter ratio of three and the width-to-diameter ratio of 6 has stronger bearing capacity.

(3) Compared with the RTD environment, the bearing capacity of the hybrid bolted-bonded joint in the ETW environment is significantly reduced. This is because thermal stress causes the molecules to arrange in a free molecular packing, which reduces the molecular bonding force. The diffusion of water molecules will intensify the chemical changes and hydrolysis processes around the holes and in the bonding area while affecting the matrix properties, which will in turn have an adverse effect on the static properties of the hybrid joint. When the end-to-diameter ratio is three and the width-to-diameter ratio is 6, the bearing capacity of the hybrid bolted-bonded joint decreases by 17.50%.

(4) Scanning electron microscopy revealed that the laminate suffered more serious damage in the ETW environment. The compression of the laminate is more severe, and the debonding of the fibers and the matrix is also intensified. There is a more obvious matrix fracture and collapse state at the edge of the hole. The number of fiber breakages and fiber bundle pull-out phenomena increased. More obvious delamination occurred around the holes, and the cracking around the holes of the laminates also increased. In the thickness direction, the fiber breakage and debonding phenomena increased in the 90° direction, and more resin debris also appeared. The adhesive exhibits greater deformation, and the debonding of the adhesive layer is more obvious. Some residual adhesive can be observed, and its bonding cracking damage is also relatively serious. The number of holes in the film has increased significantly.

Footnotes

Acknowledgments

This research is financially supported by the Tianjin Education Commission Research Plan Project (2021KJ054). The authors would like to acknowledge the editors and the anonymous referees for their insightful comments.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Tianjin Education Commission Research Plan Project (Project No. 2021KJ054).

ORCID iDs

Peng Ren

Haijian Xiao

Data availability statement

Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

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