Abstract
In this study, the effects of hygrothermal aging of composite patches and adhesives on the plasticity ahead of repaired crack with bonded composite patch were analyzed. The finite element method was used to compute the size and the shape of the plastic zone for cracks repaired with bonded hygrothermal aged composite patches and adhesives. The obtained results show that moisture absorption has a negative effect on the repair performance. Indeed, the size of plastic zone increases with an increase in the rate of water absorption whether in composite patches or in adhesives. The reduction of the composite patches and adhesive strength by the water absorption leads to a reduction of the repair performances.
Introduction
The use of advanced laminated composite materials for repair and strengthening of existing structures has emerged as a promising technology in aerospace, mechanical, and civil engineering.1–4 Performance level and life span of existing structural elements can be increased by repair and strengthening of these structural elements using advanced composite materials. The bonded repair reduces stresses in the cracked region and keeps the crack from opening, and therefore from growing.5–9
Adhesive properties play an important role in the performance of the bonded composite repair. Bachir Bouiadjra et al. 10 showed that an adhesive with high-shear modulus (with bad qualities) gives weak stress intensity factor at repaired crack tips. Hence, they recommended the use of this material for increasing the performance of bonded composite repair. In spite of the fact that higher adhesive shear modulus leads to higher adhesive stresses, this increases the risk of adhesive failure. They also highlighted the importance of the adhesive thickness and concluded that the adhesive properties must be optimized in order to improve the performance of the repair and to avoid adhesive failure.
Although adhesively bonded patch repair of cracked metallic structures has been studied extensively and service experience with repairs has been good, it appears that further work is required to address some remaining problems and to assess the full potential of the repair technique. Among the specific research objectives are the assessment of the influence of hot–wet fatigue test environments on patch efficiency, and the effect of long-term pre-exposure to hot–wet environments on such behavior. Several studies11–15 threat the effect of environmental conditions on composite materials. They showed that all polymer composites absorb moisture in humid atmosphere, and when immersed in water, the effect of absorption of moisture leads to the degradation of fiber–matrix interface region. This degradation leads to a poor stress transfer rate between the repaired metallic plate and composite patch throughout the adhesive layer The humidity absorption may also reduce the strength and the stiffness of the adhesive and leads to apparition of corrode region under the patch.
Analyses of adhesively bonded composite patches to repair cracked structures have been the focus of many studies. Most of these studies investigated the damage tolerance of the repaired structure by using linear elastic analysis.16–19 In this study, the non-linear three-dimensional finite element method is used to investigate the contour and the size of the plastic zone ahead of repaired cracks with bonded composite patch. The effects of water absorption by the adhesive and the composite patch on the plastic zone size were highlighted.
Geometrical and finite element models
Let us consider a plate in aluminum alloy 2024T3 with the following dimensions: length: L = 300 mm; width: W = 250 mm; thickness: ep = 1.3 mm and the following elastic properties: Young’s modulus: Ep = 72,400 MPa; Poisson’s ratio = 0.3. The elatic–plastic curve of the aluminum alloy 2024T3 is given in Figure 1. The plate is subjected to uniaxial tensile load giving a remote stress state of σ = 70 MPa (Figure 2). One supposes the existence of a central crack of length 2 a parallel to the loading direction. The crack is repaired with a bonded hygrothermal aged glass–epoxy composite patch with the dimensions, width Wr = 100 mm and thickness er = 0.5 mm. The material properties of the composite patch are presented in Table 1. Due to symmetry of both geometry and loading, only one-half of the structure is modeled.
Stress–strain curve of aluminium alloy 2024T3. Geometrical model. Mechanical characteristics of the bonded patch composite
20
Mechanical characteristics of the adhesive in water absorption 21
The analysis involved a three-dimensional finite element method by using a commercially available finite element code ABAQUS. The finite element model consisted of three subsections to model the cracked plate, the adhesive, and the composite patch. Because of symmetry in both geometry and loading, only one-half of the models were analyzed. The plate had four layers of elements in the thickness direction, the adhesive had only one layer of elements through thickness, and the patch had two layers of elements through thickness. The mesh was refined near the crack tip area with an element dimension of 0.062 mm using at least 16 such fine elements at the front and back of the crack tip. Figure 3 shows a typical mesh model of the repaired structure and mesh refinement in the crack tip region. The procedure used in the finite element analysis involved the following steps: the tensile stress was applied to the gripped specimen. General static ‘STEP’ option was used for analysis with ABAQUS. Automatic increment of steps was used with a maximum number of increments as 100. Minimum increment size was 10 E − 5, while the maximum increment size is 1. Figure 3 presents the finite element mesh of the patched structure.
Typical mesh model of the repaired structure.
Results and discussion
Effect of the crack length
Before analysing the effect of patch composite hygrothermal aging on the size of plastic zone ahead of repaired crack, it is considered to be useful to estimate the evolution of the plastic zone size according to the crack length without the effect of moisture absorption. Figure 4 presents the variation of the contour of the plastic zone as a function of the crack length. It is shown that the size of the plastic zone size increases when the crack length varies between 5 and 15 mm and the difference in the plastic zone size between the two crack lengths is considerably significant. Therefore, it can be noted that the stress intensity at the crack tip is not important enough and consequently the stress transfer between the repaired plate and the patch is not significant, what explain the great difference of the plastic zone size between a = 5 mm and a = 15 mm. As the crack length is important (a > 15 mm), it can be noted that the effect of the crack length on the plastic zone size ahead of repaired crack is not important. Therefore, the plastic zone size has tendency to stabilize as the crack length increases. These results confirm those of Albedah et al.
22
By comparing with the results obtained by Bachir Bouiadjra et al. for unpatched crack,
23
it can be noted that the reduction of the plastic zone by the patch is about 10 times. So we can say that the bonded repair reduces stresses in the cracked region and keeps the crack from opening, and therefore from growing.
Contour of the plastic zone for different crack length.
Effect of composite hygrothermal aging
Figure 5 presents the variation of a plastic zone contour for different rates of water absorption in bonded composite patch. According to this figure, it can be seen that the plastic zone size increases with the increase of the rate of water absorption in composite patch. These results are confirmed in Figure 6. This last figure presents the variation of the plastic zone radius according to the crack lengths for deferent rates of water absorption in the composite patch. It can be noticed that the increase in the plastic zone radius is about 10.64% when the rate of water absorption varies between 0.08% and 1.46% for crack length a = 5 mm. This deference is about 20.94% for crack length of 15 mm and 22.36% for crack length of 40 mm. This behavior is due to the fact that the moisture absorption by the composite reduces its strength and consequently the stress transfer between the repaired plate and the composite patch decreases. Therefore, this effect leads to the reduction of the fatigue life of the repaired structures. Quantitatively, the reduction of the fatigue life can be highly significant because the size of plastic zone increases proportionally to the rate of water absorption.
Contour of the plastic zone for different rates of water absorption by the composite patch. Variation of plastic zone radius vs. crack length for different rates of water absorption by the composite patch.
Effect of adhesive hygrothermal aging
An elevated temperature and relative humidity environment can accelerate many failure mechanisms of structure. It is known that the adhesive properties play a considerably important role in the repair process with bonded composite patch. The adhesive layer transfers the stresses from the damaged plate toward the composite patch. In this paragraph, the effect of environment and absorption of water by the adhesive on the size of the plastic zone ahead of repaired crack is analyzed. Figure 7 presents the variation of plastic zone contour for different rates of water absorption by the adhesive layer. It can be noted that the plastic zone size increases when the rate of water absorption by the adhesive increases too. One can thus confirm that the absorbed moisture had deleterious effects on the mechanical properties of the adhesive and can greatly compromise the role of the adhesive layer. The results of Figure 8 confirm what was advanced previously; finally, this presents the variation of plastic zone radius for different rates of water absorption by the adhesive. One can see that the increase in the plastic zone radius is 41.52% when the rate of water absorption varies between M = 0% and 1% and this deference is about 51.85% when the rate of water absorption varies between M = 0% and 4%. One can conclude that the rate of water absorption has a considerably negative effect on the mechanical behavior of the adhesive. This absorption leads to a significant reduction of the mechanical properties of the adhesive, which attenuate the stress transfer between the patch and the damaged plate. The fatigue life of the repaired structures is thus negatively affected by the moisture absorption by the adhesive.
Contour of the plastic zone for different rates of water absorption by the adhesive. Variation of plastic zone radius vs. water absorption rate in the adhesive.
Concluding remarks
In this study, the effects of water absorption on the size plastic zone ahead of repaired cracks were analyzed using the finite element method. The obtained results allow us to deduce the following conclusions:
The presence of the composite patch reduces considerably the size of the plastic zone ahead of the crack. This reduction is more significant when the crack length exceeds 15 mm. The variation of plastic zone size increases when the rate of water absorption in the composite patch increases. The moisture absorption by the composite patch has a considerably negative effect on the fatigue life of repaired structures. The variation of plastic zone radius increases when the rate of water absorption in adhesive increases. This variation can exceed 50% when the rate of water absorption is about 4%.
Footnotes
Acknowledgment
The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group no. RGP-VPP-035.
Funding
This work was supported by King Saud University.