The influence of structural parameters of fabrics on the bending strength of T-shaped composites

Abstract

Fabric-reinforced composites with an assumed cross-sectional shape due to their specific properties and the possibility of their shaping are increasingly used in many industrial sectors. The article presents the results of research on the influence of T-shaped fabric structural parameters on the bending strength of the composites reinforced with this fabric. The composites were reinforced with a designed 3D fabric, which was made in one step on a classic harness loom. The test results showed that the increase in the bending strength of the T-composite was influenced by the increase of the warp and weft density in the web and the reduction of the weft density in the base. The most important cross-sectional element influencing the bending strength of the T-shaped composite is the web element and the distribution of threads in this composite element. In addition, the T-shaped structure was reinforced by using a multilayer fabric in the appropriate cross-sectional elements. There was a three-fold increase in bending strength compared to composites with the fabric without reinforcement. The strength properties of the composite are influenced by the ‘density’ of the structure expressed by the density of threads. By modifying the construction of the reinforced fabric, the bending strength properties can be shaped.

Keywords

Textile composites woven textile T-shaped bending strength

Introduction

Modern engineering constructions place very high demands on materials. They are expected to have low specific weight and high strength, as well as resistance to fatigue load and corrosion. Textile-based composite materials can meet these requirements. In certain areas of application, composite materials reinforced with flat textile products, or formed into a spatial product by stitching or bonding, do not provide the expected properties. Such formed structures are characterized by a risk of delamination of the components under the influence of forces.¹ Therefore, new materials for the reinforcement of composites with appropriate characteristics are sought, which may be fabrics of spatial structure.

The properties of the composite are affected, apart from the type of fibres and matrix, by the geometric form of the reinforcement.^2,3 The properties of 3D woven fabric-reinforced composites⁴ are better than the commonly used 2D laminates, but due to technological limitations and production efficiency, their potential has not been sufficiently exploited so far^5,6 compared to 3D braided or flat fabric laminates.

Woven fabrics are the structures used to reinforce composites.⁷ They are mostly flat fabrics, which are the starting material for shaped laminates. In L-shaped composites, base or folding point has a major role in distortion and increasing the thickness at base changes the mechanics and ability to resist the deformations is increased.⁸ Recently, weaving techniques have been modified to adapt them to the production of 3D fabrics, including shaped fabrics with different amounts of threads in x, y and z directions.⁹ Advanced weaving technologies enable the production of structurally diversified materials for special purposes. 3D woven fabrics¹⁰ include thick multilayer fabrics in a simple regular form or made in more complex shapes, multilayer fabrics containing voids (cells) and thin 3D shells/cores of complex shapes.¹¹ 3D/spatial fabrics are fabrics that have a defined arrangement of threads in three mutually perpendicular systems.¹² However, the same conventional 2D weaving technique may also be used to produce certain spatial structures in which the fibres form a three-dimensional architecture.

T-shaped composites are an example of spatially reinforced materials in aircraft and wind turbine applications^13–15 and elements of boat hulls,¹⁶ which made it necessary to research and develop models to learn their strength characteristics.^17–22 In such composites, the point of connection of the rib (web) with the base is important,^23–25 also in composites reinforced with woven structures.²⁶ In the case of T-shaped bonded composites, delamination occurs in this point.^27–29 In the laminates formed in T-shaped composites, there are high stress concentrations in the area of rib-base connection, whereas the 3D woven structure shows relatively uniform stress distributions in the whole structure.^30,31 By means of an appropriate way of structure modification through stitching, the strength properties can be improved.³² In the T-shaped carbon composite reinforced by needling with fibres introduced in the additional Z-direction, the delamination resistance under both static and fatigue loading increased.³³ Currently, composites reinforced with natural fibres are the subject of research, due to good cost–performance ratio and environmental friendliness,¹⁰ biocompatibility and biodegradability,³⁴ and also T-shaped composites.³⁵

The use of shaped fabric as a reinforcement of T-shaped composite allows to obtain a higher initial damage load value (increase by 48%) and a higher peak value (increase by 30%) compared to the non-directional seam-bonded flat textile products. These results were obtained using the method described in ASTM D790 (Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials). The use of shaped fabric for reinforcement, that is, interlacing threads, causes an increase in delamination resistance. This is due to the fact that the fabric structure has a relatively complex thread (fibre) track, which effectively blocks the initiation of the composite cracking. The size of damage occurring in both tests was estimated using the ultrasound scanning technique.³⁶ The analysis of mechanisms of deformation and internal stress distribution indicates that both deformations and internal stresses are closely correlated with each other and can be reduced using other manufacturing methods.³⁷ In composites reinforced with woven fabrics, the reduction of the braiding angle increases the tensile and compressive strength in the braiding direction and decreases in the perpendicular direction to the braiding direction. The value of force increases with the increase of fibre volume share. Strength properties of woven T-shaped structures can be shaped by means of structural solutions by using various models of multilayer fabrics.³⁵ Understanding the mechanism of strength shaping and developing a method of modelling the bending strength of a composite reinforced with T-shaped fabric, by means of structural parameters of the fabric, allow predicting the properties of the composite at the design stage.

There are no detailed analyses of the possibility of modelling the strength properties of 3D composites, obtained based on three-dimensional fabrics made of flat fabric, in the literature data; the authors focused primarily on the study of the strength properties of composites reinforced with 3D fabrics with a structure with a multi-layered arrangement of warp and weft threads;^27,38,39 nevertheless, it confirms the fact of great interest in this field. The research aimed to assess the possibility of shaping the bending strength of a composite reinforced with a three-dimensional fabric with a T-section, using the fabric’s structural parameters. Determining the influence of particular structural parameters of the fabric will allow forecasting the strength properties of the composite at the stage of its design. The aim of the research was at the same time to improve the strength properties of the composite by strengthening the connection point of the web with the base using the developed method of its strengthening.

Materials

The mechanical properties of composites and the mechanism of their destruction also depend on the strength properties of the fibres. All the spatial fabrics constituting the basis for obtaining the research material in the form of composites were made of polyester filament yarn with increased strength properties and reduced elongation of 1060 dtex, 256 elementary fibres and 60/1m twists. The yarn was characterized by an average breaking force of 6601 cN and relative elongation at maximum force of 13.3%.

This fabric for the reinforcement was made on a laboratory conventional dobby (18 harnesses) shuttle loom (Figure 1) and is made flat using the multiple-width product fabric technique. In the element, which after unfolding constitutes the flange of the T-shape, there are two layers: upper and lower, in which, in order to obtain monolithic reinforcement, a multilayer weave is used. The weave of the fabric of the upper and lower layers (within the T-shaped flange) is identical. The upper and lower layers are not interconnected along the length of the flange. Within the web element, the lower and upper layers are combined into one, in which a weave is identical to that of the flange. The number of warp threads (288) in the whole cross-section of the reinforcement fabric was assumed as a constant structure parameter. In the fabrics, the variable parameters were the number and distribution of warp threads in each tape element. A change in the width of an element while maintaining a constant number of warp threads caused a change the warp density. When designing the fabrics, the plain weave was used in each shape element (Figure 2).

Figure 1.

Loom and way of making loom fabrics (SL8900S by CCI Tech, purchased within the Key Project No. POIG.01.03.01-00-006/08 ENVIROTEX).

Figure 2.

T-shaped structures, (a) scheme and (b) weave.

The fabrics produced in this way, differing in the number of warp threads and warp density in individual cross-sectional elements and with three weft densities (G1, G2 and G3) (Table 1), were used to make composites using the vacuum bag (Figure 3) method and the E53 epoxy resin, which was selected based on the tests of shrinkage and adhesion of the resin to the fibre. In order to obtain the intended T-shape of the cross-section of the composite, forms made of steel elements secured with anti-adhesive agents were used.

Table 1.

Summary of technological parameters of T-shaped fabrics.

Sample	Number of warp threads in individual cross-sectional elements			Weft density in individual cross-sectional elements per 10 cm			Width of cross-section an element, mm			Weight per metre, g/mb	Weight per metre of composite, g/mb
	1	2	3	1	2	3	1	2	3
	Flange		Web	Flange		Web	Flange		Web
I.a.G1	96	96	96	75	75	75	30	30	30	38.88	93.95
I.a.G2				60	60	60				37.52	129.62
I.a.G3				40	40	40				35.00	128.63
I.b.G1				75	75	75	30	30	60	40.22	101.64
I.b.G2				60	60	60				38.37	108.91
I.b.G3				45	45	45				36.02	126.99
I.c.G1				75	75	75	60	60	30	41.40	113.62
I.c.G1				60	60	60				39.50	145.52
I.c.G3				40	40	40				36.39	137.88
II.a.G1	72	72	144	75	75	75	30	30	30	38.50	90.89
II.a.G2				60	60	60				37.86	92.82
II.a.G3				45	45	45				35.03	104.53
II.b.G1				75	75	75	30	30	60	39.05	117.05
II.b.G2				60	60	60				37.48	126.29
II.b.G3				45	45	45				36.06	119.60
II.c.G1				80	80	80	60	60	30	42.41	148.50
II.c.G2				65	65	65				39.37	122.84
II.c.G3				45	45	45				37.44	144.73
III.a.G1	120	120	48	75	75	75	30	30	30	38.61	111.19
III.a.G2				60	60	60				36.55	111.69
III.a.G3				40	40	40				34.61	105.12
III.b.G1				80	80	80	30	30	60	40.87	111.89
III.b.G2				60	60	60				37.90	137.71
III.b.G3				40	40	40				35.83	117.73
III.c.G1				80	80	80	60	60	30	40.55	106.87
III.c.G2				60	60	60				38.43	107.35
III.c.G3				40	40	40				36.07	117.29

Figure 3.

The vacuum bag for composite making.

With a constant number of warp threads, the widths of the individual elements have been changed to vary the warp density (variants of changing the warp density with the same number of threads). The scheme of the distribution of warp threads is shown in Figures 4 to 6. The obtained samples differed in the value of section modulus W (analytically determined dependence of the amount of deformation on the cross-section shape of the composite subjected to bend).

Figure 4.

Scheme of distribution of warp threads in individual section elements – samples ‘a’.

Figure 5.

Scheme of distribution of warp threads in individual section elements – samples ‘b’.

Figure 6.

Scheme of distribution of warp threads in individual section elements – samples ‘c’.

Test methodology

The research methodology included preparatory work in the field of textile raw materials testing and selection of polymeric resin for the finishing of composites. The multivariance of structural solutions was worked out based on the variation in the warp density and distribution while maintaining a constant number of warp threads in the cross-section and the weft density.

Bending strength of T-shaped composites

Manufactured composites of ‘T’ cross-sectional shape was tested according to a developed procedure based on the INSTRON 3367 tensile testing machine equipped with special fixtures, shaped for bending of the tested samples (Figure 7). The distance between the fixture elements was experimentally determined at 6 cm so that during the bending test, no slipping of samples between fixture elements occurred. A fixture-bending element, wider than test samples, was fixed parallel to the base element halfway between them. The conditioned samples, cut out without damaging the structure, were placed symmetrically on the fixture base. The bending test was carried out at a constant speed of 50 mm/min. The load–strain curve was recorded until the moment of sample damage (end of test) occurred. The test result – the maximum force carried by the sample – was read.

Figure 7.

The tensile testing machine equipped with special fixtures.

The stress at which the damage of the sample occurs was determined analytically, taking into account the section modulus W and bending strength Rg, to analyse the tensile strength of composite elements.

The coefficients of filling

For the fabric samples produced, the coefficients of filling the cross-section with warp and weft threads were calculated. This coefficient depends on the thread’s density, geometric dimensions and yarn diameter. This parameter reflects the packing of threads of both systems in the cross-section of the fabric and corresponds to the volume content of fibres in the composite.

$W_{p_{o}} = \frac{Sum of warp yarn cross ‐ sectional areas}{cross ‐ sectional area along the warp} [%] W_{p_{w}} = \frac{Sum of weft yarn cross ‐ sectional areas}{cross ‐ sectional area along the weft} [%]$

The results

Bending strength of T-shaped composites

Figure 8 presents average values of bending strength. Based on the results obtained, the analysis of the effect of warp and weft density, the number of warp threads in individual sectional elements while maintaining the same width on the bending strength of the composite reinforced with T-shaped fabric was performed.

Figure 8.

Average values of bending strength.

The effect of variable warp density on bending strength

The effect of the changed warp density and section modulus while maintaining the same weft density on the bending strength of composites is shown in Figure 8. In most cases, the highest values of bending strength are observed for the samples in which warp density is the highest. When comparing all the samples, it should be stated that the lowest values of bending strength occur for the samples in which the web element has the lowest, of all samples, the number of warp threads − 48. In the samples with the lowest value of weft density, an increase in the bending strength of the composite was observed.

The effect of the variable number of warp in individual cross-section elements while maintaining the same width

The criterion for assessing the impact of structural parameters of the T-shaped fabric is influenced by the distribution of the warp threads in each geometric element of the fabric. While maintaining the same width of individual cross-section elements, the distribution of the warp threads, that is, the number of threads in individual elements, changes.

The effect of the variable number of warp in individual cross-section elements while maintaining the same width of individual cross-section elements is shown in Figures 9 to 11.

Figure 9.

Average values of bending strength – samples I. a. (G1, G2 and G3), II. a. (G1, G2 and G3), III. a. (G1, G2 and G3).

Figure 10.

Average values of bending strength – samples I. b. (G1, G2 and G3), II. b. (G1, G2 and G3), III. b. (G1, G2 and G3).

Figure 11.

Average values of bending strength – samples I. c. (G1, G2 and G3), II.c. (G1, G2 and G3), III. c. (G1, G2 and G3).

It was observed that the value of the bending strength parameter is the highest in samples where the number of warp in the web is the highest, that is, 144 in comparison with samples of the same geometry. In the samples with the lowest value weft density, an increase in the composite bending strength is observed. The values of bending strength in all the samples are the smallest in the samples‘b’, with the dimensions of flange elements of about 60 mm and web dimension of about 30 mm.

The effect of variable weft density in T-section elements

The weft density is the structural parameter that affects the bending strength of composites. With the same weave, warp thread distribution and the value of section modulus W, only the weft density changed. Samples were made with three weft densities for each structural and geometric variant of the fabric. The influence of variable weft density in the elements of the T-section is presented in Figure 12.

Figure 12.

Summary of average values of bending strength of samples with different weft densities.

With the same number and distribution of warp threads, and the same section modulus W, in most cases, the bending strength increases with the decrease in the size of weft threads. In order to determine the influence of many factors such as the geometry of the cross-section of T-shaped composite, taking into account the dimensions of individual cross-section elements, the number of warp threads in the cross-section elements and the weft density per dependent variable, the bending strength value of the T-composite reinforced with single spatial fabric, the analysis of variance was applied for the multivariate ANOVA classification (Table 2).

Table 2.

Table with the results of the multivariate analysis of variance.

Effect	F	p-value
Intercept term	2393.62	0.000000
Variable 1 section modulus	216.727	0.000000
Variable 2 number of warp thread	37.218	0.000000
Variable 3 weft density	29.018	0.000000
Variable 1/variable 2	9.749	0.000005
Variable 1/variable 3	8.177	0.000031
Variable 2/variable 3	5.617	0.000744
Variable 1/variable 2/variable 3	5.964	0.000018

Based on the calculations obtained, it was found that all factors have a statistically significant influence on the dependent variable, that is, bending strength. Both the influence of individual factors and their interaction are highly significant. In each case, the level of significance is lower than the assumed p = 0.05. Similar conclusions can be drawn in relation to the effect of all three factors on bending strength. It was found that the interaction of the three factors is highly significant (p = 0.000018).

Figures 13 to 15 illustrate the influence of the levels of one factor on the examined variable at a certain level of the other factor. Depending on the course of these curves, we can conclude that one factor interacts with the other or does not interact. The occurrence of a significant interaction, that is, modification of the interaction of one factor by the other factor, is illustrated by the crossing of the polylines.

Figure 13.

The influence of individual factors on the dependent variable.

Figure 14.

The influence of two factors on the dependent variable.

Figure 15.

The influence of all factors on the dependent variable.

On the basis of the calculations performed, a statistically significant impact was found of three examined factors such as cross-section geometry of the T-shaped composite, taking into account the dimensions of individual section elements, number of warp threads in section elements and the weft density per variable dependent on the bending strength value of T-shaped composite reinforced with a single spatial fabric.

The coefficient of filling

For the fabric tests, the coefficient of filling of the cross-section with weft and warp threads was determined (Table 3). Fibre content influences the mechanical characteristics of the composites.

Table 3.

The coefficient of filling of the cross-section with weft and warp threads.

Sample	Coefficients of filling the cross-section with warp, %			Coefficients of filling the cross-section with weft, %
	1	2	3	1	2	3
	Flange	Web		Flange	Web
I.a.G1	84.27	84.27	84.36	19.75	19.75	19.11
I.a.G2	84.27	84.27	85.74	15.80	15.80	15.54
I.a.G3	91.93	91.93	84.36	11.49	11.49	10.19
I.b.G1	81.55	79.00	66.53	19.75	19.75	29.63
I.b.G2	81.55	79.00	66.53	15.80	15.80	23.70
I.b.G3	81.55	80.25	68.95	11.85	11.85	17.78
I.c.G1	68.95	68.95	91.93	29.63	29.63	21.55
I.c.G1	67.71	67.71	101.12	23.70	23.70	18.96
I.c.G3	91.93	90.29	108.73	21.07	21.07	14.04
II.a.G1	98.07	98.07	125.36	29.63	29.63	21.55
II.a.G2	98.07	98.07	137.89	23.70	23.70	18.96
II.a.G3	101.57	101.57	137.89	17.78	17.78	14.22
II.b.G1	101.57	98.41	102.30	29.63	29.63	29.63
II.b.G2	98.75	97.06	95.60	23.70	23.70	23.70
II.b.G3	91.74	118.19	109.81	17.78	20.31	20.31
II.c.G1	78.19	78.46	125.70	50.56	50.56	25.28
II.c.G2	79.83	79.83	121.34	41.08	41.08	20.54
II.c.G3	77.13	78.46	118.19	28.44	28.44	14.22
III.a.G1	126.40	126.40	87.17	23.70	23.70	39.50
III.a.G2	122.32	126.40	87.17	18.96	18.96	31.60
III.a.G3	122.32	126.40	90.29	12.64	12.64	21.07
III.b.G1	105.33	101.94	71.55	21.07	21.07	63.20
III.b.G2	103.61	101.94	72.92	15.80	15.80	47.40
III.b.G3	126.40	114.91	68.95	12.64	12.64	31.60
III.c.G1	87.78	83.16	80.25	31.60	31.60	36.11
III.c.G2	86.18	83.16	90.29	23.70	23.70	31.60
III.c.G3	87.78	84.64	93.63	15.80	15.80	21.07
Min	67.71	67.71	66.53	11.49	11.49	10.19
Max	126.40	126.40	137.89	50.56	50.56	63.20

The increase in strength with the increase in fibre content occurs up to the limits of 62% for fabric reinforcement. Fibre content above this value causes the decrease of strength due to the impossibility of over-saturation of fibres with the warp.²

For the tested composites, no relationship between the bending strength parameter and the coefficient of filling with threads of the warp and weft for the flange elements was found. Such a relationship can be found for the warp thread filling coefficient of the web element (Table 4).

Table 4.

Summary of the coefficients of filling of the web with warp threads and the mean values of bending strength.

X₁ coefficients of filling the web with warp,%	X₂ mean values of bending strength R_g, MPa
84.360	14.168
85.743	27.536
84.360	29.359
66.526	4.678
66.526	5.110
68.945	5.486
91.927	14.591
101.120	18.292
108.731	19.026
125.355	17.844
137.891	21.340
137.891	29.198
102.302	4.632
95.597	8.797
109.807	8.475
125.702	18.189
121.344	12.080
118.192	22.045
87.172	12.958
87.172	19.285
90.286	12.638
71.547	6.946
72.923	5.344
68.945	5.857
80.254	5.412
90.286	10.893
93.630	14.519
95.353 s₁ = 21.077	13.878 s₂ = 7.547

To determine the correlation between the mean values of the web coefficient of filling with warp threads X₁ and mean values of composite bending strength X₂, Pearson’s linear correlation coefficient was used.

The value of the correlation coefficient r was calculated using the Statistica program, that is, r= 0.5639, that is, between the mean values of the coefficient of filling of the web with the warp threads X₁ and mean bending strength values of the composite X₂, there is a relationship significant at the level of probability p = 0.002 (Figure 16).

Figure 16.

Spread of the variable mean values of the coefficient of filling of the web with warp threads X₁ and mean values of the composite bending strength X₂.

Reinforced T-shaped composites

Based on the research results obtained, it was found that the structure of the web is important in the spatial composite. The conducted tests showed that increasing the number of warp and weft threads in the web allows increasing the bending strength of the composite, but at the same time, to reduce the number of threads at the base, the flexural strength is increased.

The bending strength of the composite is influenced not only by the structure of the web itself but also by the connection point of the web and the base. In many cases, when testing composite bending strength, the web was damaged/detached from the base. In connection with such observations, work was undertaken to determine the effect of additional reinforcement of the web element and the place of web-base connection on the strength of the T-composite. In the next stage of work, the composites were reinforced with a multilayer fabric.

The samples were performed with additional web reinforcement and web connection location with a flange. The additional reinforcement was made with an O/L (orthogonal layer-to-layer) multilayer weave.^40,41

The scheme of multi-level forming of reinforced T-shaped reinforcement fabric is presented in Figure 17.

Figure 17.

Scheme of multi-level forming of reinforced T-shaped reinforcement fabric and distribution of weaves in the cross-section.

The element which, when unfolded, forms the flange of the T-section, contains two layers: upper and lower, in which, in order to obtain a monolithic reinforcement, a multi-layered weave is used in technical fabrics intended for belts. The weave of the fabric of the upper and lower layers (within the T-section flange) is identical. The upper and lower layers are not connected at the length of the T-section. In the web element, the lower and upper layers are joined into one, were also a multilayer weave, identical to the weave in the flange, is used for a sufficiently long section (Figure 18).

Figure 18.

Picture of the section of multilayer OL fabric, reinforced shaped fabric and composites.

Due to the different warp crimp, especially in the multilayer weave fabric, the supply of the warp threads from two beams was used.

The results of tests on additionally reinforced composites are presented in Table 5.

Table 5.

Summary of the results of the reinforced samples.

Sample	Reinforced	Max force, N	Bending strength R_g, MPa
P0	Without reinforced	219.89^a	12.17
P0	Without reinforced	±26.55	±2.93
P1	1/2 of the web	741.87	31.70
P1	1/2 of the web	±15.59	±2.65
P2	1/3 of the web	436.63	20.31
P2	1/3 of the web	±6.54	±2.16
P3	2/3 of the web	696.43	25.59
P3	2/3 of the web	±8.25	±0.74
P4	2/3 of the web and 1/3 of the flange	1003.73	32.51
P4	2/3 of the web and 1/3 of the flange	±85.47	±2.84
P5	1/2 of the web and 1/3 of the flange	663.68	34.96
P5	1/2 of the web and 1/3 of the flange	±24.54	±2.93
P6	1/3 of the web and 1/3 of the flange	558.28	26.28
P6	1/3 of the web and 1/3 of the flange	±17.57	±0.84

^amean value ± standard deviation.

The phenomenon of destruction, damaging the composite during bending, is illustrated by the load-deflection curves and the mean values from three measurements (Figure 19).

Figure 19.

Load-deflection curve for Samples P0–P6.

The destruction process, as shown in the charts, takes place in most cases in several stages. In the first stage, as the loading force increases, the deflection increases until the first damage is visible, as shown by the momentary drop in force. Then, as the force continues to increase, the deflection value increases until the test object is destroyed. The total mechanical damage to the test object was considered to be the maximum value of force that the object was able to transfer.

The bending strength of the reinforced composite is influenced by the length of the web element reinforcement and the place where the web is connected to the flange. In all cases, there is an increase in strength compared to the sample without additional reinforcement, P0 (Table 5). The highest almost three-fold increase in the bending strength of the composite compared to the sample without reinforcement (P0–12.17 [MPa]) occurred in the test-designated P5 – 34.96 [MPa], in which the web was reinforced up to ½ of the height and the flange. In the P1, P2 and P3 tests, where only the web was reinforced, the largest almost two-and-a-half-fold increase in comparison to the test without reinforcement (P0 – 12.17 [MPa]) occurred in the P1 test (Rg – 31.70 [MPa]). In both test groups, that is, group P1–P3, that is, with only the web element reinforcement and test groups P4–P6, in which the webs of the flange elements were reinforced, the greatest increase in bending strength compared to the test without additional reinforcement P0 was characterized by tests where the web reinforcement reached up to ½ of this element. The smallest increase in bending strength occurred in tests P2 and P6, where the web reinforcement was the shortest, that is, up to 1/3 of the web height. For the P4 test, that is, the test where the web reinforcement was the longest, compared to the P0 test, the greatest, more than five-fold increase in the value of the maximum force with complete destruction occurred. In the tests with the web element reinforcement up to 1/3 of the height (P2 and P6) of the maximum force with complete destruction, the smallest increase in the maximum force occurred compared to that of P0’s force. The length of the reinforcement is important. The bending strength of the composite is most significantly influenced by the reinforcement not only of the web itself but also of the connection point between the web and the flange.

Conclusion

The strength properties of the composite are influenced by the density of the structure expressed by the weft and warp density. The increase in warp density with unchanged weft density in the element located in the bending plane along which bending takes place (warp threads in flange elements) results in the increase in bending strength (taking place along warp threads). An increase in weft density with an unchanged warp density in the element in the bending plane causes a decrease in the bending strength (along the warp threads).

In the case of composites additionally reinforced, there is an increase in the bending strength of the composite in comparison with the test without additional reinforcement. The greatest increase in strength occurred in samples where reinforcement reached ½ of the web height (three-fold increase). On the basis of the analysis of the obtained test results, it can be concluded that the most important cross-sectional element influencing the bending strength of the composite of the T-shape is the web element and the distribution of threads in this element of the composite. Increasing warp and weft density in the web allows increasing the bending strength of the composite, but if at the same time, the weft density in the horizontal element (flange) is reduced, the bending strength will be higher. Therefore, the structure of the element located in the plane perpendicular to the work-bending plane of the composite plays an important role in the construction of spatial fabrics intended to reinforce the composite.

This article is part of a dissertation and was carried out in the frame of statutory activities of the Textile Research Institute financed by the Ministry of Science and Higher Education.

Footnotes

Acknowledgements

The author would like to thank Izabela Jasinska and the team of Łukasiewicz Reserch Network-Textile Research Institute The Laboratory of Textile Raw Materials, Fabrics and Electrostatic Properties Testing for conducting the tests and Marek Snycerski from Lodz University of Technology, , Institute of Architecture of Textiles for mentoring.

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: Developed on the basis of the results of work carried out as part of the statutory activities of the Textile Research Institute financed by the Ministry of Science and Higher Education.

ORCID iD

Ewa Witczak

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