Microstructural Damage Characteristic of a Layer-to-Layer Three-Dimensional Angle-Interlock Woven Composite Under Quasi-Static Tensile Loading

2.1 Materials

Figure 1 shows the surface and cross-sectional representations of the 3DAWF which are used in this study. This fabric was made with high-silica glass filaments. Its specifications are listed in Table 1. The sketch diagram of the 3DAWF structure is shown in Figure 2. It indicates that the straight weft yarns are hold together perfectly by the weaving of the warp yarns. Such structural feature results in a very good structural stability for the 3DAWF. For this presented fabric structure, the tightly entangled structure between the warp yarns and weft yarns extends through the entire thickness direction of the material, providing the 3DAWF with excellent structural stability and ability to withstand external loads. When the 3DAWF is used as the reinforcement, to manufacture the 3DAWC prepared by the specific curing process with the high-performance resin, the obtained composites also have the advantages of excellent mechanical performance. In addition, when the 3DAWC is subjected to the specific external load, the entire material structure can be fully loaded by the load transfer capacity of the resin and the yarns; thus, the mechanical properties of the 3DAWC are significantly improved compared with the pure fabric reinforcement of 3DAWF.

Figure 1

Photographs of the 3DAWF (3DAWF, three-dimensional angle-interlock woven fabric)

Figure 2

Sketch of the layer-to-layer angle-interlock structure

Vacuum-assisted resin transfer molding (VARTM) technique was employed to manufacture the 3DAWC plate. The applied resin and curing agent were AROPOLTM INF 80501-50 polyester resin manufactured by Ashland Composite Polymers China and AKZO^® M-50, respectively. Their proportion was 100:1.5 by weight. After the manufacture of the 3DAWC plate, the 3DAWC coupons presented in Figure 3 were cut with high-pressure water jet along the longitudinal and transverse directions of the composite plate. The longitudinal direction of each coupon was aligned with the warp yarns. The size (length × width × thickness) of each coupon was 200 × 20 × 8.4 mm³. In addition, the fiber volume fraction was approximately 54%.

Figure 3

3DAWC specimen for testing (3DAWC, three-dimensional angle-interlock woven composite)

2.2 Quasi-static tension tests

As shown in Figure 4, the quasi-static tension tests of the 3DAWC coupons were conducted on an MTS 810.23 system at the speed of 2 mm/min. Aluminum tabs with the length of 50 mm, width of 20 mm, and thickness of 1.2 mm were glued at the ends of the composite coupons. The aluminum tabs will prevent the failure in the grip zones.

Figure 4

Experimental setup

2.3 FEA

FEA was employed to figure out the stress concentration effects and critical important structural areas where prone to damage for the presented layer-to-layer 3DAWC structure under the quasi-static tensile loading condition. FEA was performed using the commercial finite element software package (ABAQUS/Standard).

Geometrical model and material parameters

As shown in Figure 5(a), the simplified shell element FEA model of the 3DAWC was established. In addition, the material parameters of glass fiber and resin are listed in Table 2.

Figure 5

The quasi-static tension model of the 3DAWC (3DAWC, three-dimensional angle-interlock woven composite)

Table 2

Material parameters of glass fiber and resin

Materials	E (GPa)	Poisson’s ratio	Density (g/cm3)	Failure stress (MPa)	Failure strain (%)
Glass fiber	70	0.20	1.44	2,200	2.1
Resin	3.65	0.35	1.36	146	3.2

3.1 Tensile failure morphology

The ultimate failure morphology of the presented layer-to-layer 3DAWC under quasi-static tensile loading is presented in Figure 6, and it shows the typical damage modes of the 3DAWC. At the views of surface and cross-section, it can be found that the resin cracking, yarn breakages, and pulling out from the matrix and breakages of undulated warp yarns are the primary damage mode. In particular, the resin cracks and yarn breakages can be easily found on the surface of the sample, as well as the resin–yarn interface de-bonding is obviously observed at the cross-section.

Figure 6

Failure morphologies of the 3DAWC under quasi-static tensile loading (3DAWC, three-dimensional angle-interlock woven composite)

In particular, for the fracture failure of the yarns, the damage of undulated warp yarn system can be mainly found. Taking the internal structure of the presented layer-to-layer 3DAWC and the direction of applied tension force into consideration, the length direction of each entire warp yarn parallels to the direction of the tensile load, as well as the axial direction of the weft yarns is perpendicular to the load direction. Such kind of structural feature induces the undulated warp yarns carry the majority of loads during the quasi-static tensile loading process.

The phenomenon of de-bonding is relatively easier to occur at the stress concentration regions (as indicated by red arrows in Figure 6(b)) of warp yarn–resin–weft yarn interface, where the undulated warp yarns hold the maximum amplitude of undulation.

3.2 Key structural areas of stress concentration

Figure 7 shows the FEA contour images of the structural stress distribution of the layer-to-layer 3DAWC subjected to the displacement control of 5 mm at different time points. As time passes, it can be seen that the high stress areas become larger due to the stress concentration effect and high volume content of yarns. Besides, it shows that the inner central region carries the majority of the applied load during the loading process, as well as the stress concentration occurs at the specific regions at warp yarns, where the maximum curvature occurs and the weft yarns cross the warp yarns. These highly stressed sites are also sites of interface de-bonding. The abovementioned phenomena are in good agreement with the experimental results which are presented in Figure 6. In addition, as shown in FEA results, the maximum stress is mainly concentrated on the undulated warp yarns, i.e., the system of undulated warp yarns along the load-carrying direction is the primary load-carrying component for the 3DAWC under tensile loading.

Figure 7

Stress concentration regions of the 3DAWC structure at different time points (3DAWC, three-dimensional angle-interlock woven composite)

3.3 Structural failure mechanisms

Taking the structural characteristic of the layer-to-layer 3DAWC into consideration, the phenomena summarized above should be identified as the different dominant structural damage mechanisms among three steps.

For Step I, it was mainly attributed to the resin cracking. After the tensile load was applied on the 3DAWC, at the area of finest central surface, the resin cracking began to occur after a short period of loading time. As the loading process continued, the cracking propagated along the thickness direction of the sample. Such process consumed a short period of testing time.

For Step II, the resin cracks continued to propagate into the inner layers of the composite. More importantly, the de-bonding at resin–yarn interface was also taken place, and simultaneously the cracks propagated from the resin to the yarns as a result of stress concentration.

In addition for Step III, it was primarily identified as the progressive breakages of yarns, especially the undulated warp yarns along the load-carrying direction. As the less and less occur of the resin damage and interface de-bonding till the ultimate saturation, with the gradual breakages of yarns. The ultimate damage of the material also occurred within a short period of time.

Based on the abovementioned analysis results, suggestions on the structural optimization design for the tensile resistance property of the layer-to-layer 3DAWC can be proposed. One of the effective methods is to reduce the bending amplitude of warp yarns as much as possible, which can be achieved by reducing the number of interlaced layers of warp yarns along the thickness direction, and for each layer in the warp direction, decrease the number of weft yarns interlaced with the warp yarns. This method not only helps to reduce the stress concentration effect but also contributes to the improvement of the tensile tolerance performance.