Impact damage evolution of plain weave composites: multiscale modeling and experiments
Date
2022
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Recent developments in plain weave glass/epoxy composites have led to their increasing dual use as a structural and protective material in light armored vehicles. As such, understanding the evolution of damage due to ballistic impact is critical for improving the survivability of these materials. Ballistic impact experiments are often conducted, but post-test inspection of experimental specimens provides a picture of the end-state of damage. Diagnostic tools such as high-speed cameras have limited resolution in space and time, so often only provide insight into a part of the overall damage evolution. Thus, the spatial and temporal evolution of damage in plain weave (PW) composites following ballistic impact is not well understood. So, we turn to modeling to elaborate our understanding of this damage evolution. ☐ At the earliest timescale following projectile impact on woven composites, a stress wave propagates from the impact area through the composite thickness. At longer time scales, a transverse deformation cone forms around the projectile, and primary tows are loaded in tension, which spreads through shear to secondary tows. At the mesoscale—the length scale of a tow cross section—projectile impact causes damage including transverse cracks, tow-tow delamination cracks, tow tension and fiber failure. ☐ Projectile impact experiments were conducted, and the mesoscale damage mode tow-tow delamination was found to be a maximum near the ballistic limit velocity. These experiments were simulated with a state-of-the-art continuum model. The model predicted the projectile residual velocity reasonably well, but demonstrated a need for improved predictive capability, particularly regarding the ballistic limit velocity. Therefore, this work developed a mesoscale model, which incorporated discrete fabric architecture and showed improvement over the continuum model. However, this model was missing rate-dependent material behavior and the important mesoscale damage mode of tow-tow delamination. ☐ Simulating tow-tow delamination with the cohesive zone modeling approach required rate-dependent traction-separation laws (TSLs). These TSLs were derived using a multi-scale embedded cell modeling approach. A microscale model of fiber-matrix microstructure was embedded within a mesoscale continuum. Model inputs included rate-dependent matrix plasticity and failure and rate-dependent fiber-matrix interfacial debonding. Models were exercised in mode I and mode II to produce tension and shear cracking in the microstructure. The J-integral method was used to bridge the crack energy from the microscale to the mesoscale. The J-integral data were differentiated to derive the mode I and mode II TSLs. Bridging was demonstrated by comparing the load-displacement response of the microstructure to a mesoscale continuum cohesive crack modeled with the TSLs derived from the microscale. ☐ These TSLs were then used in a highly-resolved model of a PW representative volume element (RVE). The RVE was used to model the effects of through-thickness stress wave propagation at the earliest timescale. The damage evolution during this timescale was investigated for a range of impact velocities. It was found that tensile spall due to stress wave propagation can initiate tow-tow delamination (TTD) cracking for lower impact velocities, but higher velocity crushes the material. Delamination cracking at the projectile annulus initiates during this timescale during formation of the deformation cone and facilitates primary tow tensile loading. ☐ Single-layer ballistic perforation experiments were conducted. The PW RVE was repeated in space to build a full-scale model of the impact experiments. In these mesoscale models, TTD was modeled with the TSLs determined from the microscale. The experiments and modeling focused on the ballistic limit velocity (VBL). The mesoscale model provides more realistic deformation than the continuum model, which allowed ranking of energy absorbing mechanisms. The mesoscale model indicated two phases of penetration for impact velocities near VBL. The first phase is dominated by momentum transfer, and the second by tow-tension and pullout. The mesoscale model was used to partition energy dissipation and investigate the damage evolution during perforation. It was found that the development of TTD cracking is important for releasing the constraint on primary tows and enabling tow-elongation and frictional sliding, which dissipate additional energy. ☐ Finally, the multiscale modeling approaches developed in this work form a framework in which a materials-by-design evaluation of novel materials can be used at the lower length scales to derive properties used at higher length scales for evaluating and enhancing understanding of ballistic performance of plain weave composites.
Description
Keywords
Woven composites, Rate-dependent matrix plasticity, Representative volume element