Damage Tolerance of Thick-Section Composites Subjected to Ballistic Impact

Abstract

There are two predominate modes of failure in a compressively loaded laminate that has been subjected to ballistic damage. Analysis has shown that the stress concentration and the membrane failure are sensitive to the m-plane stiffness matrix of the damaged region, while delamination growth failure is dependent on the size and location of the damage as well as the fracture toughness of the material. An experimental study investigated means of improving damage tolerance ranging from resin toughness to through-thickness stitching. From the experimental study, it was found that improving fracture toughness by through-the-thickness stitching resulted in a reduction in damage size. Stitching may cause severe reductions in stiffness of the damage region as projectile energy is absorbed through the fiber breakage and pullout mechanisms. Thus, while stitching improves multi-impact performance, it may also lead to an increase in fiber damage, causing the compression after ballistic impact (CABI) failure mode to shift fi-om delamination growth to membrane failure. A design trade-off exists between maximum fracture toughness and stiffhess reduction induced within the ballistically damaged region. By examining the factors affecting residual strength (i.e., fracture toughness, inclusion stifmess, loading condition, and finite width effects) the methodology to develop design chart may be produced to optimize damage tolerance.

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“…Once a composite is damaged, its residual stiffness, strength, and toughness should be reevaluated to determine if the damaged composite can still meet the design specifications. Conventionally, damage tolerance in a composite structure is defined as the ability of the structure to tolerate a reasonable level of damage or defects that may be induced during manufacturing or while in service, without jeopardizing the safety of the structure and its users, i.e., structurally functional [3][4][5][6].…”

“…This approach in general involves extensive computations and has been limited to simple lay-up configurations, e.g., symmetric cross-ply laminates. Numerical simulations of highvelocity impact phenomena in three-dimensional thick-section composites is still non-existence [6]. Most of previous work on numerical modeling on high-velocity impact was mainly limited to twodimension simulations [132].…”

“…• Non-destructive evaluation of damage extent, especially the delamination area, using X-ray radiographic [110] and/or ultrasonic [111] techniques, • Estimating residual properties, especially the compressive-after-impact (CAI) strength [138] and compressive-after-ballistic impact (CABI) strength [6], residual tensile [139] and flexural [140] strength, stiffness, and ductility, residual buckling stability [141], etc.…”

“…The main goal of the proposed research is to investigate damage caused by low-velocity and ballistic-speed impact in thick-section hybrid composite joints and the ensuing issue of damage tolerance. After reviewing available literature [6,[119][120][142][143][144][145][146][147][148][149][158][159][160][161][162][163][164][165] and considering Army's need for ground combat vehicle applications and the capabilities of the laboratories at CCNY and U.S. Army TACOM, we would like to propose for this project the choice of 1/4" composites (approximately 24 layers) made of toughened epoxy (cured at 350°F, instead of 250°F) reinforced by carbon/S2 glass interwoven stitched fabrics. S2 glass fiber-reinforced toughened epoxy has been proven to be a viable material for the Composite Armored Vehicle demonstrator program.…”

“…We also would like to suggest using vacuum-assisted resin-transfer molding (VARTM) to manufacture the hybrid composites since using the VARTM technique will allow us to design the preform easily, which is a feature very desirable for hybrid composite manufacturing [166]. However we are fully aware of the existence of a new process, called co-injection resin-transfer molding (CIRTM), which was developed by Army Research Laboratory, University of Delaware, and the CAV/Crusader composite structure contractor, United Defense Limited Partnership (UDLP) [2,6,167]. If the technique is available to us, we would like to use it to make specimens.…”