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The failure behavior of cross-linked polymer epoxies were investigated under Mode I fracture at low and high loading rates. By varying the monomer choices, the properties of the epoxies can be tailored to achieve a greater resistance to cracking and high impact toughness. For these experiments, a unique four-point bending specimen was developed. High rate experiments were conducted on a modified Split Hopkinson Pressure Bar (SHPB) with pulse-shaping. High speed digital imaging was used to determine the failure initiation, and allow for use of digital image correlation (DIC) to optically measure the crack opening displacement (COD) and crack propagation velocity. The experimental results are used to obtain the energy required to initiate fracture at various loading rates. The critical energy required to initiate fracture for this epoxy shows limited rate dependence from quasi-static to dynamic loading rates, but becomes rate sensitive when loaded at dynamic rates. The experimental procedures and results are discussed in this paper.

High performance fibers such as Kevlar KM2 used in protective vests are loaded primarily in tension at high loading rates. During the weaving and crimping process of fibers/yarns to make fabrics, individual fiber tensile strength may be reduced due to pre-straining of fibers by twisting. To systematically investigate the loading rate effects under different pre-twisted stress states, a miniaturized Kolsky tension bar is used to study single Kevlar KM2 fibers subject to varying levels of shear strain at high strain rates. Low and intermediate strain rate responses of the fibers are investigated using a Bose Electroforce loading system. In this paper, tensile strength as a function of pre-loaded shear strain on a single fiber is presented for different strain rates of loading. Fracture morphology of the fibers is also investigated using a high resolution scanning electron microscope to understand the mechanisms of failure.

During injury causing events such as impact, blunt trauma, penetration or blast, the human body is subjected to high rate loading. These events will deform and damage human tissue, frequently causing tearing in soft tissues and cracking/fracture in hard tissue (bone) due to the inherent brittleness of bone. For the development of accurate fracture models for computer simulation of bone fracture and also to understand the failure mechanisms of hard tissues during blast and impact events, it is necessary to investigate the failure behavior (fracture) of hard tissues at different loading rates, including high loading rates. In this study, the Mode I fracture behavior of cortical bone from the human femur, perpendicular to the long axis, is investigated as a function of loading rate, including dynamic loading rates which are accomplished with a modified Kolsky bar. The bone fracture specimens were extracted from three different human donors, ages 36, 50, and 43. For all three donors, the fracture toughness increases as loading rate increases from quasi-static to intermediate. As loading rate increases from intermediate to dynamic rates, the initiation fracture toughness decreased on average, while remaining higher than at quasi-static rate. The observed variations in initiation fracture toughness with donor age for each loading rate did not show any specific trends. The initiation fracture toughness of cortical bone is dependent on the path of the crack relative to the internal bone structure.

The strain rate response of PMMA was investigated under uniaxial compression at room temperature at strain-rates ranging from 0.0001/sec to about 4300/sec. In addition, the temperature response of PMMA was investigated at strain-rates of 1/sec and 0.001/sec at temperatures ranging from 0°C to 115°C (below Tg). High rate experiments at room temperature (greater than 1/sec rates) were conducted using a split-Hopkinson Pressure bar (SHPB) with pulse-shaping. This is necessary to induce a compressive loading on the specimen at a constant strain rate to achieve dynamic stress equilibrium. Results conducted at room temperature show that PMMA is strain rate sensitive from quasi-static to dynamic loading. Additionally, the stress-strain response exhibits a decrease in the flow stress with an increase in temperature. These experimental data are being used to develop constitutive behavior models of PMMA.

Blunt impact experiments were conducted on single and multi-layered Kevlar KM2 (Style 706) fabric at various velocities. To complement previous experiments with multi-ply fabric layers without resins, multi-ply laminates were fabricated from single or multiple plies of Kevlar fabric hot-pressed with a blended polyolefin (PO) resin consisting primarily of low molecular weight polyethylene to form a relatively rigid panel while maintaining a high fiber volume. During impact, full-field, back surface displacements were obtained using digital image correlation (DIC) technique using a pair of high-speed digital cameras configured stereoscopically. This allowed the full-field measurement of the fabric panel in three dimensions. These experimental results are to be used to evaluate different types of fabric and composite laminate models and simulation methodologies that are in literature. In this paper, experimental measurements are presented.

For decades, ballistic gelatin has been used as a tissue surrogate to test and evaluate bullets and firearms due to its similar viscosity to natural tissue. However, the high water content in ballistic gelatin makes it unstable at room temperature, and therefore causes it to have a poor shelf life. The development of polymer-based gels has shown promise as an alternative tissue surrogate. Polymer gels such as Perma-Gel are stable at room temperature and can be stored for long periods of time. Gels often fail due to tensile stresses during penetration. The failure behavior in tension is highly influenced by the presence of defects, such as cracks and voids, in the bulk material. A mode I experimental method was developed to obtain tensile failure criteria for the initiation and propagation behavior of these types of soft materials. Digital image correlation is used to determine the full-field surface strains around the crack tip to obtain a quantitative measure of the critical strain-field required for initiation and propagation of failure due to a defect. This systematic study utilizes these experimental techniques to determine the critical criteria for crack growth initiation and crack propagation of ballistic gelatins and a polymer gel as a function of loading rate. This paper presents experimental methodologies and results from Mode I fracture experiments including measured critical energy and strain-based criteria for failure initiation and growth, as well as their dependence on the rate of loading.

The commercially available ZylonTM single fiber has been investigated to understand the effect of strain rate on the tensile response. Experiments were conducted on single Zylon fibers across a range of strain rates ranging from 0.001/s to ~1000/s. The low rate uniaxial tensile experiments were performed using a Bose TestBenchTM system. The high rate dynamic tensile experiments were performed using a modified Kolsky Bar setup. This paper discusses the experimental setup and presents the effect of strain rate on the tensile deformation and failure behavior of this single fiber. These experimental data will be used to develop continuum and micro-mechanics based fabric models and computational simulation methodologies for impact of these fabrics by projectiles; this will allow engineers to better understand the impact behavior of composite structures created with fabrics made of this fiber.

Polycarbonate (PC) and polymethyl-methacrylate (PMMA) are commonly used materials for transparent protection. For increased effectiveness against impact, PC and PMMA are typically sandwiched and bonded in multiple layers of varying thicknesses to create a composite laminate. To develop high fidelity simulation methodologies for impact behavior for this type of laminated construction, panels were fabricated with different layers of PC and PMMA, on which blunt impact experiments were conducted. A high speed digital image correlation (DIC) technique was used to obtain full-field deformation measurements including out-of-plane displacement and surface strain. The experimental results are used to evaluate constitutive models and simulation methods for these various configurations of PC/PMMA composite laminates. In this paper, experimental technique and results are presented.