a b s t r a c tProtecting a soldier's head from injury is critical to function and survivability. Traditionally, combat helmets have been utilized to provide protection against shrapnel and ballistic threats, which have reduced head injuries and fatalities. However, home-made bombs or improvised explosive devices (IEDs) have been increasingly used in theatre of operations since the Iraq and Afghanistan conflicts. Traumatic brain injury (TBI), particularly blast-induced TBI, which is typically not accompanied by external body injuries, is becoming prevalent among injured soldiers. The responses of personal protective equipment, especially combat helmets, to blast events are relatively unknown. There is an urgent need to develop head protection systems with blast protection/mitigation capabilities in addition to ballistic protection. Modern military operations, ammunitions, and technology driven war tactics require a lightweight headgear that integrates protection mechanisms (against ballistics, blasts, heat, and noise), sensors, night vision devices, and laser range finders into a single system. The current article provides a comparative study on the design, materials, and ballistic and blast performance of the combat helmets used by the US Army based on a comprehensive and critical review of existing studies. Mechanisms of ballistic energy absorption, effects of helmet curvatures on ballistic performance, and performance measures of helmets are discussed. Properties of current helmet materials (including Kevlar Ò K29, K129 fibers and thermoset resins) and future candidate materials for helmets (such as nano-composites and thermoplastic polymers) are elaborated. Also, available experimental and computational studies on blast-induced TBI are examined, and constitutive models developed for brain tissues are reviewed. Finally, the effectiveness of current combat helmets against TBI is analyzed along with possible avenues for future research.

Three different types of yarn have been subjected to transverse impact experiments in efforts to gain an understanding of local yarn failure and to provide input parameters for future transverse yarn impact simulations. Dupont™ Kevlar® KM2, DSM Dyneema® SK76, and AuTx® from JSC Kamenskvolokno were selected as representative materials, as the former two are commonly implemented into bullet resistant panels and the latter is a promising material for future impact resistant fabrics. In order to assess the effect of projectile nose shape on the critical rupture velocity range for each yarn type, three missile geometries have been implemented, namely a 0.30 caliber rounded head, a 0.30 caliber chisel nosed fragment simulation projectile (FSP), and a high-carbon steel razor blade. As opposed to one single velocity wherein yarn behavior transitions from transverse wave development to immediate local failure, a range is defined wherein progressive filament failure is detected with increasing impact velocities. Such ranges are determined for all yarn types using the three projectile geometries yielding critical velocity transition regions of increasing value when impacting via razor blade, FSP, and round projectile heads, accordingly. In addition, post-mortem fracture surfaces recovered from impact experiments have been imaged so as to elucidate the mechanism of failure throughout the range of velocities tested for each projectile type and yarn material and said fracture surfaces correlate well with impact velocity and projectile nose geometry.

No abstract

No abstract

A transversely isotropic visco-hyperelastic constitutive model is provided for soft tissues, which accounts for large deformations, high strain rates, and short-term memory effects. In the first part, a constitutive model for quasi-static deformations of soft tissues is presented, in which a soft tissue is simulated as a transversely isotropic hyperelastic material composed of a matrix and reinforcing fibers. The strain energy density function for the soft tissue is additively decomposed into two terms: a neo-Hookean function for the base matrix, and a polyconvex polynomial function of four invariants for the fibers. A comparison with existing experimental data for porcine brain tissues and bovine pericardium shows that this new model can well represent the quasi-static mechanical behavior of soft tissues. In the second part, a viscous potential is proposed to describe the rate-dependent short-term memory effects, resulting in a visco-hyperelastic constitutive model. This model is tested for a range of strain rates from 0.1 /s to 90 /s and for multiple loading scenarios based on available experimental data for porcine and human brain tissues. The model can be applied to other soft tissues by using different values of material and fitting parameters.