The currently-required backing material for standards-based ballistic resistance testing of body armor, an artist modeling clay, suffers many drawbacks as a standard material. Among those drawbacks are sensitivities to temperature and strain rates. The search for an alternative backing material to replace the current clay began in 2010. By 2018, a synthetic polymer composite emerged as the most promising candidate material from U.S. Army development efforts. The deformation behavior of the synthetic polymer composite is very similar to that of the current clay.
A process for identifying suitable replacement materials that relies only on ballistic performance validation tests would be expensive and require extensive testing, and it still would not identify material properties that are useful for formulation-development and quality-control purposes. To overcome this, a team of NIST researchers, working in collaboration with the Army Research Laboratory (ARL), developed a rheological framework to facilitate standardization of the formulations, in-situ process monitoring, and quality control evaluation of the candidate ballistic witness materials (BWMs).
Ran Tao, Kirk Rice and Aaron Forster from the Security Technologies Group in MML utilized this framework to quantify the effects of strain, strain rate, and temperature dependence on the mechanical properties of the ARL candidate BWMs compared with those of the clay. The ARL BWMs were found to exhibit many of the key properties required of the clay, but with less temperature dependence. Effects of aging and work history on material properties were also investigated, as those factors are involved in actual usage conditions. The Army BWM is found to be more aging-dependent and less work-dependent relative to the clay. By understanding the structure-property relationships for those materials, this research also shed light on future formulation and processing optimization of candidate BWMs from a fundamental material science perspective.
The Army candidate BWMs exhibit minimal temperature sensitivity and smaller variations in material properties compared to the current clay, both of which are desired for improved repeatability in backing material verification testing conducted prior to ballistic testing and during body armor ballistic resistance testing. For end-use purposes, identification of key material properties is critical to ensure that a BWM will satisfy performance requirements. Important criteria for a BWM are that it is dimensionally stable, deforms easily upon impact, and exhibits minimal elastic recovery after deformation. By performing large amplitude oscillatory shear (LAOS) measurements, the candidate BWM is identified as an elasto-visco-plastic material, similar to the current clay. This means that for applied stress below the yield stress, the material behaves as a viscoelastic solid; upon elevated stress exceeding the yield stress, the material deforms irreversibly and flows as a fluid, but with permanent deformation. This characteristic is particularly important for accurate determination of the behind-armor backface signature of a BWM, a common metric used in the performance evaluation of body armor.
There is always a desire to identify theoretical and empirical correlations of mechanical properties obtained at lower strain rates with material responses at higher strain rates. ARL previously identified a scaling relationship between the ballistic penetration depth and the effective Froude number for a soft elastomer. The effective elastic Froude number correlates the density difference between the projectile and the BWM, the effective projectile velocity, and the shear modulus of the BWM measured at low strain rates. In this work, the scaling relationship was extended to the complex ARL BWM and clay systems. Although this relationship is empirical and further study is required, this result may open doors for the bounding of BWM response at ballistic strain rates from low strain rate lab-scale measurements.
Valuable material data on candidate BWMs recently developed by the U.S. Army are reported in . Along with ARL’s report, the results from  support the synthetic polymer composite as a promising replacement BWM for the current clay for body armor performance testing. This work highlights the effectiveness of our methodologies to help optimize formulations, to set quality control guidelines, as well as to establish documentary performance standards and specifications related to next-generation BWMs for body armor testing.
 R. Tao, K.D. Rice, A.S. Djakeu, R.A. Mrozek, S.T. Cole, R.M. Freeney and A.M. Forster. Rheological Characterization of Next-Generation Ballistic Witness Materials for Body Armor Testing. Polymers 2019, 11, 447. DOI: 10.3390/polym11030447