James Zheng

The effects of off-axis transverse deflection loading on the failure strain of various high-performance fibers

Single filaments are subjected to a transverse deflection loading environment in efforts to gain insight into the failure strain of soft-body armor systems experiencing transverse impact. The fiber types utilized for all such experiments are Kevlar® KM2, Spectra® 130d, Dyneema® SK62, Dyneema® SK76, and Zylon® 555. In order to understand the effect of indenter shape, three different indenter geometries are utilized, namely a 0.30 caliber rounded head, a 0.30 caliber fragment simulation projectile (FSP), and high-carbon steel razor blades. The angle at failure is also varied in order to evaluate the presence of a stress concentration developed around such indenters through angles that would be produced during the transverse impact of single fibers/yarns. Loading with the rounded indenter yields failure strain values similar to pure longitudinal tensile experiments. Fibers loaded via a razor blade show a drastic reduction in failure strain, although the demonstrated failure strains are reasonably similar for all tested angles. Most interestingly, fibers loaded with the FSP show a reduction in failure strain with increasing loading angles, with low angle and high angle failure strains being similar to failure strains of fibers loaded with the rounded indenter and razor blade, respectively. In efforts to gain further insight into the method of fiber failure due to different loading configurations, post-mortem fracture surfaces are imaged for Kevlar® KM2 and Dyneema® SK76.

Why the Smith theory over-predicts instant rupture velocities during fiber transverse impact

The effect of multi-axial loading on single Kevlar® KM2 fibers is explored with an emphasis on correlating the results to fiber/yarn transverse impact. A 0.30 caliber fragment simulation projectile (FSP) is slightly modified to act as a transverse loading indenter. Fiber failure angles are forced between ∼0° and 50° in efforts to deduce the deleterious effect caused by such angles. Said angles are also enforced in order to create the geometry that would be produced at an impact velocity causing immediate fiber/yarn rupture in transverse impact experiments. The effect of fiber angle around the FSP indenter is experimentally studied along with an analysis of the specific angle causing immediate fiber failure. It is shown that there exists a demonstrative reduction in fiber longitudinal failure strain of KM2 filaments due to this multi-axial stress state, thereby questioning the recurrent assumption that fiber performance within a body armor system is dominated by failure in pure tension.

Effect of projectile nose geometry on the critical velocity and failure of yarn subjected to transverse impact

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.

Loading rate effects on dynamic out-of-plane yarn pull-out

In this study, the mechanical response of a single yarn pull-out from single layers of Kevlar® and Twaron® fabric under out-of-plane loading at both quasi-static and dynamic rates was experimentally investigated. In order to perform the dynamic experiments, a pendulum impact setup was designed and constructed to pull out a single yarn dynamically. The pull-out load was measured directly by a load cell and the movement of the fabric was measured to portray the load–displacement history. The effects of transverse pressure, different weave direction, and loading rates were also investigated.

Out-of-plane effects on dynamic pull-out of p-phenylene terephthalamide yarns

In this study, the mechanical response of a single yarn pull-out from single layers of Kevlar® and Twaron® fabric under out-of-plane loading at dynamic rates was experimentally investigated, as ballistic applications typically occur at higher velocities and out-of-plane directions compared to previous literature. In order to perform the dynamic experiments, a pendulum impact setup was designed and constructed to pull out a single yarn dynamically. The pull-out load was measured directly by a load cell and the movement of the fabric was measured to portray the load–displacement history. The effects of fabric length, out-of-plane versus in-plane yarn pull-out, and constraining boundary conditions were compared.

A Hierarchical Model for Kevlar Fiber Failure

Advances in materials characterization at the submicron and the nano-scales have progressed in the last decade. At the same time, computational capability for finite element analyses are also improving through technological developments in parallel computing. However, large computational models of nanostructured materials are currently limited by the lack of validation data. The work reported in this paper describes the formulation of a representative nanoscale model for Kevlar fibers based on failure section imaging that captures its fibril and microfibril structure. In this regard, a finite element model that captures the nanoscale structure of Kevlar fibers was developed to predict their macroscale response. Experimental derivation of geometrical parameters and physical properties of fibrils and microfibrils is challenging due to the sensitive nature of polymers. There are several microfibril parameters that reflect into effective fiber response, such as the microfibril constitutive behavior, length, diameter, shape, the inter-fibril shear and normal strength, and the inter-fibril normal and tangential force decay the after peak strength is achieved. This paper investigates the effect of each of the aforementioned parameters on the initial modulus, yield strength, ultimate strength, and strain rate dependence of Kevlar fibers with 10 μm average diameter. The sensitivity of the macroscale response to each microfibril parameter can be used to identify areas where experimental information can further enable the predictive capability of the computational model. A parametric study was performed to calculate the effective macroscale fiber response. Subsequently, a local gradient sensitivity method was employed to plot the sensitivity of the fiber response to each microfibril parameter.Copyright

Reverse ballistics penetration of Kevlar® fabric with different indenters at different loading rates:

In this study, the mechanical load on a bullet-shaped indenter when impacted by a single-ply Kevlar fabric was experimentally investigated using a reverse ballistics method at both quasi-static and dynamic rates. Different indenter geometries, namely the 9-mm Luger, .223 Remington, and .308 Winchester bullet geometries, were used. The penetration load of the stationary indenter was measured using a force transducer located behind the indenter, and the penetration load was then plotted against the impact velocity of the fabric sample. Different mechanisms of penetration were observed at different impact velocities. Penetration mechanisms were also found to be highly dependent on projectile nose geometry. A modified method to obtain an approximate ballistic limit based on the impact loads was used to compare the efficacy of different geometry types.

Mechanical properties of silicone based composites as a temperature insensitive ballistic backing material for quantifying back face deformation

This paper describes a new witness material for quantifying the back face deformation (BFD) resulting from high rate impact of ballistic protective equipment. Accurate BFD quantification is critical for the assessment and certification of personal protective equipment, such as body armor and helmets, and ballistic evaluation. A common witness material is ballistic clay, specifically, Roma Plastilina No. 1 (RP1). RP1 must be heated to nearly 38°C to pass calibration, and used within a limited time frame to remain in calibration. RP1 also exhibits lot-to-lot variability and is sensitive to time, temperature, and handling procedures, which limits the BFD accuracy and reproducibility. A new silicone composite backing material (SCBM) was developed and tested side-by-side with heated RP1 using quasi-static indentation and compression, low velocity impact, spherical projectile penetration, and both soft and hard armor ballistic BFD measurements to compare their response over a broad range of strain rates and temperatures. The results demonstrate that SCBM mimics the heated RP1 response at room temperature and exhibits minimal temperature sensitivity. With additional optimization of the composition and processing, SCBM could be a drop-in replacement for RP1 that is used at room temperature during BFD quantification with minimal changes to the current RP1 handling protocols and infrastructure. It is anticipated that removing the heating requirement, and temperature-dependence, associated with RP1 will reduce test variability, simplify testing logistics, and enhance test range productivity.

Parametric study using modal analysis of a bi-material plate with defects

Global vibrational method feasibility as a non-destructive inspection tool for multi-layered composites is evaluated using a simulated parametric study approach. A finite element model of a composite consisting of two, isotropic layers of dissimilar materials and a third, thin isotropic layer of adhesive is constructed as the representative test subject. Next, artificial damage is inserted according to systematic variations of the defect morphology parameters. A free-vibrational modal analysis simulation is executed for pristine and damaged plate conditions. Finally, resultant mode shapes and natural frequencies are extracted, compared and analyzed for trends. Though other defect types may be explored, the focus of this research is on interfacial delamination and its effects on the global, free-vibrational behavior of a composite plate. This study is part of a multi-year research effort conducted for the U.S. Army Program Executive Office - Soldier.

Improved quasi-static twin-fiber transverse compression of several high-performance fibers

The method of determining the quasi-static transverse compressive response of several high-performance polymer fibers was improved upon from a previous twin-fiber transverse compression setup in order to detect small initial high compliance signals while maintaining consistent diametral compression. Two fibers were laid parallel between two polished tool steel platens, and the fibers were subsequently compressed using a piezo-electric actuator at quasi-static rates. The new experimental setup ensures that the compression cycle begins when extremely small load signals are detected so that initial elastic transverse moduli may be more accurately measured. Nominal stress–strain curves were obtained for several types of high-performance fibers. The results show good agreement with previously obtained measurements. S-glass fibers exhibited a vastly different mechanical response compared to the polymer fibers.