Bazle Z. (Gama) Haque

Tailoring Interfacial Properties by Controlling Carbon Nanotube Coating Thickness on Glass Fibers Using Electrophoretic Deposition

The electrophoretic deposition (EPD) method was used to deposit polyethylenimine (PEI) functionalized multiwall carbon nanotube (CNT) films onto the surface of individual S-2 glass fibers. By varying the processing parameters of EPD following Hamakers equation, the thickness of the CNT film was controlled over a wide range from 200 nm to 2 μm. The films exhibited low electrical resistance, providing evidence of coating uniformity and consolidation. The effect of the CNT coating on fiber matrix interfacial properties was investigated through microdroplet experiments. Changes in interfacial properties due to application of CNT coatings onto the fiber surface with and without a CNT-modified matrix were studied. A glass fiber with a 2 μm thick CNT coating and the unmodified epoxy matrix showed the highest increase (58%) in interfacial shear strength (IFSS) compared to the baseline. The increase in the IFSS was proportional to CNT film thickness. Failure analysis of the microdroplet specimens indicated higher IFSS was related to fracture morphologies with higher levels of surface roughness. EPD enables the thickness of the CNT coating to be adjusted, facilitating control of fiber/matrix interfacial resistivity. The electrical sensitivity provides the opportunity to fabricate a new class of sizing with tailored interfacial properties and the ability to detect damage initiation.

Quasi-static, low-velocity impact and ballistic impact behavior of plain weave E-glass/phenolic composites:

Quasi-static, low-velocity impact (LVI) and ballistic impact loading conditions were used to find the material properties and dynamic responses of E-glass/phenolic composites. Standard American Society for Testing and Materials (ASTM) tests were used to find the density, Poisson’s ratio, tensile, compressive and shear strengths, and the elastic and shear moduli of the material. The quasi-static punch shear and crush strength tests were used to find the punch shear and crush strengths of the material. LVI tests were conducted to obtain force versus time curves for various loading conditions. Ballistic testing was conducted using a right circular cylinder (RCC) to find the V50 ballistic limit and the depth of penetration of the RCC at various impact velocities. The experimental results of this investigation can be used for structural design and to validate numerical solutions for both LVI and ballistic impact events.

A new penetration equation for ballistic limit analysis

A new penetration equation for ballistic limit analysis (BLA) of a projectile–target pair is developed and presented. A critical review of the classic BLA (CBLA) has identified that the CBLA penetration equations do not satisfy the conservation of momentum and energy principles simultaneously and completely. It has also been found that the classic definition of residual velocity of the projectile does not quantify the instantaneous rigid body velocity of the projectile at ballistic limit. A new definition of the projectile relative velocity with respect to the target in contact (and in motion) is defined, and this new definition is used to define the projectile instantaneous velocity at ballistic limit. With this new definition of the projectile instantaneous velocity, the new penetration equation is derived satisfying both the conservation of momentum and energy principles simultaneously. A general functional form of the new penetration equation is used in analyzing experimental data and the results are found to match well with the experiments. The new equation is shown to be able to explain projectile residual velocity at and around ballistic limit, can predict the jump velocity at ballistic limit (if any), and is applicable to experimental ballistic data of a wide range of thin and thick targets. The present work sets the background for further development of theoretical penetration models incorporating material properties and parameters of the projectile–target pair.

Dynamic effects of single fiber break in unidirectional glass fiber-reinforced composites:

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process that leads to stress concentrations in the interface, matrix and neighboring fibers that can propagate at high speed over long distances. To gain better understanding of this event, a fiber-level finite element model of a two-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Moreover, dynamic interfacial failure is predicted where debonding initiates, propagates and arrests at longer distances than predicted by models that assume quasi-static fiber breakage. In the case of higher strength fibers breaks, unstable debond growth is predicted. A stability criterion to define the threshold fiber break strength is derived based on an energy balance between the release of fiber elastic energy and energy absorption associated with interfacial debonding. A contour map of peak dynamic stress concentrations is generated at various break stresses to quantify the zone-of-influence of dynamic failure. The dynamic results are shown to envelop a much larger volume of the microstructure than the quasi-static results. The implications of dynamic fiber fracture on damage evolution in the composite are discussed.

Modeling transverse impact on UHMWPE soft ballistic sub-laminate

High-performance ultra-high molecular weight polyethylene (UHMWPE) soft ballistic sub-laminates ([0/90] n , SBSL) are stacked to build a soft body armor pack (SBAP) that can defeat handgun projectiles. Transverse impact on single-layer [0/90] SBSL of different size is modeled with shell elements and is solved using LS-DYNA composite material model MAT54. The finite element (FE) model is validated using 1D and 2D theories for transverse impact. The validated FE models are then used to study the perforation behavior of a [0/90] SK76/PU SBSL under constant and variable velocity impact. Results show that the basal shape of the transverse deformation cone has a diamond shape; the cone wave speed along primary material direction agrees well with 2D membrane theory, there exists a minimum perforation velocity below which the SBSL will not perforate, the peak perforation force reduces with the size of the SBSL, and the work of perforation decreases with increasing speed. Detail perforation mechanics of [0/90] SK76/PU SBSL is presented for the first time.

Investigation of penetration mechanics of PW Kevlar fiber reinforced HDPE composites

In this article, a study on the quasi-static penetration resistance behavior of plain weave Kevlar/high density polyethylene composite with varying thicknesses, i.e. HC = 3.1–9.4 mm, is presented using the quasi-static punch shear test methodology for the experiments. The penetration resistance is usually shown by a load–displacement graph, integral of which is the energy dissipated by the composite during penetration. The penetration energy varies with the diameter of the support span which can be equal or higher than the punch diameter. During tests, a flat punch of diameter 7.6 mm with a range of support spans 8.89 to 50.8 mm has been used and quasi-static punch shear test experiments are carried out for varying support span to punch diameter ratios (i.e. SPR = 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, etc.). Their damage mechanisms for different support span to punch diameter ratios and thickness are documented. Stiffness, peak force, deflection, damage area and energy dissipation results are presented in detailed form.

Multi-hit ballistic impact on S-2 glass/SC15 thick-section composites: experiments

Multi-hit ballistic impact and damage behavior of thick-section composites are of interest to many military and aerospace applications. The effect of support spans on single-hit penetration resistance and the effect of relative distance between multiple impacts on the penetration resistance are the subject matter of the present investigation. In order to study the effect of support spans on penetration resistance, single-hit ballistic experiments are conducted on two plain-weave (PW) S-2 glass/SC15 composite laminates of thickness, 13.5 mm (22 layers, 22L) and 20.4 mm (33 layers, 33L), respectively at two different support span diameters, i.e. 102 mm and 203 mm. On the other hand, the effect of multiple impacts on penetration resistance has been investigated by impacting both the laminates (22L and 33L) at four additional radial shot locations (90 degrees apart) with a support span diameter of 203 mm. In both the impact scenarios, 0.50cal fragment simulating projectiles (0.50cal FSP) were used while the composite laminates were clamped between a cover and a support plate. In addition, the maximum dynamic deflection of the composite laminates were recorded using a thin aluminum witness plate at the rear end of the laminate, and the through-thickness ballistic damage of the composite laminates was investigated by sectioning through the impact centers, dying with an ink-alcohol solution, and taking optical photographs of the cross-sections. Results show that the single-hit ballistic limit velocity increases marginally with support span diameter investigated. The curvatures of the dynamic deflection profiles at the support edge suggest that the dynamic deflection was constrained by the smaller support span while the dynamic deflection was not constrained by the larger support span. There was no noticeable difference in the single-hit maximum dynamic deflection between the two laminate thicknesses as a function of impact velocity to ballistic limit velocity ratios. Single-hit through-thickness damage extended toward the edges of the larger support spans such that the subsequent multiple impacts were on partially damaged laminates; however, the pre-existing ballistic damage showed about 4.5% and 9.0% decrease in site specific multi-hit ballistic limit and energy, respectively.

Multi-hit ballistic impact on S-2 glass/SC15 thick-section composites: finite element analyses

The effect of support spans on single-hit ballistic limit and the effect of pre-existing ballistic damage on multi-hit ballistic limit for two S-2 glass/SC15 composite laminate thicknesses (i.e. 13.6 mm and 20.5 mm) using 0.50cal fragment simulating projectiles have been experimentally investigated in a companion paper. The main objective of this paper is to simulate and correlate the multi-hit ballistic experiments using finite element analyses. One multi-hit ballistic impact scenario on two different composite laminate thicknesses is considered in the present analyses. Finite element model of the impact scenario is developed using three-dimensional solid elements and are solved using LS-DYNA and the progressive composite damage model MAT162. The MAT162 properties and parameters of plain-weave S-2 glass/SC15 composites used in the present simulations were validated in our previous work. Multi-hit impact cases are simulated by sequentially impacting the composite laminate with five different fragment simulating projectiles at five different impact locations at an interval of 200 micro-seconds. Good correlations of ballistic limit velocities between experiments and finite element analyses are obtained. In addition, finite element analyses provided time histories of projectile and laminate dynamics, and damage evolution and interactions for the multi-hit impact cases. Detailed simulation results and comparison with the experiments are presented.

Ballistic Impact Experiments and Quantitative Assessments of Mesoscale Damage Modes in a Single-Layer Woven Composite

In this work, we investigated the mesoscale impact and perforation damage of a single layer, woven composite target transversely impacted below and above the ballistic limit by a rigid projectile sized on the order of a tow width. To visualize mesoscale impact damage in woven composites, a thin translucent composite target was used, which provided access to both impact and back-face surfaces. High-resolution photography was used to visualize mesoscale damage, and impact and residual velocity data relative to the location of projectile impact on weaving architecture were quantified. It was found that impact on a tow-tow crossover requires more energy to perforate than impact on a matrix-rich interstitial site or on adjacent, parallel tows. Mesoscale damage in thin, woven composites was characterized for impact velocities below and above the ballistic limit. Four mesoscale damage modes were identified: transverse tow cracks, tow-tow delamination, 45° matrix cracks, and punch- shear. These damage modes were observed both on the surface and inside the composites. High-resolution images of these damage modes were quantified in digital damage maps whereby the output of color intensity correlated with the quantity and type of material damage. Digital maps generated for select specimens revealed characteristic damage patterns in woven fabric composites including a diamond pattern in matrix cracking and a cross pattern in tow–tow delamination. It was found that the greatest extent and quantity of mesoscale damage occurs for impact velocity just below the ballistic limit.

Stochastic micromechanical modeling of transverse punch shear damage behavior of unidirectional composites

Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber–matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber–matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature.