Alper Taşdemirci

Experimental and Modeling Studies of Stress Wave Propagation in Multilayer Composite Materials: Low Modulus Interlayer Effects

The behavior of a multilayer material at high strain rate is investigated by a combination of experimental and numerical techniques. It is shown that, although the Split Hopkinson Pressure Bar (SHPB) at first appears unsuitable for such applications, it is a valuable tool to validate finite element modeling. The feasibility and usefulness of modeling the stress wave propagation in complex multilayer materials was thus demonstrated. The one-dimensional stress state usually assumed for conventional SHPB testing is inapplicable, but it is shown that both numerical and experimental results can nevertheless be coupled for a complete understanding of the wave propagation characteristics. The specific material consists of ceramic face plate and glass/epoxy backing plate with a low modulus interlayer. It is shown that the lateral constraint of an interlayer with a significant positive Poisson’s ratio allows relatively easy transmission of the elastic compressive wave into the backing plate, whereas a low modulus interlayer with an almost zero Poisson’s ratio drastically reduces the ease of elastic wave transmission. The implications for the reduction in damage of the backing layer are discussed. Numerical modeling clearly shows that the severe stress inhomogeneities and discontinuities exist and these may have serious consequences regarding mechanical and other properties. The stress states inside the components are presented here.

Failure and tribological behaviour of the AA5083 and AA6063 composites reinforced by SiC particles under ballistic impact

The wear behaviour of two different MMCs, namely AA5083 and AA6063 reinforced by 45, 30 and 15% SiCp, respectively, are investigated under condition of high-velocity impact. The tests are carry out by firing 7.62 armour piercing rounds into these composite materials. The wear and failure mechanisms are evaluated by examining the projectile tips and the hole surfaces produced by high-velocity impact using SEM and optical microscopy. The hardness differences of the two regions on the hole surfaces, i.e. the plastically compressed regions surrounding the projectile hole is higher than unaffected matrix. The wear mechanisms on the friction surfaces of the matrix are predominantly abrasion and melt wear. It is observed that the projectile nose is plastically deformed when it impacts the armour. The projectile surface is also scratched by the SiC particles on the hole surface produced by the impact. The wear mechanism on the projectile surface is predominantly abrasive.

Stress Wave Propagation Effects in Two- and Three-Layered Composite Materials

Multilayer materials consisting of ceramic and glass/epoxy composites have been subjected to high strain rate compression testing using the Split Hopkinson Pressure Bar. The samples were extensively strain gaged so that dynamic data were generated directly from the samples during testing. Output data from the experiments were compared with numerical simulations of the same experiments and good agreement was noted. It was found that the stress distribution within samples was quite inhomogeneous and that stresses were highest in the region of the bar-sample interface. The presence of a rubber interlayer between the ceramic and glass/epoxy decreased the stress in both components but dramatically increased the degree of stress in homogeneity.

Microstructural analysis and discontinuities in the brazed zone of copper tubes

Abstract In this study, ISO Cu-DHP tubes were brazed using different filler metals such as AWS BCuP-2, BCuP-3, BCuP-5, BAg-16, and DIN L-CuP7 and L-Ag40Cd. The microstructures of the brazed zone of tubes were examined with optical and scanning electron microscopic studies. Hardness measurement and leak-tight tests were conducted. The discontinuities in the brazed zone were determined. The microstructures of the brazed zone of the filler metals generally included two or three different regions. The zones of BCuP-2 and LCuP7 consisted of round phosphorus poor dendrites in a matrix of Cu–P. Ag poor dendrites were observed in a silver rich matrix of BAg-16 filler metal. On the other hand, the microstructures of the BCuP-3, BCuP-5 and L-Ag40Cd consisted of three distinct regions. For instance, dendrites of copper solid solution, Ag–Cu–P eutectic and silver rich dendrites were observed in the brazed zone of BCuP-5 filler metal. Some discontinuities such as porosities, insufficient penetration and positioning errors were observed in the brazed zone of the different filler metals. It was observed that the positioning errors in joints strongly influenced the penetration of filler metal. Silver content decreased the ductility of the brazed zone.

Mechanical interlocking between porous electrospun polystyrene fibers and an epoxy matrix.

An epoxy matrix filled with nonwoven mats of porous polystyrene (PS) fibers processed by an electrospinning was compression tested at quasi-static (1 × 10(-3) s(-1)) and high strain (315 s(-1)) rates. The electrospun PS fibers with a diameter between 6 and 9 μm, accommodated spherical pores on the surface with the sizes ranging from 0.1 to 0.2 μm. The filling epoxy matrix with 0.2 wt % PS fibers increased the compressive elastic modulus and compressive strength over those of neat epoxy resin. The microscopic observations indicated that the surface pores facilitated the resin intrusions into the fiber, enhancing the interlocking between resin and fibers, and increased the deformation energy expenditure of the polymer matrix.

Projectile impact testing of glass fiber-reinforced composite and layered corrugated aluminium and aluminium foam core sandwich panels: a comparative study

E-glass/polyester composite and layered corrugated aluminium and aluminium foam core sandwich panels were projectile impact tested between 127 m/s and 190 m/s using a hardened steel sphere projectile. The corrugated aluminium cores, constructed from aluminium fin layers and aluminium interlayers and face sheets, exhibited relatively lower-plateau stresses and higher stress oscillations in the plateau region than aluminium foam cores. The applied brazing process resulted in reductions in the plateau stresses of the corrugated aluminium cores. The sandwich panels with 2- and 3-mm-thick composite face sheets and the epoxy-bonded corrugated aluminium sheet cores were perforated, while the sandwich panels with 5-mm-thick composite face sheets were penetrated in the projectile impact tests. On the other hand, the sandwich panels with aluminium foam cores were only penetrated. A simple comparison between the ballistic limits of the sandwich panels as a function of total weight revealed significant increases in the ballistic limits of the cores with the inclusion of composite face sheets. The determined higher impact resistance of the foam core sandwich panels was attributed to the relatively higher strength of the foam cores investigated and the ability to distribute the incident impulse to a relatively large area of the backing composite plate.

Numerical Approach to Design Process of Armored Vehicles

Today, it is imperative that armored vehicles need advanced protection kits against anti-symmetric threats more than before. The primary goal of this study was to assess benefits of explicit hydrocodes for mine protection resistance of armored vehicles. An analysis of an armored vehicle under blast loading caused by high explosive (HE) detonation is presented with comparison to a full-scale test. The problem was examined using LS-DYNA which is an explicit non-linear finite element code. Multi Material Arbitrary Lagrangian Eulerian (MM-ALE) Fluid Structure Interaction Method was selected to model the explosion domain so as to observe advancing of the shock wave in the compressed air and to investigate the effects of blast on the vehicle structure after explosion. Johnson-Cook constitutive material model, Jones-Wilkins-Lee (JWL) and Linear Polynomial equation of states were used for the problem. Results show that numerical analysis was in good agreement with the experimental result.Copyright

Experimental and numerical investigation of the effect of interlayer on the damage formation in a ceramic/composite armor at a low projectile velocity

The damage formation in a multilayered armor system without and with an interlayer (rubber, Teflon, and aluminum foam) between the front face ceramic layer and the composite backing plate were investigated experimentally and numerically. The projectile impact tests were performed in a low-velocity projectile impact test system and the numerical studies were implemented using the nonlinear finite element code LS-DYNA. The results of numerical simulations showed that the stress wave transmission to the composite backing plate decreased significantly in Teflon and foam interlayer armor configurations. Similar to without interlayer configuration, the rubber interlayer configuration led to the passage of relatively high stress waves to the composite backing plate. This was mainly attributed to the increased rubber interlayer impedance during the impact event. The numerical results of reduced stress wave transmission to the backing plate and the increased damage formation in the ceramic front face layer with the use of Teflon and foam interlayer was further confirmed experimentally.

The effect of perforations on the stress wave propagation characteristics of multilayered materials

The effect of perforated interlayers on the stress wave transmission of multilayered materials was investigated both experimentally and numerically using the Split Hopkinson pressure bar (SHPB) testing. The multilayer combinations consisted of a ceramic face plate and a glass/epoxy backing plate with a laterally constrained low modulus solid or perforated rubber and Teflon interlayer. The perforations on rubber interlayer delayed the stress rise time and reduced the magnitude of the transmitted stress wave at low strains, while the perforations allowed the passage of relatively high transmitted stresses at large strains similar to the solid rubber interlayer. It was concluded that the effect of perforations were somewhat less pronounced in Teflon interlayer configuration, arising from its relatively low Poisson’s ratio. It was finally shown that SHPB testing accompanied with the numerical simulations can be used to analyze the effect of compliant interlayer insertion in the multilayered structures.

Projectile Impact Testing Aluminum Corrugated Core Composite Sandwiches Using Aluminum Corrugated Projectiles: Experimental and Numerical Investigation

E-glass/polyester composite plates and 1050 H14 aluminum trapezoidal corrugated core composite sandwich plates were projectile impact tested using 1050 H14 aluminum trapezoidal fin corrugated projectiles with and without face sheets. The projectile impact tests were simulated in LS-DYNA. The MAT_162 material model parameters of the composite were determined and then optimized by the quasi-static and high strain rate tests. Non-centered projectile impact test models were validated by the experimental and numerical back face displacements of the impacted plates. Then, the centered projectile impact test models were developed and the resultant plate displacements were compared with those of the TNT mass equal Conwep simulations. The projectiles with face sheets induced similar displacement with the Conwep blast simulation, while the projectiles without face sheets underestimated the Conwep displacements, which was attributed to more uniform pressure distribution with the use of the face sheets on the test plates.