The effect of polyacrylate microstructure on the impact response of PMMA/PC multi-laminates

Polymer armor is widely implemented in many military and commercial applications, particularly where optical clarity is required. The impact mechanics and energy absorption mechanisms of these multi-layered structures are not well understood. This experimental study focuses on the impact response of three-layered structures consisting of poly(methyl methacrylate) (PMMA) and polycarbonate (PC) outlerlayers with various polyacrylate adhesives. Specifically, the effect of varying the microstructure of a soft interlayer incorporated into all-polymer multi-laminates is investigated. Four polyacrylates (VHB 4905, VHB 5925, VHB 4930 and VHB 4936) having a common acrylic base matrix with microstructural variations including combinations of compressible air gaps and rigid microsphere inclusions. To examine how interlayer microstructure affects impact resistance, an instrumented, compressed air driven, experimental setup is utilized to conduct intermediate velocity (9-30 m/s) normal impact testing. The setup is unique in that both force and displacement during impact are recorded independently using a shock accelerometer embedded impactor and optical displacement sensors measuring contact force and out-of-plane deflection, respectively. Quantitative metrics from the data are used to characterize and assess impact response between various configurations. Two impact velocities are tested: 12 and 22 m/s. Several correlations between adhesive microstructure and impact response are observed, including a secondary contact force decrease in multi-laminates having interlayers with microspheres and delamination in multi-laminates having interlayers without air gaps. The multi-laminates with VHB 5925 adhesives (air gaps only) showed the longest cracks and largest fracture area, which directly relates to the greatest displacements and pulsewidths, and smallest second force peaks. Increased fracture compromises structural integrity resulting in more deflection, prolonged impactor contact with the multi-laminate, and decreased elastically stored energy lessening impactor reload. It is demonstrated that this experimental methodology is capable of consistently probing and assessing the local impact response and modulation of an impact load when a multi-layered polymer includes a soft middle layer. Quantitative and qualitative effects of the interlayers’ microstructure on overall impact performance are presented.