A Hybrid Experimental-Numerical Study of Crack Initiation and Growth in Transparent Bilayers Across a Weak Interface

Abstract

Transparent layered structures are of importance to both the military and civilian communities. Their applications include but not limited to lightweight transparent armor, automotive windshields and canopies, personnel shields and visors as well as electronic displays. The introduction of adhesive interlayers is a low-cost approach for developing mechanically resilient multilayered lightweight structures. However, a rigorous mechanics based design of such architectures requires tailoring interfaces (layer thickness, adhesive properties, number of layers, interface location, etc.). Among the multitude of issues involved in this regard, grasping the mechanics of dynamic crack growth across interfaces is of paramount importance. In this context, this work builds on optical investigations of Sundaram and Tippur (J Mech Phys Solids 96:312–332, 2016) who reported dynamic crack-interface interactions related to crack penetration vs. crack branching at a weak interface when the interface was oriented perpendicular to the incoming mode-I crack in an otherwise homogeneous bilayer. A major finding of this work was that a slowly growing crack with a lower stress intensity factor penetrated the interface and grew into the next layer without branching. On the contrary, a fast growing crack with a higher stress intensity factor debonded the interface ahead of its arrival at the interface and hence branched into the interface and subsequently into the next layer as (two) mixed-mode daughter cracks creating higher fracture surface area. In order to exploit this observation and gain further insight into crack growth in multilayered structures, a hybrid experimental-numerical approach that mimics the complexities observed in the bilayer experiments is attempted. This includes optical measurement of the force histories imposed on the bilayer during impact loading of a V-notched PMMA sample impacted by a long-rod with wedge shaped tip matching the notch. Digital Gradient Sensing (DGS) method has been utilized in conjunction with ultrahigh-speed photography followed by optical data analysis to visualize and quantify the force histories. The measured force histories along with other previously determined interface and PMMA characteristics are used as input parameters into a finite element model that includes cohesive elements to benchmark the experiments. Thus validated computational model will be used to investigate a variety of parameters far too complex to emulate experimentally in multilayer architectures.