NFLUENCE OF POINT OF IMPACT ON DAMAGE IN SINGLE LAP-JOINTS OF WOVEN FABRIC COMPOSITES

Abstract

Low velocity impact on composite lap joints is a common occurrence in various applications such as aircraft wings, sporting equipment, pipes and civil engineering works etc. The response of these joints under such loading is quite complex. It involves multiple damage modes that may occur within plies (fibre and matrix damage), at interfaces between plies (delamination) and at the bond interface (joint failure). The severity of damage, the damage modes, and the interaction between them depends on multiple factors such as joint geometry, material properties of the laminate and the adhesive, impact velocity and point of impact relative to joint geometry. As a designer, considering all these factors poses a significant challenge. While the choice of material for laminate and the adhesive and the overall joint geometry is often dictated by multiple practical factors such as availability, cost and manufacturability, the designer must demonstrate that the design will be safe during operation regardless of where a projectile hits the joint. Thus the designer designs for extreme loading scenarios. In case of joints made of composite materials, it is very difficult if not impossible to determine these extreme loading scenarios based on analytical formulations. Thus the objective of this study was to investigate and quantify the effect that the change in point of impact has on the damage within a single lap joint under transverse impact. In particular, it analyses that how the delamination and debonding damage is affected by the change of impact location for a single lap joint, with the ultimate aim to provide guidelines which will help the designer in deciding the extreme loading scenarios that must be considered to demonstrate that the design is safe. This was achieved by FE modeling of impact damage for four joints having different overlap widths. For each joint, FE based explicit dynamic impact simulations were carried out for seven locations or points of impact for three different impact velocities in the low velocity regime. Delamination between the plies and disbond at the bond interface was modeled using cohesive zone approach, while the plies themselves were modeled using 3D continuum shell elements. This extensive FE based parametric study was carried out to categorize the delamination damage as a function of impact location, velocity and overlap width. In this regard, delamination percentage in the overlap width of a joint was evaluated in the form of interfacial and total 9 delamination i.e. the average of all the interfacial delaminations in a particular impact scenario divided by the total number of interfaces in the lap joint – seven in this study for each case. After detailed examination, it was established that the total delaminated area in a lap joint is insensitive to the point of impact. The same, however, cannot be said about interfacial delamination; it being highly dependent on point of impact. Based on the comparative analysis of the results achieved from 74 different impact scenarios, the authors determined the impact location at which the lap joint experiences extreme delamination failure and proposed a criterion for safe design of a lap joint