Critical bending load of CFRP panel with shallow surface scratch determined by a tensile strength model

Abstract

Carbon fibre reinforced polymer (CFRP) composite structures may experience light surface damage such as shallow surface scratch around 50 μm or less in applications. In this study, critical bending loads of 2D woven CFRP panels with and without shallow surface scratches were assessed under three-point-bending (3-p-b) conditions and then modelled by a simple strength theory for composite fracture. This closed-form tensile strength solution is a non- Linear Elastic Fracture Mechanics (LEFM) model derived specifically for heterogeneous composites. Direct tensile tests of the CFRP panels (without shallow surface scratch) were also performed, and measured tensile strength was compared with that from 3-p-b tests. The relative error between the two different tests was only around 6.7%. The ply thickness around 140 μm was selected as the controlling composite microstructure parameter or characteristic composite unit Cch in this non-LEFM model. This simple model shows that a suitable composite characteristic microstructure measurement such as Cch = ply thickness can significantly simplify composite fracture analysis. A simple statistical analysis using a 2D normal distribution methodology for strength measurements was also introduced so that the experimental scatters can be analysed. The 95% reliability band can be conveniently used in safe design of composite structures.