Experimental Specimen for Classification of Matrix Compression Damage in Carbon Fiber Reinforced Polymers

Abstract

Composite materials are becoming increasingly common in the aerospace industry. In order for simulation and modeling to accurately predict failure of composites, a material model based on observed damage mechanisms is required. Composites are commonly classified into four damage categories based on the composite constituents and their loading condition: fiber tension, fiber compression, matrix tension, and matrix compression. Previous work identified a compact compression (CC) specimen as a suitable option for isolating matrix compression damage. However upon continued testing, stable crack propagation in the specimen was limited to a relatively low material failure ratio (σCompressive/σTension). This paper presents specimen geometry that can isolate matrix compression damage in materials with a failure ratio greater than two, the limit of the compact compression specimens. Initial specimen selection used the compact compression specimens from previous research and added additional specimens based on commonly used compressions specimens for different materials. The added specimens included center notched compression (CNC), edge notch compression (ENC), and four-point bending (4PB). All specimens were evaluated experimentally with the success criteria of controlled propagation of a matrix compression crack. In addition to propagating a controlled matrix compression crack, specimens were required to have a visible region around the stress concentrator to allow for digital image correlation (DIC) image capture during the experiments. The specimens were manufactured from a carbon fiber reinforced polymer (CFRP) with a failure ratio greater than six. CC and 4PB specimens were unable to produce a compression crack before any other failure methods were present. CNC specimens produced an unstable compression crack that progressed from the notch to the edge of the specimen too rapidly to acquire meaningful crack propagation data. ENC specimens showed some ability to stably propagate a crack, however some tests resulted in an unstable crack propagation similar to the CNC specimens. In order to increase the test repeatability, a tapered thickness was added to the specimen around the notch tip. The resulting tapered ENC (TENC) produced repeatable controlled matrix compression crack propagation. Ultimately, the specimen fails when the crack has propagated through the entire width of the specimen. TENC specimens show promise for isolating matrix compression damage in materials with high failure ratios. Continued testing of CFRP with TENC specimens could be used to refine the material model for finite element analysis.