In Situ Tomography of Microcracking in Cross Ply Carbon Fiber Composites with Pre-existing Debonding Damage

摘要

Carbon fiber based composites are an essential material in weight-critical applicationssuch as in the aerospace industry. However, these materials are susceptible todamage such as matrix microcracking and fiber/matrix debonding (diffuse damage),which occurs at stresses much lower than the failure stress.A T700/M21 [0/90]s laminate was tensile loaded to introduce diffuse damage andprepared for a study on the initiation of transverse microcracks. The material wastensile loaded in a [+45/-45]s orientation to induce diffuse damage. A diffuse damageindicator was developed by measuring the decrease in shear stiffness. Samples withdiffuse damage levels of 0, 0.05, 0.10, 0.15, 0.20, and 0.25 were prepared to be tensiletested in a [0/90]s orientation to induce microcracks.A successful development of the microcracking test procedure was performed. Theedge of the material was studied with optical microscopy and x-ray to establish thestructure of the fiber bundle geometry when undamaged. A sample containing microcrackswas treated with diiodomethane dye penetrant, which successfully highlightedmicrocracks during x-ray imaging. The application time was not sufficient to produce consistent x-ray images over time, so a 45 minute soak time was recommendedinstead. The same damaged sample was subjected to a tomographic scan withouta dye penetrant and while unloaded. Transverse microcracks were successfully identified from the data, although the results were not clean enough and likely omittedsome smaller microcracks. Results are expected to be cleaner if performed duringtensile testing.Future tensile testing will quantify the induced crack density of samples containingvarious degrees of initial diffuse damage, either using x-rays with a dye penetrant orusing x-ray microtomography.