Nano Mechanical Testing for In Situ X-CT Observation of CFRP

Abstract

Carbon fiber reinforced polymer (CFRP) composites are of growing use in aircrafts owing to their high specific strength and stiffness. Micro-mechanism of damages and microscopic chemical properties of the CFRPs is a key to understand the mechanical properties and durability of these materials. Especially the initiation and propagation along fiber/polymer interfaces is of great importance. However, most experimental observations have been carried out for surfaces (cross sections) of CFRP specimens typically using electron microscopes, where the crack propagation along the fibers, i.e. along the thickness direction of a specimen, cannot be observed. Recent reports pioneered non-destructive and three-dimensional (3D) observation using X-ray computed tomography (X-CT), but their special resolutions are limited down to ca. few or sub μm. Here we have succeeded in non-destructive and 3D observation of cracks initiation and propagation with a resolution of less than 50 nm under an applied stress with a new x-ray transmission microscope (XRM) using synchrotron radiation. Optimization of apparatus and experimental techniques of mechanical testing is the key for in situ X-CT observation, which is especially a challenging task in the case of X-CT with a high resolution less than μm. We have developed techniques of nano-mechanical testing for in situ X-CT observation by indentation with a diamond edge to initiate and propagate cracks, or tensile testing. Using the nano-mechanical testing for in situ XCT, we have succeeded in non-destructive and 3D observation of the initiation and propagation of cracks with a resolution down to 50 nm under an applied stress.