Fractal analysis of mixed-mode I/II fracture for thermoplastic matrix composite

Abstract

Fracture surfaces express the sequence of energy release events with the crack propagation showing the relation among the components of the structure, local stress fields and the formation of typical textures in its relief. In the case of polymer matrix composites, the evolution of topographic features may indicate the action of load lines, the effects of environment degradation, processing defects, and others. So, with the increasing use of these materials in aircraft, quantitative fractography can expand the investigation of structural failure as a tool for the development of processes and products. Therefore, it is necessary to adapt or create approaches for the quantitative study of reliefs. This proposal is based on the combination of topography measurement tools, such as the focus of extension for reconstruction in optical microscopy or reconstruction by parallax stereo pairs obtained by scanning electron microscopy, with fractal analysis of textures formed by the relevant details, corresponding to the fracture mechanisms operating in mixed mode (I / II) loading, using a methodology already developed by our Group. It is used to evaluate the use of fractal characterization routine also developed by this Group for fractographic characterization of long fiber carbon composite materials, in the form of plates obtained by pressing with heating, loading and controlled cooling, serving aircraft construction standards.The materials are samples of carbon-polyethylene composite to be fractured under MMB tests (Mixed-Mode Bending), prepared from plates obtained by axial pressing using a press with automatic heating control, load and cooling. Investigations will be conducted primarily from image stacks obtained by optical microscopy and processed for removal of relief and distribution of textures formed in the fractured surfaces. Techniques of scanning electron microscopy will also be used, involving fractography correlative technique where maps of elevations and reconstructed images from the stacks of images obtained by optical microscopy are associated, pixel by pixel, with images obtained by scanning electron microscopy from imaging techniques such as microscopy, ultra-low voltage and variable pressure. Except for mixed mode test, all other characterization protocols and processing have been developed by the Group, which will not bring inadequate complexity of this scientific research work. The generated images are quantitatively analyzed using processing routines and analysis of digital images developed or adapted by members of the research group that the candidate is member for more than two years.