IN-SITU INVESTIGATIONS OF THE FRACTURE MECHANISMS AT VARIOUSLENGTH SCALES

摘要

In this paper the authors present results of in situ observation of crack growth in various materials, at various length scales and under various modes of loading.In order to approach better understanding of the fracture behaviour light microscope (LM) with in-situ recording device, scanning electron microscope (SEM) equipped with a tensile test device and transmission electron microscope (TEM) with a straining holder have been utilized. In situ investigations by light microscopy were carried on pure Cu of bimodal grain size with microcrystalline grains embedded in sub-microcrystalline matrix.For quantitative description of strain field the Digital Image Correlation method was used.As a result of the performed examinations it was found that a crack propagating in a material with bimodal grain size distribution meanders, due to which fracture surface is not flat.This allows presuming that materials with such structure have higher resistance to cracking than materials with homogeneous grain size distribution. In situ SEM fracture studies were performed on the coarse grain Al sample directly in the vacuum chamber of SEM, equipped with tensile test device.The sample with a thickness of 0.2 mm was pre-notched by means of electro polishing.The processes of strain localization, crack initiation and propagation have been investigated with details showing distribution of slip bands. A 316 type stainless steel for the in situ TEM dislocation pile ups against grain boundary studies as an appropriate material having low stacking fault energy has been chosen.An increase of dislocation number in the pile ups as well as a slip of the dislocation in the opposite direction was registered and analyzed in detail. The presented research results show that in situ examinations are useful in revealing phenomena occurring in the crack tip plastic zone.These investigations can be conducted at various length scales, which allows to reveal various phenomena, starting from quantitative description of deformation in case of examinations with use of light microscopy, through SEM examination of slip bands, up to an analysis of individual dislocations behaviour in case of transmission electron microscopy.