Paper 4: Electron Optical Techniques of Failure Investigation

Abstract

Fractography, the study of fracture surfaces, is useful in failure investigations as the topography and characteristic markings of such surfaces are indicative of the mechanism of fracture which operated during the initiation of failure and crack propagation. Owing to the low depth of focus of the optical microscope, interpretation of some fracture surfaces may be difficult. The microscopic topography, and its relation to the causes and basic mechanisms of fracture, may be conveniently studied by electron microfractography using non-destructive replica methods. Replicas may be taken from selected areas of the fracture surface of large, unwieldy engineering components. Complementary electron optical techniques such as electron diffraction, scanning electron microscopy, and extraction replicas are used where possible to obtain additional fine-scale information of use in the elucidation of failure mechanisms. An explanation of the various techniques and examples of their use in the work of the National Engineering Laboratory in failure investigations is given. The investigations involve fatigue, brittle fracture, corrosion fatigue, stress corrosion, welding problems, and surface phenomena.

Surface Studies in Tribological Research and Failure Investigation

“…Also, the initial stages of many surface phenomena, such as wear, corrosion and fatigue, occur on a very small scale so that electron microscopical techniques may be required for their study. Direct examination of suitable small speci-Stylus methods provide an immediate numerical char-mens can be carried out by reflection or scanning acterization of a surface, such as "center line techniques (11), but the transmission electron miaverage," but there is uncertainty about the ex-croscope requires special replica techniques of tent of damage done by the stylus itself. There-specimen preparation (3)(4)(5)(6)(7)(8).…”

“…Numbers in parentheses designate References developed (3,11) for rough surfaces. Replicas may at end of paper.…”

“…Crushed extraction replicas may be used to provide powder material for indentification by X-ray diffraction or fluorescent analysis (6). The electron-probe X-ray microanalyzer may be used to determine the composition of surface material, and contaminant films as well as replicas extracted material (11,17). A finely focussed electron beam scans the specimen and the elastically scattered, and emitted secondary electrons are collected, passed through an electron multiplier, and displayed on a cathode-ray tube in synchronism with the scan to provide a visual image.…”