Analysis of acoustic emission caused by internal cracks

Investigation and identification of fracture types of structural materials by means of acoustic emission analysis

In this study, the fracture behaviour of magnesia, magnesia chrome and magnesia spinel (MgAl2O4 and FeAl2O4) refractories under wedge splitting test are qualitatively and quantitatively investigated with the acoustic emission (AE) and digital image correlation (DIC). First of all, the concepts of characteristic widths are proposed for estimating the brittleness of refractory materials according to the shape of load-displacement curve and validated by their good correlation with the characteristic length. Besides, the AE data are analyzed with AE parameter-based approaches and offer new insight into the fracture behaviour of refractory materials, including the classification of the cracking events in grains and in matrix, the distinction between the tensile mode and shear mode damage, and the visualization of the fracture process zone development. It confirms that the pre-existing micro-crack networks in refractories are favourable for the brittleness reduction, which enhance their nonlinear fracture behaviour and thermal shock resistance.

Mechanical and fracture investigation of magnesia refractories with acoustic emission-based method

Anisotropic mechanical and acoustic emission behavior of A533B steels

The feasibility of using acoustic-emission techniques for the characterization of fracture resistance in AISI 4340 and SA533B steels is examined. The critical value of J-integral (JIC) is measured with a single small specimen (compact tension) loaded in the elastic-plastic range at room temperature. Initiation is detected during loading by acoustic emission. A new AE procedure for crack-growth monitoring in fracture-toughness specimens has been proposed. The possibility of discriminating AE signals from noncritical sources, such as void nucleation during crack-tip plastic deformation and signals from a growing crack, is discussed.

Acoustic-emission crack monitoring in fracture-toughness tests for AISI 4340 and SA533B steels: Purpose of this investigation was to examine the features of acoustic emission associated with the onset of subcritical crack extension and crack propagation

A high-strength 4340 steel fracture-toughness specimen was heat treated to give a ductile-rupture type of slow crack growth under rising load. For evaluation of the step-wise growth process, the specimen was instrumented with acoustic stress wave emission (SWE) detection equipment. The resulting crack area swept out by the advancing crack was correlated to the magnitude and number of the acoustic emission pulses. A crack growth model was developed which accounts for the direct relationship between crack area swept out and the sum of the individual SWE amplitudes, and for the experimentally observed bimodal distribution of the SWE amplitudes. The model postulates that slow crack growth takes place in a step-wise mechanism. This involves a repeated two-step process where the first step is the formation of a multitude of individual thumbnail cracks and the second step is the simultaneous interconnection of these thumbnail cracks to form a new continuous crack front.

David G. Atteridge

Papers

An acoustic emission investigation of microscopic ductile fracture