ASTM C1678-21 PDF

SIGNIFICANCE AND USE 5.1 Fracture mirror size analysis is a powerful tool for analyzing glass and ceramic fractures. Fracture mirrors are tell-tale fractographic markings in brittle materials that surround a fracture origin as discussed in Practices C1256 and C1322. Fig. 1 shows a schematic with key features identified. Fig. 2 shows an example in glass. The fracture mirror region is very smooth and highly reflective in glasses, hence the name “fracture mirror.” In fact, high magnification microscopy reveals that, even within the mirror region in glasses, there are very fine features and escalating roughness as the crack advances away from the origin. These are submicrometer in size and hence are not discernable with an optical microscope. Early investigators interpreted fracture mirrors as having discrete boundaries including a “mirror-mist” boundary and also a “mist-hackle” boundary in glasses. These were also termed “inner mirror” or “outer mirror” boundaries, respectively. It is now known that there are no discrete boundaries corresponding to specific changes in the fractographic features. Surface roughness increases gradually from well within the fracture mirror to beyond the apparent boundaries. The boundaries were a matter of interpretation, the resolving power of the microscope, and the mode of viewing. In very weak specimens, the mirror may be larger than the specimen or component and the boundaries will not be present. Eq 1 is hereafter referred to as the “empirical stress – fracture mirror size relationship,” or “stress-mirror size relationship” for short. A review of the history of Eq 1, and fracture mirror analysis in general, may be found in Refs (1)3 and (2). 5.5 A, the “fracture mirror constant” (sometimes also known as the “mirror constant”) has units of stress intensity (MPa√m or ksi√in.) and is considered by many to be a material property. As shown in Figs. 1 and 2, it is possible to discern separate mist and hackle regions and the apparent boundaries between them i... SCOPE 1.1 This practice pertains to the analysis and interpretation of fracture mirror sizes in brittle materials. Fracture mirrors (Fig. 1) are telltale fractographic markings that surround a fracture origin in brittle materials. The fracture mirror size may be used with known fracture mirror constants to estimate the stress in a fractured component. Alternatively, the fracture mirror size may be used in conjunction with known stresses in test specimens to calculate fracture mirror constants. The practice is applicable to glasses and polycrystalline ceramic laboratory test specimens as well as fractured components. The analysis and interpretation procedures for glasses and ceramics are similar, but they are not identical. Different optical microscopy examination techniques are listed and described, including observation angles, illumination methods, appropriate magnification, and measurement protocols. Guidance is given for calculating a fracture mirror constant and for interpreting the fracture mirror size and shape for both circular and noncircular mirrors including stress gradients, geometrical effects, residual stresses, or combinations thereof. The practice provides figures and micrographs illustrating the different types of features commonly observed in and measurement techniques used for the fracture mirrors of glasses and polycrystalline ceramics. FIG. 1 Schematic of a Fracture Mirror Centered on a Surface Flaw of Initial Size (a) Note 1: The initial flaw may grow stably to size ac prior to unstable fracture when the stress intensity reaches KIc. The mirror-mist radius is Ri, the mist-hackle radius is Ro, and the branching distance is Rb. These transitions correspond to the mirror constants, Ai, Ao, and Ab, respectively. 1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard. 1.3 This standard does not purport to address all of the safet...