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Standard Practice for Fractographic Analysis of Fracture Mirror Sizes in Ceramics and Glasses
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NORM herausgegeben am 1.7.2021
Bezeichnung normen: ASTM C1678-21
Ausgabedatum normen: 1.7.2021
SKU: NS-1030573
Zahl der Seiten: 16
Gewicht ca.: 48 g (0.11 Pfund)
Land: Amerikanische technische Norm
Kategorie: Technische Normen ASTM
Keywords:
ceramics, fractography, fracture mirror, fracture strength, fracture stress, fracture surface, glasses, hackle, microscope, mist, origin, residual stresses,, ICS Number Code 81.060.20 (Ceramic products)
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 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 in
glasses. Each has a corresponding mirror constant, A. The most
common notation is to refer to the mirror-mist boundary as the
inner mirror boundary, and its mirror constant is designated
A5.6?The
size predictions based on Eq
1 and the A values, or alternatively stress intensity
factors, match very closely for the limiting cases of small mirrors
in tension specimens. This is also true for small semicircular
mirrors centered on surface flaws in strong flexure specimens. So,
at least for some special mirror cases, A should be directly
related to a more fundamental parameter based on stress intensity
factors.
5.7?The size of the fracture mirrors in laboratory test specimen fractures may be used in conjunction with known fracture mirror constants to verify the stress at fracture was as expected. The fracture mirror sizes and known stresses from laboratory test specimens may also be used to compute fracture mirror constants, A. 5.8?The size of the fracture mirrors in components may be used in conjunction with known fracture mirror constants to estimate the stress in the component at the origin. Practice C1322 has a comprehensive list of fracture mirror constants for a variety of ceramics and glasses. |
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1. 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
a1.2 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 safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee. |
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2. Referenced Documents | ||||||
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