Fracture surfaces

The microscopic aspect of the surface where the crack propagates due to stress corrosion and that where the final brittle fracture occurs are rather different. 

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However, it is possible that friction events from rubbing between fractured surfaces can be generated at low load levels also during the loading part of the cycle. This is depicted in the two correlation plots of Figure 5. In the plot at the bottom, these events are marked with a rectangle. It was decided that in addition to the previous filter, another filter based in load level should be added. Acoustic emission events were thus accepted only if they occurred at a load higher than 85% of the maximum load level of the test. 

Figure C2.11.5. Scanning electron micrographs showing the microstmcture of an alumina ceramic spark-plug body (a) fracture surface and (b) polished and thennally etched cross section.

Fig. 2. RepHca electron micrograph of the fracture surface of enstatite—P-spodumene—zirconia glass-ceramic, showing twinning in the enstatite grains...

Fig. 4. Fracture surface (repHca micrograph) of fluorcanasite glass-ceramic showing interlocking blade-shaped crystals and effects of cleavage splintering...

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

Fig. 11. Micrographs of (a) a hot-pressed alumina—TiC ceramic showing a white TiC phase and a dark alumina phase (3) and (b) a fracture surface of an...

Fig. 9. (a) Optical micrograph of the fracture surface of a glass rod at 38 X. (b) Characteristic fracture markings such as mirror, mist, and hackle and the... 

Close examination revealed that the cracks originate on the external surface. Exposure of the fracture surface revealed a rough contour covered with dark copper oxide. Close examination of the internal surface revealed fewer, tighter fissures corresponding to the locations of cracks on the external surface. 

Exposure of the fracture surface revealed two distinct regions (Fig. 

It is clear from examination of the fracture surface and weld cross sections that the weld was improperly formed, resulting in an irregular plane of unbonded metal. The smoothly rippled, spherical contours in some regions of the fractured area are evidence of solidification of the weld metal along a free surface that was not in contact with the plate. Substantial porosity is apparent. 

Figure 15.27 Fracture face (right) contrasted with corresponding, intact weldment. Note that the transition from a rough to a smooth fracture surface occurs abruptly at the junction of the weld metal with the base metal.

Up to this point we have addressed primarily the flaw structure and energy concepts in stress-wave loaded solids governing the creation of new fracture surface area (or the mean fragment size) in catastrophic fragmentation events. It remains to consider a concept which is frequently the end concern in impulsive fracture applications, namely, the distribution in sizes of the particles produced in the dynamic fragmentation event. 

Slides Plastic cavitation around inclusions in metals (e.g. metallographic section through neck in tensile specimen) SEM pictures of fracture surfaces in ductile metals, glass, alkali halide crystals. 

Slides Fatigue fracture surfaces components failed by fatigue, e.g. gear teeth, half-... 

There are two basic types of cast iron white, and grey. The phases in white iron are a and FejC, and it is the large volume fraction of FejC that makes the metal brittle. The name comes from the silvery appearance of the fracture surface, due to light being reflected from cleavage planes in the FejC. In grey iron much of the carbon separates... 

But it does not usually do this. We have already seen that, if the length of the fibres is less than 2x, they will not fracture. And if they do not fracture they must instead pull out as the crack opens (Fig. 25.6). This gives a major new contribution to the toughness. If the matrix shear strength is tr (as before), then the work done in pulling a fibre out of the fracture surface is given approximately by... 

Fig. 25.6. Fibres toughen by pulling out of the fracture surface, absorbing energy os the crock opens.

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