Local deformation, specimen surface

Irregularities of a specimen s surface will result in local deformation with accompanying deformation hardening. This may lead to erroneous hardness numbers, although such errors may be small. 

For a very thin specimen i.e., with B (Kjc/ays) ), the influence of plastic deformation at the surfaces will relieve crack-tip constraint through the entire thickness of the specimen before Kj reaches Kjc. As such, the opening mode of fracture is suppressed in favor of local deformation and a tearing mode of fracture. The behavior is reflected in the load-displacement record by a gradual change in slope and final fracture, which could still be abrupt (see Fig. 4.7b), but the conditions of plane strain would not be achieved. 

Surface layers interfere with the motion of dislocations near surfaces. Among other effects, this causes local strain-hardening, creating a harder surface region which thickens with further deformation, and eventually affects an entire specimen. A specific way in which this happens is through curving... 

Fracture initiated in the tensile tested ABS samples, as noted also by Truss and Chadwick from either surface flaws or from internal flaws. Figure 33a shows an SEM picture of the tensile fracture surface of a sample broken at a comparatively high deformation rate of 12.7 cm/min. The fracture surface is unlike that of SAN (Fig. 27 a) or that of rubber modified polystyrene (Fig. 3 a). Fracture, for this specimen, has developed from both a surface source and from an internal source and fine radial flow lines emanate from both sources. The slow growth region adjacent to the source tends to develop a conical shape as has been noted This is probably a result of localized shear formation. In ABS specimens subject to creep deformation at low values of stress, the creep strain is found to be due almost entirely to shear but, at higher stresses, shear is accompanied by crazing Crazes can also be induced... 

Fig. 5.8a. Scanning electron microscopic image of the fracture surface of a brittle crack from a NCTL test spedmea The view is directed fiom the front top on the top of the lower part of the 3.2 mm wide raptured test specimen (see also Fig. 3.17). The edge of the notch is fully recognisable in the foreground. A semicircular fracture surface emerges from the edge. Only when the remaining cross section reduces to such an extent that the local stress exceeds stress at yield, the material yields and breaks after large plastic deformation...

CF experiments are hindered by several common factors. Aggressive environments are difficult to contain at a constant condition, and hinder precise measurements of specimen displacement, load, and crack size. CF is influenced by many interactive mechanical, chemical, and microstructured variables that must be factored into experimental design. It is often necessary to investigate slow-rate deformation and cracking phenomena in a realistic time experiments must be conducted for one day to one year or more. CF damage is localized at surface slip structure and near the crack tip high resolution observations are not generally available and behavior must be interpreted from indirect measurements. 

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