Fracture toughness measurement

T. D. Burchell, I. Oku and M. Eto, A Comparison of Fracture Toughness Measurement Techniques as Applied to Nuclear Graphites, Extended Abstracts and Program, International Carbon Conference, 1990, pp 278 279... 

T.D. Burchell and. T. Oku and M. Eto. A comparison of fracture toughness measurement techniques as applied to nuclear graphite. In Proc. Carbone 90, Paris, France. Pub. Groupe Francais D etude des Carbones, 1990. pp. 278 279. 

Fracture cracks often propagate in an uncontrolled manner. Hence, adhesion tests should utilize cracks which are grown carefully and slowly, as is the case with the procedures commonly used for fracture toughness measurements (ASTM D3433-85). 

Der, V.K., Holloway, D.C. and Kobayashi, T. (1978). Techniques for Dynamic Fracture Toughness Measurements. Prepared for the National Science Foundation by Photomechanics Laboratory, Mechanical Engineering Department, University of Maryland, College Park Campus. 

The characteristic values of the crack initiation and propagation parameters (Jic, Kjc and Tr) are listed in Table IV for the different ternary blends. In all cases the requirements for valid fracture toughness measurement were fulfilled. These results confirm that type B fibres are better suited as reinforcement for rubber-toughened nylon materials. But the marked reduction... 

Parvin and Williams made fracture toughness measurements on HIPS between —120° and 20 °C. Some of the measurements were made on 150 mm wide single edge (SEN) specimens, but the study concentrated on fracture in 6 mm thick... 

Fig-3- Schematic of the asymmetric double cantilever beam geometry used for fracture toughness measurements... 

The fracture toughness measurements, combined with the post-mortem surface analysis of the fracture surfaces, allow us to develop some simple micromechanical models to account for these three situations. 

The key points to be gleaned from this exercise are that the plastic zone size depends on the state of stress (or constraint) and is proportional to (Ki/oys). Its size is expected to vary through the thickness of a plate, and it would increase with increasing stress intensity factor Kj and decreasing yield strength ays. The consequence of plastic deformation on fracture behavior and fracture toughness measurements is considered briefly in the next section. 

In summary, the specimen size requirements for plane strain fracture toughness measurements are as follows ... 

Typical long-crack data for a number of ceramics are shown in Fig. 12.12, where the og dc/dN) versus log AA i curves are linear and very steep, implying high values of q. These studies also indicate that under cyclic loading, the thresholds for crack growth can be as low as 50 percent of the fracture toughness measured under monotonically increasing loads. 

Wells [11] first proposed the crack opening displacement as a measure of fracture toughness for tough metals such as the structural steels where LEFM is not valid. The concept became a standard in 1979 in the form of British Standard (BS) 5762 [12], which is now-superseded by BS 7448 Part 1. 1991. Method of determination of Afi. critical CTOD and critical J values of metallic materials [13], More recently. ASTM E1290-9,3, Crack-tip opening displacement (CTOD) fracture toughness measurement [14] was issued. It is stated that this method may be used to characterize the toughness of materials that arc (a) too ductile or fail the size conditions to be tested for or (b) likely to produce unstable crack extension that would invalidate Ji tests in accordance with the requirements of ASTM E81.3 [15]. 

ASTM El290. 1993. Crack-tip opening displacement (CTOD) fracture toughness measurement. 

Once again, the last term represents the pressure times the volume of the bulge, which is relevant to interface fracture toughness measurements conducted by injecting a fixed volume (see Section 39.4.2.2). 

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