Stress intensity factor approach fracture mechanics

Rates of CF crack propagation are uniquely defined by the linear elastic fracture mechanics stress intensity factor range that combines the effects of applied load, crack size, and geometry 17,40. The similitude principle states that fatigue and CF cracks grow at equal rates when subjected to equal values of AK [6-S]. The dal N versus AK relationship may be complex however, an effective approach is based on a power (or Paris) relationship of the form [4/]... 

Fracture mechanics A J KINLOCH Basis energy balance and stress-intensity factor approaches... 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

Linear elastic fracture mechanics (LEFM) approach can be used to characterize the fracture behavior of random fiber composites. The methods of LEFM should be used with utmost care for obtaining meaningful fracture parameters. The analysis of load displacement records as recommended in method ASTM E 399-71 may be subject to some errors caused by the massive debonding that occurs prior to catastrophic failure of these composites. By using the R-curve concept, the fracture behavior of these materials can be more accurately characterized. The K-equa-tions developed for isotropic materials can be used to calculate stress intensity factor for these materials. 

Molecular theories are fundamental in assisting material producers to develop more ESC-resistant materials. However, where load-bearing structures are concerned, an engineering approach to ESC capable of setting up suitable design criteria is required. This should result in the safety of the structure being at an acceptable level with reasonable confidence. Such criteria make use of fracture mechanics and its critical stress intensity factor paramenter, Kic. 

Another aspect to be considered is the difficulty in producing curved structures with the same fibre content as flat laboratory panels. This effect is shown in Figure 16, at the comer the laminate thickness is larger than at the flat section and fibre content is rather lower. This will affect the bending stiffness of the arm and the predicted failure load. This figure also shows the fillet, which is critical to initiation in the specimens without implanted defects. It is well known that fillets can significantly alter the load path in lap shear joints and increase the failure loads (see [1] and Figure 3 for example). If a fracture mechanics approach is to be applied this effect must be considered. Some recent studies on stress intensity factors for such cases may allow this to be addressed [22]. 

The second approach, due to lrwin is to characterise the stress field surrounding a crack in a stressed body by a stress-field parameter K (the stress intensity factor ). Fracture is then supposed to occur when K achieves a critical value K - Although, like Griffith s equation, this formulation of fracture mechanics is based on the assumptions of linear elasticity, it is found to work quite effectively provided that inelastic deformations are limited to a small zone around the crack tip. Like, however, the critical parameter remains an empirical quantity it cannot be predicted or related explicitly to the hysical properties of the solid. Like,, K. is time and temperature de ndent. 

Stress Intensity Factors. A seemingly different approach to fracture on the basis of stress intensity factors, (, 1 6, 17) is frequently encountered in the fracture mechanics literature. It is, therefore, appropriate to briefly discuss this approach and its relation to the Griffith theory. 

In the Fracture mechanics approach, subcritical debonding rate is determined, often as a function of a fracmre parameter such as the applied energy release rate, G. Paris was the first to use this method, and noted that the crack growth rate per cycle was related to the energy release rate (by way of the stress intensity factor) through a power law relationship of the form... 

Experimentalists customarily present the Ahl t rates as a function of the logarithmic stress intensity factor log Ki as defined by equation 20. The problem of relating hlAt to K was solved by using the CRC approach (48) in conjimction with the Griffith fracture mechanics including our equation 21. The result is... 

ISO CD 13586, Plastic—Determination of energy per unit area of crack (Gc) and the critical stress intensity factor (Kc), linear elastic fracture mechanics approach, 2000. 

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