Optimal Strain Gage Locations for Experimental Determination of Stress Intensity Factors

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Stress intensity factor (SIF) is an important parameter in linear elastic fracture mechanics as its limiting value decides whether an existing crack in a component grows or not. Use of linear elastic fracture mechanics principles in predicting and preventing fracture of engineering components largely depends on the availability of accurate values of SIF. Among all the experimental techniques, strain gage techniques are relatively inexpensive, simple and easy to use for the determination of the SIFs. However, the location and orientation of strain gage(s) with respect to the crack tip are extremely important for accurate determination of SIFs using strain gages. Strain gages placed either very near or very far from the crack tip might lead to inaccurate determination of SIFs due to local effects near the crack tip or incorrect representation of the strain field respectively. The present work proposes methodologies for accurate estimation of maximum permissible radial location D D max r of strain gage (s) in both mode I and mixed mode I/II, which ensures optimal strain gage locations for accurate determination of SIFs. A finite element based approach supported by strong theoretical formulations has been proposed for accurate estimation of max r for different mode I and mixed mode experimental specimens. Further, a modified strain gage technique for experimental determination of mixed mode SIFs has also been proposed in the present investigation. The effect of crack length, net ligament length, PoissonDs ratio and state of stress on the max r have been investigated in this work. Finally, the experimental verification of the theory and methodologies proposed in the present investigation has also been carried out by conducting number of experiments on both mode I and mixed mode specimens. Experimental investigations show that very accurate values of SIFs could be obtained when the strain gage(s) are placed optimally (within the max r , obtained using the proposed methodologies). It was also observed from the results of present experiments that the error in SIFs was very high when the gage(s) were not placed within the recommended value of max r , thereby reinforcing the importance of knowing the max r in experimental determination of SIFs using strain gages. The experimental results also confirm the dependence of max r on crack length and net ligament length as predicted by the proposed methodology..
Supervisor: K .S. R. KrishnaMurthy AND D Chakraborty