Elastic-Plastic Fracture Toughness Determination Under Some Difficult Conditions

The JIc value and J-R curve have been widely used to characterize elastic-plastic fracture behavior in modern engineering practice. Their determination has been standardized. To Determine the JIc value and the J-R curve using the ASTM standard test methods, three measurements, load, displacement and crack length, must be obtained simultaneously from the test. This may sometimes be difficult or even impossible due to the restrictions of test equipment, adverse environments, test conditions or test arrangements. Recently, a method of normalization has been introduced by Landes and his group as a convenient way for J-R curve determination. This method uses the principle of load separation to relate the three variables, load, displacement and crack length, during the fracture process. A functional form, named the LMN function, has been successfully used to relate the normalized load to the plastic deformation. It gives an unique relationship between load, displacement and crack length. The method of normalization provides an analytical basis to predict any one of the three variables from the other two. In this study the method of normalization was further developed for J-R curve determination under some difficult conditions. Four subjects were included. They are: • J-R curve determination from load versus displacement records without crack length measurements. • J-R curve determination from load versus crack length records without displacement measurements. • J-R curve determination under the conditions when load is difficult to measure. • J-R curve determination under dynamic loading conditions. For each subject, the new features and problems needed to be solved were described. A major problem solved for all the subjects was how to determine the LMN function accurately and efficiently under the different conditions. A large number of experimental data sets were collected to support this study. Based on the analysis results of these test data, new approaches were developed for each subject. Each approach was examined under the condition assumed for corresponding application. Results of the analysis were presented to show its adequacy on each subject. It has been demonstrated that the approaches developed in this study could greatly simplify the J-R curve test procedure, and improve the resulting J-R curve accuracy. The results are more reliable, especially under the specific difficult test conditions. These methods will likely have extensive applicability in engineering practice