Interpretation of Data for Plane Strain Fracture Toughness Testing

4 Interpretation of Data for Plane Strain Fracture Toughness Testing 

This is the most demanding part of the measurement for fracture toughness. It is recommended that the readers familiarize themselves with the discussions in ASTM STP 410 [1], which captures the historical development of the methodology, and 

ASTM Method E-399 and the supporting documents that document the evolution of the method. 

Although the onset of crack growth, on rising load, serves as the measurement point for fracture toughness, there are test records that exhibit behavior that reflects artifacts associated with microstructural inhomogeneity and severe environmental sensitivity, respectively. They serve as false (low) indicators of fracture toughness, and are deemed to be invalid. Typical examples are shown in Fig. 4.12. 

For experimental purposes, the procedure must be cast in terms of measurable quantities from the load-displacement records (namely, the test data). As such, the physical quantities, such as crack increments, must be related to the changes in displacements, or in the slopes of the load-displacement records. To establish a permissible limit for AailUo, it is assumed that Aoi should not exceed the formally computed plane strain plastic zone correction term, namely 

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