Part 1: An Analytical Comparison Of Short Crack And Deep Crack CTOD Fracture Specimens Of An A36 Steel

The effect of crack-depth to specimen-width ratio on Crack Tip Opening Displacement (CTOD) fracture toughness is an important consideration in relating the results of laboratory tests to the behavior of actual structures. Deeply cracked, three-point bend specimens with crack-depth to specimen-width ratios (a/W) of 0.50 are most often used in laboratory tests. However, to evaluate specific weld microstructures or the behavior of structures with shallow surface cracks, specimens with a/W ratios much less than 0.50 often are required. Laboratory tests reveal that three-point bend specimens with short cracks (a/W = 0.15) exhibit significantly larger critical CTOD values than specimens with deep cracks (a/W = 0.5) up to the point of ductile initiation.

In this study, finite element analyses are employed to compare the elastic-plastic behavior of square (cross-section) three-point bend specimens with crack-depth to specimen- width ratios (a/W) ranging between 0.50 and 0.05. The two-dimensional analysis of the specimen with an a/W ratio of 0.15 reveals a fundamental change in the deformation pattern from the deep crack deformation pattern. The plastic zone extends to the free surface behind the crack concurrent with the development of a plastic hinge. For shorter cracks (a/W = 0.10 and 0.05), the plastic zone extends to the free surface behind the crack prior to the development of a plastic hinge. For longer cracks (a/W > 0.20), a plastic hinge develops before the plastic zone extends to the free surface behind the crack.

Part 2: The Effects Of Crack Depth On Elastic-Plastic CTOD Fracture Toughness

The potential effects of crack depth (a/W ratio) on elastic-plastic measures of fracture toughness must be known to correlate properly the results of laboratory tests with the behavior of flawed structural components. Deep crack specimens (a/W = 0.50) are used extensively to provide the most severe crack-tip conditions and therefore conservative (lower-bound) measures of fracture toughness. A short crack specimen (a/W << 0.50) is frequently employed when a deeply cracked specimen is either inappropriate or impossible to obtain in the CTOD specimen. Specific examples are the testing of particular microstructures in weldments, local brittle zones (LBZ's), and in-service structures with shallow or short cracks, particularly surface flaws. This study compares the CTOD results of square (cross-section) three-point bend specimens with short cracks (a/W = 0.15) to the CTOD results of specimens with deep cracks (a/W = 0.50).

Part 3: A Comparison Of The J-Integral And CTOD Parameters For Short Crack Specimen Testing

Two of the elastic-plastic fracture mechanics (EPFM) test methods widely used in the transition region where linear-elastic ---, behavior is no longer applicable are the J-integral and the crack tip opening displacement (CTOD) test methods. The J-integral test procedure is restricted to temperature regions above the initiation of ductile tearing and is also limited to crack-depth to specimen-width ratios (a/W) between 0.50 and 0.75. In contrast, the CTOD test procedure can be used for testing throughout the entire temperature-toughness transition region from brittle to fully ductile behavior. Also, extensive research is being conducted to extend the CTOD test procedure to the testing of short crack specimens (a/W ratios of approximately 0.15).