Determination of Fracture Toughness Master Curve from ABI Tests
Indentation with a ball indenter generates concentrated stress (and strain) fields near and ahead of the contact of the indenter and the test surface, similar to concentrated stress fields ahead of a crack tip; albeit the indentation stress fields are mostly compressive. The high value of the stress under the ball indenter is an example of plastic constraint where the rigid material surrounding the indentation volume does the constraining. Hence, at a certain critical ball indentation depth, there is a high state of transverse and lateral stresses similar to those in front of a sharp notch in an elastic material. Although the conditions for crack initiation might be attained, the high degree of plastic constraint will prevent cracks from developing during ball indentation of ductile metallic materials. Therefore, only initiation fracture toughness, not tearing modulus, can be determined from ball indentation (Equations 6-12 of Table 1). The initiation fracture toughness is calculated from the integration of the indentation deformation energy (IDE) up to the critical depth (when the maximum pressure underneath the ball indenter equals the critical fracture stress of the steel material at the test temperature or reaches a critical strain value of 0.12, whichever occurs first).