4.1 The property K_Ic, determined by Test Method E399 or ISO 12135, characterizes a material's resistance to fracture in a neutral environment and in the presence of a sharp crack subjected to an applied opening force or moment within a field of high constraint to lateral plastic flow (plane strain condition). A K_Ic value is considered to be a lower limiting value of fracture toughness associated with the plane strain state.

4.1.1 Thermal quenching processes used with precipitation hardened aluminum alloy products can introduce significant residual stresses.5 Mechanical stress relief procedures (stretching, compression) are commonly used to relieve these residual stresses in products with simple shapes. However, in the case of mill products with thick cross-sections (for example, heavy gauge plate or large hand forgings) or complex shapes (for example, closed die forgings, complex open die forgings, stepped extrusions, castings), complete mechanical stress relief is not always possible. In other instances residual stresses may be introduced into a product during fabrication operations such as straightening, forming, or welding operations.

Note 1: For the purposes of this guide, only bulk residual stress is considered (that is, of the type typically created during a quench process for thermal heat treatment) and not engineered residual stress, such as from shot peening or cold hole expansion.

4.1.2 Specimens taken from such products that contain residual stress will likewise themselves contain residual stress. While the act of specimen extraction in itself partially relieves and redistributes the pattern of original stress, the remaining magnitude can still be appreciable enough to cause significant error in the test result.

4.1.3 Residual stress is a non-proportional internal stress that is superimposed on the applied stress and results in an actual crack-tip stress-intensity factor that is different from one based solely on externally applied forces or displacements, and residual stress can bias the toughness measurement. Conceptually, compressive residual stress in the region of the crack tip must be overcome by the applied force before the crack tip experiences tensile stresses, thus biasing the K_Q or K_Ic measurement to a higher value, potentially producing a non-lower-bound toughness value. Quantitatively, the effect depends on stress equilibrium for the continuously varying residual stress field and the associated crack tip response. Conversely, a tensile residual stress is additive to the applied force and biases the measured K_Q or K_ic result to a lower value, potentially under-representing the material “true” toughness capability.

4.1.4 Tests that utilize deep edge-notched specimens such as the compact tension C(T) are particularly sensitive to distortion during specimen machining when substantial residual stress is present. In general, for those cases where such residual stresses are thermal quench induced, the resulting K_Ic or K_Q result is typically biased upward (that is, K_Q is higher than that which would have been achieved in a residual stress-free specimen). The inflated values result from the redistribution of residual stress during specimen machining and excessive fatigue precrack front curvature caused by variable residual stresses across the crack front.6

4.2 This guide can serve the following purposes:

4.2.1 Provide warning signs that the measured value of K_Ic has been biased by residual stresses and may not be a lower limit value of fracture toughness.

4.2.2 Provide experimental methods that can be used to minimize the effect of residual stress on measured fracture toughness values.

4.2.3 Suggest methods that can be used to correct residual stress influenced values of fracture toughness to values that approximate a fracture toughness value representative of a test performed without residual stress bias.

1.1 This guide covers supplementary guidelines for plane-strain fracture toughness testing of aluminum products for which complete stress relief is not practicable. Guidelines for recognizing when residual stresses may be significantly biasing test results are presented, as well as methods for minimizing the effects of residual stress during testing. This guide also provides guidelines for an empirical correction as well as interpretation of data produced during the testing of these products. Test Method E399 is the standard test method to be used for plane-strain fracture toughness testing of aluminum alloys.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.