The property K_Ic, determined by Test Method E399 or ISO 12737, 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.

Thermal quenching processes used with precipitation hardened aluminum alloy products can introduce significant residual stresses in the product. 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 gage 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 unintentionally introduced into a product during fabrication operations such as straightening, forming, or welding operations.

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 ensuing test result.

Residual stress is superimposed on the applied stress and results in an actual crack-tip stress intensity that is different from that based solely on externally applied forces or displacements.

Tests that utilize deep edge-notched specimens such as the compact tension C(T) are particularly sensitive to distortion during specimen machining when influential 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 combination of specimen distortion and bending moments caused by the redistribution of residual stress during specimen machining and excessive fatigue precrack from curvature .

This guide can serve the following purposes:

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.

Provide experimental methods by which to minimize the effect of residual stress on measured fracture toughness values.

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 correction and 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 and health practices and determine the applicability of regulatory limitations prior to use.