DAMAGE TOLERANT ASSESSMENT OF AM 304L AND AM 316L STEEL

The damage tolerance and durability assessment of AM materials is one of the key factors required for the certification of AM parts. Unfortunately, AM materials often exhibit a large degree of anisotropy and the crack growth curves needed for damage tolerance design certification differ markedly from those associated with the conventionally manufactured material. Indeed, as noted in the USAF certification standard MIL-STD-1430D it is essential that this variability be understood. This is particularly important when performing the Risk of Failure analysis mandated in the US Joint Services Structural Guideline JSSG2006.

The anisotropy and the effect of stress relief associated with AM 304L can be quite significant. However, when assessing the damage tolerance and durability of AM304L steel these differences need to be accounted for. To this end the governing parameters, and how to account for their effect on crack growth will be explained. It will be shown that this can be done using the Hartman-Schijve crack growth equation associated with conventionally manufactured 304L steel and making allowance for the effect of the additive manufacturing process, and heat treatment process on the fracture toughness, and the fatigue threshold. It is also shown that when this is done various R ratio dependent curves associated with conventionally manufactured and AM 304L steel all collapse onto the same Hartman-Schijve master curve. Interestingly Master Curve is very similar to that of conventionally manufactured and AM 316L steel.