Comparison of 15-5PH Stainless Steel Type 1 versus Type 2 Fatigue Data for Aircraft Primary Structural Elements

15-5PH is a precipitation-hardening, martensitic stainless steel used for primary structural elements such as engine mounts where corrosion resistance, high strength, good fatigue and fracture toughness is required. The material composition is defined in AMS5659M. This alloy can be either Type 1 - vacuum arc remelt (VAR) or Type 2 - electro slag remelt (ESR) and is most commonly heat treated per SAE AMS-H-6875 or AMS2759/3 to condition H1025 (an ultimate tensile strength of 155 ksi [1070 MPa] minimum). Typically material handbooks have limited fatigue data and most data is only for Type 1. Therefore, the fatigue properties of 15-5PH H1025 stainless steel for both Type 1 and Type 2 were determined. The objective of the fatigue testing was to generate a family of S-N curves (maximum stress versus number of cycles to failure) for a series of stress ratios across the entire range of cycles to failure. All test coupons have been made from 5/8″ to 1″ [16-25 mm] diameter bar material with a typical lathe turned surface finish of 40-80 micro-inches [1-2 micro-meters] or a polished surface of 8 micro-inches [0.2 micro-meters]. The machined surface versus the polished surface resulted in a 10% reduction in fatigue strength near the endurance limit. When comparing the family of curves of Type 1 and Type 2 materials no significant differences were found. Therefore, the VAR and ESR Type materials appear to have approximately equivalent fatigue properties based on this set of fatigue data.