Advances Relating To Fatigue Calculations For Combined Random and Deterministic Loads Neil Bishop § , Paresh Murthy § , and Karl Sweitzer §§ § CAEfatigue Limited, Farnham, Surrey, UK. §§ Booz Allen Hamilton, Oakton, VA, USA Summary Techniques for calculating fatigue life from random structural responses were first proposed in the 60’s but these early methods were limited to narrow band responses (ref 1). When used for wide band responses these same techniques could become very conservative. In order to reduce this conservatism much effort was devoted from the 1980’s onwards to develop methods that worked more accurately for the wide band situation. Several methods now exist for the wide band case and these typically exist alongside FE based random analysis tools like Nastran, Ansys or Abaqus to take the PSD’s of stress response and return the Rainflow cycle count and fatigue damage (ref 2). Several problems still exist with todays design methods. Firstly, for large models, these stress transfer functions have to be generated and stored for subsequent use in the fatigue life calculation and these files can be very large. By treating the fatigue life calculation in this way, as a post processing task, the analysis of large models becomes difficult. Secondly, the processing of random stresses within an FE model is problematic when fatigue life is the required result. And thirdly, no practical way exists to simultaneously combine both random and deterministic loads. This is widely required by the test environment standards like MIL-HDBK-810G. This paper will presents a review of existing methods, particularly where a design approach has been implemented within an FE environment. This is then followed by details of a new approach, which includes novel procedures for more accurately processing the random stresses, more efficient ways of dealing with large models and new methods for combining deterministic and random loads. What is Fatigue? Fatigue is normally considered to be the, Failure under a repeated or otherwise varying load which never reaches a level sufficient to cause failure in a single application. We can think of this as a deformation based approach since it is deformations (stresses or strains) that form the primary input. We are not considering crack propagation which is, The growth of an existing crack, or growth from a pre-existing material defect, until it reaches a critical size. Damage tolerance procedures which rely on crack growth techniques are a very important durability requirement but will not be considered in this paper. The primary factor that affects (deformation based) fatigue is stress range as shown in Figure 1. The bigger the range the faster the failure and fatigue life reduces exponentially with range. 13th International ASTM/ESIS Symposium on Fatigue and Fracture Mechanics (39th National Symposium on Fatigue and Fracture Mechanics), November 13 – 15, 2013, Jacksonville, FL Copyright 2013 by CAEfatigue Limited. Published by ASTM International, West Conshohocken, PA, with permission