Issue 37

N. Zuhair Faruq, Frattura ed Integrità Strutturale, 37 (2016) 382-394; DOI: 10.3221/IGF-ESIS.37.49 382 An elasto-plastic approach to estimate lifetime of notched components under variable amplitude fatigue loading: a preliminary investigation N. Zuhair Faruq University of Sheffield, Department of Civil and Structural Engineering, Sheffield S1 3JD, UK Zfnamiq1@sheffield.ac.uk A BSTRACT . The present paper is concerned with the formulation of an elasto-plastic strain based approach suitable for assessing fatigue strength of notched components subjected to in-service variable amplitude cyclic loading. The hypothesis is formed that the crack initiation plane is closely aligned with the plane of maximum shear strain amplitude, its orientation and the associated stress/strain quantities being determined using the Maximum Variance Method. Fatigue damage is estimated by applying the Modified Manson-Coffin Curve Method (MMCCM) along with the Point Method (PM). In the proposed approach, the required critical distance is treated as a material property whose value is not affected either by the sharpness of the notch being assessed or by the profile of the load spectrum being applied. The detrimental effect of non-zero mean stresses and degree of multiaxiality of the local stress/strain histories is also considered. The accuracy and reliability of the proposed design methodology was checked against several experimental data taken from the literature and generated under different uniaxial variable amplitude load histories. In order to determine the required local stress/strain states, refined elasto-plastic finite element models were solved using commercial software ANSYS®. This preliminary validation exercise allowed us to prove that the proposed approach is capable of estimates laying within an error factor of about 2. These preliminary results are certainly promising, strongly supporting the idea that the proposed design strategy can successfully be used to assess the fatigue lifetime of notched metallic components subjected to in-service multiaxial variable amplitude loading sequences. K EYWORDS . Notched Components; Variable Amplitude; Critical Plane; Manson-Coffin Curve; non-zero mean stress. I NTRODUCTION n situations of practical interest, real engineering components are characterised by complex geometries resulting in local stress/strain concentration phenomena. They normally contain either notches or complex features that favour the initiation of fatigue cracks. The presence of stress/strain raiser results in multiaxial stress/strain states in the critical regions even if the nominal load history being applied is uniaxial. Furthermore, real mechanical components are often exposed to variable amplitude load histories. Accordingly, in the recent past, a tremendous effort has been made by the international scientific community to device specific design techniques suitable for accurately assessing the durability of notched components subjected to in-service variable amplitude load histories [1]. In this complex scenario, this paper summarises the results from a preliminary investigation aiming at developing an elasto-plastic strain based approach capable of predicting fatigue lifetime of notched components subjected to variable amplitude load histories. I