Multiaxial Fatigue Behaviour of 1050 H14 Aluminium Alloy by a Biaxial Cruciform Specimen Testing Method R.A. Cláudio 1,2 , M. Freitas 2 , L. Reis 2 , B. Li 2 , I. Guelho 2 , 1 ESTSetúbal, Instituto Politécnico de Setúbal, Campus do IPS, Estefanilha, 2910-761 Setúbal, Portugal. ricardo.claudio@estsetubal.ips.pt 2 ICEMS, Instituto Superior Técnico, UTL, Av Rovisco Pais, 1049-001 Lisboa, Portugal ABSTRACT. In this paper the mechanical behaviour of 1050 H14 aluminium alloy is investigated under in-plane biaxial fatigue. Both experimental and theoretical methods are applied to validate multiaxial fatigue criteria. In order to cover a large range of multiaxial stress states, a new biaxial testing machine was developed and applied in the fatigue tests with a specially designed cruciform specimen. Different multiaxial fatigue criteria, including those based on octahedral shear stress amplitude combined with hydrostatic pressure, and more recent models based on the critical plane approach are evaluated. A modified Minimum Circumscribed Ellipse (MCE) approach is proposed that offers a possible compromise and seems to significantly improve the assessments. INTRODUCTION Machine components and structures in service are generally subjected to multiaxial fatigue loading conditions. Fatigue life evaluation of mechanical components under complex loading conditions is of great importance in order to optimise structural design, and improve inspection and maintenance procedures. However fatigue experiments are much more easily performed under uniaxial loading and constant amplitude but most practical problems associated with metal fatigue in structural elements and machine components are associated with multi-axial loading. For example, rotor shafts in electric power plants, propeller shafts in ships, and so on. The most common multiaxial fatigue specimens and testing fixture are therefore associated with bending-torsion or tension- torsion testing machines and in-phase and out-of-phase fatigue tests are available in literature for a wide range of materials and loading paths, [1]. Less attention has been paid to fatigue tests performed under biaxial loading such those present on pressure vessels or pressurized aircraft cabins. These examples cover different circumstances of cyclic nature of loading and also variations in biaxiality including in-phase versus out-of-phase, different ratios of biaxiality, etc. The cost and availability of biaxial fatigue testing machines that can perform biaxial loading for example in cruciform specimens is certainly the cause. While considerable advances have been made in analytical modelling of stress response of materials under biaxial