1 Experimental and numerical investigations on softening behavior of POM under cyclic strain-controlled loading M. Shariati*, H. R. Epakchi, H. Hatami, H. Yarahmadi, H. Torabi Mechanical Department, Shahrood University of Technology, Shahrood, Iran Abstract In this paper, cyclic stress-strain behavior of POM was studied. Fatigue tests were carried out under symmetric uniaxial strain-controlled mode. In this case of loading conditions the material exhibited cyclic softening and with the increase of strain amplitude the total number of cycles to failure reduced dramatically. Additional fatigue tests were further performed under non-symmetrical uniaxial cyclic loading where specimens exhibited relaxation behavior and mean stress reached a zero value in continuous cycles. Experimental tests were performed using an INSTRON 8802 servo- hydraulic machine and the results obtained from the experiments were compared with numerical results. The numerical analysis was carried out by ABAQUS software using the following advanced plasticity models based on Von Mises criterion: isotropic hardening, linear kinematic hardening, and non-linear isotropic/kinematic hardening. Keywords: Polyoxymethylene (POM); Strain-controlled; cyclic loading; hardening model; FEM 1. Introduction Polyoxymethylene (POM) is one of the major engineering thermoplastics that have very good mechanical properties, such as tensile strength, flexural modulus, high creep, and good fatigue endurance and deflection temperature. The dense crystalline structure accounts for such properties as stiffness, fatigue endurance, durability and high temperature resistance [1]. Polyoxymethylene copolymer is one of the relatively new, high performance engineering polymers. POM also possesses excellent chemical resistance to a wide range of materials, comparing favorably with many thermoplastic polymers. Fine fibered products of POM may find applications in specialty filtration, such as hydrocarbon fuel filtration, and hydraulic fluid filtration. Many POM engineering components often endure compression cyclic loading during their service. The component will be subject to softening under cyclic load. Under the cyclic load, the component may fail because of its unacceptable ratcheting deformation. To establish an accurate constitutive model to evaluate the accumulated deformation of a structure, it is essential to study softening behavior farther for the materials.