A New Method Of Small Sample Test For Obtaining Low Cycle Fatigue Property Of Thin Sheet Materials

The low cycle fatigue(LCF)properties of metal materials are the key index for industrial design and safety assessment.For key engineering like aerospace,nuclear power,high-speed railway and micro-machineries key engineering etc.,a large number of structures and components work under the environment of alternating temperature or load.Therefore,the LCF properties of those materials are significant for the safety and failure prediction of in-service equipment.For a long time,large round bar specimens are recommended by domestic and foreign standards in order to obtain the LCF properties of ductile materials through symmetrical cyclic tests.With the gradual development of industrial designs and innovations of new materials,small components have been widely used in engineering.However,the traditional methods are difficult to determine the LCF properties of small components because of the limits of specimen dimensions.Based on energy equivalent method,a new method is put forward to obtain the LCF properties of thin sheet materials.The main works are introduced as follows:(1)For the funnel-shaped and ring-shaped specimens of thin sheet materials,the semi-analytical equations among the constitutive parameters of material,geometric dimensions,load and displacement are established based on the energy equivalent method.The different geometric dimensions and material parameters of specimens are carried out by numerical simulations.The accuracy and universality of the equation are examined from forward and reverse directions.(2)The design,slicing,polishing and symmetrical cyclic loading experiments of funnel-shaped and ring-shaped specimens are completed.And the corresponding cyclic stress-strain curves of GH4169,G115 and 16 Mn are obtained by the inverse prediction semi-analytical equation on the basis of the cyclic stable load-displacement hysteretic loops.The results indicate that these predicted cyclic stress-strain curves are in agreement with the experimental curves determined from traditional round bar specimens,which verify the applicability of the equations.(3)The transformation relationships between local stress and nominal stress,local strain and nominal strain are established by finite element analyses according to the cyclic stress-strain relationships determined by semi-analytical equations.Further,the fatigue life parameters of Manson-Coffin model can be predicted based on the local stress and strain.The LCF life curves of thin-slice-specimens for G115 and 16 Mn are compared with those obtained by round bar specimens.It can be observed that they agree well with each other.The validity of the new method for obtaining the LCF properties of ductile materials by thin-slice-specimens has been verified.