Theoretical Modeling And Application Of Low Cycle Fatigue And Fatigue Crack Growth For Sheet Miniature Specimen

Local permanent damage caused by monotonic or cyclic load may lead to failure for structural components in aerospace,petrochemical,nuclear power and other engineering fields,and the mechanical properties of materials under monotonic and cyclic load conditions are the basis of service safety evaluation and anti-fatigue design for structures.The low cycle fatigue(LCF)test of materials can reveal the damage evolution law or the relationship between strain amplitude,stress amplitude and fatigue life of materials under cyclic strain load,which has great significance to the prediction of fatigue crack growth(FCG)law of cracked components.Due to the limitation of material size and sampling for service structure material,it is vital to investigate the testing method to obtain LCF and FCG properties of materials by millimetersized and centimeter-sized specimen.Based on the energy density equivalence and dimensional analysis,theoretical and experimental studies on LCF and FCG testing methods of sheet miniature specimens were carried out in this paper.The main works completed are as follows:(1)A semi-analytical model for the elastic-plastic displacement-load relationship of a sheet funnel specimen is proposed,and the analytical equations for the equivalent stress and strain of the representative volume element(RVE)at the median energy density point were derived.Verifications for results were completed using 40 materials with imaginary finite element analysis(FEA)constitutive relationship and 6 geometrical size of sheet funnel specimens,and the results showed that the semi-analytical model and analytic equation are generally applicable with good accuracy.(2)The monotonic tensile tests for two materials and one geometry and symmetric strain cyclic tests with various strain amplitude for five materials and one geometry were completed for standard round bar specimens and sheet funnel specimens respectively.And the results showed that monotonic and cyclic stress-strain relationships of materials predicted by the analytical model with sheet funnel specimens agree well with those tested by standard method with round bar specimens.(3)A novel semi-analytical model describing the relationship between stress,strain,geometry and lateral relative displacement across the gap in the funnel root for the RVE of material was proposed,and then a novel LCF test method with the sheet funnel specimen was developed universally applicable to various materials and geometries.LCF tests were completed for two materials and one geometry with sheet funnel specimens and standard round bar specimens respectively,and the results showed that the Manson-Coffin law(M-C law)obtained by the novel method and the traditional test method with standard round bar specimens are in good agreement.(4)The theoretical model for calculating the stress intensity factor K(K-factor)and compliance C of mode-I cracked sheet specimens was proposed,and then the FCG test method for mode-I cracked sheet specimens was developed.The FCG life prediction model for modeI cracked sheet specimens was proposed based on the M-C law of LCF and K-factor.The finite element numerical simulation method for FCG of materials was proposed based on the M-C law obtained from LCF test as the property of material in FEA conditions.For the FCG of mode-I cracked sheet specimens for 16 Mn material,the results predicted by the developed numerical simulation method and predicted by the theoretical model in this paper and the experimental results are in good agreement.