Papers by Keyword: Low Cycle Fatigue

Abstract: This paper investigates the fatigue results in low cycle fatigue region obtained from a miniaturized specimen having a 6mm gage length, 3mm diameter and 55mm total length. Fatigue tests were performed for two type lead-free solders using horizontal-type electrical servo hydraulic push-pull fatigue testing machine. Materials employed were Sn-3.0Ag-0.5Cu and Sn-5Sb. The results from Sn-3.0Ag-0.5Cu were compared with those obtained using a bulk specimen in a previous study. Relationship between strain range and number of cycles to failure of the small-sized specimen agreed with those of the bulk specimens. The testing techniques are applicable to Sn-5Sb following the Manson-Coffin law. These results confirm that the testing technique proposed here, using small-sized specimen, is suitable to get fruitful fatigue data for lead-free solder compounds.

Abstract: In the previous papers, the author discussed the role of r-value (anisotropic parameter in plasticity) on the surface roughening after plastic deformation. In the present paper, discussion is made on the effects of mutual constraint of deformation between neighboring grains at the grain boundary on the surface roughening. A characteristic parameter called constraint ratio is introduced to express the degree of the mutual restriction between grains. The well-known Coffin-Manson relation in low cycle fatigue is deduced considering to the surface roughening after cyclic deformation.

Abstract: This paper presents grain size effect on low cycle fatigue behavior of high strength maraging steel with gain size of 20,60 and 100μm and Ti-6Al-4V alloy with grain size of 0.5,1.4 and 5.1μm. Low cycle fatigue strength of the maraging steel depends on grain size in number of cycles up to 10³.The smaller the grain size, the higher the low cycle fatigue strength was. Quasci-cleavage fracture surfaces were predominant for material with grain size of 20μm,while intergranular fracture surfaces were predominant for materials with larger grain size in number of cycles lower than 60. Striation was predominant for all tested materials in number of cycles higher than 60.Low cycle fatigue strength of Ti-6Al-4V alloy also depends on grain size in number of cycles up to 10⁴. Grain size dependent transgranular fracture surfaces were predominant for materials with ultra-fine grain size of 0.5μm and fine grain size of 1.4μm.

Abstract: The influence of cyclic loading on microstructure and hardness of a 10%Cr steel with 3%Co and 0.008%B was examined at room temperature and total strain amplitudes of ±0.25% and ±0.6%. Low cycle fatigue (LCF) curves exhibit a stress peak after a few cycles. Hardening is attributed to an increase in dislocation density; no changes in lath size were observed. Then stress tends to decrease monotonically with number of cycles that is indicative for material softening. At ε_ac =±0.25%, strain softening is attributed to decreasing dislocation density and lath coarsening under LCF, whereas at ε_ac =±0.6%, the knitting reaction between dislocations comprising lath boundaries and trapped lattice dislocation leading to the transformation of lath boundaries to subboundaries is a reason for hardness decrease and strain-induced subgrain coarsening.

Abstract: Abrasive wear and low cycle fatigue (LCF) have the greatest influence on the durability of dies. This paper presents a new criterion for estimation of the LCF of forging dies. The deformation model of Manson-Coffin is the classic model for the calculation of LCF. The pulsating cycle of loading dies and the phenomenon of cyclic thermal softening doesn’t provide implementation of the full Manson-Coffin’s model for the analysis of the tool life. At the same time the majority of researchers do not take into account the plastic strain component for estimation the fatigue durability of the dies and use only elastic component of the Manson-Coffin’s law or model of Basquin that is based on the analysis of the stress cycle. The present work uses the strain-kinetic criterion to analyze the durability of the dies that allows taking into account the elastic-plastic strain components and thermos-cyclic softening. During loading in bulk forging the first cycle is to be with plastic deformation while all remaining cycles would have to remain within the elastic limit due to metal hardening. Moreover it has been shown that cyclic softening effect may also be observed in the thermomechanical fatigue. This approach has been implemented for LCF simulation of the die in hot forging in FE program QForm by introducing a special subroutine. The comparison of results of the die fatigue failure simulation has shown good correspondence with practice.

Abstract: This paper presents a numerical method using FE damage analysis to simulate ductile tearing in CT specimen under low cycle fatigue loading conditions. To define a cyclic material properties, the nonlinear kinematic hardening model is adopted. The damage model is defined based on the ductility exhaustion concept using the multi-axial fracture strain energy concept. The proposed model is then applied to simulate two cyclic fracture toughness tests with different R-ratios. Simulated results show overall good agreement with experimental results.

Abstract: Aluminium alloys are widely used in the fields of automobile, machinery and naval construction. To investigate the effect of non-proportional loadings and corrosive environment on the fatigue resistance of 6061-T6 aluminum alloy, a set of uniaxial and multiaxial low cycle fatigue tests were carried out. Firstly, the results of uniaxial tests showed that the alloy exhibited cyclic hardening then cyclic softening. With the increase of stress amplitude the cyclic softening became pronounced. The increasing of plastic deformation was basically cyclically stable with small plastic strain amplitude accumulation when the stress amplitude was lower than 200MPa ,while it was increasing rapidly when the stress amplitude was higher than 220MPa. Secondly, it was observed that non-proportional cycle additional hardening of 6061-T6 aluminum alloy was little. While the fatigue life was badly affected by the loading paths. Thirdly ,the fatigue corrosion interactions were also talked about in details by performing the tests under the same loading conditions with corrosive environment. The experiment proved that the seawater corrosion has huge impact on fatigue life under pH 3. Finally, a multi-axial fatigue life prediction model was used to predict the fatigue life with or without the corrosive environment which showed a good agreement with experimental data.

Abstract: At room temperature, the low cycle fatigue tests for smooth specimens of TC25 titanium alloy under various stress ranges are operated at a CSS280I-20w Electro Hydraulic Servo Universal Testing Machine with a microscopic observation system, and the low cycle fatigue lifetimes are measured. Based upon the analysis of stress-strain hysteresis loop of low cycle fatigue of TC25 titanium alloy, a simplified Manson-Coffin formula is derived according to both the experimental characteristics and the stress-strain constitutive model, the fatigue lifetimes are plotted against stress ranges, and a stress-fatigue life curve for TC25 titanium alloy is obtained by the linear regression analysis method. Finally, the fracture surface morphologies of TC25 specimens are investigated using a JSM-6360 Scanning Electron Microscopy, and the fatigue fracture mechanisms of low cycle fatigue are studied. It shows that the plastic deformations are mainly formed at the accelerated fracture stage, and various shear lips can be observed on the fracture surfaces, which demonstrates that the shear stress results in the final rupture of TC25 titanium alloy. During the fracture of low cycle fatigue, the cleavage nucleation leads to the formation of fatigue crack initiation region, the fatigue crack growth exhibits a mixed transgranular and intergranular crack growth mode, and in the final rupture region, the fracture surface of low cycle fatigue of TC25 titanium alloy appears as a typical semi-brittle fracture mode.

Abstract: This paper presents results of strain energy ΔW_pl cumulation tests on cast steel specimens under constant-amplitude isothermal stress conditions and programed block stress conditions at three different temperatures (20, 400, 600°C). Programed loads (blocks) were composed of five stages of different sequences which included the same strain as during the constant-amplitude isothermal tests. Through the constant-amplitude tests the author demonstrated effect of temperature and the strain level on the fatigue life, strain energy value ΔW_pl and accumulated energy ΣΔW_pl. While no influence of the load program form on the value of the energy accumulated in the specimen was found.

Abstract: In this paper results of P91 cast steel after static and fatigue tests were presented. During the tests longitudinal and transverse strains of the specimen were measured. Basing on the results a comparative analysis of nominal stresses σ_n, true stresses σ_rz, nominal strain energy ΔW_{pl (n)} and true strain energy ΔW_{pl (n)} was carried out. It was stated that differences between determined paramters rise with increasing strain.