Fatigue Properties Of Specimens Research Articles

The Effect of Postprocessing on the Fatigue Properties of Ti-5Al-5Mo-5V-1Cr-1Fe Produced Using Electron Beam Melting.

Despite the significant potential advantages of processing Ti-5Al-5Mo-5V-1Cr-1Fe alloy (Ti-55511) using Electron Beam Melting (PBF-EB/M), when compared to conventional manufacturing technologies, the resulting internal defects are an important characteristic of such additive technologies and can highly decrease mechanical properties. One of the most dangerous defects formed during metal additive manufacturing processes are material discontinuities such as a lack of fusion. Defects of this type, due to their "flat" nature, are difficult to characterize. For cycle-loaded specimens, where the loading force acts perpendicular to the lack-of-fusion plane, defects of this type can significantly reduce fatigue properties. This paper presents the results of research aimed at improving the fatigue properties of Ti55511 alloy by reducing the influence of the lack-of-fusion defect on fatigue damage. The static and fatigue properties of specimens in the as-built state, as well as after hot isostatic pressing (HIP) treatment, were analyzed. The effect of HIP on both the reduction of pores and the degree of sphericity when using the X-ray computed tomography (XCT) system was presented. The change in the microstructure after HIP was analyzed in terms of the change in the size of individual phases, as well as the change in the phase ratio. This paper also contains a fractographic analysis of the samples after tensile and fatigue tests.

The anisotropic fatigue properties of extruded Al–Zn–Mg–Cu alloy

The effect of specimen orientation on the fatigue properties of extruded Al–Zn–Mg–Cu alloy was investigated, and the relationship between tensile properties and fatigue strength was established. The results show that the fatigue properties of specimens vary largely with the orientation. The specimens with 0° orientation (relative to extrusion direction) have the highest fatigue strength, i.e. 301.3 MPa, whereas the fatigue strength of specimens with 45°and 90° orientation was reduced by 13.3% and 33.5%, respectively. The fatigue strength is linearly proportional to the elongation. It was found that the crack propagation modes vary with specimen orientation, a result of the grain orientation and intergranular coarse second phase particles, leading to the difference in fatigue lives. In addition, Basquin equations for specimens in the three directions were proposed for fatigue life prediction.

Development of an H-shaped shear key for mutually perpendicular precast shear walls

Prefabricated reinforced concrete (PC) shear walls are widely used because of their convenience of construction. Diverse forms of wall joints for assembling precast shear walls were studied in the recent decade. However, during the assembly of mutually perpendicular precast shear walls, it is difficult to satisfy joint strength requirements. In this study, an H-shaped Shear Key (HSK) with welding connections was proposed for the connection of mutually perpendicular precast shear walls. Quasi-static cyclic loadings were conducted to test the seismic properties of HSK specimens with different welding configurations. The strength, energy dissipation capacity and fatigue properties of specimens were compared. Image measurement was adopted to measure the damage patterns and crack dimensions. The test results validated the effectiveness of the HSK specimens and appropriate configurations of HSKs are recommended for practical engineering applications.

Influence of artificial defects on fatigue strength of induction hardened S38C axles

Impact damage are critical for damage tolerance assessment of high-speed train axles. Flying ballast impact damage on railway axles produce geometry discontinuities, which become preferential sites for fatigue crack initiation and usually lead to premature failure. According to this, the formation of surface impact damage and its influence on structural integrity of induction hardened S38C axles are herein explored. In particular, the morphology of impact damage on induction hardened S38C axles were analyzed as well as fatigue properties of specimens with artificial defects, electronic discharge machine (EDM) defects and impact defects fabricated by shooting metallic balls to the surface with compressed-gas gun. Results show that surface impact damage of the axles include two types, namely scratches and notches. Fatigue strength of the specimens with EDM defects are larger than the Murakami’s model predictions due to existence of compressive residual stresses. The influence of shallower impact damage (less than 200 μm) on the fatigue strength is negligible. With a larger depth, the rim is extracted out of the original surface and fatigue crack starts from this point with a decline of fatigue strength. Microcracks and loss of materials appear at the rim of impact damage, and microcracks along adiabatic shear bands lies beneath the surface when the impact depth is about 300 μm resulting in 50% reduction of fatigue strength. Fatigue strength of impacted specimens are larger than that of EDM specimens by 200 MPa with the equivalent area due to the local residual stresses.

Effects of cold working and corrosion on fatigue properties and fracture behaviors of precipitate strengthened Cu-Ni-Si alloy

In this study, the tensile and fatigue tests were conducted on precipitate strengthened Cu-Ni-Si alloy. The fatigue properties of specimens with cold working and without cold working in air, as well as that of the specimen with cold working in salt atmosphere were investigated. The results show that the monotonic tensile strength and the yield strength are improved obviously, while the elongation decreases by cold working. The S-N curves of all the types of specimens show a continuous decrease. The fatigue strength of specimens in air is slightly reduced by cold working, however, that of the specimen with cold working at 107 cycles is largely decreased from 190 MPa to 120 MPa in salt atmosphere. All the fatigue cracks initiate from the specimen surface by slip deformation. In air condition, the specimens with cold working show shear mode fracture with transgranular crack propagation and it is caused by the obvious texture, while that without cold working show normal mode fracture with both transgranular and intergranular crack propagation. However, in salt atmosphere condition, the specimens with cold working show normal mode fracture with totally intergranular crack propagation.

Electrochemical-controlled Characterization of the Corrosion Fatigue Behavior of Creep-resistant Magnesium Alloys DieMag422 and AE42

Magnesium alloys offer a high potential for lightweight construction, however their application range is limited due to their low corrosion resistance. In the present study the corrosion fatigue behaviors of the creep-resistant alloys DieMag422 and AE42 were characterized and compared. In this context, fatigue properties of specimens in sodium chloride solutions as well as under simultaneous galvanostatic anodic polarization were assessed in constant amplitude tests. The results were correlated with the corrosion behavior of the alloys, which was investigated in instrumented immersion tests. Corrosion- and deformation-induced changes in microstructure were observed by light and scanning electron microscopy, yielding a structure-property relationship for a comprehensive understanding of corrosion fatigue processes. The reduction of the corrosion fatigue strength with increasing corrosion impact could be quantitatively correlated with the adjusted corrosion rates. However, different corrosion morphologies between both materials were found, leading to varying influence on the corrosion fatigue behavior.

Degradation and damage behaviors of steel frame welded connections

In order to study the degradation and damage behaviors of steel frame welded connections, two series of tests in references with different connection constructions were carried out subjected to various cyclic loading patterns. Hysteretic curves, degradation and damage behaviours and fatigue properties of specimens were firstly studied. Typical failure modes and probable damage reasons were discussed. Then, various damage index models with variables of dissipative energy, cumulative displacement and combined energy and displacement were summarized and applied for all experimental specimens. The damage developing curves of ten damage index models for each connection were obtained. Finally, the predicted and evaluated capacities of damage index models were compared in order to describe the degraded performance and failure modes. The characteristics of each damage index model were discussed in depth, and then their distributive laws were summarized. The tests and analysis results showed that the loading histories significantly affected the distributive shapes of damage index models. Different models had their own ranges of application. The selected parameters of damage index models had great effect on the developing trends of damage curves. The model with only displacement variable was recommended because of a more simple form and no integral calculation, which was easier to be formulated and embedded in application programs.

Fatigue properties of notched specimens made of FeP04 steel

Fatigue properties of specimens with different notches made of FeP04 steel are presented. The specimens are characterized by double symmetric lateral notches with radius of the notch root ranging from ρ = 0.2 to 10 mm. An MTS 809 servohydraulic device was used for the tests. All fatigue tests were performed under force control by imposing a constant value of the nominal load ratio (R = 0) and a load amplitude P a = 6 kN for the notch root ρ = 0.2 mm and 7 kN for the notch roots ρ = 1.25, 2.5 and 10 mm. The test frequency varied from 13 to 15 Hz. During the tests performed under constant loading, we observe the fatigue weakening of the material and an increase in strain.

237 環状切欠き材の疲労強度向上(OS 軽量化・高強度化と材料代替・材料進化)

More than 90% of failures of machines and mechanical parts are caused by fatigue. In addition, approximately 90% of the fatigue cracks initiate at a stress concentration part of structural component. Therefore, it is important to improve the fatigue limit of a notched specimen. In this study, the fatigue properties of specimens with circumferential notch have been investigated using Ono-type rotating bending fatigue test machine. The main results obtained in this study are as follows: (1) The fatigue strength of a notched-specimen considerably increases by roller-working. (2) The improvement of fatigue limit by roller-working would be attributed to work-hardening and compressive residual stress. (3) Furthermore, the non-propagating cracks are observed in the specimen subjected to the stress amplitude of fatigue limit by 1 × 10^7 cycles.