A Comparison of Amplitude-and Time-Dependent Cyclic Deformation Behavior for Fully-Austenite Stainless Steel 316L and Duplex Stainless Steel 2205

Austenite and duplex stainless steels are widely used in engineering, and the latter exhibits a more excellent combination of mechanical properties and corrosion resistance due to the coexistence of austenite and ferrite and higher nitrogen. However, fatigue failure still threatens their structural integrity. A comprehensive comparison of their cyclic deformation behavior is a major foundation to understand the role of duplex-phase microstructure and nitrogen in the safety assessment of engineering components. Thus, in this paper, the cyclic deformation behavior of fully-austenitic stainless steel 316L and duplex stainless steel 2205 was studied by a series of low cycle fatigue tests with various strain amplitudes, loading rates and tensile holding. A theoretical mechanism diagram of the interaction between nitrogen and dislocation movements during cyclic loads was proposed. Results show that the cyclic stress response of 2205 was the primary cyclic hardening, followed by a long-term cyclic softening regardless of strain amplitudes and rates, while an additional secondary hardening was observed for 316L at greater strain amplitudes. Cyclic softening of 2205 was restrained under slower strain rates or tensile holding due to the interaction between nitrogen and dislocations. The cyclic plasticity of 2205 started within the austenite, and gradually translated into the ferrite with the elevation of the cyclic amplitude, which lead to a decreased hardening ratio with the increase in amplitude and a shorter fatigue life for a given smaller plastic strain amplitude.

Cyclic Deformation of Semi-solid Processed Magnesium Alloys

To improve fuel economy and reduce greenhouse gas emissions, magnesium alloys are being considered for automotive and aerospace applications because of their high strength-to-weight ratio. The objective of this thesis was to study monotonic and cyclic deformation behavior of two semi-solid processed (thixomolded) magnesium alloys, AZ91D and AM60B. The fatigue life of these thixomolded alloys was observed to be higher than that of their die cast counterparts. As the total strain amplitude increased, the stress amplitude and plastic strain amplitude increased, while the pseudoelastic modulus decreased. The change in the modulus was attributed to the nonlinear (pseudoelastic) behavior caused by twinning-detwinning during cyclic deformation. The fatigue life increased with decreasing strain ratio, and partial mean stress relaxation occurred mainly in the initial 10-20% of the fatigue life. The fatigue life of theAM60B alloy improved after solution or solution-aging treatment, and the monotonic strength increased by aging, while the thixomolded condition itself exhibited moderate monotonic strength and fatigue life.

Cyclic Deformation of Semi-solid Processed Magnesium Alloys

To improve fuel economy and reduce greenhouse gas emissions, magnesium alloys are being considered for automotive and aerospace applications because of their high strength-to-weight ratio. The objective of this thesis was to study monotonic and cyclic deformation behavior of two semi-solid processed (thixomolded) magnesium alloys, AZ91D and AM60B. The fatigue life of these thixomolded alloys was observed to be higher than that of their die cast counterparts. As the total strain amplitude increased, the stress amplitude and plastic strain amplitude increased, while the pseudoelastic modulus decreased. The change in the modulus was attributed to the nonlinear (pseudoelastic) behavior caused by twinning-detwinning during cyclic deformation. The fatigue life increased with decreasing strain ratio, and partial mean stress relaxation occurred mainly in the initial 10-20% of the fatigue life. The fatigue life of theAM60B alloy improved after solution or solution-aging treatment, and the monotonic strength increased by aging, while the thixomolded condition itself exhibited moderate monotonic strength and fatigue life.

A New Model for Predicting Low Cycle Fatigue Behavior of Discontinuously Reinforced Metal Matrix Composites

An analytical model for predicitng the crack inititation life of low cycle fatique (LCF) of discontinuously reinforced metal matrix composites (DR-MMCs) has been proposed. The effects of the volume fraction Vf cyclic strain hardening exponent n' and cyclic strength coefficient K' on the LCF crack initiation life of DR-MMCs were analyzed. While both the lower level of the plastic strain amplitude and the lower Vf were found to increase the LCF crack initiation resistance, the effects of n' and K' were more complicated. By considering the enhanced dislocation density in the matrix and the load bearing effect of particles, a quantitative relationship between the LCF life of DR-MMCs and particle size was also derived. This model showed that a decreasing particle size results in a longer LCF life. The theoretical predictions based on the proposed models were found to be in good agreement with the experimental data reported in the literature.

A New Model for Predicting Low Cycle Fatigue Behavior of Discontinuously Reinforced Metal Matrix Composites

An analytical model for predicitng the crack inititation life of low cycle fatique (LCF) of discontinuously reinforced metal matrix composites (DR-MMCs) has been proposed. The effects of the volume fraction Vf cyclic strain hardening exponent n' and cyclic strength coefficient K' on the LCF crack initiation life of DR-MMCs were analyzed. While both the lower level of the plastic strain amplitude and the lower Vf were found to increase the LCF crack initiation resistance, the effects of n' and K' were more complicated. By considering the enhanced dislocation density in the matrix and the load bearing effect of particles, a quantitative relationship between the LCF life of DR-MMCs and particle size was also derived. This model showed that a decreasing particle size results in a longer LCF life. The theoretical predictions based on the proposed models were found to be in good agreement with the experimental data reported in the literature.

Fatigue Modeling and Numerical Analysis of Re-Filling Probe Hole of Friction Stir Spot Welded Joints in Aluminum Alloys

In the present study, the fatigue behavior and tensile strength of A6061-T4 aluminum alloy, joined by friction stir spot welding (FSSW), are numerically investigated. The 3D finite element model (FEM) is used to analyze the FSSW joint by means of Abaqus software. The tensile strength is determined for FSSW joints with both a probe hole and a refilled probe hole. In order to calculate the fatigue life of FSSW joints, the hysteresis loop is first determined, and then the plastic strain amplitude is calculated. Finally, by using the Coffin-Manson equation, fatigue life is predicted. The results were verified against available experimental data from other literature, and a good agreement was observed between the FEM results and experimental data. The results showed that the joint’s tensile strength without a probe hole (refilled hole) is higher than the joint with a probe hole. Therefore, re-filling the probe hole is an effective method for structures jointed by FSSW subjected to a static load. The fatigue strength of the joint with a re-filled probe hole was nearly the same as the structure with a probe hole at low applied loads. Additionally, at a high applied load, the fatigue strength of joints with a refilled probe hole was slightly lower than the joint with a probe hole.

Fatigue Life Improvement of Cracked Aluminum 6061-T6 Plates Repaired by Composite Patches

Bonded patches are widely used in several industry sectors for repairing damaged plates, cracks in metallic structures, and reinforcement of damaged structures. Composite patches have optimal properties such as high strength-to-weight ratio, easiness in being applied, and high flexibility. Due to recent rapid growth in the aerospace industry, analyses of adhesively bonded patches applicable to repairing cracked structures have become of great significance. In the present study, the fatigue behavior of the aluminum alloy, repaired by a double-sided glass/epoxy composite patch, is studied numerically. More specifically, the effect of applying a double-sided composite patch on the fatigue life improvement of a damaged aluminum 6061-T6 is analyzed. 3D finite element numerical modeling is performed to analyze the fatigue performance of both repaired and unrepaired aluminum plates using the Abaqus package. To determine the fatigue life of the aluminum 6061-T6 plate, first, the hysteresis loop is determined, and afterward, the plastic strain amplitude is calculated. Finally, by using the Coffin-Manson equation, fatigue life is predicted and validated against the available experimental data from the literature. Results reveal that composite patches increase the fatigue life of cracked structures significantly, ranging from 55% to 100% for different applied stresses.

A New Method for Determining the Brittle-to-Ductile Transition Temperature of a TiAl Intermetallic

Intermetallic materials typically change their deformation behavior from brittle to ductile at a certain temperature called the Brittle-to-Ductile Transition Temperature (BDTT). This specific temperature can be determined by the Charpy impact, tensile or bending tests conducted at different temperatures and strain rates, which usually requires a large number of specimens. In order to reduce the number of necessary specimens for finding the BDTT, a new methodology comprising cyclic loadings as the crucial step was studied on a fully lamellar TiAl alloy with composition Ti-48Al-2Nb-0.7Cr-0.3Si. The loading blocks are applied isothermally under strain control and repeated on the same specimen at different temperatures. The development of plastic strain amplitude with increasing temperature is analyzed to determine the BDTT of the specimen. The BDTTs found with the described method agree well with literature data derived with conventional methods. With the loading strategy presented in this study, the BDTT and additionally the effect of strain rate on it can be found by using a single specimen.

Influence of Temperature on Low Cycle Fatigue and Ratcheting Behavior of SA333 Gr-6 C-Mn Steel

The aim of this investigation is to study the influence of temperature on the cyclic plastic deformation behavior of SA333 Gr-6 steel at two loading conditions. Strain-controlled cyclic loading experiments were carried out at ± 0.5% total strain amplitude, 1×10-3 s-1 strain rate, and temperature varied from RT to 400°C, whereas stress controlled ratcheting experiments were conducted at fixed mean stress (σm) of 50 MPa and stress amplitude (σa) of 400 MPa, 115 MPa s-1 stress rate, and in the temperature range of RT to 350°C. The investigated steel shows cyclic hardening characteristic at DSA temperature regime in both the loading condition. The steel shows lower fatigue lives at 250°C and 300°C temperatures even though plastic strain amplitude is smaller. The ratcheting life of the steel increases and strain accumulation decreases with the increase in temperature up to 300°C and on further increment in temperature ratcheting life get decreased. The steel shows greater cyclic hardening at both the loading conditions at 300°C.

Low-cycle fatigue behavior of extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy at room and low temperatures

Due to the low density and high specific strength, aluminum alloys have been considered for automotive and aerospace applications. The aluminum components usually service in the conditions of low temperature and dynamic loading. Therefore, the research on the low temperature fatigue performances of Al alloys has great significance. The lowcycle fatigue tests for the extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy subjected to solution plus aging treatment have been conducted at 25°C and -40°C, respectively. The strain ratio and cyclic frequency applied in the low-cycle fatigue test were -1 and 0.5Hz, respectively. The experimental results show that at 25°C, the alloy exhibits the cyclic hardening at the total strain amplitudes of 1.0% and 1.2%, and the cyclic stabilization at the total strain amplitudes of 0.4%, 0.6% and 0.8%. At -40°C, however, the cyclic stability is observed during whole fatigue deformation at the total strain amplitudes of 0.4%, 0.5%, 0.6%, 0.7% and 0.8%. The relationship between the elastic strain amplitude, plastic strain amplitude and reversals to failure can be described by Basquin and Coffin-Manson equations, respectively. In addition, the observation results of fatigue fracture surfaces reveal that the cracks initiate at the free surface of fatigue specimen and propagate in a transgranular mode.