Plastic Strain Amplitude Research Articles

Fatigue life modeling for a low alloy steel after long-term thermal aging

Fatigue failure of materials has a considerable impact on the safety of equipment in service. In this study, axially tensile and low cycle fatigue tests were conducted on a low alloy steel after accelerated thermal aged at 450 °C for 10,000 h. The experimental results indicate that the ultimate and yield strengths increase moderately, while the fatigue life of specimens experience a slight decrease in this circumstance. The fracture analysis demonstrates that the bainite breaking facilitates the fatigue crack initiation and propagation after thermal aging, which is accompanied by a decrease in plastic strain amplitude. Therefore, the plastic strain amplitude is considered as an indicator of thermal aging in fatigue life modeling for the low alloy steel. Finally, a novel life model that incorporates both aging time and temperature was proposed for rapid prediction of low cycle fatigue life. It is assumed that this model promotes reliable fatigue life prediction in low alloy steels under various thermal aging circumstances, as well as the extrapolation of fatigue performance of the material in service.

Cell structure development during cyclic deformation of near-[001] and near-[011] copper single crystals

Dislocation cell structures significantly influence the fatigue failure process. Research on the cell structures in multiple-slip-oriented copper single crystals has focused primarily on the [1‾11] orientation, whereas formation mechanisms and development processes of cell structures in [001] and [011] copper single crystals remain unclear. Therefore, this study examines the dislocation structures of near-[001] and near-[011] copper single crystals under cyclic deformation at high strain amplitudes. The cyclic stress–strain curve of the [011] crystals showed a distinct plateau at plastic shear strain amplitudes ≤7.5 × 10−3. In the plateau region, the dislocation structures consisted of cell structures along the (111) primary slip plane and matrix wall structures. Furthermore, (111) cell structures were formed in the single-slip deformation bands (DBs). After the plateau region, several types of new DBs could be observed due to the activation of multiple-slip systems. Multiple-slip DBs have a higher degree of plastic strain concentration than single-slip DBs, leading to the preferential formation of cell structures. For the near-[001] crystals, (111) cell structures were uniformly formed throughout the matrix, without the formation of DBs. The formation mechanism of the (111) cell boundary was independent of the stress axis and resulted from the interaction between the primary and coplanar slip systems.

Effect of prefatigue-induced persistent slip bands on the strength-ductility match in -oriented Cu single crystals

To explore the effect of persistent slip bands (PSBs) formed during cyclic loading on the static mechanical behavior of materials, [1‾12]-oriented Cu single crystals are selected as target materials, and the tensile behavior of Cu single crystals prefatigued to the stress saturation stage at different plastic shear strain amplitudes (γpl) and the corresponding microstructures, surface deformation features and fracture morphologies are systematically studied. The results show that the formation of few soft PSB ladder-like structures in hard dislocation veins during prefatigue deformation at γpl = 3.7 × 10−4 below the plateau region of cyclic stress-strain (CSS) curve can promote a synchronous improvement of strength and ductility (SISD) compared with the unfatigued crystal. As γpl increases to a lower value of 9.0 × 10−4 within the plateau region, the strength and ductility reduce sharply compared with the case at γpl = 3.7 × 10−4 due to more concentrated plastic deformation caused by a high volume fraction of prefatigue-induced PSBs. With continuously increasing γpl to a higher value of 2.3 × 10−3 within the plateau region, non-uniform dislocation distribution in labyrinth structures and the short spacing between labyrinth walls as well as the fine dislocation cells formed during tension jointly give rise to a slight increase in strength and ductility compared with the crystal prefatigued at γpl = 9.0 × 10−4. As γpl is increased to the level beyond the plateau region, the ductility and ultimate tensile strength continuously increase, resulting from the co-existence of wider deformation bands and dense dislocation walls.

Fatigue and corrosion‐fatigue behavior of the β‐metastable Ti‐5Al‐5Mo‐5V‐3Cr alloy processed by laser powder bed fusion

AbstractWe performed rotating bending tests and axial (tension‐compression) load‐increase and constant amplitude high‐cycle fatigue tests in air and Hanks' balanced salt solution (HBSS) on the β‐metastable titanium alloy Ti‐5Al‐5Mo‐5V‐3Cr, processed by laser powder bed fusion (LPBF‐M), solution‐treated and aged, and shot‐peened. Rotating bending loading in air revealed a strong influence of process‐induced flaws on fatigue endurance. Especially in the high‐cycle fatigue range and the transition region, the stochastic distribution of the flaws and flaw sizes led to a high scatter of the number of cycles to failure. The axial load‐increase tests yielded a good fatigue life estimation, with a negligible difference between air and HBSS. The cyclic deformation behavior in HBSS was also strongly influenced by the local microstructure and defect distribution, and, thus, by crack formation and propagation. Plastic deformation and microcrack growth interact, and their relative amount resulted in different progressions of the plastic strain amplitude over the number of cycles for different specimens. Changes in the free corrosion potential and the corrosion current were highly sensitive indicators for fatigue‐induced damage on the rough surfaces, which was correlated to the microscopic examination, fracture surface features, and the fatigue crack development.

Experimental investigation of thermomechanical fatigue behavior in directionally solidified Ni-based superalloy under in-phase and out-of-phase conditions

The fatigue behavior of a directionally solidified (DS) Ni-based superalloy was investigated under in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) conditions. Specimens were tested under strain control with a temperature range of 450–850 °C. In some tests, an optional 5 min out-of-phase dwell (OPD) time was introduced. In the case of IP TMF, the primary damage manifestation was predominantly intergranular cracks, whereas in the OP tests, the damage was mainly transgranular cracks. The fatigue life of IP TMF was found to be significantly shorter than that of OP TMF. This indicated that the intergranular cracks propagated faster than the transgranular cracks. The dwell time did not influence the dominant damage form; however, it led to an increase in the density of the microcracks. This increase resulted in a reduced lifetime, leading to lower stress amplitudes and higher plastic strain amplitudes compared with the continuous cycle tests. In addition, cycle hardening became apparent during the initial stages of cyclic loading. The microstructures and damage evolution were examined to provide insights into the changes in fatigue life. A model for predicting the fatigue life of DS Ni-based superalloy under given test conditions has been assessed. The proposed fracture mechanics model, which outlines the failure mechanism of directionally solidified Ni-based superalloy under TMF conditions, has proven to be effective in predicting fatigue life.

Impact of plastic strain amplitude on the cyclic behavior of commercially pure titanium grade 2

The present study focuses on investigating the impact of plastic strain amplitude on the mechanical behavior of T40 (Grade 2) under low cycle fatigue (LCF) at room temperature with a strain rate of 2 × 10−3 s−1. The mechanical behavior is analyzed in terms of effective and back stress evolution, and the relationship between the microstructural observations, based on analyses using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The evolution of stress amplitude as a function of cyclic number highlights a competition between hardening and softening processes. Both effects can be related to a specific evolution of internal stresses with dislocation patterns and dislocation/oxygen interactions. Crack initiation and propagation regimes are studied using the measurement of striation spacing and TEM-FIB samples on the crack surface. Crack initiates along (10–12) twins and propagates with a crack growth rate directly proportional to the dislocation inter-wall spacing.

Experimental investigation on the low cycle fatigue performance and fractographic analysis of deep cryogenic treated 6063 aluminium alloy

The influence of soaking time in Deep Cryogenic Treatment (DCT) on the fatigue behaviour of Al 6063 aluminium alloy was investigated. The aluminium rods were subjected to the deep cryogenic treatment for 2, 4, 6, 16, and 24 hrs. The low cycle fatigue behaviour of DCT-treated samples were analyzed using the 25 kN nano fatigue testing machine at the total strain amplitude of 0.4% and cyclic frequency of 0.3 Hz. CT 6 sample retained a higher fatigue life of 2216 cycles than CT 2, 4, 16, and 24 samples which had cycles to failure of about 1392, 1723, 1249, and 364, respectively. Cyclic stabilization was observed through the stress-strain curve, and the mean stress response of DCT samples exhibited a compressive nature during fatigue loading. The plastic strain amplitude response of the CT 6 sample showed a more stabilized response among DCT samples until crack initiation. Fractography reveals the origin and propagation of cracks along with fatigue striations and final fracture of DCT samples. The final fracture of CT 16 and CT 24 samples exhibited microvoid formation due to higher immersion time, whereas CT 2, CT 4, and CT 6 samples had a transgranular fracture. XRD analysis of DCT samples confirmed the formation of Mg2Si, which leads to improved fatigue properties and hardness in DCT samples.

Effect of Shot Peening on the Low-Cycle Fatigue Behavior of an AA2519-T62 Friction-Stir-Welded Butt Joint.

In this investigation, an AA2519-T62 FSW butt joint was subjected to shot peening with an air pressure of p = 0.6 MPa, a processing time of t = 10 min (per side), and a steel ball diameter of dk = 1.5 mm. In order to evaluate the impact of shot peening on the low-cycle behavior, the samples were tested with coefficient R = 0.1 at total strain amplitudes of 0.35%, 0.4%, and 0.5%. The shot-peened welds are characterized by a higher value of stress amplitude, a lower value of plastic strain amplitude, and their fatigue life increased slightly. The cyclic strength coefficient and the cyclic strain hardening exponent were reduced by 45% and 55%, respectively, as the result of the surface layer hardening. The shot peening process had no noticeable effect on the character of crack initiation and propagation. Almost in all cases, the cracking started in the area under the weld face, located close to the boundary between the thermo-mechanically affected zone and the stir zone at the advancing side. Only at the heaviest loadings (εac = 0.5%) were cracks initiated in the heat-affected zone at the retreating side. Despite the introduction of small cracks in the stir zone, their presence did not affect the decohesion character of the welded joint. Overall, it was observed that there is a minimal, positive impact of shot peening on the properties of the investigated joints.

Prediction of thermomechanical fatigue life in RuT450 compacted graphite cast iron cylinder heads using the Neu/Sehitoglu model

The increase in engine power has resulted in a significant rise in both thermal and mechanical loads on cylinder heads, leading to the emergence of thermomechanical fatigue (TMF) as one of the significant issues. This paper proposed a parameter identification method for the Neu/Sehitoglu model, utilizing multi-objective optimization (MOO), and applied it to predict the TMF life of compacted graphite cast iron (CGI) material used in cylinder heads. The study assessed the TMF behavior of RuT450 material through various experiments, including tensile, low cycle fatigue (LCF), creep, TMF, and oxidation experiments. Employing the MOO, the unknown model parameters are estimated and optimized to predict the TMF life of RuT450 materials. Results demonstrated that over 95% of the predicted TMF life falls within twice the error band of experimental values. Furthermore, plastic strain amplitude, temperature, and strain rate significantly influenced the damage mechanism and life prediction. The TMF life under out-of-phase (OP) loading conditions was found to be smaller than that of in-phase (IP) loading. Notably, under out-of-phase thermomechanical fatigue (OP TMF), oxidation damage and fatigue damage together contributed to over 95% of the total damage in RuT450 materials. Moreover, variations in oxidation damage parameters had a considerable impact on the predicted life. This study might offer an effective method for determining Neu/Sehitoglu model parameters and establish material boundaries for TMF life assessment of cylinder heads.

Estimation of Cyclic Stress-Strain Curves of Steels Based on Monotonic Properties Using Artificial Neural Networks.

This paper introduces a novel method for estimating the cyclic stress-strain curves of steels based on their monotonic properties and plastic strain amplitudes, utilizing artificial neural networks (ANNs). ANNs were trained on a substantial number of experimental data for steels, collected from relevant literature, and divided into subgroups according to alloying elements content (unalloyed, low-alloy, and high-alloy steels). Only monotonic properties that were proven to be relevant for the estimation of points on the stress-strain curve were used. The performance of the developed ANNs was assessed using an independent set of data, and the results were compared to experimental values, values obtained by existing empirical estimation methods, and by previously developed ANNs. The results showed that the new approach which combines relevant monotonic properties and plastic strain amplitudes as inputs to ANNs for cyclic stress-strain curve estimation is better than the previously used approach where ANNs estimate the parameters of the Ramberg-Osgood material model separately. This shows that a more favorable approach to the estimation of cyclic stress-strain behavior would be to directly estimate corresponding material curves using monotonic properties. Additionally, this may also reduce inaccuracies resulting from simplified representations of the actual material behavior inherent in the material model.

Low-cycle fatigue behavior of differently matched welded joints of quenched and tempered steel

The appropriate matching of base and filler material is a complex task, where yield strength matching is the most general aspect. As the strength properties of structural steels have significantly improved in the past decades, the matching problem has become more relevant today. The mismatch phenomenon significantly affects the behavior of welded joints under dynamic and cyclic loading. Among cyclic loading, low-cycle fatigue (LCF) often occurs in welded steel constructions; furthermore, the LCF resistance of these advanced steels and their welded joints is limitedly known. In this paper, welding experiments are presented for the analysis of the LCF behavior of differently matched butt-welded joints made from two grades of quenched and tempered (Q + T) high-strength steels. For S690QL steel, matched and overmatched consumables were applied, while for S960QL steel, matched and undermatched filler materials were used. Material tests were performed to determine the mechanical properties of the different welded joints. In both examined steel grades, the welded joints tolerated a smaller number of cycles until failure than the base materials. In the case of S690QL, the LCF resistance of the matched welded joints was higher than the overmatched filler material. At S960QL in the higher strain amplitude range, the number of cycles to failure was higher at the same total and plastic strain amplitudes when undermatched filler material was used; however, an opposite ratio can be observed at lower strain values, compared to the matching filler material.

Modified Coffin-Manson equation to predict the fatigue life of structural materials subjected to mechanical-thermal coupling non-coaxial loading

With regard to the structures subjected to severe mechanical-thermal coupling fatigue conditions, the consistency between service conditions and loading conditions was the prerequisite for the fatigue properties test and fatigue life prediction. A miniature piezoelectric-driven instrument integrating with a tripodal piezoelectric actuator and envelope-type heating function was developed to investigate the mechanical-thermal coupling fatigue properties and obtain the fatigue life of materials under approximate service conditions. A physical model was described to quantitatively calculate the alternating deformation under the mechanical-thermal coupling tensile-bending combined condition. Accordingly, a fatigue life prediction method based on the Coffin-Manson equation under tensile-bending combined loading was proposed to estimate the mechanical-thermal coupling fatigue life in terms of the total coupling deformation and plastic strain amplitude. The feasibility of the proposed instrument was verified through the analysis of displacement responses of high entropy alloy specimens under the thermal-mechanical coupling tensile-bending combined condition at temperatures ranging from RT to 600 °C. The comparison between fatigue lives directly measured through thermal-mechanical coupling experiments and theoretically predicted fatigue lives indicated the availability of the modified Coffin-Manson equation.

Machine learning framework for predicting the low cycle fatigue life of lead-free solders

This study explores an efficient and reliable machine learning framework for determining the low cycle fatigue life of lead-free solders, which does not necessarily separately test different series of lead-free solder. With 1387 datasets from the published experiments and formulae, five mainstream machine learning models to date are adopted for the first time to predict the low cycle fatigue life for four different series of tin-based solders by considering the composition, loading and geometry factors. Based on feature importance and Shapley values, it is confirmed that the Boosting model is capable of capturing the nonlinear relationships of factors to influence the low cycle fatigue life of lead-free solder by greatly emphasizing the effects of plastic strain amplitude and temperature.

High-Temperature Low Cycle Fatigue of Nickel-Based Superalloy IN738LC

Polycrystalline cast nickel-based superalloy IN738LC is employed for critical parts of gas turbine components in the power industry, aircraft engines, and the marine sector. These components undergo severe degradation by low cycle fatigue caused by thermal gradients, particularly during start-up and shut-down periods. The present work reports the cyclic deformation behaviour and fatigue damage of IN738LC during high-temperature isothermal fatigue. Cylindrical specimens were cyclically deformed under strain control with constant total strain amplitude in symmetrical cycling at 800 °C and 950 °C in air. The microstructure is typical of coarse dendritic grains with carbides, eutectic, and shrinkage pores. SEM imaging revealed a γ matrix with coherent L12 γ′ precipitates with bimodal morphology. Cyclic hardening/softening curves, cyclic stress-strain response, and fatigue life diagrams were determined. An increase in testing temperature is associated with a significant decrease in stress amplitude and an increase in plastic strain amplitude. The fatigue life gradually decreases with increasing temperature. The fracture surfaces and polished sections parallel to the specimen axis were examined to study damage mechanisms in cyclic loading at high temperatures.

Low cycle fatigue properties assessment and damage influence on DP 500/800 steel sheet

A dual phase (DP500/800) steel sheet (1.6 mm thickness) was evaluated by means of low cycle fatigue (LCF). The fatigue properties of the material were assessed by means of the cyclic elastic modulus determination at half-life as well as by the plastic strain amplitude measurements. Both methods show a good correlation and high reliability, however, the plastic strain amplitude approach tends to fit better to the experimental results. From a parametric analysis performed, it was observed that the plastic strain amplitude approximation could be more suitable to assess the fatigue properties for LCF of the material due to an improved correlation of the computed cyclic elastic modulus in comparison to the cyclic elastic modulus measurements at half-life. Both methods showed that, for strain amplitudes higher than εa = 0.004, the cyclic elastic modulus tends to be lower than 210 GPa, meanwhile for lower strain amplitudes, the cyclic elastic modulus tends to increase. An evaluation by X-ray Diffraction technique allowed to determine that the increment of residual equivalent strain contributes to the cyclic elastic modulus diminution.

Characterization of dislocation microstructures in near-[formula omitted] single crystal copper using high-voltage scanning transmission electron microscopy

Metallic structures are prone to failure caused primarily by fatigue-related damage to the structural material. Fatigue behavior is strongly correlated with dislocation structures in metals. Therefore, investigating the formation of dislocation structures is essential for understanding the fatigue process. The dislocation structure in face-centered cubic crystals has a strong orientation dependence. However, the detailed characteristics and evolution process of dislocation structures of fatigued [1‾11] single crystal copper are still insufficient. This study investigated dislocation structures in near-[1‾11] single crystal copper cyclically deformed at room temperature under different constant plastic shear strain amplitude (γpl) to analyze the formation mechanism of dislocation structures and their relationship with fatigue behavior. Characteristic dislocation structures were characterized by adopting multiple planes using a high-voltage scanning transmission electron microscope. With increasing shear strain amplitude, a vein-like structure along the (1‾11) plane gradually developed into a wall structure. When γpl ≥ 1 × 10−3, deformation bands and cell bands were formed to accommodate the higher plastic strain. The state of cell-band development significantly affected the cyclic softening of the single crystal copper. A positive linear relationship existed between the shear stress amplitude and the reciprocal of the cell width.

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels.

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.

Crystal-plasticity modeling of monotonic and cyclic softening in inconel 718 superalloy

A phenomenological model is proposed to predict both monotonic and cyclic softening in Inconel 718 superalloy at 650 °C. Contrary to the current cyclic softening models consisting of a modified isotropic hardening variable, an internal state variable acting on the amplitude of the back stress is proposed and implemented in a crystal-plasticity model. Crystal-plasticity finite-element simulations are performed to validate quantitatively the model on uniaxial strain-controlled monotonic and cyclic loading for several plastic strain rates and strain amplitudes as well as to gain insights on the ratcheting phenomenon depending on the mean stress and stress amplitude values when cyclic softening occurs.

Low-cycle fatigue behavior of solutionized and aged WE43 magnesium alloys at room temperature

The low-cycle fatigue behavior of solutionized (T4) and aged (T6) WE43 magnesium alloys was studied at room temperature. The total strain amplitudes (Δεt/2) were 0.4%, 0.5%, 0.6%, 0.7% and 1.0%. Detailed microstructure evolution was characterized by scanning electron microscope (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The results showed that plastic strain amplitude decreased with the increasing cycle number in T4 alloy, which is due to the dense persistent slip bands (PSBs) and dynamic precipitates hindering <a> dislocation slip. In contrast, the plastic strain amplitude increases gradually in T6 alloy, which is attributed to the enhanced activation of pyramidal slip. The low-cycle fatigue life of T6 alloy with larger fatigue ductility coefficient is longer than that of T4 alloy. The Coffin-Manson model can accurately predict the fatigue life of T4 and T6 alloys compared to Jahed-Varvani (JV) energy model. For T4 alloy, the fatigue damage mechanism was dominated by basal slip. For T6 alloy, the enhanced pyramidal slip plays an important role to accommodate plastic deformation.

Mesoscale damage behavior and meso-macroscale correlation of low-cycle fatigue in Z2CND18.12N austenitic stainless steel

A low-cycle-fatigue damage behavior in Z2CND18.12N austenitic stainless steel is mainly studied at mesoscale, focusing on the correlation with that at macroscale. The variations of local misorientation in electron backscatter diffraction, ΔML, first increased with cyclic loading, then decreased to the minimum, and finally increased significantly, with <110> and <111> orientations having higher values than <100>. This evolution was positively correlated with the fraction of multiple slips and maximum von Mises stress, and negatively with the plastic-strain amplitude at macroscale in experiments and mesoscale in crystal-plasticity finite-element modeling. A ΔML model is proposed based on the dislocation evolution and slip mode.