High Strain Amplitudes Research Articles

Influence of friction and back stresses evolution on cyclic softening of laser powder bed fusion Ti-6Al-4V ELI alloy

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V ELI enables the fabrication of complex and individualized load-bearing patient-specific implants (PSI). These PSI are subjected to cyclic loading, which necessitates their fatigue performance evaluation. In this work, the fatigue properties of LPBF Ti-6Al-4V ELI in heat-treated conditions were characterized by studying the low cycle fatigue (LCF) behavior and underlying deformation mechanism. The fully reversed strain-controlled fatigue tests were performed at various strain amplitudes at room temperature. The ratio of fatigue life of conventional wrought alloy and LPBF Ti-6Al-4V ELI at the lowest (0.8 %) and the highest (1.8 %) strain amplitudes were found to be approximately 2.7 and 4.5, respectively. Further, the softening response during fatigue loading was correlated to the variation in the back and friction stresses. The TEM observations revealed that the dislocation pileup along α/β interfaces caused an increase in dislocation density, which drives the increase in back stress. However, the ease of slip transmission at high strain amplitude (1.8 %) reduced the heterogeneity between α and β phases, causing a decrease in the back stress. The TEM observations further suggested that the dislocation annihilation and subsequent rise in dislocation-free zones in α-phase reduced the friction stress at all strain amplitudes, which resulted in cyclic softening of LPBF Ti-6Al-4V ELI.

Probing phase transformation and dislocation evolution in high-entropy alloy under cyclic loadings

Stress-induced FCC-BCC phase transformation plays a crucial role in the mechanical behaviors of high-entropy alloys (HEAs). While there has been extensive research on this transformation during monotonic deformation, studies on fatigue behavior are extremely limited. Here, we use molecular dynamics simulations to investigate phase transformation and dislocation evolution in HEAs under strain-controlled symmetric tension–compression cycles. Our results show that cyclic deformation behavior is sensitive to strain amplitude, revealing three distinct cyclic responses. Notably, a progressive FCC-BCC phase transformation process occurs at a high strain amplitude of 4.8%. Grain boundaries and their triple junctions are identified as preferred sites for phase transformation under cyclic loading conditions. These findings provide valuable atomic-scale insights for understanding fatigue deformation in HEAs with transformation-induced plasticity.

Effect of strain level on the cyclic response and microstructure characteristics of selective laser melted 304L

Considering the relationship between austenite stability and strain level, the cyclic response curves are obtained for SLM 304L under different strain amplitude at R = 0.3. The results indicate that since the microstructure characteristics shift from stacking faults (SFs) at low strain amplitude to martensite and twins at high strain amplitude, the cyclic softening behavior transitions to cyclic hardening behavior. Besides, it also indicates that high strain levels can overcome the influence of initial microstructure on fatigue performance, presenting a new sight to understand the relationship between cyclic hardening and martensite transformation.

Improved low cycle fatigue property of Ti–6Al–4V alloy by trace Fe addition

The low-cycle fatigue properties of titanium alloys suffering cyclic loading determines the safety in service for their components used in submarine shells. It has been reported that tensile property could be improved by making use of Fe alloying element partitioning effect. However, the fatigue property and mechanism of titanium alloys with Fe addition remain unknown. In this study, the low-cycle fatigue property and cyclic deformation mechanism of Ti–6Al–4V-0.55Fe and Ti–6Al–4V ELI with a duplex microstructure were systematically investigated. With the addition of Fe element, Ti–6Al–4V-0.55Fe with longer lasting cyclic hardening behavior and lower cyclic softening rate, showed higher fatigue life under selected strain amplitudes. Under low strain amplitude, effect of Fe on dislocation motion, interface resistance, lattice distortion, and micro strain were responsible for the enhanced fatigue life. While under high strain amplitude, Fe brought out the pinning effect on interface. Besides, Fe addition also limited the fatigue crack propagation.

Phasing effects on thermo-mechanical fatigue damage investigated via crystal plasticity modeling

Nickel (Ni)-based superalloys have high strength at elevated temperatures, making them ideal candidates for aerospace components exposed to thermo-mechanical loads. Thermo-mechanical fatigue's (TMF) influence on material failure is highly dependent on the temperature-load phase profile as a consequence of path-dependent thermo-mechanical plasticity. In order to understand the direct influence phasing effects have on TMF performance, a microstructure-based fatigue model has been developed to connect damage mechanisms to crack initiation events. In this work, in-phase (IP) TMF, out-of-phase (OP) TMF, and iso-thermal (ISO) loading profiles are investigated with a temperature-dependent, dislocation density-based crystal plasticity model to pinpoint a relationship between microstructural damage and TMF phasing effects. A crystal plasticity modeling framework with capabilities to isolate phasing (IP, OP, and ISO) effects is presented to locate fatigue damage in a set of statistically equivalent microstructures (SEMs) with an energy-based damage indicative parameter. Local stress, accumulated damage, intragranular misorientiation, grain interactions, and slip alignment activity at the damaged regions are studied under the various temperature-load phase profiles to connect microstructural material damage to TMF performance. Based on the local microstructural material damage, it is postulated the material's stiffness at maximum load influences OP TMF performance, whereas the hardening/plasticity at maximum load influences IP TMF. The results from the presented framework is consistent with experimental evidence that at high mechanical strain amplitudes IP TMF has a shorter life as a consequence of hardening/plasticity influencing path-dependent thermo-mechanical plasticity, whereas at low strain amplitudes OP TMF has a shorter life due to stiffness influencing path-dependent thermo-mechanical plasticity.

Tensile and Low‐Cycle Fatigue Behavior of Laser Powder Bed Fused Inconel 718 at Room and High Temperature

This study investigates the room‐ and high‐temperature (650 °C) tensile and low‐cycle‐fatigue behavior of Inconel 718 produced by laser powder bed fusion (PBF‐LB/M) with a four‐step heat treatment and compares the results to the conventional wrought material. The microstructure after heat treatment is characterized on different length scales. Compared to the wrought variant, the elastic and yield properties are comparable at both test temperatures while tensile strength, ductility, and strain hardening capacity are lower. The fatigue life of the PBF‐LB/M variant at room temperature is slightly lower than that of the wrought material, while at 650 °C, it is vice versa. The cyclic stress response for both material variants is characterized by cyclic softening, which is more pronounced at the higher test temperature. High strain amplitudes (≥0.7%) at room temperature and especially a high testing temperature result in the formation of multiple secondary cracks at the transitions of regions comprising predominantly elongated grain morphology and columns of stacked grains with ripple patterns in the PBF‐LB/M material. This observation and pronounced crack branching and deflection indicate that the cracks are controlled by sharp micromechanical gradients and local crystallite clusters.

Influence of different cycle strain amplitudes on the microstructure and properties of 2024 aluminum alloy

The effect of varying strain amplitudes on the tensile properties and microstructure of aluminum alloy 2024 after a solution treatment (ST) of 495 °C for 1 h followed by cyclic strengthening at room temperature in the presence of various strain amplitudes is methodically investigated. Cyclic strain treatment substantially leads to the enhancement of the strength and plasticity of the alloy. Different strain amplitudes result in distinct strengthening mechanisms in the tested specimens. The specimens treated with low strain amplitudes (Δεt/2 = 0.1–0.4%) exhibit the least changes in the dislocation cell size and the dislocation density. The strengthening is mainly attributed to the precise adjustment of solute cluster precipitation. In specimens treated with high strain amplitudes (Δεt/2 = 0.5–1.1%), various strain amplitudes result in various dislocation configurations. The S-phase precipitation occurs selectively in the dislocation cell walls, forming solute cluster-composite dislocation loops within the dislocation cells, enhancing both the strength and plasticity of the alloy. The rational cyclic strain treatment not only yields the achievement of the precise control of precipitate phases, but also regulates the dislocation configurations. The obtained results reveal that the high-density cluster-composite dislocation loops are capable of resolving the conflict between increasing dislocation density and reducing dislocation pile-up, providing new insights into alloy strengthening.

Impact of crystalline domains on long-term stability and mechanical performance of anisotropic silk fibroin sponges.

Sponge-like materials made from regenerated silk fibroin biopolymers are a tunable and advantageous platform for in vitro engineered tissue culture and in vivo tissue regeneration. Anisotropic, three-dimensional (3D) silk fibroin sponge-like scaffolds can mimic the architecture of contractile muscle. Herein, we use silk fibroin solution isolated from the cocoons of Bombyx mori silkworms to form aligned sponges via directional ice templating in a custom mold with a slurry of dry ice and ethanol. Hydrated tensile mechanical properties of these aligned sponges were evaluated as a function of silk polymer concentration (3% or 5%), freezing time (50% or 100% ethanol), and post-lyophilization method for inducing crystallinity (autoclaving, water annealing). Hydrated static tensile tests were used to determine Young's modulus and ultimate tensile strength across sponge formulations at two strain rates to evaluate rate dependence in the calculated parameters. Results aligned with previous reports in the literature for isotropic silk fibroin sponge-like scaffolds, where the method by which beta-sheets were formed and level of beta-sheet content (crystallinity) had the greatest impact on static parameters, while polymer concentration and freezing rate did not significantly impact static mechanical properties. We estimated the crystalline organization using molecular dynamics simulations to show that larger crystalline regions may be responsible for strength at low strain amplitudes and brittleness at high strain amplitudes in the autoclaved sponges. Within the parameters evaluated, extensional Young's modulus is tunable in the range of 600-2800 kPa. Dynamic tensile testing revealed the linear viscoelastic region to be between 0% and 10% strain amplitude and 0.2-2 Hz frequencies. Long-term stability was evaluated by hysteresis and fatigue tests. Fatigue tests showed minimal change in the storage and loss modulus of 5% silk fibroin sponges for more than 6000 min of continuous mechanical stimulation in the linear regime at 10% strain amplitude and 1 Hz frequency. Furthermore, we confirmed that these mechanical properties hold when decellularized extracellular matrix is added to the sponges and when the mechanical property assessments were performed in cell culture media. We also used nano-computed tomography (nano-CT) and simulations to explore pore interconnectivity and tortuosity. Overall, these results highlight the potential of anisotropic, sponge-like silk fibroin scaffolds for long-term (>6 weeks) contractile muscle culture with an in vitro bioreactor system that provides routine mechanical stimulation.

Investigating the deformation and microstructural evolution of laser powder-bed fusion of Hastelloy X during high temperature fatigue loading

This study investigates the fatigue behaviour of samples made by laser powder-bed fusion of Hastelloy X (LPBF-HX) with as-built and machined surface conditions at 700 °C under fully reversed strain-controlled cyclic loading. Samples with both surface conditions exhibited initially cyclic hardening followed by cyclic softening under large strain amplitude testing, where a slight continuous hardening was observed for tests with smaller strain amplitudes. The samples with machined surfaces showed longer endurance and higher stress ranges than those with as-built surfaces. Post-fatigue-test EBSD analysis showed the formation of the Goss texture and extensive local strain accumulation in the samples tested under high strain amplitude at 700 °C. Fractography investigations revealed that early crack initiation in the samples with as-built surfaces was from stress concentrations induced by valleys on the rough surface. No evidence of crack initiation induced by pre-existing defects was observed in the machined samples, and the excessive slip activity at the surface was found to be responsible for the crack initiation.

Effects of multiaxial non-proportional cyclic pre-hardening on monotonic tensile and fatigue properties of 316L stainless steel

Effects of multiaxial non-proportional cyclic pre-hardening (CPH) treatment by performing tension–torsion cyclic test in a circular path on the monotonic tensile and fatigue properties of 316L austenitic stainless steel are studied. Results show that the circular path pre-hardening treatment significantly improves the yield strength but reduces the elongation of materials in comparison with the as-received (AR) material. Moreover, the fatigue resistance of 316L stainless steel after CPH treatment gets markedly degraded at the strain amplitude of 0.4 % with a 26 % decrease of fatigue life, however, the 62 % life increase is obtained at the lower strain amplitude of 0.2 %. Therefore, it is reasonable to believe that the CPH treatment can be regarded as an effective method to increase the fatigue resistance within a certain range of strain amplitude if the proper loading path and cycles of the multiaxial pre-hardening process are used. The different life variations at the high and low strain amplitudes are explained from the perspectives of macroscopic deformation energy and microscopic deformation modes based on EBSD and TEM characterizations.

Numerical simulation and analysis of microfracture mechanism of magnesium alloy materials

Abstract This paper takes the typical AZ31B magnesium alloy, which is increasingly widely used in engineering, as the research object and aims to study the fracture properties of magnesium alloys and their damage mechanisms under different loading strain rates and loading modes. EBSD, trace method, and other techniques are applied to study the fracture modes at a low strain amplitude of 0.42% and high strain amplitude of 1.746%, to explore and compare the type and number of microcracks at these two strain amplitudes, and to analyze the reasons for the formation of different types of microcracks. The results show that the experimental 0pass and 4pass start to fracture when they are numerically simulated impact loaded at impact velocities of 225m/s and 260m/s, and the larger the value of impact loading, the more complete the fracture occurs. The fatigue fracture occurred at TBs, GBs, and PSBs with a strain amplitude of 1.746% and 0.42%, respectively. The SF of PSBs was mainly distributed in the range of 0.8-1.0, while that of TBs was most distributed in the interval of 0-0.2 and 0.6-0.9. The larger the SF, the more prone to sprouting of microcracks. The smaller the value of m′, the larger the value of Φ, the more likely to promote microcracks sprouting at the grain boundaries.

Microstructure and damping properties of Mg–Si binary hypoeutectic alloys

The effects of Si content and solution heat treatment on the microstructure and damping properties of as-cast Mg–Si hypoeutectic alloys were investigated. Si has the capacity to refine grains of pure magnesium. The three contents of Mg–Si hypoeutectic alloys are all high-damping alloys. The damping value of the as-cast Mg–0.5 wt-%Si alloy was higher than that of the Mg–0.3 wt-%Si and Mg–0.8 wt-%Si alloys in the small strain amplitude area. Compared to the as-cast state, the solution heat-treated Mg–Si hypoeutectic alloys show an increase in damping values, which are close to each other for the three contents in the small strain amplitude area, and decrease with increasing Si content in the high strain amplitude area as in the as-cast state.

Fatigue and anisotropic behavior of wire-arc additive manufactured TC17 titanium alloy

Wire-arc additive manufacturing (WAAM) was used in aeronautical engineering due to its low equipment cost and ability to create complex shapes. However, the fatigue behavior of WAAM titanium alloy, specifically for low cycle fatigue (LCF), was poorly studied. This paper investigated the microstructure and mechanical behavior of WAAM TC17 in both vertical and horizontal orientations. Fully reversed LCF tests were conducted on specimens with varying strain amplitudes from ±0.4 % to ±1.2 %. The results found that the ultimate tensile and yield strength were similar for both vertical and horizontal samples, but the elongation in the horizontal orientation was approximately 60 % lower than that in the vertical orientation. Considering the cyclic loading behavior, both vertical and horizontal samples exhibited cyclic softening characteristics at high strain amplitudes (0.8%–1.2 %). Additionally, the cyclic softening rate (CSR) of horizontal samples exhibited a higher than that of vertical samples at high strain amplitude. The LCF cracks of WAAM TC17 were initiated from the surface and internal defects of the samples. The LCF performance of the horizontal samples was found to decrease than that of vertical samples. Specifically, the fatigue performance of the horizontal samples is lower by 36.7 % compared to the vertical samples when the number of cycles is 103.

Thermo-elasticity of multi walled carbon nanotube- natural rubber/acrylonitrile butadiene rubber blends

Natural rubber nano-composites are emerging as one of the key players in the industry and research due to its novel material properties and nano functionalities. In order to create high performance natural rubber nano-composites, new multifunctional nano-fillers can be created using a various approaches due to the advances of nanotechnology. Blends of natural rubber (NR) and acrylonitrile butadiene rubber (NBR) were used to evaluate the effects of various constitutional ratios between the two substances of fixed weight of multi walled carbon nanotubes (MWCNTs). The increase in the frequency of alternating extension (fatigue deformation) for samples (B2, B3) causes an increase in the thermo-elastic temperature change. The breakdown of the structure is thought to be prevailing at high strain amplitude and consequently may account for the observed effect. The thermo-elastic measurements also involve the determination of the thermo-elastic coefficient, change in entropy, internal energy per unit length, and Gruneisen constant. All these parameters were found to be highly dependent on both fatigue parameters and NR/NBR polymer ratios in the composite. The prepared sample (50/50 phr) can be used as thermo-elastic sensors.

Low cycle fatigue and creep-fatigue interaction behavior of 2.25CrMoV steel at high temperature

This paper presents a comprehensive investigation into the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of 2.25CrMoV steel at an elevated temperature of 455 °C. The LCF tests are conducted using a triangular waveform with varying strain amplitudes ranging from 0.4 % to 0.7 %. Furthermore, the CFI tests are conducted using a trapezoidal waveform with different hold times and hold directions. To gain further insights, the microstructure and fracture behavior of the steel are characterized using optical, scanning, and transmission electron microscopy techniques. The results reveal that the softening behavior and fatigue life degradation of the steel are dependent on the applied strain amplitude, hold time, and hold direction. Higher strain amplitudes in the LCF tests lead to an increased number of crack initiation sources. During the CFI tests, fatigue fracture is identified as the primary failure mechanism under tensile hold conditions, and an increase in hold time promotes crack propagation. The interaction between fatigue and creep is found to be more significant in compressive hold and tensile compressive combination hold conditions. Microstructure analysis demonstrates that the lath structure recovers during cyclic loading, resulting in cyclic softening. Finally, a modified energy-based model is proposed to distinguish the different roles of tensile energy and compressive energy in the fatigue process. The proposed model accurately predicts the fatigue life of the 2.25CrMoV steel, as demonstrated by the good agreement between the experimental and predicted results.

Role of geometrically necessary dislocations on the superior damping capacity and high strength-ductility of 5083Al/SUS405 laminated heterostructure composites

The laminated heterostructure composite design was implemented on 5083Al alloy. Through the process of vacuum hot pressing (HP state) followed by cold rolling and annealing (CRA state), the 5083Al/SUS405 laminated heterostructure composites (LHCs) were fabricated. The LHCs exhibit excellent combination of strength-ductility and damping capacity. The damping in the LHCs originates from the combined effects of dislocation, interface and ferromagnetic damping mechanisms. Compared to 5083Al alloy, the LHCs exhibit higher dislocation damping at relatively high strain amplitudes (ε > 0.01 %), especially for the CRA state LHC with higher density of geometrically necessary dislocations (GNDs). This suggests that GNDs not only enhance strength-ductility through the hetero-deformation induced (HDI) hardening effect, but also contribute to dislocation damping in the LHCs.

Spreading ceramic stereolithography pastes: Insights from shear- and orthogonal-rheology

We study the shear rheological behavior of a commercial stereolithography paste containing ≈50 vol. % of zirconia particles (diameter ≈ 100 nm) with the aim to clarify physical mechanisms occurring during the “scraping” step of this yield stress fluid. Beyond a flow curve characterized by a high zero-shear viscosity accompanied with an overall shear-thinning behavior, we investigate in a systematic way the transient regime through start-up experiments. We demonstrate that a structural transition occurs between 10−2 and 10−1 s−1, resulting in an apparent interruption of the shear-thinning. The corresponding transient response presents a pronounced extra-growth of the shear stress before to stabilize at high strain amplitude and a negative first normal stress difference peak, both effects become stronger at higher shear rates. These observations are rationalized based on the high interparticle friction owing to the polyhedral shape and the roughness of the particles. In addition, relaxation tests following the start-up experiments reveal that the samples submitted to shear rates higher than 10−1 s−1 cannot relax the shear stress to the same level as in low shear rate experiments, suggesting a durable structural modification likely to impact the quality of the parts prior to their debinding and densification. Finally, we utilize orthogonal superposition rheology to illustrate how the application of an oscillatory deformation during the scraping procedure could help to reduce the shear-thinning interruption and improve the stereolithography processing as already observed empirically during scraping.

Predictions of the behavior of a single droplet and blends composed of Newtonian/viscoelastic minor phase and viscous major phase subjected to oscillatory shear flow

The oscillatory behavior of Newtonian and viscoelastic droplets in a Newtonian phase and blends composed of viscoelastic minor phase in a Newtonian major phase are theoretically investigated in this work. The non-Newtonian constrained volume model predictions are compared to experimental oscillatory shearing flow data of droplet and blends that are available in the literature. For a Newtonian droplet in a Newtonian phase, the model describes experimental droplet behavior well at high viscosity ratio and high strain amplitude. For viscoelastic droplet in a Newtonian phase, the model predicts less deformation for viscoelastic droplet than a comparable Newtonian droplet. For large amplitude oscillatory shear rheological data of blends composed of Boger fluid minor phase in a Newtonian major phase, the model shows improvement in prediction of the elastic modulus at high viscosity ratio, compared to the Newtonian model. The model also shows good agreement with large and minimum strain elastic moduli and large and minimum rate dynamic viscosities for small and large viscosity ratio viscoelastic polymer blends. For the blend of Boger fluid minor phase in a Newtonian major phase at viscosity ratio larger than one, we find that elasticity contributes to total stress from small to large strain amplitude values. For the blend of Boger fluid minor phase in a Newtonian major phase at viscosity ratio smaller than one, we find that elasticity is important only at large values of strain amplitude. Moreover, for the aforementioned blend at viscosity ratio larger than one, the predicted Lissajous Bowditch plots of excess stress do not reflect droplet shape/droplet orientation, and the opposite is true for the small viscosity ratio blend. Investigation of droplet long semi axis for the large viscosity ratio blend at LAOS conditions reveals oscillations at two to three times the imposed frequency.

Deformation and damage mechanisms during clockwise diamond and counter clockwise diamond thermomechanical fatigue in Timetal 834 alloy

In the present investigation, the thermomechanical fatigue (TMF) behavior of Timetal 834 alloy was studied under clockwise diamond (CWD) and counter clockwise diamond (CCD) loading conditions. These tests were conducted within 723 K ↔ 873 K temperature range at the strain amplitudes of Δεm/2 = ±0.6% and ±1.0%. The cyclic softening behavior observed under CWD-TMF condition at both strain amplitudes, is attributed to shearing of coherent Ti3Al precipitates. Contrarily, the alloy exhibits both cyclic hardening and softening responses under the CCD-TMF conditions. This additional hardening response is attributed to the occurrence of dislocation–dislocation interactions coupled with dynamic strain aging. The present study reveals that both phase angle and strain amplitude affect the TMF life. The alloy exhibited relatively lower fatigue life under CCD-TMF condition than under CWD-TMF condition, at both strain amplitudes. Transmission electron microscopy analyses of the post-deformed specimens reveal that the dislocation arrangements are similar in both loading conditions at Δεm/2 = ±0.6%, while being distinctively different at Δεm/2 = ±1.0%. At higher strain amplitude, the fatigue life is significantly reduced under CCD-TMF condition. Fractographic analyses indicate the involvement of oxidation damage during both the TMF loading conditions. However, its effect is more pronounced under CCD-TMF than under CWD-TMF condition.

Enhanced low cycle fatigue properties of selective laser melting Ti–6Al–4V with fine-tuned composition and optimized microstructure

Improving the low-cycle fatigue (LCF) properties of additively manufactured Ti–5.6Al–3.8V alloy is critical in ensuring its service safety and represents a significant research challenge. This work discusses a solution that optimizes the alloy's microstructure and ductility by precisely controlling the over-saturated strengthening elements and heat treatment. This was accomplished using selective laser melting (SLM), heat treatment at 800 °C for 2 h, and furnace cooling on a Ti–5.6Al–3.8V alloy with tightly controlled Al, V, and O concentrations in a lower range. The results showed that the SLM-fabricated Ti–5.6Al–3.8V alloy, post-heat treatment, exhibited α laths with a width of ∼1.4 μm and β columnar grains with a diameter of ∼126 μm, without experiencing coarsening or variant selection phenomena. The alloy balanced strength and ductility post-heat treatment with a UTS of 1015 MPa and an EL of 16.5% relative to the as-deposited state (UTS of 1199 MPa and EL of 11.9%). Notably, the LCF properties of the heat-treated SLM Ti–5.6Al–3.8V alloy are superior to those of other Ti–6Al–4V alloys produced by additive manufacturing and comparable to traditional forgings. At high strain amplitudes (1–1.5%), the fatigue life of this alloy was twice that of the Ti–6Al–4V forgings. Furthermore, we comprehensively analyzed the microstructure, strength, and ductility of the SLM Ti–5.6Al–3.8V alloy to elucidate the factors influencing its LCF properties. These findings provide a solid foundation for improving the LCF properties of additively manufactured Ti–6Al–4V alloy, thereby contributing to its safe and reliable use in critical applications.