Showing papers on "Fatigue limit published in 2021"

The effects of shot peening, laser shock peening and ultrasonic nanocrystal surface modification on the fatigue strength of Inconel 718

Abstract: As most of the failures in engineering components initiate from the surface layer, applying surface treatments can play a crucial role in controlling material performance and lifetime. In this study, different surface severe plastic deformation techniques including severe shot peening, laser shock peening and ultrasonic nanocrystal surface modification have been considered. The effects of process parameters and the kinetic energy of each treatment on the microstructure, mechanical properties and fatigue behavior of nickel-based super-alloy Inconel 718 have been investigated. The results revealed that using the proper parameters to increase the kinetic energy of the applied surface treatments, it is possible to effectively promote surface grain refinement and induce a deep compressive residual stress field in Inconel 718 samples. Among the applied treatments, ultrasonic nanocrystal surface modification was found to be the most efficient one in improving the mechanical properties as it led to the most significant fatigue performance, followed by severe shot peening and laser shock peening.

High-cycle bending fatigue properties of additive-manufactured ABS and PLA polymers fabricated by fused deposition modeling 3D-printing

Abstract: Nowadays, polymer parts made by additive manufacturing methods (AM) have multiple applications in various industries. Different parameters affect the print quality of the parts and subsequently their mechanical properties. In this study, the effect of the print direction in 3D-printing on the high-cycle bending fatigue properties of polymer parts made by the fused deposition modeling (FDM) method has been investigated. The standard samples of the fatigue test were printed in both horizontal and vertical directions and were made of two polymers, PLA and ABS. The thickness of each layer was 0.15 mm and the infill percentage of the parts was 50%. The rotary bending fatigue test was performed under conditions of fully-reversed stress-controlled loading at different stress levels. The results included the stress amplitude versus the fatigue lifetime of FDM 3D-printed PLA and ABS polymers. Besides, the scanning electron microscopy (SEM) image of the fracture surface was also depicted after fatigue testing. Results showed that PLA specimens had better fatigue lifetimes than ones of ABS samples. Moreover, the fatigue strength of 3D-printed samples in the horizontal direction was higher than that of 3D-printed specimens in the vertical direction. The print direction, the material and the stress level were found to be statistically significant factors from the analysis of variances. The SEM image demonstrated beach marks on the fracture surface of fibers (3D-printed filaments) in the PLA specimen, which proved cyclic loadings. Then, cleavage facets were the result of the brittle fracture.

Investigating the fatigue and mechanical behaviour of 3D printed woven and nonwoven continuous carbon fibre reinforced polymer (CFRP) composites

Abstract: This paper evaluates the mechanical properties of woven continuous carbon fibre composites printed by additive manufacturing (AM) Comparison mechanical test studies (tensile, flexural and fatigue) were carried out with two nonwoven AM printed composites (unidirectional and multidirectional fibres), along with those of both a woven composite, as well as a composite reinforced using chopped carbon fibres Compared with the 17 MPa tensile strength obtained for the chopped fibre composite, the average strength of unidirectional (nonwoven), multidirectional (nonwoven) and woven fibre composites were 39, 13 and 19-fold higher, respectively The tensile strength of the woven composites was 52% lower than that attained by the unidirectional (nonwoven) fibre composites; and 38% higher than the multidirectional (nonwoven) fibre composites A comparison was also made between the flexural and fatigue performance of the unidirectional (nonwoven) and woven fibre composites The flexural strength of the latter was approximately 39% lower than the nonwoven composites, however, the load bearing capacity of woven fibre was superior This performance difference was supported by the fatigue testing results At 70% of maximum tensile load capacity after 2 × 105 cycles, the nonwoven composites failed, while the woven composites continued to perform until a level of 85% of maximum load capacity was reached The superior fatigue strength of the AM fabricated woven carbon fibre composites, demonstrates their potential for use in high cyclic load applications

Effects of Conventional and Severe Shot Peening on Residual Stress and Fatigue Strength of Steel AISI 1060 and Residual Stress Relaxation Due to Fatigue Loading: Experimental and Numerical Simulation

Abstract: This study investigates and compares the effects of different shot peening treatments including conventional and severe shot peening on microstructure, mechanical properties, fatigue behavior, and residual stress relaxation of AISI 1060 steel. Shot peening treatments were applied with two Almen intensities of 17 and 21 A and a wide ranges of coverage (100%–1500%). Various microstructural observations were carried out to analyze the evolution of microstructure. Microhardness, residual stress and surface roughness measurements and also axial fatigue test were performed. Moreover, the extent of the residual stress relaxation during cyclic loading was investigated by means of XRD measurements. Furthermore, numerical simulation of residual stress relaxation due to fatigue loading was carried out and validated against experimental investigations. The comparison indicated a good agreement for the surface residual stress relaxation up to 100 cycles. The experimental results indicated the efficiency of severe shot peening processes in obtaining nanostructured surface layer and achieving superior mechanical properties and fatigue behavior. Also, residual stress measurements revealed that stress relaxation started with a high rate at the initial stages of loading and gradually increased at higher number of cycles which was lower in the case of severely shot peened samples compared to the conventionally treated ones.

Effect of laser shock peening on microstructural, mechanical and corrosion properties of laser beam welded commercially pure titanium

Abstract: In the present study, laser beam welding of commercially pure titanium has been carried out at different scan speeds in the range from 2.4 m/min to 4 m/min at 2.15 kW laser power and its influence on molten pool thermal history, microstructure, residual stress, mechanical and corrosion properties has been studied. Weld thermals cycles were monitored using a single wavelength infrared pyrometer. Effect of laser shock peening on laser welded specimens was also studied. Laser shock peening inducted significant amount of compressive residual stress through plastic deformation. Strain hardening and changes in microstructure such as formation of twining lead to significant improvement in tensile and fatigue properties. Micro-hardness in the fusion zone at sub-surface region got improved by ~26% and fatigue strength increased by ~24%. Corrosion resistance also got increased due to high compressive residual stress and grain refinement after LSP.

On the effect of laser powder-bed fusion process parameters on quasi-static and fatigue behaviour of Hastelloy X: A microstructure/defect interaction study

Abstract: The mechanical performance of parts, made by laser powder-bed fusion (LPBF), under quasi-static and cyclic loadings can be altered significantly by changing the process parameters. To better understand the related changes, tensile and fatigue LPBF Hastelloy X coupons were manufactured at high and low laser scanning speeds (LSS) in the nearly full-dense processing window. The experimental results demonstrate that, although the microstructure and surface roughness of the parts varies by altering the LSS, the porosity or density of the printed samples are not changing significantly within the range studied here. High LSS samples show higher tensile strength and higher rates for strain hardening at the primary stages of the deformation. Statistical analysis of variance indicates that LSS is a significant factor affecting the fatigue life. The S-N diagram shows Basquin lines intersection at the midlife fatigue region. In the low cycle fatigue (LCF) region high LSS samples show higher cycles to failures due to hysteresis stabilization at lower strains and hence less relative pre-strain and cyclic damage. Since stress risers are important features at high cycle fatigue (HCF) due to stress concentration and associated plasticity, the low LSS samples tolerate a greater number of cycles at lower stress levels and show a higher fatigue limit obtained by the step-loading method. These findings suggest that the process parameters can be optimized to achieve the best possible performance based on the desired application in LCF or HCF for AM parts.

Recent advances on size effect in metal fatigue under defects: a review

Abstract: Structural components with different scales normally show different fatigue behaviors, which are virtually dominated by defects originated from multiple sources, including manufacturing processes. This paper reviews three types of size effects (statistical, geometrical, technological) as well as their recent advances in metal fatigue, aiming to provide a guide for fatigue strength assessment of engineering components containing defects, inclusions and material inhomogeneity. Firstly, the background of inherent defects and defect-based failure mechanism are briefly outlined, and fatigue failure analysis based on fracture mechanics as well as statistics theory are emphasized. Then, two approaches commonly applied in statistical size effect modeling, i.e. critical defect method and weakest link method, are elaborated. In addition, the highly stressed volume method is introduced for considering the geometrical size effects, and the technological (production and surface) size effect is briefly overviewed. Finally, further directions on size effect in metal fatigue under defects are explored.

Multi-scale damage analysis and fatigue behavior of PLA manufactured by fused deposition modeling (FDM)

Abstract: Fused deposition modeling (FDM) draws particular attention due to its ability to fabricate components directly from a CAD data; however, the mechanical properties of the produced pieces are limited. This paper aims to present the experimental aspect of multi-scale damage analysis and fatigue behavior of polylactic acid (PLA) manufactured by FDM. The main purpose of this paper is to analyze the effect of extruder temperature during the process, loading amplitude, and frequency on fatigue behavior.,Three specific case studies were analyzed and compared with spool material for understanding the effect of bonding formation: single printed filament, two printed filaments and three printed filaments. Specific experiments of quasi-static tensile tests coupled with microstructure observations are performed to multi-scale damage analysis. A strong variation of fatigue strength as a function of the loading amplitude, frequency and extruder temperature is also presented.,The obtained experimental results show the first observed damage phenomenon corresponds to the inter-layer bonding of the filament interface at the stress value of 40 MPa. For instance, fatigue lifetime clearly depends on the extruder temperature and the loading frequency. Moreover, when the frequency is 80 Hz, the coupling effect of thermal and mechanical fatigue causes self-heating which decreases the fatigue lifetime.,This paper comprises useful data regarding the mechanical behavior and fatigue lifetime of FDM made PLA specimens. In fact, it evaluates the effect of process parameters (extruder temperature) based on the nature of FDM that is classified as a thermally-driven process.

Fatigue performance and crack propagation behavior of selective laser melted AlSi10Mg in 0°, 15°, 45° and 90° building directions

Abstract: In the present work, selective laser melted (SLM) AlSi10Mg samples were used in rotational bending fatigue, scanning electron microscope in-situ fatigue and nanoindentation tests. The effects of building directions (0°, 15°, 45° and 90°) and molten pool boundaries (MPBs) on the fatigue performance and crack propagation behavior of the AlSi10Mg samples were studied. The results indicated that the SLM samples (from 0° to 15° building directions) were in high fatigue performance range. The fatigue fracture parameters (fatigue strength, threshold stress intensity factor range and fracture toughness) of the 0° and 15° samples were higher than those of the 45° and 90° samples. Moreover, the differences of elasticity modulus and hardness between the MPBs and molten pool cores were identified. Under cyclic loads, the mechanical property differences resulted in the MPBs severe strain concentration and plastic deformation. By finite element simulations, the fatigue crack propagation behavior of the SLM samples can be explained. Finally, the crack aspect ratios (a/b) were measured from fracture surfaces. The building directions had no obvious impact on the ratio of a/b whose constant values (a > 300 μm) were about 0.45–0.60.

Fatigue limit prediction and analysis of nano-structured AISI 304 steel by severe shot peening via ANN

Abstract: AISI 304 stainless steel is very widely used for industrial applications due to its good integrated performance and corrosion resistance. However, shot peening (SP) is known as one of the effectual surface treatments processes to provide superior properties in metallic materials. In the present study, a comprehensive study on SP of AISI 304 steel including 42 different SP treatments with a wide range of Almen intensities of 14–36 A and various coverage of 100–2000% was carried out. Varieties of experiments were accomplished for the investigation of the microstructure, grain size, surface topography, hardness and residual stresses as well as axial fatigue behavior. After experimental investigations, artificial neural networks modeling was carried out for parametric analysis and optimization. The results indicated that, treated specimens with higher severity had more desirable properties and performances.

Fatigue strength estimation methodology of additively manufactured metallic bulk material

Abstract: The objective of this research work is to scientifically contribute to the fatigue strength assessment of selectively laser melted metallic structures and aims to derive a methodology to safely estimate bulk material strength based on experimental data. Investigations focus on determination of the relevant post build and -processing residual stress condition and implementation in state of the art fatigue design concepts, alongside defect population and material properties. Additive manufacturing, especially powder-bed based laser melting, exposes material to immense thermal gradients that provoke residual stresses which bias complex mean stress states in components, may causing premature failure. X-ray diffractometry enables the measurement and allows assessment of present stresses by providing information about the superposition of residual stresses of different order by considering peak broadening effects. Residual stresses of first order overlay with load-induced stresses and significantly alter fatigue performance. Therefore, a mean stress correction of the experimentally determined fatigue strength is carried out by applying Smith-Watson-Topper’s damaging parameter. The fully reversed fatigue strength amplitude is matched by complementing Murakami’s approach by a correlation coefficient which empirically considers tensile residual stresses relative to hardness. Implementation of the introduced reduction factor to established designing concepts is found to be well applicable, considering that the developed methodology estimates fatigue strength of several test series from different materials in various post treatment conditions within a conservative region of − 3% and − 8%. The approach illustrated within this paper satisfies the demand for a holistic and conservative fatigue design approach incorporating defect population and hardness as well as the effect of the local residual stress condition.

Effects of laser shock peening on the ultra-high cycle fatigue performance of additively manufactured Ti6Al4V alloy

Abstract: Although additive manufacturing (AM) allows to fabricate metallic parts with a high static strength comparable to their forged counterparts, the fatigue strength of an AM-ed part is generally inferior, restricting AM from many critical applications. In this study, laser shock peening (LSP) was applied to modify the surface properties of selective laser melting fabricated (SLM-ed) Ti6Al4V titanium alloy. This study performs systematic tests and analyses on the fabricated alloy specimens to characterize their microstructures and mechanical properties including residual stress, tensile strength, ultra-high cycle fatigue (UHCF) strength. The results reveal that LSP can refine microstructure, suppress residual stresses, and delay crack propagation in the affected area. However, the inherent defects in an SLM-ed part, such as unmelted powders, lack of fusion and clusters of α phase, dominate the fatigue failure of the specimens especially in the UHCF regime, resulting in their poor fatigue performance. Meanwhile, The LSP processed specimens showed a lower S-N curve than that of specimens without LSP processing especially in the UHCF regime, which not only results from the inherent defects, but also the increased surface roughness and non-uniform residual stresses.