Increase In Fatigue Life Research Articles

Effect of stress ratio on the tension–tension fatigue behavior of tungsten carbide nanoparticles toughened GFRP

AbstractThe fatigue behavior of woven ply (0/90°) glass fiber reinforced plastic (GFRP) laminates filled with 2 wt.% of tungsten carbide nano‐powder at different stress ratios, 0.1 and 0.3, was investigated. Specimens were subjected to constant amplitude fatigue loading for three stress levels, 80%, 70%, and 60%, of ultimate tensile strength at each stress ratio for both neat GFRP and tungsten carbide modified GFRP (GFRP‐WC). The fatigue life was enhanced at all stress levels in neat GFRP composites with an increase in stress ratio from 0.1 to 0.3. In addition, incorporation of nanoparticles also increases fatigue life at all stress levels for both stress ratios. The addition of nanoparticles acts as a barrier to crack propagation, which leads to a significant effect on the evolution of fatigue stiffness, cyclic creep strain, and dissipated energy during cyclic loading at both stress ratios.

Synthesis of microcrystalline cellulose from abaca nonfluorescent flower pistil and its twill weaved fiber‐polyester composites: Viscoelastic, fatigue, and mechanical properties

AbstractThe development of microcrystalline cellulose from banana flower pistil and its twill weaved woven abaca polyester composites were studied in this research. The aim of this research was to explore the viscoelastic, fatigue, and mechanical behavior of surface‐treated twill weaved abaca fiber and unsaturated polyester incorporating banana flower pistil cellulose particles for various composite combinations. Hand layup process was used to construct the bio‐composite laminates, which were then tested in accordance with american society for testing and materials (ASTM) standards. The maximum tensile strength and modulus, flexural strength and modulus, Izod impact strength, as well asinter laminar shear strength (ILSS) values by addition of 3.0 vol% of cellulose were about 124 MPa, 4.48 GPa, 163 MPa, 5.36 GPa, 5.33 J/m2, and 27 MPa respectively, whereas composite having 5.0 vol% of cellulose gave better hardness of 82 shore‐D. Incorporation of cellulose by 3.0 vol% with 50 vol% of abaca fiber increases fatigue life counts up to 35,480, 29,039, 24,802, and 18,247 for 25%, 50%, 70%, and 90% of ultimate tensile strength (UTS) respectively. On comparing all other composite combinations, the composite containing 3.0 vol% of cellulose shows the highest storage modulus of up to 5.6 GPa and the lowest loss factor of 0.38. The scanning electron microscopy (SEM) fractography shows the improved adhesion between fiber and matrix due to the silane treatment on cellulose and abaca fiber. These novel functional materials could be employed in a large range of applications, including industrial, automobile, electronic and household equipment.

Post-impact Fatigue Performance of 2198-T8 Aluminum-Lithium Alloy Sheet with Pre-crack

Abstract The fatigue test with the digital image correlation system is applied to 2198-T8 aluminum-lithium alloy sheets with preset cracks and impact dents. The influence of different inserts and impact energies on residual fatigue life and failure behavior is analyzed. The results show that the shape of inserts is the main factor of the residual fatigue life at the same impact energy following the order: U-shaped > hemispherical > conical. Moreover the fatigue life decreases with the increase of impact energy. The hemispherical and U-shaped inserts cause local hardening at low impact energy 5 J, resulting in a maximum increase in fatigue life of 12.09 %. In the initial stages of fatigue failure, high strain areas are always around the preset crack where the propagations initiate. There is a multi-axial loading area around the impact dent in the middle stage. Its fatigue crack propagation mode is type I+III, resulting in a deceleration period.

Efficient optimization framework for L-PBF fatigue enhanced Ti6Al4V lattice component

Industries today face challenges in incorporating metallic additively manufactured lattice structures in critical components subjected to fatigue loading. This work explores the relationship between fatigue properties and the printing orientation of Laser-Powered Bed Fusion (LPBF) lattice structures. This relation is at the base of a cost-effective and time-efficient optimization workflow able to determine the optimal lattice printing orientation for improved fatigue life. Fatigue resistance is tested under uniaxial conditions on miniaturized specimens that mimic the lattice sub-unit elements: struts and nodes. The collected data is used as input for the optimization algorithm to determine the specimen orientation that maximizes fatigue life. The optimized specimens are manufactured, tested under three-point-bending conditions, and analysed using metrological x-ray computed tomography to verify the improvement. The proposed workflow is able to produce a 24 % increase in specimen fatigue life by simply adjusting the orientation on the printing plane.

Exceptional fatigue life and ductility of new liquid healing hot isostatic pressing especially tailored for additive manufactured aluminum alloys

Porosity and hot cracking still hinder the introduction of new additive manufactured alloys on the market, which requires high reliability. Hot Isostatic Pressing (HIP) is thus commonly applied to close these defects. Although HIP showed promising results as post-treatment on steel and titanium alloys, it is detrimental on aluminum alloys. Liquid assisted healing is thus applied during HIP to not only compress the pores, leading to stress concentration defects but to completely heal them. An Al-Mg alloy was manufactured as a proof-of-concept and X-ray nano-holo-tomography highlighted the healing of almost all the defects, even a 80 µm diameter pore, during liquid assisted HIP. This leads to the substantial increase of the fracture strain by a factor 3 without strength loss and an exceptional increase in fatigue life up to a factor 100. This healing HIP treatment is promising for post-treatment of other Al alloys but also as in-service healing treatment.

FATIGUE BEHAVIOUR OF TEMPERED AND ISOTHERMAL HEAT TREATED AISI 5160 LEAF SPRING STEEL

The oil quench and temper technique have a lot of benefits for heavy duty spring manufacture since it may expose the best balance of toughness and ductility, as well as increase fatigue life. The current study looked at the fatigue behavior of tempered AISI 5160 leaf spring steel samples at tempering temperatures of 400, 450, 500, 550, and 600 °C, as well as isothermally heat- treated steel samples at 830 °C. All leaf spring steel samples that had undergone thermal tempering and isothermal heat treatment were then tested up to fracture utilizing rotational fatigue test equipment under the effect of various stress levels. All steel samples subjected to tempering heat treatments of 400°C to 600°C showed a decrease in hardness ratings. The Rockwell hardness ratings of the steel samples that treated to isothermal heat treatment increased significantly. Experimental fatigue testing revealed that the values of fatigue resistance for steel samples tempered at (400 and 450) °C temperatures dropped by a small amount. The fatigue resistance values for steel specimens tempered at 500 °C to 600 °C temperatures decreased more than the values for steel samples tempered at 500 to 600 °C temperatures. A fatigue resistance of steel samples that were treated to isothermal heat treatments, on the other hand, increased. Steel samples that were isothermally heat-treated at 830 °C and then chilled in a salt brine solution, on the other hand, showed an increase in fatigue resistance

Fatigue design and assessment guidelines for high-frequency mechanical impact treatment applied on steel bridges

Structural design of bridges in Europe should be carried out in accordance with Eurocode regulations. However, there is no guideline demonstrating how the fatigue design is to be conducted when high-frequency mechanical impact (HFMI) is used to enhance welded joints in steel bridges. The aim of this paper is to present the different design rules and equations and apply them to some example bridges enhanced by HFMI treatment. Fatigue verification of some welded details in these bridges is carried out via either “damage accumulation” or “λ-coefficients” methods in the Eurocode. Four fatigue load models are used in the fatigue verification (FLM3 and FLM4 for road bridge assessment, and LM71 and traffic mix, for railway bridge assessment). The effect of steel grade, mean stresses, self-weight, variation in stress ratios, and maximum stress on the treatment efficiency is considered in both examples. It is found that HFMI treatment causes a significant increase in fatigue lives in all studied cases.

Remaining fatigue endurance of gusset plate connections from a dismantled railway bridge

The replacement of an existing steel railway bridge in Sweden opened up for fatigue testing connections with a history of about 70 years of in-service loading. One decisive detail in the bridge was the gusset plates welded to the flange edge of I beams. This has low fatigue resistance in the governing standards and was deemed exhausted in theoretical assessments. To estimate the remaining fatigue endurance, 20 similar samples of these were cut out from the bridge and tested. During service, they were exposed to alternating compression and tension stresses. The testing was designed to determine their category for constant amplitude fatigue loading and to evaluate the influence of compressive stresses. It is shown that the compressive stresses have an influence on the fatigue endurance and a correction function has been fitted to the results. Despite many years of loading, they had a fatigue endurances significantly greater than that estimated theoretically. A detail category of 63 was derived, which, together with the correction function, provides a significant increase in fatigue life, which should be possible to use for similar details in other existing steel structures.

The effect of multiaxial geocomposite reinforcement on fatigue performance and crack propagation delay in double-layered asphalt beams

The presented study investigates the effect of a recently developed multiaxial geocomposite made of polypropylene geogrid and non-woven fabric on the delay of crack propagation, based on four-point bending tests of large asphalt concrete beams – both for reinforced and non-reinforced specimens. Several approaches are described in this study, including analysis of stiffness modulus decrease and analysis of crack propagation using dissipated energy. The study also includes investigation of the impact of reinforcement on shear resistance of the inter-layer bond. The reinforced system displays reduced decrease in stiffness during cyclic loading; it is able to bear loads over a longer period, in contrast to the unreinforced system. An increase in fatigue life by a factor of 10–22 is possible, as compared to the unreinforced system. The factor of relative increase in the number of cycles to the first initiation of crack in lower asphalt layer when reinforcement is introduced ranges from 5 to 10.

Benchmarking of VIV numerical analysis with prototype response for fatigue assessment of inverse catenary coldwater pipelines

High density polyethylene pipe has been used to draw coldwater for low temperature thermal desalination plants since the year 2006 at Lakshadweep Islands, India. The pipeline endured in-line oscillations due to shear current in one of the desalination plants, and this gave an opportunity to observe the response parameters of the prototype pipeline. The recorded oscillation parameters are utilized as a benchmark to verify the results of the vortex induced vibration (VIV) analysis in frequency domain and time domain, and the results show good concurrence. The in-line fatigue damage assessment is carried out for a range of current profiles showing the need for mitigation of pipeline VIV. The helical strakes on a partial length of the pipeline is modeled numerically showing significant suppression of VIV response as well as substantial increase in fatigue life of the pipeline. The benchmarking of the VIV response analysis with prototype measurements enhances the reliability of using numerical VIV fatigue analysis and mitigation measures.

Repeatable self-healing of composite’s fatigue delamination

This paper investigates repeatable self-healing of composite laminates to mitigate fatigue delamination where a two-step healing mechanism is employed to heal the delamination cracks in a CFRP composite: (i) close the cracked surfaces by thermally activating dispersed shape memory polymer (SMP) filaments, and (ii) heal the closed crack by melting the thermoplastic healant (PCL) dispersed in the thermoset polymer matrix and allowing it to flow into the cracked region and solidify. Repetitive self-healing is carried out on double cantilever beam (DCB) specimens using macro fiber composite (MFC) actuators bonded to the test specimens, which generate the heat required to activate the thermoplastic healants to heal the delamination in the composite. Fatigue delamination growth rates were obtained for replicate specimens to extract statistically meaningful data on fatigue life under mode-I loading after seven healing cycles, indicating substantial retardation in delamination growth rate resulting in a significant increase in fatigue life.

Machine learning based very-high-cycle fatigue life prediction of AlSi10Mg alloy fabricated by selective laser melting

Few machine learning models are applied to investigate the influence of defect features on very-high-cycle fatigue performance of additively manufactured alloys and these models usually suffer from data scarcity. Interpolation methods are run to enlarge dataset size and machine learning models are established to investigate the synergic influence of layer thickness, stress ratio, stress amplitude, defect size, shape and location on fatigue life of selective laser melted AlSi10Mg. Results show that the increases in defect distance to surface, circularity, and layer thickness favor higher fatigue life; however, the increases in stress amplitude, stress ratio, and defect size decrease fatigue life.

Cyclic Deformation and Fatigue Failure Mechanisms of Thermoplastic Polyurethane in High Cycle Fatigue.

In this study, the main purpose is to analyze the fatigue failure of thermoplastic polyurethane (TPU) plate under tension-tension load control tests (frequency = 5 Hz, stress ratio = 0.1) and consider the change in hydrogen bond content. The results show that the S-N curve of TPU material shows a downward trend before reaching the fatigue limit (10.25 MPa), and the energy is continuously consumed during the cyclic creep process and undergoes three stages of the hard segment and the soft segment changes. The infrared spectrum study shows that the increase in fatigue life will lead to more physical crosslinking, resulting in the reduction of hydrogen bond content, and the increase in microphase separation, leading to the occurrence of fatigue fracture. In addition, the scanning electron microscope and three-dimensional confocal analysis showed that the crack originated from the aggregation of micropores on the surface of the material and was accompanied by the slip of the molecular chain, the crack propagation direction was at an angle of about 45°.

Fatigue-Damage Initiation at Process Introduced Internal Defects in Electron-Beam-Melted Ti-6Al-4V

Electron Beam Melting (EBM) is a widespread additive manufacturing technology for metallic-part fabrication; however, final products can contain microstructural defects that reduce fatigue performance. While the effects of gas and keyhole pores are well characterized, other defects, including lack of fusion and smooth facets, warrant additional investigation given their potential to significantly impact fatigue life. Therefore, such defects were intentionally induced into EBM Ti-6Al-4V, a prevalent titanium alloy, to investigate their degradation on stress-controlled fatigue life. The focus offset processing parameter was varied outside of typical manufacturing settings to generate a variety of defect types, and specimens were tested under fatigue loading, followed by surface and microstructure characterization. Fatigue damage primarily initiated at smooth facet sites or sites consisting of un-melted powder due to a lack of fusion, and an increase in both fatigue life and void content with increasing focus offset was noted. This counter-intuitive relationship is attributed to lower focus offsets producing a microstructure more prone to smooth facets, discussed in the literature as being due to lack of fusion or cleavage fracture, and this study indicates that these smooth flaws are most likely a result of lack of fusion.

Residual stress stability of HFMI-treated transverse attachments under variable amplitude loading with the P(1/3) and the linear spectrum

The potential of HFMI treatment to increase fatigue life under service loading remains in debate. However, some recent studies show that even under variable amplitude loading (VAL), fatigue strength is increased compared to untreated welds. Discussions generally focus on the stability of initial compressive residual stresses, which may be reduced during VAL due to high peak stresses. In this context, the potential of HFMI treatment is often only attributed to residual stress stability. This study presents further results on the effect of VAL with a P(1/3) and a linear load spectrum on the residual stress stability of HFMI-treated transverse stiffeners (TS) made of mild steel (S355) and high-strength steel (S700M). The impact of random, High-Low and Low-High loading sequences on the fatigue strength as well as on the residual stress behaviour of HFMI-treated joints will be discussed.

Influence of Pre-Strain on Static and Fatigue Properties of S420M Steel

This paper reports the results of static tensile and low-cycle fatigue tests on S420M steel specimens. As-received (unstrained) and pre-strained specimens were used during the tests. Based on the static tensile tests carried out, no effect of pre-strain on the basic strength parameters of the S420M steel was found. Low-cycle fatigue tests showed that the pre-strain of the specimens causes a change in the cyclic properties of the steel and a slight increase in fatigue life compared to that of the as-received specimens. The greatest increase in durability was observed at the lowest strain levels.

Study on Surface Integrity and Fatigue Properties of TC4 Titanium Alloy by Surface Ultrasonic Rolling.

In this paper, the influence of a surface ultrasonic rolling process on the surface integrity of TC4 titanium alloy and its influence on the fatigue properties were studied. By comparing and analyzing the surface roughness, microhardness, residual stress, microstructure, and fatigue fracture, the surface strengthening and modification mechanism of TC4 titanium alloy is discussed. The results show that the surface roughness of titanium alloy is observably decreased after the suitable surface ultrasonic rolling process, and the maximum Ra value can be reduced to 0.052 μm. The axial residual stress on the specimen surface can be increased to -685 MPa. The hardening rate of the surface hardness of the sample was 35%. The residual compressive stress and hardness of the sample surface increased with the increase of static pressure. However, the increase of feed rate and rational speed was less. After surface ultrasonic rolling, the sample surface exhibited obvious grain refinement, the number of high-angle boundaries increased to include the formation of nano-equiaxed grains. The fatigue strength increased by 52% from 280 MPa to 425 MPa. Under 450 MPa, the fatigue life of samples with SUR 2 was the highest, at about 7.7 times that of the original samples. The surface integrity of titanium alloy samples after surface ultrasonic rolling treatment is greatly improved, which is the reason for the significant increase in fatigue life of the samples.

Laser shock peening of laser powder bed fusion produced Ti6Al4V for potential improvements to implant performance

Titanium alloy based implants are becoming more common with medical advancements and longer global life expectancy. With its geometrical design freedom, low material wastage, and mass customisation, additive manufacturing has found growing use in biomedical applications. In this study, laser shock peening without coating (LSPwC) was investigated as a means to enhance the surface and mechanical properties of selective laser melted Ti6Al4V implants. The application of LSPwC was found to induce a favourable surface oxide layer, increase the measured surface roughness and hardness, and reverse the tensile residual stresses imparting deep, high-magnitude compressive residual stresses. The combination of oxide layer, increased roughness, and induced compressive stress will potentially improve implant osseointegration and increase fatigue life.

Study on fatigue damage and fatigue crack propagation of rubber concrete

In order to study the crack propagation process and damage mechanism of rubber concrete under fatigue loading, the fatigue test of plain concrete beam and rubber concrete beam with precast cracks was carried out, and the loading process was monitored in real time by acoustic emission. The results show that after the addition of rubber, the maximum deformation of rubber concrete under fatigue load is about twice that of plain concrete, and the fatigue life and fracture energy increase. It is confirmed by AE signals that there is hysteresis between cracks and loads under fatigue load due to the lag of energy transfer. The incorporation of rubber particles also reduces the range of changes in stress strength factors. The b-value of AE reflects the damage state and crack development of the concrete. When the b-value is reduced to 0.8, it can be used as an early warning indicator of structural failure.

Geometry Optimization for Increasing Fatigue Life of Pressing Packaged IGBT with SiC Chips and Silver Sintering Layers

A novel SiC IGBT module packaging structure is proposed. Pressing packaging with silver sintering is adopted. The thermal analysis is made with FEM. A maximum temperature of 137°C is obtained when the module is working, which leads to significant fatigue problem. The minimum fatigue life of the fundamental model is 106, and it occurs in the upper silver layer nearby the SiC. Five improved models with different silver layer edge geometry are analyzed. It turns out that the best geometry optimization in this study can lead to a fatigue life of 141, that is, 33% higher than the fundamental model. This shows that a very subtle change in the geometry of the silver layer edge can lead to significant improvement of the fatigue life of the module. This result can provide useful reference for the design the third generation IGBT device.