Fatigue Behavior Research Articles

The Influence of Shot Peening Media on Surface Properties and Fatigue Behaviour of Aluminium Alloy 6082 T6

Shot peening is generally used to improve the fatigue performance of mechanical components. However, identifying the geometrical and mechanical characteristics of the shots that improve fatigue strength is still a challenging task, as there are many variables involved in the shot peening process. The present work addresses the effect of different shot media on the fatigue behaviour of an aluminium alloy 6082 T6. Four different shot types were used: silica microspheres, alumina shots, aluminium cut wire and zinc cut wire. Axial fatigue tests were carried out to obtain the Wöhler curves corresponding to each shot peening treatment. The surface properties of the shot-peened specimens, such as grain size, hardness, residual stress and roughness were measured to determine their effect on the fatigue results. The fatigue results revealed that silica and zinc shots increased significantly the fatigue life of the alloy, whereas alumina and aluminium shots reduced its fatigue strength. Almen intensities have shown to correlate well with grain refinement and strain hardening. However, better fatigue results were obtained with the shots that generated higher surface compressive residual stresses. It is believed that small and smooth shots are preferable to sharp and irregular ones, regardless of the Almen intensity or surface hardness attained with the latter.

Experimental behaviour and fatigue life prediction of bonded joints subjected to variable amplitude fatigue

The variable amplitude fatigue behaviour of adhesive joints has not been extensively considered in literature and considerable knowledge gaps regarding its underlying mechanisms still exist, hindering accurate life prediction. This study investigates the effect of loading sequence and number of load transitions on single lap joints (SLJs) bonded with a toughened epoxy adhesive. Life prediction is also conducted using established cumulative rules: Palmgren–Miner (PM), Broutman and Sahu (BS) and, Hashin and Rotem (HR). To overcome their limitations, a modified Cortan–Dolan approach is proposed. For this, SLJs were subjected to single transition (low-to-high and high-to-low load transitions at different stages of the fatigue life and with different magnitudes) and multi-block loading spectra with different numbers of transitions.Results showed an extension of the fatigue life for high-to-low transitions. This was attributed to a combination of load interaction effects, as evidenced by an elastoplastic numerical simulation, and non-linear damage accumulation. Conversely, low-to-high transitions exhibited a damaging effect. For single transition spectra, BS and HR models were seen to improve prediction over PM by indirectly considering these effects. The proposed approach further improved this prediction. Multi-block results showed no significant influence of the number of load transitions and were satisfactorily predicted using PM.

Fatigue life assessment of thermite welded rails based on laboratory tests and field measurement data

This in-depth study assesses the fatigue life of thermite-welded 60E1 rails by combining fatigue experimental tests with field measurement data. Material characterization, conducted using X-ray Diffraction (XRD), X-ray Fluorescence (XRF), and Scanning Electron Microscopy (SEM), aimed to understand the fatigue behavior of these materials. The research was particularly focused on high-speed rail lines in South Korea, targeting areas with influence of track condition to collect strain data. By combining dynamic modulus calculations from high-speed tensile tests with the rain flow counting method for determining wheel loads, and employing the Linear Cumulative Damage method for estimating rail fatigue life, this study provides novel insights into the maintenance and durability of thermite welded rails. The integration of detailed laboratory experiments with field stress data offers a significant contribution to the predictive maintenance and lifecycle management of high-speed railway infrastructure.

Fatigue assessment of directionally solidified superalloy thin plates under bimodal random processes

During actual service, aviation aircraft often experience multiple complex alternating loads coupled with each other. If the frequency range of the excitation load spectrum covers the first two resonance frequencies of the structure, it can significantly impact the vibration fatigue life of the structure. In this study, bimodal random vibration fatigue tests were conducted on thin plates composed of DZ125L directionally solidified superalloy. The impact of low-frequency and high-frequency vibration signal intensities on the vibration fatigue behavior of DZ125L alloy thin plates was investigated separately during bimodal random processes. The crack propagation mechanism of bimodal random vibration fatigue was proposed based on the fracture morphology observed in DZ125L alloy thin plates that failed due to vibration fatigue. The study findings indicate that the number and location of crack initiation zones can influence the mechanism of fatigue crack propagation. Additionally, a new model was developed in this study to enhance the sensitivity of the frequency domain method to the intensity of low-frequency and high-frequency vibration signals in bimodal random processes. Compared to traditional frequency domain methods used for broadband and bimodal processes, the new model can effectively describe the variation characteristics of bimodal fatigue life with different component process intensities. Furthermore, it has broader applicability for predicting the vibration fatigue life of bimodal random processes with varying vibration signal intensities.

Investigating mechanical properties of 3D printed polylactic acid / poly-3-hydroxybutyrate composites. Compressive and fatigue performance

The 3D printing technique known as Material Extrusion (MEX) was initially employed for prototyping, but it has evolved to fit applications in mechanical and biomedical industries. Polylactic acid (PLA) stands out as a commonly used polymer for manufacture pieces by MEX, due to its good properties and organic origins. Pursuing renewable and biodegradable thermoplastics has led to the development materials such as composite of PLA with wood fibers and blends with poly-3-hydroxybutyrate (PHB). This study aims to characterize the effect of the most relevant printing parameters on the mechanical properties of a PLA/PHB blend, motivated by the interest to facilitate the use of this type of materials in industrial applications. To achieve it, compressive and fatigue tests were carried out, comparing the results with those obtained in previous studies for pure PLA and PLA-wood composite. Results show that the compressive behavior of PLA/PHB is influenced by the layer height, nozzle diameter and fill density. Its fatigue behavior is mainly determined by the nozzle diameter and the fill density. Moreover, the mechanical performance of PLA/PHB (Young's Modulus of 1.67 GPa, yield Strength of 33.8 MPa and maximum fatigue life of 9711 cycles) is inferior compared to pure PLA and PLA-wood composite. Despite the increase in the biodegradability that PHB introduces into PLA, the findings of this study reveal that there is statistically evidence that it can also hinder the mechanical performance of the base material.

Impact of biological environment on bending fatigue lifetime in additive-manufactured polylactic acid fabricated by 3D-printing

Polylactic acid (PLA) has become desirable for biomedical applications, particularly implantable devices. However, the degradation of PLA in biological environments under mechanical stress remains incompletely understood and requires further investigation. This study compared the plain fatigue (PF) and the biodegraded fatigue (BDF) behavior of 3D-printed PLA. For this purpose, two sets of standard fatigue specimens were additively manufactured by the fused filament fabrication (FFF) method. One set was used for plain fatigue testing, and the other was immersed for 330 days in simulated body fluid (SBF). After immersion, the samples were dried and weighed before fatigue testing. The fully reversed rotary bending fatigue tests were conducted on both sets of specimens, and the stress-lifetime (S-N) curves were obtained. Additionally, the fatigue properties of PF and BDF specimens were evaluated. Moreover, the fracture behaviors of the materials were studied using field emission scanning electron microscopy (FESEM). The outcomes implied that the weight of the samples extended during the immersion period, primarily due to water absorption by the PLA. However, after drying, the final weights did not change compared to the weights before immersion. The SBF immersion significantly reduced the fatigue performance of the biodegraded samples comparing the PF result.

Design and implementation of a driving safety assistant system based on driver behavior

<p>These days, road accidents are one of Morocco's biggest problems. Fatigue, drowsiness, and driver behavior are among the primary causes.This research aims to develop an embedded system by image processing and computer vision to ensure driving safety by monitoring driver behavior and assist drivers to awaken from micro-sleep or fatigue due to long driving hours and various other reasons. Indeed, the driver inattention, drowsiness or driver fatigue can be detected. The suggested method is designed to support drivers if needed, based on the vehicle velocity. Once the driver crosses a certain speed limit, the program starts face detection and analyzing this data to determine whether the driver is tired, sleepy, or inattentive. This activates different alarm depending on the criticality level. It can sound a voice alert to help him wake up and drive more cautiously. The system is based on AI algorithms in image processing based on OpenCV libraries and the Python language to capture the movements of the driver's eyes and head when starting the automobile. Every algorithm is run on a Raspberry-Pi 4 card, and numerous experimentation series have demonstrated overall credible performance with success accuracy of over 93% in EAR and MAR calculations.</p>

Corrosion fatigue cracking behaviors of Q690qE high‐strength bridge steel as a weld joint in simulated marine environment

AbstractCorrosion fatigue behavior and mechanism of the weld joint of Q690qE high‐strength bridge steel was investigated in a simulated marine environment. It reveals the sub‐critical heat‐affected zone (SCHAZ) and coarse‐grained heat‐affected zone (CGHAZ) are the most vulnerable sites to corrosion fatigue cracking. The CGHAZ of the Q690qE steel weld joint was prone to corrosion fatigue initiation mainly because of micro‐galvanic corrosion between CGHAZ and weld metal (WM). The corrosion fatigue failure in SCHAZ mainly resulted from local stress/strain concentration due to weld softening. The final fracture location was determined by the competition between these two effects.

Effect of microstructure rafting on deformation behaviour and crack mechanism during high-temperature low-cycle fatigue of a Ni-based single crystal superalloy

The low cycle fatigue behaviours of a microstructure rafting Ni-based single crystal superalloy have been experimentally investigated at 980 ℃. Deformation of γ/γ’ phases and the corresponding dislocation configurations were investigated, highlighting rafting γ/γ’ morphology that contributes to crack initiation and propagation, as well as macro-scale accumulated plastic strain. Unlike the discrete slip lines of a virgin superalloy, intense slips developed along the parallel {111} slip plane result in crossed slip bands in the rafting superalloy. The decreased resistance of widened γ channels to dislocation movement, along with the prevention of dislocation cutting through γ’ precipitates in pre-existing dense dislocation networks, facilitates crack propagation in the γ channel in the slightly rafting superalloy. As the rafting state increases, the dislocation network loses its protective effect by reducing coherency stress and acting as a superdislocation source, which facilitates crack propagation along the γ/γ’ interface. Finally, a microstructure-based fatigue model is developed considering the reduction of deformation resistance induced by rafting. The fatigue loading control mode effect is introduced by a combination of resolved shear stress and tensile stress effects on crack initiation. The LCF life of rafting Ni-based superalloys significantly decreases under stress-controlled conditions compared to strain-controlled conditions due to the increase in cumulative plastic strain. However, the insignificant impact of the initial surface oxide layer on LCF life is revealed.

Stochastic or deterministic: Duality of fatigue behaviour of 3D-printed meta-biomaterials

The two deformation modes of meta-biomaterials during cyclic loading have been revealed: stochastic and deterministic strut failure processes. Biomimetic Voronoi structures with a range of strut thicknesses and number of cells per unit volume are printed. We show that when the strut thickness is 200 μm or above, the fatigue fracture process of the lattice is deterministic and the fatigue scatters are below 15%. As the strut is thinned to 150 μm, the local failures occur randomly within the structure, which may lead to a high fatigue scatter (>30%). The two distinct behaviours result from the processing limit of the laser powder bed fusion technique. We demonstrate that the fatigue scatter and the location of the failure process within the lattice are related to the probability that a cluster of unconnected struts larger than a critical value can exist within the lattice. Unlike solid parts, porosity hardly triggers any damage in metallic lattices during cyclic deformation. The discovery of the Janus-like failure process opens up our understanding of meta-biomaterials and defines the pathway towards the design of mechanically durable intricate implants.

Study of the fatigue fractography of dual phase low carbon steel used in automotive industry

Fatigue properties play a crucial role as they are vital to ensuring the durability and integrity of components subjected to repeated loading conditions over long periods. The main objective of this work is to investigate the fatigue behavior of dual-phase low-carbon steels used in automotive applications using a rotating bending fatigue machine. Heat treatments were carried out to analyze the microstructure's effect on the fatigue properties, including quenching low-carbon steel samples at 800 °C and 900 °C. Hardness and tensile tests were performed, and the microstructure was inspected to examine the constitute phases. With the assistance of a scanning electron microscope, fractographic analyses were carried out to reveal the fracture features of the samples at different lifetime ranges. The results show that various failure mechanisms occur depending on the stress levels. Additionally, the specimens quenched at 900 °C exhibited higher fatigue strength.

Advancement of constant and progressive load multi‐cycle indentation method on surface properties characterization of polymers

AbstractIn recent years, polymers have been popular in industrial applications due to their lightweight, corrosion‐resistant, improved surface polish, ease of manufacturing, cost‐effectiveness, and so forth. Similarly, micro/nano‐indentation has gained popularity as a technique for assessing the surface mechanical characteristics of polymers. The present study conducted comprehensive experiments using cyclic micro‐indentation on engineering polymers, specifically poly‐ether‐ether‐ketone (PEEK), poly(methyl methacrylate) (PMMA), and poly(tetra‐fluoroethylene) (PTFE). An appropriate and optimal indentation method has been proposed after analyzing the behavior and significance of all the input parameters in evaluating the properties. Both constant load multi‐cycle (CLMC) and progressive load multi‐cycle (PLMC) were considered for this investigation. A comparative evaluation has been conducted to assess two multi‐cycle tests on these materials. From the analysis of the input parameters, including maximum loads, loading and unloading rates, and the number of cycles, the unloading rate and indentation cycle are crucial factors in determining hardness (H) and elastic modulus (E). Increasing the loading rates leads to an increase in H and a reduction in E for all three materials. This effect arises from the thermal effect, which is characterized by the creep modulus and a closed hysteresis loop. Employing the holding duration and multiple cycle data in constant load multi‐cycle can significantly influence the creep behavior and use of the hysteresis loop for fatigue behavior. Similarly, a progressive load multi‐cycle indentation with a force greater than 0.5 N and a minimum of five cycles is the most accurate approach for evaluating surface mechanical parameters.

The effects of fiber load and frictional work on fatigue behavior of water hydraulic artificial muscles in underwater environment

Water Hydraulic Artificial Muscles (WHAMs) exhibit characteristics of high pressure and substantial load capacity. In previous work, the fatigue life of WHAMs under elastic load has been investigated, and experimental results have indicated that the fatigue life is related to both fiber load and frictional work. To individually investigate the impact of fiber load and frictional work on the fatigue life of WHAMs, an experimental scheme was designed to test the fatigue life under different levels of fiber load and frictional work. Meanwhile, the working pressure and contraction range were determined for various working conditions, in order to assess their effects on the fatigue life of WHAMs. The W-test confirms that the fatigue life of WHAMs under each working condition follows a normal distribution. Based on the experimental results, the influence of fiber load and frictional work on fatigue life was analyzed. And the experimental results indicate that the impact of fiber load on the fatigue life of WHAMs is approximately three times greater than that of frictional work. Furthermore, substituting aramid 1414 fiber with aramid 3 fiber in the fiber sleeve led to a substantial increase in fatigue life, elevating the average fatigue life to more than 4 times. Based on the power-law model, a fatigue model of WHAM is established with fiber load and frictional work as factors. The relative error between the fatigue model and experimental results in previously published work demonstrates that the new fatigue model can accurately predict the fatigue life of WHAMs.

Low cycle fatigue behavior and failure mechanism of 34CrNiMo6 steel used as connecting rod for diesel engine

Abstract In this paper, for the 34CrNiMo6 steel material used in diesel engine connecting rods, the low-cycle fatigue properties and failure mechanism were studied. The metallographic structure of 34CrNiMo6 has been investigated by using optical microscopes, scanning electron microscopes, and transmission electron microscopes. Tensile tests and low-cycle fatigue experiments were performed at different levels of strain amplitudes, and the microstructural changes were monitored. The examination indicates that the structure of 34CrNiMo6 steel consisted primarily of a ferrite base and small spherical and rod-shaped carbides, with a mean particle size measuring 2 μm. It had exceptional mechanical characteristics of high strength and moderate plasticity. The total strain amplitude (TSA) of 34CrNiMo6 steel affected the cycling behavior, which was stable at low TSA (0.2% ~ 0.3%). At high TSA (0.4% ~ 0.7%), a large number of dislocations were annihilated through the dynamic recovery process, showing a typical cyclic softening phenomenon. The low-cycle fatigue failure behavior was mainly a combined mode of quasi-cleavage fracture and cross-grain fracture.

A physics-informed deep learning approach for combined cycle fatigue life prediction

The prediction of the combined high and low cycle fatigue (CCF) life of welded components was valuable for the reliability of practical engineering structures. In this study, a physics-informed Grey-deep convolutional neural networks (DCNN) model was constructed and the model parameters was determined based on the fatigue behavior characteristics of CCF, which was proved perform well in the CCF life prediction of welded components. The predicted results indicated that the proposed model can realize reliable CCF life prediction with good accuracy (MAE = 0.49) and stability (RMSE = 0.44). Further, the proposed model also exhibited good prediction performance under different loading and geometry conditions, indicating the good universality of the proposed model. And the good universality represented that the proposed model had the potential to be applied in more engineering fields.

A comparative study of self pierceing riveting and mechanical clinch of DP590-Al5754 high-strength steel and aluminum alloys

This paper presents a comparative study of two joining processes, self piercing riveting (SPR) and mechanical clinch, used to join DP590 high-strength steel and Al5754 aluminum alloy. A comparative study of microhardness, tensile shear properties, fatigue properties, and economics of joints from both processes was conducted. The findings indicate that the highest hardness levels are observed in the rivet leg region for SPR joints and in the neck region for mechanical clinch joints. The maximum failure load and energy absorption values of SPR joints were found to be 1.46 and 9.07 times higher, respectively, than those of mechanical clinch joints. The static tensile test results demonstrate that the strength and hardness of SPR joints in aluminum sheets subjected to cold work hardening are significantly lower than those of steel sheets. Therefore, the failure of SPR joints is primarily attributed to the deformation and fracture of the lower aluminum sheet. Mechanical clinch joints fail by neck fracture because the steel sheet neck becomes brittle by cold work hardening and the neck thickness is small. The fatigue behavior of SPR joints was observed to be superior to that of mechanical clinch joints. The findings of the scanning electron microscopy and energy spectrum analysis suggest that fretting wear between the upper and lower sheets results in fatigue fracture of the lower sheet in the joints. When joining the same material on a large scale, the selection of mechanical clinch over SPR can prove an effective method of reducing costs. Furthermore, the selection of a set of dies with an extended life can contribute to the reduction of joining costs.

Experimental investigation and computational modelling of 304LN stainless steel under constant and variable strain path multiaxial loading conditions

This study investigates the fatigue behavior of 304LN stainless steel under constant and variable strain path multiaxial loading conditions. Experimental methods and computational modelling were used to analyze the response of the material to different loading scenarios. Experimental investigations were conducted to gather empirical data, which were subsequently validated using a modified Ohno-Wang and Tanaka cyclic plasticity framework. Two models were used for the analysis: the MOT model and a modified model that incorporated a weighted function and fractional functions for hardening and softening. In contrast, the modified model offers significant improvements, accurately capturing primary hardening and secondary softening across all loading scenarios. Furthermore, the modified model effectively simulates transient behavior, including load alteration and recovery effects, due to the integration of load history memory regulating functions. Overall, this study emphasizes the importance of considering the complexities of cyclic loading and the sensitivity of the material behavior to the loading history.

Experimental study on the fatigue behavior of CFRP-strengthened cracked steel plates under eccentric axial tension

In this paper, an experimental investigation has been conducted to study the fatigue behavior of single edge-notch steel plates strengthened with CFRP (laminates/sheets). The plates have been subjected to eccentric fatigue under axial loading conditions. The experimental study includes fourteen specimens and has been tested under axial loading until failure. The constant parameters in the study are steel material grade, steel plate dimensions, and fatigue loading conditions. Meanwhile, the studied parameters are steel plate initial edge crack size, CFRP configurations (length, one or two-sided, and laminates or sheets), and adhesive material strength. The experimental results revealed that the fatigue life for specimens strengthened with CFRP sheets or CFRP laminates can be improved by a factor of 1.15 to 3.87 and 1.47 to 6.32, respectively, compared with the un-strengthened specimens. The results show that using CFRP can be considered an efficient way to repair the initial cracks in steel elements under eccentric axial fatigue loading.

Multiaxial fatigue life assessment of dental implants

As screwed joints, dental restorations may suffer mechanical failures such as screw loosening and implant or prosthetic screw failure due to fatigue. This work is focused on the failure of the implant and develops a numerical methodology to predict its fatigue life under cyclic loading conditions. This methodology is based on the combination of Critical Plane Methods and the Theory of Critical Distances to account for stress multiaxiality and notch effects. The obtained predictions were validated experimentally, which can be used to identify the main geometrical, assembly and operational factors affecting the fatigue behavior of dental implants. As a result, a powerful and efficient design tool for fatigue life prediction of dental implants is presented. This methodology complements a previously presented one focused on the fatigue life prediction of the prosthetic screw, thereby, offering now a complete design tool package regardless the critical component of the dental restoration, predicting accurately the fatigue response of the restoration, with no need for long-term fatigue test campaigns. This is a pioneering work since no other fatigue design methodology for dental implants with such a solid foundation and experimental validation has been published to date.

Flexural behavior of corroded RC beams repaired with high performance cementitious mortar under cyclic loading

AbstractThe understanding of the cyclic performance of reinforced concrete (RC) elements is of vital importance in relation to the extent of the service life of buildings and infrastructures. Steel rebar corrosion plays a major role in this regard because it significantly affects the overall structural integrity, especially under cyclic loads, leading to reduced stiffness and load‐bearing capacity of structural elements. Cyclic condition has the potential to accelerate the corrosion‐induced cracking and spalling, the effectiveness of the bond strength between rebar and concrete, and also the ductility and energy dissipation characteristics of the structure. The primary objective of this study is to investigate the effectiveness of a high‐performance thixotropic repairing cementitious mortar in improving the fatigue behavior of RC elements through a multiscale experimental approach. First, at the material scale of concrete specimens, two different concrete classes together with the repairing one‐component, pre‐blended, thixotropic cementitious mortar, were tested under incremental cyclic condition. Based on the results obtained from material scale, four reinforced concrete beams were exposed to different levels of accelerated corrosion by means of the impressed current technique and, subsequently, repaired by bonding a layer of the thixotropic high‐performance mortar onto the tension side. Finally, beams were tested under incremental cyclic four‐point bending test to investigate the fatigue behavior in terms of crack onset, propagation and energy dissipation. The resulting cyclic properties and cracking behavior of the structural elements were related to the level of corrosion achieved through the accelerated test and the effectiveness of the structural repair mortar was proven. In terms of code compliance, the repairing mortar was able to fulfill the requirements of frequent and quasi‐permanent combination of loads, remaining below all the threshold values provided by the Italian NTC2018 and Eurocode.