Fatigue Samples Research Articles

Laser Treatment Effect on Fatigue Characterizations for Steel Alloy Beam Coated with Nanoparticles

Previous work investigated the effect of nano coating for steel beams on the fatigue characterization and calculated that the coating for beams leads to modifying the fatiguelife and stress. Therefore, this work modifies the fatigue characterizations for beams by using laser treatment. The main aim of this work is to modify the fatigue properties of steel materials by using a coating surface, with nanoparticle materials, in addition to the laser treatment technique, at the same time. The experimental work was divided into three parts; first, the fabricated fatigue steel samples were coated by zinc oxide (ZnO) nanoparticle; second, treatment of the fatigue sample by laser technique using (low-high) and (high–low) loading; and finally, testing the fatigue samples to calculate the fatigue life and stress, with and without laser treatment. A scanning electron microscope (SEM) was used to study the surface morphology of the samples. In addition, the numerical technique, using the finite element, was employed to evaluate the fatigue characterizations for steel samples and the results were compared. The comparison of the numerical and experimental results shows that the maximum discrepancy did not exceed about 10.86%. Finally, the treatment of nanomaterials coated steel samples shows that the fatigue characterizations were more than 35%.

Combining Effective Plastic Strain and Surface Roughness for Predicting Future Propagation Path of Microstructure‐ Sensitive Crack (MSC) in Polycrystalline Nickel

ABSTRACTThis work introduces a damage index (DI) that combines the effective plastic strain and surface roughness changes to predict the future propagation path of a microstructure‐sensitive crack (MSC) in a polycrystalline nickel fatigue sample. The surface topography of a fatigue sample was measured at short fatigue intervals, from which both the effective plastic strain and surface roughness changes were calculated using digital image correlation (DIC). Three DIs based on the effective plastic strain, surface roughness changes, and their combinations were studied using two criteria, that is, the highest intensity and the confidence threshold. Comparing the crack path predicted by these DIs with the actual crack path acquired at later fatigue cycles, we demonstrated that the combined DI is more accurate and has less uncertainty in predicting the future crack path.

A STUDY OF THE EFFECT OF ANNEALING HEAT TREATMENT ON THE FATIGUE PROPERTIES OF 0.17%C HSLA STEELS: A STATISTICAL APPROACH

The present study investigated the effect of annealing heat treatment on the fatigue properties of 0.17%C of HSLA steels using a statistical approach. The Carbon equivalent (CE) was first calculated to determine the ideal starting point for annealing which is 840oC from the Iron Carbon phase diagram. The steel samples were then exposed to multi-régimes of annealing temperatures namely; 840 oC, 870 oC, 900oC, 930oC, 960oC and 990oC at 30oC interval they were then soaked for 30 minutes. Some of the samples were thereafter machined into fatigue samples before being subjected to fatigue test. Quantitative metallography was also carried to examine its microstructure; results showed that for the steel under study, higher annealing temperature improved the fatigue properties significantly. Highest number of cycles being recorded at 3.9 Ⅹ 103 and 1.3 X 103 at 321.3 MPa and 1606.57 MPa stress levels for samples annealed at 990 oC, this is followed closely by samples annealed at 960 oC which had 3.5 X 103 and 0.8 X 103 number of cycles at the same stress levels before failure. The control had the least number of cycles of about 0.9 X 103 and 0.1 X 103 at the same stress levels, and, standard deviation of 5.960 and a mean of 26.881 which shows that annealing greatly improved the fatigue life of the steel. The implemented software (ANOVA) test confirmed the results at 95% confidence which further showed that there was significant difference amid the annealing parameters and, it shows promising results.

Optical Microscopy on Machined Surfaces in Ti-6Al-4V Reveals Pore Statistics that Correlate with Fatigue

Abstract Optical microscopy on the machined surfaces of 4-point bend fatigue samples 3D printed in Ti-6Al-4V is used to characterize pores larger than ≈15 microns. A large enough number of distinct scan speeds and fatigue tests that span the process window provides a statistically useful dataset. The variation in stress at which runouts (to 10 million cycles) are observed is anti-correlated with the pore density and the same inverse relationship exists for the maximum pore size observed within the area exposed to maximum tensile stress in each test. Total fatigue life is also anti-correlated with maximum defect size albeit with considerable scatter.

Assessment of location- and orientation- dependent fatigue behaviour for as-deposited LPBF Inconel 718 using miniaturized specimens

The location- and orientation-dependence of tensile properties and low/high cycle fatigue behaviour of additively manufactured IN718 were determined on as-deposited material with the use of miniaturized specimens. Local properties were determined throughout the build height for two orientations by using 39 fatigue samples and 32 tensile test coupons excised from bulk builds. The low cycle fatigue behaviour is described with the use of the Manson-Coffin approach while the high cycle regime was analysed using a Wohler-based assessment. Extensive microstructure characterization included optical microscopy, electron microscopy with EDX and EBSD analyses, fractography and computer tomography in order to rationalize the fatigue anisotropy in certain regimes. Significant anisotropy in the low cycle (i.e. high strain) fatigue regime is attributed to the differences in microstructure, UTS, and ductility in the as-deposited material. The fatigue behaviour in the high cycle (i.e. low strain) regime was worse in Z-oriented samples, partly due to its somewhat lower UTS and detrimental orientation of lack-of-fusion defects that served as fatigue initiation sites. The complete axial fatigue behaviour, which evaluated the whole range from uniaxial tension (i.e. one cycle) through the low cycle to high cycle regime, was attempted with the use of the Stussi model. While the various modelling approaches that used either uniaxial tension or cyclic tension data captured some of the observations, the detrimental effects of process-induced defects and their orientation with respect to the loading direction prevented a complete description of the fatigue behaviour in the high cycle fatigue regime. Nevertheless, the use of miniaturized samples provides a unique capability to examine both the location- and orientation-dependent fatigue behaviour relevant to many engineering structures.

Influences of grain orientation on high-cycle fatigue performance of laser powder bed fused Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at room temperature and 500℃

The fatigue performance of titanium (Ti) alloys fabricated by laser powder bed fusion (LPBF) is a crucial factor influencing the structural safety in service. The high-cycle fatigue (HCF) of LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy at the room temperature and 500 °C was tested. Compared with the room-temperature samples, the fatigue strength of fatigue samples tested at 500 °C is reduced by about 55.8 % after 106 fatigue cycles. Crack initiation on the surface is mainly related to fatigue after fewer cycles under high stress, while interior cracks initiate after a longer fatigue life under low stress. Electron back scattering diffraction (EBSD) was adopted to characterize the orientations of grains on the crack propagation paths and the activated slip systems. All prismatic oriented grains were found to activate prismatic slip systems, the activation of which achieves symmetric slips surrounding crack tips, forms ductile striations, and increases energy absorption during crack propagation. These characteristics are conducive to improved fatigue performance. About half of the basal oriented grains activate prismatic slip systems. These activated prismatic slip systems not only have a high Schmid factor (SF) but also need to have a high geometrical compatibility factor with slip systems activated by adjacent grains. The residual β phase and α phase are aligned according to the Burgers orientation relationship (BOR), so the β phase does not alter the direction of crack propagation. The results were used to predict the fatigue crack propagation paths in LPBF Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during loading along building and transverse directions were predicted. This, to some extent, provides a basis for comparing fatigue performance of LPBF Ti alloys in different directions.

Recognizing and predicting muscular fatigue of biceps brachii in motion with novel fabric strain sensors based on machine learning

Localized muscle fatigue is of major kinematic and medical significance in the field of sports and rehabilitations. However, only few works reported fatigue recognition for field training, and via analysis of only the noise-susceptible electromyography. In this study, along with portable EMG sensors, a wearable smart arm band powered by fabric strain sensors was implemented to detect the thickness variation of biceps. Dumbbell curl training protocol was designed, and 32 subjects were enrolled, contributing to a database of 580 curl samples. Fatigue-related characteristics of muscle thickness were proposed according to muscle physiology and combined with that of sEMG signals, forming feature vectors of the samples. Clustering results showed that the fatigued cycles can be successfully recognized and labeled. For potential applications, fatigue prediction models based on supervised learning methods were further proposed and compared, of which the SVM-based model was observed with satisfactory overall accuracy of 83.3%, yet the fatigue samples can be predicted at much higher rate, with effective recall 90%, F1-SCORE 95% and AUC 98.7%. This work not only exploits feasibility of using latent physiological indicators such as muscle thickness in muscle fatigue monitoring, the result and methodologies will also inspire wider horizon of human-centered applications using novel flexible sensors.

Evaluation of microstructure and mechanical properties of additively manufactured Ti–5V–5Mo-5 Al-3 Cr alloy

Ti–5V–5Mo–5Al–3Cr (Ti-5553) is a near-beta titanium alloy commonly used in aerospace applications. It is currently being explored for additive manufacturing (AM), but the fatigue behavior of this AM alloy needs to be well-understood for adoption. This paper explores the tensile and high-cycle fatigue behavior of laser powder bed (LPBF) manufactured Ti-5553 that has been post-print heat treated with a beta anneal and age (BAA), with a focus on the microstructural evolution and effects of surface roughness and print defects. Fatigue lives of as-LPBF and BAA material are nearly identical and both are far below fatigue lives of conventionally manufactured Ti-5553. As-built samples contain columnar β grains with a [001] texture in the build direction with a negligible fraction of α phase confirmed by electron backscatter diffraction (EBSD). The BAA material contains HCP alpha-phase precipitates: grain boundary α, primary α, and secondary Widmanstätten α. Fatigue samples were printed in the vertical direction and tested with an as-printed and machined surface. Both the as-LPBF and BAA samples exhibit similar fatigue strengths despite their vastly different microstructures. Machined surfaces led to an increase in tensile strength, ductility, and fatigue strength. This work emphasizes the importance of heat treatment and minimization of flaws inherent to AM processing of near-beta Ti-5553.

High cycle fatigue response of grain refined EUROFER97

The present paper has investigated the high cycle fatigue behavior of EUROFER97 steel submitted to a novel process consisting of cold rolling with reduction ratio of 80 % followed by a heat treatment at 650 °C for 1 h. This process had already proved to be highly effective under static loads leading to a significant improvement in yield stress. The fatigue tests have been performed with 0.2 load ratio at room temperature. Results indicate that the fatigue strength of the steel submitted to the novel treatment is comparable to that of the standard EUROFER97. Moreover, the fatigued samples underwent a microstructural evolution consisting of grain size increase and texture change due to the stress-driven instability of grain boundaries, especially LAGBs. The collapse of some boundaries involves partial annihilation and re-arrangement of dislocations, and grain coalescence. As a consequence of such microstructural change the material softens with hardness variations up to 8 %.

Experimental optimization of CuO and MgO hybrid nanoparticle reinforcement ratios to enhance fatigue life of GFRP composites

AbstractThis study investigated the optimization of CuO and MgO nanoparticle reinforcement ratios within glass fiber‐reinforced polymer (GFRP) composites, and it examined the impact of the hybrid mixture of oxide powders on the mechanical properties of the GFRPs. The study aims to improve the mechanical properties of GFRPs by combining these two powerful nanofillers in various weight ratios to find the best combination to obtain the best fatigue life. Tensile‐tensile fatigue tests were conducted on 25 samples containing hybrid CuO and MgO nanoparticles ranging from 0.0 to 0.8 wt% in 0.2% increments. The test was conducted using a computer‐controlled servo‐hydraulic fatigue device. It was found that the GFRP composite reinforced with hybrid nanoparticles improved the fatigue life. The CuO proportion in the composites partially increased the brittleness properties, while the MgO proportion partially increased the stiffness according to the results. It was found that 0.6 wt% MgO–0.4 wt% CuO and 0.2 wt% MgO–0.6 wt% CuO GFRP composites demonstrated the highest fatigue life yielding at the 39.5k, and 13.7k cycle at 70%, and 80% load level, respectively. The best fatigue life was yielded at the 128k cycle with the 0.4 wt% MgO and 0.6 wt% CuO sample at the 60% load level. According to experimental hysteresis energy loss, 0.4 wt% MgO–0.2 wt% CuO GFRP composite was found to have the highest energy loss. The energy loss increased by approximately 51% in this composite compared to the non‐reinforced composite. The fracture surfaces of a fatigue sample were shown with the SEM image.Highlights The GFRP composites were produced by combining MgO and CuO as a hybrid reinforcement. Analyzed fatigue life using hysteresis curves and S–N graphs. The fatigue life of GFRP composites was improved with hybrid nanofiller content. As compared to separate reinforcements with CuO or MgO, the hybrid nanoparticle reinforcement enhanced fatigue life more.

Microstructure, microhardness, tensile and fatigue investigation on laser shock peened Ti6Al4V manufactured by high layer thickness directed energy deposition additive manufacturing

Thin layer thickness in additive manufacturing (AM) has always been a hindering factor leading to long completion times. Increasing layer thickness is accompanied by a higher tendency of surface cracks and inferior mechanical properties. These can be improved with laser shock peening (LSP). Emphasizing on the high layer thickness, this paper elucidates the high-layer thickness laser directed energy deposition (LDED) AM and the effects of LSP on the fabricated parts. Primary focus was given to the investigation of surface microcracks, microstructures, tensile and fatigue properties. Ti6Al4V bulk sample was prepared using the LDED process. Tensile, fatigue, and other samples were extracted to investigate the above-mentioned properties using different characterization tools such as optical microscope, scanning electron microscope, Vickers microhardness tester, and universal testing machine. Results showed proper depositions without major defects despite the high-layer thickness. Micro-cracks were limited to the surface only, and typical Widmanstätten patterns with parallel α lath and mixed α+β microstructures are observed. Yield strength (YS) varied from 730 MPa to 940 MPa, while ultimate tensile strength (UTS) ranged from 799.5 MPa to 953.75 MPa. Even though laser peening significantly improved micro-hardness by 49.81 % on average, not all fatigue samples observed enhanced fatigue life. A maximum improvement of 40.34 % in fatigue performance was observed in the LDED+LSPed samples. The strain hardening, plastic deformation, and grain refinement during the LSP process contributed to the enhanced mechanical properties. Some samples’ reduced fatigue life may be due to the anisotropic nature of additively manufactured parts, internal defects, and surface cracks on the specific samples. Considering high layer thickness, vertical orientation of samples, and as-prepared samples, our research shows a promising high layer thickness additive manufacturing process. The current research can be used as a base for further study on other properties and characteristics.

Thermal fatigue life prediction of brake disks based on coupled thermal simulation

The thermal-mechanical performance of friction pairs of the foundation brake gear is an important guarantee for the safe operation of high-speed trains. The thermal stress of brake disks under high temperature is the primary cause of thermal fatigue damage. In this article, the brake bench tests were carried out under braking speed of 350km/h and a thermo-mechanical-wear coupled model was established to simulate tests. By analyzing the results of bench test and finite element simulation, it shows the top temperature of brake disk was over 600°Cand the friction surfaces of brake friction pair show obvious wear. Combining the FE simulation and test results of the thermal fatigue sample, the thermal fatigue characteristics of brake disk material were studied. Based on the residual stress results, the propagation life of brake disk thermal fatigue crack was estimated. It shows that when the crack length increases to 10mm, the crack propagation life of brake disk is halved, and continues to decrease as the crack grows.

Skeletal muscle stiffness is altered by age and fatigue at the cellular level

Advanced age is associated with increased skeletal muscle stiffness at the cellular and whole-tissue levels. In older adults, fatigue increases falls risk. Our lab has previously observed fatigue-induced reductions in skeletal muscle cellular stiffness in younger adults, prompting the question of whether the same occurs in older adults. If so, it is possible that fatigue-induced reduction in cellular stiffness of older adults would reduce the rate of tendon loading and/or increase tissue compliance, ultimately contributing to increased falls risk. Therefore, the OBJECTIVE of the present study is to measure cellular passive and active stiffness in younger and older adults across fatigue conditions to test the hypothesis that fatigue induces tissue compliance that predisposes older adults to falls. Methods: 11 untrained males and females (7 younger, 20±1 yrs.; 4 older, 74±4 yrs.) performed unilateral knee extensions to fatigue, followed by bilateral biopsy of the Vastus Lateralis providing fatigued (F) and non-fatigued (NF) samples. Single fibers (146 younger, 130 older) were passively stretched to 156% initial length and passive modulus was calculated as the slope of the resulting stress-strain curve. Separately, Residual Force Enhancement (RFE) was measured in a subset of fibers (9 younger, 40 older) to assess passive contributions (titin) to active (actin-myosin) stretch. Main effects (age/fatigue) on passive modulus and RFE were tested via linear mixed model and t-tests, respectively. Results: Passive modulus was increased with age (p=0.04) and reduced by fatigue in both the younger (p<0.01) and older (p<0.01) participants. In a subset of fibers, RFE was increased in older women compared to younger (p<0.01) and older men (p<0.01). No main effect of fatigue on RFE was detected (p=0.49). However, RFE trended towards reduction with fatigue in a younger male (p=0.13) and was significantly reduced by fatigue in myosin heavy chain (MHC) I fibers from an older male (p=0.02). CONCLUSION: In agreement with existing literature, cellular passive modulus was higher in older versus younger adults. Additionally, fatigue reduced passive modulus in skeletal muscle fibers from both older and younger adults. Though no main effect of fatigue on RFE was detected, these results suggest that cellular RFE measures are age- and fiber-type specific. This work was supported by funds from the Wu Tsai Human Performance Alliance. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Low-cycle compression-compression fatigue behavior of MEX-printed PLA parts

Due to cyclic loads, additively manufactured (AM) parts can collapse at lower stress values than the strength corresponding to static loads. To date, there is limited research on the fatigue life of AM polymers. The present work investigates the fatigue behavior of material extrusion (MEX) printed Polylactic acid (PLA) parts. In this regard, the low-cycle fatigue (LCF) domain was studied and a compression-compression fatigue test setup was adopted. Initially, before the experimental tests, the MEX-printed samples were subjected to physical assessments. The printed fatigue samples presented a high accuracy with dimensional relative errors below 0.04%, while 6.66% was noted as their mass relative error. Subsequently, quasi-static and cyclic tests were performed in order to determine the mechanical behavior of the AM parts, with special focus on the low-cycle domain of the S-N (or Wöhler) curve. The quasi-static compression behavior of the PLA samples is typical of cellular materials, the samples highlighting a shear band as a failure mechanism. Five load levels were used to plot the low-cycle domain of the S-N (or Wöhler) curve with its 95% level confidence bands. The tests were performed in force-controlled mode with the stress asymmetry ratio R = 0.1 and at the test frequency of f = 10 Hz. An exponential relationship was also found between the strain at break and the number of cycles until failure. The failure of the samples in fatigue load occurred suddenly.

Low Cycle Fatigue and Damage Evaluation on 16MnCr5 Alloy Steel

The 16MnCr5 alloy steel samples are evaluated in a normalized condition and put through low cycle fatigue (LCF) under constant amplitude strains of εa = 0.2%, 0.25%, 0.4%, and 0.6%. Mechanical properties are reported based on the LCF results. Fatigue damage is checked in specimens that have reached 80% of their nominal fatigue life by two methods that are different from traditional electronic microscopy: measuring the roughness of the surface and looking at it through full‐field optical microscopy. In addition, the electric resistivity is determined for the test specimens. It presents an increment from the blank value of 6.5 × 10−8 Ωm (free of fatigue damage) to 1.25 × 10−7 Ωm for the fatigue test specimen (εa = 0.6%). Finally, X‐ray diffraction patterns are determined for analyzing the residual equivalent strain at the microscale for the fatigued samples. The residual equivalent strain exhibits a trend toward increasing. For instance, it increases from 18.5 (blank value) to 34.0 με for the fatigue test specimen with εa = 0.6%.

Defect tolerance and fatigue limit prediction for laser powder bed fusion Ti6Al4V

This study is focused on the individual effect of gas porosity size and location on fatigue strength of a laser powder bed fusion Ti6Al4V alloy. Tensile and fatigue samples were manufactured by artificially seeding gas pores either at the centre or close to surface. Seeded porosity reduced the ductility by 20% and fatigue strength by 33% but had little influence on the static strength. Despite seeded pores, 50% fatigue samples had crack initiation from process inherent defects that also caused fatigue data scatter. A modified Kitagawa-Takahashi diagram was established to predict the fatigue strength under the influence of porosity defects.

3D characterization of closure of surface and internal fatigue cracks in nodular cast iron using Digital Volume Correlation of laboratory X-ray tomography images

Digital volume correlation (DVC) of laboratory X-ray tomography images has been used to characterize the closure of artificial internal fatigue cracks in nodular graphite cast iron. DVC allows to produce crack opening displacement (COD) map from 3D tomography images at the sub-voxel scale thanks to the graphite nodules which can be used as internal natural markers. Fatigue samples containing controlled internal defects have been produced in a nodular cast iron by diffusion bonding plates containing laser-machined sharp notches. Specimens with internal cracks and surface cracks were respectively characterized by laboratory tomography at different loads during fatigue crack propagation tests. The local stress intensity factors (SIFs) have been extracted from the measured displacement fields by using Williams series, based on those results the closure levels of internal cracks and surface cracks were comparatively analyzed and discussed. Crack closure levels were found to be linearly related to the location of the surface crack front, and the level of internal crack closure was comparable to that of the surface crack in bulk.

High- and low-cycle-fatigue properties of additively manufactured Inconel 625

In the last years, additive manufacturing has become a widespread technology which enables lightweight-design based on topological optimization. Therefore, generation of lattice structures with complex geometries and small thicknesses is allowed. However, a complete metallurgical and mechanical characterization of these materials is crucial for their effective adoption as alternative to conventionally manufactured alloys. Industrial applications require good corrosion resistance and mechanical strength to provide sufficient reliability and structural integrity. Particularly, fatigue behavior becomes a crucial factor since presence of poor surface finishing can decrease fatigue limits significantly. In this work, both the low-cycle-fatigue and high-cycle-fatigue behaviors of Inconel 625, manufactured by Selective Laser Melting, were investigated. Fatigue samples were designed to characterize small parts and tested in the as-built condition since reticular structures are usually adopted without any finishing operation. Microstructural features were studied by light-optical microscopy and scanning-electron microscopy. Finally, fatigue failures were deeply investigated considering fracture mechanics principles with the Kitagawa–Takahashi diagram.

Ultrasonic fatigue testing of welds made of structural steels S355JR and S275JR

There is a limited data on VHCF for structural steels welds for over 10 million cycles. The purpose of this research is a fatigue performance comparison of the welds made of steels S355JR+AR and S275JR+AR. The goal of reaching gigacycle fatigue domain is achieved using the ultrasonic fatigue testing at 20kHz. Fatigue samples are prepared to investigate the influence of two surface conditions – polished and pre-corroded. Fatigue failures are driven primarily by the welding porosity with the fatigue life duration dependent on the size and location of pores. Visual comparison of fatigue data for steels poses a challenge, because of the massive scatter of experimental data points. Therefore, a statistical approach is used with a Fatigue Performance Parameter applied to the fatigue data for welded samples to quantify the quality of the material.

Effect of chemical milling and anodizing on the high cycle fatigue behaviour of 2024-T3 aluminium alloy

Aluminium 2024-T3 alloy, widely employed in transport aircraft wing and fuselage skin structures, necessitates thickness reduction for weight optimization. Chemical milling is utilized for this purpose; however, it removes Cladding (as a standard corrosion protection layer), prompting subsequent anodizing and sealing to provide corrosion protection. Literature indicates that anodizing or chemical milling can significantly reduce fatigue life, and limited data have been available on the life of chemically milled anodized materials. In this study, constant amplitude stress-life (S-N) testing was performed on two sets of fatigue samples: Cladded Aluminium Alloy (CAA) 2024-T3 sheet specimens (1.6 mm thick) and chemically milled, anodized and sealed (CMAS-AA) Aluminum Alloy sheet specimens (1 mm thick). S-N curves were generated at R=0.1 and room temperature under stress control. SEM micrographs showed micropores in CMAS-AA and a standard plain surface in CAA, consistent with prior findings. Both types exhibited 3-5 times shorter fatigue life than electropolished samples, as reported in MIL Handbook HDBK-5J, highlighting the importance of corrosion protection techniques for structural integrity. Residual stresses were observed, with cladded samples displaying compressive and CMAS-AA exhibiting tensile stresses. Fractography revealed no abnormal fatigue fracture features, and computed plane stress fracture toughness values estimated using measured crack size from the failed specimens matched standard values of aluminium alloys.