Interrupted Fatigue Tests Research Articles

Self-Heating and Fatigue Assessment of Laser Powder Bed Fusion NiTi Alloy with High Cycle Fatigue Mechanisms Identification

Rapid methods for assessing the fatigue properties of materials have been developed, among which the self-heating method stands out as particularly promising. This approach analyzes the thermal signal of the specimen when subjected to cyclic loading. In this research, the self-heating method was utilized for the first time with laser powder bed fusion (LPBF) of NiTi alloys, examining two specific loading conditions: loading ratios of 0.1 and 10. A thorough examination of the material self-heating behavior was conducted. For comparative purposes, conventional fatigue tests were also conducted, alongside interrupted fatigue tests designed to highlight the underlying mechanisms involved in high cycle fatigue and potentially self-heating behavior. The investigation revealed several key mechanisms at play, including intra-grain misorientation, the emergence and growth of persistent slip bands, and the formation of stress-induced martensite. These findings not only deepen our understanding of the fatigue behavior of LPBF NiTi alloys but also highlight the self-heating method potential as a tool for studying material fatigue.

Random vibration-based progressive fatigue damage monitoring in thermoplastic coupons: a preliminary investigation

In recent years there has been an increasing trend towards the utilization of composites, particularly thermoplastics, in various components of aerostructures. The prelusion of such materials has underscored the significance of investigating their fatigue behavior and developing reliable methods for detecting fatigue damage. In this context, vibration-based techniques hold significant potential as they leverage the inherent excitation provided by in-flight noise and turbulence. This study aims at assessing the progressively accumulated fatigue damage in thermoplastic coupons via random vibration signals while accounting for operational and inter-structural uncertainties. The experimental process consists of preliminary tension and fatigue tests, interrupted fatigue tests, C-Scan inspection tests, and non-contact random vibration tests. Consecutive fatigue states are obtained by performing fatigue tests at intervals of 10 000 cycles for a population of 7 coupons. At each interruption, ultrasonic C-Scan and vibration inspection tests are performed, allowing for the visualization of fatigue damage and random vibration signal analysis. Welch Power Spectral Density estimates are employed and are shown to have good potential for distinguishing among different fatigue states despite the inevitable population and experimental uncertainty. Furthermore, fatigue damage is found to progress symmetrically and laterally along the free edges of the test coupons, which is explained by the free edge effect.

Fatigue Behaviour of Medium Carbon Steel Assessed by the Barkhausen Noise Method

Abstract In this paper, an attempt to estimate the stage of the fatigue process using the Barkhausen noise method is studied. First, microstructural and static tensile tests were carried out and, subsequently, fatigue tests up to failure were conducted. After determination of the material behaviour in the assumed static and dynamic conditions, the interrupted fatigue tests were performed. Each specimen was stressed up to a different number of cycles corresponding to 10%, 30%, 50%, 70% and 90% of fatigue lifetime for the loading conditions considered. In the next step of the experimental programme, the specimens were subjected to the Barkhausen magnetic noise measurements. Various magnetic parameters coming from the rms Barkhausen noise envelopes were determined. The linear relationship betweenthe full-width at half-maximum (FWHM) of the Barkhausen noise envelope and the number of loading cycles to fracture was found. Specimens loaded up to a certain number of cycles were also subjected to a tensile test to assess an influence of fatigue on the fracture features.

Impact of interlamellar spacing and non-pearlitic features on mechanical properties and cyclic damage initiation in near-eutectoid pearlitic steels

The relationship between microstructural characteristics and mechanical properties was investigated for C72 and C82 pearlitic steel rods patented at temperatures between 500 °C and 600 °C using a lead bath. Results revealed a refinement of interlamellar spacing upon reduction of patenting temperature. At the same time, the fraction of grain-boundary ferrite and Widmanstätten ferrite as common non-pearlitic regions was slightly increased, reaching a maximum of nearly 1.3 vol.-% in the case of C72 steel patented at 500 °C. Reduction of patenting temperature was associated with strengthening, as confirmed by tensile, fatigue and hardness tests. The strengthening was explained in terms of the Hall-Petch effect for ferrite due to increase in the boundary area. The Hall-Petch coefficient, obtained by taking the apparent interlamellar spacing as the mean free path of dislocations, was in the range 70–140 MPa.μm12. To some extent, these low values could be related to the low solute interstitial content of pearlitic ferrite. More importantly, the coefficients are underestimated due to the plate-shaped morphology of pearlitic ferrite, which causes the average mean free path of dislocations be longer than the apparent interlamellar spacing. Scanning electron microscopy examinations after interrupted fatigue tests indicated the preferred occurrence of intrusions and extrusions, as precursors to fatigue crack, in the non-pearlitic ferrite. The enhancement of fatigue limit at lower patenting temperatures—in spite of slight increase in the fraction of ferrite—is justified by the stress shielding effect of pearlite and necessity of crack propagation into pearlitic regions for stable crack propagation. In effect, as long as the fraction of non-pearlitic features remains low, the interlamellar spacing of pearlite serves as the most efficient strength-controlling microstructural parameter. Furthermore, due to the preferred growth of fatigue cracks parallel to lamellae and deflection at colony boundaries, the refinement of colonies at reduced patenting temperatures is an additional contribution to the fatigue strength.

Improvement of stress corrosion cracking resistance by low cycle fatigue of a CrNiMoV steel

Low cycle fatigue often interacts with stress corrosion cracking behavior. In this work, the effect of low cycle fatigue on stress corrosion cracking susceptibility of a CrNiMoV steel was investigated by interrupted fatigue tests in air and slow strain rate tensile tests in 3.5 wt% NaCl solution at 180 °C. Microstructure evolution characterization verified cyclic softening and was found to improve stress corrosion cracking resistance with crack deflection along lath martensites due to a decrease of residual stress, phase transformation and increase of low-energy structures. As a semi-quantitative damage indicator, the microstructure evolution due to low cycle fatigue damage can be well described by the Kernel Average Misorientation (KAM) from the Electron Backscatter Diffraction technique. The effect of fatigue damage on stress corrosion sensitivity was found predictable based on the linear relationship between KAM and stress corrosion sensitivity factor of reduction of area.

Influence of the C Content on the Fatigue Crack Initiation and Short Crack Behavior of Cu Alloyed Steels

The mechanical properties of Cu alloyed steels are influenced significantly by the Cu content and the respective state of Cu precipitations as well as the C content. In this context, the effect of an increased C content on the fatigue crack initiation and growth of differently aged Cu alloyed steels with 0.005 (X0.5CuNi2-2: X0.5) and 0.21 wt.-% C (X21CuNi2-2: X21) was investigated in this study. Notched specimens were examined via SEM in interrupted fatigue tests to detect the location of crack initiation and growth. The results showed that fatigue crack initiation and growth occurred for both steels at grain boundaries, and within ferrite grains. However, a higher C content increased the incidence of crack initiation and growth at grain boundaries. This is caused by the smaller grains of X21 and especially by the presence of cementite on the grain boundaries. This explains why, in contrast to X0.5, no influence of the Cu precipitation state on the defect-based failure was observed for X21, as the precipitates are located within the ferrite grains and, thus, only have a minor impact on the fatigue failure mechanisms of X21.

Mechanical properties of aluminum to steel dissimilar spot joints produced by cold metal transfer weld-brazing

Spot joints between aluminum and steel sheets were produced by cold metal transfer arc welding using an AlSi3Mn alloy low melting point filler wire. The current study found that by using optimal welding conditions, the thickness of the joint interface was significantly reduced compared to previous studies. The interface thickness increased from a few nanometers at the center of weld to 1.7 micrometers (1.2 mm from center of weld) at periphery of weld. The reduction in joint thickness helped to achieve better joint properties. The tensile shear and fatigue properties of the spot joints were evaluated in conjunction with interrupted mechanical tests to determine the fracture mechanisms. This study investigates the fatigue properties and fracture mechanisms of cold metal transfer arc spot joints, which has not been done previously. The tensile shear fractures exhibit primarily interfacial fracture passing through an intermetallic compound layer at the joint interface. However, three different modes of fractures are observed during fatigue tests, including interfacial fracture, sheet fracture, and mixed-mode fracture. Interrupted fatigue tests reveal numerous crack initiation in pores/particles in the deposited weld metal and coagulation of those cracks leads to sheet fracture during the fatigue tests. Sheet fracture mode fractures are mainly observed during high cycle fatigue conditions.

Additive manufacturing-based repair of IN718 superalloy and high-cycle fatigue assessment of the joint

Room-temperature high-cycle fatigue (HCF) of IN718 repaired joint via laser direct energy deposition (DED) were studied with the fatigue axis perpendicular to the joint interface. Solution treated and aged (STA) were compared with directly aged (DA) conditions. The wrought IN718 substrate showed equiaxed grains with a size of ~90 μm and a high fraction of annealing twins, whereas the DED deposit revealed a mixture of equiaxed and columnar grains with an average size of ~20 μm. There was little difference between the STA and DA conditions in the grain length-scale. Micro-hardness results highlighted the need for the heat treatment as it can remove the heat-affected zone and hardness dip, creating a uniform hardness profile across the joint. Although the monolithic DED deposit had a similar tensile strength to the wrought substrate, the DED joint exhibited an overall decreased HCF performance, regardless of the heat treatment conditions. When the fatigue stress was low, the STA condition had a better HCF performance than the DA, however, the opposite trend appeared for the high stress, resulting in a cross-over point on the stress-life S - N plot. Interrupted fatigue tests, combined with microscopy and fractography, revealed that the fatigue failure occurred in the substrate for the DED joint in the DA condition, whilst in the deposit zone for the STA condition due to the distribution and fracture of the Laves and δ phases. Grain boundary cracking in the substrate near the substrate-to-deposit interface can occur in both cases, probably due to the Nb-rich liquid films. • High-cycle fatigue of AM IN718 repaired joints with loading perpendicular to the interface • Post heat treatment can effectively eliminate the heat affected zone • Directly aged joint outperforms the solution treated and aged joint under high fatigue stress • Whereas the solution treated one performs better under low fatigue stress • Such an unexpected phenomenon is reconciled using single-specimen interrupted tests

Short fatigue crack growth in an artificial-defect-containing nanostructured bainitic steel

For a full exploitation of novel steels in applications under mechanical cyclic loading, it is vital to understand the crack-microstructure interactions. In this work, the fatigue behaviour of a nanostructured bainitic steel in the short crack regime has been studied considering the structural integrity of the material and its polycrystalline nature. For that purpose, a specimen containing an artificial defect has been designed and manufactured, subjected to interrupted fatigue testing, and examined by crystallography-sensitive techniques. Results show that, depending on the loading conditions, different crystallographic boundaries can play an important role in the crack path and crack growth rate.

Fatigue behavior and modeling of chopped carbon fiber reinforced sheet molding compound composites

AbstractThe quasi‐static and fatigue behavior of chopped carbon fiber reinforced sheet molding compound (SMC) composite was investigated. The microstructure of material was inspected by X‐ray Computerized Tomography (XCT) and the fiber orientation distribution was analyzed. Quasi‐static tensile test was performed on specimens with different gauge length to study the size effect on mechanical properties and digital image correlation system was applied to record the strain distribution. The fatigue behavior of material was evaluated under both tension‐tension (R = 0.1) and tension‐compression (R = −1) loadings, in which XCT scanned specimens were tested to characterize the microstructure at failure location. Interrupted fatigue test was performed to observe the damage evolution process on the side surface of specimen by optical microscope and the dominant fatigue failure modes were identified. A representative volume element based modeling methodology was modified and applied to establish the relationship between heterogenous microstructure and macroscopic mechanical behaviors. The simulation results show good consistency with experimental observations which demonstrates the effect of local fiber orientation on durability performance of material.

Fatigue behavior of nanostructured bainite: A morphological study of crack path

Possessing the highest combination of strength and toughness ever-recorded among bainitic steels, nanostructured bainite is a significant advanced steel due to its simplicity with regards to the alloy design and processing. The outstanding monotonic properties do not necessarily correlate with an optimum fatigue performance. Therefore, prior to the industrial implementation of the microstructure, intensive laboratory-scale research should be carried out to clearly rationalize the microstructure’s behavior under cyclic loadings. This work aims at investigating the first stages of the fatigue crack propagation in a nanobainitic steel. For this purpose, interrupted fatigue testing, with crack initiation assisted by the presence a of focused ion beam (FIB) defect, was conducted, ensuring that the short-crack regime applies, and the crack path was morphologically examined.

Characterization and modeling of fatigue behavior of chopped glass fiber reinforced sheet molding compound (SMC) composite

In the present study, the quasi-static and fatigue behavior of chopped glass fiber reinforced sheet molding compound composite was investigated. Considering the stochastic meso-structure induced by processing, fiber orientation within specimens was analyzed based on the sectional images obtained by X-ray Computerized Tomography (XCT). Quasi-static tensile and compressive tests were conducted, in which the strain distribution was recorded by Digital Image Correlation (DIC) system. The fatigue behavior of material was evaluated under tension–tension (R = 0.1) and tension–compression (R = -1) loading. Interrupted fatigue test was conducted to investigate the damage evolution on the side surface of specimen using optical microscopy. A modeling method based on Representative Volume Element (RVE) was modified and applied to predict the fatigue behavior of glass fiber SMC. The simulation result showed a good consistency with experimental results on both prediction of the fatigue life and the damage mechanisms under cyclic loading.

Damage behaviour in different micro-regions of P92 steel weldment under sequential low cycle fatigue and creep loading

Understanding the evolution of microstructure and micro-region properties of weldments during cyclic loading is a challenge for the reliability assessment of high-temperature components. This work is devoted to quantitatively evaluating the variation of micro-region properties and corresponding responsible microstructural features of P92 steel weldments under interrupted fatigue tests and subsequent creep fracture. To achieve this target, high-resolution characterization techniques, including electron backscatter diffraction (EBSD) and nanoindentation tester were used. The results revealed that the microstructures in the inter-critical heat affected zone (ICHAZ) and adjacent regions, fine-grained HAZ (FGHAZ) and parent metal (PM), show significant sensitivity to fatigue loading. Notably, the grain size in the ICHAZ reaches saturation after fatigue cycles of 10% fatigue lifetime, and the weld metal (WM) keeps unchanged throughout the fatigue loading. After subsequent creep fracture, only the WM presents an increasing trend in the average grain size. Nanoindentation tests uncover that the reductions of the elastic modulus and microhardness in each region also present three cyclic softening behaviour during the fatigue process due to the evolution of martensite lath structure. However, due to nucleation and growth of cavities accelerated by carbide precipitation, relatively low elastic modulus and microhardness were observed after subsequent creep loading.

Bending fatigue behavior of thin Zr61Ti2Cu25Al12 bulk metallic glass beams for compliant mechanisms application

In this work, bending fatigue behavior of thinner Zr61Ti2Cu25Al12 (ZT1) BMG beams with thicknesses of 500 μm and 100 μm were investigated with three-point bending (3PB), to evaluate the reliability and safety of this potentially material used in compliant mechanisms under cyclic bending load. Fatigue endurance limits (FELs) of ZT1 beams with thicknesses of 500 μm and 100 μm were determined to be 470 MPa, and ∼0.3 of its tensile strength. Fatigue life and FEL of ZT1 beams in bending are substantially independent of the beam thickness, in the scale from 500 μm down to 100 μm. Fatigue cracks of all ZT1 beams initiated at the extrinsic microvoids, either indirectly derived from surface scratches during mechanical polishing or originated from as-cast process. However, fatigue crack propagation behavior is highly related to the magnitude of cyclic stress level. By interrupted fatigue tests, larger proportion about 60–70% of fatigue life was spent on crack propagation under high stress level, while crack initiation occupied larger proportion about 80–85% under low stress level. The medium stress level of about 590 MPa almost shared the crack-initiation life and crack-propagation life. In addition, there are two crack deflection mechanisms during cracks steady propagation stage. One mechanism is crack deflection along the interacted shear bands ahead of crack-tip, the other one is crack “jump” from one shear band to another shear band at a certain angle. These two mechanisms alternate dominate the crack propagation process, generating typical staircase-like crack propagation trajectory. Fatigue endurance limits of ZT1 beams in the form of either stress amplitude or strain amplitude are superior to traditional candidate materials in the field of compliant mechanisms, showing that the better reliability and larger deflection deformation can be achieved for flexible members made by ZT1 BMG even under cyclic loading.

Surface Observation of Induction-Heated 13Cr-2Ni-2Mo Stainless Steel after Interrupted Fatigue Testing under Rolling Contact Stress in Water

In order to investigate the wear behavior of induction-heated 13Cr-2Ni-2Mo stainless steel, we performed the rolling contact fatigue (RCF) tests in water. We interrupted the RCF test at each 1.0×105 cycles and measure the wear loss and observed the contact surface. After the RCF tests, we found the oxygen concentration area in the contact area.

Residual Properties of the Drawn Steel Wires for Damage-Based Fretting Fatigue Models of the Wire Ropes

This paper presents the residual properties and parameters of the damage-based fatigue life prediction models of the steel wire ropes under fretting fatigue conditions. The damage mechanics-based approach is employed to develop the predictive method for the reliability of the steel wire ropes. The elastic modulus is dependent on the fatigue load condition and the accumulated number of the load cycles. The characteristic degradation of the Young’s modulus of drawn steel wires is established through the phenomenological presentation of the interrupted fatigue test data. The samples are given a quasi-static loading followed by a block cyclic loading with various stress amplitudes and ratios. The residual Young’s modulus is calculated after each block of cycles. The effect of the different loading condition with the amplitude and mean stress on the measured fatigue life of a single wire is presented using the life parameter, χ. The residual Young’s modulus data are presented in terms of normalized quantities. Significant reduction in the elastic modulus due to fatigue is exhibited after enduring 70% of the fatigue life of the material. The fitting constants are obtained, and the fitted equation is used to describe the degradation of Young’s modulus at N number of cycles. Subsequently, the data can be applied to predict the fatigue-life of steel wire ropes and assess its reliability through fretting-induced damage models.

Fatigue damage evaluation of carbon fiber reinforced composites using nonlinear resonance spectroscopy

The present work explores the use of nonlinear resonance spectroscopy to detect early fatigue damage in carbon fiber-epoxy composite laminates. The objective is to test composite samples at mid to high cycle fatigue (>10∧3 cycles), so that the damage evolution could be tracked. The nondestructive nature of the testing method ensures that the sample can be loaded and tested till failure, i.e. interrupted fatigue testing. The acoustic nonlinear response tracked over the fatigue life of the sample includes, the nonlinear frequency shift and modal damping ratio. Additionally, slow dynamic diagnostics was also carried out to investigate the recovery rate and recovery time. The samples that were tested were all quasi-isotropic laminates under 4-point bending load. These included a pristine sample, and two samples with impact damage to introduce variability. Tracking the nonlinear parameters over fatigue life showed a non-monotonic increase in nonlinearity. Conjectures based on acoustic response to microdamage and different stages of fatigue were used to explain the observed variation.

Experimental fatigue behaviour of CFRP composites under vibration and thermal loading

The investigation of the carbon fibre reinforced plastic components under vibration fatigue testing is known to be challenging; as it involves the analyses of both the mechanical response and the inherent self-heating behaviour which emerges due to the high-frequency testing. The paper will firstly address an open question about the so-called critical event, which is a sudden change in one of the dynamic parameters measured during the fatigue test. Past experiments showed that such a change is associated with delamination, but its onset and early growth were not clarified. Therefore, a set of interrupted fatigue tests and CT scan measurements will show the onset of delaminations and how these are correlated with the vibration parameters. The second objective is to investigate the failure behaviour under vibration fatigue and elevated environmental temperatures. The research will present both the experimental set-up for achieving fatigue under environmental conditions and the vibration data obtained from testing samples at many severity levels; both in terms of applied mechanical vibrations and thermal loads. The paper will investigate how the inherent self-heating behaviour and the applied thermal load will affect the time to failure of composites. The relationship between fatigue behaviour and environmental temperature conditions are presented in two forms. The first one is a typical S-N curve, whereas the second one, the relationship between the response phase and hot-spot temperature data. Both relationships will show how relevant is the environmental temperature in terms of fatigue behaviour for the type of components presented in this paper.

Experimental and numerical multi-scale approach for Sheet-Molding-Compound composites fatigue prediction based on fiber-matrix interface cyclic damage

In this paper, a multi-scale approach is proposed to predict the stiffness reduction of a Sheet-Molding-Compound (SMC) composite submitted to low cycle fatigue (until 2.105 cycles). Strain-controlled tensile fatigue tests (R = 0.1) are carried out at various strain ranges. Damage is investigated at both macroscopic and microscopic scales through the evolutions of Young’s modulus and SEM observations, after interrupted fatigue tests at different lifetime periods. The results show that the fatigue degradation of the composite is mainly controlled by fiber-matrix interface debonding. A quantitative analysis allows determining the threshold and kinetics of the fiber-matrix interface damage during cyclic loading as a function of the orientation of fibers. Moreover, a fiber-matrix interface damage criterion, taking into account the local cyclic normal and shear stresses at the interface, is introduced in the Mori and Tanaka approach in order to predict the loss of stiffness. The parameters of this local criterion are identified by reverse engineering on the basis of the experimental results described above. Finally, the predicted loss of stiffness is very consistent with the experimental results.

Morphology-induced fatigue crack arresting in carbon fibre sheet moulding compounds

Carbon Fibre Sheet Moulding Compounds (CF-SMCs) are tow-based composite materials. Interrupted fatigue tests, combined with computed tomography, were performed here to investigate the damage mechanisms in high in-mould flow CF-SMC. The tow-based microstructure created obstacles for fatigue damage propagation, increasing the CF-SMC’s resistance against cyclic loading. Failure is shown to nucleate inside the tows, but inter-tow crack propagation tends to be hindered by the presence of the other tows. Tows oriented perpendicularly to the initial fatigue crack stop the crack itself, showing an intrinsic crack arrest mechanism. Additionally, pre-existing manufacturing cracks or voids do not propagate at all. As a result, flatter slopes of the SN diagrams were observed for CF-SMC than for other carbon or glass fibre composites with short, long and even continuous fibres.