High Cycle Fatigue Assessment Research Articles

Very high cycle fatigue assessment of thermoplastic-based hybrid fiber metal laminate by using a high-frequency resonant testing system

The prolonged fatigue lifetime of fiber metal laminates (FML) compared to monolithic metals is a key aspect for safety-relevant components, where detailed knowledge about the fatigue properties up to the very high cycle fatigue (VHCF) regime is necessary. For thermoplastic-based FMLs, offering formability, recyclability, and mass production due to short consolidation cycle times, this knowledge needs to be established.In this study, FML containing AA6082 sheets and unidirectional glass and carbon fiber-reinforced polyamide 6 was investigated. Fatigue tests up to max. 109 cycles were conducted on a servo-hydraulic and a high-frequency resonant fatigue testing system with frequencies of 10 Hz and 1,000 Hz. Frequency-induced self-heating was evaluated and limited using air-cooling to maintain the matrix properties. Fatigue progress was monitored through high-speed optical deformation analysis.Compared to HCF loads, VHCF loads lead to reduced crack and delamination occurrence. Changes in microstructure are dominated by singular crack initiation and propagation within the aluminum. The FML’s S-N curve shows a significant flattening from the HCF to the VHCF regime, simplifying the predictability of the fatigue life. By using post-stretching, the VHCF strength can be increased by over 60 %, making this thermoplastic-based FML a considerable competitor in the field of FML.

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

Very high cycle fatigue assessment at elevated temperature of 100 μm thin structures made of high-strength steel X5CrNiCuNb16-4

Many components and structures are exposed to very high number of cycles and challenging environmental conditions during operation. This study contributes to a better understanding of the very high cycle fatigue (VHCF) properties of high-strength steel X5CrNiCuNb16-4 at room temperature (RT) and 350 °C. For this purpose, conventional specimens and thin-walled structures are extensively examined with novel high-frequency fatigue testing techniques at elevated temperature. Tests with unnotched specimens at 350 °C show a 21.7% reduction in fatigue strength for 107 cycles and a different failure mechanism compared to RT. In contrast, no temperature influence is observed for mildly notched specimens and even a higher local fatigue strength is found for sharply notched specimens at 350 °C. The decrease in fatigue strength for 109 cycles is more pronounced at 350 °C (−10%) than at RT (−5%), and it is proven that notched specimens adequately represent the VHCF behaviour of structures. The transferability of specimen results to components and structures is given great attention. A new proposal for the VHCF strength assessment of structures with high stress gradients is presented, which is based on specimen results, an extended material-mechanical support factor and a VHCF reduction factor. The prediction model gives conservative fatigue strength estimates for 109 cycles with a maximum deviation of 5.8%. This demonstrates that even complex shaped structures can be successfully evaluated with suitable specimens and methods.

Fatigue failure transition analysis of load carrying cruciform welded joints by extended local approach

This study was mainly concerned with the fatigue crack initiation points transition relationship between the weld toe and weld root in Load-carrying Cruciform Weld Joint (LCWJ) considering the mismatched filler material yield strength and geometrical sizes under cyclic loading conditions. The critical failure locations were investigated by the experimental tests and numerical simulations. To analyze the fatigue characteristic indicators of the potential fatigue initiations, different configurations of mismatch ratios and local geometries (plate thickness, weld size and penetration ratio) were designed to conduct the fatigue tests and cyclic calculations of CLWJ. Subsequently, New uniform effective notch energy indicator was proposed to characterize the low and high cycle fatigue life by extending the SED method. The related analytical formulations for weld toe and weld root were established to predict the low and high cycle fatigue assessment indicator of LCWJ considering the effects of plasticity and yield strength mismatch and geometry configurations. The effective notch energy relationship between the weld toe and weld root in LCWJ was estimated by the established analytical solutions.

Additive Manufacturing-Based Repair of In718 Superalloy and High-Cycle Fatigue Assessment of the Joint

Additive Manufacturing-Based Repair of In718 Superalloy and High-Cycle Fatigue Assessment of the Joint

Notch energy-based low and high cycle fatigue assessment of load-carrying cruciform welded joints considering the strength mismatch

This study examines the Low and High Cycle Fatigue failure (LCF and HCF) of Load-carrying Cruciform Welded Joints (LCWJ) made of 10CrNi3MoV steel through a combined investigation of experimental, simulation, and analytical methods. LCF and HCF tests were conducted on LCWJ specimens with various weldment strength matching conditions and geometric configurations. Different fatigue failure modes can be observed from potential fracture points due to the discrepancy of strength and geometry in weldments, which exhibits the discrepancy of fatigue life. New uniform energy analytical formulations for Weld Toe (WT) and Weld Root (WR) are established to predict the LCF and HCF indicator of LCWJ considering the plasticity and mechanical heterogeneity effects of the weldments based on the generalized Neuber concept of Fictitious Notch Rounding (FNR). Furthermore, an effective notch energy indicator is introduced and implemented to evaluate fatigue behaviors of welded components in both LCF and HCF regimes. According to the proposed analytical solutions, experimental data from WT and WR failure under force and displacement-controlled cyclic loadings are evaluated and verified. The results show that the notch-energy indicator can express the unique relationship with fatigue life regardless of the strength mismatch ratios and geometry discrepancy.

Empirical High Cycle Fatigue Assessment Model of MEMS Devices

Abstract Many microelectromechanical systems (MEMS) devices generate large deflections and stresses under some severe loads, and this cumulative stress often causes structural fatigue when applying high cyclic loads. It affects the reliability and quality of the product and can easily damage structures in harsh environments. For example, accelerometers and gyroscopes in motion sensors always combine a movable mass component with a bending or torsion spring, which can severely deform their structure at some large accelerations or angular accelerations. After this high cycle of deformation, the spring structure will be damaged and cause device failure, we call this failure mode high cycle fatigue (HCF). This is very common for MEMS products, but it is difficult for designers to predict it at design stage. To prevent early fatigue problems and reduce product development time, we developed a simulation process and empirical prediction model to help designers predict HCF and improve fatigue strength of moving structures used in MEMS devices. In this study, a bending and a torsion beam are used as test vehicles. A combination of dynamic and electromechanical coupling simulations was developed and applied to analyze the effects of frequency under HCF testing. The test structures were fabricated using silicon-on-insulator (SOI) MEMS technique and these test structures were used to validate empirical life prediction equation developed in this research. The frequency effect of HCF was also to be included in the developed prediction model. Based on the test results, the accuracy of the empirical prediction equation is improved by including frequency effect.

High cycle fatigue assessment of steel load-carrying cruciform welded joints: an overview of recent results

In this paper, high cycle fatigue failure behavior of Load-carrying Cruciform Welded Joints (LCWJ) is assessed using Strain Energy Density (SED) approach. Different analytical solutions are proposed and applied to illustrate the effects of LCWJ geometry under cycle tension and bending according to Finite Element (FE) calculation. The Notch Stress Intensity Factors (NSIF) according to the proposed analytical solutions are validated by comparing finite element data from several simulations in terms of LCWJ models, resulting in a good agreement. A bulk of experimental data taken from tests and the literature is assessed by the proposed solutions as the forms of SED, NSIF and peak stress. The results show that the SED-based analytical solutions for steel LCWJ are effective for high cycle fatigue failure analyses.

Low and high cycle fatigue assessment of mismatched load-carrying cruciform joints

Fatigue behaviors of load-carrying cruciform joints considering geometry variations and strength heterogeneity between base metal and weldments were studied. Low and high cycle fatigue tests were performed on specimens under displacement controlled and force controlled conditions. Observation of the specimens revealed that crack propagation paths differ by different loading conditions and that failure life was dominated by crack propagation. Besides, it was found that the effect of the strength under matching on the fatigue strength becomes large in low cycle fatigue region by significantly reducing fatigue life of the specimen. The test data were also assessed by nominal strain and proposed design guidance based on effective notch strain.

A simplified method for weakest-link fatigue assessment based on finite element analysis

Widespread use of weakest-link theory for high-cycle fatigue assessment based on finite-element (FE) results is limited by the lack of suitable post-processors. By assuming the stress within each element to be a linear function of the volume, weakest-link calculations can be performed by simple operation on typical FE output. This simplified method was evaluated on FE results from five elementary components with brick element meshes of different resolution. Deviations in effective stress amplitudes were <2.5% compared with analytical solutions. In cases where analytical solutions were unavailable, solutions were compared with those of the in-house post-processor P-FAT. Deviations were around 5% for ‘fine’ meshes and increased with decreasing mesh resolutions. Deviations were always conservative and largest for materials with high fatigue strength scatter.

Comparison of Equivalent Stress Methods with Critical Plane Approaches for Multiaxial High Cycle Fatigue Assessment

Several equivalent stress methods and more advanced critical plane criteria are compared in terms of their performance in fatigue life estimations under uniaxial and biaxial loadings in which the effect of phase is investigated. For this purpose a MATLAB code is written which transforms the multiaxial cyclic stress state into a uniaxial cyclic stress to use with the equivalent stress based methods. For critical plane approaches the program searches a damage parameter on all material planes. The maximum damage parameter is then compared with a material allowable obtained from uniaxial fatigue tests for life estimation. In addition, various methods for calibration of the material coefficient k and prominent stress history enclosure methods to calculate the shear stress amplitude are also studied for determining their effect on the performance of critical plane approaches.

Fatigue behaviour of defective cast iron

With the analysis example of a cast iron component a Finite Element Analysis (FEA) based modelling and post-processing methodology is proposed for the multiaxial high-cycle fatigue assessment of surface defects. Methods are proposed for the measurement and mechanical description of surface defects using an ellipsoid simplification and the well-known size parameter from Murakami (2002). Different methods are compared for the stress-computation near surface defects: elastic FEA, elastic-plastic FEA with nonlinear kinematic hardening material model, and analytical calculations with the Equivalent Inclusion Method from Eshelby (1957). The Defect Stress Gradient (DSG) approach from Vincent et al. (2014) is applied to predict crack initiation, whereby a new method is proposed for the estimation of the stress gradient, which allows the presentation of the DSG utilisation factor results field.

Fretting High-Cycle Fatigue Assessment through a Multiaxial Critical Plane-Based Criterion in Conjunction with the Taylor’s Point Method

The critical plane-based multiaxial criterion originally proposed by the authors for plain fatigue is here applied to estimate the crack initiation life of fretting high-cycle fatigued structural components. Although fretting fatigue can be regarded as a case of multiaxial fatigue, the common multiaxial fatigue criteria have to be modified to account for the severe stress gradients in the contact zone. Therefore, the above criterion is used in conjunction with the Taylor’s point method to numerically estimate the fatigue life of Ti-6Al-4V and Al-4Cu specimens under cylindrical contacts.

High-cycle fatigue of notched plain concrete

This paper investigates the accuracy of the so-called Theory of Critical Distances (TCD) in modelling, in the high-cycle fatigue regime, the behaviour of notched plain concrete. The TCD postulates that the fatigue damage extent has to be estimated by directly post-processing the entire linear-elastic stress field damaging the material in the vicinity of the crack initiation locations. According to the TCD’s modus operandi, the high-cycle fatigue assessment is performed by using a scale length parameter which is treated as a material property. The accuracy of this method was checked against a number of experimental results generated by testing, under four-point bending, notched specimens of plain concrete. This validation exercise allowed us to prove that the TCD is successful in estimating the high-cycle fatigue strength of notched concrete beams, resulting in predictions falling within an error interval of about ±15%.

Gradient-enriched linear-elastic tip stresses to perform the highcycle fatigue assessment of notched plain concrete

Gradient Elasticity (GE) allows the stress analysis to be performed by taking into account the size of the dominant source of microstructural heterogeneity via a suitable length scale parameter. This is done by simply assuming that the material under investigation obeys a linear-elastic constitutive law, albeit equipped with additional spatial strain gradients. From a practical point of view, the most important implication of this modus operandi is that gradient-enriched linear-elastic stresses at the notch tips are always finite, this holding true also in the presence of sharp stress risers (such as cracks). In the present investigation, the accuracy of two different GE based design strategies was checked against a number of experimental results generated by testing, under cyclic four-point bending, plain concrete samples containing different geometrical features. The high level of accuracy which was obtained by directly using gradient-enriched linear-elastic notch stresses strongly supports the idea that GE is a powerful tool suitable for designing notched concrete components against high-cycle fatigue. This result is very promising also because the required stress analysis can directly be performed by using standard Finite Element (FE) solvers.