Constant Amplitude Loading Research Articles

Fatigue life prediction considering temperature effect for carbon fibre reinforced composites under variable amplitude loading

Fatigue life prediction method based on stress normalization was proposed to reduce the influence of material property dispersion on life for CFRC (carbon fibre reinforced composites). In order to consider the effect of temperature on fatigue damage, a temperature influence factor was introduced into the fatigue limit equation and the life prediction method under variable amplitude loading at high temperature. Combined with the rainflow cycle counting and Miner linear damage accumulation rule, the proposed fatigue life prediction method was verified by the experimental results of composite multidirectional laminates under constant amplitude and variable amplitude loadings at various temperatures, and the prediction errors are within a factor of 5.

Experimental and Numerical Study of Cyclic Stress–Strain Response and Fatigue Crack Initiation Life of Mid-Carbon Steel under Constant and Multi-Step Amplitude Loading

This paper investigates the fatigue cyclic deformation behavior of mid-carbon steel. Uniaxial tensile loading tests and fatigue tests under constant and multi-step amplitude loading steps are performed to characterize the influence of loading history. The material is shown to exhibit different uniaxial ratcheting behavior depending on loading history. A smooth and gradual increase in cyclic softening is observed under smaller stress/strain conditions. Based on experimental characterization, numerical investigations are carried out to reproduce the cyclic stress–strain behavior under different variable amplitude load ranges. The nonlinear material behavior is reproduced by means of an elastoplasticity model called the Fatigue SS Model (hereafter, FSS model). The main feature of the FSS model is the ability to describe the cyclic softening behavior within a macroscopically elastic stress state. The good agreement between experimental and numerical results proves the reliability of the model to catch a realistic material response in fatigue problems. Furthermore, the present study introduces a method for the prediction of fatigue crack initiation life under variable loading conditions based on cumulative plastic work.

Mechanical behavior of friction stir butt welded joints under different loading and temperature conditions

Friction Stir Welding is a process that has led to increasing usage of aluminum and its alloys in different industries, since it is an autogenous and a solid-state welding process. The work developed in this paper was focused on the analysis of the mechanical behavior of AA7075-T651 friction stir butt welded joints, of 4 mm of thickness, under different loading and temperature conditions. A selection process was applied to assure the appropriate welding parameters. The welds from which the investigated specimens were manufactured from were submitted to nondestructive testing to assess the presence of welding defects. Fatigue tests were carried out, under a constant amplitude loading regime, at stress ratios of 0.05 and 0.5 and at temperatures of 23 °C and 150 °C. The results obtained by the fatigue tests point to a higher fatigue life of the specimens when the highest stress ratio, R = 0.5, was used (for the maximum stress applied), for both loading temperatures. For the stress range, the contrary was found to be true. Room temperature was also found to result in a significantly better fatigue performance. A Scanning Electron Microscope was used to characterize the fatigue fracture surfaces.

Fatigue life evaluation method of metallic components under random loads based on a new strain-life curve

The fatigue life prediction methods of constant-amplitude load have been widely studied. However, a simple and mature random loads fatigue model is more worth exploring. In this paper, the battery box brackets were taken as the study case, the road simulation technology and statistical method are adopted to analyze the prediction accuracy of random loads which is based on ε-N curve and damage accumulation law, a fatigue damage calculation procedure of random loads was established. Firstly, both the road simulation and constant-amplitude experiments of bracket were conducted respectively on the hydraulic servo rig. Simultaneously, the strain loads were collected and the part ε-N curve was constructed. In accordance with the material and part ε-N curve, an improved double-parameter (u and v) Coffin-Manson model was proposed and a fatigue ε-N curve was plotted. Then the damage originated from machining process and geometrical uncertainty was corrected by u and v. Under random load conditions, u and v were calibrated furtherly by random load factor β. Finally, the damage of the brackets was recalculated and the result has a high precision. The double-parameter model coupled with β has strong adaptability to most fatigue uncertainties. A leaf spring experiment was also performed to verify the generality and validity of the model.

Fatigue behaviour and impact strength assessment of friction stir-welded carbon steel joints

The present study investigates the fatigue behaviour and impact strength of friction stir-welded (FSWed) carbon steel (AISI 1018) joints in comparison with the base metal (BM). The fatigue behaviour was tested using compact tension (CT) specimen of BM and FSWed joints under the constant amplitude load at different values of stress ratio (R) having a frequency of 10 Hz. A 20% increase in fatigue life along with a lower fatigue crack propagation rate was observed for FSWed specimens when compared to BM. The impact strength of BM and FSWed specimens was measured using the Charpy test. A higher value of impact strength was observed in FSWed specimens in comparison to BM. Additionally, regression analysis was carried out to model impact energy in terms of input parameters. Microstructural examination of a fractured surface of the FSWed specimen revealed the ductile fracture. A higher microhardness value was also observed in FSWed specimens when compared to BM.

Study on fatigue behavior and crack propagation life of longitudinal fillet welded lap joint

Fifty longitudinal fillet welded lap joints with main plates (12 mm thick) and overlapping plates (8 mm thick) made of Q345B high-strength low-alloy structural steel, were tested. The joint specimens were fatigued at room and low temperatures (0 °C, −20 °C, and − 40 °C) under constant amplitude loading. The test life results were modelled by S–N curves and the crack growth was modelled in detail by applied fracture mechanics. The results show that the fatigue strength of the lap joint varies insignificantly when the temperature ranges from 20 °C to − 40 °C.

Multiple linear regression parameters for determining fatigue-based entropy characterisation of magnesium alloy

This paper presents the development of the multiple linear regression approach based on the stress ratio and applied load that was assessed using entropy generation. The energy dissipation is associated with material degradation to determine the fatigue life with consideration to the irreversible thermodynamic framework. This relationship was developed by predicting a complete entropy generation using a statistical approach, where a constant amplitude loading was applied to evaluate the fatigue life. By conducting compact tension tests, different stress ratios were applied to the specimen. During the tests, the temperature change was observed. The lowest entropy generation was 2.536 MJm-3 K-1 when 3,000N load with a stress ratio of 0.7 was applied to the specimen. The assumptions of the models were considered through graphical residual analysis. As a result, the predicted regression model based on the applied load and stress ratio was found to agree with the results of the experiment, with only 9.3% from the actual experiment. Therefore, the entropy generation can be predicted to access the dissipated energy as an irreversible degradation of a metallic material, subjected to cyclic elastic-plastic loading. Thermodynamic entropy is shown to play an important role in the fatigue process to trace the fatigue life.

Evaluation of creep-fatigue life and strength for AA7001-T6 under constant amplitude loading

Aluminum alloys were widely used in the construction, automotive, marine, and aviation industries due to their low specific strength, ease of manufacture, and low weight. The fatigue behavior of aluminum alloys at different temperatures is investigated. Thanks to the rapid development of armament in recent years, 7XXX ultra-high strength aluminum alloys are now used more frequently because of their non-corrosive qualities and low weight. Aluminum alloy 7001-T6 behavior is examined at the Company State for Engineering, Rehabilitation, and Inspection (SIER) in Iraq, where chemical analysis of the AA7001 is supported. Most engineering components that operate at high temperatures will eventually fail from fatigue strain, creep damage is a time-dependent process that is primarily influenced by the history of stress and temperature applied to the component. When the two damaging factors combine their effects, This study used AA7001-T6 to conduct experiments on mechanical characteristics (UTS, YS, E, and ductility) and the interaction between creep and fatigue at four distinct temperatures: room temperature (25, 150, 280, and 330) °C, the UTS, YS, and E were lowered by 37.2, 37.2, and 24) %, respectively, as compared to the result at room temperature, but the ductility increased by 28.27 %. It has been noted that rising temperatures cause mechanical and fatigue characteristics to decline. Experimental S-N fatigue test findings showed a significant loss of fatigue strength, After 107cycles, the endurance fatigue limit was reduced from 208 MPa at (RT) to 184 MPa at 330 °C, an 11.5 % reduction. Overall, it can be said that AA7001-T6 demonstrates a significant drop in mechanical and fatigue properties at high temperatures

Effects of specimen size and stress ratio on fatigue crack growth after a single tensile overload

This paper investigates the effect of specimen size, stress ratio, and loading pattern on fatigue crack growth for centrally cracked low carbon steel specimens. Elastic-plastic calculations were conducted under cyclic loading based on FEM using the domain integral method. The computed fatigue lives were firstly verified against the experimental tests taken from the literature. The numerical calculations showed that thickness has no significant effect on crack growth at a given stress ratio under the constant amplitude loading pattern. While the thinner specimens revealed longer fatigue lives compared to the thicker specimens at a certain stress ratio due to the application of a single tensile overload. A remarkable reduction in fatigue lives with a clear acceleration in crack growth rates was noticed for specimens with a small ligament length-to-specimen width ratio. The retardation response due to the application of a single tensile overload depended on a particular combination of specimen size and stress ratio. The variation in the crack driving force due to the change in thickness and ligament length was investigated. The relation between the crack driving force and the local stress state ahead of the crack tip was also discussed.

Residual fatigue life prediction of natural rubber components under variable amplitude loads

In the present study, the residual fatigue life (RFL) of natural rubber (NR) components under variable amplitude loads is predicted. To this end, a support vector machine (SVM) model is established to estimate the fatigue life of NR specimens under a constant amplitude load. Here, the strain amplitude and strain mean of NR specimens are used as input variables while the rubber fatigue life is considered as the output variable of the SVM model. It is found the regression results and predicted life distribution of the SVM model are promising. Then the load sequence and load interaction are considered in the calculations and a nonlinear fatigue cumulative damage (NFCD) model is developed. Finally, the RFL of rubber specimens under variable amplitude load is predicted using the Miner criterion and the NFCD model. The performed experiments demonstrate that the prediction accuracy of the NFCD model is higher than that of the Miner criterion.

Characterising the stress ratio effect for fatigue crack propagation parameters of SAE 1045 steel based on magnetic flux leakage

The aim of this study was to determine the relationship between fatigue crack length, a, the number of cycles, N, the magnetic flux gradient intensity, dH/dx and the stress ratio, R. Fatigue crack growth tests were performed using a constant tensile load amplitude on SAE 1045 steel in the form of single-edge cracks. Stress ratio values of 0, 0.1, 0.2, 0.3 and 0.4 were used to study the characteristics of the magnetic flux gradient. A crack opening displacement device was used to detect the parameters of the stress intensity factor range, ΔK, fatigue crack growth rate, da/dN, and N. Meanwhile, a metal magnetic memory sensor device was used to detect the H value at the stress concentration zone when a 1 mm-increase in the crack occurred. The experimental results showed that as the fatigue cycle and crack length increased for each R, dH/dx also increased exponentially. The governing equations were derived based on the magnetic flux gradient for the fatigue crack length equation and the fatigue cycle equation, with the stress ratio. Based on a statistical analysis of the magnetic flux leakage signals, it was found that all the data were within a range based on a 90 % confidence level. Thus, the magnetic flux leakage parameters could be used to construct fatigue crack growth behaviour models for ferromagnetic materials and potentially as an alternative method for predicting the fatigue life.

A hybrid approach of modified bootstrap and physics-based methods for probabilistic fatigue life prediction considering overload effects

A hybrid approach is proposed by combining the modified bootstrap and physics-based methods, which aims to achieve a reasonable and accurate probabilistic fatigue life prediction under arbitrary overload conditions based on constant amplitude fatigue data. The fatigue crack growth model and equivalent initial flaw size (EIFS) are employed to calculate the fatigue life. The crack closure mechanism is used to account for the loading interaction effects. Furthermore, the uncertainties of EIFS, coefficient of the fatigue model and overload intervals are considered. Fatigue data are collected by performing testing of aluminum alloy 7075-T6 under constant amplitude loading with/without overloads. The bootstrap method is modified by generating the expanding samples, which is utilized to quantify the dispersion of fatigue data. The resampling is conducted based on the expanding samples, instead of the original data directly obtained from the experiment. Eventually, the fatigue lives with different failure probabilities and confidence levels are calculated. In addition, other testing data sets of Al–Li alloy 2060 are also employed to verify the effectiveness and accuracy. Good agreements are observed.

Fatigue damage assessment of metal sandwich panels under four-point bending cyclic loading conditions

This paper aims to investigate the effects on metal sandwich panels with different dimple core designs using four-point bending simulation under constant and variable amplitude loading. A core design with cavity and porosity in the body structure was found to reduce the panel structural integrity. This would lead to unstable bonding between the panels and initiate an early delamination process. Sandwich panels consisting of AR500 as the outer layer and magnesium alloy AZ31B as the core material were simulated using dimples of different diameters and depths; 5.0 / 2.5 mm, 6.0 / 3.0 mm, 7.0 / 3.5 mm and 8.0 / 4.0 mm, at a range of 50 % to 90 % of maximum strength, based on the lowest material strength for cyclic loading. The results indicate that the highest fatigue damage of 85 % occurred at the effective region of the dimple area, where force was applied to the panel. A hemispherical dimple core 6.0 mm in diameter and 3.0 mm in depth produced a reduction of failure at the effective zone of more than 85 % and the regression coefficient was over 0.89. This illustrates that a large dimple size was vulnerable to early delamination failure at the dimple region.

Multiple-order vertical vortex-induced vibration mechanism of a typical streamlined closed-box girder

Multiple-order vertical vortex-induced vibrations (VIVs) have been frequently observed in prototype bridges as well as tested models in wind tunnels, but VIV excitation and VIV suppression mechanisms need further investigation. In this study, a simplified vortex model (SVM) derived from the spatial distribution characteristics of the wind pressure around the girder surface was developed to elucidate the multiple-order vertical VIV phenomena of a typical streamlined closed-box girder with additional attachments. The results showed that there are three potential lock-in ranges of vertical VIVs. The 1st lock-in range is excited and sustained by the separated vortex formation at the trailing edge, which is referred to as trailing-edge-vortex-dominated VIV. In the 2nd and 3rd lock-in ranges, the contribution of local aerodynamic forces to the general vortex-excited forces (VEFs) over the upper surface varies with the local positions in the form of simple harmonic curves. Therefore, the local aerodynamic force can be simplified by a concentrated force with an approximately constant loading amplitude and drift velocity, which is illustrated as a simplified moving vortex; based on this, an SVM can be established and utilized to associate the vortex-drift pattern with multiple vertical VIVs in terms of Strouhal number of the separated vortex. With the help of the SVM, the 2nd and 3rd lock-in ranges are excited and sustained by the separated vortex formation at the leading edge and then by a regular vortex drift pattern over the upper surface of the bridge girder, referred to as impinging-edge-vortex-dominated VIVs. Spoilers installed on the handrails can effectively suppress the separated vortex formation both at the trailing edge and leading edge; thus, vertical VIVs in all lock-in ranges can be mitigated.

Towards three dimensional aspects of plasticity-induced crack closure: A finite element simulation

The mutual interactions between intrinsic (damage) and extrinsic (shielding) mechanisms are essential for understanding fatigue crack growth in ductile materials. In the latter case, plasticity-induced crack closure is the dominating retardation mechanism in the Paris regime. The transition between plane strain and plane stress states leads to a locally different fracture surface contact, which is difficult to access during experiments. In this work, plasticity-induced crack closure under constant amplitude loading of an AlCu4Mg (AA2024) aluminium sheet material is studied from a three-dimensional perspective. An elastic–plastic 3D finite element model of an M(T)160 specimen with a bilinear isotropic hardening model is used to study the evolution of plasticity during fatigue crack growth. Cyclic crack propagation is simulated with the releasing-constraint method.In the results, plasticity-induced crack closure is present up to a load ratio R = 0.3. Detailed investigations of the contact pressure distributions indicate a strong dependence on the stress intensity factors and the sheet thicknesses. The contact pressure distributions on the fracture surface can be divided into three characteristic regions. Near the plastic zone, plastic energy is still induced during the unloading process after the crack is already closed. However, the crack surface contact does not affect the shape of the plastic zone. Additionally, further plastic energy accumulates in the plastic wake region during crack closure within subsequent load cycles, described here as cyclic plastic wake. In summary, the finite element model can reproduce the major features of fatigue crack closure like Kop or Kcl and the three-dimensional and load-dependent evolution of plasticity-induced crack closure.

Multiaxial variable amplitude fatigue strength assessment of steel welded joints using the peak stress method

The peak stress method (PSM) is a rapid, FE-oriented method to evaluate notch stress intensity factors. In fatigue design of welded components, the PSM has been previously validated against>1300 experimental data to estimate the lifetime of steel welded joints under constant amplitude (CA) loadings. In the present work, the PSM has been extended to account for variable amplitude (VA) uniaxial as well as multiaxial loadings. To this aim, the PSM has been combined with the Miner’s Linear Damage Rule and it has been successfully validated against a large bulk of VA fatigue data taken from the literature.

Modelling fatigue uncertainty by means of nonconstant variance neural networks

AbstractThe modelling of fatigue using machine learning (ML) has been gaining traction in the engineering community. Among ML techniques, the use of probabilistic neural networks (PNNs) has recently emerged as a candidate for modelling fatigue applications. In this paper, we use PNNs with nonconstant variance to model fatigue. We present two case studies to demonstrate the developed approach. First, we model the fatigue life of cover‐plated beams under constant amplitude loading, and then we model the relationship between random vibration velocity and equivalent stress in process pipework. The two case studies demonstrate that PNNs with nonconstant variance can model the distribution of the data while also considering the variability of both distribution parameters (mean and standard deviation). This shows the potential of PNNs with nonconstant variance in modelling fatigue applications. All the data and code used in this paper are openly available.

Automated visual tracking of crack growth in coupon and component level fatigue testing using thermoelastic stress analysis

An automated system capable of visually detecting the two dimensional location of a crack in metal is presented. A peak-detection algorithm is applied to the thermoelastic signal component caused by heat diffusion, obtained from thermoelastic stress analysis using an industrial microbolometer, which is mounted on motion stages, allowing two dimensional tracking of the crack. The system is experimentally validated on coupons under constant amplitude loading, with the crack tip location estimates compared against measurements from an optical microscope. The system is also applied to monitoring crack growth on a geometrically complex section of an aircraft wing under representative flight-spectrum loading.

High‐cycle variable amplitude fatigue experiments and design framework for bridge welds with high‐frequency mechanical impact treatment

AbstractFatigue enhancement by way of high‐frequency mechanical impact (HFMI) treatment can enable effective design and construction of steel bridges. However, bridges may experience high and varying mean stresses, the effects of which are not covered today by any design recommendation or in the literature on HFMI‐treated joints. In this study, fatigue experiments were conducted with realistic in‐service bridge loading, which revealed the same high fatigue performance as for constant amplitude loading. The effect of mean stress in spectrum loading was quantified and a method to account for it in an equivalent manner is proposed. A design framework has been developed for design and engineering purposes.

Measurement-driven, model-based estimation of residual stress and its effects on fatigue crack growth. Part 2: Fatigue crack growth testing and modeling

This paper assesses the accuracy of fatigue crack growth (FCG) predictions for high-strength aluminum samples containing residual stress (RS) and complex two-dimensional cracks subjected to constant amplitude load. FCG predictions use linear-elastic, multi-point fracture mechanics. A first prediction includes RS estimated by the model described in Part 1; a second prediction includes RS measured by the contour method. FCG test data show a significant influence of RS. Ignoring the RS results in a +60% error in predicted FCG life (non-conservative). Including RS improves predictions of crack growth significantly (errors better than +26% (estimated RS) and −14% (measured RS)).