Constant Amplitude Loading Research Articles

Fatigue life investigation of non-load carrying fillet weld of structural offshore steel S460G2+M using experiment and FEM simulation

This research deals with the fatigue life investigation using experiment and FEM simulation on longitudinal fillet weld of structural offshore steel S460G2+M with a thickness of 10mm. The experimental fatigue test is conducted based on nominal stress (NS) approach while the simulation uses the effective notch stress (ENS) approach for fatigue life assessment. The investigation begins with the preparation of the longitudinal fillet weld fatigue specimen using Milling Machine based on the IIW fatigue specimen design recommendation and joined using the semi-automatic GMAW process following the AWS D1.1 procedure. Fatigue testing on non-load carrying fillet weld is conducted using the Instron Fatigue Machine with a stress ratio of 0.1 with constant amplitude loading and stress loading from 50%-75% of the yield strength of the base material. For the simulation approach, the 3D longitudinal fillet weld geometry is created using CAD based on the ENS design procedures of IIW where the sizes and dimensions are similar to the experimental fatigue specimen. The static elastic stress analysis of the model is conducted using MSC Marc/Mentat FEM software. Based on the IIW fatigue data evaluation, it is found that the natural mean curve of the longitudinal fillet weld obtained 146 MPa of FAT class which exceeds approximately 106% from the IIW FAT class recommendation for a longitudinal fillet joint. However, the characteristic curve of 97.7% failure probability of the parts only attained 95 MPa of FAT class, but still exceeds approximately 33% form the IIW FAT class. In the ENS fatigue assessment of the 3D longitudinal fillet weld, it is found that the model obtained 195MPa of FAT class which is inferior approximately 15 % from the FAT class recommendation of IIW. Also, it is found that both S-N curves of NS after conversion to ENS system have a good agreement with the S-N curve of ENS of 3D longitudinal fillet weld model and the IIW recommendation due to only 16 % and 34% lower as compared with the IIW FAT class recommendation for ENS.

Damage Tolerance Capability of Retrogression and Re-aged 7010 Aluminum Alloy Under FALSTAFF Loading

The present work deals with the damage tolerance characteristics of high strength aluminum alloy tempered in T6 and reversion condition. The fatigue experiments were carried out by applying a service simulating load spectrum, i.e., standard mini FALSTAFF loading. The crack propagation speed was found to be lower and the total crack propagation life was longer by 22% for reversion-treated alloy. The crack growth was also predicted to be using two parameter crack driving force approach. The fatigue data of these treated alloys under constant amplitude loading at various stress ratios were analyzed to obtain crack growth law. The predicted crack growth behavior was conservative and followed similar trend in both the alloys as observed in experiments. Predicted results of reversion-treated alloy also showed longer crack growth life. The modified microstructure after reversion treatment was attributed for the observed improvement in damage tolerance capability.

PREDICTION OF FATIGUE CRACK GROWTH DIAGRAMS BY METHODS OF MACHINE LEARNING UNDER CONSTANT AMPLITUDE LOADING

Important structural elements are often under the action of constant amplitude loading. Increasing their lifetime is an actual task and of great economic importance. To evaluate the lifetime of structural elements, it is necessary to be able to predict the fatigue crack growth rate (FCG). This task can be effectively solved by methods of machine learning, in particular by neural networks, boosted trees, support-vector machines, and k -nearest neighbors. The aim of the present work was to build the fatigue crack growth diagrams of steel 0.45% C subjected to constant amplitude loading at stress ratios R = 0, and R = –1 by the methods of machine learning. The obtained results are in good agreement with the experimental data.

Multiaxial fatigue behavior and life prediction of 7075-T651 aluminum alloy under two-step loading

Focusing on the loading sequential effects on the multiaxial fatigue damage accumulation, tests including two series of constant amplitude loadings and 12 groups of two-step loading spectra were conducted on 7075-T651 aluminum alloy. Under the same equivalent stress amplitude, the multiaxial fatigue life raises with the increase of stress amplitude ratio. Fatigue damage accumulation rate is decreased by the two-step loading interaction. Cyclic loading curves and fracture morphology analysis reveal that the loading step corresponding to lower fatigue life is dominant. Considering the stress amplitude ratio effects and loading interaction, a non-linear accumulative damage model is proposed based on the modified Basquin’s criterion. The proposed model provides a more accurate calculation of accumulative fatigue damage compared with the other four commonly used models. Fine life prediction results are obtained by the proposed model with all predicted data within a factor of three.

Fatigue crack growth modelling of Fão Bridge puddle iron under variable amplitude loading

This study addresses the fatigue crack growth behaviour of a material from an old riveted steel bridge, the Portuguese Fão Bridge, based on an experimental program of constant and variable amplitude loading tests. The material under consideration is a centenary puddle iron, for which information on fatigue crack growth under variable amplitude loads is not available in the literature. Additionally, the experimental program aimed at analysing the fracture surfaces of the specimens through observations under Scanning Electron Microscopy (SEM), and correlate their characteristics with macroscopic fatigue crack growth behaviour. Furthermore, a crack closure analysis was performed using electrical strain gauges applied on the back face of the CT specimens. The fatigue crack propagation along the parallel to the rolling direction (L) revealed to be higher when compared with the transverse direction (T). The experimental data obtained in this study were compared with existing fatigue crack propagation models, such as the Paris model with linear damage accumulation, according to Miner’s rule, as well as with more complex crack propagation models, with capacity for modelling retardation effects of single overloads. The Paris model with linear damage accumulation, despite its simplicity, led to the best predictions for the generality of variable amplitude random block loading. The original Wheeler model is enough to correlate the crack propagation data under overloads, since the application of more sophisticated models would not be justified by the extremely high scatter observed in these old materials.

Experimental Research on Anchoring Behavior of Chemical Anchor Bolt under Repeated Loading

To study the anchoring behavior of chemical anchors under repeated loading, 33 test pieces were designed and fabricated based on the specifications. Each 3 test pieces were 1 set, that is, 1 set of comparative test pieces and 10 sets of test pieces, respectively, for static force. Bonding and anchoring performance tests for loading and reloading. Through the analysis of the test phenomena and results, the following conclusions are drawn: when the loading load exceeded 0.9 times the ultimate bearing capacity of the anchor; when the loading load was equal to 0.6 or 0.75 times, the test piece would undergo splitting damage after the set number of loading times, and its bearing capacity would increase. As the number of loading increased, the energy consumption would gradually decrease, and the decreasing amplitude would gradually increase. Compared with constant amplitude loading and variable amplitude loading, it is found that the slip value of variable amplitude loading increased by 42.1% and the bearing capacity decreased by 12%, which indicated that variable amplitude loading has a great influence on the bonding and anchoring performance of specimens.

Real-time damage evaluation method for multiaxial thermo-mechanical fatigue under variable amplitude loading

A real-time multiaxial thermo-mechanical damage evaluation method was proposed under variable amplitude loading. In order to achieve real-time damage evaluation, a real-time multiaxial cycle counting method that can take into account temperature loading history was proposed. The proposed multiaxial cycle counting method can obtain the reversals one by one in order with the reading of the load-time history data, which can realize the damage calculation immediately as a single reversal is counted out. At the same time, to consider the effect of temperature loading on the damage accumulation, an equivalent temperature was determined for the time period of the counted reversal. A fatigue-oxidation-creep damage model that can consider the influence of the non-proportional additional hardening on the fatigue, oxidation and creep damages was used to predict the lives in cases of isothermal axial-torsional fatigue, uniaxial thermo-mechanical fatigue and axial-torsional thermo-mechanical fatigue under constant and variable amplitude loadings. The results showed that the prediction errors were within a factor of 2.

The effect of grain size on the fatigue overload behaviour of nickel

The fatigue overload behaviours of coarse grain (~30 μm), ultrafine grain (360 nm) and nanocrystalline (~30 nm) Ni are compared under constant amplitude loading at R = 0.1 after a 100% overload. Synchrotron X-ray diffraction is applied to map the elastic crack-tip strain fields at the mid-thickness of a compact tension specimen of nanocrystalline Ni at various stages through the loading cycles, from which the variation in stress before, during and after overload is estimated. Digital image correlation is used to measure the crack length and the displacement fields at the specimen surfaces for both grain sizes, from which the fatigue crack growth (FCG) rate, crack opening displacement and stress intensity factor range are determined. The FCG for coarse grain Ni is most significantly retarded whereas the nanocrystalline Ni is least affected by the overload due to an increased yield stress and a more planar crack surface morphology. As a result, FCG retardation by plasticity, surface roughness and residual stress-induced crack closure are reduced.

Surface cracks growth in aluminum alloy AW-2017A-T4 under combined loadings

The work deals with the tests results of the growth of fatigue cracks subjected to torsion and bending with torsion specimens at the constant amplitude loading and with zero or non-zero mean load. In the fatigue tests, specimens made of AW-2017A-T4 alloy with rectangular cross-section weakened by external notches were used. The results of experimental and numerical calculations were presented. During experimental investigations, heterogeneous growth of fatigue cracks at the front and rear of the specimen was noticed. Thereby, both the crack growth rate and the cracking process itself are affected. Two types of fatigue cracking were also observed: surface and passing through specimen. Crack paths were measured on the surface of the specimens using a microscope. Different lifetimes were observed for the specimens subjected only to cyclic torsion compared to those subjected to bending with torsion in the first and second periods of cracking. At the final stage of cracking, both in bending with torsion and in torsion, the difference in crack lengths for ratio a/c and a/a* is greater for R = −1 than for R = 0.

Cracking simulation‐based fatigue life assessment

AbstractA fracture mechanics numerical model is developed to simulate the collective behavior of growing short fatigue cracks originating from the surface of unnotched round specimens made of a two‐phase alloy. The specimen surface roughness is considered resembling microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Variations in mechanical and microstructural features of grains are randomly distributed. Possible activities of surface cracks are predicted against loading cycles till either fracture occurs or all existing cracks become nonpropagating. The material's S‐N curve and fatigue limit can, thus, be assessed. Published experimental data on ferritic‐pearlitic steel specimens in push‐pull constant amplitude loading (CAL) were utilized. Different specimens were randomly configured and virtually tested. Comparison of experimental results and corresponding predictions validates the model, which, further, recognizes the effect of surface roughness, specimen size, and mean stress on lives.

Prediction of SMA residual lifetime taking into account mechanical properties under constant amplitude loading

Prediction of SMA residual lifetime taking into account mechanical properties under constant amplitude loading

Ponašanje širenja prskotine na FM-laminatu posle preopterećenja zatezanjem pod konstantnim amplitudnim opterećenjem

The crack propagation behaviour on the metal laminate of the FMLs is investigated under constant amplitude loads, which is higher compared to the fatigue strength of the metal laminate, and the subsequent crack propagation behaviours after being highly overloaded. The observation was carried out on the FMLs with the various surface roughness of the metal laminate at the interface between the metal and fibre-reinforced composite laminate. The overload leads to delay of the crack propagation following the overload, and the higher surface roughness increases the number of delay cycles. The overload together with the surface roughness affects stress condition in front of the crack tip and the crack tip condition as well as the compressive residual stress in front of the crack tip after the overload. In addition, the higher surface roughness strengthens the shear strength of the interface leading to the delamination caused by cyclic loads to be more difficult to take place. Therefore, the delay cycles after the overloading become longer.

Fractographic analysis of the overload effect on fatigue crack growth in 2024-T3 and 7075-T6 Al alloys

The effect of single overload on the fatigue crack growth in 2024-T3 and 7075-T6 Al alloys was analyzed. Fatigue tests under constant-amplitude loading with overload peak were carried out on V-notched specimens. Fractographic analysis was used as a principal approach to explain the crack growth retardation due to the overload. Scanning electron microscopy (SEM) analyses were conducted on the fractured surface of failed specimens to study the retardation effect. The obtained results show that the overload application generates a plastic zone in both aluminum alloys. The generated plastic zone is three times larger in the case of 2024-T3 compared to 7075-T6, and thus, a significant crack retardation was induced for 2024-T3. The retardation effect due to the overload for 2024-T3 and 7075-T6 lasted for about 10 mm and 1 mm, respectively, from the point of overload application.

The application of transient thermal tensioning for improving fatigue crack growth resistance of AA5083-H116 FSW joints by varying secondary heating temperature

In this work, transient thermal tensioning (TTT) treatments were applied to friction stir welding of AA5083-H116 plates at various secondary heating temperatures of 100, 200 and 300 °C with the primary aim of gaining better fatigue crack growth performance. Fatigue crack growth tests were conducted with a constant amplitude load on centre crack tension (CCT) and single edge cracked tension (SECT) specimens. Results show that TTT treatments modify the distribution of residual stresses and cause microstructural changes. These two factors strongly influence fatigue performance of the weld joints and mechanisms of the fatigue crack growth are discussed in this investigation.

Fatigue crack growth behaviour in Ti6Al4V alloy specimens produced by selective laser melting

The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio $$\hbox {R}=0$$ was detected and for $$\hbox {R}=0.4$$ no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in $$\upbeta $$ phase contouring the martensitic $$\upalpha $$ phase.

Numerical and experimental validation of residual stresses of laser‐welded joints and their influence on the fatigue behaviour

AbstractIn this work laser‐welded tube‐tube specimens made of aluminium alloys AlMg3.5Mn and AlSi1MgMn T6 were experimentally tested under constant and variable amplitude loading, under pure axial and pure torsion loading. In order to evaluate the influence on fatigue behaviour of the residual stresses, because of the welding process, some specimens were subjected to postweld heat treatment and then were tested. The numerical analyses, using finite element (FE), were carried out to obtain a reliable estimation of the residual stress in the specimen. The numerical results were in a good agreement with experimental ones obtained by means of hole‐drilling method. Finally, the residual stress distribution was superimposed to stress distribution because of fatigue loads obtained by FE analyses applying local concept, to calculate the stresses in the crack initiation zone and to understand the different types of failure that occurred in as‐welded and relieved specimens.

Fatigue behaviour of stainless steel bolts in tension and shear under constant-amplitude loading

This paper experimentally investigates the fatigue performance of stainless steel bolts in tension and shear under constant-amplitude loading. The static behaviour of normal bolts and Hollo-bolts subjected to monotonic loading was initially reported aiming to obtain the ultimate strength. Periodic loading tests were thereafter performed to acquire tensile and shear fatigue life under various stress ranges in which failure modes and cycles were recorded. From test results, S-N curves were summarised through regression analysis, followed by numerical modelling which was used to investigate the stress distribution. Finally, different design codes were selected to estimate the fatigue limit of bolts in tension and shear. It was found that both coupon tests and monotonic loading tests are required to determine the material property of stainless steel bolts. Regions adjacent to bolt nuts are the most vulnerable parts in terms of fatigue in tension, and the maximum principal stress dominates the fatigue life of fasteners with threads in shear plane. The fatigue strength of Hollo-bolts is slightly higher than that of normal bolts. Moreover, although current design codes can conservatively estimate the fatigue behaviour, provisions for shear fatigue are recommended to be modified for higher accuracy and better application.

RETRACTED: Fatigue characteristic and life test of reinforced concrete T girders under cyclic loading

Aiming at the issue of reinforced concrete girder fatigue, a typical T shaped scale model girder form is selected as the test specimen based on the real structure drawings. On the basis of designing test program reasonably, 7 same batch test girders are loaded with different constant cyclic loading amplitude. Thus according to the test data, girder failure mode, fatigue life, mechanical property evolution law as well as rebar S-N curve is discussed systematically. The result indicate that the longitudinal rod fracture is the domain failure mode of girders, which can finally induces whole specimen rupture brittlely. The mode is clear and definite in the observation. On account of all observation, the longitudinal rod stress range is more regular and stable, which seems suitable for life evaluation. Using the data obtained above, the rod S-N curve equation is calculated by linear fitting method. Result shows the data fit well and all the data lie within the normal range of deviation, which may reveal the law between cyclic load and girder fatigue life.

Fatigue strength assessment of ultra-high-strength steel fillet weld joints using 4R method

A fatigue analysis on experimentally tested transverse fillet-welded non-load-carrying T- and X-joints made of S960 and S1100 ultra-high-strength steel was carried out in the present study. The test data consisted of welded joints in the as-welded, high frequency mechanical impact-treated, and tungsten inert gas-dressed conditions, that were fatigue tested using uniaxial constant amplitude loading with an applied stress ratio of R = 0.1–0.5. The weld geometry and residual stress measurements were carried out, and for each joint, the fatigue strength was assessed using a multiparametric notch stress approach, entitled the 4R method, which incorporates the consideration of four parameters, i.e. notch stress range Δσk(r), applied stress ratio R, material ultimate strength Rm, and residual stress σres in the fatigue assessment. In the 4R method, the local cyclic elastic-plastic behavior at the notch root is obtained, and the Smith-Watson-Topper parameter is applied to conduct a mean stress correction to commeasure all results into a single S-N curve. The results showed that the applied stress ratio had a distinct influence on the fatigue strength capacity for both joints in the as-welded and post-weld treated conditions when using the conventional stress-based approaches, i.e. nominal stress, structural hot-spot stress and effective notch stress concepts. Nevertheless, the 4R method resulted in a good agreement between the experimental test results and the fatigue strength assessments, regardless of the load and joint conditions.

Overview of the geometrical influence on the fatigue strength of steel butt welds by a nonlocal approach

AbstractThe paper deals with the high‐cycle fatigue behaviour of steel butt‐welded joints in the as‐welded condition, under constant amplitude loading. It tackles the problem of strength assessment by changing the geometrical shape and features of the weldment. This investigation considers the recommendations available in design standards, particularly referring to the Eurocodes, and it re‐analyses experimental data where a clear profile of the actual weld shape is available. For the available weld profiles, numerical investigations are shown by applying the Implicit Gradient approach for the evaluation of the nonlocal effective stress in the fatigue strength assessment. The results are very promising since the nonlocal approach significantly agrees with design standard recommendations in conventional cases, but it is also able to provide specific indications on the influence of the geometrical features, as well as the tip radius and opening angles, specifying their influence when no explicitly detailed indications are available in the design standards.