Fatigue Tests Of Specimens Research Articles

Fatigue Resistance of Reinforcing Steel Bars

During the last few years there has been an intensification of interest in the fatigue performance of steel reinforcement bars in concrete structures. Although fatigue has not proved to be a problem to date, loading cycles are becoming increasingly severe so that the margin of reserve strength is progressively being reduced. In this paper the authors present and discuss results of fatigue life tests performed for reinforcing steel bars which is one of the most widely used construction material in the world. Endurances can be influenced by type of steel, geometry and size of the bars, nature of the loading cycle, welding and presence of corrosion. The experimental part deals with fatigue testing of specimens for identification of the strain-life behaviour of reinforcing bars and determining the number of cycles to failure.

Influence of specimen type and reinforcement on measured tension–tension fatigue life of unidirectional GFRP laminates

It is well known that standardised tension–tension fatigue test specimens of unidirectional (UD) glass-fibre-reinforced plastics (GFRP) laminates tend to fail at end tabs. The true fatigue life is then underestimated. The first objective of this study was to find for UD GFRP laminates a test specimen that fails in the gauge section. The second objective was to compare fatigue performance of two laminates, one having a newly developed UD powder-bound fabric as a reinforcement and the other having a quasi-UD stitched non-crimp fabric as a reinforcement. In the first phase, a rectangular specimen in accordance with the ISO 527-5 standard and two slightly different dog-bone shaped specimens were evaluated by means of finite element modelling. Subsequent comparative fatigue tests were performed for the laminates with the three specimen types. The results showed that the test specimen type has a significant effect on the failure mode and measured fatigue life of the laminates. A significantly higher fatigue life was measured for the laminate with the powder-bound fabric reinforcement when compared to the laminate with the stitched reinforcement.

Material characterization of high-frequency mechanical impact (HFMI)-treated high-strength steel

Fatigue strength improvement due to high-frequency mechanical impact (HFMI) is based on compressive residual stresses, improved weld toe geometry and strain hardening of the treated surface region. In order to understand the combined effects of these factors on the weld toe elastic–plastic response and residual stress stability, the material properties in the HFMI-treated weld toe region need to be determined. In this study, five different material conditions were studied: high-strength steel base material, Gleeble simulated heat-affected zone, and three HFMI-treated material conditions. The local material characteristics were determined by microhardness measurements, electron backscatter diffraction and mechanical testing. The specimens were fatigue tested under fully reversed strain-controlled loading. Surface residual stresses in the fatigue test specimens were measured with X-ray diffraction. The results showed that the heat-affected zone had lower fatigue and yield strength than the base material due to larger grain size. HFMI-treatment increased the steel yield strength up to 1–1.5mm. This observation was supported by change in grain shape in the treated surface region. This lead to an increase in fatigue strength at low strain amplitudes and a decrease in fatigue strength at high strain amplitudes. Continuous cyclic softening was observed for all material conditions.

Fatigue life prediction for 7050-T7451 aluminum alloy plate with scratch defect

Fatigue damage of 7050-T7451 aluminum alloy plate with scratch was studied,and the fatigue life of aluminum alloy plate after scratching was predicted considering the common effects of the residual stress,plastic damage and fatigue load. The nonlinear dynamic finite element( FE) analysis was conducted to simulate scratch generation,and the residual stress field and plastic strain field near scratch were obtained.According to the equation of plastic damage,the initial damage field near scratch caused by plastic deformation in the scratching process was calculated. Based on the multiaxial fatigue damage model,the finite element numerical method was established to analyze fatigue damage,and the calibration of material parameters in the damage evolution equation was conducted. This method was used to predict the fatigue life of scratched aluminum alloy plates with synthetically considering the residual stress field,the initial plastic damage and fatigue damage. The predicted results were compared with results from the fatigue test of specimens,and the theoretical prediction was in accordance with the test results of fatigue life. A new method and a practical means for fatigue life prediction of metal component with scratch defect were provided.

Crack path for run-out specimens in fatigue tests: is it belonging to high- or very-high-cycle fatigue regime?

Fatigue tests run-out specimens up to 106 – 5x107 load cycles are used to determine the stress level named “fatigue limit”. Nevertheless, it is not clear what kind of fatigue cracking takes or will take place in these specimens. To discuss this problem, fatigue tests of titanium alloy VT3-1 specimens have been performed under tension with different values of R-ratio and under rotating-bending after various thermo-mechanical treatments (tempering, surface hardening and their combinations). Well-known S-N curves in High-Cycle- Fatigue regime have been plotted with run-out specimens usually used for “fatigue limit” determination. Then, after fatigue tests, run-out specimens have been tensed up to their failure, and fracture surface analyses have been performed for all tested specimens. It is found that run-out specimens in all combinations of treatments, for different R-ratio, have fracture surfaces for crack path in Very-High-Cycle-Fatigue regime. Based on this result, all S-N curves have been reconstructed in duplex curves for High- and Very-High-Cycle-Fatigue regime without using knowledge about “fatigue limit”. Detailed fracture surfaces analyses have been developed, and crack paths have been compared for various combinations of materials and surface states.

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( ? a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Notch Effect on the Fatigue Behavior of a Hot-Dip Galvanized Structural Steel

We investigate the effect of a galvanizing coating on the fatigue strength of S355 structural steel. Although some results on fatigue tests of unnotched specimens are available, those on notched components are scarce. A comparison is carried out between hot-dip galvanized specimens weakened by a central hole and untreated specimens of the same geometry for two nominal load ratios R = 0 and R = −1. In total, over 60 new experimental data are summarized and analyzed in the present contribution.

Texture, microstructure, and fractal features of the low-cycle fatigue failure of the metal in pipeline welded joints

Crystallographic texture and fracture features are studied after low-cycle fatigue tests of laboratory specimens cut from the base metal and the characteristic zones of a welded joint in a pipeline after its longterm operation. The fractal dimensions of fracture surfaces are determined. The fractal dimension is shown to increase during the transition from ductile to quasi-brittle fracture, and a relation between the fractal dimension of a fracture surface and the fatigue life of the specimen is found.

Evaluation of the Period of Initiation of Fatigue Microcracks on the Race of an Axle Bearing

We propose a numerical method of evaluation of the period of initiation of a fatigue microcrack at the tip of a surface cavity-like defect in the race of an axle bearing of the locomotive. In this method, we use a model based on the energy approach of fracture mechanics. To find the required mechanical characteristics of the material of the racer of an axle bearing, we perform static and fatigue tests of specimens of ShKh15 steel. It is shown that if the surface cavity-like defect is located in the most loaded place of the race, then a fatigue microcrack can be initiated near its smooth tip for a relatively short period of operation of the bearing.

Fatigue life calculation for titanium alloys considering the influence of microstructure and manufacturing defects

A model is presented for calculation of the fatigue life to crack initiation in specimens with surface stress concentrators. The model is tested for the results of fatigue testing specimens of Ti–6Al–4V titanium alloy condensate prepared by electron-beam physical vapor deposition method (EB PVD-method). The specimens had manufacturing defects, such as column defects of different diameters reaching the specimen surface. Comparison between results of calculation and experiment showed a good agreement. In order to fill the model, it is sufficient to have the results from monotonic tensile testing and characteristics of microstructure and texture of the starting material.

Resonance bending fatigue testing with simultaneous damping measurement and its application on layered coatings

The use of specimen loss factor as fatigue damage indicator of Hastelloy-X substrates with different surface treatments was investigated together with other fatigue damage indicators, namely resonance frequency and crack mouth length. The tested surface treatments included grit-blasting and plasma spraying of NiCoCrAlY bond coat and yttria stabilized zirconia (YSZ) top coat. The loss factors of fatigue test specimens were measured repeatedly during the resonance bending fatigue test using the conventional free decay method. The analysis of the damping spectra, i.e. the model describing the relation of loss factor to maximum macroscopic specimen strain εyy was drafted. The model is based on the combination of defect models developed by Göken and Riehemann (2004) and classical dislocation model of Granato and Lücke (1956). It appears, that the damping spectra can be well approximated as a combination of two defect peaks (C1 and C2) and one dislocation peak (D1). The low strain defect peak (peak C1) is sensitive to the presence of fatigue cracks. The second defect peak (peak C2) can be attributed to the remaining substrate and coating defects such as embedded grit particles, coating porosity, surface roughness and sliding in the sample clamping area. The fatigue damage detection using the C1 peak magnitude was performed and its results were related to the crack length obtained by digital image correlation (DIC) method. In the crack initiation stage I., the C1 peak height shows different behavior than the resonance frequency and therefore provides new information. The underlying processes causing C1 peak changes need to be found yet, however. In the crack growth stage II., both resonance frequency and peak height C1 correlate with the measured fatigue crack size.

Tailoring the Hardening Behavior of 18CrNiMo7‐6 via Cu Alloying

In order to improve the rolling contact fatigue (RCF) behavior of gear steels, a concept to increase their damage tolerance is developed alternatively to the conventional approach of improving the degree of steel cleanliness. For that purpose, Cu is used as a main alloying element in order to trigger the precipitation of nano‐sized Cu precipitates which shall improve the strain‐hardening rate of the martensitic matrix of Cu‐alloyed 18CrNiMo7‐6 steel surrounding a non‐metallic inclusion during plastic deformation. In this way, early component failure may be avoided and the maintenance costs of, e.g., wind energy converters may be kept low. The experimental analysis shows that nano‐sized Cu precipitates influence the material's strength, ductility, and strain‐hardening behavior under tension, depending on their coherence. Among others, the latter is related to strain‐induced martensitic transformation of coherent Cu. The structure of the Cu precipitates is studied by TEM and SANS analysis. The Cu‐alloyed steel also shows an increased hardening‐exponentCHT studied by cyclic hardness test (CHT) PHYBALCHT. Fatigue tests of specimens with coherent precipitates show cyclic hardening until a critical stress amplitude. Above that, stress amplitude cyclic softening is detected. An increased damage tolerance could be obtained for a 1 mass‐% Cu‐alloyed 18CrNiMo7‐6 steel.

Energy Accumulation in Steel, in the Conditions of Constant-Amplitude

This work presents results of fatigue tests of P91 steel specimens under constant-amplitude and programmed loading in temperature T=600°C. Two strain levels, applied in different order, were used for programmed loading. The loading program level changes were introduced for different levels of damage. The tests provided the basis for analysis of strain energy ΔWpl in the function of the number of load cycles under constant-amplitude and programmed loading. It has been found that that the form of a loading program has an influence on the life and value of energy cumulated in the specimen until crack initiation. Verification of Palmgren Miner hypothesis revealed that its effectiveness depends on the loading program and temperature.

Base material fatigue data for low alloy forged steels used in the subsea industry. Part 1: In air S–N data

This is the first of two papers presenting fatigue data for low alloy forged steels in a quenched and tempered condition. This paper considers the effect of the surface roughness, mean stress and material strength on the high cycle fatigue crack initiation life on low alloy steels having tensile strengths in the range from 600 to 800MPa. The scatter in fatigue life has been quantified for the data set consisting of 354 fatigue test results. Fatigue test specimens having surface roughness from Ra=0.3μm (polished surface) to Ra=6.3μm (coarse production surface, resulting for instance from coarse surface grinding). A method for estimating high cycle fatigue data for smooth low alloy steel from the tensile strength or the Vickers hardness is presented. The estimation method has been validated with fatigue test results and compared with other published methods for estimating strain–life fatigue properties. Part 2 of this paper addresses the effect of seawater with cathodic protection on the fatigue crack initiation life for smooth specimens.

The effect of pitting corrosion on the safe-life prediction of the Royal Australian Air Force P-3C Orion aircraft

This paper investigates if corrosion pitting invalidates the fatigue lifeing method used by the Royal Australian Air Force to manage the structural integrity of its P-3C Orion aircraft. Fatigue life predictions made using the P-3C Service Life Assessment Program method were compared with those from an Equivalent Crack Size model for pitting corrosion. The data for this comparison came from a program of variable amplitude fatigue tests of high-kt specimens of 7075-T6. This testing was performed using two stress levels on both corroded and uncorroded specimens. Analysis of the results showed that the safe-life estimates made using the P-3 life assessment method were conservative in the presence of corrosion as they were above the 99.9th percentile of fatigue life predicted by the equivalent crack size method.

Research on fatigue crack propagation of micro-alloyed steel by acoustic emission

This study focused on fatigue crack propagation of Q345 steel in tests using acoustic emission technique. The fatigue tests of the Q345 steel plate specimens were monitored with acoustic emission, and acoustic emission amplitude, energy and counts were obtained. According to the acoustic emission features, the fatigue crack growth was divided into three parts: primary stage (amplitude of 42–51 dB and energy of 1–10 V μs), steady stage (amplitude of 42–73 dB and energy of 1–80 V μs) and rapid growth stage (amplitude of 42–84 dB and energy of 1–170 V μs). The results showed that acoustic emission could be used as a non-destructive testing method to monitor and analyse the fatigue deformation of micro-alloyed steels because acoustic emission is relatively sensitive to fracture mode changes.

Wiles of Using Hollow Specimens for Fatigue Tests

The paper reports an issue found in using the hollow specimens claimed usually as unnotched. If the goal is to test them under multiaxial loading, the target is to reduce the shear stress gradient by making them thin-walled. Anyhow, the analyses described here show, that this attempt induces an issue unknown previously to authors – the maximum stress in tension loading is found on the inner surface of the tube. The individual effects concerning material and geometry are described.

Stringer-sheets fatigue cracking of helicopter-tail-boom in welding joints of 01420 AL–Li alloy

Regularities of fatigue crack appearance and growth in-service cracked stringers of helicopter Mi-26 tail-boom area were demonstrated. Statistic analyses of cracks distribution in helicopter tail-boom has shown crack appearance not only in stringers manufactured from Al–Li alloy but in frames too. Fatigue tests of complicated sheet-stringer specimens reproducing spot-welding-bonding joint assembly experienced constant cyclic loads amplitude were performed. Result of tests have shown fatigue cracking by the regular change of inter- and transgranular crack propagation. This manner of material cracking is similar case for in-service cracked stringers. The discovered mechanism of Al–Li alloy fatigue cracking is considered based on the results of fractographic investigations in scanning electron microscope. The discussed mechanism of the crack propagation reflects Li diffusion in grain boundary under stressing ahead of a crack tip during fatigue crack propagation that weakened grain boundary and accelerate crack propagation. Crack growth periods for tested specimens were estimated and compared with in-service registered crack growth duration based on regular non-destructive inspections. In-service fatigue cracking period of stringers was established and tolerance in-service duration for helicopter between two inspections has been recommended.

J2210305 シリコン単結晶における破壊の結晶方位依存性と製造歩留り

Fracture of single-crystal-silicon fatigue-test specimens during fabrication was analyzed as the result of an uniform loading due to film residual stress. Five kinds of specimens showed different fracture probability from 1.4% to 26.5% depending on the crystal orientation. The ratios between the applied stress and scale parameter were analyzed based on Weibull statistics. Applied stresses were also analyzed by finite element simulation. The adjustment of crystal orientation was shown to be effective to suppress fractures of MEMS devices of single-crystal silicon.

Strain-based Multiaxial Fatigue Life Evaluation Using Spectral Method

To describe the impact of multiaxial service loads a power spectral density matrix of the strain tensor components has been taken into account. To determine the fatigue life some selected multiaxial fatigue failure criteria have been used. The main focus of this paper is set on the comparison of the cycle counting method and the spectral method. An algorithm used to calculate the fatigue life in the time domain as well as in the frequency domain has been introduced. The proposed algorithm has been verified on the basis of fatigue tests of cruciform specimens made out of the S355J2WP steel.