Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Initiation and propagation behaviour of fatigue cracks in hard-shot peened Type 316L steel in high cycle fatigue

Observations of fatigue crack growth behaviour were made during rotating‐bend testing of hard‐shot peened Type 316L steel. From the results of these observations, the crack that developed in the axial direction was observed and the mechanism of the fatigue crack properties was clarified as follows: (1) Small circumferential surface fatigue cracks were detected at 60% of the fatigue lifetime. These cracks propagated very slowly in both the circumferential and radial directions. (2) When a radial crack reached a depth of between 150 and 350 μm, axial fatigue cracks were formed. (3) In the next stage, either the radial or the axial fatigue cracks continued propagating, or an inwards growing radial crack formed from the axial crack. (4) In the final stage, the circumferential surface crack began to grow rapidly and resulted in fracture. (5) The fracture type of hard‐shot peened Type 316L is a particular type of surface fracture.     

Applicability of life prediction methods to the low cycle fatigue of braze clad AlMn1.0Mg0.5 alloys

The original and modified universal slope methods, the uniform material law, and the modified Diercks equation have been applied to the low cycle fatigue life prediction in the braze clad AlMn1.0Mg0.5 alloys. The experimental data from clad and non-clad, pre-strained and non-pre-strained AlMn1.0Mg0.5 alloys at room temperature and at 75 and 180°C have been used to evaluate the applicability of the life estimation methods. It is likely that there is no universal method to predict the low cycle fatigue lives of all kinds of aluminium alloys. However, the modified universal slope method and the modified Diercks equation provide reasonably good predictions in these braze clad aluminium alloys.     

Improvement of fatigue properties by shot peening for Mg-10Gd-3Y alloys under different conditions

In the present study we investigated the influence of shot peening on the high cycle fatigue (HCF) performance of the Mg-10Gd-3Y magnesium alloys in four different conditions referred to as-cast, cast-T6, as-extruded and extruded-T5, respectively. The results show that shot peening can cause different degree of enhancement of fatigue performance for Mg-10Gd-3Y alloys depending on the Almen peening intensity applied; and that the Almen intensity could always be found that conferred the optimum improvement. The effect of shot peening was quantified, and for the as-extruded and extruded-T5 alloys it was found to be superior to that for the as-cast and cast-T6 alloys. The peened extruded-T5 Mg-10Gd-3Y alloy showed the highest fatigue strength at 10⁷ cycles of 240 MPa. The results of the analyses established a connection between the grain size, ductility and precipitates within the studied alloys. Microstructure affected the magnitude of the surface roughness induced by shot peening and also the maximum compressive residual stress and its relaxation during fatigue, and then determine the beneficial effect of shot peening.     

Micromechanisms of fatigue crack growth in aluminium alloys

The fatigue crack growth characteristics of high-strength aluminium alloys are discussed in terms of behaviour during mechanical testing and fracture surface appearance. For a wide range of crack growth rates, the crack extends both by the formation of ductile striations and by the coalescence of micro-voids. Dimples are observed at stress intensities very much less than the plane strain fracture toughness, and this is explained in terms of the probability of inclusions lying close to the crack tip. The striation formation process is described as a combination of environmentally-enhanced cleavage processes and plastic blunting of the crack tip.     

Influence of shot peening on notched fatigue properties of magnesium alloy ZK60

Abstract

The influence of shot peening on high cycle fatigue performance of notched specimen was investigated for ZK60 and ZK60-T5 magnesium alloys. The results show that the notched fatigue strengths (at 10⁷ cycles) for ZK60 and ZK60-T5 alloys increase from 150 and 155 MPa to 220 and 240 MPa at the optimum Almen intensity of 0·30 and 0·40 mmN respectively. In comparison to ZK60 alloy in extruded condition, higher notched fatigue performances of both unpeened and peened specimens were observed for ZK60-T5 alloy.     

Rapid determination of fatigue life based on temperature evolution

This paper presents an approach to rapidly predict high cycle fatigue life based on the temperature evolution of a specimen under cyclic loading. The initial slope of the temperature evolution was proved as an indicator for rapid estimation of fatigue life theoretically. Meanwhile, a real-time temperature measurement system based upon high-precision semiconductor temperature sensors was developed to detect the temperature evolution based upon high-precision semiconductor temperature sensors. In order to verify the presented approach, constant-amplitude fatigue tests were carried out on A7N01 aluminium alloy and the welded joints, respectively. The predicted fatigue life based on proposed approach was in good agreement with experimental results.     

Study of fatigue behaviour of 7475 aluminium alloy

Fatigue properties of a thermomechanically treated 7475 aluminium alloy have been studied in the present investigation. The alloy exhibited superior fatigue life compared to conventional structural aluminium alloys and comparable stage II crack growth rate. It was also noticed that the fatigue crack initiated from a surface grain and the crack extension was dominated by ductile striations. Analysis also revealed that this alloy possessed fracture toughness and tensile properties superior to that noticed with other structural aluminium alloys. Therefore the use of this alloy can safely reduce the overall weight of the aircraft.     

Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue

The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin²ψ technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes.     

Evaluating surface deformation and near surface strain hardening resulting from shot peening a tempered martensitic steel and application to low cycle fatigue

The plastic deformation resulting from shot peening treatments applied to the ferritic heat resistant steel FV448 has been investigated. Two important effects have been quantified: surface roughness and strain hardening. 2D and 3D tactile and optical techniques for determining surface roughness amplitude parameters have been investigated; it was found that whilst R_a and S_a were consistent, S_z was generally higher than R_z due to the increased probability of finding the worst case surface feature. Three different methods for evaluating the plastic strain profile have been evaluated with a view to establishing the variation in yield strength near the surface of a shot peened component. Microhardness, X-ray diffraction (XRD) line broadening and electron backscatter diffraction (EBSD) local misorientation techniques were applied to both uniaxially deformed calibration samples of known plastic strain and samples shot peened at intensities varying from 4A to 18A to establish the variation in plastic strain and hence the variation in yield strength. The results from the three methods were compared; XRD and EBSD profiles were found to be the most similar with microhardness profiles extending much deeper into the sample. Changes in the measured plastic strain profile after exposure to low cycle fatigue and the correlation of these changes with the cyclic stress–strain behaviour of the material are also discussed with a view to assessing the importance of the dislocation profile in component life assessment procedures.     

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm² are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.     

Electrochemical behaviour of aluminium alloys containing indium and tin in NaCl solution

The electrochemical behaviour of Al, Al–In, Al–Sn and Al–Sn–In alloys in 2 M NaCl solution has been studied using an open circuit potential, potentiodynamic polarization and ac impedance measurements as well as by optical microscopy examination. The addition of alloying components to aluminium produced in all cases a considerable activation of aluminium. The activation is manifested by shifting the open corrosion potential and the pitting potential in the negative direction (for about 0.6 V) and significant reducing of the passive potential region. The degree of activation depended on alloying element and it was found that there is an increase in the order: Al < Al–In < Al–Sn ≈ Al–Sn–In. The anodic dissolution of the Al–Sn and Al–Sn–In alloys started at open circuit potential which is only 0.45 V more positive than the thermodynamic Al³⁺/Al potential. The ac impedance measurements performed at different potentials in wide potential range (corresponding to passive and active state of each examined samples) confirmed the great activity of Al–Sn and Al–Sn–In alloys compared to aluminium.     

Synergistic effect of environmental media and stress on the fatigue fracture behaviour of aluminium alloys

The present study aims at explaining the synergistic effect of environmental media and stress/strain on fatigue lives of aluminium alloys. Rotating bending fatigue tests were carried out using four different aluminium alloys LY12‐CZ, 2024‐T4, 7475‐T7351 and 7075‐T651, at air state, 3.5% and 5.0% NaCl aqueous solutions. These results indicated that synergistic actions of the environmental media and cyclic loading accelerated the fatigue crack propagation of aluminium alloys. Furthermore, various influence factors (such as solution concentration, cyclic numbers, high (low) strength aluminium alloys etc.) of the fatigue life at synergistic actions of the environmental media and stress were quantificationally discussed in this paper.     

Gel electrode imaging of fatigue cracks in aluminium alloys

Previous papers have described a gel electrode technique recently devised for detecting and imaging fatigue cracks in aluminium tested in simple bending. In this study, the technique is shown to be applicable to testing in both bending and torsion and to high strength aluminium alloys 7075-T6, 2024-T3 and 2024-T4. Fatigue cracks as short as 10 μm in length are consistently detected and located. The flow of charge during image formation under standard conditions provides a quantitative measure of crack length, which is independent of alloy composition. A crack 100 μm long can be reliably detected by charge flow measurement; thus, this approach is not as sensitive as the information contained in the actual images.     

Modeling fatigue crack nucleation at primary inclusions in carburized and shot-peened martensitic steel

The mechanics of high cycle fatigue crack nucleation (formation of a stable crack that can grow away from the influence of the notch root of the inclusion) at subsurface primary inclusions in carburized and shot-peened martensitic steel subjected to cyclic bending is investigated using three-dimensional (3D) finite element (FE) analysis. FE models are constructed using a voxellation technique to address the shape, size, and distribution of primary inclusions within clusters. The critical depth for fatigue crack nucleation is predicted considering the gradient in material properties arising from carburization, prestrain and compressive residual stress distribution due to shot peening, and the gradient of applied bending stress. The influence of inclusion shape and interface condition (intact or debonded) with the matrix on the driving force for fatigue crack nucleation is examined. It is observed that the inclusion shape has minimal influence on the predicted results while the effect of the interface condition is quite significant. For partially debonded interfaces, the predicted critical depth from surface for fatigue crack nucleation agrees qualitatively with experimental observations.     

Fatigue behaviour of SiCp-reinforced aluminium composites in the very high cycle regime using ultrasonic fatigue

The fatigue behaviour of a 2009/SiC/15p‐T4 DRA composite has been examined in the very high cycle fatigue (VHCF) regime where 10⁷≤N_f≤ 10⁹ cycles. Ultrasonic fatigue was used to achieve the very high cycle counts. Careful processing yielded a composite with a very homogeneous particle distribution with minimal clustering. Fatigue crack initiation was observed almost exclusively at AlCuFe inclusions with no crack initiation observed at SiC particle clusters. Fatigue lives at a given stress level exhibited minimal scatter and subsurface crack initiation was observed in all cases. This behaviour is consistent with the presence of a low number density of critical inclusions that are responsible for crack initiation very early in fatigue life.     

Probabilistic prediction of high cycle fatigue reliability of high strength steel butt‐welded joints

The aim of this paper is to develop a probabilistic approach of high cycle fatigue (HCF) behaviour prediction of welded joints taking into account the surface modifications induced by welding and the post‐welding shot peening treatment. In this work, the HCF Crossland criterion has been used and adopted to the case of welded and shot peened welded parts, by taking into account the surface modifications which are classified as follows: (i) the compressive residual stresses, (ii) the surface work‐hardening, (iii) the geometrical irregularities and (iv) the superficial defects. The random effects due to the dispersions of: (i) the HCF Crossland criterion material characteristics (ii) the applied loading and (iii) the surface modifications parameters are introduced in the proposed model. The HCF reliability has been computed by using the ‘strength load’ method with Monte Carlo simulation. The reliability computation results lead to obtain interesting and useful iso‐probabilistic Crossland diagrams (PCD) for different welding and shot peening surface conditions. To validate the proposed method, the approach has been applied to a butt‐welded joint made of S550MC high strength steel (HSS). Four types of specimens are investigated: (i) base metal (BM), (ii) machined and grooved (MG) condition, (iii) As welded (AW) condition and (iv) as welded and shot peened (AWSP) condition. The comparison between the computed reliabilities and the experimental investigations reveals good agreement leading to validate the proposed approach. The effects of the different welded and post‐weld shot peened specimen's surface properties are analysed and discussed using the design of experiments (DoE) techniques.     

X-ray analysis of polycrystalline aluminium subjected to fatigue cycling

Variations in the halfwidth values of X-ray reflections from fatigue-cycled, polycrystalline aluminium samples have been analysed. An oscillatory variation of the halfwidths with fatigue cycling has been observed. Analysis of the diffraction line profiles indicates that broadening arises mainly because of the build-up of microstrains during fatigue cycling. The present data indicate that (i) broadening due to fatigue cycling increases with glancing angle; (ii) changes in halfwidth and integral widths, due to fatigue cycling, are comparable and (iii) (b/b ₀) versusN curves for fatigue cycling under constant stress amplitude and flight loading conditions are comparable.     

Thermo-mechanical behaviours of light alloys in comparison to high temperature isothermal behaviours

Abstract

In this article, out-of-phase thermo-mechanical fatigue (TMF) behaviours of light alloys were investigated in comparison to their high temperature low cycle fatigue (LCF) behaviours. For this objective, strain based fatigue tests were performed on the A356 aluminium alloy and on the AZ91 magnesium alloy. Besides, TMF tests were carried out, where both strain and temperature changed. The fatigue lifetime comparison demonstrated that the TMF lifetime was less than that one under LCF loadings at elevated temperatures for both light alloys. The reason was due to severe conditions in TMF tests in comparison to LCF tests. The temperature varied in TMF test but it was constant under LCF loadings. As the other reason, the tensile mean stress occurred under TMF loadings, in comparison to the compressive mean stress under LCF loadings. At high temperatures, the cyclic hardening behaviour occurred in the AZ91 alloy and the A356 alloy had the cyclic softening behaviour.