Nondestructive detection of fatigue damage in austenitic stainless steel by positron annihilation

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a ⁷²Se/⁷² As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β⁺ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed.     

Recent developments in ultrasonic fatigue

The recently increased interest in very high cycle fatigue properties of materials has led to extended use and further development of the ultrasonic fatigue testing technique. Specimens are stimulated to resonance vibrations at ultrasonic frequency, where the high frequency allows collecting lifetime data of up to 10¹⁰ cycles and measuring crack propagation rates down to 10^?12 m per cycle within reasonable testing times. New capabilities and methods of ultrasonic testing and outstanding results obtained since the year 1999 are reviewed. Ultrasonic tests at load ratios other than R = ?1, variable amplitude tests, cyclic torsion tests and methods for in situ observation of fatigue damage are described. Advances in testing at very high temperatures or in corrosive environments and experiments with other than bulk metallic materials are summarized. Fundamental studies with copper and duplex steel became possible and allowed new insights into the process of very high cycle fatigue damage. Higher cyclic strength of mild steels measured at ultrasonic frequency because of plastic strain rate effects are described. High‐strength steels and high‐alloy steels are less prone to frequency influences. Environmental effects that can lead to prolonged lifetimes in some aluminium alloys and possible frequency effects in titanium and nickel and their alloys are reviewed.     

The fatigue properties of dynamically consolidated aluminium

Dynamic consolidation produces material made up of work-hardened particles bonded together by ultra rapidly solidified welds. Obviously such a structure will have unique mechanical properties. It has been found that the fatigue strength at 10⁷ cycles of dynamically consolidated aluminium is significantly above that of wrought ingot metallurgy material of similar composition. Annealing the compacts at 300° C only slightly reduces the fatigue strength. However, there is an appreciable reduction after annealing at 400° C and above. The fatigue strength appears equal to one-half the yield strength; a more complex relationship exists for the ultimate strength. The fatigue strength, like the yield strength, is probably related to the ratioR, which predicts the degree of interparticle melting. The value of this ratio may be calculated. The compacts undergo an insignificant amount of cyclic softening during low-cycle fatigue tests, although some softening does occur in specimens annealed at 300° C. Microscopy indicates that failure is of a ductile interparticular nature with some evidence of small areas of fatigue striations and “ridges”. The results, especially the reduction in properties on annealing, suggest that the improved fatigue properties reported for aluminium are a direct result of dynamic consolidation. Application to other alloys is, therefore, logical. The degree will depend upon the amount of work-hardening and of ultra rapidly solidified welding the alloy experiences.     

Positron lifetime studies and coincidence Doppler broadening spectroscopy of Al–6Mg–<Emphasis Type="Italic">x</Emphasis>Sc (<Emphasis Type="Italic">x</Emphasis> = 0 to 0.6 wt.%) alloy

Positron annihilation spectroscopy (PAS), comprising of both positron lifetime and coincidence Doppler broadening measurements, has been employed for studying the phase decomposition behaviour of scandium doped Al–6Mg alloys. Micro structural and age hardening studies have also been conducted to substantiate the explanation of the results of PAS. Samples with scandium concentration ranging from 0 to 0.6 wt.% have been studied. The measured positron lifetimes of undoped alloy reveal that GP zones are absent in the as-prepared Al–6Mg alloy. The observed positron lifetimes and the results of coincidence Doppler broadening measurements largely stem from the entrap of positrons at the interface between aluminium rich primary dendrites and the magnesium enriched interdendritic eutectic mixture of Mg₅Al₈ (β) and the primary solid solution of aluminium (α). The study also provides evidence of the formation of scandium vacancy complexes in Al–6Mg alloys doped with scandium upto a concentration of 0.2 wt.%. However such complex formation ceases to continue beyond 0.2 wt.% Sc; instead, the formation of fine coherent precipitates of Al₃Sc is recorded in the as prepared alloy containing 0.6 wt.% scandium. The positron annihilation studies coupled with CDBS have also corroborated with the fact that the fine coherent precipitates of Al₃Sc are formed upon annealing the Al–6Mg alloys doped with scandium of concentration 0.2 wt.% and above. Transmission electron microscopic studies have provided good evidence of precipitate formation in annealed Al–6Mg–Sc alloys. Elevated temperature annealing leads to dissociation of the scandium-vacancy complexes, thereby leading to the enhancement of the mobility of magnesium atoms. This has facilitated fresh nucleation and growth of Mg₅Al₈ precipitates in the above alloys at 673 K.     

Fracture and Fatigue Behaviour of Aluminium Matrix Composite Automotive Pistons

The fracture and fatigue behaviour of prototype automotive pistons produced in an aluminium alloy matrix composite in industrial conditions has been studied. Fracture toughness increased when the testing temperature rose from 20° to 75°C and kept near constant up to 250°C, when a significantly lower value was recorded. A change in the failure operating mechanism, which can explain this trend, was observed by analysing the fracture surfaces in the scanning electron microscope. Room temperature fatigue tests performed with R = 0.1 stress ratio led to an average value of the Paris law exponent higher than those reported in aluminium alloys but low for an industrially produced brittle composite. A higher exponent and a much larger scattering were observed in those fatigue tests carried out under R = 0.5 stress ratio.     

Influence of the microstructure of aluminium alloys on their residual impact properties after a fatigue loading program

Consecutive loadings of fatigue and impact have been carried out on aluminium alloys. The aim of this study is to quantify the influence of the microstructure on the residual impact behavior after a prior fatigue loading. Two alloys with different chemical compositions and hardening modes have been investigated: 2017A-T3 used in the aircraft industry and 5454-O used in automotive applications.The fatigue pre-loadings were carried out under fully reversed tensile-compression with several pairs (stress level, number of cycles) in the high cycle fatigue zone (10⁵– 10⁶cycles). The residual impact behavior was determined under tensile loading, in the range of medium strain rates (about ). To assess the prior fatigue damage and to follow its evolution during the impact loading, observation of the specimens (surface and fracture surfaces) were made.From this study, two conclusions have been highlighted: (1) there is no direct correlation between a given prior loading and residual behavior, whatever the material; (2) the material aspect is fundamental. At the mechanical (macroscopic) scale, the Al–Mg alloy (5454-O) remains insensitive to the prior fatigue loading whereas the Al–Cu alloy (2017A-T3) undergoes a large modification in its residual performance. At a lower scale, the pre-damage signature appears for the insensitive as well as for the sensitive material. The prior damage and its contribution to the process fracture appear to be strongly linked with the material’s microstructure.     

Effect of a commercial hard anodizing on the fatigue property of a 2014-T6 aluminium alloy

The effect of a commercial hard anodizing on the fatigue property of 2014 Al alloy which has been solution heat treated and age hardened prior to the hard anodizing of approximately 10 μm in thickness have been investigated. The results indicated that fatigue life in high stress region for aluminium alloy samples hard anodized was shorter than that of the other material condition which has been solution heat treated and then age hardened (T6 heat treated), while the life in low stress region was longer than that of the material (T6 heat treated). However, such a coating to the aluminium substrate gives rise to a significant increase in fatigue strength of 10⁷ cycle in comparison with the as cast condition, but a much less increase in comparison with T6 heat treated samples. From the microscopic point of view, it has been observed that fatigue fracture of samples hard anodized initiated in the coating in high stress region in opposition to low stress region in which fatigue fracture initiation started on the interface between coating and substrate.     

Post-β″ phases and their influence on microstructure and hardness in 6xxx Al-Mg-Si alloys

Starting from solid solution (T4) or a condition with β″ precipitates (T6), three Al-Mg-Si alloys with similar total solute content (1.3 at%), but different Si/Mg ratios (2, 1.25 and 0.8) were isothermally heat-treated at 250 or 260°C and investigated by transmission electron microscopy. The result microstructure for all alloys and conditions consisted of metastable, needle-shaped precipitates growing along 〈100〉 directions in aluminium. Each of the phases β″-Mg₅Si₆, β′-Mg_1.8Si, U1-MgAl₂Si₂ and U2-MgAlSi could be identified as main precipitate in the alloy with its solute Si/Mg ratio closest to the same ratio in the composition of that particular phase: The highest Si content alloy produced coarse needles of the trigonal U1-phase coexisting with finer precipitates of hexagonal B′-phase. The most common phase in the Mg-rich alloy is coarse needles of hexagonal β′-type. The Si/Mg ratio of 1.25 in one alloy is similar to the Si/Mg ratio in β″. Here the microstructure changes from that of fine β″ needles to fine needles of the orthorhombic U2-phase. This material remains strongest during heat-treatment. Nucleation on dislocations, mainly by the B′-phase, was observed to be significant in the case of Si-rich alloys heat-treated from T4-condition.     

Non‐destructive evaluation of fatigue damage and fatigue crack initiation in type 316 stainless steel by positron annihilation line‐shape and lifetime analyses

Rotating bending fatigue tests were conducted using type 316 stainless steel. The fatigue tests were periodically terminated, and fatigue damage and fatigue crack initiation were non‐destructively and sequentially evaluated by positron annihilation line‐shape and lifetime analyses. The counter‐jig and anticoincidence methods were used for positron annihilation line‐shape and lifetime analyses, respectively, to enhance the analytical precision. The fatigue crack lengths were monitored by a plastic replication technique, and related to the parameters in both analyses. S‐parameter obtained in the line‐shape analysis increased with increasing fatigue damage, while it was difficult to detect fatigue crack initiation and subsequent small fatigue crack growth. That was because the precision of line‐shape analysis was limited. On the other hand, both fatigue damage and fatigue crack initiation were successfully detected by lifetime analysis. Positron annihilation lifetime also increased with increasing fatigue damage, and lifetime was longer at the notch root with fatigue crack than at the smooth section without crack. It was considered that the precision of lifetime analysis was high enough to detect high dislocation density areas at the fatigue crack tips.     

Nonlocalized Fatigue Damage of Metals and Alloys. Part 1. Inelasticity,Investigation Methods,and Results

Analysis of data presented in the literature and results of unique studies revealed that nonlocalized fatigue damage of metals and alloys, which manifests itself in the initiation and propagation of a large number of microcracks distributed arbitrarily in the bulk of the material, can be considered with the use of the characteristics of inelastic deformation such as inelastic strain and inelastic strain energy per cycle. The author studied the nature of the material inelasticity and methods of its investigation. Analysis has been made of the hysteresis loop shape, width, and area, as well as of the main regularities of inelastic deformation of metals and alloys. __________ Translated from Problemy Prochnosti, No. 4, pp. 5 – 32, July – August, 2005.     

Characterization of polymer films using a slow positron beam

Polymer structures have been investigated using positron annihilation lifetime spectroscopy (PALS) with a slow positron beam as well as a conventional radioactive source (²²Na). The properties of the free volume holes near the polymer surface were studied as a function of the positron implantation energy. The longest lifetime was associated with ortho-positronium (o-Ps) annihilation in the free volume holes. In polytetrafluoroethylene film, the lifetime of o-Ps was observed to decrease with increasing positron implantation depth, and a significant change in the o-Ps lifetime was found at a short distance (about 10 nm) from the surface, while its intensity increased. This result implies that near the polymer surface the free volume holes become larger that in the bulk. The effect of temperature on the polymer sub-surface layers was also studied. For high molecular weight polystyrene, the glass transition temperature for the sub-surface was lower than that for the bulk and thermal expansion coefficient of the sub-surface layers was found to be larger than the bulk value. Electronic Publication     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Thermal stability and decomposition kinetics of Li2Al4CO3(OH)12·3H2O

The thermal stability of lithium containing hydrotalcite, a material that has potential application for providing atmospheric corrosion protection to aluminium alloys, was investigated. Lithium aluminium carbonate hydroxide hydrate (Li₂Al₄CO₃(OH)₁₂·3H₂O) coatings were prepared by immersion of an 1100 aluminium alloy into a lithium carbonate-lithium hydroxide solution, and the bulk material was prepared by precipitation in the same solution. Thermal stability of the coatings and the bulk material existed to around 150 °C. Above this temperature, interlayer water was expelled, followed by loss of structural water and carbon dioxide. The kinetic parameters for interlayer water loss have been determined, and water loss can be described by a Johnson-Mehl-Avrami rate equation.     

Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.     

Fatigue behaviour in a plastic strain-controlled mode of an austenitic stainless steel treated by explosive

A surface-treatment technique using primary explosive was applied to a 316L type stainless steel. Some characterizations of the induced mechanical or metallurgical effects are given such as surface roughness, microhardness, residual stresses, microstructure. Fatigue tests were performed in tension-compression in the plastic strain-controlled mode with amplitudes in the range 10^–3 to 5×10^–3. The cyclic behaviour of the treated samples is characterized by a higher cyclic stress amplitude than the untreated material and a shorter fatigue life. The surface damage has been analysed by counting the secondary microcracks after failure. The cyclic behaviour and the damage are discussed taking into account the different induced effects and assuming the treated material to be a composite one, with a highly strengthened surface layer and a quasi-untreated bulk     

Prediction of creep–fatigue crack growth rates in inert and active environments in an aluminium alloy

This paper reports on a study on creep–fatigue crack growth resistance of a precipitation hardened 2650 T6 aluminium alloy selected for fuselage panels of a future civil supersonic aircraft. The objective is to develop a methodology to predict crack growth under very low frequency loading at elevated temperatures. With this aim, creep crack growth rates (CCGRs), fatigue crack growth rates (FCGRs), creep–fatigue crack growth rates (CFCGRs) have been measured at 130 °C and 175 °C in laboratory air and in vacuum at R = 0.5 under different load frequencies and waveshape signals. It is shown that, for a given temperature, CFCGRs are unaffected by frequency below a critical value of the load period T_c. Above this value CFCGRs are directly proportional to the load period. This time-dependent crack growth regime is assisted by a significant creep damage as indicated by the large amount of intergranular decohesions induced by cavitation on fracture surfaces. CFCGRs are calculated under the assumption that fatigue damage and creep damage can be linearly summed. In vacuum the predictions are in good agreement with experimental data at both temperatures. In air however a discrepancy is observed for low frequency loading, suggesting the occurrence of a creep–fatigue–environment interaction. As a consequence the time-dependent crack growth behaviour affected by this interaction is different from creep crack growth behaviour, although the reasons for this are still unclear. A methodology is then proposed to predict CFCGRs in air. This methodology, if assessed by very low frequency experimental results, could be extended to different structural components made of aluminium alloys operating at elevated temperatures, provided that the mechanisms are unchanged.     

Identification of vacancies in electron irradiated GaSb by coincidence Doppler broadening spectroscopy

In this paper we present the results of coincidence Doppler broadening (CDB) measurements and positron lifetime spectroscopy (PLS) on the semiconductor material GaSb. Gallium vacancy with positron lifetime of about 283 ps (V_{Ga, 283 ps}) was identified in as-grown sample by CDB technique and PAS technique. For electron irradiated samples with dosages of 10¹⁷ cm^− 2 and 10¹⁸ cm^− 2, the PAS showed almost the same defect-related positron lifetime of about 285 ps. CDB experiments indicated that defects in irradiated samples were related to Ga vacancies.     

THE DEVELOPMENT OF STRESSES AND THEIR DISTRIBUTION IN TWO-PHASE ALLOYS DURING CYCLIC LOADING

A finite element program calculates the cyclic behaviour of the individual component phases of a multiphase material using a master curve observed in uniaxial cyclic stress tests. The fatigue behaviour of the two-phase alloys was characterized by visualizing the evolution of the phase stress (denoted by an average effective stress and an hydrostatic stress) during cyclic loading. The evolution procedure shows a unique fatigue behaviour of the in situ component phases, which differs from that observed in uniaxial or multiaxial fatigue tests of the single phase material. The fatigue damage on a microstructural scale was identified by the distributions of the plastic strain accumulated during cyclic loading and the stress triaxility in the component phases.     

Extrusion of vapour deposited Al–7·5Cr–1·2Fe (wt-%) alloy (RAE Alloy 72)

Abstract

Techniques and equipment were developed for the extrusion of vapour deposited RAE Alloy 72. The alloy was extruded at temperatures from 300 to 420°C and extrusion ratios from 2:1 to 25:1. Room and elevated temperature strengths and smooth S–N (stress–number of cycles to failure) fatigue properties were determined for a range of extrusions. The best extrusions gave room and elevated temperature strengths that were comparable to those of rapidly solidified aluminium alloys. The fatigue strength/tensile strength ratio of >0·5 was higher than would be expected for an aluminium alloy.

MST/1350     

Fatigue behaviour and crack growth rate of cryorolled Al 7075 alloy

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life, and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stress controlled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing ΔK) technique. The microstructural characterization of the alloy was carried out by using Field emission scanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloys showed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa, 530 Mpa, 21 J, 24 Mpa m^1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppression of dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. The cryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy due to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been found be higher, especially at higher values of applied stress intensity factor ΔK The reasons behind such crack growth retardation is due to diffused crack branching mechanism, interaction between a propagating crack and the increased amount of grain boundaries (GB), and steps developed on the crack plane during crack-precipitate interaction at the GB due to ultrafine grain formation.