Time dependent crack growth of type 304 stainless steel in elevated temperature sodium

The crack growth characteristics, under constant load and in elevated temperature liquid sodium, were studied for Type 304 stainless steel through using linear elastic fracture mechanics. The crack growth rates are represented by the Arrhenius equation. Furthermore the crack growth rates in sodium, at 873 K and oxygen concentrations of 0.71 and 15 ppm, were examined. The crack growth rates seem to increase in accordance with increased oxygen concentration, however the difference is slight. Scanning electron microscope observations were made of corrosion products on the fracture surface at 15 ppm oxygen. However, at 0.71 ppm the corrosion products could not be observed. So the mechanism of crack growth seems to be related to impurities in sodium.     

Stress intensity factors for surface cracks at countersunk holes

Fatigue crack growth from countersunk fastener holes loaded in remote tensile loading was studied using the transparent polymer PMMA. A single edge corner crack at the bottom of the plate and a single internal surface crack at the sharp intersection between the bore and the countersink were induced in the PMMA specimens by pre-cracking. The specimens were then fatigue tested under constant amplitude remote tensile loading and the ‘back-calculation’ method was used to determine stress intensity factors at several crack front locations. When variations in fatigue crack closure were taken into account, the experimental stress intensity factors agreed well with the computational results at selected crack fronts.     

Experimental and numerical analysis of thermal striping in automotive brake discs

In the present study, thermal striping development on friction bands of brake discs is investigated through an experimental and numerical analysis. A test consisting of a series of several hundred severe brakings was carried out on a specific bench at PSA Peugeot Citroën. The experimental observations of the crack network evolution and a numerical analysis of a brake disc with a single crack helped to propose a macroscopic criterion capable of predicting the criticality of the network, namely its ability to lead to the complete failure of the structure. In addition, a numerical analysis of the crack shielding process is developed, whose results are compared with experimental findings.     

Measurement of crack closure after the application of an overload cycle, using moiré interferometry

The crack closure behaviour on the application of a single overload cycle was studied in a Ti-6Al-4V specimen. Moiré interferometry with photoresist gratings was used to measure crack displacements. During the overload cycle a large crack opening displacement was observed at the maximum load. This was similar to predictions from a Dugdale-type crack closure model. When the load was taken back to zero, the crack was open at the crack tip due to the high levels of plastic deformation during the overload cycle. As the crack was grown there was some evidence of the deformed material on the crack faces.Moiré interferometry provided displacement data close to the crack faces, even when the crack had grown to over two-and-a-half times the overload crack length. When the overload was applied the crack bifurcated, and the Dugdale-type model under-predicted the crack opening.     

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC-ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep-fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.     

A new method for modelling the coalescence and growth of two coplanar short cracks of varying lengths in AA7050-T7451 aluminium alloy

The growth, interaction and coalescence of two coplanar short cracks of varying lengths in AA7050-T7451 aluminium alloy were studied under low amplitude cyclic loading. Fractographic studies showed that the way in which the fracture surfaces developed was dependent on the relative crack sizes, however interactions between the cracks did not significantly affect the crack growth rates before the tips of the cracks touch to form a single crack. Subsequently, the longitudinal growth rates of the crack were retarded for a period which appeared necessary for the newly coalesced crack to form a single semi-elliptical shape before resuming growth rates of a single crack. A new mathematical model was developed to account for this behaviour.     

Abrasive wear and craze breakdown in polystyrene

The role of craze breakdown during the fracture process of abrasive wear in glassy polystyrene was investigated. At first, the wear resistance, _w, was compared with the craze breakdown strain as a function of molecular weight and diluent concentration. It was found that _w increases with molecular weight and decreases with the diluent concentration. Although craze breakdown strain also increases with molecular weight and decreases with the diluent concentration, the wear data do not converge into a single curve in a plot against the craze breakdown strain. Selected specimens were then studied by micro-indentation and micro-scratching experiments. An analysis of the scratch patterns and contact load at the polymer surface indicated that a critical stress criterion, rather than a critical strain criterion, may be suitable for the onset of the failure process in brittle polymer wear. With this criterion, the critical load for crack opening, _c, can be related to the craze breakdown strain and Young's modulus, and the observed deviation between the craze breakdown strain and _w can be explained.     

In situ observations of crack propagation and role of grain boundary microstructure in nickel embrittled by sulfur

In situ observations of crack propagation in sulfur-doped coarse-grained nickel were performed for the specimens with grain boundary microstructure pre-determined by SEM/EBSD analysis. The role of grain boundary microstructure was studied in the crack propagation in nickel embrittled by grain boundary segregation of sulfur. It was found that the main crack tends to predominantly propagate along random boundaries, and the crack propagation rate can be locally accelerated at the grain boundary network with a high connectivity of random boundaries. On the other hand, the cracks can propagated along fracture-resistant low-Σ coincidence site lattice (CSL) boundary only when the trace of the grain boundary is arranged being almost parallel to slip bands in the adjacent grains. The local crack propagation rate was found to become lower when a crack propagated along low-Σ CSL boundaries. Moreover, when the crack propagation is inhibited by low-Σ CSL boundaries, the branching of propagating crack occurs at partially cracked triple junctions. The crack propagation can locally slow down due to the occurrence of crack branching. The optimum grain boundary microstructure for the control of sulfur segregation-induced brittle fracture is discussed on the basis of new findings obtained from the in situ observations on crack propagation and fracture processes in polycrystalline nickel.     

THE EFFECTS OF CATHODIC POTENTIAL AND CALCAREOUS DEPOSITS ON CORROSION FATIGUE CRACK GROWTH RATE IN SEA WATER FOR TWO OFFSHORE STRUCTURAL STEELS

The effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels. These materials were cathodically polarised in seawater and 3% sodium chloride solution at three potentials of -0.8, -1.0 and -1.1 V(SCE). The corrosion fatigue crack growth rate in seawater was greater than that in air and increased with more negative potentials. The maximum acceleration of crack growth rate in seawater was observed at the crack growth plateau which was independent of ΔK. Calcareous deposits precipitated within the cracks resulted in an increase of crack opening level and contributed to a reduction of the corrosion fatigue crack growth rate. Such a corrosion-product-wedging effect could be evaluated by using an effective stress intensity range, ΔK_eff. The estimation of corrosion fatigue crack growth rate in terms of ΔK_eff clarified the effect of hydrogen embrittlement under a cathodic potential. Thus the processes of cracking in seawater at cathodic potentials resulted from mechanical fatigue and hydrogen embrittlement with calcareous deposits reducing the crack growth rate. All these three mechanisms were mutually competitive.     

Passivity and Pitting Susceptibility of type 304 Stainless Steel in Acidic Sulphate Solutions

Passivation and its breakdown reactions of type 304 stainless steel have been studied in acidic sodium sulfate solutions using different electrochemical techniques. Results from potential and impedance measurements under open circuit conditions confirmed the formation of a bilayer as a primary passive film. The results from cyclic polarization curves showed that E_pit−E_prot increases with increasing Cl⁻ concentration indicating failure of the steel sample due to pitting. Impedance spectra recorded at different passivation potentials showed a maximum at 0.4 V due to film thickening. Above 0.4 V film breakdown occurs.     

Evolution of residual stresses with fatigue crack growth in integral structures with crack retarders

Bonded straps are investigated for their ability to retard a growing fatigue crack in metallic structures. The evolution of the residual stresses in the vicinity of the strap with fatigue crack growth has been studied. Cracks were grown in single edge-notched tension (SEN(T)) specimens reinforced with either a titanium or a carbon fibre reinforced plastics (CFRP) strap. The residual stress evolution has been measured in situ during crack growth using neutron diffraction, and modelled with a finite element approach. The peak residual stresses induced by the mismatch of the coefficient of thermal expansion between the strap and plate materials were seen to be fairly constant with crack growth. Good correlation between the experimental and the modelling results was found, except at very long crack lengths for a specimen that exhibited considerable fracture surface roughness at long crack lengths. The difference was attributed to wedging of the fracture surface changing the expected stress state, rather than any effect of the strap.     

Compressive fatigue crack growth behaviour of alumina and glass

Cylindrical specimens of polycrystalline alumina and sodium glass with a deep circumferential notch were loaded cyclically under compression, and the fatigue crack-propagation processes in the notch root were studied. The studies showed that debris and a fatigue crack are formed in the notch root of alumina, but not in glass. There, only fragmentation takes place at the crack tip; glass chips are formed which surround the notch. The influence of microstructural inhomogeneities on fatigue crack formation and propagation as well as the consequences for material scientists and engineers are discussed on the basis of a simple model.Dedicated to the 60th birthday of Professor Dipl. Ing Dr. Peter Ettmayer, Technical University, Vienna.     

TRANSIENT FATIGUE CRACK GROWTH BEHAVIOUR FOLLOWING SINGLE OVERLOADS AT HIGH STRESS RATIOS

Fatigue crack growth tests with constant amplitude loading and single overload have been performed on a long mode I crack in 2017-T3 aluminium alloy at various stress ratios from 0 to 0.7. Two crack tip parameters of σ_op and σ_tt were evaluated using a finite element analysis for a growing crack under these loading conditions. The former is the crack opening stress and the latter is the applied stress level at which the stress at the crack tip becomes tensile. It was found that transient crack growth behaviour following single overloads at high stress ratios was significantly different from that at a low stress ratio: at higher stress ratios, following the application of the overload, there was a rapid retardation which was followed by an acceleration in growth rate and then a faster return to the steady state level at baseline loading. The experimentally observed transient post-overload behaviour is discussed in terms of the two effective stress range ratios of U_op and U_tt, which are determined from σ_op and σ_tt, respectively. For the stress ratios and overload ratios studied, the results indicate that the changes in U_tt with crack extension after the overload are reasonably consistent with the crack growth rate trends. The stress distribution at minimum applied stress would account for the transient changes in U_tt.     

Impact-echo from a unilateral interlayer crack. LCP–BEM modelling and neural identification

The reflection of elastic waves from subsurface interlayer unilateral cracks, with the possibility of crack closing (contact) and frictional effects, is studied by dynamic boundary element method (BEM) combined with a linear complementarity problem (LCP) formulation. The effect of the unilateral crack on the impact–echo response is studied by means of numerical experiments. Further, a neural network model is proposed and tested for the classification of the data based on the numerically generated waveforms. The position or the size of the sublayer crack can be identified from the trained neural network.     

Influence of sodium oxide content on bioactive glass properties

The rate of in vivo degradation and level of bioactivity of bioactive glasses are composition dependent [1]. By altering bioactive glass composition, the rate of resorption can be controlled. The network connectivity of a glass can be used to predict various physical properties of the glass including its solubility and, hence, its bioactivity [2]. Glass solubility increases as network connectivity is reduced. Glasses in the soda-lime phosphosilicate system were studied. The initial choice of composition was based on phosphate content and low network connectivity. A systematic substitution of calcium oxide for sodium oxide on a molar basis was made in order to examine the influence of sodium oxide content on the glass properties while keeping the network connectivity constant. The glass transition temperature and the peak crystallization temperature were seen to decrease linearly with increasing sodium oxide content. Thermal expansion coefficient and glass density were also seen to be related to sodium oxide content. Preliminary in vitro biocompatibility studies revealed that the glasses of higher sodium oxide content were associated with a cytotoxic response. The measurement of media pH indicated that this cytotoxic effect was due to ion exchange reactions at the glass surface.     

镍基单晶高温合金热机疲劳断裂特征

为了进一步提高镍基单晶高温合金的热机疲劳性能,通过微观结构解析研究了合金热机疲劳断裂特征.通过金相和扫描电子显微镜研究了热机疲劳断裂的断口特征和微观结构.研究表明:裂纹起源于形变孪晶与试样外表面的交截处,过程中的氧化有助于裂纹的长大;裂纹尖端的应力场诱发出大量形变孪晶,而形变孪晶的存在为裂纹进一步沿着孪晶界扩展提供了便利条件;镍基单晶高温合金的疲劳断裂主要是由于形变孪晶的形成以及裂纹沿孪晶界的扩展造成的.形变孪晶与高温合金疲劳断裂密切相关.     

An experimental investigation on fatigue crack growth of AL6XN stainless steel

The crack growth behavior of AL6XN stainless steel was experimentally investigated using round compact tension (CT) specimens. The influences of the R-ratio (the ratio of the minimum load over the maximum applied load in a cycle), the tensile and compressive overloads, and the loading sequence on crack growth were studied in detail. The results from the constant-amplitude experiments show a sensitivity of the crack growth rate to the R-ratio. The application of a tensile overload has a profound effect on crack growth, resulting in a significant retardation in the crack propagation rate. A compressive overload (underload) leads to a short-lived acceleration in crack growth. Results from the two-step high-low loading reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. A crack driving force parameter together with a modified Wheeler model is found to correlate the crack growth experiments well.     

CONSTANT AMPLITUDE AND POST-OVERLOAD FATIGUE CRACK GROWTH BEHAVIOR IN PM ALUMINUM ALLOY AA 8009

Abstract— The fatigue crack growth behavior of a recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was studied at room temperature in laboratory air. Constant amplitude/constant Δ K and single spike overload conditions were examined. Alloy 8009 exhibited high fatigue crack growth rates and low closure levels in comparison to typical ingot processed aluminum alloys. It was proposed that minimal crack roughness and crack path deflection, along with limited slip reversibility, resulting from the ultra-fine microstructure were responsible for the relatively poor (versus ingot processed aluminum alloys) d a /d N versus Δ K performance of AA 8009.     

Experimental and numerical study of fatigue crack growth of aluminum alloy 2024-T3 single lap simple bolted and hybrid (adhesive/bolted) joints

In this paper, the effects of tightening torque (clamping force) on the fatigue crack growth rate and stress intensity factors in cracked single lap simple bolted and hybrid (adhesive/bolted) joints have been studied experimentally and numerically. To do so, series of fatigue crack growth tests for two different amounts of tightening torque in Aluminum alloy 2024-T3 pre-cracked joints have been carried out to record the fatigue crack growth and also the fatigue life of specimens. In the numerical part, finite element method was employed to obtain the stress intensity factors and also the effective stress intensity factor ranges for different crack lengths to explain the behavior of fatigue crack propagation. It was found that the hybrid joint has longer fatigue crack growth life compared to the simple bolted joint at a given bolt tightening torque. The results also showed that a higher bolt tightening torque provides improved fatigue crack growth life for both types of the joints.     

High-temperature electrical breakdown in single crystal and polycrystalline MgO

Electrical breakdown strengths of single crystal and polycrystalline MgO were measured at 1000 to 1400C. Temperature and thickness dependences of the breakdown strength of single crystal MgO firmly indicated that the electrical breakdown was of thermal type. A high correlation between the ionic conductivity and the breakdown strength was found, which together with the observed melted track suggested that the electrolytic decomposition took place. The breakdown strength of single crystal MgO decreased with increase in the dislocation density. The breakdown strength of polycrystalline MgO indicated a significant effect of the microstructural factors, though a quantitative evaluation was impossible in the present study.