Showing papers in "Fatigue & Fracture of Engineering Materials & Structures in 1980"

A quantitative metallographic assessment of structural degradation of type 316 stainless steel during creep‐fatigue

Abstract: — Optical and scanning electron microscopy have revealed the existence of grain boundary cavities in a series of specimens which had been cyclically deformed in the strain range ±0.25% with hold times ranging from 0 to 1000 min. The way in which these defects increase in size and number has been measured and found to correlate with the creep strain accumulated during the hold periods. A further observation is that a critical amount of deformation is required before either fatigue or creep type damage is nucleated. These observations lead to an alternative method to the currently accepted linear damage summation rule for estimating a lower bound of the creep-fatigue endurance.

Estimation of the fatigue limit by progressively‐increasing load tests

Abstract: — On the basis of fatigue data derived from a large number of experiments, the load progression methods of Locati and Prot as well as a method operating on a thermometric basis have been comparatively evaluated in an experimental and analytical (statistical) study. The last-named thermo-method can provide useful estimates for a fatigue limit corresponding with ca. 0 to 5% probability of fracture with only 1 to 3 experiments. The Prot-method, set up and evaluated in a certain way, provides a useful estimate of the fatigue limit and, additionally, an approximate indication of the width of the transition range of the corresponding Wohler-type tests after experiments on not less than about ten specimens. The load progression, contrary to the original Prot-concept, must not necessarily be applied in a continuous mode, but can occur stepwise with increments Δσ smaller than ca. 15 Nm-2.

A simple theory for low cycle multiaxial fatigue

Abstract: — A theoretical development based on a simple physical model is proposed to help the designer predict high strain multi-axial fatigue behaviour. This approach hypothesises that the maximum shear strain γ*, on planes driving the crack through the thickness, controls the fatigue crack propagation rate and hence the life. The direct strain δ*n acting normal to the plane of γ* can exert a secondary modifying influence. Experimental results from several research laboratories have been analysed in this manner with some success.

Incomplete self‐similarity of fatigue in the linear range of crack growth

Abstract: —The application of dimensional analysis and similarity methods to the study of the speed of fatigue crack growth is considered. It is shown that the Paris range of the crack propagation diagram is an intermediate-asymptotic stage of the crack growth process. Over this stage the influence of the initial conditions on the process of fatigue crack growth has disappeared but the influence of the instability has not yet intruded. So-called incomplete self-similarity prevails at this stage with respect to a basic similarity parameter, equal to the ratio of the stress intensity factor amplitude to the fracture toughness. It is shown that for a certain material under fixed external loading conditions the exponent in the Paris power law is a universal function of the ratio of specimen thickness to the ultimate size of the cyclic plastic zone. Processing of available experimental data confirmed the results obtained by this approach.

Stress gradients around notches

Abstract: — Stress gradients at the root of a notch are significant for the notch effect and the size effect of fatigue properties. Usually the gradient of the stress distribution in the minimum section is considered. In the present paper the variation of the tensile stress along the edge of the notch is considered. Calculations are made for a variety of notches. The results indicate a remarkable conformity of stress distributions at the notch root if the same peak stress and notch root radius (ρ) apply. Consequently K1and ρare highly characteristic for the stress distribution around the notch. Along the edge of the notch the stress decreases at a much slower rate than in the minimum section going away from the material surface. For the stress along the edge of the notch a stress gradient coefficient is defined. The variation of this coefficient is fairly small for several notches and K1, values. A 5% lower stress as compared to the peak stress at the notch root is obtained at about 0.02 ρbelow the material surface and at a distance of about 0.18 ρalong the material surface.

A model for predicting the effect of frequency on fatigue crack growth behavior at elevated temperature

Abstract: —An analytical model to represent and predict the influence of frequency on the elevated temperature fatigue crack growth behavior of structural alloys is described This model was formulated by considering the time dependence of stress and strain rate in the crack tip region due to creep deformation Fatigue crack growth rate data were generated on ASTM grade A470 class 8 (CrMoV) steel at 538°C (1000°F) for several frequencies to evaluate the model Based on these results and data taken from the literature on 304 stainless steel, it was concluded that the proposed model is capable of accurately representing and predicting the effect of frequency over several orders of magnitude on the fatigue crack growth rates at elevated temperature

High strain fatigue of a type 316 stainless steel

Abstract: — High strain fatigue tests of Type 316 stainless steel have been carried out at room temperature and 600°C. Cold work reduces the life relative to the annealed condition when considered in terms of plastic strain per cycle but not when the total strain range is used. It also renders the material far more resistant to cyclically induced changes in strength than the annealed state in which the strength is more than doubled at 600°C. Strain ageing causes further instability and enhances the hardening and loss of ductility in the annealed material. Because of the large number of variables involved, comparison with other work is unable to identify the parameters most influential in determining endurance. It is argued that where a fatigue–creep interaction occurs, then cyclic hardening will promote a higher creep resistance in a sequential interaction and favour a fatigue dominated failure in a simultaneous interaction.

Some creep/fatigue properties of type 316 steel at 625°c

Abstract: — Strain controlled continuous cycling fatigue tests are reported on three casts of Type 316 steel; the results obtained are shown to be consistent with published data. Strain controlled creep/fatigue tests are reported involving a hold period per cycle of between 0·2 and 168 h on one batch of Type 316 steel. An empirical extrapolation of the data and one involving a linear damage summation suggest that the existing Code Case N47 creep/fatigue design curve and rules are over-conservative. A stress relaxation/fatigue endurance correlation shows some promise for predictive purposes. Although many of the mechanical test and metallurgical features observed are consistent with a fundamental crack growth extrapolation approach, further consideration of detailed aspects are necessary before it can be used with confidence. It is concluded that there are no entirely reliable methods of extrapolation currently available for Type 316 steel and longer term tests are therefore essential to reduce the extrapolation uncertainties.

Effect of temperature on the fatigue‐crack propagation behaviour of inconel x‐750

Abstract: — Linear-elastic fracture mechanics techniques were used to characterize the effect of temperature on the fatigue-crack propagation behaviour of precipitation heat-treated. Inconel X-750 in an air environment over the temperature range 24 to 649°C. In general, crack growth rates were found to increase with increasing temperature, particularly at the highest test temperature (649°C). The effect of stress ratio on the fatigue-crack growth behaviour of Inconel X-750 was examined at 538°C, and results indicated that the elevated temperature fatigue response of this nickel-base superalloy was relatively insensitive to stress ratio level at the growth rate levels studied. Metallographic and electron fractographic examination of Inconel X-750 fatigue fracture surfaces revealed operative crack growth mechanisms to be a function of temperature and prevailing stress intensity factor. Under room temperature and intermediate temperature conditions (up to 538°C), all fatigue fracture surfaces exhibited a faceted crystallographic morphology at low crack growth rates followed by striations in the higher growth rate regime. At the highest test temperature (649°C), the fatigue crack was found to propagate by an intergranular mechanism.

Fatigue crack tip plastic strain in high-strength aluminum alloys

Abstract: — Plastic zone size and shape and the distribution of strain within the plastic zone are determined for the high-strength aluminium alloys 2024-T4, 6061 T6, and 7075-T6 using the technique of selected area electron channeling. Plastic zone size is found to correlate with the work done in creating a unit of new crack surface and the yield stress, rather than with the stress intensity factor and yield stress. Plastic strain distribution is found to be a logarithmic function of distance from the crack tip, in agreement with the mathematical analysis for a moving crack in plane strain.

BIAXIAL HIGH STRAIN FATIGUE TESTING OF 1Cr‐Mo‐V STEEL

Abstract: — A test facility has been developed in which a thin walled tube can be subjected to fully reversed fatigue cycles for all biaxial stress or strain ratios. Twin loop servo-control allied to high resolution gauge length extensometry, load and pressure transducers permits stress or strain controlled testing. The rig is unique in that cyclic stress-strain hysteresis loops can be obtained for both axial and circumferential directions during a test. Results from a strain controlled series of low cycle fatigue tests on a ferritic 1Cr-Mo-V steel obeyed the Manson-Coffin relationship with the exponent being sensibly constant for all strain ratios. For a given maximum principal biaxial strain, the damaging effect increased as the strain ratio φ increased from –1 to +1. By comparing results from solid and hollow cylindrical specimens, geometry effects on fatigue life were revealed. Data on shakedown and cyclic softening under biaxial strain conditions were also obtained.

Environmental effects on fatigue fracture mode transitions observed in aluminium alloys

Abstract: — Fatigue crack propagation tests were carried out in different environments on 7075–T6 and 2024–T3 centre-cracked sheet specimens. Observations were made on the macroscopic transition from tensile mode to shear mode. The transition is suppressed by an aggressive environment, whereas it is promoted by an inert environment. As a consequence there is no unique correlation between the state of stress and the mode of cracking. Both the state of stress and the environment have a significant effect on the mode of cracking. A simple model for the effect of environment on fatigue crack growth is presented. The implications for crack growth under corrosion fatigue conditions are discussed.

Comparison of plastic work of fatigue crack propagation in low carbon steel measured by strain‐gages and electron channeling

Abstract: — The plastic work to propagate a fatigue crack by a unit area, U, measured by the foil strain gage technique requires an extrapolation to estimate the contribution closer than 100 μm to the crack tip. This is due to the size of the strain-gages used, 200 × 210 μm. Conversely, the electron channeling technique for determining U is useful mainly close to the crack tip where subgrains form. In the present work U was measured by both techniques in the same low carbon steel at ΔK= 8 MN/m3/2. The contribution to U from closer than 100 μm of the crack tip was determined to be 1·7 × 106 J/m2 using electron channeling and 2·0 × 106 J/m2 by extrapolation. The measured contribution to U from further than 100 μm from the crack tip was 3·6 × 106 J/m2 giving 5·3 × 106 J/m2 for U. Thus, a large amount of energy is absorbed outside the region where sub-grains form. The non-hysteretic plastic work was found to be about four orders of magnitude smaller than the hysteretic plastic work, and may be neglected. A map of the plastic zone results from the strain-gage measurement. Rice's theory predicts the measured plastic zone sizeif the proper material's strength is employed in the formula.

An analysis of low cycle fatigue based on hysteresis energy

Abstract: — Cyclic deformation behaviour of medium carbon steel, armco iron and copper has been investigated. A model based on the energy absorbed for the fracture process at the crack tip, given in the form (Δw*p)y where Δw*p is the hysteresis energy at the crack front and the exponent y decides the energy spent for the fracture process, is proposed for the crack growth in low cycle fatigue. The value of the exponent y has been found to be around 0·26 to 0·28 for medium carbon steel and armco iron and around 0·52 for copper. These values of y give good prediction of the relations between the total fracture energy and the stress amplitude and the total fracture energy versus the fatigue life. The energy based approach has been extended to predict the cumulative damage in a single step stress variation. Good correlation has been obtained between the experimental data and the theoretical prediction.

A calculation of the threshold stress intensity range for fatigue crack propagation in metals

Abstract: — In this paper the strain distribution and the plastic zone size at a fatigue crack tip are discussed. From this analysis and considering the critical condition for plastic-blunting, a simple analytical formula for calculating the threshold stress intensity range for fatigue crack propagation at R= 0 is proposed. It is found that calculated values are in good agreement with existing experimental results.

An improved a.c. potential drop method for detecting surface microcracks during fatigue tests of unnotched specimens

Abstract: —The detection and measurement of surface microcracks in unnotched specimens are becoming more important, both from the point of view of description of crack growth by linear elastic fracture mechanics and from the classical endurance limit approach. Theoretical analyses of the current distribution in a cylindrical test specimen show promise that the a.c. potential drop method will become more sensitive for surface microcracks when higher current frequencies are applied. This effect was experimentally affirmed during fatigue tests on unnotched cylindrical specimens. With a 40 kHz current frequency semicircular surface microcracks with an area of 0.0066 mm2 (0.05% of the specimen cross-section) were detected. For accurate and reproducible crack growth measurements, a 5 kHz current frequency is preferable.

Some observations on small fatigue cracks in a superalloy

Abstract: —An investigation into the applicability of linear elastic fracture mechanics to very small fatigue cracks growing in a powder metallurgy nickel base superalloy is described. An unusual specimen was designed to facilitate the study of these small cracks. The stress intensity factor for the specimen was estimated and then calculated from the plastic zone size as determined by interferometry. The crack tip deformation field was also observed in the SEM and by stereoscopic viewing. These observations showed that the macroscopic deformation field was the same for both the long and short cracks, and was controlled by the stress intensity. The fatigue crack tip was found to interact strongly with the material microstructure and the localgrowth rate cannot be correlated with fracture mechanics quantities.

Tongue-shaped crack extension during fatigue of high strength aluminium alloys

Abstract: — Fracture surfaces of both service and laboratory fatigue fractures frequently show dark tongue-shaped marks. In fatigue tests on 7075–T6 specimens such tongues were produced by high peak loads. Measurements indicated that a tongue is not formed during a single burst of crack extension but is the result of a number of successive pop-ins requiring an increasing load. Therefore tongue formation is a quasi-stable phenomenon. The tunnelling fracture in the centre of plate thickness is accompanied by unfailed ligaments at the plate surfaces which reduce the stress intensity at the crack tip. The effect of different material conditions and loading direction on tongue forming was studied. A new model was developed to describe the growth of a tongue. The model was in good agreement with the various test results. The analysis of the problem has some relevance to validity requirements for K1c. A formula pertaining to tongues proposed by Forsyth was slightly modified and found to be approximately correct for the present results.

Low‐cycle fatigue and cyclic creep of metals

Abstract: — This paper deals with the effect of accumulated (oriented) plastic deformation (cyclic creep) on the life of metals under conditions of cyclic loading typically in the range of low-cycle fatigue. Specific features of cyclic creep of aluminium, titanium alloys and structural steels were investigated for the lives ranging from 05 to 2 × 105 cycles to rupture as well as specific features of transition from quasistatic to fatigue fracture associated with the variation in the mode of deformation of metals. It is shown that under conditions of repeated stress-controlled loading in a wide range of low temperatures, the cyclic creep processes are the predominant ones determining life of metals over a wide range of cycles, and when calculating life under these conditions it is necessary to take into account the rate of these processes irrespective of the type of fracture: quasistatic or fatigue. An equation was obtained for calculating life under low-cycle loading conditions which takes into account the interrelationship between the life, the accumulated plastic strain, steady-state cyclic creep rate and the resistance of the material to cyclic creep.

Formation and growth of fatigue cracks in reinforcing steel for concrete

Abstract: — –The da/dNvsΔKrelation for Torbar Steel is obtained by testing specimens cut from 32 mm bar. This relation is compared with available data for Unisteel 410 and with medium strength steels. Also, the pattern of fatigue cracking in Torbar is studied by inspecting bars retrieved from reinforced concrete beams which were subjected to repeated loading. It has been observed that fatigue crack locations in the bars coincided with the locations of concrete cracks traversing the bars. Tension tests on the fatigue cracked bars show that the cracks considerably reduce the ductility of the bars.

Fatigue thereshold in a 2618a aluminium alloy

Abstract: — This work is concerned with the determination of the fatigue threshold ΔKth for an aluminium alloy. No dependency of ΔKth with grain size was observed. Crack closure concepts were applied in order to explain the effect of mean stress and environment. The opening load Pop was determined by the compliance method. Results show no significant variation of Pop with environment (air or vacuum) but partially explain the influence of mean stress on crack growth rates. Fractographic analysis (SEM) of fracture surfaces and evolution of compliance diagrams suggest that a change of mechanism in crack propagation occurs when stage I changes to stage II growth.

A method of estimating biaxial fatigue growth rates

Abstract: —Conventionally, fatigue growth rate data are correlated in terms of the range ΔKof the applied stress intensity factor. It is argued that a correlation involving both ΔKand the constant stress Tclose to the tip of the equivalent elastic crack should be adequate for moderate stress, biaxial fatigue data. Practical application of these ideas would involve the preparation of “T -term” compendia similar to those already available for K -calibrations. “T -term” data are presented for the case of an edge-cracked strip in tension.

An assessment of crack closure in fatigue using the westergaard stress function

Abstract: — The residual stress fields at the tip of a fatigue crack have been evaluated using the Westergaard stress function. Using these values the loads at which the crack tip begins to open have been calculated to give U, Elber's effective stress range factor, as a function of stress ratio R, the ratio of applied stress range to yield stress, Δσ/σy, and the cyclic strain hardening exponent n. The computed values of U are shown to be in good agreement with measured values, and the predicted effect of prior high load cycles on the crack propagation rate at a lower cyclic load, on taking crack closure effects into account, is in excellent agreement with experimental results.

Fractographic investigation of fretting-wear and fretting-fatigue in aluminium alloys

Abstract: A fully age hardened Al-4%CuPl%Mg alloy has been subjected to plain-fatigue and fretting-fatigue and the resulting fracture surfaces compared. Crack initiation in the plain-fatigue sample occurred at cracked inter-metallic inclusions and subsequent crack growth was at 90" to the surface. Crack initiation in the fretting-fatigue sample occurred at the edge of wear scar and subsequent crack growth was at 45" to the surface. Measurement of fatigue striation spacing on the fracture surfaces showed that, for the same applied stress, the initial crack propagation rate was an order of magnitude higher in the fretting-fatigue sample. This accelerated crack propagation rate was maintained until the crack reached a depth of 0.5-1.0 mm; thereafter the growth rate was the same as that in the plain-fatigue sample.

Fatigue crack closure and residual displacement measurements on aluminum alloys

Abstract: —Ultrasonic bulk measurement technique was used to measure crack closure and a clip-on gage surface measurement technique was used to measure residual displacement at the crack tip of a propagating fatigue crack in 2219 and 7075 aluminum alloys. Experiments were carried out in vacuum, humid air, dry O2 gas, N2 gas, and deuteriated water vapor environments to evaluate effects of environments on residual strain and crack closure loads. It was observed that the closure load detected by ultrasonics decreased with introduction of humidity while the residual strain parameter dincreased with the humidity. The results are modeled as a build-up of oxide layer in addition to strengthening of asperities on the fracture surface. In the light of these observations, the effectiveness of surface and bulk measurements is evaluated. The results demonstrate that surface measurements are not always representative of the bulk behavior and use of such measurements in predictive analysis for fatigue can be misleading.

THE INFLUENCE OF TEST TEMPERATURE ON IMPACT FATIGUE CRACK GROWTH BEHAVIOR IN QUENCHED AND TEMPERED Cr‐Mo ALLOY STEELS WITH DIFFERENT PRIOR AUSTENITE GRAIN SIZES

Abstract: — Impact fatigue tests were carried out using a rotating-disk type impact fatigue testing machine The influence of prior austenite grain size, ductile-brittle transition temperature and test temperature on impact fatigue crack growth rate was investigated by means of fracture mechanics and fractography in quenched and tempered Cr-Mo alloy steel in which the prior austenite grain size was varied from 8–3 to 25-4 μm The results in impact fatigue tests were compared to those under non-impact conditions The crack growth rates associated with striation formation were insensitive to the change in prior austenite grain size, ductile-brittle transition temperature and test temperature regardless of impact and non-impact fatigue When the material was in the brittle condition, impact fatigue gave rise to a transition from striation formation to intergranular and cleavage cracking Such a transition will result in the acceleration of crack growth rate The Paris Law exponent values in impact fatigue were reasonably expressed by the ratio of test temperature to ductile-brittle transition temperature

Fatigue macrocrack growth in tempered hy80, hy130, and 4140 steels:threshold and mid‐δk range

Abstract: — –The rate of propagation of macrofatigue cracks down to near threshold was measured in air in three tempered martensitic steels; HY80, HY130 and 4140 (650°C temper). The value of ΔKth was determined by the load-shedding technique in center notched panel specimens. Of the three steels, 4140 tempered at 650°C had the lowest ΔKth, 3–5 MN/m3/2, while HY80 had the highest, 4.2 MN/m3/2. The 4140 (650°C temper) is intermediate in strength between HY80 and HY130. The results are discussed in terms of a recent theory of one of the authors. The fatigue crack propagation rates in the mid-ΔKrange in HY80 and HY130 in argon were also studied by measuring, with foil strain gages, the cyclic plastic work to propagate a fatigue crack by a unit area, U.HY80 has a lower crack propagation rate and correspondingly higher U.This was attributed in part to the higher yield strength of HY130 but the dislocation structure and carbide composition and morphology also play roles. Microstructural changes due to cyclic plastic deformation inside the plastic zone in HY80 and HY130 were observed by TEM of thin foils. SEM studies of the fracture surfaces at ΔK= 20 MN/m3/2 indicate a more ductile fracture mode for HY80 than for HY130. The fatigue crack propagation rate of HY130 is substantially higher in laboratory air (47% relative humidity) than in dry argon. This is not the case for HY80.