Showing papers on "Paris' law published in 1970"

The Effect of Biaxial Stresses on Fatigue and Fracture

Abstract: The results are presented of an analytical and experimental program which was conducted to determine if a biaxial stress field produces a significant effect on the fatigue and fracture behavior of thin plates. The materials tested were 6061-T4 and 6061-T6 aluminum sheets and plexiglas sheets. The experimental program included fracture tests with various magnitudes of biaxial load at fracture and fatigue tests with sinusoidal loading normal to the crack and either constant or sinusoidal stresses applied parallel to the crack. The effect of nonsingular stresses on the behavior of a crack is examined from both a linear elastic and an elastic-plastic viewpoint. The experimental study indicates that a biaxial stress field does affect the behavior of a crack in a thin sheet. An increase in the apparent fracture toughness with increasing biaxial load was observed experimentally but as yet cannot be adequately explained using linear fracture mechanics theory. Biaxial stresses were found to produce a shift in the fatigue crack growth rate data, and it is shown that this shift can be predicted using several empirical fatigue crack propagation models.

Environmentally Controlled Fatigue Crack-Growth Rates in SAE 4340 Steel—Temperature Effects

Abstract: The temperature dependence of sustained-loading and corrosion-fatigue crack-propagation behavior of SAE 4340 steel immersed in distilled water was investigated using the fracture mechanics approach. During the load cycle, stress-intensity histories had values higher than the static threshold stress intensity (KIscc ). For a stress-intensity range of 20 ksi in., a factor of 30 increase in the corrosion-fatigue crack-growth rate was noted for a temperature increase of 70 deg C. The mechanical variables were found to have much less influence than the environmental reaction rate. In contrast with dry air results, the fatigue crack-growth rate in a distilled water environment was dependent on the mean stress-intensity for stress-intensity histories above KIscc . This dependence varied with temperature level.

Residual Stresses Induced By Hole Cold Expansion

Abstract: This paper describes an X-ray diffraction technique for measuring residual stresses. The results of measurements at locations in the vicinity of plain and cold expanded holes in an aluminium alloy are presented. Residual stresses are shown to vary significantly in all three dimensions and two dimensional analyses commonly used for residual stress determination are shown to be inadequate. The results of a fatigue test programme are also presented in which simple aluminium alloy specimens containing plain and cold expanded holes were subjected to constant amplitude fatigue loading. The results show that cracks from plain holes continuously increase in growth rate to failure whilst cracks from cold expanded holes decrease in growth rate and frequently arrest. The arrested crack lengths are different on either face of the cold expanded specimens and this is equated to the different residual stress fields present. Fatigue crack growth rates predicted using a Green's function technique are compared with those measured experimentally.

Effect of a Tensile Mean Stress on the Alternating Stress Required to Propagate an Edge-Crack in Various Materials

Abstract: The conditions governing growth or dormancy of edge-cracks in plate specimens of twelve materials subjected to the general tensile loading cycle σm ± σa, where σm ≥ σa and σm + σa < yield stress, have been determined. It was found that, as in the case of zero mean load, these conditions were defined by the value of the parameter σa3l; if σa3l < Cm a crack would grow whereas if σa3l < Cm it would remain dormant, where Cm depended on the material and on the value of the ratio σm/σa. Above a certain value of σm/σa reached a minimum value for a given material. In general, this minimum value agreed with the value derived from the known fatigue crack growth characteristics of the material by assuming that during one loading cycle a crack will remain dormant if it cannot extend at least one atomic spacing.

The Growth Of Short Fatigue Cracks: A Review

Abstract: The behavior of short fatigue cracks differs from that of long cracks because of greater sensitivity to the microstructure, a greater size of the plastic zone relative to crack length, and a lesser extent of crack closure. Advances have been made in the understanding of the fatigue crack growth process of short cracks, and this understanding has been employed in the development of analytical treatments of short fatigue crack growth. The present paper reviews this progress and also discusses the relevance of short fatigue crack behavior to technologically significant areas.

Non-propagating fatigue cracks in an aluminium-1/2% magnesium alloy

Abstract: Non-propagating fatigue cracks have been observed at the roots of sharp notches in specimens of a single-phase Al-1/2% Mg alloy. The rate of fatigue crack growth, and the extent of cyclic deformation on either side of the crack, decrease with increase in length of crack. The process of formation of non-propagating cracks can be repeated in the same specimen by means of intermediate anneals. The results are discussed in terms of the cyclic strain-hardening of the material.

Fatigue crack growth in Si-Fe

Abstract: Fatigue crack growth rates were measured in vacuum in the temperature range from 100° to 250°C. At 200°C and above, crack growth occurs by a ductile mechanism. The ductile crack growth rate is proportional to the stress intensity factor amplitude raised to the 5/2 power, is equal to about one-tenth of the crack opening displacement per cycle, and is not measurably dependent on the peak stress intensity factor in the range measured. Local crack growth occasionally becomes arrested by large scale blunting of the crack tip. The fracture surface has many “river markings” and often adheres to specific growth planes on a scale of the grain size. The overall crack growth is approximately in the plane of maximum tensile stress. Inclusions and grain boundaries are not of importance in the growth mechanism in the range of crack growth rates measured. Growth rates in air at 200°C are at most twice those in vacuum. Although a mechanism, using continuum mechanical concepts, can explain the external features of the ductile crack growth rate, the local features showing a combination of all three modes of straining suggests that the actual process of crack growth must be far more complicated than hitherto realized. At 150°C and below, the ductile growth mechanism is augmented by the wholesale cleavage of single grains or grain clusters. The growth rates at these temperatures are higher and more sensitive to stress intensity factor amplitude.

Markov approach to fatigue crack growth under stochastic load

Abstract: Under variable amplitude loading the fatigue crack growth rate to the load cycles following an overload is reduced. Under stochastic loading the retardation phases occur randomly with random intensity and duration depending on the last random overload. In the paper the load process is assumed to be stationary and its extremes are modelled as a Markov sequence. The fatigue crack growth appears to form a random sequence of retardation and post- retardation phases. The drift and diffusion parameters of a Markov approximation for the whole crack growth process are evaluated from a numerical simulation within retardation and post-retardation blocks. This mixed, numerical and analytical, approach allows us to efficiently investigate the effect of different load and retardation model assumptions on fatigue structural lifetime.

Linear Fracture Mechanics - What It Is, What It Does

Abstract: : This report is intended to assist non-specialists in understanding papers on linear elastic fracture mechanics. The main concepts are described, and the various terms and conventions used are explained. Advanced mathematical techniques are used in fracture mechanics; some of which are described briefly, but results rather than methods are emphasized. Fracture mechanics developed from the study of brittle fractures which had taken place below general yield; it is found that such failures always originate at some type of crack or flaw. Previously, the problem of brittle fracture was tackled by empirical methods based on service experience but the problem can only be properly understood using fracture mechanics, which is the applied mechanics of crack growth starting from a flaw. Stress analysis of cracked parts leads to the concept of stress intensity factor, which describes the elastic crack tip stress field. A critical value of this factor at which crack growth starting from a flaw. Stress analysis of cracked parts leads to the concept of stress intensity factor, which describes the elastic crack tip stress field. A critical value of this factor at which crack growth starts is a useful measure of the fracture toughness, or resistance to brittle fracture of a high strength material, which can be used for material development, quality control, design failure analysis. The stress intensity factor can be applied to other types of crack growth such as stress corrosion and fatigue crack growth.

Fatigue Crack Growth Mechanism Of MMC AtRoom And Higher Temperatures

Abstract: In order to clarify the fatigue failure mechanism of an alumina short fiber reinforced aluminum alloy (hereafter, MMC for brevity), a series of fatigue strength and fatigue crack growth tests was carried out at room and elevated temperatures. Aluminum alloy A6061 was used as a reference material. Results of fatigue strength tests showed that the fatigue strength of the MMC was superior to that of A6061 alloy in long life region and at higher temperature. Similar trend was also found from the results of fatigue crack growth tests, that is, the MMC showed higher threshold level and relatively lower dependence of the crack growth rate on the temperature when compared with the case of A6061 alloy. Moreover, it was found that the fatigue crack growth rate of the MMC was governed by both K^ and AK^ and its temperature dependence was expressed in terms of the 0.2% proof stress, Young's modulus and a parameter relating to the crack closure at each temperature.

A Study Of Fatigue Crack Growth Laws InSmall And Large Cracks Based On COD

Abstract: Fatigue crack growth tests were carried out on an annealed 0.45%C steel. Crack opening displacement ranges (ACOD's) along a crack were measured in the neighborhood of the crack tip from the SEM photographs taken of positive metal replicas of surface cracks under the maximum and minimum loads. The physical background of the two representative crack growth laws: dl/dN — C\o™l for small cracks under high nominal stresses and dl/dN — CA/i™ for large cracks under low nominal stresses, which formally contradict each other, was discussed on the basis of the measured ACCWs. In small and large cracks having the same growth rate, the geometries of measured ACOD's near the crack tips were also almost the same. Therefore, the ACOD distribution near the crack tip can be considered as a foundamental factor controlling consistently the growth rate of a crack ranging from small to large cracks.

Influence Of Changing Of Microstructure ByThe Heat Treatment On Fatigue Crack GrowthProperties Of The Carbon Cast Steel

Abstract: An influence of the surface heat treatment on the GSN 422660 carbon cast steel, which is equivalent to the German GS-60 steel (DIN 1681), with a various content of Al on fatigue crack growth rates and threshold values of physically long cracks was studied. The microstructure is the most important factor, which determines the fatigue crack growth mechanism and resistance. Oxide debris on the crack faces reduce significantly the threshold values.

Subcritical crack growth in several titanium alloys

Abstract: : Studies of low-cycle fatigue crack propagation in air and in salt water environments and salt water stress-corrosion cracking (SCC) were conducted on several intermediate-to-high strength, very low interstitial oxygen, titanium alloy plate materials. The alloys studied included one sample each of Ti-7Al- 2Cb-1Ta and Ti-7Al-2.5Mo, and two samples each of Ti-6Al-4V and Ti-6Al-6V-2Sn. The yield strengths of these alloys ranged from 110 to 150 ksi (77.3 to 105.5 kgf/sq mm) and included both as-received and heat-treated conditions. Fatigue and SCC data were obtained from single-edge-notched (SEN) cantilever specimens, and the results were analyzed in terms of crack tip stress-intensity parameters. Most of the alloys investigated exhibited low resistance to low-cycle fatigue crack propagation. The most favorable results were obtained from the Ti-6Al-4V alloys. However, several alloys were highly resistant to environmentally- accelerated crack growth in salt water. Several alloys also showed a correlation between fatigue crack growth behavior in salt water and the threshold stress- intensity for SCC to occur (K sub Iscc). Examination of fatigue surfaces by electron fractography revealed that the predominant mode of separation was microvoid coalescence, regardless of environment.

Fatigue crack growth studies on constant compliance specimen

Abstract: Fatigue crack growth tests in the presence of tensile cyclic stressing have been conducted on 18% Nickel Maraging steel and 70/30 Brass. The stress intensity level is restricted to values which maintain plane strain conditions at the crack tip. Crack growth has a linear relationship to maximum stress intensity and the results predicted a minimum stress intensity below which crack growth should not occur.

Fatigue Crack Growth In Welded Joints

Abstract: Recent developments in crack measurement technique ACPD/ACFM (alternating current potential difference/alternating current field measurement) and parametric equations for weld toe nonlinear stress distributions has allowed the validation of early fatigue crack growth models for tubular welded connections. A series of fatigue tests on X and multi-planar nodes have been conducted and these have provided early crack growth data. This information has been utilised to validate crack growth models for tubular welded connections.

Probabilistic Predictions Of Fatigue CrackBehaviour

Abstract: It is shown that reasonable predictions of fatigue crack growth can be obtained using damage mechanics concepts. A model that considers the growth of a main crack as a sequence of failures of sub-elements along the crack path is used to perform computer simulations of crack propagation in centre notched specimens. Probabilistic criteria are introduced from the statistical analysis of S/N curves for fatigue life. Scattering of the data is reproduced in sub-elements ahead of the crack tip. The result is a set of simulations for fatigue crack propagation through the same track. In order to check the model, fatigue crack growth tests were carried out in grade II titanium sheet samples, using a servohidraulic MTS machine. The results of experimental curves are compared to those of simulated curves. The good agreement observed between the sets of curves shows the possibilities of damage mechanics to give reliable lifetime predictions. Transactions on Modelling and Simulation vol 22, © 1999 WIT Press, www.witpress.com, ISSN 1743-355X

Fatigue and fracture of a bituminous paving mixture

Abstract: THE FATIGUE BEHAVIOR OF SIMPLY SUPPORTED SAND-ASPHALT BEAMS IS EXAMINED BY USING EXPERIMENTAL AND ANALYTICAL METHODS OF FRACTURE MECHANICS. THE FATIGUE CRACK GROWTH RATES CORRELATE WELL WITH THE STRESS-INTENSITY FACTOR IN ACCORDANCE WITH PARIS'S LAW THAT STATES THE DC/DN = AK, TO THE FOURTH POWER WHERE DC/DN IS THE CRACK PER CYCLE, A IS A CONSTANT OF A MATERIAL, AND K IS THE STRESS-INTENSITY FACTOR, WHICH IS DEPENDENT ON THE LOAD, GEOMETRY, AND BOUNDARY CONDITIONS. IT IS POSTULATED THAT IN A MATERIAL SUCH AS A SAND-ASPHALT MIXTURE, WHICH IS ABUNDANTLY ENDOWED WITH FLAWS, FATIGUE DAMAGE IS INITIATED AT THE FIRST LOADING CYCLE, SO THAT THE FATIGUE LIFE IS THE NUMBER OF CYCLES OF REPEATED LOADING TO PROPAGATE A "STARTER FLAW," INTO A CRACK OF CRITICAL SIZE. THE STARTER FLAW IS A MATERIAL'S CONSTANT BUT IS SUBJECT TO STATISTICAL VARIATION AND IS BELIEVED TO BE PRINCIPALLY RESPONSIBLE FOR THE STATISTICAL VARIATION OF FATIGUE LIFE. THE CRACK REACHES THE CRITICAL STRESS-INTENSITY FACTOR, WICH IS A CONSTANT FOR A GIVEN MATERIAL. THE CRITICAL STRESS-INTENSITY FACTOR IS THE FAILURE CRITERION FOR BOTH STATIC FRACTURE AND FATIGUE. A FORMULA FOR FATIGUE LIFE IS GIVEN. METHODS FOR DETERMINING THESE CONSTANTS ARE PRESENTED, AND THE FATIGUE LIVES, DETERMINED EXPERIMENTALLY, ARE COMPARED WITH THOSE PREDICTED FROM THE FORMULA AND SHOW GOOD AGREEMENT. /AUTHOR/

The stress corrosion resistance and fatigue crack growth rate of a high strength martensitic stainless steel, AFC 77

Abstract: The plane-strain stress corrosion thresholdK Iscc and fatigue crack growth rate have been determined for a high strength martensitic stainless steel, AFC 77, in both conventionally processed and strain-aged conditions. TheK Iscc (in 3.5 pct sodium chloride solution) is markedly affected by both the tempering temperature and the degree of strain aging. The highestK Iscc of 105 ksi $$\sqrt {in} $$ . was obtained by tempering at 500°F and the lowestK Iscc of 10 ksi $$\sqrt {in} $$ . by tempering at 1100°F. Retained austenite raisedK Iscc at tempering temperatures up to 1000°F, which was the highest tempering temperature at which austenite could be maintained. Fatigue crack growth rates in both dry air (<10pct relative humidity) and 3.5 pct sodium chloride solution were at a maximum for material tempered at 700°F. Over the range of stress intensity studied, retained austenite reduced fatigue crack growth rate in salt solution but increased it in dry air.

Fatigue Crack Growth Of Semi-elliptical CracksUnder Bending Combined With Steady Torsion

Abstract: An analysis of the influence of steady torsion loading on fatigue crack growth of semi-elliptical cracks under rotating and reversed bending is presented. The crack growth has been studied in two different fatigue machines which perform mixed mode (i+m) loading obtained by cyclic mode I (AKj) with superimposed static mode III (%J loading. Tests were carried out on cylindrical specimens in Ck45k steel and the results are compared in both loading conditions: rotating and reversed bending with and without steady torsion. Results show that the static mode III loading superimposed to mode I leads to a significant reduction on the crack growth rates. The influence of these phenomena could be related with an increased "crack closure effect" associated to interlocking oi rough fracture surfaces, friction and fretting debris, leading to a decrease of the AK effective at the crack tip. This work provides a contribution for a better understanding of the crack growth rate on shafts under mixed mode load conditions in order to predict remaining life times arid to estimate the risks of precracked rotor shafts.

Variation in Fatigue Crack Growth due to the Geometrical and Loading Effects

Abstract: The problem of crack growth is a major issue in the prediction and maintenance of aerospace structures, as well as other structural elements in mechanical engineering. Fatigue crack growth as consequence of service loads depends on many different contributing factors. Due to the number and complexity of the mechanisms involved in the fatigue crack growth problem, no universal solution exists yet and there is no general agreement among researchers for any of the available models. Most of the results reported are dealing with geometry with some factors separately. This paper simulates the factors affecting the fatigue crack growth of metallic materials under cyclic loading. For the simulation purpose, three points bend (TPB) with span to width ratio 8:1 and compact tension (CT) specimen geometries were used. There are many factors affecting the fatigue crack growth in structures, such as initial crack length, stress ratio, aspect ratio and type of geometry. The behavior of such cases is shown using Forman model. The fatigue crack growth obtained from the two geometries was compared. Different values of these factors showed different effects on the fatigue crack growth. For further study need to validate the modelling procedure with experimental work as well as take into account the other factors such as; other types of geometries with fatigue crack models and environmental effects towards a universal solution.