Showing papers in "Fatigue & Fracture of Engineering Materials & Structures in 1981"

Fatigue crack propagation behaviour of short cracks; the effect of microstructure

Abstract: — –Fatigue cracks shorter than some critical length tend to propagate anomalously quickly. This paper examines the concept of a ‘critical length’, identifying three regimes of behaviour for different crack lengths. Some published work is examined, covering a wide range of different materials. It is concluded that there is an approximate correlation between the critical length for short crack behaviour and the scale of the microstructure. LEFM is difficult, if not impossible, to apply to cracks shorter than this critical length because the material surrounding a crack cannot be assumed to approximate to a homogeneous continuum. Suggestions are made for a fatigue design philosophy which incorporates short crack behaviour.

A critical review of crack tip stress intensity factors for semi-elliptic cracks*

Abstract: Several crack tip stress intensity factor solutions have been published for semi-elliptic, surface breaking cracks in plates subjected to tension or bending forces. These solutions do not agree with each other particularly well and the basis for choosing which one is the best has not been established. In this paper, the development of fatigue crack shape is used as a diagnostic tool to test the accuracy of these theoretical stress intensity solutions in predictive fatigue crack growth calculations. Those solutions giving the best engineering estimate of crack tip stress intensity factors are identified. Single equations are also given for each loading case at the deepest point or surface intersection point of semi-elliptic cracks in order to facilitate calculations on programmable calculators. A rational basis for calculating the progress of a crack which snaps through the thickness and continues to propagate in a stable way by fatigue is suggested.

Fatigue life behavior of copper single crystals. part i: observations of crack nucleation

Abstract: In low strain fatigue, cracks often form in persistent slip bands (PSB) in a wide variety of materials. Observations of crack nucleation involving PSB have been made in copper single crystals by careful interferometric studies. The width, length, step height and thus strain localization in the PSBs remain very stable during saturation, even up to three-quarters of the life. For a given applied strain amplitude, the specimen will form PSBs having a certain distribution of slip offsets. The fatal crack is found to nucleate in the group of micro-PSBs having the largest offsets, and the largest strain localization. The PSB volume fraction for a given strain amplitude depends only very slightly on crystal orientation, and the slip offsets not at all. Thus the kinetics of crack nucleation are not affected by orientation. The mechanism of nucleation is concluded to be one of random walk in the slip offsets of PSBs.

Damage accumulation during initiation and short crack growth regimes

Abstract: — The birth and growth of short cracks is analysed from an elastic-plastic fracture mechanics viewpoint. Low to high cumulative damage tests from the low stress to high strain regime indicate that there is no crack initiation period in the metallurgical sense and that cracks grow from the first cycle, but at a slow rate. The initiation phase terminates when one crack starts to dominate and accelerates to failure, its initial size being given by for the medium carbon steel tested here, of grain size 56 μm.

Fatigue life behavior of copper single crystals. part ii: model for crack nucleation in persistent slip bands

Abstract: A crack nucleation model is constructed based on the random slip of the dislocations within the fatal persistent slip band (PSB). From such a model, we derive the notch-peak probability function. The crack is considered to nucleate when a critical notch depth is attained. This depth is constant for applied strains corresponding to the plateau of the cyclic stress- strain curve because the stress is constant. The PSBs behave in such a way that the more concentrated their localized slip the fewer the cycles required to attain the critical notch depth. Using the experimental relation between the applied strain and the magnitude of the slip offsets (reported in Part I) in conjunction with observations of the notch distributions along the gage surface, we have obtained the relation between the applied strain and the cycles for crack nucleation. A log-log plot of this relation shows a slightly different slope from that of the Coffin-Manson life data, previously reported. However, by allowing for cycles spent in Stage II propagation, good agreement is obtained.

Creep‐fatigue behaviour of four casts of type 316 stainless steel

Abstract: — Low-cycle fatigue tests including hold periods up to 24 h at maximum tensile strain have been conducted on four casts of Type 316 stainless steel in the temperature range 570 to 625°C. These resulted in failure times up to 4000 h. In general, fatigue life is reduced as the hold period increases, but the extent of life reduction varies from material to material and with test temperature. For two casts material this initial life reduction is followed by an increase in life as hold periods are extended. The results are rationalised in terms of prevailing fracture processes, and the implication for the extrapolation of short-term data is discussed.

Electrical potential monitoring of crack formation and subcritical growth from small defects

Abstract: —An experimental method was developed to quantify the formation and subcritical propagation of small cracks emanating from artificial surface defects. Continuous crack depth information was obtained from dc electrical potential measurements employing an analytical model. Fatigue experiments were conducted to evaluate the crack monitoring procedures for conditions relevant to the elevated temperature defect tolerance of superalloys. Cracking progressed uniformly and predictably from small surface defects (0.1 mm deep and 1.4 mm long) in A286, 304 and 10Ni steels and in Rene 95 and MP–159 superalloys. Crack depths, computed based on analytical calibration of measured electrical potentials, agreed to within ±18% of corresponding values measured optically. Similar results were obtained for a penny-shaped defect, 75 μm deep. Applications of the technique were investigated, including characterizations of fatigue crack formation and growth in Rene 95.

Stress-ratio effect of fatigue crack propagation in a biaxial stress field

Abstract: Fatigue crack propagation tests were conducted under conditions of equibiaxial, uniaxial and shear loading by using a cruciform specimen in a servo hydraulic testing machine. The effect of non-singular stress cycling on the fatigue crack propagation rate was examined based on the observation of crack opening behavior. The crack propagation rate was significantly influenced by the non-singular stress parallel to the crack when it was correlated to the stress intensity range. The crack closure behavior was greatly affected by the non-singular stress. The crack propagation rate was uniquely correlated to the effective range of the stress intensity factor except for the case of completely reversed shear loading where significant plasticity was detected. The crack opening displacement range was concluded to be a parameter controlling the crack propagation rate for all the stress conditions examined in the present experiments. Some discussion is made on the effect of material anisotropy on fatigue crack propagation in a biaxial stress field.

The engineering fatigue properties of wrought copper

Abstract: — The paper presents a comprehensive review, supplemented by original data, of the engineering fatigue behaviour of copper. Variations in manufacturing route and softening treatments are shown to have little effect on the fatigue of annealed copper but the high cycle fatigue strength is increased by cold work. The high strain fatigue behaviour is defined in terms of the plastic strain range and the cyclic stress-strain characteristics are documented. Fatigue behaviour in bending and torsion is defined by data and related to that in tension by simple design rules. Notches are found to reduce the laboratory measured fatigue strength of copper by ∼ 30% and the effect of surface finish, surface distortion and surface residual stress is defined in the literature. Fatigue crack growth is defined in terms of stress intensity factor range ΔK by an upperbound law and, together with the conditions for non-growth (ΔK0), shown to relate to the equivalent conditions for steels via the ratio of the respective elastic moduli. The effect of environment on the fatigue of copper has received scant attention in the literature, such results as exist suggesting little if any reduction in strength to be brought about by gaseous or aqueous environments. The most dramatic change is the improvement of about an order of magnitude which results when tests in vacuum are compared with equivalent tests in air. Results of fatigue tests on copper in ammoniacal environments are conspicuously absent from the literature. As the test temperature is reduced below room temperature there is a predictable increase in high cycle fatigue strength, a reduction in fatigue strength occurring above room temperature. High strain fatigue test results presented in terms of plastic strain range appear insensitive to temperature although at very low strain rates and high temperatures a reduction in fatigue strength occurs. A linear life fraction cumulative damage creep-fatigue law appears sometimes to be non-conservative but much more testing is needed to evaluate fatigue damage summation laws generally for copper. Numerical data are given in support of all the aspects of the engineering fatigue behaviour reviewed in the paper.

Fatigue crack growth under mixed mode loading

Abstract: —Mixed mode fatigue crack growth is analysed using Sih's strain energy density approach. A centre crack panel geometry loaded under uniaxial cyclic tension is considered. The crack angle is varied from 30° to 90°. A procedure for the determination of crack propagation life is outlined. The crack trajectory due to cyclic loading is predicted. The crack growth rate, the cyclic life and the cyclic life ratio are discussed, for an aluminium alloy and a steel, as a function of initial crack angle, crack length, stress amplitude, and the strain energy density factor.

Low‐ and high‐cycle fatigue properties of various steels specified in jis for machine structural use

Abstract: —Low-cycle fatigue properties were investigated on four carbon steels and five low alloy steels specified in JIS (Japanese Industrial Standard) for machine structural use, which are the most commonly used in Japan. Several different heats from each of several representative manufacturers were sampled so as to represent the average fatigue characteristics of current materials. The cyclic deformation behaviour of material was denned by comparing the monotonie yield stress on the extrapolated tensile work hardening curve with the cyclic yield stress in the cyclic stress-strain curve determined by incremental step test. The normalized ferrite-pearlitic steels cyclically hardened, while the quench-tempered martensitic were cyclically stable or softened. The S–N relations derived from the strain-controlled low-cycle tests were compared with the results obtained by load-controlled high-cycle tests. The extrapolated S–N curves based on the cyclic stress-strain curve predicted the fatigue strength in the high-cycle range to be stronger for cyclic-hardening steels, but weaker for cyclic-softening steels. The predicted S–N curves for stable steels coincided with the high cycle test data. The fatigue limit had a proportional relationship with cyclic yield stress, slightly depending on the cyclic deformation behaviour. On the other hand, the cyclic yield stress was found to exhibit a very good linear correlation with the monotonie tensile strength, independent of cyclic deformation behaviour. This explains the empirical law that the fatigue limit is approximately proportional to the tensile strength.

An understanding of fatigue thresholds through the influence of non‐metallic inclusions in steel

Abstract: —In recent years the low fatigue growth rate regime has been the subject of extensive study revealing its complex and often seemingly inconsistent nature. The value of alternating stress intensity at threshold ΔKTh) is sensitive to a range of variables which include mean stress, stress history, monotonie and cyclic yield strength, grain size and environment. A degree of coherency may be achieved by interpreting the data in terms of a crack closure mechanism which is a function of surface roughness. Experiments, in which the surface roughness (φ) has been altered by the introduction of non-metallic inclusions, give insight into the significance of crack closure in this regime.

A thermal shock fatigue study of type 304 and 316 stainless steels

Abstract: — –Thermal fatigue crack initiation and propagation promoted by thermal quenches have been studied in AISI 316 and 304 stainless steels and correlated with isothermal strain cycling fatigue. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 250°C to 500°C and symmetrically water-quenched on the narrow faces to give equivalent surface strain ranges from 2.8 ×10−3 to 5.4 ×10−3. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8×10−3 equivalent to a thermal amplitude of 150°C with no cracking being produced in 500,000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrical quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. The results indicated a good correlation with the fracture behaviour observed from isothermal strain cycling fatigue behaviour in an air environment.

Crystallographic fatigue crack growth in nimonic ap1

Abstract: Recently a controversy has developed over whether crystallographic crack growth near threshold in nickel-base superalloys occurs along right brace 111 left brace slip planes or right brace 100 left brace planes at room temperature. In this work crack propagation is shown to occur on both right brace 100 left brace and right brace 111 left brace planes. The most common facet plane is right brace 111 left brace and this is the only orientation observed at the lowest stress intensities, but at higher stress intensities occasional right brace 100 left brace facets are also produced. This behavior is compared with similar results in aluminum alloys.

Cyclic strain hardening of 316 stainless steel at elevated temperatures

Abstract: The cyclic strain hardening occurring during fatigue testing at 550°C was studied in terms of the dislocation structures developed. The dislocations are collected in dense tangles which form the boundaries of a cell structure. The stress required to push through, or eject, a dislocation from the cell boundary predicts the flow stress. These values showed good correlation with the cyclic shear stresses obtained by applying the Tresca criterion, which underlies its usefulness in design under cyclic loads. Carbide particles are shown to precipitate on grain and cell boundaries.

Fractographic analysis in the understanding of corrosion fatigue mechanisms

Abstract: — Fractographic analyses have been used to explain the cyclic crack growth behaviour of A533-B, Ducol W30, a C-Mn steel and type 304 stainless steel in simulated light water reactor environments at ambient temperature. Fractographic observations have offered an explanation for anomalous crack growth behaviour and have also indicated where micro structural or environmental variables dominate in producing certain fracture modes and crack growth rates. An understanding of the operative corrosion fatigue mechanisms has been formulated through these fractographic analyses. Environmental crack growth in the ferritic steels has been described by a model involving both anodic dissolution and hydrogen embrittlement. Conditions where only one of these mechanisms would dominate have been identified and limits to their effect postulated. A crystallographic mode of failure observed in the austenitic type 304 stainless steel has also been explained by a selective dissolution process.

Stress intensity and fatigue crack growth in multiply-cracked, pressurized, partially autofrettaged thick cylinders*

Abstract: : Stress intensity factors are determined for internally and externally cracked, pressurized thick cylinders with partial autofrettage (less than 100% overstrain). The solutions are based on a superposition of existing solutions which does not involve any loss of accuracy. Implications of the stress intensity factor results for the safe-life design of gun tubes are discussed. Various suggestions for future work are presented. (Author)

A d.c. potential drop method to monitor crack growth in notches subjected to torsion

Abstract: — A d.c. potential drop formula has been established for monitoring crack growth in circular notched members subjected to torsion. Changes in crack depth of 20 μm can be detected in cracks growing up to 2 mm in depth.

Intrinsic material scatter in fatigue crack propagation

Abstract: The statistical nature of fatigue crack growth rate has been examined for G41400 steel exposed to different heat treatments and for different loading and environmental conditions. Techniques for determining the intrinsic value of the crack propagation rate are discussed. The intrinsic scatter due to the material has been evaluated and considered in terms of its effect on fatigue life prediction, both normal and log normal distributions of fatigue crack growth data being considered. It appears that the intrinsic material scatter in crack propagation rate is closely related to the scatter in total life.

Analysis of combined static and fatigue crack growth data

Abstract: —Three methods are presented, whereby' static’(R-curve) and fatigue crack growth data may be combined. In the first, a single curve is fitted through both sets of data, making due allowance for residual stresses from prior unloading and stress ratio. For the other two methods, crack growth is divided into tearing and ductile striation formation components. These components may be combined either by adding crack growth rates associated with specified values of stress intensity factor or by adding crack growth resistances associated with specified crack extensions. The results are compared with test data obtained from compact tension specimens of high strength aluminium alloys, BS.L97 (2024–T3) and DTD.5120 (7010–T7651).

FATIGUE THRESHOLDS AND DUCTILE-BRITTLE TRANSITIONS IN Ti-30Mo

Abstract: — Substantial effects of hydrogen are found on the DBTT and cleavage fracture stress in both cathodically and gas phase charged samples of a BCC Ti alloy. Fatigue thresholds when plotted as a function of temperature exhibit a peak and this peak shifts towards higher temperatures when the hydrogen level is increased. An elastic calculation of static threshold stress intensities using experimentally observed fracture stress parameters show a similar trend to fatigue threshold stress intensities.

The impact fatigue properties of pearlitic plain carbon steels

Abstract: — A literature search has revealed that the most extensive study of impact fatigue reported is still that of Stanton and Bairstow (1908) who were the first to work on this topic. Their results, which were on pearlitic plain carbon steels, have been reanalyzed in an attempt to deduce from them the effects of chemical composition on impact fatigue in this class of materials. The results show that when the number of impacts to failure, Nf, is greater than about 103, Nf and the energy absorbed per impact, Ei, are related by the equation: Ei, =Eo+EkNf−p, where Eo appears to be the impact endurance limit. It is suggested that Ek and p be named respectively the “impact fatigue parameter” and “impact fatigue exponent”. Ek is independent of composition except that it is lower by a factor of about four for two materials which probably failed by an intercrystalline rather than transcrystalline fracture; p has a value of 0.6; and Eo increases monotonically with silicon content but does not vary systematically with the carbon, manganese, sulphur or phorphorus contents. The significance of these results is discussed.

Fatigue life calculation and testing of full-scale welded i-beams

Abstract: —Fatigue tests under four-point bending were carried out on low alloy steel welded I-beams under constant and variable amplitude loading. Four different types of stiffeners welded to flanges and webs were analysed. Particular attention was paid to the effect of configuration at the stiffener-ends. It was shown that the stiffener-ends were the most critical points. Fatigue cracks initiated and propagated in both tension and compression flanges. The fatigue life to crack initiation was calculated on the basis of the local stress-strain analysis. Both theoretically and experimentally estimated stress concentration factors were used for calculation. The predicted fatigue lives were compared with the respective values obtained in experiments.

A method for predicting the lifetime of gas turbine blades

Abstract: — –The article describes a method developed at ONERA for predicting the lifetime of gas turbine blades. This method makes use of non-linear viscoplastic constitutive laws, the problem being solved from a plane cross section assumption and through a time-step linearization. The initiation of the first macroscopic crack is calculated from a stabilized cycle condition. A blade test rig has been developed for checking the method under load and temperature conditions that are as close as possible to operational ones. The results of two series of tests on convection cooled IN100 blades prove the method to be acceptable to design engineers.

The applicability of strain‐range partitioning to high temperature low cycle fatigue life prediction of “in 738” alloy

Abstract: — The results of high temperature low cycle fatigue (HTLCF) on IN 738, obtained from two different sources, were used to study the applicability of strain-range partitioning (SRP) to this alloy. In spite of the difficulty in separating the plastic-plastic strain component from the different strain-life relations, an attempt was made to determine the coefficients and exponents for each of these relations from both sets of data. The observation that compressive creep, balanced by tensile plastic deformation, was more damaging than the reverse, could be related to the behaviour of the material. The evidence of the effect of temperature on the plastic-plastic strain-life relation, reported by some researches, was shown to be not decisive in the reported temperature range. The reasonable agreement and the similarity in the material's behaviour, obtained from the analysis of the high temperature low cycle fatigue of the two sets of data, substantiates the applicability of the strain-range partitioning method to IN 738 alloy.

EFFECT OF STRESS-STRAIN BEHAVIOR ON LOW-CYCLE FATIGUE OF α-β TITANIUM ALLOYS

Abstract: — Tensile specimens of Ti-6A1-4V with four levels of interstitial oxygen content and a Ti-8Al-1Mo-1V alloy with different heat treatments to alter grain size and/or microstructural character are subjected to slow strain-controlled cyclic deformation leading to rupture in the 5-500 cycle range. Indication of crack initiation as well as rupture life are compared, relative to the plastic excursion strain. On this basis, the effects of grain size and oxygen content are not clearly discriminated. Yet, some of the materials exhibit markedly superior performance. This improvement seems to be related to a characteristic evolution in the shape of the cyclic stress-strain curve. Here, relative to a full convex hysteresis loop of early cycles, the later cycles exhibit a reduced stress level, or cyclic softening, in the first half of the excursion, followed by a resurgence of strength to initial stress levels in the latter portion. The enhanced strain hardening rate enabling this terminal strength restoration is thought to stabilize the deformation, reducing the amount of stress-relaxation-induced tensile strain. Taking such strain as an increment of damage in a cumulative cyclic creep strain criterion provides a correlation between the evolving shape of the cyclic stress-strain curve and the low cycle fatigue endurance. Results indicate the absolute increase in the terminal plastic strain hardening rate to be a constant of a material, independent of the cyclic strain excursion.