CRACK GROWTH BEHAVIOUR OF FATIGUE MICROCRACKS IN LOW CARBON STEELS

The behaviour of low carbon steels has been studied, in particular the initiation and growth of the major crack which led to final fracture of smooth specimens, via surface replica and photomicrograph techniques. From this study, the fracture process and fatigue life prediction characterized by the growth of surface microcracks have been analysed by a new approach unifying the conventional approaches based on the final fracture of materials with the fracture mechanics approach. Knowledge of fracture parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life prediction and assessment.     

INTRINSIC MATERIAL SCATTER IN FATIGUE CRACK PROPAGATION

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.     

CRACK INITIATION AND PROPAGATION BEHAVIOUR OF A HEAT-TREATED CARBON STEEL IN CORROSION FATIGUE

Abstract— When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air.     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

MICROSTRUCTURAL AND ENVIRONMENTAL EFFECTS ON FATIGUE CRACK PROPAGATION IN DUPLEX STAINLESS STEELS

Abstract— Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.     

SCATTER CHARACTERISTICS OF FATIGUE LIFE AND THE BEHAVIOUR OF SMALL CRACKS

Usually, there is large scatter in fatigue data and this should be evaluated quantitatively when fatigue data are applied to the design of machines and structures. Consequently it is important to clarify the physical basis of scatter in fatigue life. In this present study, rotary bending fatigue tests were performed on an annealed 0.21% carbon steel. At least sixteen smooth specimens were fatigued at each of three stress ranges and successive observations of the surface were studied for all the specimens using the plastic replica method. By examining the initiation and propagation behaviour of cracks the physical basis of scatter in fatigue life is analysed and discussed. To estimate the scatter characteristics quantitatively, the distributions of crack initiation life, propagation life, fatigue life and crack length were individually studied by assuming a Weibull distribution for each set of data.     

FATIGUE LIFE PREDICTION OF HEAT-TREATED CARBON STEELS AND LOW ALLOY STEELS BASED ON A SMALL CRACK GROWTH LAW

Abstract— Since heat-treated high strength steels are often used as materials for machines and structures that operate under severe service conditions, it is important to evaluate their fatigue life. Hence the growth law of a small fatigue crack must be known in order to estimate the fatigue life of machines and structures since the life of such members is controlled mainly by the behaviour of a small crack. The growth rate of a small crack can not be predicted usually by linear elastic fracture mechanics, but can be determined uniquely by the term σⁿ_a l , where σ_a is stress amplitude, l is crack length and n is a material constant. In this paper, the small-crack growth law of heat-treated carbon steels and low alloy steels was studied. An effective and convenient method based on a small-crack growth law, d l /d N = C ₃(σ_a/σ_a)ⁿ l is proposed, where σ_u is the ultimate tensile strength, for predicting the small crack propagation life of heat- treated steels with different tensile strength levels, together with a method for determining the fatigue life of plain members.     

SHORT CRACK FATIGUE BEHAVIOUR IN A MEDIUM CARBON STEEL

The initiation stage and short crack behaviour in torsional fatigue of a 0.4% C steel was investigated by a replication technique. The fatigue cracks initiated and propagated in the ferrite phase which is located at the prior austenite grain boundaries in the form of long allotriomorphs. At this stage of crack development it is proposed that crack growth rate depends on the extent and intensity of plasticity at the tip of the crack. Crack growth per cycle is correspondingly proportional to the strength of the slip band. The ferrite-pearlite boundaries are strong barriers to crack propagation, which is manifested by a deceleration of growth and possible arrest. On raising the stress level the previously non-propagating cracks may continue to grow by branching or joining with other cracks in the ferrite phase. This process is repeated until the stress fields of one or more dominant cracks attain a critical value to sustain continued growth that leads to failure.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF SMALL CRACKS AND FATIGUE LIFE IN CARBON STEELS WITH DIFFERENT FERRITE GRAIN SIZES

Abstract— In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propagation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks.     

CRACK TIP BEHAVIOUR AND CRACK PROPAGATION IN DUCTILE MATERIALS

Dilatational plastic equations, which can include the effects of ductile damage, are derived based on the equivalency in expressions for dissipated plastic work. Void damage developed internally at the large-strain stage is represented by an effective continuum being strain-softened and plastically dilated. Accumulation of this local damage leads to progressive failure in materials. With regard to this microstructural background, the constitutive parameters included for characterizing material behaviour have the sense of internal variables. They are not able to be determined explicitly by macroscopic testing but rather through computer simulation of experimental curves and data. Application of this constitutive model to mode-I cracking examples demonstrates that a huge strain concentration accompanied by a substantial drop of stress does occur near the crack tip. Eventually, crack propagation is simulated by using finite elements in computations. Two numerical examples show good accordance with experimental data. The whole procedure of study serves as a justification of the constitutive formulation proposed in the text.     

CRACK INITIATION AND SMALL FATIGUE CRACK GROWTH BEHAVIOUR OF SQUEEZE-CAST Al-Si ALUMINIUM ALLOYS

Abstract— Fatigue strength, crack initiation and small crack growth behaviour in two kinds of squeeze-cast aluminium alloys, AC8A-T6 and AC4C-T6 were investigated using smooth specimens subjected to rotatary-bending fatigue at room temperature. Fatigue resistance of these alloys was almost the same as that of the wrought aluminium alloys because of their fine microstructure and of the decrease in defect size due to squeeze-casting. Fatigue crack initiation sites were at the eutectic silicon particles on the surface of specimens or at internal microporosity in the specimens. Crack initiation life, defined as a crack length of 50 μm on the specimen surface, was successfully estimated from an evaluation of initiation sites using fracture mechanics and the statistics of extrema. Small fatigue crack growth in the two kinds of alloys obeys the relation proposed by Nisitani et al. , namely that d(2c)/d N = C (σ_a/σ_B)ⁿ· (2 c ), where C is a constant and σ_B is the ultimate tensile strength. It is pointed out that an improvement in fatigue strength of cast aluminium alloys can be expected by refining the eutectic silicon rather than by an increase in static strength.     

A COMPARISON OF NEAR-THRESHOLD FATIGUE CRACK PROPAGATION IN TWO HIGH STRENGTH STEELS

Abstract— Fatigue crack growth rate data for long and short cracks were obtained from two high strength steels used in the aerospace industry. Differences in the behaviour of the two cracks were noted and explained in terms of the three-dimensional nature of short cracks compared to two-dimensional long cracks, the interaction between cracks, the applicability of Linear Elastic Fracture Mechanics (LEFM) and their effect of crack closure.     

STUDY ON FATIGUE THRESHOLD BEHAVIOUR AND FATIGUE CRACK PROPAGATION IN A CAST Co-Cr-Mo ALLOY USED FOR SURGICAL IMPLANTS

Abstract— —Fatigue crack propagation rates (d a /d N ) and fatigue crack thresholds (Δ K _th) have been studied in a cast Co-Cr-Mo alloy used for surgical implants with various grain sizes. Results for materials with average grain sizes of about 400 and 60μm respectively are presented. Threshold values close to 10–15 MPam have been measured with decreasing values observed on increasing the grain size. Similar effects of grain size are found on the crack propagation behaviour at higher growth rates, where a coarse grain size material show a higher crack growth rate than a fine grain size material at the same Δ K levels. The effects of microstructure on fatigue properties of the cast Co-Cr-Mo alloy are caused not only by grain size variation but are also attributed to the microstructural differences: a coarse-grained material with a directionally grown dendritic structure vs a fine-grained material with an equiaxed grain structure.     

THE ROLE OF PRIOR AUSTENITE GRAINS IN FATIGUE CRACK INITIATION AND PROPAGATION IN LOW CARBON MARTENSITE

Abstract— A study has been made of the role of Prior Austenite Grains (PAG) in fatigue crack initiation and propagation in low carbon martensite. The occurrence of ferrite precipitation along prior austenite grain boundaries during rapid cooling will lead to the formation of intergranular cracking. This intergranular cracking has a close relationship with the reduction of the endurance limit for fine grained low carbon martensite.     

FATIGUE CRACK GROWTH BEHAVIOUR AND FRACTURE RESISTANCE IN CERAMICS

Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and a silicon carbide whisker-reinforced alumina composite (Al₂O₃-SiC_w) at room temperature in air using a combined loading technique for stabilizing crack growth, and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Subsequently, the specimens were used for quasi-static crack growth tests under a monotonic loading condition. The R-curves were determined in this experiment; however, fracture resistance did not increase markedly with crack growth. Detailed observations of the crack growth behaviour revealed that the flat R-curve was attributed to the shielding effect of the fatigue crack tip wake. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growth. It was found that the cyclic loading produced fretting damage in the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed as is the significance of K_th as a material parameter.     

FATIGUE CRACK PROPAGATION AND CRACK CLOSURE BEHAVIOR IN POLYCARBONATE AND FIBRE REINFORCED POLYCARBONATE

Abstract— Fatigue crack propagation was investigated in polycarbonate and glass fibre reinforced polycarbonate and the effect of stress ratio and glass fibre content determined. The addition of glass fibre increases the tensile strength, but does not always contribute to an increase in fatigue crack propagation resistance. For polycarbonate the effect of stress ratio can be partly explained by using crack closure concepts as other researchers have suggested, but for glass fibre reinforced polycarbonate this was not possible. Fractography revealed a void growth process, which occurred by decohesion at the interface of the glass fibres and the base material, which was dependent on the maximum stress intensity factor. The process of linking the voids and the main crack growth behavior depended on the stress intensity factor range, Δ K. A proposed crack propagation model can explain the effect of stress ratio on crack propagation in fibre reinforced polycarbonate.     

NEAR THRESHOLD FATIGUE CRACK PROPAGATION BEHAVIOUR OF A HETEROGENEOUS MICROSTRUCTURE

The threshold regime of a heterogeneous, pearlitic-ferritic microstructure was investigated at R= 0.1. Not only did the ΔK_th values vary depending on the test procedure in that da/dN vs ΔK curves before and after ΔK_th determination were not identical but transition ΔK values also differed. The observations are correlated with inherent micro- structural features.     

THE INFLUENCE OF AGE-HARDENING ON FATIGUE CRACK PROPAGATION BEHAVIOUR IN 7075 ALUMINIUM ALLOY IN VACUUM

Abstract— The influence of age-hardening on the middle and low crack growth rates of a 7075 Al alloy is studied in vacuum. A transition in fracture surfaces morphology and crack growth curves is observed with the T 651 and T 7351 treatments in the near-threshold regime. Measurements of crack closure show its dependance on surfaces roughness and explain the lack of dependance of ΔK_th with load ratio, except for the T 7351 alloy. An equation of crack growth rate to the fourth power of ΔK_eff is in good agreement only with the crack propagation curves obtained for microstructure with an homogeneous deformation mode.     

SMALL FATIGUE CRACK GROWTH IN A LOW CARBON STEEL UNDER TENSION–COMPRESSION AND PULSATING-TENSION LOADING

The growth behaviour of small fatigue cracks has been investigated in a low carbon steel under axial loading at the stress ratios R of –1 (tension-compression) and 0 (pulsating-tension). Crack closure was measured to evaluate the effects of stress ratio and stress level on small crack growth. Except for the accelerated growth at stress levels close to the yield stress of the material, at R=–1 small cracks grow faster than large cracks below a certain crack length, but at R= 0 the crack growth rates for small cracks are coincident with those for large cracks in the whole region of crack length investigated. The critical crack length, 2c_c, above which the growth behaviour of small cracks is similar to that of large cracks depends on stress ratio, being 1–2 mm at R=–1 and smaller than 0.7 mm at R=0. The 2c_c value at R=–1 agrees with that obtained under rotating bending (R=–1). The small crack data are closely correlated with large crack growth rates in terms of the effective stress intensity range, ΔK_eff; thus ΔK_eff is found to be a characterizing parameter for small crack growth including the growth at the higher stress levels.