SHORT FATIGUE CRACK GROWTH BEHAVIOUR OF A LOW CARBON STEEL UNDER CORROSION FATIGUE CONDITIONS

The effect of an aqueous chloride environment upon the development and growth of short fatigue cracks from smooth specimen surfaces has been studied under fully reversed torsional fatigue loading conditions. Crack initiation and growth has been monitored using a plastic replication technique enabling a full history of cracking characteristics to be recorded. Corrosion fatigue conditions were achieved by complete immersion in a 0.6 M NaCl solution, of nominal pH value 6.0, with specimens corroding at the free corrosion potential. Variations to these conditions were obtained by the addition of concentrated hydrochloric acid enabling test solution pH values to be altered, typically pH values of 3.5 and 2.0 were obtained. Further information regarding the effects of the environment on the early stages of crack development were obtained by conducting two stage alternate immersion type testing conditions. Evaluation of these effects through previously established Elastic-Plastic Fracture Mechanics models shows that the environment plays a major role during the early stages of microstructure-dominated crack growth particularly as cracks approach major barriers to propagation and at decreasing levels of applied shear stress.     

SCATTER IN SMALL CRACK PROPAGATION AND FATIGUE BEHAVIOUR IN CARBON STEELS

In order to investigate the physical basis of scatter in fatigue behaviour, rotatory bending of smooth fatigue specimens was carried out using two steels with different microstructures (ferrite/pearlite and tempered martensite). Fifteen or sixteen specimens were fatigued at each stress, and the initiation and propagation behaviour of a crack which led to the final fracture were examined for all the specimens. The emphasis was to investigate the scatter characteristics of fatigue behaviour based on the scatter of small crack propagation data. The data were analysed by assuming the Weibull distribution, and a statistical investigation of the physical basis of scatter was performed, i.e. the distributions for fatigue life, crack propagation life and growth rate of small cracks were established and the relation between the scatter in those distributions was studied.     

STATISTICAL INVESTIGATION OF THE BEHAVIOUR OF MICROCRACKS IN CARBON STEELS

Abstract— In order to clarify the statistical behaviour of microcracks, rotating bending fatigue tests of plain specimens were carried out using two kinds of carbon steel with different microstructures, normalized 0.21% C steel and heat-treated 0.45% C steel. The emphasis is to investigate separately the statistical properties of the initiation and propagation of microcracks through successive observations of the specimen surface. Results show that the microcrack initiation life distributions of normalized and heat-treated steels are expressed by a two-parameter Weibull distribution and a Weibull distribution of the mixed type, respectively. On the other hand, the microcrack propagation life distributions of these steels are represented by three-and two-parameter Weibull distributions, respectively. The three-parameter Weibull distribution is well fitted to the crack length distribution for both the steels.     

FATIGUE MICROCRACKS IN A LOW CARBON STEEL

Abstract— The behaviour of a low carbon steel has been studied, in particular the initiation, growth and coalescence of fatigue microcracks on the surfaces of smooth specimens via surface replicas and photomicrographs. From the study, quantitative information on the initiation period, growth and coalescence of cracks, statistical distributions of crack length and crack depth, density of cracks, distribution pattern and characteristics of the major crack, were obtained. Knowledge of these parameters is critical for non-destructive inspection during service life and the application of fracture mechanics to life assessment.     

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.     

INFLUENCE OF TEMPERATURE ON FATIGUE CRACK GROWTH BEHAVIOUR OF STEELS AT ULTRASONIC FREQUENCY

Fatigue crack growth rates have been measured at 22 kHz for two types of carbon steel, between 20 and 500°C under symmetrical push-pull loading. The crack length was determined from the decrease in the resonant frequency of the specimen. For ?SN 412013 steel, an increased crack propagation resistance was observed between 250 and 300°C. The values of the constants C and n in the Paris equation, and K_a,th are dependent on temperature. The fatigue crack growth rate and C both increase with temperature, while n and K_a,th decrease with increasing temperature. Electron scanning and light microscopy have shown that intercrystalline fracture does not occur in ?SN 415313 steel at elevated temperature. Intercrystalline fracture was observed for specimens fatigued at 20°C and also in ?SN 412013 steel at temperatures of 200 and 500°C. The width of the plastic zone and the height of the surface relief around the fatigue crack increased with temperature.     

INFLUENCE OF AN OVERLOAD ON THE FATIGUE CRACK GROWTH IN STEELS

Abstract— This study is concerned with the influence of a single-peak overload and the overload ratio on the subsequent rate of growth of a fatigue crack in steels. Retardation increases with increasing overload ratio.
The crack opening load was also measured during all tests. It is shown that the Elber's crack closure concept is not able to explain the effect of overloads. The importance of the material yield stress was evaluated by testing steels of different strength. It seems that the residual stress state induced by the overload is the major factor causing retardation. Two models are analyzed.     

FATIGUE CRACK GROWTH BEHAVIOUR IN DISSIMILAR BONDED MATERIALS

Abstract The fatigue crack growth behaviour of dissimilar bonded materials was studied using explosively bonded naval brass clad steel plate and roll bonded nickel clad and naval brass clad steel plates. When the residual stresses produced during manufacturing processes were small, the fatigue crack growth rates showed a good correlation with the true ΔK-values evaluated for the dissimilar bonded materials; apparent ΔK-values, determined on uniform material, failed to be a correlating variable. The residual stresses strongly affected the fatigue crack growth rate of the dissimilar materials. Some microscopic and fractographic observations were made on the fatigue-fractured dissimilar materials.     

工程机械用钢焊后疲劳断裂特性研究

为了研究高强钢在工程机械焊接结构中的疲劳特性,对HQ70、1E0170和WH60钢焊接接头的疲劳裂纹扩展特性进行了较系统的研究。研究中应用了一种柔度法疲劳裂纹自动测量系统进行裂纹长度测量。用三点弯曲试样对这些焊接结构的疲劳特性进行研究。用三参数方程和Paris公式回归得出这些钢种焊缝的裂纹扩展规律及门槛值,对研究结果进行讨论。HQ70钢焊接件的疲劳特性较其它两种钢都好。焊接质量对疲劳裂纹扩展特性影响很大。由于焊接接头的裂纹扩展速率波动较大,测量门槛值时应特别小心。     

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.     

CORRELATION OF FATIGUE CRACK GROWTH BY CRACK TIP DEFORMATION BEHAVIOUR

Growth of a long mode I crack under variable fatigue loading was experimentally investigated on mild steel specimens. A dynamic elastic-plastic two-dimensional finite element program, purposely developed for the simulation of cyclic crack tip deformation, was utilised to model the transient effects on crack tip advance. The model accommodated crack tip opening displacement and both crack tip and crack edge closure. Fifty one different cycle patterns were analysed to include the application of a single overload, a single underload, a single cycle having a combined overload and underload and finally loading blocks of different sequences. Correlations of experimental fatigue crack growth rates were made from knowledge of crack tip deformation behaviour, including the use of data found in the literature. Specimens of eight materials and different geometries were analysed to determine the validity of the present approach.     

FATIGUE CRACK GROWTH BEHAVIOUR OF METALLIC BIOMATERIALS IN A PHYSIOLOGICAL ENVIRONMENT

Fatigue crack propagation (FCP) in Ringer's solution has been studied on four metallic biomaterials, i.e. pure titanium (TP35H), stainless steel (SUS304), dual-phase stainless steel (SUS329J4L), and titanium alloy (Ti–6Al–4V). For TP35H and SUS329J4L, the crack growth characteristics in Ringer's solution were almost the same as those in room air in the entire ΔK region examined, and thus the effect of Ringer's solution was not observed. The crack growth rates of SUS304 were enhanced in Ringer's solution in the region of ΔK ≥ 25  MPa m as compared to room air, while below this ΔK level they were similar in both environments. Also in Ti–6Al–4V, similar crack growth behaviour in Ringer's solution was found. From the comparison among the da/dN–ΔK relationships of the four materials in Ringer's solution, TP35H exhibited the fastest growth rates and then Ti–6Al–4V, SUS304, SUS329J4L in the order of decreasing growth rate. On the contrary, if the data were plotted in terms of ΔK_eff /E, Ti–6Al–4V and TP35H showed superior crack growth resistance to the stainless steels.     

A STUDY OF SHORT CRACK GROWTH IN TORSIONAL LOW CYCLE FATIGUE FOR A MEDIUM CARBON STEEL

A study of short crack growth for a medium carbon steel in low cycle torsional fatigue has been carried out in order to measure crack length and derive growth laws. Comparison with a previous analysis of uniaxial tests in low cycle fatigue for the same material provides a tentative basis for formulating an equivalent stress approach to describe uniquely the propagation of short cracks under multiaxial stress.     

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.     

SLOW FATIGUE CRACK GROWTH AND THRESHOLD BEHAVIOUR IN IMI 685

Abstract— Fatigue thresholds and crack growth rates up to 10⁻⁴ mm cycle ⁻¹ have been measured in β processed IMI 685. The results obtained in laboratory air for material having an aligned α microstructure and a random basketweave microstructure displayed a pronounced load ratio dependence which increased with decreasing ΔK. This sensitivity to mean load was also apparent from the threshold results determined in a vacuum of 5 ± 10⁻⁶ torr.
Fractographic observations, compliance measurements, pd output and crack path replication have indicated that contacts can be made between the fracture faces at a number of points behind the crack tip during the load cycle. These contacts wedge the crack open, thus preventing the stress intensity from falling to the value associated with the minimum applied load. A critical stress intensity, K _op, has been determined which relates to the crack being fully "open" and the results are reanalysed and discussed in terms of an effective stress intensity range, Δ K _eff.     

CYCLIC FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR IN ALUMINA CERAMICS

Cyclic fatigue crack growth behaviour in alumina ceramics is investigated and the effect of grain size discussed. Special attention is given to crack closure effects. Cyclic fatigue tests were carried out using four-point bend specimens, and the load–strain and load–differential strain curves were monitored. These curves show hysteretic behaviour probably related to frictional sliding of bridging grains, and also include non-linearities due to crack closure. The crack opening load is determined from the load–differential strain curve by using a method introduced in this study. Growth rates can be successfully described by the relationship da/dN = C[ΔK_{eff /(1 ? Kmax /KIC )]m} which is proposed in this study to account for the effects of crack closure and the maximum stress intensity factor. Irrespective of grain size, growth rates can be well represented by the above relationship, implying that the grain size exerts an influence on growth rates not only because of crack closure behaviour but also the material fracture toughness. The growth rate curve based on the proposed relationship shows a sigmoidal form for ceramic materials, which is similar to metals.     

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.     

A STUDY OF SHORT FATIGUE CRACK GROWTH BEHAVIOUR IN TITANIUM ALLOY IMI 685

Abstract— The fatigue growth of short cracks in coarse grained IMI 685, having an aligned a microstructure, has been monitored using a two stage replication technique. Crack growth rates are presented in terms of the failure mechanism and compared with standard data obtained from through cracked, compact tension specimens. The maximum difference, of up to four orders of magnitude, between long and short crack growth rates is associated with separation along specific crystallographic planes resulting in a cleavage-like fracture appearance. The rate of short crack growth is also shown to be dependent on applied stress and a dwell at maximum load during the fatigue cycle.