COMPARISON OF SHORT AND LONG FATIGUE CRACK GROWTH IN 7075-T6 ALUMINUM

The fatigue crack growth rates of physically-short cracks (0.5 ≤a≤ 1.0 mm), intermediate cracks (1 < a≤ 2 mm) and long cracks (7 < a < 25 mm) were compared using SEN type tensile specimens in 7075-T6 aluminum alloy with load ratios, R, of 0.05, ? 1 and 0.5 under constant amplitude testing at room temperature. It was found that the short cracks grew much faster than long cracks based on applied δK with da/dN≤ 10^?7 m/cycle. Even the intermediate cracks grew faster than the long cracks below 10^?7 m/cycle. The transition crack lengths where similitude with δK existed was between 1 and 2 mm. Mean stress effects were similar for R= 0.05 and ? t, but R= 0.5 caused increased crack growth rates. The above differences are partially attributed to crack closure effects. Based upon plastic zone sizes, LEFM was justifiable with all the experiments.     

PREDICTING FATIGUE LIFE IN COMMERCIALLY PURE ALUMINIUM USING A SHORT CRACK GROWTH MODEL

Abstract— Advances in manufacturing techniques and improvements in non-destructive inspection procedures have reduced the size of initial defects in fatigue-critical components to the scale of the microstructure. Unfortunately, the growth of these so-called short fatigue cracks is not properly characterised by conventional linear elastic or elastic-plastic fracture mechanics. However, the Navarro-de los Rios crack growth model does account for many aspects of their growth and has proved successful in predicting fatigue life of medium carbon and C-Mn steels. In consequence, the growth of short fatigue cracks in commercially pure aluminium, under fully reversed uniaxial loading, has been studied, using a plastic replication technique and modelled, using the Navarro-de los Rios crack growth equation. The predicted lifetimes are in good agreement with the actual experimental lifetimes, and compare favourably with predictions based on two previously reported crack growth models.     

SiC2(W)增强 Al 合金疲劳失效的特征

用扫描电镜观察了 SiC 晶须增强 Al 基复合材料疲劳裂纹的形成扩展特征,研究了 SiC 晶须分布、晶须与基体界面及基体中的缺陷对疲劳裂纹形成的影响。     

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.     

SHORT FATIGUE CRACK GROWTH IN PLAIN AND NOTCHED SPECIMENS OF AN 8090 Al-Li ALLOY

Abstract— Fatigue tests on notched and plain specimens were carried out under a combined acoustic and optical microscope. Crack initiation in plain specimens occurs invariably at inclusions which act as sites of stress concentration. Short crack propagation behaviour shows the usual pattern of accelerations and retardations associated with microstructural variations. Growth can only be sustained if the applied stress is sufficiently high to overcome the microstructural barriers. For the notched specimens the extent of the notch tip and crack tip plastic zone sizes control crack propagation. A short crack will continue to propagate only if its own plastic zone can sustain growth as the crack tip extends beyond the notch zone, otherwise it becomes non-propagating. Notch plastic zones are calculated using both approximate and accurate solutions. Some microstructural effects are discussed.     

A THEORETICAL AND EXPERIMENTAL STUDY OF ENVIRONMENTAL HYDROGEN-ASSISTED SHORT FATIGUE CRACK GROWTH IN AN Al-Li ALLOY

Abstract— The initiation and propagation of fatigue cracks in an Al-Li 8090 alloy in a vapour environment of 0.6 M NaCl solution was investigated. A severe degradation of the resistance to short crack growth was exhibited. Preliminary work carried out to establish the susceptibility of the material to hydrogen embrittlement demonstrated a close correlation between the deformation mode of this alloy and hydrogen absorption. The combination of highly localized slip and highly localized hydrogen fugacity creates a high susceptibility to hydrogen-assisted crack growth.
On the basis of current micro-mechanical models, it is suggested that hydrogen trapping induces a reduction of the friction stress acting in the crack tip plastic zone. Consequently, enhanced plasticity at the crack tip due to the decrease in friction stress leads to an increase in crack growth rate.
An exact solution for a surface crack in a semi-infinite plane is obtained based on a dislocation crack model. Using this solution a computer method is developed to calculate the time-dependent short crack growth rate and fatigue lifetime. Both solutions show good correspondence with the experimental results.     

FATIGUE CRACK GROWTH IN A SINTERED TUNGSTEN ALLOY

Abstract— Room temperature fatigue crack propagation in a sintered tungsten alloy was studied. The fatigue crack growth rates were found to be identical for the material in the sintered and forged and as sintered conditions. The propagation rates are slower when compared with other metals due to the relatively high Young's modulus of tungsten. The value of the exponent m in Paris' power law equation was found to be 12 which is higher than for most metals. This was ascribed to the activity of a cleavage mechanism through some of the tungsten grains along with the ductile decohesion fatigue mechanism.     

THE EFFECT OF HYDROGEN ON SHORT FATIGUE CRACK GROWTH IN AN Al-Li ALLOY

Abstract— Experimental results are presented to illustrate the degree of susceptibility to hydrogen embrittlement in an Al-Li 8090 alloy. The ductility of the alloy (as recorded in slow strain rate tests) is reduced by hydrogen charging which induces microcrack formation on the surface of the hydrogen treated specimens. Results of fatigue tests show that small fatigue cracks propagate 1.5 to 10 times faster in specimens charged with hydrogen and fractographic evidence shows that short crack growth in hydrogen precharged specimens is transgranular, along persistent slip bands. The effect of hydrogen is reversible, the embrittlement effect being eliminated by holding the specimens for 24 hr at 470°C.     

THE EFFECT OF GRAIN SIZE ON FATIGUE CRACK GROWTH IN AN ALUMINIUM MAGNESIUM ALLOY

Abstract Fully reversed uniaxial fatigue tests were performed on aluminium magnesium alloy Al 5754 with four different grain sizes in order that the effect of grain size on fatigue crack growth could be examined. Surface cracks were monitored by a plastic replication technique. Fatigue strength was shown to improve with a decrease in grain size. The endurance stress is a function of the inverse square root of the grain size and is described empiricdty by a Hall-Petch type relation. The effect of grain size on fatigue crack growth is most significant when the crack length is of the order of the microstructure. Fluctuations in the growth rate of microstructurally short cracks are most marked in a fine grained microstructure and may be related to the need to transfer slip to adjacent grains. Crack path deviation is greatest in the coarsest grained microstructure and SEM fractography reveals a more pronounced crack surface roughness in the coarser grained alloy than in the finer grained alloy.     

CONVERSION OF SHORT FATIGUE CRACKS INTO A LONG CRACK

This paper presents a method for the statistical processing of results of initiation and propagation of short fatigue cracks. The distribution of the lengths of these cracks followed a Weibull distribution function. The value of the size parameter of this distribution grows with the number of cycles, but the value of the shape parameter declines as the number of cycles increases. A study of the modal value of fatigue crack lengths as a function of the number of cycles yielded a statistical criterion defining the conversion of the short fatigue crack initiation and propagation stage into the long fatigue crack propagation stage. One condition for this conversion is a zero magnitude of the first derivative of the modal value of fatigue crack lengths by the number of cycles. When the dependence of the Weibull distribution shape parameter and size parameter upon the number of cycles was inserted in this condition, solution of the resultant expression revealed the critical number of cycles that is necessary for the formation of a long fatigue crack.     

A PROBABILISTIC MODEL OF SHORT FATIGUE CRACK GROWTH

Abstract— A probabilistic model is presented that draws a direct link between stochastic microstructure and the statistics of measured growth rates. The model is formulated as a semi-Markov process. The underlying Markov process describes the evolution of a growth control variable as an explicit function of crack length. The growth control variable is open to a variety of interpretations, depending on the mechanisms known to control growth in any given application. Elapsed fatigue cycles and the distribution of times to failure are calculated by invoking an empirical or postulated law of growth rate. This law is either a deterministic or probabilistic relationship between the growth control variable and the crack velocity. It may, and usually does, contain parameters that are evaluated by calibration against available statistical data. This process guarantees a high level of robustness of the model's predictions. The computational generality of the formulation facilitates the treatment of spectral loading.     

SHORT FATIGUE CRACK GROWTH BEHAVIOUR IN WASPALOY AT ROOM AND ELEVATED TEMPERATURES

Short fatigue crack growth tests have been carried out at room and elevated temperatures using the nickel-based superalloy known as Waspaloy. A fully automated computer controlled system has been developed and employed for controlling the testing and monitoring of the growth of freely initiated surface short cracks on smooth specimens. Surface cracks as small as 10 um in length have been detected and recorded at temperatures up to 700°C. Anomalous short crack propagation behaviour was observed when comparisons were made with the corresponding long crack behaviour. Some aspects of mechanical, microstructural and environmental effects on the short fatigue crack growth behaviour of the material are discussed.     

A MICRO-MECHANICS ANALYSIS FOR SHORT FATIGUE CRACK GROWTH

Crack initiation and early growth of fatigue cracks in a fully annealed 0.4% carbon steel was investigated using plastic replicas and torsion loading. In a structure consisting of a 70/30 mixture of pearlite and ferrite the cracks are seen to develop and grow initially along slip bands in the ferrite phase. Energetic considerations lead to the formulation of a model which, while characterizing short crack growth rate, also considers those microstructural variables relevant to fatigue crack initiation and early crack growth. The driving force for crack growth is provided by the energy of the slip band; correspondingly crack growth per cycle is proportional to the strength of the slip band. In the short fatigue crack region, cracks grow initially at a fast rate but deceleration occurs quickly and, depending on the stress level, they either arrest or are temporarily halted at a critical length. This critical length is shown to coincide with the value of the threshold length for crack growth under LEFM conditions.     

COMPRESSION FATIGUE CRACK GROWTH BEHAVIOUR OF AN ALUMINIUM ALLOY: EFFECT OF OVERLOADING

Abstract— The behaviour of fatigue cracks in an Al-alloy under cyclic compression, either with or without overloads, was studied. For constant-amplitude compressive cycling, a non-catastrophic (saturation) character of the fatigue crack behaviour was confirmed, with the final depth of a crack depending on the applied load level. Single (tensile or compressive) intermittent overloads were shown to re-activate a previously arrested crack while reversed (tensile—compressive or compressive—tensile) ones were also shown to maintain continual fatigue crack extension under otherwise fully compressive cycling.     

ENVIRONMENTAL SENSITIVITY OF SMALL CRACK GROWTH IN 7075 ALUMINIUM ALLOY

Fatigue propagation tests on artificial short cracks (initial length ? 0.15 mm) were performed in vacuum and in nitrogen containing small traces of water vapour (? 3 ppm) on a high strength aluminium alloy type 7075 in two aged conditions (T651 and T7351) at a load ratio of 0.1 and a frequency of 35 Hz. A predominant influence of environment was determined for short crack growth. This behaviour has been discussed in terms of crack growth rate versus the effective stress intensity factor range relationship previously determined for long cracks. The results obtained suggest the absence of closure at the early stage of short crack growth with an enhanced environmental influence as compared to long crack behaviour at the same load ratio. As the crack grows the effect of closure increases progressively and the short crack effect disappears after a crack growth of the order of 1 mm.     

FATIGUE LIFE AND SURFACE CRACK PENETRATION BEHAVIOUR OF AN ALUMINIUM ALLOY

Fatigue life and crack retardation behaviour after penetration were examined experimentally using CT specimens and surface pre-cracked specimens of aluminium alloy 5083-0. The fatigue crack shape before penetration is almost semicircular, and the measured aspect ratio is larger than the value obtained by calculation using K values proposed by Newman-Raju. It was found that crack growth behaviour on the back side after penetration is unique, and can be divided into three stages. The Wheeler model retardation parameter was used successfully to predict crack growth behaviour after penetration. By using a crack propagation rule, the change in crack shape after penetration can be evaluated quantitatively.     

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.