THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

GROWTH OF FATIGUE CRACKS EMANATING FROM NOTCHES IN SiC PARTICULATE ALUMINUM COMPOSITE

Abstract— The initiation and growth of cracks emanating from blunt notches in 6061-Al alloy reinforced with 25% particulate Sic metal matrix composite were investigated. To elucidate the role of aging condition of the matrix on the fatigue behavior, the studies were carried out at T6 and overaged conditions. The results show that the number of cycles required for initiation of fatigue cracks are not influenced with the notch severity and the aging condition of the matrix. The overaging heat treatment resulted in slower fatigue crack growth rates. The failure of the Sic particles during the fatigue process is given as the reason for the both observed initiation and crack growth characteristics. It is also shown that the growth rate of cracks emanating from blunt notches can be accurately described by an equivalent stress intensity factor range Δ K _eq. This could provide an adequate engineering method for design against fatigue failure from various stress concentrations for this composite system.     

Vorhersagemodell für die Wachstumsraten kurzer Risse auf der Basis von Kmax‐konstant‐Versuchen an M(T)‐Proben

Prediction model for the growth rates of short cracks based on K_max‐constant tests with M(T) specimens The fatigue crack growth behaviour of short corner cracks in the Aluminium alloys Al 6013‐T6 and Al 2524‐T351 was investigated. The aim was to determine the crack growth rates of small corner cracks at stress ratios of R = 0.1, R = 0.7 and R = 0.8 and to develop a method to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T)‐specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The pre‐cracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a Focussed Ion Beam. The results of the fatigue crack growth tests with short corner cracks were compared with long fatigue crack growth test data. The short cracks grew at ΔK‐values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK‐values and the same stress ratios. A model was developed on the basis of K_max‐constant tests with long cracks that gives a good and conservative prediction of the short crack growth rates.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

Crack initiation and crack propagation under thermal cyclic loading

Theoretical and experimental investigations of crack initiation and crack propagation under thermal cyclic loading are presented. For the experimental investigation a special thermal fatigue test rig has been constructed in which a small circular cylindrical specimen is heated up to a homogeneous temperature and cyclically cooled down under well defined thermal and mechanical boundary conditions by a jet of cold water. At the end of the cooling phase the specimen is reheated to the initial temperature and the following cycle begins. The experiments are performed with uncracked and mechanically precracked specimens of the German austenitic stainless steel X6CrNi 1811.

In the crack initiation part of the investigation the number of load cycles to initiate cracks under thermal cyclic load is compared to the number of load cycles to initiate cracks under uniaxial mechanical fatigue loading at the same strain range as in the cyclic thermal experiment. The development of initiated cracks under thermal cyclic load is compared with the development of cracks under uniaxial mechanical cyclic load.

In the crack propagation part of the investigation crack growth rates of semi-elliptical surface cracks under thermal cyclic loading are determined and compared to suitable mechanical fatigue tests made on compact-tension and four-point bending specimens with semi-elliptical surface cracks. The effect of environment, frequency, load shape and temperature on the crack growth rate is determined for the material in mechanical fatigue tests.

The theoretical investigations are based on the temperature distribution in the specimen, which is calculated using finite element programs and compared to experimental results. From the temperature distribution, elastic and elastic-plastic stress distributions are determined taking into account the temperature dependence of the material properties. The prediction of crack propagation relies on linear-elastic fracture mechanics. Stress intensity factors are calculated with the weight function method and crack propagation is determined using the Paris relation.

To demonstrate the quality of the crack growth analysis the experimental results are compared to the prediction of crack propagation under thermal cyclic load.     

Short crack initiation and growth at 600 °C in notched specimens of Inconel718

The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.     

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.     

Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10^?10 m/cycle, and extremely slow rates ranging from 10^?13 to 10^?11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.     

Short fatigue crack growth from a blunt notch in plate specimens

Short crack growth behavior from a notch including crack closure and load ratio effects was investigated. Experiments and analyses were carried out using four-point bending specimens made of SAE 1045 steel, using a blunt notch keyhole specimen geometry. The lower the load ratio, the more notch effect on short crack growth behavior was observed. Short cracks in the notch affected zone had higher growth rates than long cracks. After the crack grew out of the notch effect field, short crack growth rates merged with the long crack growth rates. Several parameters were used to correlate the short crack growth rates including stress intensity factor range, effective stress intensity factor range, and stress intensity factor range based on notch root stress.     

Cyclic fatigue of long and short cracks in alumina

The cyclic fatigue behaviour of long and microstructurally short cracks in a 10 μm grain-size alumina has been investigated. This material was found to be stress sensitive, a modest drop in applied stress resulting in a considerable lifetime enhancement. The growth of long cracks was studied using the circular compact tension geometry and was found to follow a Paris law behaviour. The crack path was entirely intergranular in this material with long fatigue crack growth governed by the degradation of crack-wake bridging. Short-crack growth was investigated using indented discs in a biaxial flexure geometry. Short cracks were observed to grow at lower values of applied ΔK than long cracks, increasing with crack length as bridging of the crack wake increased. The fatigue crack growth of AD90 alumina was also investigated by in situ testing within the specimen chamber of an SEM. The long-crack behaviour was found to be similar to the 10 μm grain-size alumina and other data reported in the literature. However, the crack path followed a mixture of transgranular and intergranular fracture and discontinuous in nature with frequent arrests. The crack-advancement mechanisms in these two alumina materials are different and affect the short-crack behaviour. However, in both cases the long-crack behaviour is dominated by crack-wake effects. This revised version was published online in November 2006 with corrections to the Cover Date.     

SHORT CRACK BEHAVIOUR IN NODULAR CAST IRON

Fatigue crack growth rates have been determined on standard specimens containing long cracks (∼5–10mm) and on specimens containing two-dimensional short cracks (∼0.10–0.50mm). Large differences have been observed indicating that at a given stress intensity factor short cracks propagate much faster than long cracks. Mouth opening displacement measurements for both specimen geometries have shown that the crack closure effect is largely responsible for the observed effect. These results are used to rationalize the behaviour of short cracks initiated from natural sites which were either graphite nodules or microshrinkage pores. The three-dimensional aspect of these natural small cracks is analysed and discussed in detail.     

Effect of notch geometry on short fatigue crack growth in 8090 Al–Li alloy

The short-crack propagation behaviour of 8090 Al–Li alloy under different ageing conditions has been investigated. The effect of notch geometry on short fatigue crack growth was also studied. The results show that the geometrical configuration of the notch significantly affects the growth behaviour of the short crack, the growth rates of notched short cracks being much higher than those of long cracks at the same stress intensity factor range ΔK level. The orientations of the specimens had a stronger effect on the growth rate of long cracks than on that of short cracks. This revised version was published online in November 2006 with corrections to the Cover Date.     

SHORT AND LONG FATIGUE CRACK GROWTH IN A SiC REINFORCED ALUMINIUM ALLOY

Fatigue crack growth behaviour in a 15 wt% SiC particulate reinforced 6061 aluminium alloy has been examined using pre-cracked specimens. Crack initiation and early growth of fatigue cracks in smooth specimens has also been investigated using the technique of periodic replication. The composite contained a bimodal distribution of SiC particle sizes, and detailed attention was paid to interactions between the SiC particles and the growing fatigue-crack tip. At low stress intensity levels, the proportion of coarse SiC particles on the fatigue surfaces was much smaller than that on the metallographic sections, indicating that the fatigue crack tends to run through the matrix avoiding SiC particles. As the stress intensity level increases, the SiC particles ahead of the growing fatigue crack tip are fractured and the fatigue crack then links the fractured particles. The contribution of this monotonic fracture mode resulted in a higher growth rate for the composite than for the unreinforced alloy. An increase in the proportion of cracked, coarse SiC particles on the fatigue surface was observed for specimens tested at a higher stress ratio.     

SHORT CRACK PROPAGATION AND CLOSURE EFFECTS IN A508 STEEL

Abstract— Fatigue crack growth measurements are usually made on standard specimens containing long cracks (～10 mm) although in most practical situations, a large part of the fatigue life is spent with much shorter dimensions. The purpose of the present study is a comparison of crack growth behaviour for long cracks (～13–16 mm) in CT specimens and smaller ones (～0.3–0.5 mm) in four point bend specimens. Large effects are noticed indicating that, at a given stress intensity factor amplitude, the crack growth rate is significantly higher in specimens with short cracks. Mouth displacement measurements for both specimen configurations show that the crack closure phenomenon accounts for the observed effect. Crack closure is likely to be associated with fracture surface roughness as shown by partly machining the material left behind the crack tip in CT specimens.     

Crack initiation in cyclic compression and its applications

The characteristics and applications of crack initiation under far-field cyclic compressive loads are examined in notched specimens of a lower strength steel. The fatigue cracks, initiated at the notch root due to residual tensile stresses, grow at a progressively decreasing velocity before arresting completely. Simple correlations are explored between the total fatigue crack growth distance and the size of an effective damage zone estimated from the initial crack growth rate. Some important applications of crack initiation in cyclic compression in both long and short fatigue crack problems are demonstrated. Experimental results are presented to show that crack initiation in cyclic compression offers some interesting possibilities for obtaining accelerated estimates of slow fatigue crack growth rates, while minimizing some of the inherent uncertainties arising from the artifacts of conventional test techniques. Possible applications of this method are also discussed with respect to the study of physically-short flaws and of the evaluation of the progressive development closure during fatigue.     

THE EFFECT OF GRAIN SIZE ON THE FATIGUE OF COMMERCIALLY PURE ALUMINIUM

Abstract— Fully reversed uniaxial fatigue tests were performed on polished hour-glass specimens of commercially pure aluminium with three different grain sizes, in order to examine the effect of grain size on fatigue. The growth of surface cracks was monitored by a plastic replication method. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crack initiation and growth is a feature of the fatigue of aluminium, while the grain size influences the specific detail of crack coalescence. Crack path deviation is greatest in the coarse grained microstructure and crack surface roughness is more pronounced. SEM fractography reveals that crack initiation and early crack growth takes place along crystallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blunting.     

The effect of steady torsion on fatigue crack growth in shafts

Most of catastrophic mechanical failures in power rotor shafts occur under cyclic bending combined with steady torsion: Mode I (ΔK_I) combined with Mode III (K_III). An analysis of the influence of steady torsion loading on fatigue crack growth rates in shafts is presented for short as well as long cracks. Long cracks growth tests have been carried out on cylindrical specimens in DIN Ck45k steel for two types of testing: rotary or alternating bending combined with steady torsion in order to simulate real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from the cylindrical specimen surface, has been measured for several loading conditions and both testing types. Short crack growth tests have been carried out on specimens of the same material DIN Ck45k, under alternating bending combined with steady torsion. The short crack growth rates obtained are compared with long crack growth rates. Results have shown a significant reduction of the crack growth rates when a steady torsion Mode III is superimposed to cyclic Mode I. A 3D Finite Element analysis has also shown that Stress Intensity Factor values at the corner crack surface depend on the steady torsion value and the direction of the applied torque.     

INITIATION AND PROPAGATION OF SHORT FATIGUE CRACKS IN A WELD METAL

Abstract— Fatigue tests were performed using a purpose designed triangular shaped specimen to investigate the initiation and propagation of short fatigue cracks in a weld metal. It was observed that short fatigue cracks evolved from slip bands and were predominantly within ferrite grains. As the test progressed, the short crack density increased with minor changes in crack length. The growth of short cracks, in the early stage resulted mainly from coalescence with other existing cracks. The mechanism of short crack behaviour is discussed.     

微观组织与裂纹闭合效应对40CrNiMo钢疲劳短裂纹扩展的影响

对具有粗、细晶粒组织的40CrNiMo 钢进行了疲劳短裂纹扩展试验研究。试验和分析结果表明,短裂纹扩展的偏折强烈,裂纹闭合效应较小。粗晶组织比细晶组织的裂纹偏折更大,粗糙度诱发闭合效应更强,因而裂纹扩展较幔。微观组织通过对闭合效应的作用进而影响了疲劳短裂纹的扩展行为。     

Transverse fatigue crack propagation behavior in equine cortical bone

Stress fractures stem from the initiation and propagation of fatigue cracks through bone, and fatigue damage may play a role in many other orthopaedic problems, such as hip fractures in the elderly. The objective of this investigation was to measure fatigue crack propagation rates in cortical bone. Specific aims were to determine fatigue crack growth rate, da/dN, as a function of alternating stress intensity factor, K, for equine third metacarpal cortical bone tissue; to determine whether the resulting data followed the Paris law; and to test the hypothesis that crack growth rates differ between dorsal and lateral regions. Compact type specimens oriented for transverse crack growth were subjected to fatigue under Mode I loading. The da/dN vs. K data for the dorsal specimens revealed a Paris law exponent of 10.4 (R ² = 0.82), comparable to that for ceramics. These data also exhibited an apparent threshold stress intensity factor of 2.0 MPa · m^1/2. It was not possible to obtain similar results for lateral specimens because all cracks deviated from the desired transverse path and ran longitudinally in spite of the use of side grooves to constrain the crack path. However, the results for lateral specimens were not due to a failure of the test method, but reflect dramatic differences in fatigue crack propagation resistance between the two cortical regions. These results are consistent with clinical observations that stress fractures in the third metacarpus typically occur in the mid-diaphysis of the dorsal cortex, but not in the lateral cortex.