Low Stress Fatigue Research Articles

Fatigue-crack initiation in IMI 829 caused by high-temperature fretting

Fretting fatigue tests have been carried out on a recently developed near-α creep-resistant alloy, IMI 829. The fretting fatigue characteristics have been determined at temperatures of 400 and 600° C under conditions of high and low fatigue stresses. In each case fatigue-crack initiation has been examined and related to the fretting damage on the surface of the specimen, and the microstructure of the alloy.

Comparative electron fractographic study of fatigue crack propagation in peak-aged and overaged Al-Zn-Mg alloy

Electron fractographic studies have been made on peak-aged and overaged Al-Zn-Mg alloy specimens tested in low stress fatigue. The distinct and obvious difference in microscopic features on the fracture surfaces of two specimens indicates that a peak-aged material does not overage in the crack tip plastic zone. The microfeatures on the overaged specimen further suggest that fatigue cracks at intermediate and high values of Δk in this material may have grown by a mechanism which does not result in formation of fatigue striations.

銅の疲労初期き裂の破面観察

To study the behavior of cracks in the earlier stage of fatigue in copper, observations were made on both longitudinal sections and fracture surfaces using optical and scanning electron microscopes.The observation on longitudinal sections showed that transcrystalline crack paths along the slip bands were predominant at low fatigue stresses, but intercrystalline crack paths were frequent at high fatigue stresses.Two kinds of fracture surfaces were observed beneath the specimen surface; one is lying in a slip band and the other along a grain boundary. The former was usually said to be featureless, but steps of about one micron in the direction of crack growth as well as clear slip traces were found on the flat surface. Matching of the steps on mating fracture surfaces was fairly good in such manner that hills on one surface fit into valleys on the other, but matching of the slip traces was not good. On the smooth fracture surfaces lying along grain boundaries, slip traces were observed in most areas, but clear and regular striations were found in some cases. Such topography observed on the fracture surface of intercrystalline crack was similar to that observed within the interior.The observations above suggest that both unslipping and the local plastic blunting process are responsible for the growth of the crack lying in the slip bands, and that intercrystalline cracks grow in tensile mode even in the earlier stage.

Characterization of fretting fatigue damage by SEM analysis

Since the earliest studies of fretting fatigue were initiated surface damage and debris produced during the process have been of particular concern. The Scanning Electron Microscope (SEM) has proven to be a useful tool in the analysis of fretting damage mechanisms and their relationship to the degradation of fatigue properties. Results of analysis performed on fatigue specimens which have undergone simultaneous fretting damage and fatigue are presented. The results show that irreparable damage was not produced during the first fatigue cycles. Damage also was classified as initial or advanced by characteristic debris and surface appearance. Fretting damage produced at high fatigue stress was found to be similar in appearance to damage produced at low fatigue stress. Details of the damage and its relationship to the fretting process are discussed in the paper in some depth.

Failure of polystyrene in tensile and cyclic deformation

The failure of polystyrene in cyclic deformation has been examined and compared with the fracture mechanisms involved in simple tension. The fatigue response can be divided into three discrete life ranges. In the short and intermediate life regions, fracture occurs by the processes of craze formation, craze growth, crack nucleation and crack propagation in a manner analogous to tensile failure. The primary influence of reversed straining is manifest as an acceleration of the crack formation stage of failure. In long life (low stress) fatigue, failure modes dissimilar to the documented craze breakdown pattern of crack nucleation are noted.

Vibration amplification by repetitive pulsing of rapidly heated cylinders

The fuel elements of a repetitively pulsed fast reactor are subjected to rapid heating in a time short with respect to their mechanical response time. As a result, vibration of the fuel elements occurs and, under certain conditions, these vibrations may be amplified considerably. This work is an experimental investigation of the amplification which occurs when a thin wall, hollow, stainless steel cylinder is repetitively heated internally by Joule heating. The results, which compare well with a theoretical analysis of this problem performed by another author, show that significant amplification of the vibrations can occur. The experimental technique is a variation of the resonance method for measuring natural frequency and internal damping. The apparatus could be adapted for use as a materials testing facility for high cycle, low stress fatigue studies since relatively large stresses could be induced with a rather small energy deposition per pulse.

The role of mechanical properties in low-stress fatigue crack propagation

An experimental investigation was undertaken to study the relationship between mechanical properties and low stress fatigue crack propagation. Attention was focused on the “fatigue” or “reversed plastic zone” at the crack tip, since it was felt that material properties in this region were of prime importance in the crack propagation process. An effort was made to simulate this region through fully reversed strain-cycling tests on tensile specimens. Mechanical properties obtained from a number of materials before and after strain cycling were correlated with crack propagation data from the same materials. Evidence indicated that while monotonic tensile properties are inadequate for correlation purposes, the cyclic strain-hardening coefficient, the cyclic yield strength, and the elastic modulus appear to be important parameters. This was felt to be an indication of the importance of strain cycling in the reversed plastic zone in influencing the rate-governing mechanisms in fatigue crack growth.

The growth of fatigue cracks in aluminium

Experiments have been performed in order to assess quantitatively the rates of fatigue crack propagation in metals. Low stress fatigue tests have been performed on aluminium specimens under conditions of constant load and constant stress. The crack growth rates are found to be proportional to the fourth power of the appropriate stress intensity factors. Methods of processing fatigue test data are briefly discussed.

Some Observations on Fatigue in Copper Single Crystals

The surface features and dislocation substructures present in copper single crystals subjected to slow cyclic plastic shear strains of ± 1% are reported and discussed. Thin-film electron microscopy has revealed that at no time during the hardening stage are dislocations grouped into a uniform, three-dimensional cell structure; they are arranged in dislocation walls on the glide plane, the plane normal to the primary b, and a plane at ∼ 45° to both these. After cycling at saturation the crystal breaks down into two regions, one exhibiting a misorientated, three-dimensional cell structure and the other showing an intensification of the regular structure generated during the hardening stage. No difference was observed between the dislocation structure at the specimen surface and that in the interior. From optical and scanning electron microscopy studies of the surface it is postulated that slip at saturation occurs preferentially in one of the two regions observed. The marked similarity between the regular structure described and the structure of persistent slipbands, seen in conventional low-stress fatigue tests, suggests that these regions carry most of the saturation strain.

Dislocations and Persistent Slip Bands in Copper Single Crystals Fatigued at Low Stress Amplitude

AbstractPersistent slip bands in fatigued copper single crystals are studied by means of transmission electron microscopy of surface thin foils, both parallel and nonparallel to the surface. Comparing the observations from the different nonparallel sections a three‐dimensional model of the internal structure may be constructed. Dislocations with all six possible Burgers vectors of the type 1/2 〈0011〉 are found in the structure. This demonstrates the microscopic action of the secondary slip systems during the low stress fatigue. The macroscopic action of the secondary systems, which is connected with the cell structure formation, is typical for the high stress fatigue. Surface extrusions are found to be related to the dislocation structure below the surface.

A mechanism for the production of intrusions and extrusions during fatigue

Abstract The outline of a model for intrusion and extrusion production has been devised which combines the recent calculations of dipole-dislocation strain field interactions with the observation that large numbers of dipoles are formed during low-stress fatigue. Large numbers of dipoles are produced near to and roughly parallel to the surface during one-half of the stress cycle. When the stress is reversed, these dipoles are pushed to the surface by dislocations moving outward from the crystal interior. If most of the dipoles are vacancy type, then an intrusion will be produced. If interstitial dipoles are more numerous, an extrusion will result. The model is a simple development of the mechanism proposed by Partridge.

Fatigue studies of metal crystals

Single crystals of silver and zinc were fatigued in alternating bending at constant strain amplitude of 0.12 per cent and 0.2 per cent for silver and 0.01 per cent for zinc. They were studied by a combination of several X-ray techniques (double-crystal diffractometer, divergent beam, X-ray reflection microscopy), electron transmission microscopy and hardness measurements. The experimental results indicate that the fatigue process may be divided into three stages. The initial stage can be identified with cyclic work-hardening and is attributed to a general increase in dislocation density. The second stage represents a period of dynamical recovery. It consists in the removal of dislocations from distorted lattice domains and leads to a decrease of coarse lattice misalignments. At this stage cross-slip seems to play a fundamental role, as a process by which dislocations can escape from their locked positions. When cross-slip was obstructed by a high-angle boundary in silver or a twin boundary in zinc a premature fatigue failure occurred at the obstruction. The third stage is characterized by an extensive fragmentation of the structure. Due to the interaction of basal and pyramidal slip in zinc the structure resulting from this process shows considerable differences compared to that of silver. It appears that the high-stress fatigue process (below yield stress) is in principle not different from the low-stress fatigue mechanism except for a higher rate. It is believed that fatigue failure may perhaps result from the obstructions to cross-slip by internal barriers consisting of lattice domains of high dislocation density, the latter being formed by dynamical recovery.

Crack propagation in high stress fatigue

Abstract An examination of the fracture surfaces of ductile metal specimens broken in high stress fatigue has revealed the occurrence of fracture ripples similar in appearance to those resulting from low stresses, but considerably larger. By sectioning specimens strained to various stages of the fatigue stress cycle, it has been shown that crack propagation and fracture ripple formation are the consequences of the successive rounding and sharpening of the crack tip during each stress cycle. The hypothesis that high stress fatigue is different in character from low stress fatigue and results from a ‘delayed static fracture’ is not in accord with the present observations.

Difference between High- and Low-Stress Fatigue

RECENT papers by Porter and Levy1 and Benham and Ford2 have suggested that the conventional S/N curve is composed of two separate curves, a discontinuity occurring at some particular stress-level. The former authors concluded from a large number of rotating bending tests on copper that the discontinuity occurred at about ± 21,500 lb./sq. in.; for lower stresses fatigue cracks originating in slip bands; for higher stresses cracks developing from L-shaped nuclei. Kemsley3 also found that with copper specimens tested at ± 15,000 lb./sq. in. (giving fracture after long endurances) cracks formed in slip bands and were transcrystalline, while at ± 25,000 lb./sq. in. (giving fracture with low endurances) slip bands were virtually absent from the specimen and cracks appeared to be inter crystalline. Benham and Ford2, who tested mild steel in direct stress, found for large stress amplitudes that the specimen gauge-length extended permanently each cycle and a ‘necked-out’ fracture resulted. They termed this phenomenon ‘cyclic creep’ and such failures occurred while the stresses corresponded to the upper portion of the composite S/N diagram. For stresses in the lower portion of the S/N diagram typical fatigue fractures occurred and they suggested that, if the fatigue mechanism could be suppressed as the cyclic stress decreased, the upper curve might flatten out to a ‘cyclic creep limit’.

Dislocation arrangements in aluminium deformed in tension or by fatigue

Abstract A comparison has been made of transmission electron microscope observations on polycrystalline aluminium deformed in tension or by fatigue. The dislocation distribution in aluminium deformed in tension, was studied as a function of stress. At stresses up to 3 kg mm−2 the distribution is extremely irregular; at higher stresses up to the fracture stress, polygon boundaries are formed which become increasingly well developed with increasing stress. Observations have been made on specimens fatigued at high stress, giving a life of 105 cycles, and low stress, giving a life greater than 106 cycles. The dislocation distribution in the former is similar to that in the unidirectionally deformed specimens. The low stress fatigue specimens show two new features. Firstly, polygonization does not occur despite a high dislocation density, and secondly a high density of dislocation loops similar to that found in quenched specimens is observed in some areas. The latter observation suggests the formation of large...