Fatigue strength and damage progression in a circular-hole-notched GRP composite under combined tension/torsion loading

Fatigue strength and failure processes in a hole-notched glass-fiber-reinforced plastic (GRP) material under combined tension/torsion cyclic loading have been investigated. Thin-walled tubular specimens were used in the experiment. The reinforcing fibers (plain-weave glass cloth) were aligned parallel to the longitudinal and circumferential axes of the specimen. The damage state at each biaxiality stress ratio (: the ratio of normal stress to shear stress) was observed macroscopically and microscopically.

The fatigue damage progression is dependent on the combined stress ratio. In the case of = 1/0, damage spreads radically from the hole, but covers an area of only π/4 on either side of the circumferential axis. Damaged areas are symmetrical and opposite each other. When = 0/1, damage progresses radially along both longitudinal and circumferential axes. These areas of damage progression are called ‘fatigue bands’. Macro- and microscopical observations reveal that the damage state under combined loading has mixed features from the damage states at = 1/0 and = 0/1.     

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10⁶ fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.     

Low cycle fatigue of a titanium 829 alloy

The fatigue behaviour of titanium 829 in its oil quenched (‘basketweave’) and air cooled (‘aligned’) microstructural forms has been examined at 600°C and room temperature under fully reversed, total strain controlled conditions. Identical endurances are observed for each microstructure together with a low transition life. Similarly, almost perfect cyclic stability is exhibited irrespective of microstructure, temperature, strain range and rate. This is tentatively attributed to the ability of the aligned colonies present in both microstructures to accomodate plasticity. It is argued that a reduction in strain rate shortens life due to environmental effects. Multiple crack initiation is generally associated with facet-like features, with later growth surfaces bearing striations only after fatigue at 600°C.     

A curvilinear integral method for multiaxial fatigue life computing under non-proportional, arbitrary or random stressing

Some popular concepts for reducing three variable stress components σ_x(t), σ_y(t), τ_xy(t) to one equivalent amplitude spectrum, and the use of the linear damage accumulation hypothesis, have been evaluated as not fully correct when these components vary non-proportionally and arbitrarily. A different approach is suggested: computing damage accumulation by means of an integral directly on the non-radial arbitrary path, called the ‘trajectory’, described in the σ_x−σ_y plane when τ_xy(t) = 0, in the σ_x−τ_xy plane when σ_y(t) = 0, or in a special coordinate space where this trajectory is invariant of stress directions x, y. If the trajectory is random, it may be replaced by a statistical two-dimensional density of distribution. The integrand, called the R-function, is derived from various S−N fatigue curves under different determined loadings. Thus the traditional S−N function is replaced by the R-function for direct damage summation with differential analysis, which allows the loading to be arbitrary (non-cyclic, multiaxial and non-proportional). The method works by means of computer programs and is applicable to real structures.     

Some fracture modes in metal fatigue

This paper describes scanning electron-microscope studies made in an attempt to resolve current confusion in the literature about how fatigue cracks begin. It has been said that they normally grow by a Stage I process of shear, on the grounds that they form along slip bands, which are known to tend to directions of maximum shear. It is found that in fact cracks started by slip do not necessarily follow the Stage I shear rule, but may follow random paths; and that optimum condition for Stage I cracking is indeed absence of slip. Further, it is shown that much of the confusion disappears if a distinction is made between ‘plastic’ metals, in which cracking in particular conditions can be started by slip, and ‘elastic’ metals in which cracking is started rather by local inhomogeneous strains and internal stresses and may have little to do with slip.     

The effect of overloading sequences on landing gear fatigue damage

In service, landing gear can be subject to unexpected hard landing load, which is beyond the design domain. The consequences due to overload can affect the design life of a landing gear to some extent. In this paper, the effect of overload and different loading sequences in random spectra on fatigue damage are investigated, using strain–life based fatigue analysis methods. The discussions are emphasised on the effect of loading sequences on residual stress and mean stress, especially the effect of overload on the fatigue damage of subsequent cycles. In addition, different fatigue analysis techniques in commercial fatigue analysis packages are reviewed and compared. The analysis indicates that the overload effect is stress state dependent and dominated by local residual and mean stress. A ‘Begin’ overload in a load spectrum would cause more damage in the local compressive yield area and an ‘End’ overload within a spectrum will worsen the tensile yielding area. It is suggested that the load sequence effect should be considered in common fatigue analysis if local yielding would exist before or after overloading.     

Weld tool travel speed effects on fatigue life of friction stir welds in 5083 aluminium

This paper reports the results of a study into the influence of weld tool travel speed (in the range 80–200 mm/min) on the occurrence of ‘onion-skin’ forging-type defects (similar to the root defects known as ‘kissing bonds’) in single pass friction stir (SP FS) welds, and on the effect of these defects on fatigue crack initiation and overall life. Results indicate that such defects are generally not associated with fatigue crack initiation, but may act to reduce fatigue life by providing easy linking paths between two fatigue cracks. It is likely that their influence on fracture toughness of SP FS welds would be higher, as they occur more readily when growth rates and levels of plastic deformation are higher.     

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.

An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ˜0.3 for the Cu–Ni coated pad against an as-received specimen and ˜0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.     

Fatigue crack propagation of bainitic nodular cast iron

The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a ‘bullseye’ casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode.     

Cyclic response of a 1Cr-Mo-V low alloy steel

The high strain fatigue behaviour of a bainitic (‘Non-heat-treated’) and a tempered bainitic (‘Heat-treated’) alloy steel has been studied at room temperature and 565°C. Cyclic softening is observed in all cases and is associated with dislocation redistribution, alignment and channelling of precipitates, and their growth at 565°C. Comparison of the appropriate monotonic and cyclic stress/strain curves enables a tentative estimate of the relative significance of these mechanisms to be made.     

Establishing criteria for root and toe cracking of load carrying cruciform joints of pressure vessel grade steel

In this investigation an attempt has been made to establish a criterion to forecast the possible crack initiation region (toe or root) in double fillet welded load carrying cruciform joints and also to know the probable failure mode. Cruciform joints were fabricated from pressure vessel grade (ASTM 517 ‘F’ grade) steel using shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes. Fatigue crack growth experiments were carried out in a mechanical resonance vertical pulsator (SCHENCK 200 kN capacity) with a frequency of 30 Hz under constant amplitude loading (R=0). It was found that the fatigue crack initiation lives (root crack or toe crack) were relatively lower in the joints fabricated by FCAW process than the joints fabricated by SMAW process.     

Fatigue behaviour of porous polysulphone surface coatings for orthopaedic applications

Porous polysulphone (PPSF) has recently been selected as a candidate material for use as a prosthetic surface coating in orthopaedic applications to acommodate ‘bone ingrowth’ for fixation to the skeletal system. The tension-loaded fatigue behaviour of PPSF was characterized based on the interpretation of stress/life (S/N) data. Results of this investigation indicate that the fatigue life of PPSF is governed by crack propagation across the sintered necks of the porous structure. In addition, experimental data revealed that the fatigue strength-reduction factor, K_f, remains equivalent to the tensile notch strength-reduction factor for a wide range of fatigue lives. However, analytical results indicate that the material has a low notch sensitivity and that a single application of a theoretical elastic stress-concentration factor in a fatigue analysis would be inappropriate.     

A criterion for omission of variable amplitude loading histories

The standard random loading sequence EC9 and the cycle-by-cycle sequences TWIST and MiniTWIST have been edited by a procedure based on notch or local (hot spot) stress, and the results of tests on specimens of FE 690 and AIMg4.5Mn using original and edited spectra are compared. Omission of small cycles below a filter level of about 50% of the materials' constant amplitude endurance limit was found to be ‘allowable’; this figure was confirmed by a variety of literature data. Using this level, the crack initiation lives (compared with the fatigue life under the unedited sequence) were longer by 10–30% for EC9 and by 20–55% for TWIST and MiniTWIST. In addition, for the latter two spectra, the crack propagation lives were longer by up to 90% with a detrimental influence of higher loading levels. Total life of the notched aluminium specimens increased by 30–70% using a filter level of 50% of the material's endurance limit.     

Seismic risk evaluation of steel structures based on low-cycle fatigue

This paper outlines a methodology to incorporate low-cycle fatigue in seismic risk evaluation of steel structures. The method is based on a simple cumulative damage model in which damage due to each plastic excursion is approximated by a Coffin-Manson type relationship. The parameters in this model are treated as random variables with distributions that are obtained from studies on the nonlinear seismic behavior of bilinear single-degree-of-freedom oscillators. The method is used to obtain an equivalent ‘single-excursion’ ductility demand which represents the same damage state as the low-cycle fatigue model. The proposed approach presents a realistic means of seismic risk evaluation and will provide a basis for establishing safer and more reliable steel structures.     

Threshold range and opening stress intensity factor in fatigue

The fatigue threshold, ΔK_th, is strongly influenced by the stress-ratio, ie by the loading conditions. Results for a Ti6A14V alloy show that a ΔK exists for non-propagating fatigue cracks which is independent of loading conditions. This ΔK is called the fatigue tolerance range and is denoted by ΔK^K. The fatigue tolerance range corresponds to that part of the ΔK_th during which the fatigue crack is open. Arguments that the fatigue tolerance range has to be explicitly incorporated in equations predicting fatigue crack growth rates are presented.     

Reliability updating of welded joints damaged by fatigue

The paper introduces a probability-based fatigue assessment model for welded joints in steel bridges. The approach is based on a modelization of the fatigue phenomenon issued from the principles of fracture mechanics theory. The safety margin includes the crack growth propagation and allows us to treat fatigue damage in a general manner. Damaging cycles and non-damaging cycles are distinguished. The reliability calculus is performed by a FORM technique. The sensitivity study of the different parameters shows that some variables can be taken as deterministic. Applications are made on a welded joint ‘bottom plate/stiffener’ of a typical steel bridge. The model is then used for taking into account inspection results. A sensitivity analysis of different non-destructive inspection (NDI) methods is carried out for measuring their uncertainty levels. The different types of inspection results (no detection, detection with no measurement, detection with measurement) are analysed and a general methodology for updating reliability levels is given. The results show their ability to be inserted in a maintenance strategy for optimizing the next inspection time, the need to repair or to replace the joint, and, the eventual possibility of no action.     

Ultrasonic evaluation of fatigue damage in metal matrix composites

This paper describes an experimental nondestructive technique for fatigue damage assessment in metal matrix composites by measuring ultrasonic phase velocity and attenuation. A [0/90] SiC/Ti---15V---3Cr---3Al---3Sn metal matrix composite is considered as a model system. Cyclic loading at 50 and 70% of the ultimate sample strength were used until failure. The ultrasonic phase velocities and attenuations were measured periodically and found to be very sensitive to fatigue damage. The fatigue-induced changes in the composite elastic constants were calculated from the measured ultrasonic velocity data. For samples heat treated prior to fatigue (815°C) above the matrix β transus (about 760°C), the dominant damage mechanism is debonding of the fiber/matrix interface. We found that when samples were fatigued for less than 50% of the lifetime, the reduction of the composite moduli was linearly dependent on the number of fatigue cycles, which is explained by extension of interfacial partial debonds. This was supported by micromechanical analysis based on a partial disbond model. The rate of decrease in the composite moduli in the second half of the fatigue life was found to be lower, which may serve as a basis for estimation of the remaining fatigue life of the composite from ultrasonic velocity and attenuation measurements. The attenuation data was obtained in directions perpendicular to the fiber. A single-fiber scattering model has been used to explain the effect of the fiber/matrix interface on attenuation. Good correlation between attenuation and moduli measurements was observed.     

Hydrogen embrittlement contributions to fatigue crack growth in a structural steel

The detrimental effects of a hydrogen atmosphere on the fatigue resistance of BS 4360 steel have been assessed by a comparison of crack growth rates in air and hydrogen at a low cycling frequency (0.1Hz), and at a number of temperature (25, 50 and 80 °C). The crack propagation rates in air are almost independent of temperature over this range, but those measured in hydrogen differ by more than an order of magnitude between 25 and 80 °C. The greatest enhancement is seen at 25 °C and at high values of ΔK, the maximum occurring between 40–45 MPa √m at each temperature. There is little hydrogen contribution to crack growth at values of ΔK below 20 MPa √m for R = 0.1.

The enhancement of crack growth rates is reflected by the presence of ‘quasi-cleavage’ facets on the fatigue fracture surfaces of specimens tested in hydrogen. These are most apparent where the greatest increases in growth rate are recorded. The facets show linear markings, which run both parallel and perpendicular to the direction of crack growth. The former are analogous to the ‘river’ lines noted on brittle cleavage facets, and reflect the propagation direction. The latter are more unusual, and indicate that facet formation by hydrogen embrittlement during fatigue is a step-wise process.     

The curvilinear integral method: computer realization and testing 1 (under non-roportional reversed axial force and torque)

The curvilinear integral method seems to be the first not to search for loading cycles; damage accumulation (and the corresponding fatigue life) is computed in terms not of cycles but of differentials of loading (on differential level). Following the classical mathematical differential-integral method, and considering the mutual change of the loading components, a differential of load change ds during dt has been defined as a short element of a ‘trajectory’ (path in appropriate coordinates). The damage differential on ds is dD. Integrating dD to D, the loading is allowed to be arbitrary: non-cyclic, nonproportional (with different component variations), and random. This feature of universality makes the method applicable to real structures for life prediction. The concept of the curvilinear integral has been published previously; this paper provides additional information and details about the mathematical and computer realization of the method and its first testing using a new program. Experimental data from LBF-Darmstadt Germany on the cast iron material GTS45 have been used, under reversed (alternating) axial and shear stresses: in phase, out of phase and of different frequencies.     

In situ RHEED control of self-organized Ge quantum dots

In situ registration of high-energy electron diffraction patterns was used for constructing the diagram of structural and morphological states of the Ge film on the Si(100) surface. The following regions identified in the diagram: two-dimensional (2D)-growth, ‘hut’- and ‘dome’-clusters, ‘dome’-clusters with misfit dislocations at the interface. Variations in the lattice constants of the Ge film during the MBE growth on the Si(100) surface were determined. An increase in the lattice constant at the (100) surface was attributed to the elastic deformation at the stage of 2D growth and formation of ‘hut’-clusters and to the plastic relaxation for the ‘dome’-clusters. As a result, epitaxial silicon structures with germanium quantum dots of 15 nm base size at the density of 3×10¹¹ cm⁻² were synthesized. The total electron structure of the hole spectrum of Ge quantum dots in Si was established.