Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

Abstract

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.     

Crack path and fracture analysis in FSW of small diameter 6082-T6 aluminium tubes under tension–torsion loading

This paper reports part of the work done in a research project aimed at developing an optimised process to join 38 mm diameter tubes of 6082-T6 aluminium alloy using friction stir welding (FSW), and then to determine the fatigue performance under tension, torsion and tension–torsion loading conditions. The final outcome of the project is intended to be guidance for fatigue design of small diameter aluminium tubes joined by FSW, and this paper presents information on crack path and defects under the various loading conditions. Crack path analysis was performed using both low magnification stereo microscopy and scanning electron microscopy, in order to identify crack initiation sites, the direction of crack propagation and the interrelated influence of microstructure and weld geometry on the crack initiation path.     

Tension–tension fatigue behavior of layer-to-layer 3-D angle-interlock woven composites

The tension–tension fatigue behavior of (the layer-to-layer) three-dimensional angle-interlock woven composite (3DAWC) was investigated using the acoustic emission (AE) technique, examination of damaged regions via light microscopy and micro-CT, and finite element analysis (FEA). AE events occurred during the entire fatigue process, and damage modes were determined based on cumulative fatigue damage of the 3DAWC undergoing tension-tension cyclic loading. FEA was employed to determine stress distribution and specific regions of stress concentrations within the composite structure. Debonding occurred at the warp tow-matrix-weft tow interface where the warp tows exhibited a maximum in amplitude of undulation. In addition, interrupted tests were performed to investigate fatigue damage modes of the 3DAWC at specific points in the fatigue life. Fatigue damage occurred in three distinct stages with characteristic damage modes.     

Influence of load ratio on the biaxial fatigue behaviour and damage evolution in glass/epoxy tubes under tension–torsion loading

An extensive experimental campaign was carried out to understand the influence of the multiaxial stress state and load ratio on the matrix-dominated damage initiation and evolution in composite laminates under fatigue. Tubular glass/epoxy specimens were tested under combined tension–torsion loadings with different values of the load ratio and biaxiality ratio (shear to transverse stress ratio). Results are reported in terms of S–N curves for the first crack initiation and Paris-like diagrams for crack propagation, showing a strong influence of both parameters. Fracture surfaces were also analysed to identify the damage mechanisms at the microscopic scale responsible for the initiation and propagation of transverse cracks. Eventually, a crack initiation criterion presented by the authors in a previous work is applied to the experimental data showing a good agreement.     

Improving fretting fatigue behaviour of Al 7075-T6 bolted plates using electroless Ni–P coatings

In this paper, the effect of electroless nickel–phosphorous coatings on the fatigue and fretting fatigue behaviour of Al 7075-T6 bolted plates has been investigated. A double-lap bolted joint specimen was designed and manufactured from the aluminium plates and subsequently coated with Ni–P coatings of 40 μm in thickness with a high phosphorous content of 10–13 wt.%. Then, different tightening torques were applied to clamp the plates together with the aim of studying the effect of clamping force on the fretting fatigue life of the joints. Ni–P coatings were found to protect Al 7075-T6 clamped plates against fretting fatigue damages even at low fatigue loads where the uncoated joints showed a large reduction in their fatigue life due to the fretting effects. Fretting fatigue life of moderately and firmly clamped plates was successfully improved by approximately 30–40% and 50–60% respectively after the application of Ni–P coatings. Furthermore, fatigue cracks were found at the outer surface of the aluminium substrate at the edge of the hole associated with nodular defects whose deleterious effect was more pronounced at high cyclic loads. The Ni–P deposit presented a very good adhesion to the substrate at low and moderate loads; however, considerable delaminations and fracture of the coating film at high cyclic loads was observed.     

Effect of stress ratios on tension–tension fatigue behavior and micro-damage evolution of basalt fiber-reinforced epoxy polymer composites

The tension–tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R?=?σ_min/σ_max of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios.     

Tension–tension fatigue of hybrid composite rods

The tension–tension fatigue behavior was investigated for a hybrid composite rod comprised of a unidirectional carbon fiber core and a glass fiber shell. Fatigue tests were performed at three R-ratios and four maximum applied stress levels (MAS) while recording the secant modulus at each cycle, and acoustic emission (AE) sensors were employed to monitor the activation of fatigue mechanisms. Fatigue failure occurred when the composite rod was no longer able to support the applied cyclic load. For a MAS level of 70% of the ultimate tensile stress (UTS), composite rods tested at higher R-ratios showed AE activity through a larger percentage of fatigue life, but exhibited a greater resistance to fatigue failure, whereas samples cycled at lower R-ratios displayed AE activity only near the end of fatigue life, and showed a lower resistance to fatigue failure. The hybrid composite showed modes of progressive fatigue damage at high R-ratios and low strain amplitudes in the form of longitudinal splitting of the GF shell. In contrast, failure of the CF core was catastrophic and non-progressive. The fatigue resistance and damage mechanisms of the composite rod were dependent on the MAS level and R-ratio. Fatigue cracks initiated because of fretting between the GF shell and grip surface, which led to the observed longitudinal splitting of the GF shell. Fatigue damage occurred along the GF/CF interface where non-uniform strains developed because of the clamping force of the grip on the GF surface. At an R-ratio of 0.85, a fatigue stress of 70% UTS caused catastrophic fatigue failure, while at lower stresses, composite rods did not fail and withstood cyclic loads up to 1 million cycles. The research conducted is the first to investigate the degradation in fatigue performance arising from grip/composite rod interactions and suggests that the results from the study provide new information for composite materials in industries that utilize unidirectional composites in cylindrical form.     

Fretting–fatigue behavior of steel wires in low cycle fatigue

The effect of strain amplitude on fretting–fatigue behavior of steel wires in low cycle fatigue was investigated using a fretting–fatigue test rig which was capable of applying a constant normal contact load. The fretting regime was identified based on the shape of the hysteresis loop of tangential force versus displacement amplitude. The variations of the normalized tangential force with increasing cycle numbers and fretting–fatigue lives at different strain amplitudes were explored. The morphologies of fretting contact scars after fretting–fatigue tests were observed by scanning electron microscopy and optical microscopy to examine the failure mechanisms of steel wires. The acoustic emission technique was used to characterize the fretting–fatigue damage in the fretting–fatigue test. The results show that the fretting regimes are all located in mixed fretting regimes at different strain amplitudes. The increase in strain amplitude increases the normalized tangential force and decreases the fretting fatigue life. The abrasive wear, adhesive wear and fatigue wear are main wear mechanisms for all fretting–fatigue tests at different strain amplitudes. The accumulative total acoustic emission events during fretting–fatigue until fracture of the tensile steel wire decrease with increasing strain amplitude. An increase of the strain amplitude results in the accelerated crack nucleation and propagation and thereby the decreased life.     

The effect of electroless Ni–P coatings on the fatigue life of Al 7075-T6 fastener holes with symmetrical slits

In this paper, the fatigue behaviour of Al 7075-T6 fastener holes with symmetrical through slits was studied. The holes were coated with electroless nickel (EN) plating with a high phosphorous content of 10–13 wt% and a thickness of 40 μm. Uncoated open-hole, EN coated open-hole, uncoated bolted hole and EN coated bolted hole specimens were fatigue tested. Bolted samples were clamped with a high tightening torque of 7 Nm. The established S–N curves showed 282–1348% improvements in the fatigue life due to the combined effect of EN coating and bolt clamping, depending on the level of maximum alternating stress. Excellent adhesion was observed between the coating and the aluminium substrate along the crack path. Tensile tests results showed a considerable reduction of 54% in the ductility of the coated material while both the yield and ultimate strengths were found to slightly increase by approximately 6% in comparison with the uncoated aluminium alloy.     

Crack onset and propagation at fibre–matrix elastic interfaces under biaxial loading using finite fracture mechanics

A new fracture criterion able to predict crack onset and propagation at interfaces between solids is formulated, implemented in a computational code and applied to a particular problem in composites on a microscale. More specifically, this criterion is used to study the debond onset and propagation in mixed mode in the case of a single fibre subjected to a biaxial remote loading. The fracture criterion formulation is based on the Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with a Finite Fracture Mechanics (FFM) approach, where the stress and energy criteria are suitably coupled. Each of these criteria is a necessary but not sufficient condition for crack onset and propagation. Two empirical mixed-mode fracture criteria are considered and tested: the interface fracture toughness law by Hutchinson and Suo and the quadratic stress criterion. The FFM + LEBIM approach developed offers an adequate characterization of the interface stiffness in contrast to the too restrictive, original LEBIM formulation.     

Influence of specimen type and reinforcement on measured tension–tension fatigue life of unidirectional GFRP laminates

It is well known that standardised tension–tension fatigue test specimens of unidirectional (UD) glass-fibre-reinforced plastics (GFRP) laminates tend to fail at end tabs. The true fatigue life is then underestimated. The first objective of this study was to find for UD GFRP laminates a test specimen that fails in the gauge section. The second objective was to compare fatigue performance of two laminates, one having a newly developed UD powder-bound fabric as a reinforcement and the other having a quasi-UD stitched non-crimp fabric as a reinforcement. In the first phase, a rectangular specimen in accordance with the ISO 527-5 standard and two slightly different dog-bone shaped specimens were evaluated by means of finite element modelling. Subsequent comparative fatigue tests were performed for the laminates with the three specimen types. The results showed that the test specimen type has a significant effect on the failure mode and measured fatigue life of the laminates. A significantly higher fatigue life was measured for the laminate with the powder-bound fabric reinforcement when compared to the laminate with the stitched reinforcement.     

Low cycle fatigue life of Ti–6Al–4V alloy under non-proportional loading

Multiaxial low cycle fatigue life of Ti–6Al–4V under non-proportional loading was studied. Strain-controlled multiaxial fatigue tests at room temperature were carried out using tubular specimens. The strain paths employed were push–pull loading, reversed torsion loading, and two kinds of 90° out-of-phase loadings. The former two loadings are proportional loading tests where the principal directions of stress and strain are fixed in the cycle. The latter two are non-proportional loading tests where there is a 90° phase difference between axial and shear loadings, and the principal directions are cyclically rotated continuously. Failure lives are reduced obviously by non-proportional loadings in comparison with those in proportional loading tests. This paper focuses on determining a suitable fatigue model for evaluating the failure lives of Ti–6Al–4V under multiaxial loading.     

Three-point bending fatigue behavior of WC–Co cemented carbides

WC–Co cemented carbides with different WC grain sizes and Co binder contents were sintered and fabricated. The three-point bending specimens with a single edge notch were prepared for tests. In the experiments, the mechanical properties of materials were investigated under static and cyclic loads (20 Hz) in air at room temperature. The fatigue behaviors of the materials under the same applied loading conditions are presented and discussed. Optical microscope and scanning electron microscopy were used to investigate the micro-mechanisms of damage during fatigue, and the results were used to correlate with the mechanical fatigue behavior of WC–Co cemented carbides. Experimental results indicated that the fatigue fracture surfaces exhibited more fracture origins and diversification of crack propagation paths than the static strength fracture surfaces. The fatigue fracture typically originates from inhomogeneities or defects such as micropores or aggregates of WC grains near the notch tip. Moreover, due to the diversity and complexity of the fatigue mechanisms, together with the evolution of the crack tip and the ductile deformation zone, the fatigue properties of WC–Co cemented carbides were largely relevant with the combination of transverse rupture strength and fracture toughness, rather than only one of them. Transverse rupture strength dominated the fatigue behavior of carbides with low Co content, whilst the fatigue behavior of carbides with high Co content was determined by fracture toughness.     

Energy dissipation capacity of fibre reinforced concrete under biaxial tension–compression load. Part I: Test equipment and work of fracture

The objective of this research was to analyse the differences in the dissipated energy under uniaxial tension and biaxial tension–compression load of fibre reinforced concretes using the Wedge Splitting Test. Under biaxial load the specimens were subjected to compressive stress ratios from 10% to 50% of the concrete compressive strength perpendicular to the direction of the tensile load.Under biaxial tension–compression load the energy dissipation capacity of the specimens decreases compared to the uniaxial tension load case on average 20–30%. It is believed that the decrease is a result of the damage mechanism of the concrete matrix and deterioration of the fibre–matrix and/or aggregate–cement paste interfaces in case the section is additionally loaded with compression stresses. This indicates that dimensioning of concrete elements under biaxial stress states using material parameters obtained from tests conducted on specimens under uniaxial tensile load is unsafe and could potentially lead to a non-conservative design.In the second part of this paper the extent of the fracture process zone under uniaxial tension and biaxial tension–compression load will be examined with the Acoustic Emission technique and the reasons for decrease of the energy dissipation capacity under biaxial load will be further discussed.     

Interface debond crack growth in tension–tension cyclic loading of single fiber polymer composites

Fiber/matrix interface debond crack growth from a fiber break is defined as one of the key mechanisms of fatigue damage in unidirectional composites. Considering debond as an interface crack its growth in cyclic loading is analyzed utilizing a power law, where the debond growth rate is a power function of the change of the strain energy release rate in the cycle. To obtain values of two parameters in the power law cyclic loading of fragmented single fiber specimen is suggested. Measurements of the debond length increase with the number of load cycles in tension–tension fatigue are performed for glass fiber/epoxy single fiber composites. Analytical method in the steady-state growth region and FEM for short debonds are combined for calculating the strain energy release rate of the growing debond crack. Interface failure parameters in fatigue are determined by fitting the modeling and experimental results. The determined parameters for interface fatigue are validated at different stress levels.     

High and low cycle fatigue behavior of linear friction welded Ti–6Al–4V

Linear friction welded Ti–6Al–4V was investigated in fatigue at various stress amplitudes ranging from the high cycle fatigue (HCF) to the low cycle fatigue (LCF) regime. The base material was composed of hot-rolled Ti–6Al–4V plate that presented a strong crystallographic texture. The welds were characterized in terms of microstructure using electron backscatter diffraction and hardness measurements. The microstructural gradients across the weld zone and thermomechanically affected zone of the linear friction welds are discussed in terms of the crystallographic texture, grain shape and hardness levels, relative to the parent material. The location of crack nucleation under fatigue loading was analyzed relative to the local microstructural features and hardness gradients. Though crack nucleation was not observed within the weld or thermomechanically affected zones, its occurrence within the base material in LCF appears to be affected by the welding process. In particular, by performing high resolution digital image correlation during LCF, the crack nucleation site was related to the local accumulation of plastic deformation in the vicinity of the linear friction weld.     

Effect of orientation and presence of hydride on the fatigue crack growth behavior of Zr–2.5 wt% Nb

Fatigue crack growth (fcg) behavior of cold-worked and stress relieved Zr–2.5 Nb was studied in the longitudinal (with and without hydrides) and transverse direction at ambient temperature and load ratio of 0.1 using compact tension samples. Fatigue loading in the transverse direction (distribution of both hard and soft grains) showed facet formation on the fracture surface and the highest ΔK_th whereas loading in the longitudinal direction (distribution of primarily soft grains) showed no facet formation and a lower ΔK_th. Hydrided Zr–2.5 Nb loaded in the transverse direction showed large facets with the lowest ΔK_th. Texture influenced fcg at low ΔK but not at higher ΔK.     

Modelling of Surface Crack Growth under Lubricated Rolling–Sliding Contact Loading

The paper describes modelling approach to computational simulation of surface crack growth subjected to lubricated rolling–sliding contact conditions. The model considers the size and orientation of the initial crack, normal and tangential loading due to rolling–sliding contact and the influence of fluid trapped inside the crack by a hydraulic pressure mechanism. The motion of the contact sliding load is simulated with different load cases. The strain energy density (SED) and maximum tangential stress (MTS) crack propagation criteria are modified to account for the influence of internal pressure along the crack surfaces due to trapped fluid. The developed model is used to simulate surface crack growth on a gear tooth flank, which has been also experimentally tested. It is shown that the crack growth path, determined with modified crack propagation criteria, is more accurately predicted than by using the criteria in its classical form.