Fatigue crack growth in heat-treated aluminium alloys

Fatigue crack propagation tests have been performed in several heat-treated aluminium alloys under constant amplitude loading. All experiments were performed, in load control, in a servo-hydraulic closed-loop mechanical test machine. The tests were carried out using middle tension, M(T), specimens. The influence of stress ratio and thickness were analysed. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. A strong stress ratio and material dependence effects on the fatigue crack growth were observed. These effects are discussed in terms of the different dominant closure mechanism. The crack growth behaviour of heat-treated aluminium alloys depends mainly on whether the dominant closure mechanism is plasticity-induced or roughness-induced. The enhancement of roughness-induced closure promotes higher crack growth resistance in these alloys. Roughness-induced closure dominates crack closure in aluminium alloys age hardened by naturally ageing and also artificially aged alloys with higher contents of Mn and Cr elements. In alloys aged hardened by artificially ageing and simultaneously with a lower content of these alloying elements plasticity-induced closure is dominant.     

Fatigue crack propagation in cellular metals

Two types of cellular metals were investigated: a closed-cell aluminium foam with a cell size of about 3.5 mm and densities ranging from 0.25 to 0.40 g/cm³ and hollow sphere structures made of a stainless steel (316L) with sphere sizes of 2 and 4 mm and a density of about 0.3 g/cm³. Fatigue and fatigue crack propagation tests were performed on these materials using an electro-dynamic resonance fatigue testing machine. The crack extension was monitored by a potential drop technique. Additionally, investigations were carried out inside the scanning electron microscope (SEM) using an in situ loading device. All tests were accompanied by local deformation measurements and fracture surface analyses. From the fatigue crack propagation tests it is evident that these materials show a relatively high Paris-Exponent m in the range of 6 to 25 compared to common ductile solid metals. Additional tests were performed to estimate the influence of crack closure, crack bridging and micro cracking on the da/dN versus ΔK curve for these materials. The in situ fatigue tests and the fracture surface analyses revealed a difference in the fatigue crack propagation mechanisms between the closed-cell foam and the hollow sphere structure: in the closed-cell foam a contiguous fatigue crack can be found, where in the case of the hollow sphere structure the fatigue crack propagation is concentrated in the vicinities of the sintering necks.     

Mikrostruktureinfluß auf das Rißfortschrittsverhalten bei nichteinstufigen Schwingbelastungen

Influence of Microstructure on Crack Propagation During fatigue crack propagation under service loads, pronounced load sequence effects can be observed. Only little knowledge exists, how far these sequence effects are influenced by the microstructure of the material and the resulting microstructural fatigue crack propagation mechanisms. Test series are discussed, where with different aluminium alloys and heart treatments varitions of the fatigue crack propagation mechanisms were performed. Additional attention was given to the influence of the environment on the fatigue crack propagation behaviour. Tests were performed in vacuum and, as examples of more aggressive environments, in laboratory air and in a 3.5% NaCl-solution. An attempt is made to explain the observed behaviour on the basis of continuum mechanics considerations and the microstructural properties of the aluminium alloys.     

Fatigue life and crack closure in specimens subjected to variable amplitude loads under plane strain conditions

The propagation of fatigue cracks in specimens subjected to variable amplitude loading under plane strain conditions was investigated experimentally and numerically, to find the influence of the variable amplitude loading on the stabilised crack closure level. Experiments on four-point-bend specimens with a Gurney block load scheme, showed that the crack closure level depends on the crack length but not on the stress range of the fluctuations. Numerical simulations performed in the fatigue crack growth program FASTRAN-II showed good agreement with the experimental results. In addition, statistical uncertainty analyses performed on the fatigue life show that, for technical applications, the uncertainties in initial crack length and load levels have a greater influence on the uncertainty in fatigue life, than the fluctuation level of the load.     

Environment effects and surface roughness on fatigue crack growth at negative R-ratios

Fatigue crack growth has been widely studied, since it plays an important role on the damage tolerance analysis of mechanical components and structures. The environment, material properties and stress ratio significantly influence the fatigue crack growth behaviour of materials. Experimental tests were performed in M(T) specimens of a normalized DIN Ck45 steel at constant load ratios for R = 0.7, 0.5, 0, −1, −2, −3, in ambient air and vacuum conditions, using a new and patented chamber of vacuum. Special emphasis is given to the study of environment effects, stress ratios and related effects of crack roughness. Fracture surface roughness and crack closure effect were systematically measured for all tests in order to compare the influence of different environment and R-ratios. Results have shown that fatigue crack growth rates are higher in air than in vacuum and the fracture surface roughness is also higher in air than in vacuum for comparable stress ratios. The effect of the environment on fatigue crack growth rates seems to be more significant than any mechanical contributions such as plasticity, oxide and roughness which can induce the so-called crack closure.     

FGA formation mechanism for X10CrNiMoV12‐2‐2 and 34CrNiMo6 for constant and variable amplitude tests under the influence of applied mean loads

The aim of the present work is to clarify the fine granular area (FGA) formation mechanism in two steels (tempered 34CrNiMo6 and X10CrNiMoV12‐2‐2) causing grain refinement in the early state of fatigue for internal crack initiation and propagation in the very high cycle fatigue regime at pure tension‐compression loading (R = ?1) and for applied mean stresses (R ≠ ?1). Fatigue tests were performed with constant and variable amplitude at several R values using ultrasonic fatigue testing setups. Failed specimens were investigated using high‐resolution scanning electron microscopy and focused ion beam technique with special attention paid to the crack origin and the surrounding microstructure. To prove models for FGA formation proposed in literature, a numerical model to evaluate effective R values and contact stresses between the fracture surfaces depending on the crack length has been realised. The aim of these investigations is to estimate the influence of crack closure effects on FGA formation. FGA formation due to repeating contact of the fracture surfaces according to the model postulated by Hong et al correlates well with the findings for numerical simulations.     

Fatigue crack closure: From LCF to small scale yielding

Most of the research on crack closure has been devoted to crack propagation under small scale yielding. In this paper, the effect of different length scale from micro-crack to long cracks and different loading conditions from low cycle fatigue, LCF, to small scale yielding on crack closure are considered. The main focus is on LCF crack closure behaviour which is studied by in situ fatigue experiments in a scanning electron microscope. The results demonstrate the importance of crack closure for the explanation of the LCF behaviour. The change of crack closure from LCF to high cycle fatigue and their consequences for lifetime prediction will be discussed.     

Creep–fatigue crack growth of intermetallic compound Ni3 Al(B) at elevated temperatures

Abstract

The creep–fatigue crack growth of Ni₃ Al(B) alloy was investigated at elevated temperatures in air under four different loading waveforms. Two types of time dependent damage mechanisms have been identified: oxidation and creep effects. As compared with fatigue crack growth in air at room temperature, the effect of oxidation at the crack tip on the crack growth acceleration is significant. Creep effects, on the other hand, are dominant for tensile holding and slow–fast loading waveforms. The complicated interaction between creep–fatigue, oxidation induced embrittlement, and oxide induced crack closure determined the different fatigue crack growth behaviours for different loading waveforms at elevated temperature. The relationship between the constants C and m in the Paris formula and loading waveform were examined, and the influence of loading waveform on the crack propagation were also discussed.     

Influence of monotonic and cyclic predeformation on fatigue crack propagation of high-strength aluminum alloys

Regarding the plastically deformed volume ahead of a propagating fatigue crack two types of plastic zones can be distinguished, the K_max-zone which is generated during the load increase upon maximum loading in a cycle and where monotonie tensile deformations occur, and the Δ K-zone, where the crack tip material is reversed plastically deformed.

In order to achieve similar deformation structures as they are present within these two types of plastic zones a predeformation was applied on the bulk material. The deformation structure of the K_max-zone was simulated by defined cold rolling and the cyclic predeformation structure by reversed cyclic plastic deformation. The crack propagation behaviour of the monotonically and cyclically predeformed material and of the completely non-predeformed material was observed. The tests showed that predeformation influenced the crack propagation rate: an increase in the predeformation led to an increase in the crack propagation rate.

In tests with variations in the loading conditions, e.g. a change from a high to a low loading level, considerable sequence effects were observed. The tests with predeformed and non-predeformed materials showed less retardation for the monotonically as well as for the cyclically predeformed material. The ranges in the crack lengths where retardation occurred after the decrease in the loading level, were smaller in the predeformed material. The test results give a clear indication of the significance of the plastic zone behaviour regarding sequence effects after variations in the loading conditions.     

Fatigue crack paths in AA2024-T3 when loaded with constant amplitude and simple underload spectra

It is well known that variable amplitude fatigue loading produces progression marks on fatigue crack surfaces that are related to the loading sequence. These marks are generally a local change in the crack path. The same pattern of loading can produce a pattern of progression marks that have differences from material-to-material or from heat treatment-to-heat treatment, yet the crack path changes that produce these markings are not considered in the prediction of the crack growth behaviour. These path changes can be used to: investigate how the crack grows, aid crack growth measurement and shed light on the mechanism that forms striations. Consequently, an understanding of these path changes and their fundamental cause in structurally significant alloys is important to the explanation of how fatigue cracks grow and how their life can be predicted.In this paper, a number of simple loading sequences were used to investigate the influence that underloads have on the crack path, to develop a better understanding of the formation of fatigue striations and the coarser crack path changes associated with loading changes. The material chosen was aluminium alloy (AA)2024-T3. The results from the tests reported here were compared to previously investigated AA7050-T7451 specimens that were loaded in a similar manner. It is shown that the fatigue crack surfaces that were produced here were the direct consequence of the applied loading interacting with the crystal structure of the material. By changing the loading, via the addition of underloads it was possible to produce fatigue crack surfaces that where composed of not only striations but ridges, depressions and fissures. These features give an indication of the crack growth mechanism. Although, AA2024-T3 and AA7050-T7451 have different chemical compositions, mechanical properties and micro-structures, it was shown that both materials share essentially similar fracture features corresponding to crack propagation at the cycle-by-cycle level. It also appears that despite the above noted differences, similar failure mechanisms may take place.     

Fatigue crack growth assessments in welded components including crack closure effects: Experiments and 3-D numerical modeling

This work provides a numerical and experimental investigation of fatigue crack growth behavior in steel weldments including crack closure effects and their coupled interaction with weld strength mismatch. A central objective of this study is to extend previously developed frameworks for evaluation of crack closure effects on FCGR to steel weldments while, at the same time, gaining additional understanding of commonly adopted criteria for crack closure loads and their influence on fatigue life of structural welds. Very detailed non-linear finite element analyses using 3-D models of compact tension C(T) fracture specimens with center cracked, square groove welds provide the evolution of crack growth with cyclic stress intensity factor which is required for the estimation of the closure loads. Fatigue crack growth tests conducted on plane-sided, shallow-cracked C(T) specimens provide the necessary data against which crack closure effects on fatigue crack growth behavior can be assessed. Overall, the present investigation provides additional support for estimation procedures of plasticity-induced crack closure loads in fatigue analyses of structural steels and their weldments.     

CRACK FACE INTERACTIONS AND NEAR-THRESHOLD FATIGUE CRACK GROWTH

Rough fracture surfaces usually influence substantially the fatigue growth properties of materials in the regime of low growth rates. Friction, abrasion, interlocking of fracture surface asperites and fretting debris reduce the applied load amplitude to a smaller effective value at the crack tip (“sliding crack closure”, or “crack surface interaction” or “crack surface interference”). The influence of these phenomena on the fatigue crack growth properties of structural steel is discussed and compared for the two kinds of mixed mode loading employed in this work. Mixed mode loading was performed by (A): cyclic mode III + superimposed static mode I and (B): cyclic mode I + superimposed static mode III loading. Such loading cases frequently occur in rotating load-transmission devices. Several differences are typical for these two mixed-mode loading cases. A superimposed static mode I load increases the crack propagation rate under cyclic mode III loading whereas cyclic mode I fatigue crack propagation is retarded when a static mode III load is superimposed. Increase of the R -ratio (of the cyclic mode III load) leads to an insignificant increase of fracture surface interaction and subsequently to a small decrease of the crack growth rate for cyclic mode III loading, whereas higher R -values during cyclic mode I+ superimposed static mode III loading lead to a significant reduction of the crack growth rates.     

Manifestation of Hydrogen and Low-Temperature Brittleness in Near-Threshold Cyclic Crack Resistance of Materials

We experimentally show that the realization of conditions of plane deformation at the tip of a fatigue crack is not sufficient for guaranteeing the unique dependence of the crack growth rate on the range of the stress intensity factor, which is explained by the effect of crack closure. We describe advantages and disadvantages of the effective range of the stress intensity factor as a parameter that determines the mechanical conditions for the propagation of a fatigue crack. We analyze the phenomenon of positive influence of strengthening factors (a decrease in the temperature of testing and hydrogenation) on the cyclic crack resistance of materials in a low-amplitude range of loading determined with regard for the effect of crack closure. The decrease in the crack growth rate and the increase in fatigue thresholds are intensified as the level of loading decreases and the ductility of materials increases. Differences in the influence of strengthening factors in low- and high-amplitude ranges of loading are explained by different mechanisms of fracture controlled by the shearing strength and the tensile strength, respectively. We give several examples of the mechanical behavior of materials that show the inversion of the influence of hydrogen on the resistance to fracture: fatigue fracture of smooth steel specimens in gaseous hydrogen, high-temperature corrosion fatigue of preliminary hydrogenated titanium alloys, and the influence of hydrogenation on the wear resistance of structural steels in the process of friction and cavitation and on the parameters of cutting of a tool steel.     

Creep-fatigue crack propagation in lead-free solder under cyclic loading with various waveforms

Crack propagation tests of lead-free solder were conducted using center-notched plate specimens under cyclic tension-compression of three load waveforms: pp waveform having fast loading and unloading, cp-h waveform having a hold time under tension, and cc-h waveform having a hold time under tension and compression. In the case of fatigue loading, i.e. pp waveform, the path of crack propagation was macroscopically straight and perpendicular to the maximum principal stress direction, showing tensile-mode crack propagation. The introduction of the creep components by hold time in cc-h and cp-h waveforms promoted shear-mode crack propagation. For fatigue loading of pp wave, the crack propagation rate was expressed as a power function of the fatigue J integral and the relation was identical for load-controlled and displacement-controlled conditions. The creep component due to the hold time greatly accelerates the crack propagation rate when compared at the same values of the fatigue J integral or the total J integral (the sum of fatigue J and creep J integrals). The creep crack propagation rate was expressed as a power function of the creep J integral for each case of cp-h and cc-h waveforms. The crack propagation rate for cp-h waveform is higher than that for cc-h waveform. The predominant feature of fracture surfaces was striations for pp waveform and grain boundary fracture for cp-h waveform. Grain fragmentation was abundantly observed on the fracture surface made under cc-h waveform.     

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.     

Fatigue crack initiation and propagation under cyclic contact loading

A computational model for contact fatigue damage analysis of gear teeth flanks is presented in this paper. The model considers the conditions required for the surface fatigue crack initiation and then allows for proper simulation of the fatigue crack propagation that leads to the appearance of small pits on the contact surface. The fatigue process leading to pitting is divided into crack initiation and a crack propagation period.The model for prediction of identification of critical material areas and the number of loading cycles, required for the initial fatigue crack to appear, is based on Coffin-Manson relations between deformations and loading cycles, and comprises characteristic material fatigue parameters. The computational approach is based on continuum mechanics, where a homogenous and elastic material model is assumed and results of cyclic loading conditions are obtained using the finite element method analysis.The short crack theory together with the finite element method is then used for simulation of the fatigue crack growth. The virtual crack extension (VCE) method, implemented in the finite element method, is used for simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The relationship between the stress intensity factor K and crack length a, which is needed for determination of the required number of loading cycles N_p for a crack propagation from the initial to the critical length, is shown.     

Fatigue crack propagation after overloading and underloading at negative stress ratios

This paper intends to evaluate the influence of the intrinsic properties of the materials, namely plastic and cyclic plastic properties, on the overloading/underloading effect on crack propagation rate, at baseline negative stress ratios, under plane strain conditions. The importance of the negative loading part of the fatigue cycle on crack propagation rate has been shown by previous works of this same author. In those works has also been shown that under baseline negative stress ratios there exists negative open loads and crack propagation rate does not correlate properly with the crack closure concept. These features were shown to be strongly related to plastic properties and cyclic plastic properties of the materials. It has been concluded that the Bauschinger effect may be the explanation for the different sensitivity to negative loads. Thus, some materials may be very sensitive to negative loads and some others may not be so sensitive. Tensile overload and compression underload tests, at positive and negative baseline stress ratios were made in different materials, with different plastic properties, in order to predict their influence on crack propagation rate. The main emphasis in this paper is the importance of the compressive part of the loading cycle under negative baseline R ratios on overloads/underloads effect on crack propagation rate. Results will show that the effect of overloads and underloads on crack propagation rate, at baseline negative stress ratios, are not fully accounted for by crack propagation models and that the generalized accepted behaviour of OL/UL may not be the same at baseline negative stress ratios. It will be shown that Overloads may produce acceleration instead of the accepted retardation effect. A physical understanding on the effects of OL/UL is also provided in the paper.     

Fatigue crack propagation along interfaces of selective laser melting steel hybrid parts

Selective laser melting (SLM) is an emerging additive manufacturing technology, capable of producing complex geometry components. The current work studied both the effect of substrate material and mean stress on the fatigue crack growth behaviour along interfaces of bi‐material specimens, substrate, and part by SLM. Fatigue tests were carried out in agreement with ASTM E647 standard, using 6‐mm‐thick compact specimens. The substrate steel has only a negligible effect both on the fatigue crack propagation rate and on the crack path. The failure occurs in the material additively manufactured by SLM, near the interface. The mean stress produced only a reduced influence on the fatigue crack propagation rate in the Paris regime. For larger values of ΔK, where K_max approaches K_Ic, a significant influence of the mean stress was observed. In spite of nondetection of crack closure, the application of overloads promoted significant fatigue crack retardation, quite similar for both substrate materials, probably due to the crack bifurcation during the overload.     

An investigation of crack closure and the propagation of semi-elliptical fatigue cracks in Q1N (HY80) pressure vessel steel

The results of an experimental investigation of the effect of crack closure on the propagation of semi-elliptical fatigue cracks are presented. Load-shedding fatigue threshold tests were carried out at stress ratios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surface and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth behaviour are explained by considerations of the effects of the transition from plane stress conditions at the surface to plane strain conditions at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in asperities coming into contact to induce roughness-induced crack closure.     

Size effects in the biaxial tensile-compressive behaviour of concrete: physical mechanisms and modelling

This paper analyses failure mechanisms and apparent size effects in the biaxial tensile-compressive behaviour of concrete. To this end, a probabilistic discrete crack model is used to numerically determine failure surfaces in the tensile-tensile and tensile-compressive loading range, on account of size effects being considered as volume effects. The results (failure mechanisms, crack patterns, volume effects, etc.) are discussed in some detail with respect to the applied loading state. Size effects on the failure surface are quantified in terms of stress-invariant ratios at peak load for 8 loading paths. It is found that size effects decrease with increasing hydrostatic pressure,i.e. when passing from the tensile loading range into the tensile-compressive range. This can be explained by the activation of friction at the crack lips in a stable crack propagation, which regularises mechanical volume effects, and thus apparent size effects.