Fatigue, monotonic and fracture toughness properties of a Cr–Mn–N steel

The low-cycle fatigue, monotonic and fracture toughness behaviour of E3949, a Cr–Mn–N austenitic stainless steel, used for drillcollar connections was studied. Low-cycle fatigue tests were carried out at room temperature under total strain control in the range of 0.40 to 1.50% using Companion Specimens Test (CST) and Incremental Step Test (IST) methods. Cyclic softening without saturation was observed in all tests. Massing cyclic stress–strain behaviour was observed only with the IST method. The fatigue life behaviour obeyed Basquin and Coffin–Manson relationships and the high value obtained for ′_f imparts a significant improvement in fatigue resistance of this alloy compared to AISI 304LN. The J–R curves and J_IC values were obtained at room temperature and at 150°C by using single specimens and the elastic compliance technique for crack length measurement. The observed decrease in crack initiation fracture toughness at 150°C is proposed to be due to a dynamic strain ageing effect, which impairs ductility.     

Cyclic behaviour of 12% Cr ferritic‐martensitic steel upon long‐term on‐site service in power plants

The strain‐controlled and stress‐controlled low‐cycle fatigue behaviour of served 12% Cr ferritic–martensitic steel is conducted at room temperature. Continuous softening is observed at both control modes, and the fitting results show that the fatigue properties of 12% Cr steel are not reduced significantly after 230 000 h service at 550 °C/13.7 MPa. Scanning electron microscopy has been employed to investigate the microstructure evolution after long‐term service. It is proved that the decomposition of martensite laths structure and the coarsening of carbides at grain/lath boundaries are the main reasons why the pipe bursts after 180 000 h service at 550 °C/17.1 MPa. The fracture under both control modes has been observed by using scanning electron microscopy, and it indicates coarse carbides along grain/lath boundaries are favourable sites for micro‐crack nucleation and the secondary cracks along the fatigue striations are formed by the connection of micro‐cracks nucleated during fatigue behaviour.     

On the dynamic embrittlement of copper-chromium alloys by sulphur

The high temperature ductility of copper-chromium alloys has been examined in the temperature range of 25–700°C. Copper-chromium alloys exhibit severe intermediate temperature loss in ductility. The loss in ductility is associated with the ingress of sulphur along the grain boundaries under the influence of tensile stress, followed by decohesion of grain boundaries. Integranular facets of the fracture surface exhibit striations, indicative of quasi-static, step-wise crack growth process.     

Fatigue crack growth behavior of X2095 Al–Li alloy

Microstructures and micro-textures of X2095 Al–Li alloy in as-received/superplastic state were characterized by means of SEM/BDS, X-ray diffraction and orientation imaging microscopy (OIM). It was observed that the microstructure of the alloy was typical of a particulate-reinforced composite material, consisting of aluminum matrix and homogeneously distributed T_B(Al₇Cu₄Li) particles with a volume fraction of about 10%. Brass-type texture was the dominant texture component. Both constant amplitude and near-threshold fatigue crack growth rates of the alloy in the L–T and T–L orientations were determined at different stress ratios. Particular attention was paid to the role of the T_B phase in the fatigue crack growth. When a fatigue crack approached a T_B particle, the crack basically meandered to avoid the particle. The T_B particles thus provided a strong resistance to the propagation of fatigue crack by promoting crack deflection and the related crack closure effects. The fatigue crack propagation behavior has been explained by the microstructural features, micro-textures, cracking characteristics and crack closure effects.     

Fatigue crack initiation in Hastelloy X – the role of boundaries

In polycrystalline metals, microstructural features such as grain boundaries (GBs) influence fatigue crack initiation. Stress and strain heterogeneities, which arise in the vicinity of GBs, can promote the nucleation of fatigue cracks. Because of variations in grain size and GB types, and consequently variations in the local deformation response, scatter in fatigue life is expected. A deeper quantitative understanding of the early stages of fatigue crack nucleation and the scatter in life requires experimental and modelling work at appropriate length scales. In this work, experiments are conducted on Hastelloy X under fatigue conditions, and observations of fatigue damage are reported in conjunction with measurements of local strains using digital image correlation. We use a recent novel fatigue model based on persistent slip band–GB interaction to investigate the scatter in fatigue lives and shed light into the critical types of GBs that nucleate cracks. Experimental tools and methodologies, utilizing ex situ digital image correlation and electron backscatter diffraction, for high resolution deformation measurements at the grain level are also discussed in this paper and related to the simulations.     

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q_a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The ₂(Ti₃Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/₂ interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.     

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

The short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.     

Mechanical properties of the hot-rolled Mg–12Gd–3Y magnesium alloy

Tensile and high cycle fatigue (HCF) properties of the hot-rolled Mg–12Gd–3Y (wt.%) magnesium alloy have been investigated. The magnesium alloy exhibits a fatigue strength of about 150 MPa, which is much higher than that of the commercial Mg–8Al–Zn alloy AZ80. Aging heat-treatment (T5) improved the fatigue life of the Mg–12Gd–3Y alloy. Fatigue cracks nucleated at the intense slip bands in the as-rolled alloy. After T5 treatment, however, the fatigue crack nucleation site shifted to the phase boundaries between MgGdY particles and Mg matrix. T5 heat-treatment retarded the crack initiation and thus improved the fatigue life of the Mg–12Gd–3Y alloy.     

Prediction of creep–fatigue crack growth rates in inert and active environments in an aluminium alloy

This paper reports on a study on creep–fatigue crack growth resistance of a precipitation hardened 2650 T6 aluminium alloy selected for fuselage panels of a future civil supersonic aircraft. The objective is to develop a methodology to predict crack growth under very low frequency loading at elevated temperatures. With this aim, creep crack growth rates (CCGRs), fatigue crack growth rates (FCGRs), creep–fatigue crack growth rates (CFCGRs) have been measured at 130 °C and 175 °C in laboratory air and in vacuum at R = 0.5 under different load frequencies and waveshape signals. It is shown that, for a given temperature, CFCGRs are unaffected by frequency below a critical value of the load period T_c. Above this value CFCGRs are directly proportional to the load period. This time-dependent crack growth regime is assisted by a significant creep damage as indicated by the large amount of intergranular decohesions induced by cavitation on fracture surfaces. CFCGRs are calculated under the assumption that fatigue damage and creep damage can be linearly summed. In vacuum the predictions are in good agreement with experimental data at both temperatures. In air however a discrepancy is observed for low frequency loading, suggesting the occurrence of a creep–fatigue–environment interaction. As a consequence the time-dependent crack growth behaviour affected by this interaction is different from creep crack growth behaviour, although the reasons for this are still unclear. A methodology is then proposed to predict CFCGRs in air. This methodology, if assessed by very low frequency experimental results, could be extended to different structural components made of aluminium alloys operating at elevated temperatures, provided that the mechanisms are unchanged.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

航空铝合金原位腐蚀疲劳性能及断裂机理EI北大核心CSCD

为了研究不同腐蚀条件下2024铝合金的疲劳性能,首先设计搭建原位腐蚀疲劳平台,然后分别进行无腐蚀疲劳、预腐蚀疲劳和原位腐蚀疲劳实验,分析不同腐蚀疲劳条件下2024铝合金的疲劳断裂行为,最后利用扫描电镜(SEM)表征宏、微观断口特征,探究失效机理。结果表明:相同腐蚀环境和时间下,预腐蚀和原位腐蚀疲劳寿命分别为无腐蚀疲劳寿命的92%和42%;在原位腐蚀疲劳条件下,滑移带挤入、挤出导致表面粗糙度增加,吸附较多腐蚀介质,加剧蚀坑演化,易于裂纹萌生并形成多个裂纹源。裂纹的连通形成更大尺寸的损伤,并在材料内部快速扩展。预腐蚀和原位腐蚀疲劳试件断口观察到大量脆性疲劳条带,并且原位腐蚀疲劳条带平均间距约为无腐蚀疲劳条带间距的2倍,说明原位腐蚀疲劳条件下裂纹扩展速率更快。     

Elucidation of fatigue characteristics and fracture mechanism of friction stir spot‐welded tension–shear joint steels

The fatigue life and fracture mechanism of friction stir spot welded tension–shear joints using 590‐MPa class steel as a base material under constant‐amplitude conditions were investigated with focus on welding dimension variations caused by tool wear. The fatigue limit of the friction stir spot welding (FSSW) joint used for this study is significantly low compared with the static tensile strength of the joint itself. It was clarified that the FSSW joint in this study exhibited two different failure morphologies regardless of the applied load level: base metal fracture and weld area fracture. Although the welding state changes due to the tool wear phenomenon that produce two types of fracture modes in relation to the welding rip diameter, they have no effect on the fatigue strength, regardless of the applied load.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

Effects of trace zirconium on the initiation and propagation behavior of fatigue cracks in Cu–6Ni–2Mn–2Sn–2Al alloy

The effects of trace Zr on the fatigue behavior of Cu–6Ni–2Mn–2Sn–2Al alloy were studied through the initiation and growth behavior of a major crack. When stress amplitude was less than σ_a = 350 MPa, the fatigue life of Zr-containing alloys was about 2 times larger than that of alloy without Zr. When σ_a = 400 MPa, the effects of Zr addition on fatigue life disappeared. Increased fatigue life due to Zr addition resulted from an increase in crack initiation life and microcrack growth life. Zr addition generated strengthened grain boundaries (GBs) that developed from the precipitation of SnZr compounds. Strengthened GBs contributed to the increase in crack initiation life. The effects of Zr addition on fatigue behavior were discussed with relation to the behavior of microcracks.     

Fracture behaviour of 2D-weaved, silica–silica continuous fibre-reinforced, ceramic–matrix composites (CFCCs)

Significantly improved fracture resistance (in terms of fracture toughness and fracture energy) can be imparted to monolithic ceramics by adopting composite design methodology based on fibre reinforcement technology. The present paper describes the fracture behaviour of one such fibre-reinforced material, namely the silica–silica based continuous fibre-reinforced, ceramic–matrix composite (CFCC) in two orthogonal notch orientations of crack divider and crack arrester orientations. Different fracture resistance parameters have been evaluated to provide a quantitative treatment of the observed fracture behaviour. From this study, it has been concluded that the overall fracture resistance of the CFCC is best reflected by total fracture energy release rate (J_c), which parameter encompasses most of the fracture events/processes. The J_c values of the composite are found to be more than an order of magnitude higher than the energy values corresponding to the plane strain fracture toughness (J_KQ, derived from K_Ic, the plane strain fracture toughness) and >200% higher than elastic–plastic fracture toughness (J_Ic). Apart from this, the composite is found to exhibit high degree of anisotropy in the fracture resistance and also, a significant variation in the relative degree of shear component with crack extension.     

Fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P deposit

An investigation has been carried out in order to study the fatigue and corrosion–fatigue behavior of a 7075-T6 aluminum alloy coated with an electroless Ni–P (EN) deposit, in the as-plated condition, of approximately 38–40 μm in thickness and a high P content, of approximately 18 wt%. The results obtained, show that the EN coating can give rise to a significant improvement in the fatigue and corrosion–fatigue performance of the substrate, depending on the testing conditions. When the coated system is tested in air, it is observed that the increase in fatigue properties decreases as the alternating stress applied to the material increases. At stresses of the order of 0.4 σ_0.2% the increase in fatigue life is more than about 100%. However, as the stress increases to values in the range of 0.7 σ_0.2%, no improvement in the fatigue performance of the system is observed and the behavior is similar to that of the uncoated substrate. Under corrosion–fatigue conditions, the fatigue life is observed to increase between approximately 60% and 70%, depending on the stress applied. It is shown that fatigue cracks are associated with nodular-like defects present on the surface of the coated samples. The deleterious effect of such defects seems to be more pronounced as the alternating stress applied to the material increases. A crude estimate of the yield strength of the EN coating from tensile measurements indicates that such a parameter is in the range of 3.8 GPa, in agreement with the computation of the absolute hardness of the deposit, of about 4 GPa, by means of Meyer’s law. It is also shown that the EN deposit has a very good adhesion to the substrate even when the system is subjected to tensile stresses greater than the yield strength. Such characteristics as well as the higher mechanical properties of the EN coating in comparison with the aluminum alloy substrate and the preservation of its integrity during fatigue testing contribute to the better fatigue performance of the coated system.     

Fatigue crack growth resistance of 7075 Al alloy after equal channel angular pressing

This work presents experimental results on effects of severe plastic deformation (SPD) and subsequent natural ageing on tensile mechanical properties and fatigue crack growth resistance of fine‐grained 7075 Al alloy. The alloy was subjected to equal channel angular pressing (ECAP) after solution treatment. Fatigue crack propagation tests were conducted in room condition, at load ratio R = 0.1 and different load ranges on small disk shaped compact tension specimens. Fatigue fracture surface is also investigated using scanning electron microscopy observations and showed more ductile fatigue crack growth in the unECAPed specimen. Despite the increased tensile strength after ECAP, the ductility that controls low‐cycle fatigue behaviour has decreased. It is found that ECAP has resulted in a remarkable change in Paris regime parameters and a significant increase in fatigue crack growth rate. The decrease in fatigue crack growth resistance and ΔK_c after ECAP can be attributed to the decrease in alloy's ductility.     

Damage propagation mechanism in low-cycle creep fatigue of Cu–Cr–Zr alloy

This paper describes a characteristic damage propagation mechanism in low-cycle creep–fatigue of Cu–0.7Cr–0.09Zr (mass%), as investigated by creep–fatigue tests including strain controlled fatigue and stress-holding type creep, and following microstructural observations by scanning electron microscopy (SEM). The total stress-holding time until rupture in the creep–fatigue test was shorter than one-tenth of the rupture life in the simple creep test, and the rupture life of the specimen in the creep–fatigue test was shorter than half of that in the simple fatigue test. The SEM images suggest that the connection between fatigue crack propagating along grain boundaries and intergranular creep voids rapidly accelerates crack propagation.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable.

In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result.

The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation.

While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.