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In many fields some structural materials are subjected to strain aging, which gives rise to inhomogeneous yielding such as Piobert-Lüders’ bands and Portevin — Le Chatelier instabilities. These phenomena occur in steels containing interstitial elements in solid solution such as carbon or nitrogen which segregate to dislocations during aging and pin them. Strain aging in ferritic steels induces a loss in ductility and fracture toughness at mild temperatures [ 1 ]. To study the influence of strain aging on ductile tearing, it is possible to simulate plastic strain instabilities using a local constitutive equation [ 2 ] taking into account the interaction between solute atoms and dislocations responsible for strain aging [ 3 ]. This study first requires the identification of the model parameters, which needs to correlate the experimental values obtained from tensile tests to the intrinsic behavior of the material and then requires a good interpretation of the upper and lower yield stresses in the presence of static strain aging. So we have performed a numerical study of the Lüders’ plateau featuring the influence of several parameters such as meshing, boundary conditions or behavior law on the nucleation and growth of Lüders’ bands. The influence of the local band development on the plateau has been investigated. Correlations between the plateau stress level and the local behavior will be discussed.

Since the first investigations four decades ago, a large number of experiments on ferritic steels has confirmed the existence of a warm pre-stress (WPS) effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following the application, at a higher temperature, of a stress intensity factor (SIF) which exceeds the fracture toughness of the virgin material at low temperature. These experiments allowed for the establishment of the so-called ‘conservative principle’, which states that no fracture will occur if the applied SIF decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. In structural integrity assessments involving a prior overload or a thermal transient, such as that of a nuclear pressure vessel subjected to a pressurized thermal shock (PTS) consecutive to a loss of coolant accident (LOCA), such a principle is of great importance in the evaluation of the safety margins. Three main reasons have been advanced to explain the WPS effect: the blunting of the crack tip at high temperature, the formation of high compressive stresses on elastic unloading, and a change in the cleavage fracture micromechanisms induced by plastic deformation. While all these factors certainly contribute to the effective toughness enhancement following WPS, their relative incidence on the fracture risk is not easily established. In this paper, we choose to mainly focus on the cleavage fracture micromechanisms following WPS, as a first step towards better quantifying the individual contributions of crack tip blunting and residual stresses.

Occurrence of intergranular initiation of brittle fracture could be taken as significant simple measure of the negative influence of impurities content. This micromechanism of failure has been usually responsible for strong decrease of mechanical properties, anomalous fracture behaviour or, at least, comparably larger and in structural applications unacceptable data scatter.

Applications of Mg-based alloys at elevated temperatures are limited by the low melting point of Mg. This difficulty can be overcome by an addition of rare earth elements. A number of novel promising Mg-based hardenable alloys with high creep resistance at elevated temperatures have been developed, e.g. Mg-Gd, Mg-Mn-Sc etc. Despite the favorable strength and thermal stability, a disadvantage of these alloys consists in a low ductility, which is not sufficient for industrial applications. Grain refinement is known as a way how to improve ductility. It has been demonstrated that an extreme grain size reduction can be achieved by methods based on severe plastic deformation (SPD). In the present work we used high pressure torsion (HPT), which is the most efficient in grain size reduction among the SPD-based techniques, for preparation of selected Mg-based alloys with ultra fine grained (UFG) structure. Microstructure investigations and defect studies of HPT deformed UFG Mg-based alloys are presented in this paper. The extraordinary properties of UFG materials are closely related with defects (grain boundaries, dislocations) introduced by HPT. Positron lifetime (PL) spectroscopy [ 1 ] is a well-established non-destructive technique with high sensitivity to open volume defects. It enables identification of the defect types present in the material studied and determination of defect densities. Thus, PL spectroscopy represents an ideal tool for defect studies of UFG materials. In the present work PL spectroscopy was combined with X-ray diffraction (XRD), microhardness measurements, and direct observations of microstructure by TEM.

Ab initio calculations of elastic moduli and theoretical uniaxial strength of composite lamina having continuous nano-fibre reinforcements are performed using pseudo-potential plane-wave code of Kresse et al. [ 1 ]. Obtained results are used to verify validity of macro-scale empirical relations for composites (rules of mixtures) [ 2 ] on the nano-scale. All quantities are computed from the dependence of crystal energy on a suitable deformation parameter. Results for tungsten nano-fibres in niobium matrix will be presented as a particular example of the ab-initio analysis.

When subjected to severe shear deformation by ECAP (Equal Channel Angular Pressing), microstructure of Al2024 becomes extremely refined. To measure the strength of this ultra fine grained metal, the small punch (SP) testing method was employed as a substitute for the conventional uniaxial tensile testing since the size of metal bar processed by ECAP were limited to 12 mm in transverse direction as shown in Fig. 1. The small punch tests were performed with specimens in longitudinal and transverse directions of Al 2024 ECAP metal. For comparing the strength values with those assessed by SP tests, the uniaxial tensile tests were also conducted with specimens in longitudinal direction, in which specimen sampling is possible. Failure characteristics were investigated using scanning electron microscopy (Fig. 2). The surfaces of the tested SP specimens showed that failure mode was shear deformation (Fig. 3). Based on this observation it was argued that the conventional equations proposed for assessing the strength of the material by the SP test were improper to assess those of Al2024 ECAP metal.

Fatigue crack initiation stage is competitive and requires a full screening of potential initiation sites. In a more comprehensive view, the role of the mechanical driving force is generally broadened, thus, deformation/environment interactions become significant. Besides better fundamental understanding as related to the fatigue process other considerations emerged like the distinction between initiations or propagation controlled processes. This has direct implications on the fatigue life assessment. The current phenomenological study selected polycrystalline, pure copper of 99.98 %, a typical FCC model system in the intensive volume of fatigue investigation. The material with various impurity traces included at the most, aluminum of 100ppm. After heat treatment microstructure of 50–80 µm grain size was tested at ambient temperature. Standard mechanical properties were determined and strain controlled fatigue tests were performed. The latter in the range of about 10^−4 to 10^−3 plastic strain amplitudes. The specimens consisted of uniform and cylindrical geometry, 3mm in diameter and tension/compression cyclic tests of load ratio, R=–1 at 3Hz were conducted. Four fold interrelated information levels were searched namely, mechanical response, low energy dislocation structure development, slip upset and crack initiation tracking. These were achieved by closed loop strain controlled devices, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Micro Probe Microscopy (MPM). In addition crack on a micron scale resolution has been tracked by one stage replication technique. For some isolated cases crack initiation tracking was assisted by Acoustic Emission (AE) spectra. The slip upset was observed by centering on fine scale features that enabled a quantitative approach to be analyzed by defining the local strain.

In order to solve the global warming problem, development and commercialisation of fuel cell (FC) systems are being promoted. In the FC systems, many stainless steels are used for the components such as the liners of ultra-high pressure vessels, piping, bearings, and springs. Such components are directly exposed to hydrogen environments under cyclic loading. It has been reported that hydrogen degrades the mechanical properties, especially ductility of metallic materials [ 1 – 3 ]. Moreover, the degradation of fatigue strength of metallic materials due to hydrogen intrusion is a matter of concern from a practical point of view. In our previous investigations [ 4 – 7 ], hydrogen was artificially (electro-chemically) charged into the specimens of several candidate materials, and it was revealed that hydrogen affects the slip band morphology and fatigue crack growth behaviour. In this study, hydrogen intrusion and fatigue crack growth behaviour in stainless steels exposed to high pressure hydrogen environments were investigated at room temperature and in laboratory air to clarify the effect of the intruded hydrogen on the fatigue crack growth.

The adhesive fracture energy, G _c, of adhesive joints may be readily ascertained from linear-elastic fracture-mechanics (LEFM) methods, and indeed a British Standard (BS7991-2001) now exists for the LEFM Mode I value, G _Ic, largely as a result of the efforts of the European Structural Integrity Society (ESIS) TC4 Committee, as described by Blackman and Kinloch [ 1 ]. Notwithstanding, the LEFM test specimens are relatively complex and expensive to make and test, and many industries would far prefer to deduce the value of G _c from the very common and widely used ‘peel test’.

The eXtended Finite Element Method (XFEM) has been successfully used for several years for the numerical analysis of cracked structures under statically and later under dynamically loadings. Based on Partition of Unity Method, the XFEM was developed at Northwestern University (Moes et al. [ 1 ]) firstly as a method for analysing crack growth without remeshing, using special enrichment functions to model discontinuous displacement fields. Since, the method was continuously improved and applied to various domains of fracture mechanics. The dynamic crack propagation is one of them and an important contribution for its modelling by XFEM was given by Belytschko et al. [ 2 ].