Plastic pre-compression and creep damage effects on the fracture toughness behaviour of Type 316H stainless steel

The influence of inelastic damage in the form of plastic pre-strain and creep damage, on fracture toughness of Type 316H stainless steel has been examined. Creep damage has been introduced into the 8% pre-compressed material by interrupting creep crack growth tests. Comparisons have been made between the fracture toughness test results from the as-received, pre-compressed and creep damaged materials. Furthermore, the effects of creep crack discontinuities on the crack tip strain fields have been examined by digital image correlation measurements. Inelastic damage was found to reduce the fracture toughness of the material, with creep damage having more severe effects than pre-strain

The evolution of galaxy clustering since z=1 from the Calar Alto Deep Imaging Survey

We present results from an investigation of the clustering evolution of field galaxies between a redshift of z~1 and the present epoch. The current analysis relies on a sample of ~3600 galaxies from the Calar Alto Deep Imaging Survey (CADIS). The redshift distribution extends to z~1.1, with formal errors of sigma_z~0.02. Thus the amplitude of the three-dimensional correlation function can be estimated by means of the projected correlation function w(r_p). We developed a new method to overcome the influence of redshift errors on w(r_p). We parametrise the evolution of the clustering strength with redshift by a parameter q, the values of which give directly the deviation of the evolution from the global Hubble flow. From a subsample of bright galaxies we find q=-2.28+-0.31 for Omega_m=0.3, Omega_Lambda=0.7, that is a significant growth of the clustering strength between z=1 and the present epoch. From linear theory of dark matter clustering growth one would only expect q=-2. Moreover, we establish that the measured clustering strength depends on galaxy type: galaxies with early type SEDs (Hubble type: E0 to Sbc) are more strongly clustered at redshifts z>0.2 than later types. The evolution of the amplitude of the two-point correlation function for these ``old'' galaxies is much slower (q=-0.85+-0.82 for Omega_m=0.3, Omega_Lambda=0.7).Comment: 15 pages, 9 figures, accepted for publication by A&

Determination of plasticity following deformation and welding of austenitic stainless steel

Intergranular strain has been associated with high-temperature cracking of welded pipework in 316H austenitic stainless steel material used in nuclear power plant heat exchangers. In this study, neutron diffraction has been used to study the development of intergranular strains in plastically-deformed and welded 316H stainless steel. Measurements have been made of the intergranular strain evolution with increasing plastic strain in base material, and correlated with further measurements made in samples extracted from welded pipes, where the pipes were welded following plastic deformation to different levels of plastic strain. Strong tensile strain evolution was seen on the compliant 200 grain family. The results were correlated with various proxy measures of plastic strain, including hardness and diffraction peak width, and excellent agreement was obtained

Measurement of local creep properties in stainless steel welds

A high temperature measurement system for creep deformation based on the digital image correlation (DIC) technique is described. The new system is applied to study the behaviour of a multi-pass welded joint in a high temperature tensile test and a load controlled creep test at 545°C. Spatially resolved tensile properties and time dependent creep deformation properties across a thick section type 316 stainless steel multi-pass welded joint are presented and discussed. Significantly lower creep strain rates are observed in the HAZ than in the parent material which is attributed to the introduction of substantial plastic strain in the parent material on initial loading. The weld metal shows the fastest creep rates and a variation that appear to correlate with individual weld passes. The visual information provides not only the local creep strain distribution but also the reduction of area and true stress distribution based on strains measured in the transverse direction. The results demonstrate the capability of the DIC technique for full field measurement of displacement and strain at high temperature long term creep tests

An investigation of irregular crack path effects on fracture mechanics parameters using a grain microstructure meshing technique

Electronic version of an article published as Journal of Multiscale Modeling, Vol. 4, Iss. 1, atricle 1250001, 2012, http://dx.doi.org/10.1142/S1756973712500011 © World Scientific Publishing Company, http://www.worldscientific.com/worldscinet/jmmA sub-grain size finite element modelling approach is presented in this paper to investigate variations in fracture mechanics parameters for irregular crack paths. The results can be used when modelling intergranular and transgranular crack growth where creep and fatigue are the dominant failure mechanisms and their crack paths are irregular. A novel method for sub-grain scale finite element mesh consisting of multiple elements encased in ~50–150 μm-sized grains has been developed and implemented in a compact tension, C(T), mesh structure. The replicated shapes and dimensions were derived from an isotropic metallic grain structure using representative random sized grain shapes repeated in sequence ahead of the crack tip. In this way the effects of crack tip angle ahead of the main crack path can be considered in a more realistic manner. A comprehensive sensitivity analysis has been performed for elastic and elastic-plastic materials using ABAQUS and the stress distributions, the stress intensity factor and the J-integral have been evaluated for irregular crack paths and compared to those of obtained from analytical solutions. To examine the local and macroscopic graph path effects on fracture mechanics parameters, a few extreme cases with various crack-tip angles have been modelled by keeping the macroscopic crack path parallel to the axis of symmetry. The numerical solutions from these granular mesh structures have been found in relatively good agreement with analytical solutions

Effect of prior cold work on the mechanical properties of weldments

Heat exchanger units used in steam raising power plant are often manufactured using many metres of austenitic stainless steel tubes that have been plastically formed (bent and swaged) and welded into complex shapes. The amount of plastic deformation (pre-straining) before welding varies greatly. This has a significant effect on the mechanical properties of the welded tubes and on the final residual stress state after welding. The aim of the present work was to measure and understand the combined effects of pre-straining and welding on the properties and residual stress levels in stainless steel tubing weldments. Effects of plastic deformation were simulated by plastically straining three identical stainless steel tubes to different strain levels (0%, 10% and 20%). Then each tube was cut into two halves and welding back together. The variation in mechanical properties across weldments was measured using digital image correlation (DIC) and a series of strain gauges (SG). Residual stresses were measured on the 0% (undeformed) and 20% prestrained and welded tubes by neutron diffraction. It was found that the welding process had a marked effect on the tensile properties of parent material within 25mm of the weld centre-line. Evidence of cyclic strain hardening was observed in the tube that had not been pre-strained, and evidence of softening seen in the 10% and 20% pre-strained tubes. Macroscopic residual stresses were measured to be near zero at distances greater than 25 mm from the weld centre-line, but measurements in the 20% pre-strained tube revealed the presence of micro residual stresses having a magnitude of up to 50 MPa

The role of prior fabrication and in service thermal ageing on the creep life of AISI Type 316 stainless steel components

A significant factor in creep life of AISI Type 316H austenitic stainless steel components such as headers, and tubes is the initial microstructure. These components typically have a comparable specified composition but different thermo-mechanical fabrication histories. The variations in composition within the nominal range result in initial microstructures which become increasingly divergent during ageing. In this paper we explore effect of these contributions on the long term service aged microstructure and discuss the resulting impacts on the overall creep life of these components. The microstructure of specific regions has been characterised with a range of techniques, including high resolution transmission electron microscopy imaging and chemical analyses undertaken using a JEOL ARM instrument operating at 200 KeV fitted with an energy dispersive spectrometer. This provides a unique identification of the service aged precipitates and the distribution of alloying and impurity elements. The results are discussed with respect to the initiation of creep cavities and the associated creep damage accumulation in the context of lifetime assessment of these AISI Type 316H austenitic stainless steel boiler components

The effects of long range residual stress, elastic follow-up and applied load on creep crack incubation and material toughness

Creep crack incubation of Type 316H stainless steel at 550 °C is explored in this article. Fracture mechanics specimens, subjected to combinations of residual and applied loads and in the presence of elastic follow-up, are tested. The design of two new test rigs is described. The rigs introduce planned levels of elastic follow-up together with combined residual and applied loading conditions to the specimens. A series of high-temperature elastic–plastic and elastic–plastic–creep experiments are undertaken to compare the experimentally determined values of elastic follow-up with the theoretical values. A further series of fracture mechanics tests are performed to measure creep crack incubation and material toughness for samples subjected to constant load and for tests under combined loading with elastic follow-up. It is demonstrated that for tests subjected to the same initial reference stresses, longer incubation times are attained for elastic follow-up tests compared to constant load tests. Also, combined loading tests exhibit longer creep crack incubation times based on the same measured material toughness obtained from constant load tests. This suggests that not all the available strain energy provided by combined loading to a specimen at high temperature contributes to creep crack incubation. </jats:p