Role Of Stress State Research Articles

The Role of Stress States on the Hysteric Behavior of Expansive Soil under Multiple Drying-Wetting Cycles

Expansive soils in the field are typically exposed to cyclic wetting and drying due to climatic fluctuations and subjected to a variety of stress conditions in nature or when used as compacted layers for the construction of hydraulic barriers or waste disposal facilities. The hysteric behavior of the soil-water characteristic curve (SWCC) is a key parameter for understanding, modeling, and interpreting the unsaturated behavior of these soils under such conditions. This study investigates the effect of stress states on the hysteresis behavior of soil-water characteristic curves (SWCCs) for compacted highly expansive clay over a range of matric suction between 0 and 1500 kPa. Two test series were performed, the first test series investigated the effect of stress states on the hysteresis of SWCCs during a single drying-wetting (DW) cycle. The second test series studied the combined effect of stress applied and multiple drying-wetting cycles on the SWCC hysteresis. For the sake of comparison, the overall SWCC hysteresis due to drying-wetting cycles was quantified using the average degree of hysteresis in terms of volumetric water content (ADHθ). Furthermore, contributors to the observed hysteresis were defined using two newly proposed measures; namely, average degree hysteresis in terms of gravimetric water content (ADHw) and in terms of volume change (ADHe*). The outcomes of this study indicate that consideration of stress states on the hysteresis of SWCC for expansive clay is of great importance. The results show a dual trend for the variation of ADHθ with applied vertical stress. Furthermore, multiple DW cycles induced a significant reduction in the hysteresis (ADHθ) under low- and high-stress states up to a certain level of DW cycles, then, no further changes in the hysteresis trend were detected. It was also found that hysteresis loops under a low-stress state were concentric in shape while hysteresis loops for specimens under a high-stress state were non-concentric, with a downward shift in hysteresis loops with the increase in DW cycles.

The role of stress-state on the low cycle fatigue resistance of non-hydrided beta-treated Zircaloy-4

Reactor operation results in cyclic stresses that produce fatigue. Increased strain to failure under monotonic loading has been observed for Zircaloy-4 under a stress-state of shear defined by a low triaxiality. Triaxiality (η) is defined as the ratio of hydrostatic stress to von Mises stress and typically ranges from values of -0.33 for compression, to 0 for torsion loading and to 0.9 for notched, axial loaded specimens. Low Cycle Fatigue (LCF) testing of Zircaloy-4 was performed under uniaxial and torsion loading in air at room-temperature. Both smooth and notched specimens were tested to vary the value of positive triaxiality. An increase in LCF resistance with less notch sensitivity is observed under torsion loading. Cyclic stress-strain curves are developed that show a higher strain to failure and lower flow stress under torsion loading. This suggests a lower triaxiality produces increased strain to failure capability under both cyclic and monotonic loading that is shown to generally follow a Rice-Tracey model for void nucleation, growth and coalescence. Fractography examinations are used to understand the mechanism for crack extension.

Role of stress state on dynamic recrystallization behaviour of Ni during hot deformation: Analysis of uniaxial compression and plane strain compression

In the present work, the role of deformation stress state (uniaxial compression (UC) and plane strain compression (PSC)) on work hardening, softening and dynamic recrystallization (DRX) behaviour of Ni during hot deformation is investigated. Hot deformation was performed through UC and PSC to a von Mises equivalent true strain of 0.7 at 973 K, 1023 K and 1073 K at strain rates of 0.01 s−1, 0.1 s−1 and 1 s−1 in a thermomechanical simulator. Analysis of flow curves revealed rapid work hardening followed by softening in PSC samples. In UC samples, continuous work hardening at a slower rate was observed and at 1123 K, steady state was achieved. Analysis of the work hardening parameter ‘h’ and dynamic recovery parameter ‘r’ from the flow curve data showed that the h and r values of PSC deformed samples are higher than UC deformed samples. The DRX behaviour is dependent on temperature and strain rate and significant difference in DRX fraction was observed between UC and PSC samples deformed at strain rate of 0.1 s−1. Evaluation of microstructure from electron back scatter diffraction maps showed that DRX fraction is significantly higher in PSC deformed samples. Samples deformed through PSC have higher annealing twin density and number of twins per grain. It is reasoned that annealing twin formation is activated by the presence of shear stress component during PSC mode of deformation. Annealing twin formation and growth in turn facilitate DRX and explain the observed differences between UC and PSC modes of deformation.

The role of stress state and stress triaxiality in lifetime prediction of solder joints in different packages utilized in automotive electronics

This work presents an overview on the role of the stress state and stress Triaxiality Factor (TF, see Eq. (1)) in lifetime prediction of solder connections. According to various literature sources, the TF is one of the most important factors influencing initiation of ductile fracture (Bao and Wierzbicki, 2004; Davis and Connelly, 1959). It is widely reported that lifetime of the ductile materials decreases under hydrostatic tension when combined with high TF-values. Recent investigations report that the compressive hydrostatic stress state combined with a high shearing load and low (or even zero) TF-values also contributes to failure (Bao and Wierzbicki, 2005; Kweon, 2002; Nielsen and Tvergaard, 2011).Two package types, the Loss Free Packaging (LFPAK) and the Plastic Ball Grid Array (PBGA), were investigated by means of FE-simulation on Board- and System-Level, and presented damage prediction was compared with an experimental data. In the LFPAK and BGA solder joints the regimes of hydrostatic tension and compression during temperature cycles are evaluated and compared with distribution of equivalent von Mises stress, stress intensity (maximum shear stress) and triaxiality factor. The multiaxial effects were included in both, lifetime prediction and fracture location: the damage related variables, inelastic strain and energy density, were modified in an APDL post-processing routine based on the state of hydrostatic stress and corresponding TF for each time increment. Further, using a simplified simulation approach, the path of the crack propagation was calculated according to the distribution of the TF-modified and non-modified inelastic strain. It is shown that when including multiaxial effects by modification of damage related variables a better correlation between calculated and experimentally observed crack path is achieved.

The role of stress-state on the deformation and fracture mechanism of hydrided and non-hydrided Zircaloy-4

Zircaloy-4 was tested at room-temperature over a range of hydrogen content between 10 and 200 ppm, and stress-states between a triaxiality of −0.23 and 0.9. Triaxiality (η) is defined as the ratio of hydrostatic stress to von Mises stress and was controlled through use of select mechanical test specimen designs. Testing of smooth and notched tensile specimens (η = 0.33 to 0.9) results in an increase in the stress to initiate plastic deformation and a decrease in strain to failure at higher values of η. Increases in triaxiality are shown to have a more significant effect on reducing the strain to failure when the material is hydrided. Increases in strain to failure are observed at lower values of triaxiality for dual keyhole specimens (η = 0.1) and compression specimens (η = −0.17 to −0.23), with hydrided material showing much less decrement in strain to failure at these lower triaxialities. Examinations of microstructure are used to show that a change in mechanism for deformation and fracture with triaxiality can explain these differences in mechanical behavior and a model is developed to describe the observed changes in strain to failure with stress-state.

Fracture Characterisation of Reactor Core Graphite under Biaxial Loading

The role of stress state on the fracture properties of a quasi-brittle material are explored using reactor core Gilsocarbon graphite. The objective of the experiment was to study the initial propagation of cracks. Cruciform specimens have been tested by a biaxial flexural loading method. Pre-slots of 10 mm width and up to a quarter of the depth of the specimen were introduced at the centre of the specimen by electric discharge machining. The slots are located between two through-thickness holes, which are designed to guide crack propagation. A loading jig has been designed and built that allows a range of biaxial loading states to be applied by variation of the length of the loading arms. Clip gauges are used to measure the crack mouth opening displacements. Preliminary tests have studied the fracture of specimens under different loading conditions.

The role of stress state on the fracture toughness and toughening mechanisms of wrought molybdenum and molybdenum alloys

Rolling unalloyed molybdenum, molybdenum alloys, and Oxide Dispersion Strengthened (ODS) molybdenum into sheet produces microstructures with elongated, pancake shaped grains that can result in anisotropic mechanical properties. In this work, unalloyed molybdenum, molybdenum alloys, and ODS molybdenum are rolled to thinner sheet and then subjected to tensile and fracture toughness testing and examination of the toughening mechanism. The ductile laminate toughening mechanism observed for wrought molybdenum results from a lower toughness in the short-transverse orientation that leads to separation of the layers of sheet-like grains of the microstructure along the grain boundaries in the regions of stress concentration. This splitting of the microstructure results in the formation of ligaments of grains, or non-constrained laminates, that are stretched to failure under a plane stress-state with large amounts of plastic deformation. The thinner specimens exhibit higher fracture toughness values and lower Ductile to Brittle Transition Temperature (DBTT) values than for thicker specimens machined from thicker starting material from the same alloy. The lower constraint of the thinner specimens tested in this work results in higher toughness and lower DBTT values. The finer grain size, finer precipitate size, and state of plane stress achieved for the thinner sheet specimens appears to enhance the ductile laminate toughening to result in higher fracture toughness and lower DBTT values. The detrimental effect of crack initiation from brittle carbides, oxides, and second phases is also observed to be diminished under a stress-state of plane stress.

Deformation induced by ethanol substitution in normal and tension wood of chestnut (Castanea sativa Mill.) and simarouba (Simarouba amara Aubl.)

Wood deformation in the longitudinal and tangential directions induced by ethanol substitution and oven-drying was measured in normal wood (NW) and tension wood (TW) of chestnut (Castanea sativa Mill.) (TW with G-fibres) and simarouba (Simarouba amara Aubl.) (TW without G-fibres). The results show that with increased concentration of ethanol solution TW tends to contract, regardless of the presence or absence of G-fibres. In contrast, both NW and opposite wood expand at different rates in the longitudinal direction. These results are discussed and explained by the role of stress state at cell wall level.

The effect of specimen thickness and stress ratio on the fatigue behavior of polycarbonate

AbstractThe cyclic residual stresses ahead of fatigue cracks are known to play an important role in the fatigue fracture response of engineering materials. The size of these residual stresses are directly affected by the stress state of the component. In this paper, we examine the effect of stress state for fully compressive and fully tensile cyclic loading of polycarbonate. The role of stress state is studied using two different specimen thicknesses, one thickness will represent a near‐plane stress condition and the other will represent a near‐plane strain condition. In cyclic compressive loading, it will be shown that the near‐plane stress specimen with its larger zone of residual tension will exhibit enhanced crack saturation lengths. While for cyclic tensile loading, the larger zone of residual compression upon unloading will result in crack retardation for the same unloading stress intensity. A series of systematic experiments on the effects of mean stress on fatigue fracture is reported, and the results of the experiments are rationalized with the aid of scanning electron microscopy of the fracture surfaces. The results of this study have strong implications for both constant amplitude fatigue loading and variable amplitude fatigue loading as well as applicability to other engineering materials.

The role of stress state on hydrogen cracking in Fe-Si single crystals

While state-of-stress effects have long been known to play a major role in hydrogen embrittlement, there has been little direct evidence outside of thickness effects on threshold stress intensities. Also, there have been recent mechanisms proposed suggesting that enhanced plasticity effects may be more dominant than enhanced decohesion effects. If true, this would mandate different state of stress effects. The goal here was to settle this issue in at least one material, that being a semi-ductile, iron base single crystal. This was achieved by examining the crack initiation site under various loading and hydrogen interaction conditions in precracked Fe-3wt.%Si single crystals oriented in {001}<010> and {001}<110>. A strong dependency of hydrogen-induced cracking on the state of stress was evidenced by the consistent nature of a mid-section initiation stage and tunneling behaviour during crack propagation. This plane stress vs plane strain effect was quantified by analysing both small scale yielding and discretized dislocation computer solutions of the stress field. It was shown that the former did not provide sufficient stress and hydrogen in the local sense while the latter provided both high stress and hydrogen enrichment to cause decohesion. Finally, an assessment is made as to whether surface observations are helpful in sorting out the micromechanical aspects of cracking. This is addressed by examining surface plasticity with respect to the interior crack profile.

The role of stress state on the creep failure of 1%Cr0.5%Mo steel

Creep tests were carried out at 848 K on 1%Cr0.5%Mo steel tubes subject to internal pressures of either 17.7 or 20.3 MN m −2 with, and without, end loading. The steel was heat treated to produce a bainitic microstructure with a prior austenite grain size of 40 μm. Quantitative scanning electron fractography was used to assess the distribution and amount of cavitation on the grain boundaries. The measured area fractions of grain boundary cavitation are compared with that predicted from a knowledge of the grain boundary stresses, and this is used to explain the distribution of cracks within the variously-loaded, failed tubes. Furthermore, the measured creep life of the tubes is compared with that predicted using a grain boundary stress criterion.