Effect Of Stress Ratio Research Articles

Functional Behavior of Pseudoelastic NiTi Alloy Under Variable Amplitude Loading

Abstract Cyclic loading of superelastic NiTi shape memory alloy (SMA) causes forward and reverse austenite–martensіte transformations, and also increases the volume of stabilized martensite. This appears in the change of stress-strain curve form, the decrease of dissipation energy, and increase of residual strain, that is, named transformation ratcheting. In real structures, the SMA components in most cases are under the action of variable amplitude loading. Therefore, it is obvious that the loading history will influence the functional fatigue. In the present work, the effect of stress ratio on the functional properties of superelastic NiTi shape memory alloy under variable amplitude loading sequence with two blocks was investigated. The studies were carried out under the uniaxial tension of cylindrical specimens under load-full unload and load-part unload. The change of residual strain, strain range, dissipation, and cumulative dissipation energy density of NiTi alloy related to load sequences are discussed. Under both stress ratios, the residual strain in NiTi alloy is increased depending on the number of loading cycles on the high loading block that is similar to the tests at constant stress or strain amplitude. An unusual effect of NiTi alloy residual strain reduction with the number cycles is found at a lower block loading. There was revealed the effect of residual strain reduction of NiTi alloy on the number of loading cycles on the lower amplitude block. The amount of decrement of the residual strain during a low loading block is approximately equal to the reversible part of the residual strain due to the stabilized martensite. The decrease of the residual strain during the low loading block is approximately equal to the reversible part of residual strain due to the stabilized martensite. A good correlation of the effective Young’s modulus for both load blocks with residual strain, which is a measure of the volume of irreversible martensite, is observed.

Monotonic and Cyclic Behavior of Salt-Encrusted Flat (Sabkha) Soil

Sabkha (salt-encrusted flat) soils are problematic because they lose strength due to wetting, and they have liquefaction potential. These soils are spread in North Africa, Australia, and most of the Eastern Province of Saudi Arabia. Owing to a lack of experimental studies, the cyclic behavior of sabkha soils is relatively unknown. The monotonic and cyclic behaviors of sabkha soil were studied based on effective stress (50, 100, and 150 kPa) and cyclic stress ratio (CSR) (0.15, 0.35, and 0.65) using cyclic triaxial and bender element tests. Results indicate that the sabkha exhibits ductile behavior with the cohesion value of 9.33 kPa and a friction angle of 33°. The maximum shear moduli are 18,900, 49,500, and 63,500 kPa for effective confining pressures of 50, 100, and 150 kPa, respectively. Furthermore, the shear modulus tended to decrease with shear strain for different cyclic stress ratios. On the other hand, the damping ratio depends on the level of the cyclic stress ratio. At a cyclic stress ratio of 0.15, the damping ratios remained constant with shear strain. For a cyclic stress ratio of 0.65, the damping ratios decreased with shear strain. However, at a cyclic stress ratio of 0.35, the damping ratio varied with shear strain depending on effective stress.

Numerical Prediction of the Fatigue Crack Growth Rate in SLM Ti-6Al-4V Based on Crack Tip Plastic Strain

This study presents a numerical model to predict the fatigue crack growth (FCG) rate in compact tension specimens under constant amplitude cyclic loadings. The material studied is the Ti-6Al-4V titanium alloy produced by selective laser melting, which was submitted to two different post-treatments: (i) hot isostatic pressing, and (ii) heat treatment. The developed finite element model uses the cumulative plastic strain at the crack tip to define the nodal release. Two different FCG criteria are presented, namely the incremental plastic strain (IPS) criterion and the total plastic strain (TPS) criterion. The calibration of the elasto-plastic constitutive model was carried out using experimental data from low cycle fatigue tests of smooth specimens. For both proposed crack growth criteria, the predicted da/dN-ΔK curve is approximately linear in log-log scale. However, the slope of the curve is higher using the TPS criterion. The numerical predictions of the crack growth rate are in good agreement with the experimental results, which indicates that cyclic plastic deformation is the main damage mechanism. The numerical results showed that increasing the stress ratio leads to a shift up of the da/dN-ΔK curve. The effect of stress ratio was dissociated from variations of cyclic plastic deformation, and an extrinsic mechanism, i.e., crack closure phenomenon, was found to be the cause.

Compressive strength of collar plate reinforced SHS T-joints: Effect of geometrical parameters and chord stress ratio

The collar plate (CP) is applicable for the reinforcement of both new designed and in-service joints, yet the design approach for the reinforcement requires further investigation. The current paper investigates the effect of various geometrical parameters as well as chord stress ratios on the compressive capacity of the collar plate reinforced (CPR) square hollow section (SHS) T-joints. The finite element (FE) models of the CPR SHS T-joints are built and validated using experimental data. Extensive parametric studies are carried out to evaluate the influence of parameters such as the brace to chord width ratio, the width/length and thickness of the CP, as well as the chord stress ratio. The brace to chord width ratio and the CP thickness are identified as the key parameters, and three typical failure modes are observed. Finally, bearing mechanism of CPR T-joints is discussed and design equations are proposed based on the failure modes. The failure-mode-based equations agree well with both parametric data and orthogonal analysis results, which provides reference for the practicing engineers.

Effects of tensile stress ratio and amplitude on tension-torsion fretting-corrosion-fatigue behaviors of non-perpendicularly crossed steel wires

Steel wires, non-perpendicularly crossed in the hoisting rope, are subjected to tension-torsion fretting-corrosion-fatigue during coal mine hoisting. The tensile stress ratio and amplitude greatly affects tension-torsion fretting-corrosion-fatigue behaviors of non-perpendicularly crossed steel wires. Hysteresis loops of tangential force versus displacement amplitude and torque versus torsion angle, and coefficient of friction were presented to explore dissipated energy and friction characteristics, respectively. Wear scar profiles were observed by scanning electron microscope and white light interferometer to discuss wear mechanisms. Crack propagation morphologies of steel wires were observed using X-ray computed tomography to obtain crack propagation rates. The fractal theory was innovatively introduced to quantitatively demonstrate surface complexities of wear scars. Meanwhile, by the innovative introduction of acceleration factor and establishment of tension-torsion fretting-corrosion-fatigue life estimation model, influence degrees of tensile stress ratio and amplitude on the crack propagation rate, tension-torsion fretting-corrosion-fatigue life and electrochemical corrosion rate were explored. X-ray photoelectron spectroscopy was employed to analyze element compositions and valence distributions of reaction products of wear scars. The electrochemical analyzer was used to investigate electrochemical polarization curves and impedance spectroscopy. The results show that increases of tensile stress ratio and amplitude cause overall increases in the dissipated energy, coefficient of friction, maximum wear depth, crack propagation rate, influence degree on the life, fractal dimension and surface complexity. Experimental results coincide well with results from estimation models. The electrochemical corrosion damage decreases with increasing stress ratio and with decreasing stress amplitude.

The Effect of Stress Ratios on the Very High Cycle Fatigue Behavior of 9%Cr Turbine Steel at 630 °C.

Effects of the stress ratio on the very high cycle fatigue behaviors of 9%Cr turbine steel have been investigated at 630 °C. The experimental results show that the S–N curve has a continuous downward trend and has no fatigue limit with the increasing in the cycles at 630 °C. Meanwhile, according to the analysis of microstructure, there are two failure modes that were observed at different stress ratios (R = −1 and 0.1), including surface crack failure and internal crack failure, respectively. Besides, the theoretical threshold value of the crack growth is compared with the calculated value of the fracture surface. To decrease the difference between the threshold value of internal crack initiation and the corresponding theoretical value, a new model for the crack growth threshold of the interior-induced fracture at different stress ratios is proposed and discussed.

Influence of different strain rates on hydro-mechanical behaviour of reconstituted unsaturated soil

The hydro-mechanical behaviour of a reconstituted unsaturated soil under different suctions and strain rates was studied through various rate-controlled unsaturated/undrained triaxial tests. The fully saturated reconstituted specimens were desaturated to four different initial suctions (s0 = 0, 100 kPa, 200 kPa and 300 kPa) and then triaxially sheared (conventional triaxial compression) at three different strain rates in undrained conditions ( $$\dot{\varepsilon }_{1} = 0.001$$ h−1, 0.01 h−1, and 0.1 h−1). The observed hydro-mechanical behaviour during shearing including the volumetric strain, deviatoric stress, degree of saturation and suction is presented and discussed in this paper. The results indicate that when the strain rate rises at the given initial suctions (or pore water pressures), the maximum deviatoric stress (qmax), critical net stress ratio (M) and critical state suction (sc) increase but the degree of saturation (Src) and volumetric strain at the critical state (evc ) reduce. The critical effective stress ratio (M′) is not dependent on the strain rate for saturated and unsaturated samples. The critical state lines for unsaturated soils with the constant strain rates are parallel with each other in the e − lnp′ space.

Deformation behaviors of hydrate-bearing silty sediment induced by depressurization and thermal recovery

Hydrate dissociation during gas production would lead to severe deformation of hydrate-bearing sediments and further induce the damage of the production well and possible reservoir instability. Predicting the deformation response of hydrate-bearing silty sediments during hydrate dissociation enable appropriate design and anticipate risk due to extraction process of gas from silty reservoirs which exist widely and stores large amounts of hydrates. To that end, this paper designs a series of triaxial tests, which were followed by thermal injection or depressurization, on artificial hydrate-bearing silty sediments. Deformation response of silty sediments during gas production is analyzed, and the respective effect of hydrate dissociation and pore-pressure-drop on the volume deformation of hydrate-bearing silty sediments is predicted. The results suggest that hydrate-bearing silty sediments are not broken during the depressurization process; instead, severe deformations occur. The axial and volume deformations of hydrate-bearing silty sediments are governed mainly by the effective stress, the effective stress ratio and the hydrate saturation. The mean effective stress plays the main role in the volume deformation induced by pore-pressure-drop, and the effective stress and effective stress ratio both plays important role in the volume deformation induced by hydrate dissociation. In contrast, collapse occurs for all hydrate-bearing silty sediments with various hydrate saturations when the thermal recovery method is applied.

Fatigue life prediction of woven composite laminates with initial delamination

AbstractAn engineering approach for fatigue life prediction of fibre‐reinforced polymer composite materials is highly desirable for industries due to the complexity in damage mechanisms and their interactions. This paper presents a fatigue‐driven residual strength model considering the effect of initial delamination size and stress ratio. Static and constant amplitude fatigue tests of woven composite specimens with delamination diameters of 0, 4 and 6 mm were carried out to determine the model parameters. Good agreement with experimental results has been achieved when the modified residual strength model has been applied for fatigue life prediction of the woven composite laminate with an initial delamination diameter of 8 mm under constant amplitude load and block fatigue load. It has been demonstrated that the residual strength degradation‐based model can effectively reflect the load sequence effect on fatigue damage and hence provide more accurate fatigue life prediction than the traditional linear damage accumulation models.

Effect of Stress Ratio on Fatigue Behaviour of Non-Crimp Fabric Composites at Room and Elevated Temperatures

The fatigue behaviour of a biaxial carbon/epoxy Non-Crimp-Fabric (NCF) composite is evaluated at room temperature and 130o C for automotive applications. A new specimen geometry is developed allowing three stress ratios R = 0.1, R = −1 and R = 10 to be tested at 130o C without the use of anti-buckling fixtures for axial loadings. The three stress ratios show good agreement in fatigue life using an equivalent stress amplitude. The fracture surfaces from failed fatigue test specimens are compared to those of monotonic loading to evaluate the changes to failure modes under the different loading conditions. Tensile dominated loading such as uniaxial tension and fatigue testing with R = 0.1 result in delamination of the NCF material coupled with fibre failure along the loading direction as the primary failure modes. Compressive fatigue failures at 130 °C occur at an inclined angle through thickness due to a shearing process along with fibre kinking. The matrix material exhibits more ductile characteristics at 130o C altering the fracture surfaces at elevated temperature.

Characteristics and life expression of fatigue fracture of G105 and S135 drill pipe steels for API grade

Characteristics and life expression of fatigue fracture of drill pipe steels for API G105 and S135 grade were investigated experimentally. S135 steel with higher tensile strength has higher fatigue strength and fatigue limit than that of G105 steel with lower tensile strength. A modified model for the S-N data, revealing the correlation between the fatigue life and fatigue limit, equivalent stress amplitude reflecting the effect of stress ratio, is suggested for predicting the fatigue damage. The fatigue limit and the fatigue strength coefficient are correlated to the tensile properties. The mechanism of the fatigue fracture was examined by scanning electron microscopy.

Behavior of Steel Fiber-Reinforced Concrete under Biaxial Stresses

Biaxial behavior of various types of concrete is essential to be considered in construction design because construction structures normally experience multiaxial stresses rather than uniaxial stress. Research on biaxial behavior of steel fiber-reinforced concrete (SFRC) has been conducted in the past decades. Most of the research, however, is only limited to biaxial compression, whereas information regarding biaxial tension and biaxial tensioncompression on SFRC is relatively scarce. This study presents a simple biaxial experimental setup to investigate the biaxial behavior of SFRC with 0.5, 1.0, and 1.5% steel fiber under biaxial tension and biaxial tension-compression. It is found that the smaller stress ratio enhanced the deformability and tensile capacity of SFRC under biaxial tension-compression, whereas the effect of stress ratio on biaxial tensile behavior of SFRC is negligible. The addition of steel fiber eventually enhanced the concrete strength by 15 to 41% under tension-compression compared with plain concrete.

Effects of different stress ratios on fatigue crack growth of rice husk fibre-reinforced composite

Polymers and polymer composites are susceptible to premature failure due to formation of cracks and microcracks throughout their service. Evolution of cracks and microcracks induces catastrophic material failure. Hence, detection/diagnostics, as well as effective repair of cracks and microcracks, is essential to ascertain performance reliability, cost efficiency, and safety for polymer structures. Upon adopting the Paris relation for empirical data, this study incorporated a mathematical model after weighing in cracks initiation and propagation in rice husk (RH) polymer structures, along with the several viable techniques for life prediction and fracture observation. The specimens contained 35% RH fibres and were produced via an injection molding process. Fatigue cracks were evaluated for stresses between 80 and 90% from ultimate tensile strength (UTS) for R = 0.1, 0.3, and 0.5. The outcomes signified that the increment in R value enhanced the growth rate of the crack. Upon elaborating the fracture analysis, this study discusses in detail both fracture mechanics and formation.

Numerical appraisal of rock mass anisotropy effect on elastic deformations of a circular tunnel

The paper describes effects of anisotropic mechanical properties of rock masses on elastic behaviour of a circular tunnel under both hydrostatic and non-hydrostatic in situ stress states. This study is based on field data obtained from two actual case studies. In both cases, the rock masses have transversely isotropic structures. Hence, a 2D finite element modelling based on the equivalent continuum approach is used for the analysis. The tunnel deformation behaviour has been investigated for both isotropic and transversely isotropic conditions. To evaluate the degree of anisotropy of rock mass, an “anisotropy index” and a “normalized displacement ratio” have been defined. The effect of orientation of the isotropic planes is further investigated. The results show that in a hydrostatic stress state, the maximum displacement always occurs in a direction perpendicular to the isotropic planes. In this case, three empirical equations have been developed to compute the normalized displacement ratio, the deviation, and the direction of displacement vector at any arbitrary point on the tunnel periphery. The results further show that if the anisotropy index increases, the displacement difference (the difference between the maximum and the minimum displacements) on the tunnel walls increases too. For the non-hydrostatic stress state, simultaneous effects of stress ratio, anisotropy index, and orientation of isotropic planes on normalized displacements have been investigated. In this case, the location of maximum displacement inclines towards the direction of major principal stress. This effect is more noticeable when the isotropic planes are oriented at an angle of 90° relative to the direction of the major principal stress. The paper also provides an empirical equation to determine the location of maximum displacement on the tunnel walls. Finally, the practical application of the results is further illustrated by an actual case study.

Effects of the Deviatoric Stress Ratio on the Coal Failure and Permeability During Deep Mining

Practices show that the triaxial stress change of mining-disturbed coal deeply affects its failure and permeability characteristics and easily cause the coal and rock dynamic disasters during deep underground mining process. The combined mechanic-damage-permeability experiments of gas-bearing coal were carried out under the mechanical paths of conventional triaxial compression (CTC), fixing axial pressure and unloading confining pressure (FAPUCP) and loading axial pressure and unloading confining pressure (LAPUCP). Then the effects of the deviatoric stress ratio (DSR) on the damage and permeability characteristics of coal were analyzed. The results proved that the distinguishing deformation and strength characteristics of coal were showed due to the different mechanical paths and the coal specimens were converted from brittle deformation to plastic deformation with the effect of higher confining pressure. The coal specimens of the three mechanical paths generated the main shear failure, tensile-shear failure and ‘X’ conjugate shear failure, respectively, and the fracture angles decreased in order. The damage and permeability of coal specimens could be more and more severe in the order of CTC, FAPUCP and LAPUCP paths, which was due to the different DSR. Thus, the deep mining-disturbed coal of lager DSR is seriously prone to cause the coal and rock dynamic disasters. The corresponding measures should be taken to reduce the DSR of mining-disturbed coal and then avoid the dynamic disasters.

Effect of load stress ratio on nominal and effective notch fatigue strength assessment of HFMI-treated high-strength steel cover plates

The International Institute of Welding published IIW-recommendations for the HFMI-treatment, which covers fatigue assessment procedures and quality assurances guidelines. This paper contributes to the validation of the detrimental effect of high R-ratios on HFMI-treated cover plate specimens made of high-strength steel S700. Applying the nominal stress approach, the recommended S/N-curves agree well to the experiments in both cases, R = 0.1 and R = 0.5. In case of the effective notch stress approach, only for R = 0.1 a sound agreement is observed, whereas for R = 0.5 a more pronounced reduction of the fatigue strength may be considered within the recommendations to ensure an overall conservative assessment.

Constitutive Relationship Proposition of Marine Soft Soil in Korea Using Finite Strain Consolidation Theory

This paper proposes representative constitutive relationship equations of dredging and reclamation soft soil in Korea. The marine soft soils were sampled at 23 dredged-reclaimed construction sites in the Busan, Gwangyang, and Incheon regions in Korea; then, laboratory tests were carried out. The consolidation property was classified as LL = 60% for Busan and Gwangyang marine soft soil and LL = 30% for Incheon marine soft soil by conducting basic physical property tests and consolidation tests. Busan soft soil showed a slightly higher consolidation settlement property than Gwangyang soft soil. Incheon soft soil showed the lowest consolidation settlement property among the three regions. In particular, 77 consolidation simulations were carried out at a high void ratio using the centrifugal experiment to realize high water content and in-field stress conditions. The constitutive relationship equations of each of the 23 specimens were analyzed with regard to the void ratio–effective stress and void ratio–permeability coefficient through the back analysis of finite consolidation theory from the experimental results. The constitutive relationship equation for Korean soft soil was determined to be a reasonable power function equation. The representative constitutive relationships for soft soils in the three regions were estimated using six equations, which were classified by physical and consolidation properties. The representative constitutive equations were compared to those in previous studies on high void ratio conditions of marine soft soil, and the results showed a similar range.

Evaluation of dynamic compaction to improve saturated foundation based on the fluid-solid coupled method with soil cap model

Abstract This paper presented a dynamic fluid-solid coupled finite element (FE) method incorporating the soil cap yield hardening model to analyze the improvement on saturated foundation under dynamic compaction (DC). Biot’s dynamic u–U–p formulation was employed to describe the coupling of pore fluid and solid phases, which was discretized into finite elements by application of the Galerkin method and viscous Cartesian connectors. The proposed numerical model showed reasonably good agreement with the existing analytical solutions of one-dimensional transient loading problems and field measurement of dynamic compaction. The effects of groundwater table and soil permeability were examined via the development of excess pore water pressure, effective soil stress and void ratio. Results showed that groundwater table had a significant effect on the foundation improvement by DC. Higher groundwater table resulted in larger excess pore water pressure, and effective soil stress was not found to develop under the groundwater table during the compaction process. The decrement in the void ratio was only limited to soils located above the groundwater table, and a critical depth of groundwater table existed for the DC reinforcement in fine soil foundation, which was suggested to be 6 m for the DC reinforcement with the tamping energy of 2500 kN·m per blow. Four soil permeability coefficients of k = 10−2 m/s, 10−3 m/s, 10−5 m/s and 10−7 m/s were chosen in the parametric analysis, which represented gravel, coarse sand, silt and silty clay, respectively. Higher permeability resulted in lower excess pore water pressure and larger decrease in void ratio. However, the saturated foundation was not suitable to reinforce by dynamic compaction, no matter how much the soil permeability was. Compared to the increase in soil permeability, it was more crucial to lower the groundwater table before implementing dynamic compaction. Moreover, the gravel columns were preferred to be used to accelerate the drainage of groundwater than the sand columns.

Characterization of pore pressure, fracture pressure, shear failure and its implications for drilling, wellbore stability and completion design – A case study from the Takouazet field, Illizi Basin, Algeria

This work presents the assessment of overburden stress, pore pressure and fracture pressure distribution in the Paleozoic stratigraphic units in the Takouazet field, eastern Illizi basin, Algeria. We studied a vertical exploration well, where significant hydrocarbon discoveries have been established in Devonian and Ordovician sandstone reservoirs. Overburden stress interpreted from density log has an average 1.07 PSI/ft (24204.04 Pa/m) gradient. Pore pressure derived from sonic log reveals that hydrostatic conditions prevail in Devonian and Ordovician units, whereas abnormal pore pressure with a maximum 0.66 PSI/ft (14929.60 Pa/m) gradient was recorded against the marine Silurian shale. Fracture pressure was modeled using effective stress ratio models as well as a Poisson's ratio based approach. Rock property based fracture pressure estimate is more sensitive to lithological variations and hence this estimate was used for determining safe drilling mud window and casing placements. The estimated pore pressure and fracture pressure were calibrated with direct downhole formation pressure measurements and leakoff test (LOT)measurements, respectively. Shear failure using Mohr- Coulomb rock failure criterion detected the minimum allowable mud pressure to avoid wellbore instability issues and the result was validated with caliper log observations. It indicated that solid production from the reservoir is very likely during initial production from the reservoir and a solid control mechanism has been suggested. Also widening of breakouts and deteriorating wellbore interpreted from time lapse formation imaging logs strongly suggest against open-hole completion. Based on interpreted pressure gradients and image log analyses, optimum drilling (mud and casing design) and completion policies were recommended for safe and successful operational planning.

Influence of crack driving force on correlating stress ratio effects in fatigue crack growth rate of a nickel base super alloy IN720

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material with nullified stress ratio effects. Several types of crack driving force parameters have been proposed in the past to correlate stress ratio effects on fatigue crack growth rates in various materials. In the present investigation, an effort was made to use four different crack driving force parameters to eliminate stress ratio effects in a nickel base super alloy IN720. Constant amplitude fatigue crack growth rate of Inconel 720 (IN720) was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. All the tests were performed at room temperature and in laboratory air condition in a 100-KN servo-hydraulic test machine using compact tension specimen and following ASTM test standard procedures. As expected, increasing the stress ratio increased crack growth rates and decreased threshold stress intensity factor range, ΔKth. The conventional crack growth rate, da/dN versus stress intensity factor range, ΔK data was modified and re-plotted as a function of four different crack driving force parameters, viz., (a) Kujawski’s K*, (b) two parameter ΔK*, (c) Walker’s ΔKw and (d) Foreman’s ΔKf. It was observed that Kujawski’s crack driving force parameter, K*, was correlating stress ratio effects better than all other models in all the three regimes of crack growth rates in this material. Further, all these models were employed to predict fatigue crack growth behaviour under a truncated FALSTAFF spectrum load sequence. It was observed that the total fatigue crack growth life estimated varied from about 25 blocks to about 75 blocks. Thus, the use of appropriate crack growth model appears to influence accuracy of life prediction under spectrum loads.