Effect of Shear/Axial Stress Ratio on Multiaxial Non-Proportional Loading Fatigue Damage on AISI 303 Steel

In this paper, we investigate the cyclic response of AISI 303 stainless steel subjected to non-proportional loads with different amplitude ratios between shear stresses and normal stresses. Based on the experiments, a relationship between the proportional reference load and a varied range of non-proportional loads was established. To achieve this objective, an experimental program was implemented to evaluate the non-proportional parameter Y. Then, the evolution of this parameter was analyzed with the number of cycles to failure and with the ratio between shear and normal stresses, finally, the evolution of the non-proportional parameter Y was mapped by two functions. The results show that the non-proportional response of the AISI 303 can be estimated using the two functions obtained. This allows the estimation of the relationship between non-proportional and proportional stresses as a function of the number of cycles to failure together with the relationship between shear and normal stresses. The results obtained have direct application in the evaluation of accumulated damage, assessed in real-time, resulting from variable amplitude loading spectra. This is of particular interest for the evaluation of structural health monitoring of structures and mechanical components.

Constant stress arches and their design space

It is generally accepted that an optimal arch has a funicular (moment-less) form and least weight. However, the feature of least weight restricts the design options and raises the question of durability of such structures. This study, building on the analytical form-finding approach presented in Lewis (2016. Proc. R. Soc. A 472 , 20160019. ( doi:10.1098/rspa.2016.0019 )), proposes constant axial stress as a design criterion for smooth, two-pin arches that are moment-less under permanent (statistically prevalent) load. This approach ensures that no part of the structure becomes over-stressed under variable load (wind, snow and/or moving objects), relative to its other parts—a phenomenon observed in natural structures, such as trees, bones, shells. The theory considers a general case of an asymmetric arch, deriving the equation of its centre-line profile, horizontal reactions and varying cross-section area. The analysis of symmetric arches follows, and includes a solution for structures of least weight by supplying an equation for a volume-minimizing, span/rise ratio. The paper proposes a new concept, that of a design space controlled by two non-dimensional input parameters; their theoretical and practical limits define the existence of constant axial stress arches. It is shown that, for stand-alone arches, the design space reduces to a constraint relationship between constant stress and span/rise ratio.

Compressive behavior of stirrup-confined concrete columns: size effect and a size-dependent stress-strain model

Numerous studies have indicated the existence of size effect on axial compression behavior of stirrup-confined concrete columns. However, most of these studies have been stressed in terms of nominal compressive strength. The investigation on the size effect of axial strain (at peak load) and descending branch was limited. In this study, the size effect behavior of square stirrup-confined concrete columns under axial compression was explored, by using 3-D mesoscale simulation method. Based on the numerical and available experimental results, the influence of specimen size on the peak axial stress (i.e., the compressive strength), the corresponding strain and the softening rate were explored. Moreover, the quantitative relationships between specimen size and the peak axial stress, the corresponding strain and the softening rate for circular and square stirrup-confined concrete columns were derived. Finally, considering the size effect of peak axial stress, the corresponding strain and the softening rate, a novel stress-strain model describing the axial compression behavior of stirrup-confined concrete was developed. The proposed model was verified by comparing with the available experimental results and the existing models provided.

Stiff String Casing Design: Tortuosity and Centralisation

The forces and stresses along casing strings are modeled using a stiff string torque and drag model. The effect of wellbore tortuosity and centralization are quantified in preplanning phase in addition to the effect of 3D orientated casing wear. A realistic case study is presented to show the resulting effect on axial, burst, collapse and Von Mises equivalent (VME) safety factor as well as VME body and connection design envelopes. While running a tubular downhole, a smooth wellbore is normally assumed when performing a torque and drag calculation. In reality, the inherent tortuosity of the wellbore which is caused by the drilling process can cause significant local doglegs. When applying a soft-string torque and drag model, the stiffness, radial clearance and high frequency surveys needed to fully model local doglegs are rarely modeled. The stiff string torque and drag and buckling model can model these effects, as well as the addition of rigid and flexible centralisers. This study involves the comparison of different casing design load cases, under different centralizer programs and tortuosity taking into account a 3D orientated casing wear. The results show that there can be significant differences in overall axial stress depending on the centraliser program and tortuosity used. The soft string model doesn't directly account for bending stress, normally this is estimated using a Bending Stress Magnification Factor (BSMF). In contract the stiff string model can directly calculate the additional bending stress. This additional stress can be particularly prevalent while RIH casing with centralisers and high tortuosity. The reduction in American Petroleum Institute (API) and VME stress envelope is also quantified using a 3D orientated casing wear model. A better understanding of axial stress state reduces risk of well integrity issues. This paper will show the benefits of using a stiff string model, considering additional contact points, bending stress as well as the benefits of modelling tortuosity and centralizer program early in the design process. During extended reach drilling (ERD) and high-pressure, high temperature (HPHT) wells, this information can be critical when correctly assessing the axial stress state.

Fatigue Strength Evaluation of Welded Structure on Aluminum Alloy Car Body Based on Multi-Axial Stress

With the continuous development of the subway, the demand for its safety and stability is getting higher and higher. It is of great significance to accurately evaluate the fatigue life of the carbody to ensure the subway's safe operation. In this paper, the finite element model of a subway head carbody was established, and the fatigue strength of the welded structure on the carbody was evaluated based on Multi-axial stress. The local coordinate system was defined according to the geometrical characteristics of the welds. Local stresses perpendicular and parallel to the weld seam were obtained to calculate the stress ratio, stress range, and allowable stress value corresponding to the stress component. According to the joint fatigue resistance, the components of the degree of utilization and comprehensive degree of utilization are calculated to evaluate the structural fatigue strength under the survival rate of 97.5% and load cycles of 10 7 . The evaluation of the fatigue strength of the pivotal weld joints shows that the fatigue strength of the aluminum alloy carbody meets the design requirements, the weld of the carbody has a strong ability to resist fatigue damage. The fatigue strength of the weld is mainly affected by the normal stress component, while the shear stress has little effect on the fatigue strength of the structure. In addition, compared with the filleted weld joint and the butt-welded joint, the normal stress parallels to and perpendicular to the weld direction and shear stress have the greatest effect on the lap-welded joint. Meanwhile, the comprehensive degree of utilization of the lap-welded joint is the largest at 0.49. The introduction of multi-axial stress for the fatigue strength evaluation is beneficial when considering the material utilization degree in multiple structural directions. This research results provide a reference for fatigue strength evaluation of subway carbody's welded structure.

Experimental and Numerical Investigation of High-Temperature Multi-Axial Fatigue

Gas turbines and aircraft engines are dominated by cyclic operating modes with fatigue-related loads. This may result in the acceleration of damage development on the components. Critical components of turbine blades and discs are exposed to cyclic thermal and mechanical multi-axial fatigue. In the current work, planar-biaxial Low-Cycle-Fatigue tests are conducted using cruciform specimens at different test temperatures. The influence on the deformation and lifetime behaviour of the nickel-base disk alloy IN718 is investigated at selected cyclic proportional loading cases. The calculation of the stress and strain distribution of the cruciform specimens from the experimental data is difficult to obtain due to complex geometry and temperature gradients. Therefore, there is a need for Finite Element Simulations. A viscoplastic material model is considered to simulate the material behaviour subjected to uniaxial and the selected planar-biaxial loading conditions. At first, uniaxial simulation results are compared with the uniaxial experiment results for both batches of IN718. Then, the same material parameters are used for simulating the biaxial loading cases. The prediction of FE simulation results is in good agreement with the experimental LCF test for proportional loadings. The equivalent stress amplitude results of the biaxial simulation are compared with the uniaxial results. Furthermore, the lifetime is calculated from the simulation and by using Crossland and Sines multi-axial stress-based approaches. The Crossland model predicts fatigue life significantly better than the Sines model. Finally, the simulated lifetime results are compared with the experimental lifetime

Axial Stress Provides a Lower Bound on Shear Wave Velocity in Active and Passive Muscle

ABSTRACTUltrasound shear wave elastography can be used to characterize mechanical properties of unstressed tissue by measuring shear wave velocity (SWV), which increases with increasing tissue stiffness. Measurements of SWV have often been assumed to be directly related to the stiffness of muscle. Some have also used measures of SWV to estimate stress, since muscle stiffness and stress covary during active contractions. However, few have considered the direct influence of muscle stress on SWV, independent of the stress-dependent changes in muscle stiffness, even though it is well known that stress alters shear wave propagation. The objective of this study was to determine how well the theoretical dependency of SWV on stress can account for measured changes of SWV in passive and active muscle. Data were collected from six isoflurane-anesthetized cats; three soleus muscles and three medial gastrocnemius muscles. Muscle stress and stiffness were measured directly along with SWV. Measurements were made across a range of passively and actively generated stresses, obtained by varying muscle length and activation, which was controlled by stimulating the sciatic nerve. Our results show that SWV depends primarily on the stress in a passively stretched muscle. In contrast, the SWV in active muscle is higher than would be predicted by considering only stress, presumably due to activation-dependent changes in muscle stiffness. Our results demonstrate that while SWV is sensitive to changes in muscle stress and activation, there is not a unique relationship between SWV and either of these quantities when considered in isolation.

Effects of Fillet Weld Size and Sleeve Material Strength on the Residual Stress Distribution and Structural Safety While Implementing the New Sleeve Repair Process

The process optimization and structural safety improvement of the in-service repair welding of the X80 pipeline are very important. In this paper, the temperature, microstructure, and stress distribution were analyzed using the combination of TMM (thermal-metallurgical-mechanical) simulations and the corresponding verification experiments. The effects of the sleeve material strength and the fillet weld size were discussed. The results showed that the fillet weld zone was mainly composed of ferrite and bainite when the material of the sleeve pipe was Q345B. Furthermore, the sleeve pipe’s HAZ (heat affected zone) was dominated by lath martensite, lath bainite, and granular bainite. Moreover, granular bainite and a small amount of ferrite were found in the HAZ of the X80 pipe. It was found that, as the fillet weld size increased, the welding residual stress distribution became more uniform. The hoop stress at weld toe reduced from ~860 MPa of case A to ~680 MPa of case E, and the axial stress at weld toe reduced from ~440 MPa of case A to ~380 MPa of case E. From the viewpoint of welding residual stress, fillet weld size was suggested to be larger than 1.4T. The stress concentration and the stress distribution showed a correlation with the cracking behavior. Weld re-solidification ripples on the weld surface and weld ripples between welding passes or near the weld toe could cause stress concentration and the corresponding crack initiation. Furthermore, when the material of the sleeve pipe changed from Q345B to X80, the high-level tensile stress zone was found to be enlarged. The hoop stress at weld toe increased from ~750 to ~800 MPa, and the axial stress at weld toe increased from ~500 to ~600 MPa. After implementing the new sleeve repair welding process where X80 replaces the material of sleeve pipe, the cracking risk in sleeve pipe will improve. From the perspective of the welding residual stress, it was concluded that the fillet weld size reduction and the sleeve material strength improvement are harmful to in-service welded structures’ safety and integrity.

Dependence of deformation indices on initial yield strength and hardening intensity of wire material in the passage of drawing route

The model of hardening is considered, which provides for dependence of the current yield strength σт on the product of the initial yield strength σт0 and drawing coeffi cient μ to the degree of hardening coeffi cient k (σт = σт0μk). Difference of axial stress increase from action of anti-tension in absence and action of hardening depending on hardening coeffi cient is determined. Dependencies of drawing stress, safety factor I.L. Perlin and stress state index V.L. Kolmogorov on hardening coeffi cient at different values of initial yield strength are built. Limit hardening factor k is defi ned, at which safety factor is equal to 1. Dependencies of deformation indices on initial yield strength of σт0 at different deformation parameters are constructed.