Stress Strength Research Articles

Hydraulic fracture vertical propagation behavior in transversely isotropic layered shale formation with transition zone using XFEM-based CZM method

The transition zone was defined as an interface with certain thickness, strong heterogeneity and anisotropy in the vertical profile for layered formations, which had great effect on hydraulic fracture (HF) height propagation. In this paper, a numerical model for transversely isotropic layered shale with transition zone was established by utilizing the extended finite element method (XFEM) based on cohesive zone model (CZM). The effects of in-situ stress, dip angle, anisotropy and tensile strength of transition zone, and anisotropy of shale matrix, and injection rate on fracture vertical propagation behavior were investigated. Dimensionless fracture offset distance in transition zone (DFODT) and dimensionless fracture height (DFH) were defined to quantitatively evaluated the results of hydraulic fracture vertical propagation. Numerical results showed that the transition zone could induce the HF to turn and distort within and between layers, significantly improving the degree of fracture tortuosity. Also, the greater the DFODT, the smaller the DFH. The anisotropy of transition zone and coefficient of effective vertical stress difference had the greatest influence on DFODT; anisotropy of shale rock and injection rate had the greatest influence on DFH; while tensile strength of transition zone had a relatively weaker influence on both DFODT and DFH.

Estimation of the reliability system in model of stress- strength according to distribution of inverse Rayleigh

In many real-life scenarios, stress-strength model is a significant level that notices efficincy in reliability system. Thus, this paper considers the stress–strength model with reliability estimation R=P(X<Y) based on the Distribution of Inverse Rayleigh (IRD). Few classical methods of estimation such as; Likelihood as Maximum (MLE), Uniformly Unbiased Minimum Variance estimator (UMVUE), and Moment method (MOM), and three types of shrinkage weight factors estimation methods were compared. Also, a simulation of Monte Carlo is utilized for comparing among proposed methods based on Mean Square Error (MSE).

Estimation of the reliability system in model of stress- strength according to distribution of inverse Rayleigh

In many real-life scenarios, stress-strength model is a significant level that notices efficincy in reliability system. Thus, this paper considers the stress–strength model with reliability estimation R=P(X<Y) based on the Distribution of Inverse Rayleigh (IRD). Few classical methods of estimation such as; Likelihood as Maximum (MLE), Uniformly Unbiased Minimum Variance estimator (UMVUE), and Moment method (MOM), and three types of shrinkage weight factors estimation methods were compared. Also, a simulation of Monte Carlo is utilized for comparing among proposed methods based on Mean Square Error (MSE).

Approximation of continuous random variables for the evaluation of the reliability parameter of complex stress–strength models

In many management science or economic applications, it is common to represent the key uncertain inputs as continuous random variables. However, when analytic techniques fail to provide a closed-form solution to a problem or when one needs to reduce the computational load, it is often necessary to resort to some problem-specific approximation technique or approximate each given continuous probability distribution by a discrete distribution. Many discretization methods have been proposed so far; in this work, we revise the most popular techniques, highlighting their strengths and weaknesses, and empirically investigate their performance through a comparative study applied to a well-known engineering problem, formulated as a stress–strength model, with the aim of weighting up their feasibility and accuracy in recovering the value of the reliability parameter, also with reference to the number of discrete points. The results overall reward a recently introduced method as the best performer, which derives the discrete approximation as the numerical solution of a constrained non-linear optimization, preserving the first two moments of the original distribution. This method provides more accurate results than an ad-hoc first-order approximation technique. However, it is the most computationally demanding as well and the computation time can get even larger than that required by Monte Carlo approximation if the number of discrete points exceeds a certain threshold.

Characterization of coarse aggregate morphology and its effect on rheological and mechanical properties of fresh concrete

In order to clarify the influence of coarse aggregate morphology on the rheological property and mechanical property of fresh concrete, the morphological characteristics of coarse aggregate were quantitatively characterized, the core parameters of coarse aggregate morphology were screened, the standard parameters for quantitative characterization of coarse aggregate morphology were established, and the influence of coarse aggregate morphology on the rheological property and mechanical property of fresh concrete was explored. The results show that among the 8 parameters that represent the morphological characteristics of coarse aggregate, the flatness, angularity and roughness of aggregate have the greatest influence on the rheological properties and mechanical properties of fresh concrete. The weight of flatness on plastic viscosity is 50.54%, the weight of angularity on 28 days compressive strength is 48.95%, the weight of roughness on yield stress and flexural strength is 52.69% and 64.12%. When only the morphological composition of coarse aggregate is changed, the yield stress and plastic viscosity show the opposite variation law, the greater the yield stress, the smaller the plastic viscosity, and vice versa. When coarse aggregate with flatness less than 0.2 and roughness more than 0.4 are excluded, the yield stress is reduced by 28.58%, the plastic viscosity is increased by 23.15%, and the compressive strength is still over 60 MPa. The quantitative characterization of the coarse aggregate based on the characteristic parameters can provide a more effective method for researchers to predict the influence of coarse aggregate on the rheological properties and mechanical properties of concrete.

Additive Manufacturing of Porous Ti6Al4V Alloy: Geometry Analysis and Mechanical Properties Testing

This work is devoted to the research of porous titanium alloy structures suitable for use in biomedical applications. Mechanical properties were examined on six series of samples with different structures and porosity via static compressive test to identify the type of structure suitable for elimination of the “stress shielding” effect. In addition, high porosity is desirable due to the overgrowth of bone tissue into the internal structure of the implant. The samples were made of titanium alloy Ti6Al4V by using selective laser melting (SLM) additive manufacturing. The series of samples differ from each other in pore size (200, 400, and 600 µm) and porous structure topology (cubic or trabecular). The actual weight of all samples, which plays an important role in identifying other characteristics, was determined. Compressive tests were focused on the detection of maximum stress. The highest porosity and thus the lowest weight were achieved in the samples with a trabecular structure and 600 µm pore size. All tested samples reached optimal values of maximum stress and tensile strength. The most appropriate mechanical properties were observed for samples with a 200 µm pore diameter and cubic structure.

Process Signature for Laser Hardening

During many manufacturing processes for surface treatment of steel components heat will be exchanged between the environment and the workpiece. The heat exchange commonly leads to temperature gradients within the surface near area of the workpiece, which involve mechanical strains inside the material. If the corresponding stresses exceed locally the yield strength of the material residual stresses can remain after the process. If the temperature increase is high enough additionally phase transformation to austenite occurs and may lead further on due to a fast cooling to the very hard phase martensite. This investigation focuses on the correlation between concrete thermal loads such as temperature and temperature gradients and resulting modifications such as changes of the residual stress, the microstructure, and the hardness respectively. Within this consideration the thermal loads are the causes of the modifications and will be called internal material loads. The correlations between the generated internal material loads and the material modifications will be called Process Signature. The idea is that Process Signatures provide the possibility to engineer the workpiece surface layer and its functional properties in a knowledge-based way. This contribution presents some Process Signature components for a thermally dominated process with phase transformation: laser hardening. The target quantities of the modifications are the change of the residual stress state at the surface and the position of the 1st zero-crossing of the residual stress curve. Based on Finite Element simulations the internal thermal loadings during laser hardening are considered. The investigations identify for the considered target quantities the maximal temperature, the maximal temperature gradient, and the heating time as important parameters of the thermal loads.

The Effect of Y Addition on the Microstructure and Work Hardening Behavior of Mg-Zn-Zr Alloys

The effect of adding different amounts of Y on the microstructure, mechanical properties, texture and work hardening behavior of the extruded Mg-6Zn-0.5Zr-xY alloys (x = 0, 1, 2, 3 wt.%) was investigated. According to the results, the microstructure of all alloys is composed of α-Mg grains and Mg7Zn3 particles. By adding Y, in addition to grain refinement, Mg24Y5 and Mg3Y2Zn3 particles are formed in the microstructure. Among the alloys studied, the ZK60-3Y alloy has the highest strengths, as the yield stress and ultimate tensile strength of 318 and 366 MPa, respectively, were obtained for that alloy. This is due to the finer grains and higher volume fraction of the particles in the ZK60-3Y alloy. Meanwhile, reducing the grain size by Y addition affects the work hardening behavior; Y addition reduces the saturation stress, hardening capacity, and work hardening exponent by increasing the dynamic recovery; i.e., the more Y is added, the greater is the drop in the work hardening parameters. The effects of Y addition on work hardening behavior and dynamic recovery were investigated by examining microstructural developments, the volume fraction of particles and texture evaluation. The results of the texture evaluations showed that the addition of Y changes the texture component and intensity of the basal planes and can cause work hardening loss by activating slip on the non-basal planes.

Microstructure and mechanical behaviour of Ti-Cu foams synthesized via powder metallurgy technique

In the present study titanium based copper alloy foams were synthesized using acrawax as a space holder through powder metallurgy route. The effects of distinct pore size range and Cu content on the microstructural features, density, compressive and flexural behaviour was investigated. Results show that the amount of microporosity generated was more in case of foams with coarser pores than finer ones and also it increased with increase in Cu content. The presence of fine pores and lower amount of Cu led to generation of minimal amount of microporosity; resulting into greater relative density of Ti-Cu foams. The plastic collapse stress, plateau stress and bending strength of Ti-Cu foams decreased while densification strain increased with increase in Cu content. The maximum value of plastic collapse stress is reported for Ti-3Cu foam with fine pores. The plateau strength and bending strength of Ti-3Cu foam with fine pores was found to be higher than that of Pure Ti foams and was found to be the most optimized in terms of the characteristic features. The higher values of relative density, plateau and bending strength of Ti-3Cu foams makes them more preferable over pure Ti foams for implants applications.

Hardening of Al–Si–Cu–Mg Cast Alloys: Role of Ag and Zn addition

The present work was undertaken with the aim of studying the microstructural changes and intermetallic phases in Al-Si base 413.0 alloys as well as variations in tensile properties, including ultimate stress, yield strength and elongation at rupture, resulting from the addition of alloyed elements: strontium (Sr), magnesium (Mg), copper (Cu), silver (Ag), zinc (Zn) and nickel (Ni), to the base alloy 413.0, under different conditions of heat treatments, i.e., solutionizing and artificial aging (in the temperature range of 155 °C–240 °C). The results obtained in relation to microstructural observations and tensile tests reveal that the addition of alloying elements, in particular Mg, Cu, Ag, Ni, Zn and Sr, leads to an increase in the ultimate stress and yield strength values with a decrease in the strain levels of the base alloy 413.0, following hardening during artificial aging. Both Ag and Zn do not form specific phases during solidification at a slow rate of ~ 0.8 °C/s but only segregate to the α-aluminum cell boundaries. Addition of 0.73% Ag improves the values of yield strength at all aging temperatures. Also, it enhances the alloy resistance to softening during aging in the temperature range 200–240 °C. Similar behavior was exhibited by the addition of 2.5% Zn.

Bayesian estimation of stress strength reliability P[X&gt;Y] of Lomax and exponential distribution based on right censored sample

The present paper is concerned with the estimation of stress strength reliability R=P[X>Y] when X and Y are the random variables following Lomax and Exponential Distribution based on right censored sample. The main aim of this article is to estimate the Maximum Likelihood estimates of R and Bayesian Estimates of R under Squared Error Loss function, Linex Loss function and Entropy Loss function. Finally the performance of the estimators are evaluated by simulation study.

Scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints

From the perspective of macroscopic scale, the majority of natural rock mass should be categorized as non-penetrating jointed rock mass. The existing researches in the field of scale effect of joint properties were mainly implemented on penetrating joints, which contradicts engineering practice, and is of high possibility to make the strength estimation of large natural jointed rock mass inaccurate, leading to serious loss of life and property. In response to such case, a series of numerical calculations of direct shear test on non-penetrating horizontal rock-like joints with different scales were carried out by PFC in this paper, to investigate the scale effect of shear mechanical properties of non-penetrating horizontal rock-like joints. First, the model microparameters were calibrated by three physical experiments to guarantee the precise reproduction of the mechanical performances of target rock and joint. Next, the particle parameters (average particle size dave and radius ratio μ) of model were changed, the effect of particle size on joint strength was studied by direct shear calculation, and the determining method for the values of dave and μ was suggested. Then, based on two distribution forms of non-penetrating horizontal rock-like joint (type I and type II joints), the numerical shear tests were conducted on jointed rock models with different persistence rations and model scales, and the variations of shear stress displacement curve and strength characteristics were analyzed. The results indicate: The lower the persistence ration λ of the joint, the more obvious the negative scale effect of joint shear strength. Besides, the scale effect of shear strength gradually decreases when λ > 0.5 for type I joints while λ > 0.8 for type II joints and the scale effects of joint strength parameters only emerge in the case of λ < 0.2.

Reliability estimation of a stress-strength model with non-identical component strengths under generalized progressive hybrid censoring scheme

ABSTRACT Generalized progressive hybrid censoring schemes have become quite popular depending progressive hybrid censoring scheme cannot be applied when few failures occur before pre-determined time T. Otherwise, due to the necessity of more realistic stress strength models, this article considers to estimate the reliability of an s-out-of-k system with non-identical component strengths when component strengths and stress follow Weibull distributions under generalized progressive hybrid censoring scheme. The maximum likelihood estimation of the reliability of such a system is obtained with corresponding asymptotic and bootstrap confidence intervals. Further, Bayesian estimations are derived by using the Lindley's approximation and Markov Chain Monte Carlo method with the Metropolis-Hasting algorithm. The corresponding highest posterior density confidence intervals of the Bayes estimates are obtained. In Bayesian estimations symmetric and asymmetric loss functions are evaluated. Comparisons for the performances of the estimators are made with simulation studies and a real data example.

Objective Bayesian analysis for generalized exponential stress–strength model

In reliability studies, a stress–strength model is often used to analyze a system that fails whenever the applied stress is greater than the strength. Statistical inference of reliability is widely used in a number of areas, such as engineering, clinical trials, and quality control. In addition to the common stress–strength model with one stress and one strength, the reliability of more complex systems has also been studied. In this study, we consider the reliability of a generalized stress–strength model that consists of a serial system with one stress and multiple strengths. We then develop the probability matching priors and reference priors for a generalized exponential stress–strength model. We demonstrate that the two-group reference prior and Jeffreys prior are not a matching prior. Through a simulation study and real data example, we also demonstrate that the proposed probability matching priors match the target coverage probabilities in a frequentist sense even for a small sample size.

The Transmuted Generalized Odd Generalized Exponential-G Family of Distributions: Theory and Applications

We propose a new generator of continuous distributions, so called the transmuted generalized odd generalized exponential-G family, which extends the generalized odd generalized exponential-G family introduced by Alizadeh et al. (2017). Some statistical properties of the new family such as; raw and incomplete moments, moment generating function, Lorenz and Bonferroni curves, probability weighted moments, Rényi entropy, stress strength model and order statistics are investigated. The parameters of the new family are estimated by using the method of maximum likelihood. Two real applications are presented to demonstrate the effectiveness of the suggested family.

Effect of Strain Rate on the Tensile Mechanical Properties of Electron Beam Welded OFE Copper and High-Purity Niobium for SRF Applications

An investigation of the tensile mechanical properties of electron beam welded OFE copper and high-purity niobium sheets is presented. Specimens were deformed in tension at strain rates ranging from 10−3 to ~ 1600 s−1. The 0.2% yield stress and ultimate tensile strength (UTS) of the welded niobium specimens are similar to those of unwelded specimens at strain rates lower or equal to 20 s−1. At higher strain rates, these mechanical properties are lower for welded niobium specimens. The 0.2% yield stress of welded OFE copper specimens is consistently lower than unwelded specimens over the range of strain rates studied, while the UTS is comparable at all strain rates. The elongation to failure of welded OFE copper specimens remains unchanged at all strain rates while the ductility of niobium specimens reduces at strain rates greater or equal to 20 s−1 and reaches a minimum at ~ 400 s−1. The effects of the weld on a non-standardized short specimen geometry, developed for this study to obtain strain rates in the order of 103 s−1, are more pronounced for niobium due to large grain sizes (up to 1200 μm) in the fusion region. However, comparable strength and ductility trends, with respect to a standard specimen, were measured at low strain rates. The conservation of strength and the relatively high ductility of the welded sheets, especially for OFE copper, suggest that bent and electron beam welded tubes could be used for the fabrication of seamless superconducting radiofrequency (SRF) cavities. These results are promising for the use of high-speed forming techniques, like electro-hydraulic forming, for the manufacturing of parts using welded tubes and sheets.

Interval estimation for inverse Gaussian distribution

AbstractIn this paper, we consider interval estimation for the inverse Gaussian (IG) distribution. Using generalized pivotal quantity method, we derive the generalized confidence intervals (GCIs) for the model parameters and some quantities such as the quantile, the reliability function of the lifetime, the failure rate function, and the mean residual lifetime. We verify that the GCI of the scale parameter is the same as its commonly used exact CI. We also obtain the generalized prediction intervals (GPIs) for future failure times based on the observed failure data set. In addition, we get the GCI for the reliability of the stress–strength model when the stress and strength variables follow the IG distributions with different parameters. We compare the proposed GCIs and GPIs with the Wald CIs and bootstrap‐ CIs by simulation. The simulation results show that the proposed GCIs and GPIs are superior to the Wald CIs and the bootstrap‐p CIs in terms of the coverage probability. Finally, two examples are used to illustrate the proposed procedures.

Association of yogic breathing with perceived stress and conception of strengths and difficulties in teenagers

Mental health problems are increasing at an alarming rate throughout the world, and teenagers are no exception. They experience high levels of stress in their formative years which often leads to poor social behavior. In the present study, we examine the effect of Sudarshan Kriya Yoga (SKY), a yogic breathing technique on perceived stress and social behavior of school going teenagers. A cross-sectional survey was conducted. Teenagers who practiced SKY daily constituted the study group and teenagers who did not practice any form of yoga or meditation formed the control group. Child Perceived Stress scale (C-PSS) and Strength and Difficulty questionnaire (SDQ) were used to evaluate the mental health and social behavior of both groups. Lower stress scores were observed among students who practiced SKY compared to their peers. A significant difference was observed between the groups with respect to emotional problems, conduct problems, peer problems, and pro-social behavior. Gender wise comparison highlighted that the females from the SKY group scored lower on emotional problems sub-scale compared to their counterparts from control group. Results also highlighted an improved peer to peer interaction among both boys and girls in the SKY group. The results indicate the practice of SKY is associated with reduced stress, improved self-awareness, and social behavior.

The Direct 3D Printing of Functional PEEK/Hydroxyapatite Composites via a Fused Filament Fabrication Approach.

The manufacture of polyetheretherketone/hydroxyapatite (PEEK/HA) composites is seen as a viable approach to help enhance direct bone apposition in orthopaedic implants. A range of methods have been used to produce composites, including Selective Laser Sintering and injection moulding. Such techniques have drawbacks and lack flexibility to manufacture complex, custom-designed implants. 3D printing gets around many of the restraints and provides new opportunities for innovative solutions that are structurally suited to meet the needs of the patient. This work reports the direct 3D printing of extruded PEEK/HA composite filaments via a Fused Filament Fabrication (FFF) approach. In this work samples are 3D printed by a custom modified commercial printer Ultimaker 2+ (UM2+). SEM-EDX and µCT analyses show that HA particles are evenly distributed throughout the bulk and across the surface of the native 3D printed samples, with XRD highlighting up to 50% crystallinity and crystalline domains clearly observed in SEM and HR-TEM analyses. This highlights the favourable temperature conditions during 3D printing. The yield stress and ultimate tensile strength obtained for all the samples are comparable to human femoral cortical bone. The results show how FFF 3D printing of PEEK/HA composites up to 30 wt% HA can be achieved.

Matrix stiffening by self-mineralizable guided bone regeneration

Collagen membranes produced in vitro with different degrees of intrafibrillar mineralization are potentially useful for guided bone regeneration (GBR). However, highly-mineralized collagen membranes are brittle and difficult for clinical manipulation. The present study aimed at developing an intrafibrillar self-mineralization strategy for GBR membrane by covalently conjugating high-molecular weight polyacrylic acid (HPAA) on Bio-Gide® membranes (BG). The properties of the self-mineralizable membranes (HBG) and their potential to induce bone regeneration were investigated. The HBG underwent the progressive intrafibrillar mineralization as well as the increase in stiffness after immersed in supersaturated calcium phosphate solution, osteogenic medium, or after being implanted into a murine calvarial bone defect. The HBG promoted in-situ bone regeneration via stimulating osteogenic differentiation of mesenchymal stromal cells (MSCs). Hippo signaling was inhibited when MSCs were cultured on the self-mineralized HBG, and in HBG-promoted MSC osteogenesis during in-situ bone regeneration. This resulted in translocation of the transcription co-activators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) into the nucleus to induce transcription of genes promoting osteogenic differentiation of MSCs. Taken together, these findings indicated that HBG possessed the ability to self-mineralize in situ via intrafibrillar mineralization. The increase in stiffness of the extracellular matrix expedited in-situ bone regeneration by inactivating the Hippo-YAP/TAZ signaling cascade. Statement of significanceGuided bone regeneration (GBR) membranes made of naturally derived collagen have been widely used in the bone defect restoration. However, application of collagen GBR membranes run into the bottleneck with the challenges like insufficient stress strength, relatively poor dimensional stability and unsatisfactory osteoinductivity. This study develops a modified GBR membrane that can undergo progressive self-mineralization and matrix stiffening in situ. Increase in extracellular matrix stiffness provides the mechanical cues required for MSCs differentiation and expedites in-situ bone regeneration by inactivating the Hippo-YAP/TAZ signaling cascade.