Journal of Stress Analysis
Volume:5 Issue: 1, Spring Summer 2020

Phase-field method is one of the recent developed fracture simulation methods which has attracted much interest in the last decade. Phase-field method can precisely simulate the crack nucleation as well as crack propagation path in complicated geometries. In general, phase filed method is a nonlocal theory that defines the cracks and other defects as a continuous part of the geometry with defining a length scale parameter. The major deficiency of this method is that it computationally is very time consuming. In this paper, a new numerical method based on finite element method was proposed to diminish the computational cost. The suggested numerical method was coded in Abaqus/Standard using an UEL subroutine. The simulations of different two-dimensional geometries demonstrate the capability of this method to predict the fracture process of brittle material. Results show that the proposed numerical method could significantly decrease the solution time in comparison to other methods.

In this study, a novel hybrid method was presented by considering the strengths and weaknesses of the two methods of the direct sensitivity method (DSM) and the complex variables method (CVM) and combining them to calculate shape sensitivity. The most of methods available are highly dependent on the values of step size variation related to the type of the problem. To validate the proposed method, some examples were analyzed by using the written finite element code. The comparison of results at solved problems indicated the independency of the proposed method from step size and only need to select an arbitrary small step size and the rounding error is negligible. It is a sign of its high computational performance which converges to reliable, stable, and high-precision results and saves calculation time compared to the other methods. The other advantages of the proposed method are the low volume of occupied memory and simplicity of implementation and its application in a wide range of engineering problems having simple and complicated equations.

In this paper, the swelling of the photo-thermal sensitive cylindrical polyelectrolyte hydrogel micro-valve has been studied. For this purpose, a modified constitutive model that considers the polyelectrolyte nature of the photothermal sensitive hydrogels is used. The analytical solution for swelling of the hydrogel cylinder due to temperature and light intensity changes was presented. Then, in order to confront problems with realistic complicated boundary conditions, the finite element (FE) tool was implemented in ABAQUS software by scripting a UHYPER subroutine. Using the FE tool, the swelling of the hydrogel cylinder and contact of the micro-valve with the wall of the channel was investigated. Then, the temperature and the light intensity at which the channel was closed was obtained. Finally, opening valve parameter was studied for analyzing the geometrical influence of the under-study actuator, and the obtained results were discussed.

The mechanical behavior of a fluid-saturated functionally graded porous piezoelectric material (FGPPM) rotating disc with variable angular velocity and thickness placed in a constant magnetic field was investigated. Due to variable angular velocity, the disc was subjected to Lorentz force in two directions: radial and circumferential. It was assumed the disc is power-law functionally graded in the radial direction. The disc is uniformly porous and its thickness varies as a function of radius. First, three coupled governing partial differential equations were converted to ordinary differential equations using the separation of variable technique. Then, equations were solved using Runge-Kutta and shooting methods for the case of fixed-free boundary condition. The effect of variable angular velocity, thickness profile, inhomogeneity index, porosity and magnetic field was investigated. The results demonstrate that considering angular acceleration for the disc has a considerable effect on the Lorentz force resulted by the magnetic field. Besides, the angular velocity constant has a significant effect on the stresses and displacements in the presence of the magnetic field.

Lubrication as a friction reduction technique has been used in variety of mechanisms and machines. The contacting surfaces depart from each other by the pressure produced in lubricant due to surface wedge shape. In some applications pressure is such high that deforms the contacting surfaces and more space is provided for the lubricant. Increasing the film thickness leads to misestimating of friction coefficient. Greases are usually used when the lubricating area is unreachable easily or there is not enough space for oil recirculation. In this paper, Herschel–Bulkley's model was used for isothermal non-Newtonian grease lubrication under point contact elastohydrodynamic condition. A good agreement between experimental and simulation results is shown. The effect of different type of grease is compared according to lubricant film thickness and friction coefficient. Results show that threshold yield stress does not significantly affect tribological parameters but the power law exponent does. Higher load and lower entraining velocity cause thinner film as well as higher friction coefficient in spite of the grease type.

In this paper by using the three-point bending and low velocity impact tests, the impact damage response of Polyurethane cored sandwich panels with hybrid nanocomposites face-sheets is investigated. The face-sheets are made of epoxy/woven-fiberglass/ nano-silica composite. Three-point bending test is used for determination of static threshold delamination force, and static and dynamic Interlaminar Shear Strength has been calculated. Furthermore, low velocity impact tests are performed on a sandwich panel and contact forces history, lateral deflection of the contact point and the absorbed energy of top face-sheets are obtained. The dynamic threshold delamination force has been used to predict the delamination damage mode in low velocity impact tests on sandwich panel. Finally, the delamination damage area is investigated theoretically and experimentally and the correction factor is associated with allowable shear stress is determined. Moreover, the effect of nano-silica particles on delaminations threshold forces, Interlaminar Shear Strengths, contact force, contact duration, deflection of contact point, energy absorption of top face-sheet and damage area caused by delamination is studied.

The AZ31 alloy containing nanopowder SiO2, in comparison to other magnesium alloys, can be utilized for manufacturing extruded parts with a high loading rate. The main goal of the present study is to investigate the compressive flow stress for AZ31 alloy reinforced with 2 % SiO2 nano particles (with mean diameter of 35±2 nm) in three different temperatures of 473, 493, and 513 K and three strain rates of 0.0002, 0.002 and 0.02s-1 using ring compression test. The stress-strain curve at three temperatures and three strain rates were obtained by implicating the bulge and numerical correction factors. Having drawn stress and strain curve, a relation between stress and strain using the Zener-Hollomon equation, which is based on activation energy from plastic forming, was found. The coefficients of the Zener-Hollomon equation were computed for achiving the activation energy.

Investigation of repaired crack in pressure vessels plays a critical role in the maintenance of cylindrical vessels which are under static loads. Pressurized vessels are critical elements in many industries that may be subjected to degradation and cracking due to working conditions. There are several methods to repair such reservoirs, including welding of damaged points, but this is not possible in some conditions, such as the presence of inflammable materials inside the reservoir. One of the most reliable ways to repair these types of vessels is using composite materials. In this research, crack created in the reservoir is repaired using carbon / epoxy composites. Results of this study confirm the association between the strength of vessels walls and thickness of composite materials. In this paper, the effect of different parameters such as thickness and radius of the reservoir, and composite thickness are investigated. The results show that increasing the thickness of the composite is effective, so by doubling the thickness of composite, stress intensity factor decreases by 13%. Moreover, the use of composites in thin wall tanks is much more effective than the case of a thick reservoir wall.

This paper deals with the fracture toughness of three different asphalt mixtures of HMA (Hot Mix Asphalt), WMA (Warm Mix Asphalt) and WMA-RAP (i.e. mixture of WMA and reclaimed asphalt pavement). Fracture tests were performed on SCB (Semi-Circular Bend) specimens to obtain the fracture properties (i.e. fracture toughness and fracture energy) of asphalt mixtures under three different modes of loading (i.e. pure mode I, pure mode II and mixed mode I/II) at the two test temperatures of −15◦C and 25◦C. The results exhibit that the WMA (i.e. WMA and WMA-RAP) mixtures provide greater fracture toughness than the HMA one under any mode of loading and temperature conditions; however, the WMA-RAP shows the highest fracture toughness. Furthermore, the fracture energies of the WMA and WMA-RAP mixtures are higher than the HMA mixture; however, the WMA mixture demonstrates the greatest value of fracture energy compared to other mixtures. Both the fracture toughness and fracture energy of the mixtures at −15◦C are also found to be higher than those at 25◦C.

The main purpose of this article is three-dimensional finite element analysis of defected tubes under gas detonation loading. The variation of stress intensity factor through the semi elliptical crack front for different defect profiles versus the simulation time is studied. The structural linear elastic response of tube in presence of an embedded defect is considered. The results show that the traveling pressure loads can affect the distribution of fracture factors along the defect shape. It can be seen that the moving pressure loads lead to occur the mixed mode stress intensity factor in thick walled tubes. The dependency of the stress intensity factors on the crack configuration, time period of loading and typical pressure-time profile is investigated. It can be seen that the schematic of stress/strain-time trace profiles strongly dependents on the location of points through the wall thickness. The capability of finite element modeling in analysis of three dimensional dynamic transient problems has been shown.

This paper aimed to study the process of shot peening using the combination of the Finite Element Analysis (FEA) and the Response Surface Methodology (RSM). The shot velocity, shot diameter, coverage percentage and thickness are selected as process parameters. Residual compressive stresses and roughness are considered as response variables. Using FEA, shot peening is simulated and RSM is employed to determine the governing models between the response variables and the input parameters. The statistical analysis of the results reveals that: (1) the induced surface stress depends upon the coverage percentage and sample thickness, and it is independent of the shot velocity and shot diameter, (2) the maximum compression stress depends on the coverage percentage and shot diameter respectively, (3) the depth of maximum compressive stress depends on shot velocity and shot diameter respectively, (4) the depth of compressive stress is dependent on all four factors, (5) the roughness, Ra, is only dependent on the shot velocity. The results are in good agreement with the experimental data of the literature.

The fully classical coupled thermoelasticity problem in a thick hollow cylinder is solved using analytical methods. Finite Hankel transform, Laplace transform and a contemporary innovative method are used to solve the problem and presenting closed-form relations for temperature and stress distribution. To solve the energy equation and the structural equation, on the inner and the outer surfaces of the cylinder, time-dependent thermal and mechanical boundary conditions are applied. The Dirichlet boundary condition which represents temperature, is considered to solve the energy equation and the Cauchy boundary condition which represent traction, is considered for the equation of motion. Two cases are studied numerically, pure mechanical load and pure thermal load. In plotting the results for the case of prescribing pure mechanical load in spite of not applying any thermal load induced temperature can be seen in the temperature history figure. Due to solving the elastodynamic problem, the elastic and the thermoelastic stress wave propagation into the medium and the reflection were observed in the plotted results.