Stress Analysis Model Research Articles

Highly reactive energetic films by pre-stressing nano-aluminum particles.

Energetic films were synthesized using stress altered nano-aluminum particles (nAl). The nAl powder was pre-stressed to examine how modified mechanical properties of the fuel particles influenced film reactivity. Pre-stressing conditions varied by quenching rate. Slow and rapid quenching rates induced elevated dilatational strain within the nAl particles that was measured using synchrotron X-ray diffraction (XRD). An analytical model for stress and strain in a nAl core–Al2O3 shell particle that includes creep in the shell and delamination at the core–shell boundary, was developed and used for interpretation of strain measurements. Results show rapid quenching induced 81% delamination at the particle core–shell interface also observed with Transmission Electron Microscopy (TEM). Slower quenching elevated dilatational strain without delamination. All films were prepared at approximately a 75 : 25 Al : poly(vinylidene fluoride) PVDF weight ratio and were 1 mm thick. A drop weight impact test was performed to assess ignition sensitivity and combustion. Stress altered nAl exhibited greater energy release rates and more complete combustion than untreated nAl, but reaction dynamics and kinetics proceeded in two different ways depending on the nAl quenching rate during pre-stressing.

Elevated Standoff Heights of Solder Joint Interconnections Can Result in Appreciable Stress and Warpage Relief

Abstract The “head-in-pillow” (HnP) defects in lead-free solder joint interconnections of Integrated Circuit (IC) packages with conventional (small) standoff heights of the solder joints, and particularly in packages with fine pitches, are attributed by many electronic material scientists to the three major causes: attributes of the manufacturing process, solder material properties, and design-related issues. The latter are thought to be caused primarily by elevated stresses in the solder material, as well as by the excessive warpage of the Printed Circuit Board (PCB)-package assembly and particularly by the differences in the thermally induced curvatures of the PCB and the package. In this analysis, the stress and warpage issue is addressed using an analytical predictive stress model. The model is a modification and an extension of the model developed back in 1980s by the first author. It is assumed that it is the difference in the postfabrication deflections of the PCB-package assembly that is the root cause of the solder material failures and particularly and perhaps the HnP defects. The calculated data based on the developed stress model suggest that the replacement of the conventional ball grid array (BGA) designs with designs with elevated standoff heights of the solder joints could result in significant stress and warpage relief and, hopefully, in a lower propensity of the IC package to HnP defects as well. The general concepts are illustrated by a numerical example, in which the responses to the change in temperature of a conventional design, referred to as BGA, and a design with solder joints with elevated standoff heights, referred to as column grid array (CGA), are compared. The computed data indicated that the effective stress in the solder material was relieved by about 40% and the difference between the maximum deflections of the PCB and the package was reduced by about 60%, when the BGA design was replaced by a CGA system. Although no definite proof that the use of solder joints with elevated standoff heights will lessen the package propensity to the HnP defects is provided, the authors nonetheless think that there is a reason to believe that the application of solder joints with elevated standoff heights could result in a substantial improvement in the general IC package performance, including, perhaps, its propensity to HnP defects.

Material model for stress analysis of nuclear graphite component subject to irradiation

Nuclear graphite is used as a solid moderator in many fission nuclear reactors such as gas-cooled reactors and molten salt reactors. The role of graphite is to slow down neutrons emitted by fission, thus enhance their probabilities of interacting to produce further fission reactions. Graphite also acts as a major structural component in nuclear reactors, providing channels for the coolant and control rods. Graphite undergoes significant changes in dimensions and material properties due to the effects of irradiation damages. These irradiation-induced changes can lead to build up of significant stresses and deformation in the graphite components. Throughout reactor life it is essential that the graphite core structure remains sufficiently strong and undistorted in order to maintain fuel cooling, permit loading and unloading of the fuel and allow the necessary movements of control rods, in both normal and fault conditions. To perform structural integrity assessments, the definition of the constitutive equation relating stress and strain in the irradiated graphite material is required. Apart from the elastic and thermal strain, graphite in nuclear reactors also experiences additional strains due to fast neutron irradiation. Irradiation creep and irradiation induced dimensional changes are two most important of these irradiation strains. Nuclear graphite component structural integrity assessments are usually carried out using the finite element method. To model the dimensional change, irradiation creep and material properties a complex material model is required. In this paper a simple mathematical material model for isotropic graphite under irradiation condition is derived here. The model largely relies on empirical graphite property data obtained from material test reactor experiments. The model can be used to assess the nuclear graphite component structural integrity throughout the life of reactors, taking account of increasing irradiation fast neutron fluence and operating temperature. Also the model is a useful tool for designing a suitable graphite component for a nuclear reactor.

Effects of Substrate Preheating Temperatures on the Microstructure, Properties, and Residual Stress of 12CrNi2 Prepared by Laser Cladding Deposition Technique.

The 12CrNi2 alloy steel powder studied in the present paper is mainly used to manufacture camshafts for nuclear power emergency diesel engines. Laser cladding deposition is of great significance for the manufacture of nuclear power emergency diesel camshafts, which has the advantages of reducing material cost and shortening the manufacturing cycle. However, due to the extremely uneven heating of the components during the deposition process, a complex residual stress field occurs, resulting in crack defects and residual deformation of the components. In the present paper, 12CrNi2 bulk specimens were prepared on the Q460E high-strength structural steel substrate at different preheating temperatures by laser cladding deposition technique, and a finite element residual stress analysis model was established to investigate the effects of different preheating temperatures on the microstructure, properties, and residual stress of the specimens. The results of the experiments and finite element simulations show that with the increase of preheating temperature, the content of martensite/bainite in the deposited layer decreases, and the ferrite content increases. The proper preheating temperature (150 °C) has good mechanical properties. The residual stress on the surface of each specimen decreases with the increase of the preheating temperature. The longitudinal stress is greater at the rear-end deposition part, and the lateral residual stress is greater on both sides along the scanning direction.

Elevated Stand-Off Heights in Solder Joint Interconnections of Surface Mounted IC Packages Result in Appreciable Stress and Warpage Relief

Abstract The “head-in-pillow” (HnP) defects in lead-free solder joint interconnections of IC packages with conventional (small) stand-off heights of the solder joints, and particularly in packages with fine pitches, are attributed by many electronic material scientists to the three major causes: 1) attributes of the manufacturing process, 2) solder material properties and 3)design-related issues. The latter are thought to be caused primarily by elevated stresses in the solder material, as well as by the excessive warpage of the PCB-package assembly and particularly to the differences in the thermally induced curvatures of the PCB and the package. In this analysis the stress-and-warpage issue is addressed using an analytical predictive stress model. This model is a modification and an extension of the model developed back in 1980-s by the first author. It is assumed that it is the difference in the post-fabrication deflections of the PCB-package assembly that is the root cause of the solder materials failures and particularly and perhaps the HnP defects. The calculated data based on the developed analytical thermal stress model suggest that the replacement of the conventional ball-grid-array (BGA) designs with designs characterized by elevated stand-off heights of the solder joints could result in significant stress and warpage relief and, hopefully, in a lower propensity of the IC package to HnP defects as well. The general concepts are illustrated by a numerical example, in which the responses to the change in temperature of a conventional design referred to as ball-grid-array (BGA) and a design with solder joints with elevated stand-off heights referred to as column-grid-array (CGA) are compared. The computed data indicated that the effective stress in the solder material is relieved by about 40% and the difference between the maximum deflections of the PCB and the package is reduced by about 60%, when the BGA design is replaced by a CGA system. Although no proof that the use of solder joints with elevated stand-off heights will lessen the package propensity to the HnP defects is provided, the authors think that there is a reason to believe that the application of solder joints with elevated stand-off heights could result in a substantial improvement in the general IC package performance, including, perhaps, its propensity to HnP defects.

Effects of design parameters and tension on behavior of a coil using finite element analysis

During the thin strip coiling process, it is necessary to use a sleeve with a mandrel to prevent excessive deformation of the strip. Stress distribution in the sleeve and in the strip is an important factor that determines the quality of the coil. However, owing to the accumulation of high pressure, it is difficult to determine the stress distribution through experimentation. Thus, stress analysis of the strip coiling process was conducted. Finite element analysis was used to investigate the effects of the weight of the strip and the mandrel on the stress distribution and stress concentration near the starting point of the coil. The radial stress was predicted for a coil with a stacked thickness of 384 mm, which corresponds to a strip length of 1486 m, using the stress analysis model developed in a preceding research. A method was presented to reduce the weight and radial stress of a strip coil. It was found that the deformation of the sleeve can be reduced by decreasing the gap between the mandrel segments. The thickness of the sleeve can be reduced from 120 to 106 mm using the stress analysis results. Furthermore, coiling tension can be reduced by 44% compared to the existing value considering the interlayer slip of the strip coil.

Simplified analytical model for adhesive-bonded tubular joints with isotropic and composite adherends subjected to tension

Many studies have investigated adhesive tubular joints subjected to axial load. However, previous analytical studies were only limited to the joints with single-layer isotropic or orthotropic materials, and the corresponding mathematical models were highly complicated. Thus, the aim of this article is to develop a simplified but comprehensive model for studying adhesive-bonded tubular coupler joints subjected to tension. The joint adherends can be made of isotropic, orthotropic, or multilayer composite materials. This study presents the formulation of the mathematical model for stress analysis in coupler joints. Assumptions of an axisymmetric joint with linearly elastic adherends and adhesive materials are employed. A thin adhesive layer is considered such that only the out-of-plane adhesive stresses are concerned and are treated to be uniform through the layer thickness. Using elasticity theory and the newly developed finite-segmented method, stress distributions in both adherends and the adhesive can be evaluated. The influence of fiber angle in the cases of composite adherends and joint geometries are studied. The results provide a good guideline in designing adhesive-bonded cylindrical joints with isotropic and composite adherends under axial loads.

Computational Verification of the Optimum Boundary Condition of the Brazilian Tensile Test

The present research analyses the configuration of jaws to avoid the premature failure of the disc in the Brazilian test. The objective is to depict the loading device configuration that most likely produces results comparable to the Hondros’ analytical stress solution. To this end, several numerical analyses have been carried out for different contact angles with the finite element method. It was deduced that the final contact angle plays an important part in the success of the Brazilian test and that the Griffith criterion can be fulfilled if the equivalent stress is calculated. Additionally, the orientation of the forces in the contact between the loading device and the disc has been studied for different friction conditions. According to the numerical results, it was found that a loading arc configuration of 20° shows the best agreement with the probable values given by the analytical stress model when the uncertainty of its magnitudes is taken into account. The study also demonstrates that the friction in the contact between the optimal loading configuration and the disc does not seem to significantly affect the theoretical predictions in the centre of the disc.

Estimation of the perforation force for polymer composite conveyor belts taking into consideration the shape of the piercing punch

Due to the improvement of the mechanical properties of polymer composite belts used in vacuum belt conveyors, its perforation process causes a lot of technical issues for manufacturers worldwide. The objective of this paper is to analyze the belt punching process with two cutting edges and present the influence of the piercing punch shape on the perforation force. Based on the analysis, the analytical stress model was derived and validated by using both empirical and FEM tests. The application of the proposed model was proved by presenting the methodology used to estimate the perforation force for the flat piercing punch based on the mechanical properties of the belt obtained from simple strength tests (uniaxial tension, compression, and shear), with an error between 4 and 15%. In this report, the analysis of the piercing punch profiles was made and eight different piercing punch profiles were tested. Presented results confirmed that the spherical bowl punch may be considered as a most effective tool for belt punching, because it reduced the perforation force by 60% and the precision of the created holes was the best among the tested punch profiles for all three groups of polymer composite belts. By combining the obtained results, in the form of shape factors β, with the perforation force approximation model, it is possible to calculate peak force value for the specified tool profile and belt type and use this data in the design process of the punching dies.

Emplacement of silica veins at a brittle shear zone in the Ahar region, NW Iran: Insights from structural analysis, analogue and numerical modeling

Post Eocene silica vein emplacement at a brittle shear zone in NW Iran is associated with Au, Cu and Mo mineralization. The orientation and abundance of veins are dominantly controlled by the fault and fracture pattern. Within this context, we have analyze the spatial and geometrical properties of the faults and fractures affecting vein emplacement and compare the results with analogue and numerical models to distinguish the proper area for the emplacement of ore bearing silica veins in the evolution of a brittle shear zone. For this purpose regional lineament trends have been extracted from Aster satellite image. The trends of this lineament are between N80°W to N60°W. Field observations revealed two dominant fracture sets with strike of N40°E and N20°W; as well as the silica veins general trend is N15°W. The brittle shear zone analogue model indicated that the first structure to appear in the D1 stage of deformation were the shear fracture. With the progress of deformation in D2 stage, shear fractures eventually grew, rotated and were connected to each other, which created surface ruptures in the models. The main surface rupture displays a misalignment with the basement shear zone. In the D3 stage, fractures rotated with increasing displacement along the strike slip faults. The numerical stress analysis model shows that in the D1 stage, compressive stress is localized in the central part of the shear zone, and the rupture developed. In D2 stage, stress in the central part significantly decreased from 0.14 to 0.09 MPa (67% of maximum value) and an extensional regime was dominant. In the D3 stage, the tips of the model were primarily influenced by extensional stress, whereas the central part was subdivided into two zones: one is high stress and compressive and the other is low stress and extensional. Shearing along strike slip faults led to subsequent deformation in D4. Our modeling data indicate that heterogenous stress distribution along the shear zone instigated the frequent inversion of the extensional basins to compressional and vice versa. The D2 stage of evolution of the shear zone was favorable time for the emplacement of ore bearing veins, because of open fractures and the prevailing extensional regime in the central part. Thus the young shear zone open fractures of the central part and the mature shear zone tension fractures in the tips are appropriate areas for emplacement of veins.

An Integrated Prognostics Approach for Pipeline Fatigue Crack Growth Prediction Utilizing Inline Inspection Data

Fatigue cracking is a key type of defect for liquid pipelines, and managing fatigue cracks has been a top priority and a big challenge for liquid pipeline operators. The existing inline inspection (ILI) tools for pipeline defect evaluation have large fatigue crack measurement uncertainties. Furthermore, the current physics-based methods are mainly used for fatigue crack growth prediction, where the same or a small range of fixed model parameters is used for all pipes. They result in uncertainty that is managed through the use of conservative safety factors such as adding depth uncertainty to the measured depth in deciding integrity management and risk mitigation strategies. In this study, an integrated approach is proposed for pipeline fatigue crack growth prediction utilizing ILI data including consideration of crack depth measurement uncertainty. This approach is done by integrating the physical models, including the stress analysis models, the crack growth model governed by the Paris’ law, and the ILI data. With the proposed integrated approach, the finite element (FE) model of a cracked pipe is built and the stress analysis is performed. ILI data are utilized to update the uncertain physical parameters for the individual pipe being considered so that a more accurate fatigue crack growth prediction can be achieved. Time-varying loading conditions are considered in the proposed integrated method by using rainflow counting method. The proposed integrated prognostics approach is compared with the existing physics-based method using examples based on simulated data. Field data provided by a Canadian pipeline operator are also employed for the validation of the proposed method. The examples and case studies in this paper demonstrate the limitations of the existing physics-based method, and the promise of the proposed method for achieving accurate fatigue crack growth prediction as continuous improvement of ILI technologies further reduces ILI measurement uncertainty.

Analytical model of Li-ion diffusion-induced stress in nanowire and negative Poisson's ratio electrode under different operations

Diffusion-induced stress is one of the critical challenges in developing lithium ion battery technologies, which may lead to the huge volume expansion, nucleation of cracks, electrode mechanical degradation and even pulverization. It has been testified nanostructured electrodes with the consideration of surface effect and property of negative Poisson's ratio can restrain volumetric expansion generation and relieve the hazard of stress-induced crack nucleation. In this paper, an analytical model of diffusion-induced stress for nanowire electrode under different operations is established and a relationship between stress and the nanowire radius is formed. Through changing the morphology of nanowire electrode, the stress equations for negative Poisson's ratio electrode can be obtained. By comparing surface effect influence on diffusion-induced stress between the nanowire and negative Poisson's ratio electrodes under different operations, it is found diffusion-induced stress, especially the tensile stress, can be decreased greatly and even converted to compressive one by the nanostructured electrode and the mechanism of negative Poisson's ratio, which may improve the mechanical property of battery and be in good agreement with experimental observations. Our work provides reference conditions for the improvement of battery to mitigate the mechanical degradation and prolong the cycle life as well as avoid failure.

Cross laminated timber at in-plane beam loading – Prediction of shear stresses in crossing areas

Cross Laminated Timber (CLT) at in-plane beam loading conditions present a very complex stress state and many failure modes need to be considered in design. The work presented here aims at finding improvements of a specific analytical model for stress analysis and strength verification that has been suggested in literature and which is also suggested as a basis for design equations for the next version of Eurocode 5. Although the model has appealing properties it suffers from some drawbacks related to the assumed distributions of internal forces which, based on comparison to finite element analysis, appear to be inaccurate. The main focus in this paper is on model predictions regarding the distribution and magnitude of internal forces acting in the crossing areas between longitudinal and transversal laminations. The proposed modified model assumptions regarding the distribution of lamination shear forces, which in turn influence the forces acting in the crossing areas, are suggested to be taken into account in design of CLT beams.

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

Although a large volume of mudcake filtration test data is available in the literature, effects of mudcake on wellbore strengthening cannot be quantified without incorporating the data into a stress-analysis model. Traditional models for determining fracture initiation pressure (FIP) either consider a wellbore with an impermeable mudcake or with no mudcake at all. An analytical model considering permeable mudcake is proposed in this paper. The model can predict pore pressure and stress profiles around the wellbore, and consequently the FIP, for different mudcake thickness, permeability, and strength. Numerical examples are provided to illustrate the effects of these mudcake parameters. The results show that a low-permeability mudcake enhances FIP, mainly through restricting fluid seepage and pore pressure increase in the near-wellbore region, rather than by mudcake strength. Fluid loss pressure (FLP) should be distinguished from FIP when a mudcake is present on the wellbore wall. Fracture may occur behind the mudcake at FIP without mudcake rupture. The small effect of mudcake strength on FIP does not mean its effect on FLP is small too. Mudcake strength may play an important role in maintaining integrity of the wellbore once a fracture has initiated behind the mudcake.

Large Strain Viscoelastic Material Model for Deformation, Stress and Strain Analysis of O-rings

Primary aim of this paper is to investigate the mechanical behavior of an O-ring seal made of NBR rubber material using finite element method. Further aim is to identify the O-ring’s critical locations where it can fail under normal operating conditions. For these a large-strain viscoelastic material model was set up. The recommended strain range where the material model should be sufficient is 0-100 % for normal conditions, but when the effect of overload or the temperature is the question, the strain range should be 0-150 %. From designer point of view, a large-strain viscoelastic material model with a 5 parameter Mooney-Rivlin term is appropriate. To make sure the material model′s accurate response, uniaxial tension, pure shear and equibiaxial measurements are needed, completed with dynamic measurements with a DMTA machine. The results show two critical locations where the failure of the O-ring is expected. One is on the edge of the O-ring near the gap, when friction is present, the other one is under the edge near the gap.

A progressive fatigue damage model for composite structures in hygrothermal environments

An improved progressive fatigue damage model (PFDM) involving hygrothermal effects was proposed to predict fatigue failure of composite structures in hygrothermal environments. In the improved PFDM, a unified model was established to evaluate the variation of composite material properties caused by hygrothermal environments. The hygrothermal-induced material properties were utilized in the stress analysis model, material degradation models and fatigue failure criterion. The hygrothermal strains were introduced into the constitution equation to account for the hygrothermal effects. A residual-strain-based gradual material degradation model and a micromechanics-based sudden material degradation model were enhanced to describe the damage of composite materials in hygrothermal environments. Besides, fatigue tests and progressive fatigue damage analyses of an open-hole laminate were performed in hygrothermal environments. The good consistency between the numerical and experimental results validated the improved PFDM. Both the experimental and numerical outcomes indicate that the effective moisture equilibrium considerably reduces the compressive strength and compressive fatigue life of the open-hole laminate made of T800 carbon/epoxy composites.

Transient thermo-mechanical analysis of steam generators for solar tower plants

In solar power tower plants, fast start-ups and/or load changes are mandatory to increase the power plant dispatchability. The high temperatures of the working fluids and the partial-load operation will reduce the lifetime of the thick-walled components at the steam generator. Therefore, a proper heat exchanger design should consider the stress evolution during the transient operation of the plant.This work addresses, for the first time, a methodology to determine the dynamic behavior of all heat exchangers of a steam generator train. The methodology proposed here is a powerful tool for the design of solar power plants. The stress analysis model identifies the most important components of the steam generation train during transient operation. The methodology consists of the combination of analytical models to obtain the coupled response of the steam generation train from the following dynamic variables: temperature, pressure and stress.An example of this methodology is presented for two start-up initial conditions: the assumption of non-isothermal and isothermal temperature profiles of the heat exchangers. A steam generator train based on conventional shell and tube heat exchangers is analyzed. The analysis shows that the non-isothermal condition takes approximately 50 min to reach nominal conditions, whereas the isothermal condition takes approximately 110 min, requiring 600 tons and 716 tons of hot salt to perform the start-up procedure, respectively.

Development of numerical-experimental model of connecting lugs and application on the lugs calculation of container terminals

The present paper describes numerical and experimental methodology and development of the model for stress and deformation state analysis of the connecting lug of container terminals. Numerical analysis was conducted by applying the finite element method in a ‘’KOMIPS“ software package. Experiments were performed at the Laboratory for stress and deformation measurements, Faculty of Mechanical Engineering, Belgrade University, using ‘’GOM“ equipment and ‘’ARAMIS” software application. This paper demonstrates how it is possible to anticipate the results by applying FEM. This paper will present how experimental results can be predicted using the finite element method. The paper presents an overview of the existing research and review of previous results achieved in this field. Container terminal used to supply electrical energy and management system of conveyor belts that are used in the exploitation of the mining basins. Stress and deformation state analysis of the connecting lugs is carried out at the loading of the container.

Safety assessment for In-service Pressure Bending Pipe Containing Incomplete Penetration Defects

Incomplete penetration defect is a common defect in the welded joint of pressure pipes. While the safety classification of pressure pipe containing incomplete penetration defects, according to periodical inspection regulations in present, is more conservative. For reducing the repair of incomplete penetration defect, a scientific and applicable safety assessment method for pressure pipe is needed. In this paper, the stress analysis model of the pipe system was established for the in-service pressure bending pipe containing incomplete penetration defects. The local finite element model was set up to analyze the stress distribution of defect location and the stress linearization. And then, the applicability of two assessment methods, simplified assessment and U factor assessment method, to the assessment of incomplete penetration defects located at pressure bending pipe were analyzed. The results can provide some technical supports for the safety assessment of complex pipelines in the future.

Stress solutions for joints with nonlinear adhesives

AbstractA simple predictive stress analysis model for adhesive joints with nonlinear adhesives is presented. The model allows for the treatment of various joint designs with shear‐flexible laminate or isotropic adherends. Both elastic‐plastic and hyperelastic adhesives are covered. Using a general sandwich‐type model of the adhesive bondline, the governing differential equations are derived analytically and solved numerically. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)