Severe Stress Concentrations Research Articles

Regularization of torsional stresses in tubular lap bonded joints by means of functionally graded adhesives

This paper describes the analytical stress analysis of a tubular single lap joint under torsion with a functionally graded modulus adhesive (FGA). The adhesive technology offers several advantages in structural applications, such as bonding dissimilar materials, but suffers from severe stress concentrations in the bondline which often act as a trigger for fracture phenomena. To overcome these problems, FGA with nanoparticles distributed inside the polymer can be used, as proposed recently in technical literature. The aim of the work is to retrieve which is the optimal stiffness of the adhesive layer to regularize the stresses. The peculiar geometry analyzed permits a straightforward analytical approach since pure torsion on tubes do not cause any bending and no Poisson's effect creates peel stress in the adhesive, therefore only shear stress are to be considered. The work first develops the equations which govern the shear stress distribution in the adhesive and then by forcing the shear stress to be constant is able to find out which is the stiffness profile along the bondline. The axial distribution of the stiffness of the FGA layer along the overlap is provided and the dependence on the elasto-geometrical parameters is discussed. The findings of the paper can be used to tailor the reinforcement distribution, under the hypothesis of a continuously changing adhesive stiffness.

Optimal topology design of continuum structures with stress concentration alleviation via level set method

SUMMARYAlthough the phenomenon of stress concentration is of paramount importance to engineers when they are designing load‐carrying structures, stiffness is often used as the solely concerned objective or constraint functional in the studies of optimal topology design of continuum structures. Sometimes this will lead to optimal designs with severe stress concentrations that may be highly responsible for the fracture, creep, and fatigue of structures. The aim of the present work is to develop some effective numerical techniques for designing stiff structures with less stress concentrations. This is achieved by introducing some specific stress measures, which are sensitive to the existence of high local stresses, in the problem formulation and resolving the corresponding optimization problem numerically in a level set framework. Our study indicates that with use of the proposed numerical schemes, some intrinsic difficulties in stress‐related topology optimization of continuum structures can be overcome in a natural way. Copyright © 2012 John Wiley & Sons, Ltd.

A Fatigue Model for Spur Gear Contacts Operating Under Mixed Elastohydrodynamic Lubrication Conditions

This paper presents a model to predict the crack formation fatigue lives of spur gear contacts operating under mixed lubrication conditions where surface roughnesses introduce intermittent metal-to-metal contacts and severe stress concentrations. The proposed model consists of several submodels, including (i) a gear load distribution model to determine the normal tooth force distribution along the tooth surface, incorporating any profile modifications and manufacturing deviations, (ii) a mixed elastohydrodynamic lubrication model customized to handle transient contact conditions of gears, (iii) a stress formulation that assumes the plane strain condition to compute the transient elastic stress fields on and below the tooth surface induced by the mixed lubrication surface pressure and shear stress distributions, and (iv) a multi-axial fatigue model to predict the crack nucleation life distribution. The proposed spur gear fatigue model is used to simulate the contacts of gear pairs having different surface roughness amplitudes. The predictions are compared to the measured gear fatigue stress-life data for each surface condition to assess the model accuracy in the prediction of the crack nucleation fatigue lives as well as the location of the critical failure sites.

Shear bond strength test using different loading conditions - a finite element analysis

Aim: This study evaluated the stress distribution at the bond interface during shear bond strength testing for three loading conditions. Methods: A three-dimensional model was created of a specimen for evaluation by the shear bond strength test, using three cylindrical volumes representing the dentin, adhesive system and composite resin. A linear analysis was performed to calculate the stress distribution at the dentin-adhesive interface. Three models simulating different loading conditions were prepared: chisel, orthodontic-looped wire and stainless steel tape. Results: Chisel presented severe stress concentrations near the loading site (-10681 to 637 MPa). Wire presented stress concentrations along the radial loading axis (-382 to 216 MPa). Tape presented more uniform stress distribution (-83 to 21 MPa). Conclusions: The loading with stainless steel tape allowed more uniform stress distribution at the bond interface, and was a more reliable way to evaluate the bond with regard to the aim of the shear bond strength test.

Surface Effects on Elastic Fields Around Surface Defects

There are always severe stress concentrations around surface defects like grooves or bugles, which might induce the failure of solid materials and structures. In the present paper, we consider the elastic fields around nanosized bugles and grooves on solid surfaces. The influence of surface elasticity on the elastic deformation is addressed through a finite element method. It is found that when the size of defects shrinks to nanometer, the stress fields around such defects will be affected significantly by surface effects.

A modified moment resisting connection for ductile steel frames (Numerical and experimental investigation)

Abstract The seismic behavior of steel moment resisting building systems are directly affected by the local behavior of beam to column connections. In welded connections, severe stress concentrations at the connection edge coupled with imperfections of the weld roots, may contribute to significant reduction of the seismic performance. In this study, a reduced plate section connection is proposed to shift the stress concentration from the connection face. With this aim, the authors propose drilled holes at cover plates to create an intentional weak point. Applying this method, the stress concentration at the weld roots is significantly diminished and the failure mechanism is altered from premature weld fracture to ductile plate yielding around the holes. This technique eliminates unfavorable local beam failures present in some similar methods and also forces the cover plate to behave as the replaceable fuse of the connection system. As it is revealed from the finite element and experimental investigations, the strength and ductility capacity of the considered connection type is significantly improved and the behavior of the connection seems to be far less dependant of the weld root quality. This might improve the structural reliability of the connection due to the guided failure mechanism.

A Novel Joint between Dissimilar Materials Inspired by the Mechanics of Trees

ABSTRACTAn integrated experimental and numerical investigation was conducted for removing the free-edge stress concentrations in dissimilar material joints. A convex interface/joint design, inspired by the shape and mechanics of trees, allows for least stress concentrations at bi-material corners for most engineering material combinations. In-situ photoelasticity experiments on convex polycarbonate-aluminum joints showed that the free-edge stress concentration was successfully removed. As a result, the new design not only improves the static load transfer capacity of dissimilar material joints, but also yields more reasonable interfacial tensile strength evaluation. For convex polycarbonate-aluminum and PMMA-aluminum joint specimens, the ultimate tensile load increased up to 81% while the total material volume reduced by at least 15% over that of traditional butt-joint specimens with severe free-edge stress concentrations.

Modeling of Tapered Sandwich Panels Using a High-Order Sandwich Theory Formulation

A newly developed high-order sandwich theory formulation is presented, which enables the analysis of sandwich beams or plates with variable core thickness. The faces are assumed to be of constant thickness and may be inclined arbitrary angles α 1 and α 2 , respectively, relative to the sandwich panel reference plane. The core thickness may change linearly over the length of the sandwich panel. The core is modeled as a specially orthotropic solid possessing stiffness in the out-of-plane direction only, thus including the transverse core flexibility in the modeling. The faces are modeled as laminated beams or plates including bending-stretching coupling and transverse shear effects. To validate the proposed high-order theory, the numerical results are compared with results obtained from finite element analysis, and a close match is observed. Furthermore, to demonstrate the features of the developed high-order sandwich theory formulation, numerical results obtained for two different types of tapered sandwich beams in three-point bending are presented. The characteristics of the elastic responses of the two sandwich panel configurations are compared with special emphasis on the complicated interaction between the faces through the core material. The analyses show that severe localized bending effects are displayed in the vicinity of load introduction and support points and in the vicinity of points/locations of abrupt geometric changes. These localized bending effects induce severe stress concentrations and may severely endanger the structural integrity of the sandwich panels under consideration.

Fatigue failures of austenitic stainless steel orthopedic fixation devices

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.

Strengthening of RC slabs with circular composite patches – a high-order approach

A new approach for the strengthening of existing reinforced concrete slabs using circular patches made of composite materials, and a mathematical model for the analysis of such strengthened slabs are presented. The model follows the concepts of the High-Order theory and fulfills all equilibrium and compatibility requirements as well as boundary and continuity conditions. The field equations and the boundary and continuity conditions are derived via the variational principle of virtual work assuming axisymmetric behavior. The constitutive relations of laminated materials are used for the composite patch and unique constitutive relations are derived for the RC member. The proposed model is used for the investigation of a square slab strengthened with circular patches of various materials and sizes through a sub-structuring approach. The results include the overall behavior of the strengthened slab and the localized shear and vertical normal stresses near the edges of the bonded patch throughout the entire loading process. The effects involved with pretensioning of the circular patch are also examined. The study reveals that the proposed strengthening approach improves both the serviceability and the load resistance of the slab, however it is associated with severe stress concentrations near the edge of the bonded patch.

Interface Failure at Ply Drops in CFRP/Sandwich Panels

This paper presents the results of an investigation of delamination failure at "exterior" ply drops in tension loaded sandwich panels with laminated composite face sheets. The thickness of composite laminates used as face sheets of sandwich panels is often increased locally around highly loaded locations such as joints, rivets, bolts or inserts. At the stations where such ply drops are made, the local bending stiffness of the face sheet laminates change discontinuously, thus inducing severe local bending effects. These local bending effects induce severe stress concentrations in the interfaces between the dropped plies/sub-laminates and the remaining face sheet laminates, and this may initiate delamination failure. The present paper suggests a simple approach for stress analysis and a simple "characteristic length" stress criterion for the prediction of delamination failure. An experimental investigation has been carried out, and the measured delamination loads compare well the theoretical predictions.

Processing flaws in PZT transducer rings

In industrial fabrication, many PZT transducer rings fail to meet the specifications of physical properties, and some of them even fracture during poling, the final stage of fabrication. The microscopic examination of fractured surfaces, using SEM, shows the presence of pores and microcracks that intersect with pores of size significantly larger than the average grain size. Evaluation of the semi-finished products at various stages of fabrication indicates that the pores formed during pressing and the microcracks were initiated during poling. The strong correlation of microcracks with sharp wedges, each formed by two adjacent grains at the edge of a pore, suggests that these microcracks originated from the sharp wedges where severe stress concentrations usually occur. Significant inhomogeneities in composition of the as-sintered samples were dectected using EDS, and these inhomogeneities are attributed to the inhomogeneous evaporation of Pb and the presence of a liquid phase during sintering. The x-ray diffraction patterns indicate a reduction of the Ti/Zr molar ratio during sintering that, together with observation of second-phase particles at some triple grain junctions, suggests the presence of a liquid phase during sintering.

Sandwich plates with ‘through-the-thickness’ and ‘fully potted’ inserts: evaluation of differences in structural performance

A high-order sandwich plate theory, which includes the transverse flexibility of the core, is introduced. The theory includes separate descriptions of the face sheets and the core material, and general specification of loading and boundary conditions. The theory has been adapted especially for the analysis of sandwich plates with hard points in the form of inserts, and special attention is focused on the problem of sandwich plates with inserts of the ‘through-the-thickness’ and ‘fully potted’ types. The governing equations are formulated as a set of coupled first-order differential equations, which are solved numerically using the ‘multi segment method’ of integration. Numerical results obtained for sandwich plates with ‘through-the-thickness’ and ‘fully potted’ inserts subjected to out-of-plane loading are presented. Special emphasis is focused on describing the active load transfer mechanisms, and it is demonstrated that insert/potting and potting/honeycomb interaction effects play an important role in the build-up of local stress concentrations. A detailed comparison of the results obtained for ‘through-the-thickness’ and ‘fully potted’ inserts is presented, and it is shown that the mechanical response for the latter insert type generally exhibits much more severe stress concentrations than is the case for the first insert type. The paper is concluded with a discussion of design aspects including a few guidelines for design.

Localized bending effects in sandwich panels: photoelastic investigation versus high-order sandwich theory results

The problem of local bending in sandwich panels with a ‘soft’ core often encountered in laterally loaded sandwich panels is investigated by conducting a series of photoelastic experiments. The experimental results are compared with analytical results obtained using the high-order sandwich panel theory (HSAPT). The experimental set-up consists of a symmetrically supported sandwich beam in three-point bending. The skins and the core of the sandwich specimens are made from optically active i.e. temporary double refractive, epoxy resin and polyurethane rubber, respectively, giving a elastic modular ratio of about 1260. The investigation includes two specimens differing from each other only by the thickness of their skins. The results of the analytical and experimental studies reveal that severe local stress concentrations are induced in the near vicinity of the support points and the point of external load application. The comparison between the analytical and the experimental results is presented in the form of contour line maps through the entire length and depth of the core material, and at the upper and lower fibres of the skins. The comparative study demonstrated a very good agreement between the photoelastic measurements and the predictions based on the HSAPT.

Effect of Slide-to-Roll Ratio on Interior Stresses Around a Dent in EHL Contacts

The present, study extends the transient EHL point contact model and subsurface stress field calculation model to examine the influence of a surface dent on interior stresses in an EHL point contact under various slide-to-roll conditions. Results revealed that under the pure rolling condition the effect of a surface dent on the stresses is quite negligible. The presence of a shallow surface dent is unlikely to reduce the contact fatigue, life so long as pure rolling motion and good lubrication conditions are maintained. Unfortunately, the same cannot be said of the contact if it is operating in the boundary lubrication regime. When sliding was introduced, the surface indentation generated significantly high pressure spikes with a strong directional preference. These high-pressure spikes cause severe stress concentrations either below the trailing edge of the dent, if it moves faster than the opposing surface, or below the leading edge of the dent, if it moves slower than the opposing surface. The maximum ...

Finite extension and torsion of a cylindrical solid-fluid mixture featuring a rigid core

In this work, the finite extension and torsion of a swollen elastic cylinder bonded to an inner rigid cylindrical core are investigated in the context of the theory of interacting continua. The boundary value problem is solved numerically in order to determine the variation of the stretch-ratios, the stresses and also the volume fraction of the solid. The results of these investigations not only demonstrate significant gradients in the stretch ratios and severe stress concentrations but also stress reversals under certain conditions. These stress reversals which occur in the neighborhood of the interface of the inner and outer cylinders can have a significant impact on damage studies pertaining to fiber/matrix debonding in fiber-reinforced composites, for example.

Three-dimensional analyses of interface crack front fields

Three-dimensional finite element computation is performed for a through edge crack lying on an interface between an elastic-plastic material and a perfectly rigid substrate. The modified Gurson model suggested by Tvergaard and Needleman is used in the calculation. The distributions of stresses and microvoid volume fraction in an edge-cracked specimen subjected to a remote tensile stress are presented. It is found that the severe stress and void concentrations occur in the corner where the crack front runs into the free surface. The maximum microvoid volume fraction appears on the free surface when the remote stress is low, but appears inside the free surface at the higher remote stress.

Changes in lipoprotein profiles during intense military training.

The effects of intense military training on lipoprotein concentrations were monitored in a group of 44 Navy trainees. Dietary intakes and lipoprotein profiles were obtained before and after 5 weeks of physical conditioning and after 5 days of continuous, extremely intense physical and psychological stress. Body weight did not change significantly and dietary intakes were consistently high in saturated fat and cholesterol. After physical conditioning, serum total cholesterol and triglyceride concentrations were unchanged, while high-density lipoprotein (HDL) concentrations increased 31% (p less than 0.05). After 5 days of severe stress, total cholesterol and low-density lipoprotein concentrations decreased 17.2% and 30%, respectively (p less than 0.05), whereas HDL concentrations increased 12.1%. These data indicate that marked changes in lipoprotein profiles occur, not only with long-term physical conditioning, but also with strenuous training that lasts several days, despite high energy, fat, and cholesterol intakes.

On the non-homogeneous finite swelling of a non-linearly elastic cylinder with a rigid core

The finite of a non-linearly elastic cylinder bonded to a rigid core is presented in the context of the Theory of Interacting Continua. The results of this investigation for large deformations demonstrate that the constraint imposed by the rigid core induces non-homogeneous swelling characteristics with significant gradients in the stretch ratios and severe stress concentrations at the bond interface. These results could have important implications for a variety of fiber-reinforced composites featuring hygroscopic polymeric matrix materials, for example.

On crack arrestability by a circular hole based on computational crack path prediction

In stiffened plate structures, cracks are often formed at the intersections of the stiffeners, where severe local stress concentrations and welding residual stresses exist. Since the local stress distribution is rather complicated along the crack trajectory, crack curving is one of the essential features of this type of problems. Cracks initiated and growing in the stiffened plate structures are difficult being detected by small leakage of fluid, and the crack arrest design concept which is different from the leak before break (LBB) concept could be introduced in preventing the catastrophic failure of such structures.A computational scheme has been developed for the crack path prediction, which is performed by the step-by-step stress analysis ahead of the crack tip and the prediction of a curved increment of the crack growth by the use of analytically expressed asymptotic crack path in the neighbourhood of the crack tip. As a fracture criterion we use the locally symmetric condition which requires the Mode II stress intensity factor vanishing along the crack path.In this paper, holes are considered as crack arresters and their capability is examined by computational crack path prediction and experiments. We consider the case where a crack is approaching a circular hole. When the crack tip approaches in the neighbourhood of the hole, where the shortest distance between the crack tip and edge of the hole is within the radius of the hole, they attract with each other and the crack penetrates into the circular hole resulting arrest at the hole.