Emerald | International Journal of Structural Integrity | Table of Contents

Finite element simulation of a rollover protective structure

literatinetwork@emeraldinsight.com (Tiago R. Cesa, Branca F. Oliveira) — Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The purpose of this paper is to present a finite element simulation to validate the strength and energy absorption capacity of a rollover protective structure (ROPS) of agricultural tractors. The test consists of four steps: rear loading, crushing of the rear columns, side loading and crushing of the front columns. In this study a new design of a cabin for narrow tractors was simulated and from it the computational test was run for validation of the cabin. The simulation was performed using ANSYS software, considering the nonlinear characteristics of the materials, since during the test the plastic limit is reached. With the computational results, it was possible to predict the behavior of the structure before the real test. These results were used to propose design and materials changes that significantly improved the energy absorption, making it more efficient. The proposed cabin design reaches the energies and forces required in each step of the computational simulation of the ROPS test and the deformation needed to achieve them does not cause any cabin part to enter the operator survival space inside the cabin. Design/methodology/approach – In the present study, numerous finite elements simulations have been carried out on a narrow cab for tractors. Findings – It was conclude that it is possible to virtually test the ROPS, making material and design changes in order to have a more adequate structure before a first real test. Originality/value – The present study is part of Tiago R. Cesa's Master thesis, an original research work.

Assessing the dynamic failure response of MEMS structures

literatinetwork@emeraldinsight.com (Gilad Sharon, Rachel Oberc, Donald Barker) — Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The development of micro-electro-mechanical systems (MEMS) for use in military and consumer electronics necessitates an analysis of MEMS component reliability. The understanding of the reliability characteristics of SCSi within MEMS structures should be improved to advance MEMS applications. Reliability assessments of MEMS technology may be used to conduct virtual qualification of these devices more efficiently. The purpose of this paper is to create a simple, inexpensive test methodology to use the dynamic fracture strength of a MEMS device to predict its reliability, and to verify this method through experimentation. Design/methodology/approach – The dynamic fracture strength of single crystal silicon (SCSi) was used to model MEMS devices subjected to high shock loading. Experimentation with SCSi MEMS structures was performed following the proposed test methodology. A probabilistic distribution for bending of Deep Reactive Ion Etching (DRIE) processed SCSi around the <110> directions was generated as a tool for assessing product reliability. Findings – Post shock test inspections revealed that failures occurred along {111} planes. Additional experiments provided preliminary estimates of the fracture strength for bending of DRIE processed SCSi around the <100> directions in excess of 1.1?GPa. Originality/value – This paper proposes a test methodology for an efficient method to assess the reliability of processed SCSi based on dynamic fracture strength.

Interaction of two cracks in an orthotropic lamina subjected to uniaxial loading

literatinetwork@emeraldinsight.com (Shashikant J. Joshi, Smita Manepatil) — Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The determination of stress intensity factors (SIF) is of fundamental importance in prediction of brittle failure using linear elastic fracture mechanics. The presence of a crack in the vicinity of another crack induces an interaction effect. The purpose of this paper is to determine the SIF for an orthotropic lamina subjected to uniaxial loading and containing two cracks. The solution is obtained for one crack being horizontal and located in the centre of lamina while the other crack is inclined to first one. The effect of angle of the second crack, fibre angle is studied. Also, for the case of two parallel cracks, effect of eccentricity in x and y directions is observed. Design/methodology/approach – Boundary collocation method is used and stress functions satisfying governing equations in the domain and ensuring stress singularity at the crack tips are defined. The boundary condition on the edges of lamina and the crack is satisfied to determine the complex coefficients in the stress functions. Findings – For the given fibre angle, orientations of second crack which result in increase/decrease in the SIF at the most dangerous crack tip are found out. Originality/value – Boundary collocation method which is simple and efficient is extended for studying two crack problem in orthotropic materials.

On the application of the theory of critical distances to the structural integrity assessment of stress risers

Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The purpose of this paper is to present and validate a methodology for the structural integrity assessment of components containing a variety of stress risers and subjected to static conditions. Design/methodology/approach – The methodology is based on the use of the apparent fracture toughness prediction provided by the theory of critical distances (in this case, the line method), together with a well-known, widely-used engineering tool in structural integrity assessments: failure assessment diagrams. In order to validate the proposed methodology, an experimental programme has been conducted, testing 38 specimens made of aluminium alloy Al7075-T651, each of them containing a certain stress riser. The comparison between the experimental results and the corresponding predictions provided by the proposed assessment methodology has also allowed the situations for which the theory of critical distances provides accurate predictions to be defined. Findings – The results show that the methodology provides accurate results as long as the Neuber number, defined as the notch radius divided by the critical distance (L), is sufficiently low. In order to extend the validity to situations where the Neuber number is higher, it is necessary to calibrate L by using notched specimens with similar radii to those found in the defects being analysed. Originality/value – The present study is part of Virginia Madrazo's doctoral thesis, an original research work.

Numerical and experimental study on buckling behaviour of multiple delaminated composite plates

Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – With the widespread use of the composites over other metallic materials in different fields of engineering, studies on damages of composite structures have assumed great importance. Among various kinds of damages, delamination is of very serious concern to composite applications. It may arise as a consequence of impact loading, stress concentration near a geometrical or material discontinuity or manufacturing defects. The presence of one or more delaminations in the composite laminate may lead to a premature collapse of the structure due to buckling at a lower level of compressive loading. So the effect of delamination on stability of composite structures needs attention and thus constitutes a problem of current interest. The purpose of this paper is to deal with both numerical and experimental investigations on buckling behaviour of single and multiple, delaminated, industry driven, woven roving glass/epoxy composite plates on clamped free clamped free (CFCF) rectangular plates. Design/methodology/approach – For numerical analysis, a finite element model was developed with an eight noded two dimensional quadratic isoparametric element having five degrees of freedom per node. The elastic stiffness matrices were derived using linear first order shear deformation theory with a shear correction factor. Green's nonlinear strain equations are used to derive the geometric stiffness matrix. The computation of buckling load based on present formulation is compared with the experimental results for the effect of different parameters on critical load of the delaminated composite panels. In the experimental study, the influences of various parameters such as delamination area, fiber orientations, number of layers, aspect ratios on the buckling behaviour of single and multiple delaminated woven roving glass/epoxy composite plates were investigated. Buckling loads were measured by INSTRON 1195 machine for the delaminated composite plates. Findings – Comparison of numerical results with experimental results showed a good agreement. Both the results revealed that the area of delaminations, fiber orientations, number of layers and aspect ratio have paramount influence on the buckling behaviour of delaminated plate. Originality/value – The present study is part of Jayaram Mohanty's doctoral thesis, an original research work.

Improvement of fatigue limit by shot peening for high-tensile strength steel containing a crack in the stress concentration zone

literatinetwork@emeraldinsight.com (Keiji Houjou, Koji Takahashi, Kotoji Ando) — Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The purpose of this paper is to investigate the effect of shot peening (SP) on the fatigue limit of high-tensile-strength steel containing a crack in the stress concentration zone. Design/methodology/approach – An artificial semi-circular slit was introduced into the bottom of notch, and SP was performed. Bending fatigue tests were then carried out. Findings – First, the fatigue limits of specimens containing a slit of 0.2 or 0.3?mm in depth were improved up to approximately twice their original values. Second, in the case of shot-peened specimens with a crack of 0.2?mm in depth, the fractures occurred from outside the slit. Moreover, the specimens recovered to fatigue limits up to those of non-slit specimens. Finally, the effect of stress concentration (Kt=1.9) on the slit size could be rendered harmless by SP was not found in the fatigue test. Originality/value – There are very few examples of evaluations of the fatigue limit of materials containing crack-like surface defects after SP has been performed in the stress concentration zone. The study elucidated the effect of SP on the fatigue limit in such materials, compared with that of a smooth zone.

Comparison between positive and negative bias stress on N-channel VDMOSFET transistors

Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The purpose of this paper is to apply a negative gate bias stress in order to study instabilities of threshold voltage in N-channel power vertical double-diffused metal-oxide-semiconductor field effect transistor (VDMOSFET). Variations in gate oxide trapped charge and interface trap densities are also calculated. Design/methodology/approach – A threshold voltage shift is detected; the oxide and interface trap densities were evaluated based on a direct measurement of the gate to source capacitance and conductance. Findings – Results presented show that the threshold voltage is decreasing with stress time, the capacitance and conductance curves are altered by applied stress, also the oxide traps and the interface traps densities are increasing with stress time. Originality/value – The positive bias stress seems to be more destructive in the case of the studied devices.

Tensile properties of corroded embedded steel bars B500c in concrete

literatinetwork@emeraldinsight.com (Ch. Alk. Apostolopoulos, Vassilios Kappatos) — Fri, 24 May 2013 00:00:00 +0100

Abstract

Purpose – The corrosion of reinforcing steel bars reduces significantly the life and durability of concrete structures. This critical concern causes great losses to the economy and industry. The purpose of this paper is to estimate the effects of corrosion on the tensile mechanical properties of embedded steel bars B500c in concrete. Design/methodology/approach – The concept is based on the curve fitting modelling, as well the mathematical correlation of the tensile mechanical properties between corroded bare and corroded embedded steel bars. In order to achieve this, extensive experiments were carried out on both bare (Ø8, 10, 12, 16 and 18?mm) and embedded (Ø8?mm) steel bars B500c, which were subjected to artificially accelerated corrosive conditions in a chloride-rich atmosphere for several exposure times. Findings – The research results show that the estimation method is available and effective in simulating the tensile mechanical behaviour of corroded reinforcing steel bars B500c. Originality/value – As far as is known, this is the first time that an advanced data processing technique has been employed to try to find the mathematical correlation of the existing corrosion damage on the residual tensile properties between bare and embedded steel bars. It is argued that these models can be developed in order to reduce the need for expensive experimental investigation in materials.

Editorial

literatinetwork@emeraldinsight.com (Chris A. Rodopoulos) — Fri, 24 May 2013 00:00:00 +0100