Investigation into the loosening mechanism of bolt in curvic coupling subjected to transverse loading

It has been widely observed that fasteners turn loose when subjected to dynamic loads in the form of shock, vibration or transverse cyclic loading. This reduces the preload force of bolt and leads to joint failure. Such failures can be catastrophic in safety critical applications. In this paper, the self-loosening of bolts in curvic coupling is analyzed based on the self-rotation of nut in the cases of cyclic torque loads on discs after the preload of bolts. The three-dimensional finite element model for curvic coupling and threads is established in commercial finite element software ANSYS to study the details of the self-loosening process of bolt. Such processes are characterized by microslip at the curvic, the bolt head and the thread contact surfaces. It is found that due to the application of the cyclic transverse load, the nut rotation can occur for only localized slip without complete slip at the bolt head contact surface and the thread surface. However, the curvic surface always shows complete slip under all external loads. The microslip on all contact surfaces are identified to be the major mechanisms responsible for the self-loosening of a curvic coupling. The results obtained agree quantitatively with the experimental observations.     

Localised strain and stress in bonded concrete overlays subjected to differential shrinkage

Bonded concrete overlays are widely used for repair and strengthening of existing structures as well as for precast elements which receive an in-situ topping. The performance of such overlays relates mainly to their resistance to cracking and debonding. Associated failure mechanisms are a result largely of differential volume changes between substrate and overlay. The objective of this paper is to provide an analytical tool to facilitate the design of bonded overlays for crack-resistance when subjected to shrinkage restraint. Fundamental strain characteristics of composite members are identified and existing analytical models for the prediction of strains and stresses in bonded overlays are evaluated. Results from experimental work indicate that existing models, which are based on simple beam theory, are deficient in modelling overlay strains realistically. The degree of overlay restraint was found to depend far less on relative section dimensions of substrate and overlay than is commonly assumed. Based on fundamental aspects concerning strain characteristics of bonded overlays, an analytical prediction model is introduced, based on localised strain conditions at the interface.     

Elastic T-stress for cracks in test specimens subjected to non-uniform stress distributions

Finite element analyses have been conducted to calculate elastic T-stress solutions for cracked test specimens. The T-stress solutions are presented for single edge cracked plates, double edge crack plates and centre cracked plates. Uniform, linear, parabolic or cubic stress distributions were applied to the crack face. The results for uniform and linear stress distributions were used to derive weight functions for T-stress for the corresponding specimens. The weight functions for T-stress are then verified against several linear and non-linear stress distributions. The derived weight functions are suitable for the T-stress calculation for cracked specimens under any given stress field.     

Notch root strain and stress distributions in power hardening materials

Based on the results of an elastoplastic analysis for a pure bending V-notched bar, computations of the effective plastic strains at 11 hardening exponents are carried out. It is found that a further modified form of the slip-line solution can satisfactorily describe in the plastic zone the variation of the maximum principal stress below the notch root in power hardening materials.

---

Résumé En se basant sur les résultats de l'analyse électro-plastique d'un barreau à entaille Vé en flexion pure, on calcule les déformations plastiques réelles correspondant à 11 valeurs du module d'écrouissage. On trouve qu'une modification supplémentaire de la forme de la solution par bandes de glissements permet de décrire de manière satisfaisante la variation dans la zone plastique des contraintes principales maximum sous la base de l'entaille, dans des matériaux sensibles au durcissement.

     

Substandard reinforced concrete members subjected to compression: FRP confining effects

The investigation focuses on the effectiveness of fiber-reinforced polymer (FRP) confinement in upgrading ductility and strength of reinforced concrete members under axial monotonic compression. An experimental program is presented that extends available database to address the behavior of old type members with square section, having extremely low concrete strength and potential longitudinal bars’ premature buckling. Reinforced concrete specimens were strengthened by carbon or glass FRP wraps while plain FRP confined concrete specimens were also constructed and tested to evaluate comparatively the confining effects of steel stirrups, FRP wraps, or of dual confinement. The achieved strength, ductility and energy absorption levels of the specimens were quantified to assess the effect of the longitudinal bars. Finally, a handy design-oriented empirical strength model is proposed. According to the proposed approach, no estimation of effective stress or strain at failure of FRP jacket is necessary. The satisfactory accuracy of the predictions of the proposed model is demonstrated through comparison against existing models and over a large database of results on uniform confinement as well as over presented specimens.     

Lubricant effects on strain fields of seven wire strands subjected to tensile load

As a part of an extensive analysis on strand behaviour the lubricant effect in strain fields was examined. The strains were measured by using electrical resistance strain gauges. The results obtained, by loading the unlubricated and lubricated sample both statically and quasi-statically, showed relevant effects caused by different contact loads developed among the wires. By examining the strains measured during the severing of one helical wire the inapplicability to the strands of one Italian regulation relative to wire ropes, was also confirmed.     

Flexoelectric effects: Charge separation in insulating solids subjected to elastic strain gradients

After a brief historical introduction this article will present a summary of experimental work carried through at Penn State to explore the flexoelectric coefficients μ _ijkl in ferroelectric, incipient ferroelectric and relaxor ferroelectric perovskites. The initial objective was to understand the magnitude of flexoelectricity in these systems to see whether it would be possible to develop a piezoelectric composite containing no piezoelectric element, which nonetheless could have practically useful properties. Recent discussions of the thermodynamic converse effect, ie. the generation of elastic strain by an electric field gradient, now suggest that such composites might be designed to have unique properties such as a direct but no converse effect, or vice-versa, and materials with this character could have important practical application. Present data already suggest that the direct effect may make an important contribution to the properties of epitaxial thin films where mismatch can give rise to very steep elastic strain gradients. Clearly, more work is needed to fully quantify the flexoelectric behavior. It will be important to measure single crystals in the ceramic systems which have been studied and to characterize the converse effect as a check of the measured values.     

Shear coupling effects of the core in curved sandwich beams

We consider a composite package formed by two curved external Euler-Bernoulli beams, which sandwich an elastic core with negligible bending strength but providing the shear coupling of the external layers. This coupling considerably affects the gross response of the composite structure. There is an extensive literature on straight sandwich beams of this type, but very little attention has been paid to the effects of curvature. Here, an analytical linear elastic model is proposed for beams with arbitrary variable curvature. Equilibrium equations and boundary conditions are obtained through a variational approach. Useful simplifications are possible for the case of moderately curved beams and beams with constant curvature.     

Critical moments in hinge-connected beams subjected to transverse impact loading

The dynamic system consists of two uniform, vertical beams, one above the other, with a hinge between them and an achor hinge at the base. For a transverse load suddenly applied anywhere along the upper beam, the critical or peak moment in the lower beam of length l occurs at a distance y_c = l/√3 from its base. When this impact load is applied to the lower beam in the neighborhood of the hinge, a primary critical moment always occurs at the impact point; but a secondary critical moment may also occur at a predictable distance y_c<l/√3 which depends on the relative masses of the two beams. Experimental data support these predicted results. The solutions, presented in closed form, are applicable to the design of impact resistant articulated buoys subject to the collision of ships and floating ice.     

Time resolved plastic deformations in rods subjected to transverse impact

This paper describes experiments to record deformation profiles versus time in metallic rods subjected to transverse ballistic impacts. Rod materials included a tungsten alloy, four alloys of steel, titanium, and copper. The deformation profiles were analyzed to determine time resolved transverse plastic deformation wave speeds in all the rod materials and to estimate effective fracture shear strains in some of the materials. These material parameters are key elements in an analytical model to estimate deformations and failure in long rod penetrators subjected to transverse loads arising from non-ideal plate penetration geometries (yaw and obliquity). The rod materials were also subjected to tensile tests and Taylor anvil impact tests to measure static tensile and dynamic compressive strengths and longitudinal plastic wave speeds. The Taylor anvil and transverse rod impact tests together furnish the required dynamic material properties for comparing the penetration effectiveness of candidate rod materials and geometries.     

Effects of short cracks produced at blunted pre-crack tip on stress and strain distributions

The present work investigates problems: (1) How are the plastic strain and the stress (triaxiality) re-distributed after a short crack initiated, extended and blunted at the pre-crack tip? (2) How do the above changes put a crucial effect on the triggering of the cleavage fracture? Based on the previous observations of configuration changes and fracture surfaces of pre-crack tips, Finite element method (FEM) simulations of a short crack initiated, extended and blunted at a pre-crack tip and calculations of distributions of stress, strain and triaxiality are carried out for 3PB pre-cracked HSLA steel specimens tested at -130^°C. The results reveal that: as long as the fatigue pre-crack is only blunted, in its vicinity a region where the accumulated strain is sufficient to nucleate a crack, and a region where the stress (triaxiality) is sufficient to propagate a crack nucleus are separated by a distance. The nucleated crack cannot be propagated and the cleavage fracture cannot be triggered. While a short crack produced at the fully blunted fatigue pre-crack, the strain retains, the stress (triaxiality) is rebuilt. An initiated and significantly extended and then blunted short crack makes a tip configuration, which on one hand is much sharper than that of the fully blunted original pre-crack tip, on other hand is wide enough to spread its effects into the high stress covered region. This sharpened crack tip configuration re-builds a ‘sharper’ distribution of stress (triaxiality) and makes two regions metioned above closer. Finally the two regions overlap each other and a cleavage crack can be initiated and propagated at a distance ahead of the blunted fatigue pre-crack.     

Failure mechanisms on composite specimens subjected to compression after impact

Composite panels are widely used in aeronautic and aerospace structures due to their high strength/weight ratio. The stiffness and the strength in the thickness direction of laminated composite panels is poor since no fibres are present in that direction and out-of-plane impact loading is considered potentially dangerous, mainly because the damage may be left undetected. Impact loading in composite panels leads to damage with matrix cracking, inter-laminar failure and eventually fibre breakage for higher impact energies. Even when no visible impact damage is observed at the surface on the point of impact, matrix cracking and inter-laminar failure can occur, and the carrying load of the composite laminates is considerably reduced. The greatest reduction in loading is observed in compression due to laminae buckling in the delaminated areas. The objective of this study is to determine the mechanisms of the damage growth of impacted composite laminates when subjected to compression after impact loading. For this purpose a series of impact and compression after impact tests were carried out on composite laminates made of carbon fibre reinforced epoxy resin matrix. An instrumented drop-weight-testing machine and modified compression after impact testing equipment were used together with a C-scan ultrasonic device for the damage identification. Four stacking sequences of two different epoxy resins in carbon fibres representative of four different elastic behaviours and with a different number of interfaces were used. Results showed that the delaminated area due to impact loading depends on the number of interfaces between plies. Two buckling failure mechanisms were identified during compression after impact, which are influenced more by the delamination area than by the stacking sequence.     

Mapping microstructure inhomogeneity using electron backscatter diffraction in 316L stainless steel subjected to hot plane strain compression tests

Abstract

The microstructure inhomogeneity in 316L stainless steel subjected to hot plane strain compression tests has been assessed using electron backscatter diffraction (EBSD). Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed isothermally at 950°C at nominal equivalent tensile strain rates of 0·01 and 1 s^?1. Low and high friction conditions have been simulated by applying both a glass based lubricant and a boron nitride spray respectively. Results suggest that friction causes a variation in microstructure from the surface to the midplane of the deformed specimen. Several methods used to quantify and represent this inhomogeneity are presented in the present paper. Electron backscatter diffraction measurement issues are discussed. A grain size mapping method using a two-dimensional moving average has been developed to overcome the difficulties associated with the visualisation of measurement results over large areas on EBSD maps. It has proved to be a powerful tool for the spatial statistics of large quantity data obtained by EBSD.     

Delamination vs matrix cracking in layered composites subjected to transverse loading

An algorithm is presented to compute the distribution of the strain energy release rate along the crack front of a penny-shaped delamination in a layered orthotropic body. The method applies a finite element recently proposed for three-dimensional analysis of layered orthotropic circular plates. The algorithm is economical even though it treats a full three-dimensional state of stress. The method requires only a single virtual crack extension to accurately compute the strain energy release rate at a point along the crack front. The method is applied to the study of delamination crack growth in a nine layer cross-ply laminate. The variation of strain energy release rate, G, along the crack front, is determined. The significance of the plate aspect ratio, as well as length scale, on the fracture process is studied. The establishment of a loading case where a distributed transverse compressive loading causes delamination growth is given.     

Stress and strain in a plate subjected to intensive drying

The study was carried out to investigate the development of a stress-strain state in a thin porous plate subjected to intensive drying. A narrow evaporation zone (an evaporation front) divides the plate into two regions with different structures and rheological behavior. The complete saturation area is described by Kelvin-Voigt's viscoelastic model, and the dry area, by Hooke's elasticity law. Shrinkage is suggested to be a function of the evaporation zone velocity. The influence of variable shrinkage on the stress distribution across the plate is studied.N. G. Chebotarev Research Institute of Mathematics and Mechanics, Kazan State University, Kazan. Translated from henerno-Fizicheskii Zhurnal, Vol. 63, No. 2, pp. 237–241, August, 1992.