Crackling noise in three-point bending of heterogeneous materials

We study the crackling noise emerging during single crack propagation in a specimen under three-point bending conditions. Computer simulations are carried out in the framework of a discrete element model where the specimen is discretized in terms of convex polygons and cohesive elements are represented by beams. Computer simulations revealed that fracture proceeds in bursts whose size and waiting time distributions have a power law functional form with an exponential cutoff. Controlling the degree of brittleness of the sample by the amount of disorder, we obtain a scaling form for the characteristic quantities of crackling noise of quasi-brittle materials. Analyzing the spatial structure of damage we show that ahead of the crack tip a process zone is formed as a random sequence of broken and intact mesoscopic elements. We characterize the statistics of the shrinking and expanding steps of the process zone and determine the damage profile in the vicinity of the crack tip.Comment: 11 pages, 15 figure

Characterization of the bending stiffness of large space structure joints

A technique for estimating the bending stiffness of large space structure joints is developed and demonstrated for an erectable joint concept. Experimental load-deflection data from a three-point bending test was used as input to solve a closed-form expression for the joint bending stiffness which was derived from linear beam theory. Potential error sources in both the experimental and analytical procedures are identified and discussed. The bending stiffness of a mechanically preloaded erectable joint is studied at three applied moments and seven joint orientations. Using this technique, the joint bending stiffness was bounded between 6 and 17 percent of the bending stiffness of the graphite/epoxy strut member

Feasibility study of fusion bonding for carbon fabric reinforced Polyphenylene Sulphide by hot-tool welding

In recent years, there is a growing interest in joining techniques for thermoplastic composites as an alternative to adhesive bonding. In this article, a fusion bonding process called hot-tool welding is investigated for this purpose and the used material is a carbon fabric reinforced polyphenylene sulphide. The welds are first observed through a microscope, after which the quality is experimentally assessed using a short three-point bending setup. A comparison is made between the welded specimens and the equivalent hot pressed specimens. It can be concluded that the hot-tool welding process is very promising for the welding of material under study and that the short three-point bending setup proves interesting for evaluating bonds between composite specimens

Stress intensity and crack displacement for small edge cracks

The weight function method was used to derive stress intensity factors and crack mouth displacement coefficients for small edge cracks (less than 20 percent of the specimen width) in common fracture specimen configurations. Contact stresses due to point application of loads were found to be small but significant for three-point bending and insignificant for four-point bending. The results are compared with available equations and numerical solutions from the literature and with unpublished boundary collocation results

Stable incremental deformation of a strip to high strain

This paper presents the effect of combined stretching and bending on the achieved strain in\ud incremental sheet forming ISF. A simple two dimensional model of strip undergoing stretching and\ud travelling three point bending in cyclic form is used. The numerical model presents the effect of the\ud ratio of stretching velocity to roll-set speed on the achieved strain and its distributio

Geometry functions for edge cracks in steel bridge under three- and four- point bending with various span

Fatigue cracks are found during the regular structural inspections. To precisely describe/suggest propagation of fatigue cracks throughout structure and it’s designed service life, the knowledge of geometry functions describing the stress situation in front of the crack tip for relative crack lengths are important. The cracks usually propagate/initiated from the edge or the surface of the structural element, where the maximum value of applied load is achieved. The theoretical model of fatigue crack propagation is based on linear fracture mechanics (Paris law). Steel structural elements are subjected to various bending load (three-, four- point bending and pure bending etc.). The geometry functions for the edge cracks are calculated for various span according to real steel bridge elements and appropriate polynomial functions independent on the distance are proposed for three- and four- point bending load

Stress intensity and displacement coefficients for radially cracked ring segments subject to three-point bending

The boudary collocation method was used to generate Mode 1 stress intensity and crack mouth displacement coefficients for internally and externally radially cracked ring segments (arc bend specimens) subjected to three point radial loading. Numerical results were obtained for ring segment outer to inner radius ratios (R sub o/ R sub i) ranging from 1.10 to 2.50 and crack length to width ratios (a/W) ranging from 0.1 to 0.8. Stress intensity and crack mouth displacement coefficients were found to depend on the ratios R sub o/R sub i and a/W as well as the included angle between the directions of the reaction forces

Numerical simulation of three-point bending tests : two distinct approaches

In this work are presented and discussed the numerical simulations carried out for indirect tensile tests of steel fibre reinforced self-compacting concrete specimens. The post-cracking behaviour was modelled with two distinct approaches. Within the scope of the first one, the s-w relationships were obtained by an inverse analysis procedure up to distinct ultimate crack widths. The other approach consisted on modelling the behaviour of the composite based upon the micro-mechanical behaviour of the fibres. For this purpose the composite was modelled as two-phase material under the FEM basis, with an unreinforced concrete matrix phase (paste + aggregates) and a fibre phase. The fibre phase comprises information about fibre density and orientation depending on where and how the material is applied

Determination of wood strength properties through standard test procedures

In this study a review of existing recognised standards for wood mechanical testing was conducted. This review considers tensile, compressive, bending and shear test methodologies from a range of sources. In addition, values for wood mechanical properties were obtained through controlled experimentation using a universal material testing machine. Selected standard procedures were used to obtain wood strength properties both along and across the grain. These consist of a three point bending procedure used to evaluate the wood strength across the grain and a longitudinal shear procedure used to evaluate the wood strength along the grain. Strength properties obtained through controlled experimentation are compared to values available in existing literature with little discrepancy

Permanent indentation characterization for low-velocity impact modelling using three-point bending test

This paper deals with the origin of permanent indentation in composite laminates subjected to low-velocity impact. The three-point bending test is used to exhibit a non-closure of matrix crack which is assumed as a cause of permanent indentation. According to the observation at microscopic level, this non-closure of crack is produced by the blocking of debris inside matrix cracking and the formation of cusps where mixed-mode delamination occurs. A simple physicallybased law of permanent indentation, ‘‘pseudo-plasticity’’, is proposed. This law is qualitatively tested by three-point bending finite element model and is lastly applied in low-velocity impact finite element model in order to predict the permanent indentation. A comparison between low-velocity impact experiments and simulations is presented