Tensile strength of cohesive powders

Measurement and prediction of cohesive powder behaviour related to flowability, flooding or arching in silos is found to be very challenging. Previous round robin [52] attempts with ring shear testers did not furnish reliable data and have shown considerable degrees of scatter and uncertainty in key measurements. Thus studies to build a reliable experimental database using reference materials are needed in order to evaluate the repeatability and effectiveness of shear testers and the adopted procedures.In this paper, we study the effect of particle size on the yield locus for different grades of limestone (calcium carbonate). We use the nonlinear Warren Spring equation to obtain the values of cohesion C, tensile strength T, and the shear index n. We recover linear (n = 1) yield loci for $d_{50}>70$ μm with respectively small C and T, with consistent, finite macroscopic friction C/T = 0.7. With particle size decreasing below 70 μm the response becomes more and more cohesive and non-linear ($1<n<2$).Then we compare the values of the parameters $C,T$ and n obtained from two different shear testers (Schulze and Brookfield PFT). Both testers run at positive confining stresses (slightly different ranges) and give identical results for large fractions (weakly cohesive). For strongly cohesive samples, the PFT results are very similar to the ring shear tester, with slightly smaller values for T, C, and n. Further experiments with a variety of cohesive powders are needed to confirm or rebut this systematic difference the two testers display for cohesive powders.Finally, we compare the (extrapolated) values of T with a direct, transverse measurement running at negative stresses, using the Ajax tensile tester, and found a very good agreement, which validates the Warren Spring equation for negative stresses.     

Tensile strength of hybrid composites

This paper presents an approximate, statistical analysis for the tensile strength of unidirectional hybrid composite materials consisting of two-dimensional arrays of alternating low elongation and high elongation fibres in a common matrix. Expressions for ineffective length and fibre strain concentration factor in hybrid composites are developed. The analysis identifies a number of important material properties that affect the failure process in hybrids: statistical fibre tensile failure strain characteristics, and fibre extensional moduli and cross-sectional areas. The influence of these parameters on the failure process is examined and differences between failure mechanisms in hybrids and composites containing only one type of fibre are considered. The analysis predicts that, in general, the failure strain of a hybrid should be different from those of composites reinforced with either of the parent fibres alone. It is found that the theory can explain the hybrid effect that has been observed by several authors: hybrids made by combining high elongation and low elongation fibres, such as graphite and glass, often display tensile failure strains greater than those of composites made from the low elongation fibres alone. Predicted failure strains are compared with available experimental data. Suggestions for future work in the area are presented.     

Tensile strength of ceramic powders

The tensile strength of granular materials has been extensively studied and is of great technological importance. Several theoretical or experimental relationships between density and strength have been proposed. This paper compares these models with experimental data from zirconia powders. It is shown that the most important parameter determining the strength of a green compact is not density but compaction pressure. A micromechanical model, which gives a qualitative understanding of ceramic powder behaviour under tensile stress, is also presented.     

Tensile strength of butt-joined epoxy-aluminum plates

Tensile tests conducted on butt joined epoxy-aluminum plates containing single cracks along the bond surfaces showed that there exists a characteristic crack length below which the fracture strength of the composite is mainly influenced by the stress concentrations at the bond edges. For specimens with cracks longer than the characteristic length the fracture of the composite is entirely controlled by the stress concentrations at crack tips. In the latter case the stress intensity factors defined by Sih and Rice, and Erdogan appeared to agree reasonably well with test results.

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Zusammenfassung Zugproben an stumpf aneinandergefügten, mit einzelnen Rissen nach den Bindeflächen behafteten Epoxyharz-Aluminiumplatten zeigen, daß es eine charakteristische Rißlänge gibt, unterhalb deter die Bruchfestigkeit des Verbundhalbzeugs maßgeblich von den Spannungskonzentrationen an den Bindekanten beeinflußt wird. Bei Probestücken mit Rissen, die gegen die charakteristische Länge groß sind, wird der Bruch des Verbundhalbzeugs gänzlich von den Spannungskonzentrationen an den Spitzen der Risse bestimmt. Im letzteren Fall scheinen die von Sih und Rice bzw. von Erdogan defmierten Intensitätsfaktoren vertretbar gut mit den Versuchsergebnissen übereinzustimmen.

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Résumé Des essais a la traction, effectués sur plaques d'époxyde-aluminium jointes par rapprochement et contenant crevasses individuelles le long des surfaces de joint, montrent qu'il existe une longueur caractéristique de crevasse, au-dessous de la résistance à la rupture du composé est essentiellement influencée par les concentrations de tractions aux arêtes de joint. Pour échantillons avec crevasses dont la longueur est grande devant la longueur caractéristique, la rupture du composé est déterminée entièrement par les concentrations de tractions an bout des crevasses. Dans ce dernier cas, les coefficients d'intensité de traction, définis par Sih et Rice et par Erdogan, semblent s'accordei plutôt bien avec les résultats des essais.

     

Tensile strength of discontinuous fibre-reinforced composites

A stochastic Monte-Carlo approach, based on Eyring's chemical activation rate theory, is used to study the factors controlling the tensile strength of discontinuous fibre-reinforced composites. The model explicitly takes into account the local distribution of stress near fibre ends. Both the fibre and the matrix are allowed to break during fracture of the composite. The stress-strain curves and the modes of failure of the composite are found to be strongly dependent on the volume fraction and aspect ratio of the fibres. The importance of adhesion at the fibre/matrix interface is also studied. The results are compared with available experimental data.     

Tensile strength of fibrous composites at elevated temperature

Abstract

This paper describes an analysing methodology to simulate the tensile strength of a unidirectional fibrous composite under thermomechanical loads using properties of constituent fibre and matrix materials and fibre volume fraction only. The stress states in the constituent phases at every mechanical load level are explicitly determined by making use of a bridging matrix, whereas the thermal stresses in the constituents are obtained based on Schepery's formulae. A composite failure is assumed when any constituent material attains its ultimate stress state. The maximum normal stress theory of isotropic materials is used to detect the constituent failure. A unidirectional alumina fibre reinforced aluminium matrix composite at a number of temperatures from room temperature to 773 K subjected to off axial loads has been analysed. The predicted off axial strengths are comparable with the experiments of Mutsuda and Matsuura.     

Tensile strength and failure mechanics of fibre composites

This paper is concerned with the prediction of the tensile strengths of fibre composites where the fibres are aligned in the direction of tensile loads, and are flawed to some extent. A theory is derived for predicting the strengths and failure mechanisms of such composites. The theory agrees reasonably well with experiments, and may be qualitatively applicable to composites containing randomly aligned fibres.     

Tensile strength of UD-composite laminates with multiple holes

The residual strength of glass fibre reinforced vinyl-ester laminates with multiple holes was investigated through an experimental programme. Different types of structured hole patterns and hole densities were investigated and analysed using digital image correlation strain measuring technique. Three different failure modes could be observed when the hole patterns and the hole densities were a altered. These three failure modes were used as the foundation for a simple yet effective analytical model in order to predict the residual strength of damaged composite specimens. Finally, a number of laminates with randomly distributed holes were tested experimentally. The analytical model can predict the failure mode and failure strength of the experiments with sufficiently good fidelity.     

Tensile strength and crack nucleation in boron fibres

Boron fibres of about 100m diameter have been tested to fracture in a micro-tensile testing machine. The fracture surfaces have been investigated using a scanning electron microscope and classified according to the fracture nucleation type. Comparison with the measured fracture stresses proved a significant correlation with the type of crack nucleation. Transverse cracks, nucleated at the edge of radial cracks along the fibre axis direction, are the most critical weakening feature. Also crack nucleation at the external fibre surface implies a low tensile strength, and is discussed in terms of a notch effect of the well-known nodular surface topography.     

Tensile strength enhancement in interground fiber cement composites

Interground fiber cement (IFC) is a new process where fibers are ground in with the cement clinker during the dry cement manufacturing process. With IFC considerable strength enhancement can be achieved compared to ordinary cement even at a fiber volume as low as 0.2% due to homogeneous fiber distribution and fiber surface modifications associated with the milling process. The cracking mechanisms associated with the strength enhancement were observed in real time during load application using a custom designed loading device. The homogeneous fiber distribution stabilizes crack growth. Formation of multiple, stable secondary microcracks was observed during the strain hardening regime, enhancing the strain capacity at ultimate strength. Fiber pullout was the dominant toughening mechanism in the strain softening regime. For fibers inclined to the propagating crack, fiber pullout was preceded by secondary microcrack formations along the fiber/matrix interface.     

Tensile strength,ductility and fracture of magnesium-silicon alloys

Tensile tests were performed between 293–573 K in order to investigate the mechanical properties of cast and extruded Mg-Si alloys. For the cast materials, Mg-high Si ( 10 wt%) alloys showed lower values of the highest tensile strength at temperatures up to 373 K, as compared to pure Mg and Mg-low Si (<10 wt%) alloys, whereas the strength at 573 K increased with increasing Si content. The addition of aluminum and zinc to the alloys was effective in increasing the strength. The fact that the Mg-high Si alloys showed lower strength than the Mg-low Si alloys was because a high volume of Mg₂Si embrittled the Mg-Si alloys. Microstructural investigations revealed that the particles of Mg₂Si were coarse for the cast materials and fracture of the particles was caused by deformation. The mechanical properties of the cast materials were improved by hot extrusion. Microstructural refinement by hot extrusion was responsible for the improvement of the mechanical properties.