Showing papers on "Compressive strength published in 1981"

Flexural strength and porosity of cements

Abstract: A curious feature of hydraulic cements, such as those based on calcium silicate, calcium aluminate and calcium sulphate, is that they exhibit similarly low flexural strengths, typically between 3 and 10 MPa, despite their differing chemical composition, varying degrees of hydration and contrasting setting mechanisms1–3. Because of these low strength values, unreinforced cements are never used in flexure or tension, and studies of cement strength are usually confined to compression. Those few studies which have considered flexural or tensile failure have concluded that hydraulic cements have an intrinsic maximum tensile strength of about 20 MPa4,5. Here we demonstrate that the commonly observed flexural weakness of cement is due to the presence of large voids which are largely undetected by conventional methods of pore analysis such as gas adsorption and mercury porosimetry. The removal of such macro-defects results in flex strengths up to 70 MPa, despite the large volume of gel pores remaining in the material. These strength figures, comparable with those of sintered ceramics, have been achieved without the use of elevated pressures or temperatures, and without fibrous reinforcement.

Preparation, microstructure and mechanical properties of dense polycrystalline hydroxy apatite

Abstract: Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture toughness under dry conditions are 115 MPa and 1.0 MPa m1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.

Mechanisms Responsible for Strain‐Rate‐Dependent Compressive Strength in Ceramic Materials

Abstract: Compressive strength of SiC and Al2O3 is determined over a wide range in loading rate. The results are interpreted in terms of the tensile growth of axial microcracks, controlled by two distinct mechanisms. The first is a material-dependent, thermally activated process operative at strain rates 102s; the other is a relatively material-independent, strain-rate-sensitive inertial process which controls failure at higher loading rates.

Shear Strength of Rockfill

Abstract: A practical method for estimating the shear strength of rockfill is developed. The peak drained friction angle π(prime) is found to be closely related to that of rock joints. In both cases the values of π(prime) are dependent on sample size, stress level, surface roughness, and on the uniaxial compression strength of the rock. Friction angles are therefore higher for smaller samples, and very high where stresses are low, as at the toe or near the face of a rockfill dam. Test data reviewed shows that the value of π(prime) for rockfill can be quantified by a equivalent roughness (R), and an equivalent particle strength (S). The value of (R) depends on the porosity following compaction, and on the degree of particle roundedness and surface smoothness. A practical method is proposed for physically measuring the full-scale shear strength of in-place compacted rockfill.

Determination of the Shear Strength of Shock Compressed 6061-T6 Aluminum

Abstract: The strength of 6061-T6 aluminum was assessed over the stress range of 8–40 GPa using velocity interferometry to measure reloading and unloading profiles from the initial shocked state. These results show that the shear stress which can be supported in the shocked state increases by about a factor of five over this range. This observation is in agreement with previous investigations. However, an important new observation is that a substantial increase in shear stress occurs during reloading, resulting in a well-defined elastic precursor. This result indicates a significant departure from the elastic-plastic model and suggests that softening occurs during initial shock compression.

A theoretical framework for the compressive properties of aligned fibre composites

Abstract: New experimental results have made necessary the reformulation of the theory for compressive strength. The new theory is based on the precept that a number of different mechanisms can cause composite failure. The active one in a particular situation is that which gives the lowest failure stress. Thus composite strength can be dominated by fibre strength when the fibres are ductile, or controlled by matrix yielding when the matrix is soft. Lack of linearity in the fibres has a very important effect also, as does the adhesion between the matrix and the fibres. Modulus is affected as well as strength. Governing equations are developed for six different mechanisms and the agreement with experiment is very good. It is concluded that to make composites with good compressive properties the fibres should be hard, as straight as possible and well bonded to the matrix. The matrix should have a high yield stress, tensile strength and compressive strength. Hybridization is useful to improve the compressive strength of Kevlar composites, but should be avoided with brittle fibre systems due to unfavourable “hybrid effects”.

Mechanical properties of poly(methyl methacrylate) bone cements

Abstract: Samples of low viscosity poly(methyl methacrylate) (PMMA), graphite reinforced PMMA, and graphite reinforced low viscosity PMMA were evaluated for their compression strength and fracture toughness. These results were compared with two currently used plain PMMA bone cements. There were no statistically significant differences in compression strength between the five cements. Graphite reinforcement of plain cement produced a 32% increase in fracture toughness over plain cement. Graphite reinforcement of low viscosity cement also produced a significant increase in toughness (31%) over low viscosity cement with fiber reinforcement. However, low viscosity cement demonstrated significantly less fracture toughness than plain PMMA.

High early strength cement

Abstract: A rapid reaction of high alumina cement, gypsum, lime and water produces cementitious compositions in which the only significant factor contributing to strength during the very early stages of hydration (i.e., a few minutes to a few hours) is the formation of an amount of ettringite equal to from about 40% to about 60% of the weight the paste of water, the high alumina cement, the gypsum source, and the lime source. A substantially impermeable concrete having a 4-hour compressive strength of about 8000 p.s.i. may be produced from the cement powder of this invention.

Shear Strength of R/C Beam-Column Connections

Abstract: Strength and load reversal tests were conducted on reinforced concrete beam-column connections. Fourteen tests investigated the influence of the column axial compressive stress, the amount of hoop reinforcement in the connection, the size and location of transverse beams, and the geometry of the connection region. The test results indicate that the current state-of-the-art design procedure underestimates the capabilities of concrete in the beam-column connection and overestimates the contribution of hoop reinforcement in the connection. It appears that satisfactory load reversal behavior can be achieved if the shear stress in the connection is about half of the monotonic shear strength.

Temperature-strain rate dependance of compressive strength and damage mechanisms in aluminium oxide

Abstract: The results of compression tests of Al3O3 performed over a wide range of temperatures and strain rates are interpreted in terms of dominant damage mechanisms. It is shown that compressive failure in Al2O3 is caused by one of three different mechanisms, each based on tensile (Mode I) growth of predominantly axial microcracks, and each characteristic of a specific temperature-strain rate regime. The concepts developed should be applicable to other strong ceramics.

The compression strength of composites with kinked, misaligned and poorly adhering fibres

Abstract: Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction,V f=0.4 with Kevlar and high strength carbon fibres. The effective fibre strength atV f<0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10° for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.

Kinking and tensile, compressive and interlaminar shear failure in carbon-fibre-reinforced plastic beams tested in flexure

Abstract: The first stage of the failure process in pultruded, 60% volume fraction, type III carbon fibre-epoxide beam specimens with span-to-depth ratios of 5, 15 and 40 deformed in flexure at atmospheric pressure was the initiation of kinking by the “compression” roller. Kink growth during the non-linear part of the load-deflection curve was followed by kink propagation at peak load. Acoustic emission and load-unload tests to detect irrecoverable deflection supported direct microscopic observations of damage. Kink growth with decreasing load, increasing deflection and accompanying redistribution of stresses led to two types of failure, commonly referred to as “flexural” and “interlaminar”. In the former, tensile failure was concurrently initiated to give the characteristic tensile and compressive zones on the failure surfaces. In the latter, the growing kink initiated interlaminar cracks in resin-rich zones as it propagated (with decreasing load) towards the convex surface. Kinking was associated with triaxial compressive stresses in the contact zone of the “compressive” roller or rollers (in the case of four-point bend specimens). When hydrostatic pressure was superposed on flexure, at pressures between 150 and 300 MNm−2 depending on the type of specimen, kinking was inhibited and eventually suppressed to give tensile failures, even in the so-called interlaminar shear strength type of specimen. When non-linear deflections were not large, the maximum principal tensile stress in the beams was close to the tensile strength of the carbon-fibre-reinforced plastic (∼1.8 GN m−2).

Tensile strength of highly oriented polyethylene

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Rate sensitivity of compressive strength of columnar-grained ice

Abstract: Methods of selecting and preparing specimens and details of test procedures, including those for microstructural examination, are described for the investigation of uniaxial compressive strength of columnar-grained ice. It is shown that specimen strain rates are not constant for constant cross-head-displacement rate and that consequently the results are not representative of the constant strain-rate condition. Analysis shows that constant cross-head-displacement tests are more closely representative of the constant stress-rate condition. The paper also discusses failure strains, failure times, mode of failure and possible dependence of strength results on stiffness of the test system.