Showing papers on "Compressive strength published in 1998"

Plastic-Damage Model for Cyclic Loading of Concrete Structures

Abstract: A new plastic-damage model for concrete subjected to cyclic loading is developed using the concepts of fracture-energy-based damage and stiffness degradation in continuum damage mechanics. Two damage variables, one for tensile damage and the other for compressive damage, and a yield function with multiple-hardening variables are introduced to account for different damage states. The uniaxial strength functions are factored into two parts, corresponding to the effective stress and the degradation of elastic stiffness. The constitutive relations for elastoplastic responses are decoupled from the degradation damage response, which provides advantages in the numerical implementation. In the present model, the strength function for the effective stress is used to control the evolution of the yield surface, so that calibration with experimental results is convenient. A simple and thermodynamically consistent scalar degradation model is introduced to simulate the effect of damage on elastic stiffness and its recovery during crack opening and closing. The performance of the plastic-damage model is demonstrated with several numerical examples of simulating monotonically and cyclically loaded concrete specimens.

A characterization of first-generation flowable composites

Abstract: A plethora of new low-viscosity composite resin materials, or flowable composites, have been marketed during the last two years, but little has been published about them. The authors describe research in which they compared the properties--filler, depth or cure, flow, wear, compressive strength, diametral tensile strength, indented biaxial flexure strength and toughness--of flowable and hybrid composites. Mechanical property tests (ISO 4049, ISO/DIS 6872) of eight flowable composites and two hybrid composites were conducted. The flowable composite with the least flow was similar to traditional composites. Mechanical properties were generally about 60 to 90 percent of those of conventional composites. The authors conclude that flowable materials should be used with caution in high-stress applications for restorative dentistry.

Porosity of Ceramics: Properties and Applications

Abstract: Introduction to the microstructural dependence of properties introduction to, and perspective on, the porosity dependence of properties porosity dependence of elastic properties at low to moderate temperatures porosity dependence of fracture energy and toughness, and related crack propagnation at 22C porosity dependence of tensile strength and reliability at low to moderate temperatures porosity dependence of hardness, compressive strength, wear and related properties at low to moderate temperatures porosity dependence of thermal and electrical conductivities and other nonmechanical properties at low to moderate temperatures porosity dependence of mechanical and other physical properties of related materials at 22C porosity dependence of properties at higher temperatures, including effects of porosity on thermal shock summary of porosity dependence, NDE, applications and engineering of porous ceramics.

Interface Property and Apparent Strength of High-Strength Hydrophilic Fiber in Cement Matrix

Abstract: This study addresses the characterization of fiber-matrix interfacial properties and the apparent strength of high-strength hydrophilic fibers. Single-fiber pullout bond tests and single-fiber pull-to-rupture strength tests were conducted by employing polyvinyl alcohol (PVA) fibers. The pullout bond tests showed that these fibers have surprisingly high chemical and frictional bond strengths. The chemical bond strength was relatively stable independent of a water-to-cement ratio of matrix and the fiber type tested, contrary to the friction bond strength. The pull-to-rupture strength tests revealed that the apparent strength of the PVA fibers in cementitious composites is considerably lower than that in standard fiber strength tests. The apparent strength was further reduced with inclining angle of fiber alignment. This effect was captured by a simple phenomenological model in this study, which introduces the apparent strength reduction factor. The combined effects of high bond strength and degraded fiber strength will likely contribute to composite performance less than would be expected from a high-performance fiber.

Mechanical properties of silica aerogels

Abstract: Aerogels are extremely low density solids that are characterized by a high porosity and pore sizes in the order of nanometers The mechanical behavior of silica aerogel was investigated with hardness, compression, tension and shear tests The influences of testing conditions, storage environment and age were examined, with particular attention paid to the effects of processing parameters, including fiber-reinforcement Good correlation was found between hardness and compressive strength over a wide range of processing parameters Increasing fiber reinforcement generally retarded shrinkage during fabrication and yielded smaller matrix densities for a given target density For a given fiber content, hardness, compressive strength and elastic moduli increased and strain at fracture decreased with increasing matrix density In the lower ranges of matrix density, fiber reinforcement increased strain at fracture and elastic moduli The mechanical response was also sensitive to environment and storage history With age, the compressive strength and elastic moduli increased while the strain at fracture decreased Tension and shear results indicate that shear strength of aerogels exceeds tensile strength which is consistent with brittle materials response

In vivo degradation of a poly(propylene fumarate)/β-tricalcium phosphate injectable composite scaffold

Abstract: This study was designed to investigate the in vivo biodegration and biocompatibility of a poly(propylene fumarate) (PPF)-based orthopedic biomaterial. The effects of varying the PPF to N-vinyl pyrrolidinone ratio and PPF to β-tricalcium phosphate content were studied. The composite mechanical properties and local tissue interactions were analyzed over 12 weeks. An initial increase in both compressive modulus and strength was seen for composite formulations that incorporated β-tricalcium phosphate. The samples incorporating a higher PPF to N-vinyl pyrrolidinone ratio reached a maximal compressive strength of 7.7 MPa and a maximal compressive modulus of 191.4 MPa at 3 weeks. The lower PPF to N-vinyl pyrrolidinone ratio samples gained a maximum compressive strength of 7.5 MPa initially and a compressive modulus of 134.0 MPa at 1 week. At 6 weeks, all samples for formulations incorporating β-tricalcium phosphate crumbled upon removal and were not mechanically tested. Samples that did not incorporate β-tricalcium phosphate were very weak and insufficient for bone replacement at the 4-day time point and beyond. Tissue interactions resulted in a mild inflammatory response at the initial time points and mature fibrous encapsulation by 12 weeks. © 1998 John Wiley & Sons, Inc. J. Biomed Mater Res, 41, 1–7, 1998.

Strength and Related Properties of Concrete: A Quantitative Approach

Abstract: This book addresses concrete strength. It contains the quantitative tools necessary to understand, assess and improve concrete quality. Chapter headings include the following: 1. Compressive Strength of Hardened Concrete; 2. Other Concrete Strengths; 3. Strength Development of Portland Cement; 4. Structure of Hardened Cement Paste and Concrete; 5. Relationship Between Composition and Strength of Concrete; and 6. Elastic Deformation of Concrete. The book also contains a floppy IBM PC disk which holds computerized formulas for strength testing.

Properties of Artificially Cemented Carbonate Sand

Abstract: Naturally cemented materials often have inherent variabilities in density and degree of cementation. The influence of this variability on the properties of cemented materials is investigated by producing artificially cemented specimens at dry unit weights ranging from 12 to 19 kN/m3 and gypsum cement contents ranging from 0% to 20%. Tests have been performed to investigate the index strengths, the behavior in isotropic and K0 compression, and the responses from standard triaxial compression tests over a wide range of confining pressures. The index properties, and the compression and stiffness parameters of the cemented sands are presented, with particular attention given to the influences of density and degree of cementation. For the artificial soil, the effects of the bonding are only significant for stresses below an apparent preconsolidation stress. The strength and stiffness increase with increasing density and cement content, but the influence of the cementation decreases as the density increases.

Potential for Using Waste Glass in Portland Cement Concrete

Abstract: Local recycling pressures are providing an impetus to examine the use of unconventional materials in construction such as waste glass in Portland cement concrete. It is well known that the silica in glass can be highly reactive with the alkalis in cement paste and that this reaction can lead to expansion and cracking of the concrete (alkali-silica reaction or ASR). The potential is examined for controlling ASR and achieving concrete of suitable strength and durability that uses waste glass as the aggregate. Laboratory and field research have been completed with concrete mixtures designed primarily for paving or flat-work applications. Various gradations of glass particle sizes added at various fractions of total aggregate content have been studied. Compressive strength and wet prism expansion are two of the parameters monitored. The study has spanned several years and has revealed both glass aggregate concrete mixtures that show great promise as well as mixtures that fail rapidly. Continued monitoring of the performance of the successful mixes over a period of several more years will provide an important step toward defining those situations where waste glass can be used in concrete.

Strain rate behavior of composite materials

Abstract: The effect of strain rate on the compressive and shear behavior of carbon/epoxy composite materials was investigated. Strain rate behavior of composites with fiber waviness was also studied. Falling weight impact system and servohydraulic testing machine were used for dynamic characterisation of composite materials in compression at strain rates up to several hundred per second. Strain rates below 10 s −1 were generated using a hydraulic testing machine. Strain rates above 10 s −1 were generated using the drop tower apparatus developed. Seventy-two-ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the longitudinal and transverse directions and [(0 8 /90 8 ) 2 /0 8 ] s crossply laminates were characterised. Off-axis (30 and 45°) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress–strain behavior. The 90° properties, which are governed by the matrix, show an increase in modulus and strength over the static values but no significant change in ultimate strain. The shear stress–strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly with increasing strain rate. The 0° and crossply laminates show higher strength and strain values as the strain rate increases, whereas the modulus increases only slightly over the static value. The increase in strength and ultimate strain observed may be related to the shear behavior of the composite and the change in failure modes. In all cases the dynamic stress–strain curves stiffen as the strain rate increases. The stiffening is lowest in the longitudinal case and highest in the transverse and shear cases. Unidirectional and crossply specimens with fiber waviness were fabricated and tested. It is shown that, with severe fiber waviness, strong nonlinearity occurs in the stress–strain curves due to fiber waviness with significant stiffening as the strain rate increases.

The compressive strength of articular cartilage

Abstract: Articular cartilage provides the smooth bearing surfaces in freely moving (synovial) joints. Its mechanical properties are important because structural failure of cartilage is closely associated with joint disorders, including osteoarthritis. Some mechanical properties of cartilage are well characterized, but little is known about its compressive strength. A technique for measuring cartilage compressive strength is evaluated, and an overview of experiments which relate strength to stiffness and tissue hydration is given. Specimens of bovine articular cartilage-on-bone, approximately 15 mm square, were loaded on a hydraulic materials testing machine using flat impermeable indentors. Linear-ramp loading/unloading cycles of 1 s duration, and of increasing severity, were applied until failure was evident on force-displacement graphs. Some specimens were tested following a 30 min period of creep loading. Inkstaining and histology were used to locate the site of initial damage to each specimen. Specimen failure occurred first in the cartilage surface layer at a nominal applied stress of 14-59 MPa (mean 35.7 MPa). Mechanical properties were little affected by specimen or indentor size, provided both remained within defined limits, and compressive strength could be measured to an accuracy of approximately +/- 5 per cent. Compressive stiffness was a significant predictor of strength, but only if it was measured at high levels of stress. Strength increased following creep-induced water loss, and initial mechanical damage could propagate under moderate cyclic loading. This technique for measuring cartilage compressive strength has potential for investigating the causes of cartilage failure in vivo.

High - strength concrete subjected to triaxial compression

Abstract: The behavior of high-strength concrete subjected to multiaxial states of stress was studied. An experimental program was undertaken to quantitatively determine the failure surface for high-strength concrete. Results from this study provide the means to predict the failure condition for high-strength concrete under combined stresses. The experimental program was comprised of testing high-strength concretes at three different compressive strength levels. The three strength levels included concretes with compressive strengths of 6 ksi (42 MPa), 10 ksi (69 MPa), and 15 ksi (103 MPa). The triaxial tests were performed on 4-by-8 in (100-by-200 mm) cylindrical specimens. The confining pressures employed in the experiments ranged from 1,200 psi (8.3 MPa) to 12,000 psi (82 MPa). A series of uniaxial tension and compression tests were also performed to develop the necessary data for establishment of the failure criterion. Empirical relationships were developed for prediction of axial strength as a function of confining pressure. In general, the axial strength of high-strength concrete increases with increased confining pressure. However, in comparison with the normal-strength concrete, the effect of confining pressure on the failure strength of high-strength concrete is less pronounced.

Laser-shock processing of aluminium-coated 55C1 steel in water-confinement regime, characterization and application to high-cycle fatigue behaviour

Abstract: 55C1 steel was irradiated with a high-power neodymium-glass laser with application to induce plastic shock waves within targets, through the expansion of a laser-induced surface plasma. Laser-shock processing experiments were conduced in the plasma-confined regime with water to increase the laser-induced peak stresses. Physical, mechanical and processings aspects were reviewed, such as the characterization of stress waves in coated steels with a VISAR velocimeter system, and the mechanical changes induced in 55Cl in terms of compressive residual stresses or work-hardening levels. With the use of convenient protective coatings, some 7-8 GPa peak stress levels could be achieved which authorized the generation of high compressive residual stress levels (nearly 80% of the compressive yield strength), but preserved the surface integrity from detrimental roughening. Surface modifications performed under different shock conditions were shown to display some 30% increase on the bending fatigue limits of 55C1 at R=0.1.