Mechanical properties of

The ultimate values for compressive strength, Young's modulus, and toughness of cylindrical specimens of unitary aspect ratios and uniform grain-size distributions were extrapolated for hydroxyapatite (HAP) to 70 MPa, 9.2 GPa, and 0.36 J cm-3, and for tricalcium phosphate (TCP), to 315 MPa, 21 GPa, and 2.34 J cm-3. For total volume porosities of 50%, the corresponding values were determined: for HAP, 9.3 MPa, 1.2 GPa, 0.042 J cm-3, for TCP, 13 MPa, 1.6 GP, 0.077 J cm-3. Porosities of HAP specimens ranged from 3%–50%; TCP from 10%–70%. Two pore-size distributions were employed. Exponential dependencies of the mechanical properties were found upon porosity (p0.0001). No differences in measured mechanical properties, as determined in compression, could be attributed to pore size. The superiority of TCP increases with density and suggests that a larger or more selective pore-size distribution could be effectively employed in TCP biological implants. This work also suggests the dominant role of secondary calcium phosphates in increasing compressive strengths. © 1999 Kluwer Academic Publishers     

Mechanical properties of hafnium

The mechanical properties of iodide hafnium and a hafnium-molybdenum alloy are studied between 20 and 1000°C. The mechanical properties of hafnium are found to exhibit anomalous behavior in the range 600–800°C. The origin of the anomalies is discussed in terms of strain aging and gas adsorption.     

Mechanical properties of thermosets

The tensile properties of a DGEBA (diglycidylether of bisphenol A)-norbornene anhydride network (T _g130±5 °C), were studied in the range (220 K-T _g); 4×10^–4 to 14×10^–3S^–1. The viscoelastic spectrum (1 Hz) reveals a low transition at 220 K. The bulk modulus is practically constant between 200 K andT _g — 20 K. The Poisson's ratio increases very slowly untilT _g — 30 K. Then it increases rapidly to reach its asymptotic value (0.5) near toT _g. The tensile (E) and shear (G) moduli display the classical behaviour linked to viscoelasticity. Plastic yielding occurs atT 80 °C, the elongation at yield is almost temperature and strain rate independent (G3_y = 0.035), whereas the yield stress obeys Kambour's relationship: _y = 1.1 (T _g —T) and Eyring's law (activation volume = 914cm³mol^–1). Physical ageing at 120 °C strongly affects the yield stress and the ductility. The maximum draw ratio, obtained atT T _g, is _RC = 1.35, which seems to be consistent with the network's crosslink density.     

Mechanical properties of organic nanofibers

Intrinsic elastic and inelastic mechanical properties of individual, self-assembled, quasi-single-crystalline para-hexaphenylene nanofibers supported on substrates with different hydrophobicities are investigated as well as the interplay between the fibers and the underlying substrates. We find from atomic-force-microscopy-based rupture experiments a rupture shear stress of about 2 x 10(7) Pa for an individual fiber. Deflecting a nanofiber suspended across a gap results in a Young's modulus of 0.65 GPa. Translational motion of intact nanofibers across the surface is demonstrated for fibers on a silicon substrate with a low-adhesion coating, whereas such motion on a noncoated substrate is limited to very short (sub-micrometer) nanofiber pieces due to strong adhesive forces.     

Mechanical properties of cord-rubber composites

Concepts concerning the mechanical properties of cord-rubber composites are examined. The rôle of boundary conditions in the calculation of the effective composite properties is discussed. It is shown that certain effective properties are significantly dependent on the applied boundary, conditions on the model and on the test specimen. Properties calculated from models, ranging from one dimensional analysis to the three dimensional finite element approach, are compared with some published experimental data.     

Mechanical properties of amorphous nanosprings

Helical amorphous nanosprings have attracted particular interest due to their special mechanical properties. In this work we present a simple model, within the framework of the Kirchhoff rod model, for investigating the structural properties of nanosprings having asymmetric cross section. We have derived expressions that can be used to obtain the Young's modulus and Poisson's ratio of the nanospring material composite. We also address the importance of the presence of a catalyst in the growth process of amorphous nanosprings in terms of the stability of helical rods.     

Mechanical properties of nanocrystalline materials

The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall-Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume is already low in the conventional polycrystalline regime. In fatigue, it seems that the S-N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grain size, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than that of conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation after crossing them.     

Mechanical properties of FRP-FW pipes

This paper presents an analytical approach for predicting the effective elastic moduli and breaking strength of FRP-FW pipes. The FW pipe was modelled as a laminate consisting of N laminae of unidirectional fibre composite. The elastic modulus of each lamina composed of aligned, continuous transversely isotropic fibres, and an isotropic matrix was derived based on the equivalent inclusion method of Eshelby. The effective stress-strain relation of the laminate was formulated assuming the plane stress state. The breaking strength of the laminate subjected to uniaxial tension or compression was estimated by applying the quadratic failure criterion proposed by Tsai and Wu. Numerical results were compared with experimental results obtained for CFRP-FW and GFRP-FW pipes. Good agreements were obtained both in Young's modulus and in breaking strength.     

Mechanical properties of In-Situ Composites

In this paper, author's results of several years of research work on the mechanical properties of directionally solidified (In-Situ) composites are reviewed. Alloy systems investigated were the fibrous Al-Ni, Fe-MnS and the cobalt base superalloy Co-Cr-C and the lamellar Al-Cu and Co-W. The mechanical behavior of the above systems were studied under both static and dynamic loadings. Static loading involved tension, compression and 3-point bending and the dynamic loading involved rotating bending fatigue, fatigue crack propagation and strain controlled fatigue. It was found that the tensile fracture stress and toughness and the ultimate compressive stress were generally enhanced by increasing growth rate and/or temperature gradient. However, at very high growth rates, the properties were found to decrease due to misalignment of the structure. Models were suggested to describe the static behavior of the composites investigated. Good agreement was found between the model predictions and the experimented results which indicate that the static properties are structural sensitive. On the contrary, the fatigue life of the Al-Al₃ Ni was insensitive to structural changes caused by varying the growth rate. The fatigue crack propagation response of the Co-Cr-C composites was found to follow the Paris Erdogan relation. Examination of the fracture surface confirmed a brittle mode of fracture with fiber cleavage and matrix shearing to link up fiber breaks.     

Mechanical properties of polycrystalline diamonds

Abstract

The room temperature mechanical properties of polycrystalline diamonds, i.e. tensile strength, transverse rupture strength, compressive strength, impact strength, fracture toughness, and elastic constants, have been determined. The applied test techniques are described and the results compared with those obtained by other authors. The fracture mode under the present experimental conditions was primarily transgranular. A grain size dependence, where strength increases with decreasing grain size, has been found. Fracture toughness was found to go through a maximum for grain sizes between 10 to 30 μm. The modulus of elasticity increases with increasing grain size. An influence of cobalt content on strength and modulus of elasticity has been found, while no significant influence on toughness could be determined. Increasing the cobalt content increases strength, but has the inverse effect on the modulus of elasticity. The results of strength, toughness, and elastic constants measurements are discussed in terms of available models and theories of polycrystalline ceramic materials. It can be seen from the results that polycrystalline diamonds behave in a manner similar to that of most engineering ceramics, but have the distinct advantage of a higher fracture toughness.

MST/596     

Mechanical properties of barium hexaferrites

The mechanical properties of typical isotropic barium ferrites have been determined. A statistical analysis has shown that fracture toughness is the best quality parameter. It changes with variation of production operation conditions. The fracture statistics follows the Weibull distribution. A relationship is found between the influence of the Fe2O3/BaO molar ratio on mechanical and magnetic properties. A maximum of mechanical and magnetic properties has been observed when the SiO2content in sintered ferrites is about 0.9 percent.     

Mechanical properties of molybdenum-sealing glass-ceramics

Elastic constants, thermal expansion, strength, and fracture toughness were determined for a glass-ceramic which bonds to molybdenum and matches its thermal expansion. Mechanical properties of the glass-ceramic were related to microstructure following two different crystallization treatments and moderate changes in composition. Crystallization increases the toughness and modulus of the parent glass even while the residual glass properties decline. Fracture toughness of the composite is shown to depend primarily upon properties of the separate phases; however, internal stresses are shown to decrease toughness without decreasing Young's modulus.     

Mechanical properties of ZnS nanobelts

Mechanical properties of ZnS nanobelts were measured at room temperature by direct nanoindentation experiments. It was found that the ZnS nanobelts achieve 79% increase in hardness but 52% decrease in elastic modulus compared to bulk ZnS. The nanobelts were found to exhibit creep under indentation. Indentation cracking was preferred along the belt growth direction. Indentation deformation behavior and fracture mechanisms of the ZnS nanobelts are discussed in conjunction with their crystalline structure, size effect, and surface-to-volume ratio.     

Mechanical properties of atmospheric ice

Mechanical properties of atmospheric ice obtained in a wind tunnel are measured. The ice is grown from supercooled droplets on a rotating aluminium cylinder of 31.5-mm diameter and 6.5-μm rugosity. Compressive strength is measured at two speeds of deformation (0.76 and 26 mm/min) for glaze and rime samples, as a function of air temperature for different atmospheric conditions (0.4 and 0.8 g/m³ liquid-water contents, 20 and 40 μm mean volume droplet diameters, and 4, 8, 15, and 20 m/s air velocities). These values of compressive strength are compared to the adhesive strength on aluminium, measured at a 26 mm/min speed of deformation. The ratio of compressive to adhesive strength has a maximum value of 135 for hard rime accreted at −10°C, with a wind velocity of 15 m/s, a liquid-water content of 0.8 g/m³ and a mean droplet diameter of 40 μm. The maximum compressive strength measured for the lower speed of deformation is 17395 kPa and 10745 kPa for the higher speed of deformation. The maximum adhesive strength measured is 181 kPa. On the other hand, compressive strengths measured at deformation speeds varying from 0.015 to 288 mm/min show that atmospheric ice has a ductile-brittle behaviour approaching that reported for snow ice and fine-grained lake ice.