On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy

Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material. The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format. Three loading amplitudes (/2=0.10%, 0.15, 0.20%) and three load ratios (R=–1, 0, 0.5) are considered. From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, _max, with respect to all possible shear strain planes. For certain inhomogeneities, the _max was as high as ten times the far field strains. Bonded inclusions have _max values two orders of magnitude smaller than voids, cracked, or debonded inclusions. A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle. Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation. Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio). Increasing the size of an inclusion by a factor of four increases _max by about a factor of two. At low R ratios (–1) equivalent sized voids and debonded inclusions have comparable _max values. At higher R ratios (0, 0.5) debonded inclusions have _max values twice that of voids.     

Glass transition temperature and molecular weight distribution of irradiated polymethylmethacrylates

A mechanical torsion pendulum has been used to observe changes in viscoelastic behaviour resulting from modification of polymer structure. The linear dependence of transition temperature on reciprocal average molecular weight has been confirmed for irradiated polymethylmethacrylate polymers. Gel permeation measurements are consistent with simultaneous end-linking and chain scission at low radiation levels, leading to apparently anomalous behaviour in irradiated Perspex.     

Fracture behaviour of Cu-Zn-Co and Cu-Zn-Ni alloys during superplastic deformation

Cavitation and fracture behaviour in two commercial /gb brasses, one modified with 2wt% Co (Cu-Zn-Co) and the other with 2wt% Cr (Cu-Zn-Cr), have been investigated in Region II of superplastic flow. These alloying elements form cobalt-rich (0.3 m average diameter) and chromium-rich (5 m average diameter) precipitate particles which are distributed uniformly in the matrix and which play an important role in cavitation and inhibiting grain growth during deformation. Void size distributions, volume fraction of voids and the number of voids per unit area have been measured as a function of strain in Region II and the results show a very marked difference in the degree of cavitation in Cu-Zn-Co and Cu-Zn-Cr alloys. Experiments show that the deformation is quasi-uniform with little or no necking in the specimens of Cu-Zn-Co alloy in Region II, and the final fracture occurred by the growth and interlinkage of internal voids. On the other hand, in the specimens of Cu-Zn-Cr alloy a sharp or localized neck developed early in the deformation in Region II and the specimen pulled down to a fine point leading to failure by necking. The importance of diffusion or slip accommodation of grain boundary sliding in void formation during superplastic flow is discussed and a criterion for failure is suggested.     

Strength of irradiated polymethyl methacrylate

The effect of the form of radiation (reactor and-radiation, X-rays and ultraviolet light) and absorbed energy on the strength of polymethyl methacrylate was studied. It was shown that the strength of this polymer depends in the first place on the size of radiation-induced defects and not on their distribution across the specimen thickness. When therefore the absorbed radiation energy is nonuniformly distributed along the specimen cross section, the mechanical properties of irradiated materials are determined by the radiation dose absorbed by the specimen surface layers.     

Effect of X-ray irradiation on the thermal expansion of polymers

The coefficient of linear thermal expansion (a) of ordinary (non-X-ray irradiated) and X-ray irradiated samples of polyester, glass-fibre reinforced polyethylene and polytetrafluoroethylene, has been determined in the temperature range 30 to 200° C using a two-terminal capacitance technique. The temperature corresponding to the sudden increase in of the X-ray irradiated samples are higher than those corresponding to the ordinary samples. The values of for X-ray irradiated samples are smaller than those of ordinary samples for polyester and glass-fibre reinforced polyethylene over the entire temperature range of this study. For polytetrafluoroethylene the values of of the X-ray irradiated sample are lower than those corresponding to the ordinary samples at lower temperatures, and this trend is reversed at higher temperatures. The results are explained in the light of the change in the polymer structure brought about by X-ray irradiation.     

Filamentary sapphire

An experimental study of the continuous melt growth of filamentary c-axis sapphire is presented. The filament had a nominal 0.025 cm diameter and was grown at rates of 2.5 to 7.5 cm/min using the edge-defined, film-fed growth technique. Filament growth was recorded and characterised using both single frame photography and 16 mm motion picture photography. Samples were examined after growth using transmitted light metallography. The primary structural characteristic of the material at all growth rates is the presence of 1 m voids, arranged in patterns which are dependent on both growth rate and temperature. In samples grown at rates up to 6.0 cm/min these voids assume nearly conical arrays which are directly related to the crystallographic rhombohedral planes in sapphire. At 7.5 cm/min the voids outline the edges of cell walls. Based on the hypothesis that they are shrinkage voids and the end result of liquid entrapment behind the advancing solid interface, mechanisms are proposed by which this entrapment might occur at different growth speeds. Using experimental data the stability of the growth process with respect to the constancy of the filament diameter is quantitatively related to both growth rate and temperature.     

On the factors affecting strength of portland cement

This paper reports mechanical property measurements for Portland Cement paste free from fabrication artifacts (e.g. bubble-type voids), and compares them to published results both for normal and new high strength cement. Removal of large voids (above 100m) by vacuum de-airing leads to an increase of 15% in mean flexural strength and a small decrease in fracture toughness. This increase in flexural strength is predictable from the tied-crack model previously proposed to explain the notch-sensitivity behaviour of hardened cement paste, and for which direct experimental evidence was obtained. It is suggested that factors such as moisture content are at least as important as large voids in controlling mechanical properties. It is concluded that the much increased strength of the new polymer-containing cements must result from improvements to the microstructure other than the simple elimination of voids.     

Influence of sterilization upon a range of properties of experimental bone cements

Bone cement, used to fix prostheses into the bone, must be sterilized prior to implantation. Two sterilization techniques, and radiation, were investigated, examining the influence upon molecular weight, static and dynamic mechanical characteristics and rheological properties. A number of experimental cements were studied prepared from methylmethacrylate (MMA) co-polymers, either single powders or powder blends, mixed with MMA monomer. It was found that with both and radiation, there was a decrease in molecular weight of all powders, including a MMA/styrene co-polymer, in relation to the radiation dose. This fall in molecular weight resulted in a drop in tensile strength, Youngs modulus and strain to failure of all cements tested. However, the deterioration in mechanical strength was highlighted by the dynamic testing. Fatigue lives of cements after testing in tension–tension, at 2 Hz under load control and irradiated with 25 kGy radiation, displayed significant decreases. This result indicated the utmost importance of conducting such tests upon experimental bone cements prior to in vivo use. The rheological time profiles of curing cements were also found to be influenced by 25 kGy radiation, with a reduction of complex viscosity after sterilization.     

Void formation in an Al-Mg-Si alloy under different precipitation conditions after irradiation with 100 keV Al ions

The influence of different precipitation treatments upon the radiation-induced void formation was studied on an Al-Mg-Si alloy and the results were compared with those obtained on high-purity Al. The changes in the microstructure and the void formation were investigated by TEM methods for the dose range 0.8 to 80 dpa and the temperature range 55 to 250° C. The irradiations were carried out using 100 keV Al ions. Highpurity Al showed void formation over the whole temperature range investigated with a maximum volume increase V/V of 3.4% at 150° C. The behaviour of the Al-Mg-Si alloys depends strongly upon the thermal pretreatments. The alloy in its homogenized state shows no voids between 55 and 140° C. Trapping of vacancies on solute Si atoms could be the explanation. An ageing treatment leading to coherent precipitates results in the complete suppression of void formation for the temperature region of highest swelling (0.35 T _sT0.44 T _s). On the other hand, treatments which cause incoherent or partially coherent precipitates result in swelling. The amount of swelling, however, is lower compared to pure Al and the temperature of the swell maximum is shifted to lower temperatures. These results could be explained by the mechanism of trapping vacancies by solute atoms, on the one hand, and the coherent precipitates acting as recombination centres, on the other.     

Microdeformation in partially compatible blends of poly(styrene-acrylonitrile) and polycarbonate

Dilute 3-component 1-phase solutions in methylene chloride of poly(styrene-acrylonitrile) PSAN and polycarbonate PC are used to cast phase separated thin films. Films of pure PSAN, pure PC and five intermediate compositions are examined. The films are bonded to copper grids and strained at a constant rate of 4.1 × 10^–6sec^–1. The median tensile strain _v for void formation is determined using an optical microscope and the regions surrounding the voids are examined by TEM. At room temperature and slow strain rates both PSAN and PC plastically deform by shear yielding. For pure PSAN _v was found to be 0.13 whereas for PC _v exceeds 0.23. The addition of the more ductile polymer PC to PSAN at weight fractionsx forx 0.4 decreases _v. In this case voids form in crazes at the boundaries between the PC-rich inclusion and the PSAN-rich matrix. When the PC content is increased tox = 0.8, _v approaches 0.23. The effect of physical ageing (annealing belowT _g the glass transition temperature) on the mode of plastic deformation was also examined over the same compositional range. Physical ageing was found to suppress shear deformation and favour crazing in PSAN and PSAN-rich phases. Because crazes are more susceptible to breakdown than DZ's (shear deformation zones), physical ageing results in a marked decrease in _v. The breakdown statistics of these phase separated partially compatible blends was found to follow a Weibull distribution in strain from which two parameters may be extracted: the Weibull modulus and _w the Weibull scale parameter. is a measure of the breadth of distribution of void initiation and _w is a measure of the median strain to void formation in the films. The behaviour of _w was found to approximately mirror _v. The Weibull modulus appears to be primarily controlled by the matrix phase.     

Studies on the effect of electron beam radiation on the molecular structure of ultra-high molecular weight polyethylene under the influence of α-tocopherol with respect to its application in medical implants

Ultra-high molecular weight polyethylene (UHMW-PE) is being used successfully for articulating surfaces in joint endoprostheses, especially for cups of total hip endoprostheses. Sintered specimens containing various amounts of -tocopherol (vitamin E) as a biocompatible stabilizer, were irradiated in nitrogen atmosphere as well as in air with various dosages of electron beam radiation. Size exclusion chromatography (SEC) was used to analyze the soluble fractions of the UHMW-PE samples according to their molecular weight distribution prior to and after irradiation. In nitrogen atmosphere the radiation-induced crosslinking showed to be dependent on the added amount of -tocopherol in the sintered specimens. With an increasing content of -tocopherol, the stabilizer acted as a scavenger for free radicals. Thus, the crosslinking was more and more hindered. The same effect was observed on the samples irradiated in air, where, in addition to the crosslinking process, oxidative molecular degradation occurred. The highest extent of crosslinked material was yielded with unstabilized samples in nitrogen atmosphere.     

Application of the Griffith criterion to fracture of boron fibres

Tensile tests were performed on boron fibres and the resulting fracture stresses _f recorded. The fracture surfaces were investigated by SEM, with emphasis on the type of flaw nucleating the fracture. It was found that pre-existing, so-called proximate voids, in the boron mantle near the core-mantle interface, have transverse irregularities, large enough to create stress concentrations. The radial extensions, 2c, of fracture initiating proximate voids in mirror zones were measured from the micrographs. The obtained 2c _f relation was found to be in good agreement with the Griffith criterion for brittle fracture. In addition, the experimental log 2c-log _f plot gives some information on the value of the surface energy,, of boron.     

An introduction to high-voltage electron microscopy

The various features of high voltage electron microscopy are examined in some detail, including basic features of the instrument such as lens aberrations, etc and more particularly, the effect of the microscope voltage on the behaviour of crystalline specimens, both in respect of the formation of diffraction contrast from defects and the production of radiation damage.High accelerating voltages (i.e. 500 to 1000 kV) enable thicker foils (1 to 2 m of medium atomic number materials and 4 to 8 m of aluminium) to be used so that the foil behaviour is more typical of bulk material. In addition, diffraction patterns can be obtained from smaller precipitates and sharp dark field micrographs can be produced without beam tilting.The major disadvantage is probably the formation of radiation damage by displacement of atoms when the electron energy exceeds a threshold value.     

Characteristics of a Self-Initiating Pulse-Periodic Discharge in a Kr/SF6 Gas Mixture

The characteristics are given of a low-pressure, volume, pulse-periodic discharge in a krypton/SF₆ gas mixture, accompanied by the formation of a domain in a spherical anode–flat cathode system of electrodes. It is shown that, when a dc voltage of U _ch 1.0 kV is applied to the anode, a volume pulse-periodic discharge with a pulse repetition rate of 0.1 to 120 kHz is ignited in the interelectrode gap. The static and dynamic current–voltage characteristics of the discharge are investigated, as well as the plasma radiation spectra in the range from 130 to 350 nm, oscillograms of voltage across the discharge gap, of the current and radiation of plasma, the dependence of the radiation brightness of the 249-nm KrF(B–X) band on the mean discharge current, and the resource characteristics of radiation on the electronic-vibrational transitions of an excimer molecule. It is demonstrated that the domain plasma may be used in a low-pressure pulse-periodic excimer lamp emitting in the 230 nm KrF(D–X)/249 nm KrF(B–X) band system. The active medium proper plays the role of the commutator in the lamp, and a dc voltage source is used to ignite the discharge. Such lamps can be used in polymer chemistry, biology, potable water purification systems, for the sterilization of medical instruments, and in micro- and quantum electronics.     

The Evolution of Erosion Laser Plasma of Antimony

The time dependence of the population of excited states of particles is investigated under conditions of a target of pure antimony being acted upon by pulsed laser radiation (w = (3–5)×10⁸ W/cm², _g = 1.06 mcm, = 20 ns, and f = 12 Hz). The ways are analyzed of increasing the accuracy of measurements of the laser plasma parameters by emission spectroscopy. The evolution of the mechanisms of formation of excited atoms and ions of antimony in the plasma of a laser jet is investigated under conditions of moderate effect of infrared laser radiation on a target in vacuum.     

Experimental Investigation of the Transparency of Optical Materials for UV Radiation under Conditions of Radiation–Gasdynamic Effect

The optical properties of a number of crystals such as quartz leucosapphire and calcium and barium fluorides are experimentally investigated in the spectral range = 110–250 nm under conditions of their direct contact with an intensely emitting gas–plasma medium. The broadband UV radiation of power density I ₀ 10⁶– 4 × 10⁷ W/cm² is generated with the aid of light-erosion and plasmadynamic injectors in a vacuum and gases of different chemical compositions. The possibility is demonstrated of effective transport of powerful fluxes of UV radiation through the given materials during the formation at their surface of a gas layer optically dense for the short-wave UV radiation.     

Determination of the local angular radiation coefficients in certain two-body systems

A method is shown and formulas are derived by which local angular radiation coefficients can be determined in certain two-body systems where the configuration is arbitrary but one of the bodies is either a cylinder or a rectangular plate.Notation _int radiation vector of body 1 - _E ^int intrinsic radiation intensity of body 1 - _x, _y, _z components of the geometrical radiation vector along rectangular coordinates - r₀=x²+z² shortest distance from point M(x, y, z) to linear radiator - ₀ , ₀ ^' angles subtending the two segments of the linear radiator from point M(x, y, z) on area element 2 of irradiated surface - l length of the cylinders - x, y, z space coordinates of point M Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 22, No. 6, pp. 1080–1088, June, 1972.     

Secondary grain growth in ammonium nitrate crystallization

Crystallization and ageing of pure ammonium nitrate crystals was studied by scanning electron microscopy. Ageing the crystals beyond 1 h led to the growth of secondary grains along the grain boundaries of primary grains (140–200 m). After 6 h ageing secondary growth along grain boundaries was not observed; instead, distinct clusters with morphologies similar to the secondary grains were formed. The ancillary growth obeys approximately the parabolic relation L=(Kt)^1/n+1 where n=1 justifies grain growth in the pure crystals, and its formation is ascribed to the dissolution of dendrites, edges and corners. The high interfacial activity and interparticle voids can affect the storage and mechanical properties of the material.     

Craze intersections in biaxially-stressed polystyrene

Crazes are produced on two orthogonal planes in both thin film and macroscopic samples of polystyrene by sequentially applying two orthogonal tensile strains ₁ and ₂. Although many crazes produced by the second strain ₂ (secondary crazes) are stopped when they meet a primary craze, some intersections occur. The fraction of craze meetings resulting in intersection increases from 20% at ₁= ₂=3% to 55% at ₁= ₂=5%; intersections also occur preferentially in thin regions of primary crazes. The craze fibril structure in the intersection has a much lower fibril volume fraction, v _f, than either of the two crazes from which it formed. The fibril volume fraction in the intersection is approximately given by the product of the fibril volume fractions of the two crazes, in agreement with a prediction based on the surface drawing mechanism of craze thickening. At higher strain levels the v _fs of the intersections are lower, leading to higher fibril stresses and enhanced fibril fracture; an increasing fraction of intersections breaks down to form large voids at these higher strain levels. Fractography of macroscopic samples containing intersecting crazes demonstrates that voids formed at the intersections can act as nuclei for cracks causing premature fracture of the material.     

X-ray-Induced Paramagnetic Centers in SiO2Xerogels

SiO₂xerogels prepared by a sol–gel process followed by sintering at 350–1100°C were irradiated with 30-keV x-rays at a dose rate of 80 R/s. The room-temperature electron spin resonance (ESR) spectra of the xerogels showed signals arising from radiation-induced intrinsic defects (E" centers and terminal oxygens) and organic radicals. By recording ESR spectra in a vacuum of 10^–2Pa, surface E" centers differing in relaxation time from the volume centers were identified for the first time. The ESR signal from the E" centers was measured as a function of x-ray dose. The possible mechanisms for the formation of E" centers under x-ray irradiation are discussed.