Showing papers on "Stress concentration published in 1970"

A statistical theory of material strength with application to composite materials

Abstract: A Statistical theory of material strength is proposed. Materials are considered to be imperfect heterogeneous continua composed of discrete volume elements whose characteristics are related to material structure and imperfections. The strength of the elements is assumed to be a statistic a quantity, and as the material is loaded elements fracture randomly throughout the body causing localized stress concentrations. The accumulation of these breaks results in overall failure. By relating strength to material structure this theory attempts to bridge the gap between the microscopic and continuum approaches to fracture mechanics. The theory is applied to composite materials reinforced with whiskers and continuous fibers. Comparisons with experimental data show good agreement. Results for whisker-reinforced composites appear to provide a good prediction of strength and an explanation of the disparity between the strength of individual whiskers and the strength of the composites made from them.

The biomechanics of torsional fractures. The stress concentration effect of a drill hole.

Abstract: To determine the effect of a drill hole on the strength of long bones and on the fracture resulting from rapidly applied torsional loads, paired canine femora were tested, and the following results were obtained: 1. The presence of a 2.8-millimeter or a 3.6-millimeter drill hole in the femoral mid-shaft significantly weakens the bone as measured by a mean reduction in energy-absorbing capacity of 55.2 per cent. 2. The drill hole produces a significant increase in local stresses as demonstrated by a calculated stress concentration factor of 1.6. 3. Changes in the ratio of drill hole area to bone area from .12 to .28 are not accompanied by significant changes in the bone strength or in the stress concentration factor. 4. The resulting spiral fractures occur along planes of maximum tensile stress, and the drill hole does not alter their orientation. 5. In drilled specimens the fracture is localized to the region of the drill hole and is less comminuted.

Fatigue and fracture of cylindrical shells containing a circumferential crack

Abstract: Fatigue and fracture in elastic cylindrical shells with circumferential crack under axial tension, noting precracked specimens

The Direct Measurement of the Strength of Metals on a Sub-Micrometre Scale

Abstract: Four types of experiment have been carried out to investigate the strength properties of annealed metals when the stressed volume is small enough to lie between the existing dis­locations in a crystal. These are (i) indentation experiments of a soft metal surface with a hard stylus, (ii) blunting of a soft metal tip against a hard surface, (iii) compression of individual metal crystals, and (iv) bending of thin filaments. The experiments were performed in either a scanning electron microscope or a transmission electron microscope with the use of micro­-loading devices capable of applying loads down to 0.1 mgf (10 -6 N). In the blunting experi­ments carried out in the transmission electron microscope it was possible to observe disloca­tions directly in the tip during loading. The majority of the experiments were carried out on gold. The results showed that strengths similar to the theoretical value can be achieved but in the experiments in which the stress was applied at an external surface (experiments (i), (ii) and (iii) above) the strength was strongly dependent on the condition of the interface. The strength that could be sustained by a region of perfect crystal in contact with a hard metallic surface was about five times less than the theoretical strength. These relatively low strengths are probably due to interfacial tractions producing very high local stresses. The introduction of some polymeric or amorphous layer at the interface raised the strength to the theoretical level. It is suggested that this is due to the fact that the polymeric layer removes most of the stress concentration. Transmission electron microscopy through aluminium tips during blunting showed that plastic deformation could take place at quite low stresses in a dislocation-free crystal with no dislocations remaining in the crystal during the early stages of blunting. Dislocation build up only occurred in the later stages of deformation. The yield stress was found to decrease with plastic strain in all the experiments, and could fall to values which were not substantially greater than those observed in large specimens. Comparison of the compressive strength of two spherical gold crystals, 0.5 μ m and 2 mm in diameter respectively, showed that the small crystal was only twice as strong as the large crystal after they had both undergone equal amounts of compression. The maximum strengths observed for gold ( μ /20) are greater than those calculated by Kelly (1966) ( ca . μ /50) and are nearer the value deduced by Brown & Woolhouse (1970) for the generation of dislocations around precipitate particles in alloys. The low strengths observed on deformed crystals are considered in the context of dislocation generation in small volumes and it is concluded that although a source mechanism of the Frank–Read type may be able to operate on this scale, some other source mechanism may also exist.

The Effect of Biaxial Stresses on Fatigue and Fracture

Abstract: The results are presented of an analytical and experimental program which was conducted to determine if a biaxial stress field produces a significant effect on the fatigue and fracture behavior of thin plates. The materials tested were 6061-T4 and 6061-T6 aluminum sheets and plexiglas sheets. The experimental program included fracture tests with various magnitudes of biaxial load at fracture and fatigue tests with sinusoidal loading normal to the crack and either constant or sinusoidal stresses applied parallel to the crack. The effect of nonsingular stresses on the behavior of a crack is examined from both a linear elastic and an elastic-plastic viewpoint. The experimental study indicates that a biaxial stress field does affect the behavior of a crack in a thin sheet. An increase in the apparent fracture toughness with increasing biaxial load was observed experimentally but as yet cannot be adequately explained using linear fracture mechanics theory. Biaxial stresses were found to produce a shift in the fatigue crack growth rate data, and it is shown that this shift can be predicted using several empirical fatigue crack propagation models.

Fatigue crack propagation in polymeric materials

Abstract: In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response Fatigue crack propagation rates between 5×10−6 in cycle−1 (127 nm cycle−1) and 4×10−4 in cycle−1 (10 300 nm cycle−1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics

The initiation of fatigue cracks from notches in mild steel plates

Abstract: Pulsating tension fatigue tests have been carried out on edge-notched specimens of a mild steel and measurements of electrical potential drop across notches have been used to determine the number of cycles to initiate cracks. The results can be described by using the range of stress intensity factor for the sharp crack situation modified to take into account the notch root radius. The models of plastic relaxation at a crack tip and analyses of the elastic stress distribution at cracks and notches are used to discuss the results. Limited evidence in the literature indicates that the stress intensity approach may provide a general description of crack initiation and propagation in notched specimens.

Enhanced oxidation of molybdenum disilicide under tensile stress: relation to pest mechanisms.

Abstract: The mechanical disintegration of molybdenum disilicide in air or oxygen at temperatures of 300° to 600°C, known in the literature as “pest”, is ascribed here to stress enhanced oxidation at the tips of Griffith flaws, eventually leading to brittle fracture. Stress-free single crystals of molybdenum disilicide, which are not ordinarily subject to pest, were shown to exhibit delayed failure in four-point bend tests under the conditions that lead to pest in polycrystals. The delayed failure times showed the same temperature and oxygen pressure dependence as the times to onset of pest disintegration in polycrystals. Pest in polycrystals occurs even in the absence of an applied stress, due to the high residual stresses which are introduced upon cooling this highly anisotropic material from the melt during fabrication. Nonetheless, the superposition of an applied stress, over and above the residual stress, was shown to decrease the time to pest failure compared to unstressed specimens. The absence of pest above 600°C, and the longer times required for pest disintegration as temperature is increased above about 500°C, is explained by the presence of plastic deformation in the matrix, which tends to reduce stress concentration at the tips of flaws and to counteract the flaw sharpening effect of stress enhanced oxidation. Slip was readily observed at 600°C. The delayed failure results were consistent with the general Charles-Hillig model for the mechanical failure of brittle materials that react chemically with their environment.

The stress distribution around a crack perpendicular to an interface between materials

Abstract: The plane strain problem of a crack terminating perpendicular to a planar interface between two isotropic half spaces with different elastic constants is solved to obtain the distribution of stress in the vicinity of the crack tip. The relative elastic constants are shown to strongly affect the relative magnitudes of the various stress components as well as their radial drop off with distance from the crack tip. The implications of the results with regard to failure modes in composite materials are discussed.

Creep of Doped Polycrystalline Al2O3

Abstract: Creep behavior of polycrystalline A12O3 doped with MgO + NiO, both as-hot-pressed in graphite dies and additionally annealed, was determined for 1300° to 1470°C and for 1000 to 15,000 psi in compression. The deformed specimens contained intergranular separations. Creep rates were proportional to stress to the 1.1 and 1.3 powers and were independent of grain size changes occurring during creep. The suggested creep mechanism is localized plastic deformation at stress concentration points accommodated by grain-boundary separations initiated by grain-boundary sliding.

Effects of strain-gradient on the stress-concentration at a cylindrical hole in a field of uniaxial tension

Abstract: The solution of the plane-strain problem of a circular cylindrical hole in a field of uniaxial tension is obtained in the linear theory of elasticity in which the potential energy function depends on both the strain and the gradient of the strain The stress-concentration factor at the surface of the cylindrical hole and the stress-concentration away from the hole are found and they are compared with the analogous results obtained in couple-stress theory and in classical elasticity

Note: The Optimum Profile for a Lap Joint.

Abstract: IN LAP JOINTS, stress concentrations in the adhesive layer can arise from differences in elastic moduli and abrupt variations in thickness of the adherends and the adhesive layer1. Various attempts have been made to design joints in which these stress concentrations are minimised. Mylonas and de Bruyne2 suggested that the stress concentrations could be reduced by tapering the ends of the adherends, so that a more even distribution of strain along the joint could be obtained. Hennig3 suggested that the same objective would be attained by using a high modulus adhesive in the centre of the joint and a lower modulus adhesive at the ends of the joint; he reported that an increase in joint strength of 20% could be obtained by this method. Segerlind4 discussed the variation of the magnitude of the stress concentrations in a lap joint with the dimensions of the joint and showed that since the increase in joint strength with increase in overlap effectively fell to zero above a given length of the overlap (dependen...

Twisting of an elastic plate containing a crack

Abstract: The stress distribution caused by twisting an infinite plate containing a finite crack is analyzed in terms of Reissner's theory for the bending of thin plates. The singular character and the detailed structure of the stresses near the ends of the crack are determined in closed form. Numerical results are given for the magnitudes of the stress couples and stress resultants for a range of plate thicknesses.

Observations on the Stress Corrosion Crack Propagation Characteristics of High Strength Steels

Abstract: The relationship between stress corrosion crack velocity and crack-tip stress intensity is discussed. In most high strength steels, there is a wide range of stress intensity over which crack velocity is essentially constant. Methods of estimating this velocity are described. Values for a variety of high strength steels are presented and the effects of metallurgical variables are indicated. Implications with regard to testing procedure, crack morphology, and service performance are outlined.

Dislocation arrays and elastic-plastic cracks in a two-phase system

Abstract: Elastic-plastic shear cracks propagation in two phase composite materials using dislocation model, determining critical fracture strength

Stress concentration in silicon-insulator interfaces

Abstract: Edges of silicon oxide or nitride diffusion masks produce in the silicon substrate stresses of thermal or intrinsic nature. When the stress field is evaluated in terms of an elastic two-dimensional model, with the proper boundary conditions, stresses attain a maximum value of the order of 10 9 dy/cm 2 . The shear component is particularly important. Experiments with monochromatic infra-red transmission are reported. They show a marked anomaly in transmittance in the region near the mask edge, where stresses are presumably large. To the first order this anomaly appears to be caused by a piezo-absorption phenomenon. The implications of the large stress field for semiconductor device behavior are analyzed qualitatively, while the limitations of the calculations caused by limitations in the model, are emphasized. These results nevertheless, on balance, point to the serious possibility that some reported phenomena in semiconductor devices can be understood as edge-stress dependent processes.

Comparison of Fracture Mechanics and Nominal Stress Analyses in Stress Corrosion Cracking

Abstract: Since the development of the fatigue cracked cantilever beam stress corrosion specimen, the fracture mechanics approach has been widely used to analyze the stress corrosion behavior of mat...

Mechanical Properties and Fracture Behavior of Chemically Bonded Composites

Abstract: The effect of chemical bonding between phases of a glass matrix-metal composite on strength and fracture behavior was investigated. When no chemical bonding occurs, strengthening can be achieved through the mechanical formation of an interface between the dispersant and matrix. Even greater strengthening can be obtained by the formation of a chemical bond. Strengthening occurs by the limitation of the Griffith flaw size and is controlled by micromechanical stress concentrations developed on loading. Internal stresses developed on cooling from the fabrication temperature control the path of fracture. A chemical bond counteracts the micromechanical stress concentration and therefore increases the strength.