Strains In Excess Research Articles

Shear Strength and Stiffness of Silty Sand

The properties of clean sands pertaining to shear strength and stiffness have been studied extensively. However, natural sands generally contain significant amounts of silt and/or clay. The mechanical response of such soils is different from that of clean sands. This paper addresses the effects of nonplastic fines on the small-strain stiffness and shear strength of sands. A series of laboratory tests was performed on samples of Ottawa sand with fines content in the range of 5–20% by weight. The samples were prepared at different relative densities and were subjected to various levels of mean effective consolidation stress. Most of the triaxial tests were conducted to axial strains in excess of 30%. The stress-strain responses were recorded, and the shear strength and dilatancy parameters were obtained for each fines percentage. Bender element tests performed in triaxial test samples allowed assessment of the effect of fines content on small-strain mechanical stiffness.

Volume strain within The Geysers geothermal field

During the 1970s and 1980s, The Geysers geothermal region was rapidly developed as a site of geothermal power production. The likelihood that this could cause significant strain within the reservoir, with corresponding surface displacements, led to a series of deformation monitoring surveys. In 1973, 1975, 1977, and 1980, The Geysers region was surveyed using first‐order, class I, spirit leveling. In 1994, 1995, and 1996, many of the leveling control monuments were resurveyed using high‐precision Global Positioning System receivers. The two survey methods are reconciled using the GEOID96 geoid model. The displacements are inverted to determine volume strain within the reservoir. For the period 1980–1994, peak volume strains in excess of 5×10−4 are imaged. There is an excellent correlation between the observed changes in reservoir steam pressures and the imaged volume strain. If reservoir pressure changes are inducing volume strain, then the reservoir quasi‐static bulk modulus K must be <4.6×109 Pa. However, seismic velocities indicate a much suffer reservoir with K = 3.4 × 1010 Pa. This apparent discrepancy is shown to be consistent with predicted frequency dependence in K for fractured and water‐saturated rock. Inversion of surface deformation data therefore appears to be a powerful method for imaging pressure change within the body of the reservoir. Correlation between induced seismicity at The Geysers and volume strain is observed. However, earthquake distribution does not appear to have a simple relationship with volume strain rate.

Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y 2O 3-doped -Si 3N 4

The uniaxial tensile and compressive creep rates of an yttria-containing hot-isostatically-pressed silicon nitride were examined at several temperatures between 1316 and 1399°C and found to have different stress dependencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both tensile and compressive stresses. The model also depicted the asymmetric creep deformation using exponential and linear dependence on tensile and compressive stress, respectively. Unlike other models which represent either tensile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deformation rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model’s regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavitation accounted for the creep strain asymmetry between them, and that cavitation initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0·1%.

Trends in the Antimicrobial Susceptibility of Bacterial Respiratory Tract Pathogens - Findings of the Alexander Project 1992-1996

The Alexander Project is an ongoing, multicenter surveillance study of the antimicrobial susceptibility of community-acquired lower respiratory tract bacterial pathogens with testing undertaken in a central laboratory. During the period 1992-1995, isolates were collected from geographically separate centers in countries of the EU and various states in the USA. In 1996, the project was extended to centers in Mexico, Brazil, Saudi Arabia, South Africa, Hong Kong and other European countries not previously included. Within Europe, France and Spain are established as centers with a high prevalence of both penicillin-intermediate (MIC 0.12-1 mg/l) and resistant (MIC > 2 mg/l) strains of Streptococcus pneumoniae, with combined resistance rates in excess of 40% in Toulouse and Barcelona in 1996. Combined rates of intermediate and resistant strains in excess of 10% were found in 1996, the first year of sampling, in Belgium, Switzerland, the Slovak Republic and Hungary. Penicillin resistance has evolved in the USA during the period of study, with rates for combined pneumococcal isolates increasing from 5.6% in 1992 to 16.4% in 1996. Of the new, non-European centers joining the project in 1996, Mexico (intermediate 31.4%, resistant 15.7%) and, in particular, Hong Kong (intermediate 9.1%, resistant 50%) are centers with a high prevalence of penicillin resistance. Macrolide resistance has increased generally among pneumococcal isolates examined during the study period, both in penicillin-susceptible and resistant isolates, and was evident in 16.5% of the 2160 isolates collected during 1996. In four centers (London, UK; Genoa, Italy; Pokfulum, Hong Kong; Leuven, Belgium), macrolide resistance rates exceeded those of combined penicillin-intermediate and resistant strains; in 12/19 centers (63.2%) macrolide resistance was more prevalent than penicillin resistance. In 1996, macrolide resistance was found in excess of 10% of isolates in Poland, Hungary, London, UK, combined USA isolates, the Slovak Republic, Barcelona, Spain, Genoa, Italy, Mexico, Toulouse, France and Pokfulum, Hong Kong. β-lactamase production was the principal mechanism of resistance found among isolates of Haemophilus influenzae, with rates in 1996 of around 20% or more in France, Belgium and Spain, and in excess of 10% in the UK and the Czech Republic. In the same year in non-European centers, Mexico (25%), Saudi Arabia (27.9%), Hong Kong (37.1%) and the USA (30.4% of combined isolates) had a high prevalence of β-lactamase production. Isolates of β-lactamase-negative, ampicillin-resistant H. influenzae were generally very uncommon, with only Barcelona, Spain consistently associated with rates in excess of 1%. β-lactamase production in Moraxella catarrhalis was observed in over 90% of isolates tested in 1996.

Flow Deformation of Sands Subjected to Principal Stress Rotation

In order to investigate the undrained deformation behaviour of sand subjected to principal stress rotation, two series of tests were performed using a torsional cylinder shear apparatus. The following conclusions were obtained from the tests. Pore pressures and strains in excess of 5% were accumulated under the cyclic rotation of principal stresses despite the deviatoric stress remaining constant. The form of deformation could be classified into non-flow, limited flow or full flow deformation. Flow deformation occurred when the effective stress state of samples reached a critical stress ratio. If the effective stress reached the steady state line after the critical stress ratio was reached, samples demonstrated full flow deformation. If on the other hand, the effective stress reached the phase transformation line after the critical stress ratio was attained, samples demonstrated limited flow deformation. The critical stress ratio was defined as the stress ratio at which strain softening commenced, and was obtained from undrained shear tests at various relative densities and principal stress directions. It was observed that the existence of a critical stress ratio was a function of both relative density and principal stress direction. The test results in this paper demonstrate the importance of investigating the effects not only of the density and confining pressure, but also of the principal stress direction and its rotation on the flow deformation of sand.

The Role of Dilatation in the Nonlinearly Viscoelastic Behavior of PMMA under Multiaxial Stress States

In order to better understand phenomena related to “yield-like” behavior of polymers, the nonlinear thermo-mechanical behavior of Polymethyl Methacrylate (PMMA) under combined axial (tension, compression) and shear stress states (torsion) is investigated on thin walled cylindrical specimens at temperatures between 22 and 110°C. In contrast to the mutual independence of shear and dilatational response under conditions appropriate for linearized viscoelasticity, one observes an increasingly strong influence of the first stress or strain invariant on shear creep at shear strains in excess of 0.5%. While shear stresses alone elicit nonlinear response in creep (rates) as “intrinsically nonlinear shear response”, the superposition of small positive dilatation accelerates shear deformations while negative dilatation retards it in qualitative agreement with free volume arguments when comparison is effected via maximum shear. In addition, an isochronal representation of the “intrinsically nonlinear shear response” demonstrates that the nonlinear behavior becomes more pronounced the closer one approaches the glass transition temperature from below.

Scaling laws for the reduction of threading dislocation densities in homogeneous buffer layers

In the heteroepitaxial growth of films with large misfit with the underlying substrate (linear mismatch strains in excess of 1%–2%) the generation of misfit dislocations and threading dislocations (TDs) is ubiquitous for thicknesses well in excess of the equilibrium critical thickness. Experimental data suggest that the TD density in relaxed homogeneous buffer layers can be divided into three regimes: (i) an entanglement region near the film/substrate interface corresponding to TD densities of ∼1010–1012 cm−2; (ii) a falloff in TD density that is inversely proportional to the film thickness h, applicable to densities in the range ∼107–109 cm−2; and (iii) saturation or weak decay of the TD density with further increase in film thickness. Typical saturation densities are on the order of ∼106–107 cm−2. In this article, we show that the TD reduction may be described in terms of effective lateral motion of TDs with increasing film thickness. An analytic model is developed that successfully predicts both the 1/h scaling behavior and the saturation of TD densities. Long-range fluctuations in the net Burgers vector content of the local TDs is a cause for saturation behavior. These models are supported by computer simulations.

Cyclic Simple Shear Apparatus for Low-Strain Soil Tests

A testing system has been developed to study the behavior of saturated sand under low-level cyclic shearing strains. The system has been used to determine threshold shear strain levels for fabric destruction in sand aged for different time periods. The system includes a special soil chamber and a direct simple shear (DSS) machine. To impose very small shearing strains, the DSS machine was designed to apply and measure horizontal deformations as small as 0.0005 mm (2 × 10−5 inches). Data obtained to date support the results of previous investigators who performed triaxial tests on freshly deposited samples, indicating a threshold cyclic shear strain level of approximately 0.01 percent. At strains in excess of those levels, destruction of the sand fabric occurred, as evidenced by a reduction in shear modulus at low strain levels. Subsequent modest increases in shear modulus were observed after the specimens were allowed to recover for 24 hours and then tested again. During the recovery period, drainage valves were left open to allow for dissipation of excess pore pressures and for potential consolidation during the short aging period. The DSS system was found to work well for low strain measurements. Furthermore, since shear strains are measured directly under DSS conditions (as opposed to triaxial conditions), the DSS system shows much promise as a device for studying parameters that may influence threshold shear strain levels and fabric evolution and destruction in sands.

Cyclic Simple Shear Apparatus for Low-Strain Soil Tests

A testing system has been developed to study the behavior of saturated sand under low-level cyclic shearing strains. The system has been used to determine threshold shear strain levels for fabric destruction in sand aged for different time periods. The system includes a special soil chamber and a direct simple shear (DSS) machine. To impose very small shearing strains, the DSS machine was designed to apply and measure horizontal deformations as small as 0.0005 mm (2 × 10−5 inches). Data obtained to date support the results of previous investigators who performed triaxial tests on freshly deposited samples, indicating a threshold cyclic shear strain level of approximately 0.01 percent. At strains in excess of those levels, destruction of the sand fabric occurred, as evidenced by a reduction in shear modulus at low strain levels. Subsequent modest increases in shear modulus were observed after the specimens were allowed to recover for 24 hours and then tested again. During the recovery period, drainage valv...

Influence of solution treatment on the microstructure and crystallographic texture of cold rolled and recrystallised low carbon steel

The effects of solution treatment on the texture and microstructure of cold rolled (e = 80 %) and recrystallised (T = 650 °C, salt bath) low carbon steel (mass content of 0.032 % C) were studied using both specimens having a maximum amount of carbon in solution and specimens in the as-hot-rolled state. For the investigations X-ray texture measurements and TEM observations were performed. Deformation was more inhomogeneous in specimens with carbon in solution. In such specimens a strong tendency to the formation of shear bands was observed at plastic strains in excess of 50 %. The specimens with little carbon in solution showed only few shear bands. Increasing contents of carbon in solution led to a degradation of the orientation density of the typical cold rolling texture components. This was attributed to the inhomogeneity of deformation, e.g. the formation of shear bands. The highest tendency to shear band formation was observed in {111} oriented grains. The development of shear bands in the deformation microstructure was interpreted in terms of Taylor theory. The {111} recrystallisation texture of samples with carbon in solution was weaker than in conventionally prepared samples. This observation was explained in terms of random nucleation in the inhomogeneously deformed microstructure and solute drag effects.

Experimental data on oxide fracture

Protective oxide scales on high temperature materials often fail in service leading to increased oxidation rates. Thus, a knowledge of oxide failure behaviour and failure strains is required. This paper reviews the measured failure strains of chromium oxide and aluminium oxide scales at room and oxide growth temperatures under applied strain. Data on iron and nickel oxide scales are also included for modelling purposes. Failure strains in tension show a power-law dependence on the measured void size within the layer, as expected from fracture mechanics models. Residual tensile or compressive growth and cooling strains prior to loading can increase or decrease the measured failure strains depending on the loading mode, i.e. tension or compression. At growth temperature, lateral oxide growth and creep may act to increase the failure strains, leading in some cases to measured failure strains in excess of 2%. However, the effective failure strains in tension, after taking account of the residual strains, oxid...

Correlation of Ultrastructure and Tensile Properties of a Collagenous Composite Material, the Egg Capsule of the Dogfish, Scyliorhinus spp, a Sophisticated Collagenous Material

The egg capsules of dogfish are largely constructed from laminae of parallel orientated collagenous fibrils arranged longitudinally, near-diagonally, and near-transversely. The fibrils show an axial (D) period of about 40 nm. These characteristics facilitated the observation of fibril rotation and strain and the construction of an exploratory model of the mechanical behaviour based on composite theory. Low angle X-ray diffraction demonstrated reversible changes in material strained up to 24%; the D period extended by 5% with a loss in transverse order. Electron microscopy confirmed this fibril strain and demonstrated that failure of the interfibrillar matrix at moderate global strains was conspicuous in the near-transverse laminae and less marked in the near-diagonal ones. Light microscopy demonstrated that rotation of the near-diagonal fibrils occurred during global strain. The results of mechanical testing are described. The model helps to explain the double S-shaped stress/strain and unrecoverable deformation plots. The tensile behaviour up to strains of 10% appeared dominated by failure of the interfibrillar matrix in the near-transverse laminae. The behaviour of the material at strains in excess of this was thought to result from a combination of rotation and straining of matrix and fibrils in the near-longitudinal and near-diagonal laminae. The egg capsule is thought to function as an efficient shock absorber reducing the likelihood of injury from both wave action and attempted predation by fish.

YIELD DETECTION IN ALUMINUM WELDED JOINTS USING PHOTOSTRESS

A) Photostress is very useful to provide a full field and point-by-point characterization of the behavior of welded joints having the weld beads on or ground off. B) Use of very thin coatings allows for measurement of strains in excess of 0.04. C) Poured PL-1 coatings 0.02 inches thick may be used successfully if extreme care in handling is exercised. D) PC-8 adhesive is quite satisfactory for measuring strains in excess of 0.04. E) Photostress is quite useful for detecting discontinuous yielding since the slip-hold-slip process produces instantaneous jumps in the fringe pattern.

Hybrid grid-moire method for measuring strain and displacement

A hybrid grid-moire method for measuring strain is presented. The method employs a line grating attached to a plane surface of the specimen. Images of undeformed and deformed gratings are acquired and stored using a video system and a frame digitizer. The magnification is adjusted to record 10 to 30 lines of a grating with a pitch of 4 μm over the frame width. A fast Fourier transform (FFT) of the intensity data gives a close estimate of the spatial line frequency. A moire function is formed as the product of a reference sine function and the image intensity-position function. FFTs of the moire function yield an improved estimate of the line frequency. The line frequency is related to an average strain over the recorded image. The method was demonstrated at a magnification of 1500 x with a grating pitch p s = 4 μm. The accuracy achieved depended on strain. For strains in excess of 0.1%, the errors were usually less than 5% when compared to strain gauge results. For plastic strains, in excess of 1%, the differences between two comparable measurements were less than 3%.

Measurement of soil strength in simple shear tests: Reply

Measurement of soil strength in simple shear tests: Reply

Grain boundary sliding in Ni 3Al(Ta,B) bicrystals

Grain boundary sliding refers to displacements produced when individual crystals slide over each other at or in a region directly adjacent to their common interface. Grain boundary sliding has been found to have both beneficial and detrimental roles in deformation and fracture processes that occur in polycrystalline materials. For example, extensive grain boundary sliding makes superplastic forming possible where strains in excess of 1000% may be achieved. In contrast, the occurrence of grain boundary sliding may be detrimental in high temperature components by contributing strongly to the overall creep strain. It has also been shown that stress redistribution resulting from grain boundary sliding can significantly enhance the damage mechanisms that facilitate high temperature fracture. Thus it is important to develop an understanding of grain boundary sliding in materials considered for service at elevated temperatures. Polycrystalline nickel aluminide (Ni[sub 3]Al) is a promising matrix material for use at elevated temperatures as a result of its anomalous positive temperature dependence of strength, good oxidation resistance, low density compared to superalloys, and good ductility with the addition of a small amount of boron. However, Ni[sub 3]Al alloys exhibit a transition from ductile to brittle behavior at elevated temperatures. This decrease in ductility ismore » shown in this paper.« less

Durability of cement sheets reinforced with fibrillated polypropylene networks: 10 year results

Cement sheets reinforced with layers of networks of fibrillated polypropylene film have been exposed to natural weathering, underwater storage and storage in laboratory air for periods up to 10 years. The effect of these exposure conditions on the stress-strain performance of the composite in uniaxial tension is described, and different characteristics are apparent for each exposure condition. The bond strength between film and cement has been shown to be independent of time and storage condition, and the film strength also shows little change with time. Important considerations from the potential user's point of view are that 10 years of freezing and thawing under UK winter conditions have not significantly affected the composite as measured by the initial elastic modulus, and the material has remained ductile with a failure strain in excess of 5% after 10 years underwater.

Deformation of an extruded nickel beryllide between room temperature and 820 °C

The mechanical properties of nickel beryllide, NiBe, have been investigated in the temperature range 20–820 °C. The room temperature properties were studied using tension, bending, and compression tests, while the elevated temperature properties were characterized in compression only. NiBe exhibits some ductility at room temperature; the strains to failure in tension and compression are 1.3% and 13%, respectively. Fracture is controlled primarily by the cohesive strength of grain boundaries. At high temperatures, NiBe is readily deformable—strains in excess of 30% can be achieved at temperatures as low as 400 °C. Strain hardening rates are high, and the flow stress decreases monotonically with temperature. The high temperature strength of NiBe is as good or better than that of NiAl, but not quite as good as CoAl.

A study of shear deformation and work hardening of metals under pressures to 2100 MPa

Work-hardening curves have been obtained from thin disks being sheared undirectionally between Bridgman anvils under normal pressures up to 2100 MPa (= 300 000 lbf in −2 + 21 kbar). The materials tested were hard-rolled aluminum, aluminum alloy 2124-T6 and the same alloy reinforced with 20% chopped SiC fibers with the fibers initially both parallel to and normal to the disks. Averaged shear strains in excess of 100, i.e. 10 4%, were routinely achieved. Selected samples were subsequently examined by scanning electron microscopy. Negative work-hardening rates were observed in the mechanical tests of both the unreinforced matrix alloy and the composite but not the aluminum, and the micrographs showed the concentration of deformation into shear bands which is to be expected from such work softening. The results obtained permit conclusions among others regarding the tribological behavior of these materials. The equivalent coefficients of friction, which would have been observed had these materials been used as solid lubricants, ranged from 0.25 at 700 MPa pressure to 0.14 at 2100 MPa.

Strain localization behaviour in experimental shear zones

A torsional deformation apparatus has been developed which allows the evolution of microstructures and flow patterns in analogue materials to be observed to shear strains in excess of γ=100 while the loading conditions are kept constant. Using a computer-based video microscopy system the displacement paths of marker particles have been followed in real time, and from these the local strain states in the samples have been calculated. A series of velocity stepping experiments in octachloropropane and norbornene determined the shear localization behaviour displayed by these materials. Both of these materials underwent two phases of localization, a primary localization at the start of an experiment, and a secondary localization following increases in the imposed strain rate. The secondary localization is less pronounced, but is reversible, in the sense that lowering the imposed strain rate widens the zone. An inverse relationship between the fixed imposed strain rate and the width of the zone of localized deformation was established. This velocity dependant localization is ascribed to the softening effect of dynamic recrystallization.