Constant Displacement Rate Research Articles

Impact of hydraulic suction history on crack growth mechanics in soil

The mechanics of crack formation and the influence of soil stress history were described using the crack tip opening angle (CTOA) measured with fractography. Two soils were studied: a model soil consisting of 40% Ca‐bentonite and 60% fine silica sand and a remolded paddy soil with similar clay content and mineralogy. Fracture testing used deep‐notch bend specimens formed by molding soils at the liquid limit into rectangular bars, equilibrating to soil water suction ranging from 5 kPa to 50 kPa (with some 50 kPa specimens wetted to 5 kPa), and inserting a crack 0.4× specimen thickness. Bend tests at a constant displacement rate of 1 mm min−1 provided data on applied force and load point displacement. The growth and geometry of the cracks were quantified from a series of images to determine the CTOA. Modulus of rupture, evaluated from the peak force, increased as water suction increased. However, rewetting did not alter the peak stress from the 50 kPa value, indicating that shrinkage‐induced consolidation was more important than the soil water suction at the onset of testing. CTOA measured during stable crack growth decreased with drying. CTOA decreased even further when specimens equilibrated initially to 50 kPa were rewetted to 5 kPa. These results suggested that CTOA was primarily governed by the stiffness, although rewetting probably altered the capillary stresses in advance of the crack tip. Our future work will combine CTOA with a model that couples hydrological and mechanical processes to take into account the dependency of CTOA on the soil water regime so that crack propagation in soil can be predicted.

High-temperature deformation of volcanic materials in the presence of water

We describe an experimental apparatus used to perform deformation experiments relevant to the volcanological sciences. The apparatus supports low-load, high-temperature deformation experiments under dry and wet conditions on natural and synthetic samples. The experiments recover the transient rheology of complex (melt ± porosity ± solids) volcanic materials during uniaxial deformation. The key component to this apparatus is a steel cell designed for high-temperature deformation experiments under controlled water pressure. Experiments are run under constant displacement rates or constant loads; the range of accessible experimental conditions include: 25–1100 °C, load stresses 0 to 150 MPa, strain rates 10 −6 to 10 −2 1/s, and fluid pressures 0–150 MPa. The apparatus is calibrated against standard values of viscosity using constant-load experiments on cores of NIST (NBS) 717a borosilicate glass. We also report results of constant-displacement rate (~10 −6 m/s) experiments on porous (~70%) sintered cores of ash from the Rattlesnake Tuff. The cores of volcanic ash were deformed in experiments under ambient (“dry”) and elevated water pressures (“wet”). Dry experiments at ~870 °C show an increase in effective viscosity (10 9.5 to 10 10.4 Pa·s) with increasing strain (~30%) due to porosity reduction during compaction. Experiments under ~1–3 MPa P H 2 O recover lower values of apparent viscosity (10 9.2 to 10 9.4 Pa·s) despite being run at lower temperatures (640–665 °C). The wet experiments also do not show a rise in viscosity with increased strain (decreasing porosity) as observed in dry experiments. Rather, the presence of an H 2 O fluid phase expands the window of viscous deformation and delays the onset of strain hardening that normally accompanies porosity reduction.

Biomechanical evaluation of rotator cuff repairs in a sheep model: Suture anchors using arthroscopic Mason-Allen stitches compared with transosseous sutures using traditional modified Mason-Allen stitches

Background The optimal method for rotator cuff repair of the shoulder is not yet known. The aim of this study was to compare the time-dependent biomechanical properties of the traditional open transosseous suture technique and modified Mason-Allen stitches (group 1) versus the double-loaded suture anchors technique and so-called arthroscopic Mason-Allen stitches (group 2) in rotator cuff repair. Methods Eighteen adult female sheep were randomized into two groups: in an open approach in which the released infraspinatus tendon was repaired with group 1, and with group 2. Animals were sacrificed at 6, 12, or 26 weeks; shoulders were harvested and magnetic resonance imaging was performed. Eight untreated contralateral shoulders served as controls. Tendons of 16 additional unpaired cadaver shoulder joints of adult female sheep were identically treated for analysis at time zero. In a biomechanical evaluation all specimens were loaded to failure at a constant displacement rate using a standard universal testing machine. The load-to-failure and stiffness of the healed bone-tendon interface were calculated. Results Magnetic resonance imaging analysis showed cuff integrity in all cases, and no evidence of foreign body reaction to the anchors. Load-to-failure and stiffness data did not indicate any significant difference between the two treatment groups, neither at 6 weeks nor at 12 or 26 weeks. However, at time zero the group 2 had a higher load-to-failure in comparison to the group 1 ( P < 0.010), but there was no difference for the stiffness ( P < 0.121). Conclusions This in vivo study showed that, postoperatively, the group 2 technique provides superior stability and after healing would gain strength comparable to the group 1 technique.

In Vitro Biomechanical Comparison of Equine Proximal Interphalangeal Joint Arthrodesis Techniques: Prototype Equine Spoon Plate Versus Axially Positioned Dynamic Compression Plate and Two Abaxial Transarticular Cortical Screws Inserted in Lag Fashion

To compare in vitro monotonic biomechanical properties of an equine spoon plate (ESP) with an axial 3-hole, 4.5 mm narrow dynamic compression plate (DCP) using 5.5 mm cortical screws in conjunction with 2 abaxial transarticular 5.5 mm cortical screws (DCP-TLS) inserted in lag fashion for equine proximal interphalangeal (PIP) joint arthrodesis. Paired in vitro biomechanical testing of 2 methods of stabilizing cadaveric adult equine forelimb PIP joints. Cadaveric adult equine forelimbs (n=18 pairs). For each forelimb pair, 1 PIP joint was stabilized with an ESP (8 hole, 4.5 mm) and 1 with an axial 3-hole narrow DCP (4.5 mm) using 5.5 mm cortical screws in conjunction with 2 abaxial transarticular 5.5 mm cortical screws inserted in lag fashion. Six matching pairs of constructs were tested in single cycle to failure under axial compression with load applied under displacement control at a constant rate of 5 cm/s. Six construct pairs were tested for cyclic fatigue under axial compression with cyclic load (0-7.5 kN) applied at 6 Hz; cycles to failure were recorded. Six construct pairs were tested in single cycle to failure under torsional loading applied at a constant displacement rate (0.17 radians/s) until rotation of 0.87 radians occurred. Mean values for each fixation method were compared using a paired t-test within each group with statistical significance set at P<.05. Mean yield load, yield stiffness, and failure load for ESP fixation were significantly greater (for axial compression and torsion) than for DCP-TLS fixation. Mean (+/- SD) values for the ESP and DCP-TLS fixation techniques, respectively, in single cycle to failure under axial compression were: yield load 123.9 +/- 8.96 and 28.5 +/- 3.32 kN; stiffness, 13.11 +/- 0.242 and 2.60 +/- 0.17 kN/cm; and failure load, 144.4 +/- 13.6 and 31.4 +/- 3.8 kN. In single cycle to failure under torsion, mean (+/- SD) values for ESP and DCP-TLS, respectively, were: stiffness 2,022 +/- 26.2 and 107.9 +/- 11.1 N m/rad; and failure load: 256.4 +/- 39.2 and 87.1 +/- 11.5 N m. Mean cycles to failure in axial compression of ESP fixation (622,529 +/- 65,468) was significantly greater than DCP-TLS (95,418 +/- 11,037). ESP was superior to an axial 3-hole narrow DCP with 2 abaxial transarticular screws inserted in lag fashion in resisting static overload forces and cyclic fatigue. In vitro results support further evaluation of ESP for PIP joint arthrodesis in horses. Its specific design may provide increased stability without need for external coaptation support.

An Experimental Investigation of Interface Fracture Characteristic of APS Thermal Barrier Coating System under Bending at 1000 °C

This paper investigated the thermo-mechanical bending failure characteristic of air plasma-sprayed (APS) thermal barrier coating (TBC) system at 1000 oC by three-point bending test. A through-width surface pre-crack on the centre surface of the top ceramic coating (TC) in TBC sample was firstly designed and made by air plasma spraying technology. The fracture characteristic and spallation phenomena of the TBC were investigated under bending test in air at 1000 oC. The effects of constant displacement rate, span width of bending equipment and thermal aging time on the thermo-mechanical bending failure of the APS TBC system were slight at 1000 oC. Using SEM observations and EDX analysis, the interface cracks initiation, propagation and coalescence were discussed in detail. Furthermore, the fracture surface located within the TBC close to the TBC/TGO interface for all APS TBC samples under bending at 1000 oC.

Plastic flow of two‐phase marbles

To quantify the effect of rigid inclusions on the flow behavior of a creeping matrix, we fabricated a suite of two‐phase marbles by hot isostatically pressing mixtures of calcite and quartz powders and subsequently deformed these synthetic rocks at 300 MPa confining pressure, up to 100 MPa pore pressure, temperatures ranging from 873 to 1073 K, strain rates and stresses ranging from ∼10−7 s−1 to ∼10−3 s−1 and ∼4 MPa to 190 MPa, respectively. In constant displacement rate tests performed to axial strains of up to about 30%, we observed deformation at approximately constant stress for samples with 20% or less quartz. Even small additions of second phase significantly strengthen the aggregate, although the magnitude of the strengthening varies significantly with temperature and imposed strain rate. Investigation of the microstructure of starting samples reveals that initial porosity directly varies with quartz content and that matrix grain size inversely varies with the amount of second phase. Strength decreases modestly with increasing porosity, and consequently, strength increases as the samples compact. The effect of matrix grain size on the aggregate strength depends on the deformation conditions. The majority of tests exhibit an inverse relation between strength and matrix grain size (i.e., similar to a Hall‐Petch relation), but fine‐grained samples deformed at the highest temperatures exhibit decreasing strength with decreasing grain size. The addition of the quartz particles seems to result in both structural strengthening owing to decreased grain size and to transfer of load from the flowing matrix to the rigid quartz particles.

Numerical simulation of mechanical behaviour of asphalt mix

The present paper attempts to develop a numerical model for predicting the mechanical behavior of asphalt mixes. In this work asphalt mix samples are modeled with the finite element method incorporating irregular as well as for random elliptical aggregates. The irregular shaped aggregates are obtained by tracing out the original aggregate shapes from cross-sectional images of asphalt mix samples and detecting the aggregate boundaries by image processing techniques. The elliptical aggregate samples are prepared by generating non-overlapping ellipses of random shapes and orientations. The asphalt binder, air voids and small sized aggregates together are assumed to behave like a viscoelastic material idealized by a Prony series and its properties are determined from relaxation tests performed in the laboratory on such a mix. The cylindrical asphalt mix samples are subjected to a line load at constant displacement rate and its response is studied for variations in constituent proportions and morphology. Similar conditions are simulated in the finite element model. The overall behavior of the asphalt mix in terms of displacement at failure and location of crack initiation exhibit good match between experimental and computational results.

Expansion of aftershock areas caused by propagating post-seismic sliding

SUMMARY It is known that the aftershock areas of large earthquakes often expand over time. This expansion indicates that the stress-increase associated with a main shock gradually propagates outward from the main shock rupture area. It is likely that this propagating stress-increase is caused by the propagation of post-seismic sliding, which has been detected for many large earthquakes from geodetic observations, mainly global positioning system (GPS). In the present study, I perform a numerical simulation of the expansion of the aftershock area caused by the propagation of post-seismic sliding. The model fault exists within an infinite elastic medium and is loaded at a constant displacement rate. The frictional stress on the fault obeys a laboratory-derived rate- and state-dependent friction law. Non-uniformity of the frictional constitutive parameters is introduced to the model fault plane to represent a large asperity for a large earthquake (main shock) and many possible nucleus sites for aftershocks around the main shock asperity. Negative values of A − B are assigned to areas of seismic slip for the main shock and aftershocks, while positive values of A − B are assigned to regions where aseismic sliding occurs. Here, A − B is defined by dτ ss/d ln V that describes the velocity (V ) dependence of steady-state friction stress τ ss. Seismic slip may be nucleated for negative A − B, and aseismic slip occurs for positive A − B. Slip histories on the model fault plane are simulated through numerical integration of the friction law under the condition of uniform shear loading of a constant velocity. The simulation can reproduce aftershocks triggered by stress-increases related to postseismic sliding and expansion of the aftershock area. The rate of expansion of the aftershock area decreases with increasing distance from the main shock asperity or increasing value of A − B in the velocity-strengthening region. This finding suggests that the A − B value for a plate interface can be estimated from aftershock data. The simulation indicates that the 7-day and 30-day aftershock areas are 14‐22 and 24‐39 per cent larger, respectively, than the 1-day aftershock area. These numbers are approximately consistent with observations from large earthquakes at subduction zones.

Testing environmentally assisted cracking of reactor materials using pneumatic servo-controlled fracture mechanics device

The degradation mechanism, known as stress corrosion cracking or environmentally assisted cracking (EAC) has long been recognized as a major cause of failure of components and structures but the basic mechanisms of this process are not still fully understood and the development of standardized and practicable methods of testing is a key element in a strategy to understand the kinetics of failure and to provide for mitigation measures. At the JRC-IE, Institute for Energy, a new laboratory for EAC testing of nuclear power engineering materials, in particular core internals materials, has been recently commissioned. The main part of the facility consists of a pneumatic servo-controlled fracture mechanics loading system for 3-point-bending tests of fatigue pre-cracked SEN(B) specimens of different sizes, in a high temperature and pressure autoclave. A first test campaign have been carried out and focused on comparison of the results obtained on 10×10×55 and 3×4×27 mm 3 SEN(B) 316L stainless-steel specimens loaded by constant displacement rate increase tests in typical BWR conditions. The results led to determination of the influence of the loading rate on crack initiation, comparison of the results measured on different size specimens, and verification of the usability of LEFM and EPFM parameters for evaluation of the results.

Large-strain deformation and strain partitioning in polyphase rocks: Dislocation creep of olivine–magnesiowüstite aggregates

Aggregates composed of olivine and magnesiowüstite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning, phase segregation and possible localization of deformation in a polyphase material. Samples with 20 vol.% of natural olivine and 80 vol.% of (Mg 0.7Fe 0.3)O were synthesized and deformed in a gas-medium torsion apparatus at temperatures of 1127 °C and 1250 °C, a confining pressure of 300 MPa and constant angular displacement rates equivalent to constant shear strain rates of 1–3.3 × 10 − 4 s − 1 . The samples deformed homogeneously to total shear strains of up to γ ∼ 15. During constant strain rate measurements the flow stress remained approximately stable at 1250 °C while it progressively decreased after the initial yield stress at the lower temperature. Mechanical data, microstructures and textures indicate that both phases were deforming in the dislocation creep regime. The weaker component, magnesiowüstite, controlled the rheological behavior of the bulk material and accommodated most of the strain. Deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowüstite and may have developed due to the presence of the second phase. At 1127 °C, olivine grains behaved as semi-rigid inclusions rotating in a viscous matrix. At 1250 °C, some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a-slip on (010) resulted in a weak foliation and a texture typical for pure dry olivine aggregates. Both a-slip and c-slip on (010) were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions.

Small punch testing of P91 steel

Results of small punch tests on miniaturized thin discs of P91 steel are compared with relevant conventional tests on bulk specimens. Small punch tests under constant displacement rate were performed at room temperature and 873 K. The results were correlated with results of conventional stress–strain tests. Results of small punch tests under constant force were compared with results of conventional creep tests at temperatures of 823, 848 and 873 K. It is shown that there are simple empirical relationships between the parameters determined from small punch and conventional tests. Some problems in the method for performing small punch tests are also presented and discussed.

Modified Prusik knot versus whipstitch technique for soft tissue fixation in anterior cruciate ligament reconstruction: a biomechanical analysis

Appropriate graft tension and secure graft incorporation in bone tunnels are essential for successful anterior cruciate ligament (ACL) reconstruction using hamstring tendon autografts. Permanent viscoplastic elongation in response to cyclic loading in the early postoperative period and the interposition of suture material in the tendon-bone interface might negatively affect graft function and rigid graft incorporation in the bone tunnels. A modified Prusik knot is an alternative option to the commonly used whipstitch technique for soft tissue fixation in ACL reconstruction. This is a controlled laboratory study. Sixteen formalin-fixed human cadaver semitendinosus tendons were armed with a modified Prusik knot or a whipstitch and tested in a load-to-failure test with a constant displacement rate of 1 mm/s, 14 in the cyclic loading test with 100 cycles from 10 to 50 N followed by 100 cycles from 10 to 75 N. The modified Prusik knot showed smaller force-induced displacements and higher stiffness of the entire construct in the load-to-failure test. Smaller preconditioning displacements were the only significant differences in the cyclic loading test. The modified Prusik knot has equal or superior mechanical properties and provides a larger area in the tendon-bone interface without suture material compared with the whipstitch technique.

Predicting the Crack Growth Behavior in a Filled Elastomer

In this study, single-edge cracked uniaxial specimens with an initial crack length of 0.1 in. or 0.3 in. and wedge-shaped sheet specimens with an initial crack length of 0.3 in were tested at a constant displacement rate of 50 in/min under 1000 psi confining pressure. The specimens were made of a highly filled polymeric material, containing 86% by weight of hard particles embedded in a rubbery matrix, which was made of polybutadiene-acrylic acid-acrylonitrile rubber. The uniaxial crack growth data were used to develop a crack growth model, relating crack growth rate da/dt and Mode I stress intensity factor KI. The developed crack growth model was used to predict the crack growth behavior in the wedge-shaped specimen. The results of the analysis indicated that the predicted crack growth rate compared well with the experiment

Spherical indentation load-relaxation of soft biological tissues

Elastic-viscoelastic correspondence was used to generate displacement–time solutions for spherical indentation testing of soft biological materials with time-dependent mechanical behavior. Boltzmann hereditary integral operators were used to determine solutions for indentation load-relaxation following a constant displacement rate ramp. A “ramp correction factor” approach was used for routine analysis of experimental load-relaxation data. Experimental load-relaxation tests were performed on rubber, as well as kidney tissue and costal cartilage, two hydrated soft biological tissues with vastly different mechanical responses. The experimental data were fit to the spherical indentation ramp-relaxation solutions to obtain values of short- and long-time shear modulus and of material time constants. The method is used to demonstrate linearly viscoelastic responses in rubber, level-independent indentation results for costal cartilage, and age-independent indentation results for kidney parenchymal tissue.

Factors affecting the pullout behaviour of extruded geogrids embedded in a compacted granular soil

In order to study the factors affecting the behaviour of reinforcement geogrids embedded in granular compacted soils, a large-scale pullout test apparatus has been designed. More than 40 pullout tests have been performed, at constant displacement rate, on three different HDPE extruded geogrids embedded in a compacted granular soil by varying the specimen lengths and the applied vertical effective pressures. The different geogrids used in the research have been tested using unconfined tensile tests performed at different speeds, and, in particular, at the same speed of the pullout tests; granular soil have been characterized through classification, Proctor and shear tests. The pullout test results showed the influence of the different parameters studied on pullout behaviour. Moreover, on the basis of the test results it was possible to evaluate the peak and the residual pullout resistance and the apparent coefficient of friction mobilized in the same conditions.

Experimental evaluation of biological samples impact response

The optimal method for rotator cuff repair of the shoulder is not yet known. The aim of this study was to compare the time-dependent biomechanical properties of the traditional open transosseous suture technique and modified Mason-Allen stitches (group 1) versus the double-loaded suture anchors technique and so-called arthroscopic Mason-Allen stitches (group 2) in rotator cuff repair.Eighteen adult female sheep were randomized into two groups: in an open approach in which the released infraspinatus tendon was repaired with group 1, and with group 2. Animals were sacrificed at 6, 12, or 26 weeks; shoulders were harvested and magnetic resonance imaging was performed. Eight untreated contralateral shoulders served as controls. Tendons of 16 additional unpaired cadaver shoulder joints of adult female sheep were identically treated for analysis at time zero. In a biomechanical evaluation all specimens were loaded to failure at a constant displacement rate using a standard universal testing machine. The load-to-failure and stiffness of the healed bone-tendon interface were calculated.Magnetic resonance imaging analysis showed cuff integrity in all cases, and no evidence of foreign body reaction to the anchors. Load-to-failure and stiffness data did not indicate any significant difference between the two treatment groups, neither at 6 weeks nor at 12 or 26 weeks. However, at time zero the group 2 had a higher load-to-failure in comparison to the group 1 (P < 0.010), but there was no difference for the stiffness (P < 0.121).This in vivo study showed that, postoperatively, the group 2 technique provides superior stability and after healing would gain strength comparable to the group 1 technique.

Modelling constant displacement rate experiments of asphalt concrete using a thermodynamic framework

This study is concerned with the constitutive modeling of asphalt concrete mixtures. The response of the asphalt concrete pavement depends on its internal structure. The internal structure of the asphalt concrete mixture evolves during the loading process. Here, we develop a single constituent model for asphalt concrete mixture by associating different natural configurations (stress-free configurations) with distinct internal structures of the body. The evolution of the natural configurations is determined using a thermodynamic criterion, namely the maximization of the rate of dissipation. Making appropriate assumptions concerning the manner in which the body stores and dissipates energy, the constitutive relations for the stress is deduced. Constant displacement rate experiments are carried out at different confinement pressures on asphalt concrete specimens made of two different aggregates—granite and limestone. The efficacy of the model in predicting the mechanical response of asphalt concrete mixtures is shown by corroborating the model predictions with the experimental results.

Strain localisation in bimineralic rocks: Experimental deformation of synthetic calcite–anhydrite aggregates

Abstract Deformation of synthetic calcite–anhydrite aggregates to large shear strains (up to γ = 12.4 at 600 °C, 300 MPa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10 − 3 s − 1 ) resulted in the first experimental observation of strain localisation from initially homogeneous rocks. In contrast to experiments on pure calcite and anhydrite, which deformed homogeneously to large strains ( γ ≥ 5), all experiments on calcite–anhydrite mixtures resulted in heterogeneous deformation at γ > 1 and the formation of narrow localised bands in the microstructures at γ > 4. In these bands, the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align, thereby forming microstructural layering similar to planar fabrics in natural mylonites. A switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and/or grain boundary sliding occurs simultaneously with strain localisation. It is concluded that deformation-induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite–anhydrite mixtures. The study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly-mineralic mylonites.

Laterally Loaded Pile Study in Permafrost of Northern Québec, Canada

This paper describes a study undertaken at Umiujaq, in northern Quebec, on laterally loaded piles in permafrost subject to a constant displacement rate. Test results of bending moments, shear load, soil reaction, and pile displacements demonstrate the complex nature of creep of frozen soils. FEM simulations show a good concurrence with field test results and confirm a clear distinction between stationary and nonstationary creep.

Numerical modelling of lithosphere evolution in the North China Block: Thermal versus tectonic thinning

In an attempt to understand the dynamics of lithospheric thinning in the North China Block (NCB), two-dimensional finitedifference numerical models have been constructed to simulate the lithosphere thinning process in the NCB. The models consider the theory of conductive thermal transfer on earth and explore how lithospheric thickness will vary in response to the changes in the thermal conditions of the lithosphere. As a process independent of thermal thinning, the effect of mechanical tectonic extension on lithospheric thickness has been explored by incorporating a constant extensional displacement rate at the lateral edges of the model. The results of a model involving only mechanical extension show that the thinning of the lithosphere is still quite limited to not more than 50 km even under a 25% extension. In contrast, the results of the thermal model quantitatively demonstrate that for a given thermal conductive rate and radioactive heating condition, the increase of the mantle thermal flux at the base of the lithosphere can trigger a great deal of changes in both the thermal state and the lithospheric thickness. The lithospheric thickness in the NCB could be thinned from an original thickness of 200 km to the new thickness of <100 km, when the mantle thermal flux was increased to 35–40 mWm −2 . Therefore, thermal perturbation seems to be the predominant mechanism responsible for the significant lithosphere thinning in the NCB. The effects of mechanical tectonic extension may become more profound on lithosphere thinning at a later stage when the lithosphere had already been heterogeneously thinned. A temporary and genetic link between the NCB lithospheric thinning and the Cretaceous global event is put forward to interpret the lithospheric evolution throughout the eastern China in late Mesozoic.