Biaxial Apparatus Research Articles

Liquefaction behaviour of aluminium and plastic rod assemblies using bi-axial apparatus with application of image analysis

It has recently been shown that the overall deformation characteristics of granular materials during liquefaction seems to be governed by local void ratio characteristics. The aim of this study is to investigate the relationship between mechanical properties and local void ratio changes during liquefaction in two materials of different weights (aluminium-type and plastic-type rod assemblies). A series of constant-volume cyclic bi-axial loading tests was conducted together with the image analysis obtaining local void behaviour. The experimental results demonstrate that the first liquefaction resistance of the plastic sample is higher than the aluminium one, corresponding to its smaller initial void ratio and less contractive behaviour observed in the early stage of drained monotonic compression. However, after experiencing the first liquefaction and subsequent re-consolidation histories, a liquefiable assembly of aluminium and plastic rods both became either denser (densification) or more homogenous (homogenization) in terms of local void ratio, suggesting an increase in liquefaction resistance in the next liquefaction stage. This work also includes a preliminary analogous study using a semi-3D discrete element method (DEM) with one single layer of spherical particles to simulate the similar liquefaction behaviour observed in the laboratory tests.

Investigating Dynamic Weakening in Laboratory Faults Using Multi‐Scale Flash Heating Coupled With mm‐Scale Contact Evolution

AbstractFlash‐weakening models typically show good agreement with the total magnitude of weakening in high‐speed rock friction experiments, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, remain unresolved. Here, we incorporate inhomogeneous mm‐scale normal stress evolution into a model for flash heating and weakening to resolve outstanding transient and hysteretic friction observed in laboratory experiments and to identify unique solutions to constitutive parameters. We conduced 37 rock friction experiments on Westerly granite using a high‐speed biaxial apparatus outfitted with a high‐speed infrared camera. We initiated velocity steps from quasi‐static rates of 1 mm/s to sliding velocities ranging from 300 to 900 mm/s and conducted both constant‐ and decreasing‐velocity tests following the velocity step. Two sliding surfaces geometries were used to control mm‐scale life‐times and rest‐times. Constant‐strength sliding is achieved within 2–3 mm of initiating the velocity step in all constant‐velocity experiments. Macroscopic surface temperature is inhomogeneous and increases with slip distance, velocity, and decreasing rest‐time. Weakening increases with sliding velocity and decreasing rest‐time. We combine thermal models with measured surface temperatures to constrain the evolution of local normal stress at the mm‐scale and incorporate this evolution into a flash‐weakening model that considers weakening at both the µm‐ and mm‐scale. The flash‐weakening model improves when the effects of mm‐scale wear processes are incorporated and multi‐scale weakening is considered, however some transient friction remains undescribed. Models will be advanced by further incorporating wear processes and by considering processes at the mm‐scale and above.

Varying Distance Between Suture-Endobuttons Does Not Improve Stability of the Transected Syndesmosis in a Motion Tracking Cadaveric Model

Category: Ankle; Sports Introduction/Purpose: A growing body of evidence favors the use of dynamic fixation techniques such as suture endo button over rigid screw fixation for syndesmotic injuries. However, specific dynamic fixation techniques have been poorly evaluated. The purpose of this study was to compare single suture-endobutton fixation (single) to double fixation with suture-endobuttons placed one centimeter (cm) apart (double 1 cm) and double fixation with suture-endobuttons placed three cm apart (double 3 cm). Methods: Twenty-four lower leg specimens from twelve cadavers were tested using a custom biaxial apparatus. Internal rotation, external rotation, and anterior and posterior translation of the tibia and fibula were examined using motion tracking systems after transection of deltoid and syndesmotic ligaments to compare three syndesmotic repair techniques: 1) a single suture-endobutton placed 1 cm above the joint line, 2) two suture suture-endobuttons placed 2 and 3 cm above the joint line, and 3) two suture- endobuttons placed 1 and 4 cm above the joint line. Results: All three endobutton constructs improved the stability of the syndesmosis by reducing aberrant motion after transection. There was no clear superiority of either single suture-endobutton (SEB), double suture-endobuttons placed 2 and 3 cm above the joint line (DSEB 1), or double suture-endobuttons placed 1 and 4 cm above the joint line (DSEB 3). DSEB 3 and DSEB 1 showed equal median laxity for tibia internal rotation (2.7 degrees, DSEB 1 (2.0 – 3.2) degrees, DSEB 3 (2.3 – 2.8) degrees), tibia external rotation (-0.2 degrees, DSEB 1 (-0.4 - 0.4) degrees, DSEB 3 (-0.4 - 0.3) degrees), and fibula external rotation (-0.3 degrees, DSEB 1 (-0.7 - 0.3) degrees, DSEB 3 (-0.7 - 0.7) degrees). Conclusion: Cadaveric biomechanical studies are critical to evaluate various suture-endobutton constructs prior to widespread use in patients. Our results demonstrate that the added cost of a second suture-endobutton may not confer substantial additional stability to the syndesmosis. Likewise, increased spread between suture-endobuttons appears to have minimal impact of syndesmotic stability.

Development of a biaxial apparatus for jamming profiles of photoelastic granular media.

We describe a two-dimensional biaxial apparatus that is used to conduct the experimental study of the jamming of granular media. The setup is designed based on the photoelastic imaging technique, which allows us to detect force-bearing contacts among particles, calculate the pressure on each particle according to the mean squared intensity gradient method, and compute contact forces on each particle [T. S. Majmudar and R. P. Behringer, Nature 435, 1079-1082 (2005)]. Particles float in a density-matched solution to avoid basal friction during experiments. We can compress (uniaxially or biaxially) or shear the granular system by an entangled comb geometry by moving the paired boundary walls independently. A novel design for the corner of each pair of perpendicular walls is described, which allows for independent motion. We control the system using a Raspberry Pi with Python code. Three typical experiments are described briefly. Furthermore, more complicated experiment protocols can be implemented to achieve specific granular materials research goals.

Surviving in Ocean Worlds: Experimental Characterization of Fiber Optic Tethers across Europa-like Ice Faults and Unraveling the Sliding Behavior of Ice

As an initial step toward in situ exploration of the interiors of Ocean Worlds to search for life using cryobot architectures, we test how various communication tethers behave under potential Europa-like stress conditions. By freezing two types of pretensioned insulated fiber optic cables inside ice blocks, we simulate tethers being refrozen in a probe’s wake as it traverses through an Ocean World’s ice shell. Using a cryogenic biaxial apparatus, we simulate shear motion on preexisting faults at various velocities and temperatures. These shear tests are used to evaluate the mechanical behavior of ice, characterize the behavior of communication tethers, and explore their limitations for deployment by a melt probe. We determine (a) the maximum shear stress tethers can sustain from an ice fault, prior to failure (viable/unviable regimes for deployment), and (b) optical tether performance for communications. We find that these tethers are fairly robust across a range of temperature and velocity conditions expected on Europa (T = 95–260 K, velocity = 5 × 10−7 m s−1 to 3 × 10−4 m s−1). However, damage to the outer jackets of the tethers and stretching of inner fibers at the coldest temperatures tested both indicate a need for further tether prototype development. Overall, these studies constrain the behavior of optical tethers for use at Ocean Worlds, improve the ability to probe thermomechanical properties of dynamic ice shells likely to be encountered by landed missions, and guide future technology development for accessing the interiors of (potentially habitable ± inhabited) Ocean Worlds.

Development and Experimental Verification of a High-Temperature and In-Plane Biaxial Testing Apparatus

Given the lack of primary data on heat-resistant composites under high-temperature conditions, the focus of this paper is the development of an in-plane biaxial apparatus under high temperatures and complex loads. Besides loading complex loads up to 80 kN, the apparatus can load under high-temperatures up to 2500 °C. A C/C tensile/compression test at 1700 °C illustrates the successful use of high-temperature digital speckle pattern technology to evaluate the in-plane mechanical properties of heat-resistant composites at 1700 °C under biaxial stress. A high-temperature impact test of a graphite specimen at 2500 °C shows that this apparatus can load at a high temperature in a vacuum and inert gas atmosphere. The yield characteristics of the Q235 steel sheet under in-plane stress show that the apparatus can conduct various mechanical loads, including tension–tension, tension–compression, and compression–compression loads. The proposed equipment can measure the in-plane mechanical properties of composite materials, particularly heat-resistant composites. The obtained results can be applied to structural design, life prediction, and reliability evaluation, as well as for the development, research, and design of aerospace instruments and critical materials.

The Influence of Roughness on Experimental Fault Mechanical Behavior and Associated Microseismicity.

Fault surfaces are rough at all scales, and this significantly affects fault‐slip behavior. However, roughness is only occasionally considered experimentally and then often in experiments imposing a low‐slip velocity, corresponding to the initiation stage of the earthquake cycle. Here, the effect of roughness on earthquake nucleation up to runaway slip is investigated through a series of dry load‐stepping biaxial experiments performed on bare rock surfaces with a variety of roughnesses. These laboratory faults reached slip velocities of at least 100 mm/s. Acoustic emissions were located during deformation on bare rock surfaces in a biaxial apparatus during load‐stepping experiments for the first time. Smooth surfaces showed more frequent slip instabilities accompanied by slip bursts and larger stress drops than rough faults. Smooth surfaces reached higher slip velocities and were less inclined to display velocity‐strengthening behavior. The recorded and localized acoustic emissions were characterized by a greater proportion of large‐magnitude events, and therefore likely a higher Gutenberg‐Richter b GR‐value, for smoother samples, while the cumulative seismic moment was similar for all roughnesses. These experiments shed light on how local microscopic heterogeneity associated with surface topography can influence the macroscopic stability of frictional interfaces and the associated microseismicity. They further provide a laboratory demonstration of roughness' ability to induce stress barriers, which can halt rupture, a phenomenon previously shown numerically.

A new biaxial apparatus for tensile tests on Poly Ethylene Terephthalate optimized specimen at stretch blow molding conditions

Characterization of polymer behavior over or near the glass transition temperature in condition close to industrial process remains a scientific goal. In particular being able to manage multiaxial loading at high strain rate and large strains is particularly interesting to understand Poly Ethylene Terephthalate (PET for short) behavior at condition near to the blowing process. In this paper, we present a new vertical biaxial tensile machine that has been designed and built up in our laboratory. In particular, (i) thermal regulation enables a fast and homogeneous heating of the specimen at temperature close to Tg, (ii) strain rate up to 5s−1 can be reached and (iii) a cruciform specimen geometry has been optimized to ensure large biaxial strain up to 2 or more in each direction. These two last are lower but close to the strain rate and elongation observed in stretch blow molding. The digital image correlation technique is used to obtain the displacement field and a thermal camera gives the evolution of the temperature field during the test. Dimension and height enables this apparatus to be transportable for mechanical tests under SAXS or WAXS. Here we present results of tests managed on PET and the identification procedure that is carried out to fit parameters of our orthotropic thermo-visco-hyperelastic model. Results are presented, discussed and compared to previous measures obtained from apparatus that are more complex.

Drainage Instabilities in Granular Materials: A New Biaxial Apparatus for Fluid Fingering and Solid Remodeling Detection

In this study, the effect of fluid fingering on the solid remodeling of a granular material during the drainage phenomenon is investigated. A new biaxial apparatus, endowed with two transparent windows and adapted to unsaturated soils, is used to capture the effects of hydraulic instabilities on the mechanical response, by means of high-resolution cameras. A specimen of (40 × 50 × 11) mm3, of Fontainebleau sand NE 34 initially saturated by water, is connected to a pressure-controlled gas source to inject the gas into the sample. During the injection phase, fluid instabilities are detected and filmed. Using imaging techniques, the grain remodeling and strain localization due to the two-phase fluid flow are measured.

The role of shale content and pore-water saturation on frictional properties of simulated carbonate faults

The presence of weak phyllosilicates in mature carbonate fault zones has been invoked to explain weak faults. However, the relation between frictional strength, fault stability, mineralogical composition, and fabric of fault gouge, composed of strong and weak minerals, is poorly constrained. We used a biaxial apparatus to systematically shear different mixtures of shale (68% clay, 23% quartz and 4% plagioclase) and calcite, as powdered gouge, at room temperature, under constant normal stresses of 30, 50, 100 MPa and under room-dry and pore fluid-saturated conditions, i.e. CaCO3-equilibrated water. We performed 30 friction experiments during which velocity-stepping and slide-hold-slide tests were employed to assess frictional stability and to measure frictional healing, respectively. Our frictional data indicate that the mineralogical composition of fault gouges significantly affects frictional strength, stability, and healing as well as the presence of CaCO3-equilibrated water. Under room-dry condition, the increasing shale content determines a reduction in frictional strength, from μ = 0.71 to μ = 0.43, a lowering of the healing rates and a transition from velocity-weakening to velocity-strengthening behavior. Under wet condition, with increasing shale content we observe a more significant reduction in frictional strength (μ = 0.65–0.37), a near-zero healing and a velocity strengthening behavior. Microstructural investigations evidence a transition from localized deformation promoted by grain size reduction, in calcite-rich samples, to a more distributed deformation with frictional sliding along clay-enriched shear planes in samples with shale content greater than 50%. For faults cutting across sedimentary sequences composed of carbonates and clay-rich sediments, our results suggest that clay concentration and its ability to form foliated and interconnected networks promotes important heterogeneities in fault strength and slip behavior.

A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions.

The hot stamping and cold die quenching process is increasingly used to form complex shaped structural components of sheet metals. Conventional experimental approaches, such as out-of-plane and in-plane tests, are not applicable to the determination of forming limits when heating and rapid cooling processes are introduced prior to forming for tests conducted under hot stamping conditions. A novel in-plane biaxial testing system was designed and used for the determination of forming limits of sheet metals at various strain paths, temperatures, and strain rates after heating and cooling processes in a resistance heating uniaxial testing machine. The core part of the biaxial testing system is a biaxial apparatus, which transfers a uniaxial force provided by the uniaxial testing machine to a biaxial force. One type of cruciform specimen was designed and verified for the formability test of aluminum alloy 6082 using the proposed biaxial testing system. The digital image correlation (DIC) system with a high-speed camera was used for taking strain measurements of a specimen during a deformation. The aim of proposing this biaxial testing system is to enable the forming limits of an alloy to be determined at various temperatures and strain rates under hot stamping conditions.

Comparison of Femoral Head Rotation and Varus Collapse Between a Single Lag Screw and Integrated Dual Screw Intertrochanteric Hip Fracture Fixation Device Using a Cadaveric Hemi-Pelvis Biomechanical Model.

This study compared the stabilizing effect of 2 intertrochanteric (IT) fracture fixation devices in a cadaveric hemi-pelvis biomechanical model. Eleven pairs of cadaveric osteopenic female hemi-pelves with intact hip joint and capsular ligaments were used. An unstable IT fracture (OTA 31-A2) was created in each specimen and stabilized with a single lag screw device (Gamma 3) or an integrated dual screw (IDS) device (InterTAN). The hemi-pelves were inverted, coupled to a biaxial apparatus and subjected to 13.5 k cycles of loading (3 months) using controlled, oscillating pelvic rotation (0-90 degrees) plus cyclic axial femoral loading at a 2:1 body weight (BW) ratio. Femoral head rotation and varus collapse were monitored optoelectonically. For specimens surviving 3 months of loading, additional loading was performed in 0.25 × BW/250 cycle increments to a maximum of 4 × BW or failure. Femoral head rotation with IDS fixation was significantly less than the single lag screw construct after 3 months of simulated loading (P = 0.016). Maximum femoral head rotation at the end of 4 × BW loading was 7× less for the IDS construct (P = 0.006). Varus collapse was significantly less with the IDS construct over the entire loading cycle (P = 0.021). In this worst-case model of an osteopenic, unstable, IT fracture, the IDS construct, likely owing to its larger surface area, noncylindrical profile, and fracture compression, provided significantly greater stability and resistance to femoral head rotation and varus collapse.

Frictional behavior of talc‐calcite mixtures

AbstractFaults involving phyllosilicates appear weak when compared to the laboratory‐derived strength of most crustal rocks. Among phyllosilicates, talc, with very low friction, is one of the weakest minerals involved in various tectonic settings. As the presence of talc has been recently documented in carbonate faults, we performed laboratory friction experiments to better constrain how various amounts of talc could alter these fault's frictional properties. We used a biaxial apparatus to systematically shear different mixtures of talc and calcite as powdered gouge at room temperature, normal stresses up to 50 MPa and under different pore fluid saturated conditions, i.e., CaCO3‐equilibrated water and silicone oil. We performed slide‐hold‐slide tests, 1–3000 s, to measure the amount of frictional healing and velocity‐stepping tests, 0.1–1000 µm/s, to evaluate frictional stability. We then analyzed microstructures developed during our experiments. Our results show that with the addition of 20% talc the calcite gouge undergoes a 70% reduction in steady state frictional strength, a complete reduction of frictional healing and a transition from velocity‐weakening to velocity‐strengthening behavior. Microstructural analysis shows that with increasing talc content, deformation mechanisms evolve from distributed cataclastic flow of the granular calcite to localized sliding along talc‐rich shear planes, resulting in a fully interconnected network of talc lamellae from 20% talc onward. Our observations indicate that in faults where talc and calcite are present, a low concentration of talc is enough to strongly modify the gouge's frictional properties and specifically to weaken the fault, reduce its ability to sustain future stress drops, and stabilize slip.

A Robust Calibration Technique for Acoustic Emission Systems Based on Momentum Transfer from a Ball Drop

We describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events. Unlike previous calibration tech- niques, it does not require modeling of the wave propagation, sensor response, or signal conditioning. Rather, this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical Green's function. To correctly apply this technique, we develop mathematical expressions that link the seismic mo- ment M0 of internal seismic sources (i.e., earthquakes and acoustic emissions) to the impulse, or change in momentum Δp, of externally applied seismic sources (i.e., me- teor impacts or, in this case, ball impact). We find that, at low frequencies, moment and impulse are linked by a constant, which we call the force-moment-rate scale factor C F _ MM0=Δp. This constant is equal to twice the speed of sound in the material from which the seismic sources were generated. Next, we demonstrate the calibration technique on two different experimental rock mechanics facilities. The first example is a saw-cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 MPa confining pressure. The second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels. Using the empirical cali- bration technique, we are able to determine absolute source parameters including the seismic moment, corner frequency, stress drop, and radiated energy of these magnitude −2:5 to −7 seismic events.

A New Method for Testing Formability in Sheet Metal Forming at Biaxial Tensile State

This paper presents a new method concerning testing formability in sheet metal forming, especially focuses on clarifying the divergence of the experiment and a variety of theoretical predictions on biaxial tensile state. Up to now, there are many different fracture criteria appeared. All researches have presented their experimental data which could justify the criterion they presented. However, the experimental results and predictions in the first quadrant of the forming limit diagram (FLD) often diverge. Today, limiting dome height test is commonly used for FLD experiment, but specimens are rubbed and bended during the test, both influencing the experimental results.In order to provide for convincible experimental data, this paper presents a new experimental method to establish the first quadrant of FLD. In this method, cruciform biaxial tensile specimen and biaxial tensile apparatus have been developed. The proposed specimen has the feature of thickness reduction and contour design to ensure the fracture location is in the central region, so that accurate biaxial tensile state can be obtained. Through this method, there is an opportunity to obtain the whole FLD using uniaxial tensile testing machine, which is a low-cost alternative in compared with limiting dome height test. Besides, the experimental results can be utilized to clarify the divergence between various theoretical predictions and experimental results in the first quadrant of the FLD.

A biaxial apparatus for the study of heterogeneous and intermittent strains in granular materials

We present an experimental apparatus specifically designed to investigate the precursors of failure in granular materials. A sample of granular material is placed between a latex membrane and a glass plate. A confining effective pressure is applied by applying vacuum to the sample. Displacement-controlled compression is applied in the vertical direction, while the specimen deforms in plane strain. A diffusing wave spectroscopy visualization setup gives access to the measurement of deformations near the glass plate. After describing the different parts of this experimental setup, we present a demonstration experiment where extremely small (of order \(10^{-5}\)) heterogeneous strains are measured during the loading process.

Conception and Construction of Biaxial Apparatus for Identification of Axial Orthotropic Material

Plastbloc is a new process. Its principle is to use compressed and ligatured blocks made of plastic materials. These blocks could be used to build lightweight embankments, to minimize loads on underground pipes and behind retaining walls. Because the compressing during the making of blocks is unidirectional, this material is made up of stake of layers. In this case, it has an axial orthotropy. With the purpose to study the rheological behavior of this material, a large number of simple compression tests were carried out. But the identification of all parameters is possible by realization of biaxial tests. A biaxial apparatus conceived specially for this type of material was built. Preliminary studies showed the good function of this apparatus. Then, biaxial tests were carried out and the characteristic functions were identified.

Impact Behavior of Biaxially Compressed Wood

The wood bar with square cross-section was biaxially compressed with respect to the cross-sectional plane and the impact 4-point bending test was performed using the compressed wood. The wood material used was sap wood of Sugi, which is a kind of Japanese cedar. The biaxial compression apparatus which enabled to prevent the gap generation between the material and the tool was used. The fixation of wood specimen was also processed. The bar length was 100 mm and the edge length in cross-section was 15 ~ 25 mm. In the 4-point bending test, the distance between the supports and the stress points were 80 mm and 40 mm. The stress points were impacted by a light-weight drop-hammer with 3.0 kg mass. The impact velocity was 2 m/s. The surface strain was measured by a strain gage. When the Young’s modulus was estimated by assuming a linear elastic property, it attained about 32 GPa, which was more than 4 times of the uncompressed wood. The stress – strain hysteresis loop was more remarkable under the impact condition. It was found that the compressed wood provided not only a high rigidity, but also a good property in dissipation of vibration energy.

Anisotropic visco-hypoplasticity

Apart from time-driven creep or relaxation, most viscoplastic models (without plastic and viscous strain separation) generate no or a very limited accumulation of strain or stress due to cyclic loading. Such pseudo-relaxation (or pseudo-creep) is either absent or dwindles too fast with increasing OCR. For example, the accumulation of the pore water pressure and eventual liquefaction due to cyclic loading cannot be adequately reproduced. The proposed combination of a viscous model and a hypoplastic model can circumvent this problem. The novel visco-hypoplasticity model presented in the paper is based on an anisotropic preconsolidation surface. It can distinguish between the undrained strength upon triaxial vertical loading and horizontal loading. The strain-induced anisotropy is described using a second-order structure tensor. The implicit time integration with the consistent Jacobian matrix is presented. For the tensorial manipulation including numerous Frechet derivatives, a special package has been developed within the algebra program MATHEMATICA (registered trade mark of Wolfram Research Inc.). The results can be conveniently coded using a special FORTRAN 90 module for tensorial operations. Simulations of element tests from biaxial apparatus and FE calculations are also shown.

Effects of shear velocity oscillations on stick‐slip behavior in laboratory experiments

We report on laboratory friction experiments in which simulated faults are exposed to shear velocity oscillations at different amplitudes and frequencies. Granular layers are sheared in a servo‐controlled biaxial apparatus at constant normal stress and background shear velocity, with a shear velocity sinusoid superimposed. Correlation between oscillations and stick‐slip events is determined by the timing of dynamic failure with respect to the oscillation phase. Schuster's test is used to calculate the statistical likelihood of phase recurrence. We find that correlation of failure with the shear stress oscillation depends on oscillation frequency and amplitude at low frequencies and solely on oscillation amplitude at high frequencies. The frequency boundary between these two regimes is proportional to the inverse of the time needed to displace the frictional critical slip distance. We evaluate changes in failure characteristics, including failure strength, recurrence time between events, creep, and phase of the oscillation at failure, to assess the effects of stressing rate oscillations. Failure occurs at maximum shear stressing rate at low frequencies and lags peak stressing rate in the high‐frequency regime. Friction at the onset of dynamic failure decreases with increasing frequency. The distribution of events through time depends on the frequency of the shear oscillation; low‐frequency oscillations produce bimodal distributions and high‐frequency oscillations produce unimodal distributions. If the transition between the failure regimes depends on the critical displacement length as our experiments imply, the critical frequency will vary for faults with different gouge layer thicknesses and total displacement.