Controlled Loading Rates Research Articles

Experimental study on microstructure characteristics of saturated remolded cohesive soil during consolidation

In this paper, drained consolidation tests of saturated remolded cohesive soil were carried out at different loading rates, and the samples at different loading stages were measured by nuclear magnetic resonance (NMR) testing. Through qualitative analysis of the relationship between the transverse relaxation time T2, peak area and porosity, the deformation micro-response of the saturated remolded cohesive soil was studied. The results showed that the transverse relaxation time T2 of the saturated remolded cohesive soil samples during the initial stage consisted of two spectral peaks, representing pores with large and small pore diameters, respectively. As loading progressed, the pore diameters in each sample group gradually shifted to smaller sizes, and the final form of the T2 spectrum was unimodal, in which the pores became denser and more uniform. In the initial stage of loading, the T2 spectrum of the soil sample with faster loading showed no obvious change trend due to the influence of drainage lag. After a sufficiently long dead load time, the final shape of the T2 spectrum was very similar regardless of loading rate, indicating that the loading rate only affected the intermediate state of the soil sample, and the final state was determined by the initial state of the sample. At the same time, after a certain dead load time, the changes in pore diameter were no longer obvious, indicating that the flowing fluid in the pore was drained and that drainage consolidation was complete. According to the positive correlation between the loading rate and maximum pore pressure, a boundary rate was obtained. We could reasonably infer that if the controlled loading rate was less than this value, drainage consolidation was complete in the main consolidation stage.

Effect of temperature variation on the behaviour of Bushveld rocks: comparison of laboratory test and numerical modelling results

ABSTRACT This paper presents the result of the research carried out on the effect of temperature on the Bushveld Igneous Complex (BIC), South Africa. The BIC has high virgin rock temperature. If the platinum mines advance to ultra-deep levels, then the virgin rock temperature at a depth of 5 km will be approximately 140°C. Hence, there is a need to improve the understanding of the behaviour of rock under high temperatures. Five rock types were tested in a 2600 kN servo-controlled testing machine. The testing was done at a controlled loading rate and a confining pressure 0, 10, and 30 MPa within temperature range of 20–140°C. Comparison was made between the results obtained from the laboratory experiments and numerical modelling. It was observed from the stress–strain behaviour of the samples that an increase in temperature results in the reduction of the Young’s modulus and peak strength.

Prepeak energy evolution properties of sandstone subject to diametrical splitting with different loading rates

To understand the energy evolution properties during rock splitting under different loading rates, four groups of Brazilian test with a loading-unloading cycle corresponding to four orders of magnitude of controlled loading rate (10−1, 100, 101, 102 kN/min) were conducted on red sandstone using the MTS 322 testing system. The energy characteristics were interpreted by mathematically integrating the force-displacement responses of the tested rock specimens. It was found that at each loading rate, the input energy was a linear function as the elastic energy and dissipated energy, while these three types of energy nonlinearly grew with the unloading level. Importantly, the loading rate was found to exert an insignificant influence on the evolution of the linear energy properties (referring to the linear functions between the three types of energy); the energy storage coefficient (ESC) and/or energy dissipation coefficient (EDC) remained constant under various loading rates, exhibiting an approximate independency on loading rate. Based on this, a unified formula for describing the rate-independence of linear energy properties was presented in the tested loading rate range. It is also noted that rock mechanical parameters such as the peak force and peak displacement and the ESC and EDC are closely comparable versus the increasing loading rate, and hence the energy parameters are likely to be used to characterize the rock mechanical ones. Finally, the relevancy between energy allocation and rock failure patterns was discussed.

Effect of loading rate on flexural behavior of concrete and reinforced concrete beams

The elasto-plastic characteristics of plain concrete are inevitably affected by the loading rate. This paper presents an experimental investigation on the effect of loading rate on flexural behavior of concrete and reinforced concrete (RC) beams, which was carried out with Walter+bai electro-hydraulic servo system. Three-point bending tests on 100 × 100 × 400 mm prismatic concrete samples and 80 × 120 × 1100 mm RC beams with different displacement controlled loading rates of 0.01 mm/min, 0.1 mm/min, and 3 mm/min were imposed. Based on the test results, the effects of loading rates on the load-displacement curve, cracking, and ultimate load-carrying capacities of RC beams were evaluated.

Extended Applications of the Depth-Sensing Indentation Method.

The depth-sensing indentation method has been applied for almost 30 years. In this review, a survey of several extended applications developed during the last three decades is provided. In depth-sensing indentation measurements, the load and penetration depth data are detected as a function of time, in most cases at controlled loading rates. Therefore, beside the determination of hardness and Young’s modulus, different deformation mechanisms and many other dynamic characteristics and phenomena, such as the dynamic elastic modulus, load-induced phase transition, strain rate sensitivity, etc. can be studied. These extended applications of depth-sensing indentation measurements are briefly described and reviewed.

Inferring material parameters from imprecise experiments on soft materials by virtual fields method

Inferring material parameters of soft materials, especially soft tissues from experiments, has always been a challenge because of the inaccuracy of specimen geometry, difficulty in proper gripping and difficulty in obtaining reliable displacement data. If one is able to obtain full field data with Digital Image Correlation or other similar techniques, we show that in spite of poor quality experimental data, it is still possible to get the material parameters using Virtual Fields Method with carefully chosen virtual fields. We demonstrate the approach by two cases. The first is Ecoflex (silicone rubber) under biaxial deformations at controlled loading rates, and the second is rat skin samples with imperfect shape under biaxial deformations at controlled loading rates. In both cases, rather than trying to obtain homogeneous deformation conditions, we use a technique based on the Virtual Fields Method to extract the material properties from the deformation of the specimen collected by Digital Image Correlation (DIC) and the force load measured by the load sensor. In order to apply loads in two principal directions simultaneously, a custom biaxial set-up was built and mounted into a uniaxial Instron tensile set-up. We show here that in spite of significant inhomogeneity in the deformation, errors, and missing data in the measured displacement field, we are still able to recover the material properties of the soft solid by suitable choices of virtual fields.

Temporal Characteristics of Failure in High-Speed Tests

The unstable behavior of generally accepted strength characteristics of a material makes it difficult to evaluate the dynamic strength of a material in high-speed loading problems. This work reveals a new effect, which in a number of practically important cases of tests with a controlled loading rate for evaluating the dynamic strength allows us to offer a convenient time characteristic of the incubation processes of failure under high speed loads. It is shown that this time characteristic does not depend on the strain rate or loading rate and can be determined from general experiments. This makes it possible to enter a material parameter that can be used for criteria for evaluating the strength of a material under high-speed loads, including for calculating the rate and time dependence of critical failure stresses.

Impact of Body Mass Index on Biomechanics of Recreational Runners.

Some recreational runners with obesity successfully train or compete without musculoskeletal injury. Insight into the key kinetic strategies of injury-free heavier runners is necessary to appropriately guide development of safe training programs for this population. To determine key biomechanical strategies of running in individuals with body mass index (BMI) values above and equal to and higher than 30 kg/m2 . This was a case-control study. Runners with obesity (n =18; 42.7 years, 38.9% women) who were matched by sex, age, footstrike type, footwear characteristics, and running speed with healthy runners (n = 36; 41.7 years, 32.5% women). Research laboratory affiliated with an academic medical center. A seven-camera optical motion analysis system was used to capture running kinematics and an instrumented treadmill captured kinetic data. Main outcomes were temporal spatial parameters, joint excursions, peak ground reaction forces (GRFs), joint moments, vertical average loading rate (VALR), impulses, and vertical stiffness (Kvert ). Runners with obesity demonstrated 15% less vertical excursion of the center of mass, 18% wider strides, and 3% longer stance times than nonobese runners (P < .05). Normalized peak GRFs and VALRs were higher in the nonobese group. GRF impulse was higher in the group with obesity compared to the nonobese group (means ± SD; 339.6 ± 55.2 Ns vs. 255.0 ± 45.8 Ns; P = .0001). Kvert was higher in the obese group compared to the nonobese group (238.6 ± 50.3 N/cm vs. 183.1 ± 29.4 N/cm; P = .0001). Peak hip moments were higher in runners with obesity in the sagittal and frontal planes (P < .05). Runners with obesity dampened impact forces and controlled loading rate more than nonobese runners by increasing lower body stiffness and constraining vertical displacement.

Piezoelectric Stack Actuator for Measurement of Interfacial Shear Strength at High Strain Rates

Limited methods exist for the characterization of interfacial properties in fiber reinforced composites under dynamic loading conditions since conventional techniques are designed for the measurement of a composite’s bulk properties rather than that of the fiber-matrix interface. Here, a novel experimental method is reported for the measurement of interfacial strength across various loading rates using a single fiber. The experimental apparatus uses a single fiber pullout approach along with a piezoelectric stack actuator to enable a single fixture to generate a wide range of controlled loading rates. The voltage-strain relationship is measured and the applied strain rate is controlled using resistors with varying magnitudes within a properly designed discharge circuit. The piezoelectric actuator is protected against backflow current surges using a silicon controlled rectifier, and against parasitic inductance through a toggle switch. It is shown that the proposed method accommodates single fiber specimens while providing similar performance to a Split-Hopkinson bar by reaching strain rates of up to 104 s−1, however with greater flexibility of loading rate. Single aramid fibers are tested at various strain rates and show expected interfacial shear strength independent of the embedded fiber length. The results indicate the potential of the proposed experimental method as a simple and fast technique to measure the fiber matrix interfacial properties through direct pullout at high strain rates.

Loading rate conditions and specimen size effect on strength and deformability of rock materials under uniaxial compression

In this study, various rock and cementitious rock-like material specimens with same ratio of length to diameter and different sizes were tested under various deformation controlled loading rate (mm/min) and load controlled rate (kN/s) conditions. According to the results obtained from 93 specimens of 5 different types of rock material and 2 different rock-like materials (cement paste and a concrete mix including sand) tested in this study, uniaxial compressive strength (UCS) values were found to significantly decrease with an increase in the specimen size under the condition of a load controlled rate (kN/s) selection. To practically remove the size effect on UCS values, the uniaxial compression test is suggested to perform selecting the loading rate as strain controlled (s−1) and proportional to diameters of specimens with different sizes and same geometry. In addition to the UCS values, Modulus of elasticity values, stress and strain graph shapes and deformation characteristics were found to significantly change with the change of the specimen size and loading rate. According to the results, both modulus of elasticity values and brittleness were found to notably increase as a result of increase in loading rate values.

Adhesion strength of die attach film for thin electronic package at elevated temperature

Adhesion strength of a thin film for electronic packaging was investigated. The effects of temperature and loading rate on the adhesion were observed considering the viscoelasticity of adhesive. Various temperature conditions over the glass transition temperature of the adhesive were applied with controlled loading rates. A small hot plate was specially designed to control the temperature. Loading rate was controlled by a servo motor. A cantilever specimen was fabricated by two rectangular silicon chips. The adhesion was measured by a modified single cantilever beam method. Bending force was applied to the cantilever using rotatable jig. Adhesion strength was found to strongly depend on the temperature and loading rate. Below the glass transition temperature (Tg), the adhesion strength was increased with increasing loading rate. Near the Tg, the adhesion strength was decreased with increasing loading rate. Above the Tg, the adhesion strength did not significantly depend on the loading rate.

Determination of Traction Separation Curve for Copper Using Essential Work of Fracture (EWF) Approach

The essential work of fracture concept has become very popular to characterize the plane stress toughness of ductile material. The essential work characterizing the resistance of a structure to ductile fracture is important for many applications, from structural design to impact protection. The main objective of the present work is to experimentally derive the traction separation curve also known as cohesive law for copper (UNS C11040) under plane stress condition. To this end a double edge notched tension (DENT) specimens of thickness 1.5 mm were prepared. The ligament of the specimens varied between 6 to 24 mm and each specimen was tested under displacement controlled loading rate of 0.5 mm/min using miniature tensile tenting machine with a load cell capacity of 20 KN. The load displacement data were recorded and the traction separation curve was derived using normalization process suggested by Cottrell et al., 2005. The area under the traction separation curve is believed to the specific essential work of fracture and this fact was further confirmed from the experiments.

Mechanical properties and strain fatigue lives of insulation polymers

Insulation polymers are not well characterized for their mechanical properties particularly in terms of fatigue strains. This article aims to examine the durability and strain fatigue lives of three commonly used cable insulation polymers, viz., (1) polyvinyl chloride (PVC), (2) crosslinked polyethylene (XLPE), and (3) polyphenylene ether (PPE) under selected strain and temperature ranges. The tensile properties of these materials were measured using an Instron testing machine at constant and controlled loading rates. Fatigue tests were performed at three selected temperatures, −40, 25, and 65 °C, to characterize the temperature effects on fatigue life. From the tensile test results, it was observed that PVC and XLPE are ductile and exhibit significantly more elongation prior to breaking, while PPE exhibits brittle behavior. When the loading rate is increased, there is an improvement in the tensile strength of PPE and elastic modulus of PPE and PVC. The durability of XLPE under strain fatigue testing was the largest, followed by PVC and PPE. The strain fatigue lives of PVC and XLPE decreased drastically at −40 °C and demonstrated a noted increase at 65 °C compared to fatigue lives at room temperature. This trend was not observed in PPE where the strain fatigue life showed improvement at both lower and higher temperatures.

Temperature Dependence of the Biotin−Avidin Bond-Rupture Force Studied by Atomic Force Microscopy

We report for the first time an atomic force microscopy (AFM) investigation of the dependence of unbinding force on temperature at controlled loading rate. AFM force measurements of biotin−avidin interactions were acquired at various temperatures ranging from 13 to 37 °C using force loading rates that are slow enough to ignore dissipative friction and to assume thermal equilibrium. With our established Poisson statistical analysis method, the unbinding force at a fixed loading rate between an individual biotin−avidin pair was extracted and was found to decrease by ∼5-fold with increasing temperature over this range. On the basis of a thermodynamic model similar to that of Bell, but developed here with additional insights, the bond stiffness, effective unbinding length, and the critical unbinding energy per biotin−avidin complex were estimated. This work takes quantitative AFM studies to the next level by allowing energies to be determined from force measurements.

Simple computer controlled apparatus for in vitro mechanical testing of connective tissues

Connective tissues are viscoelastic and so a series of experiments, at carefully controlled loading rates, is required to determine their mechanical properties. This paper describes a simple, inexpensive apparatus for performing these experiments; it is based on a ball screw driven by a microcomputer controlled stepper motor.

A comparison of ankle taping methods

Four different types of ankle tapings were applied to a model of the human ankle joint. The model was constructed so that it contained a hinge to represent the ankle joint and had an external shape identical to a real ankle and foot. A mechanical testing machine was used to apply moments to the model at a controlled loading rate. From these tests, the deflection and torque to failure and the tangent stiffness were determined. Loadings of an ankle joint in vivo revealed that the angular deflection to initiate pain was approximately 8 degrees. Only the figure eight and full tapings could withstand 8 degrees of angular displacement on the ankle model prior to failure. Analysis of athletic trauma revealed that torques of 420 Nm could be applied to the ankle joint. Only the figure eight taping with three or more wraps has adequate strength to withstand this moment, and thus this taping is recommended. In practice, tapings failed by shearing away from the surface of the foot of shrinking rather than by rupture of the tape.

Stabilization of Organic Carbon and Nitrogen in Consolidating Benthal Deposits

The objective of this research was to develop and validate a predictive model of the benthal stabilization of organic carbon and nitrogen in deposits of waste activated sludge solids formed at the bottom of an aerated water column, under conditions of continual deposition. A benthal model was developed from a one-dimensional, generalized transport equation and a set of first-order biological reactions. For model verification, depth profiles of the major interstitial carbon and nitrogen components were measured from a set of deposits formed in the laboratory at 20°C and a controlled loading rate. The observed sequence of volatile acid utilization in each benthal deposit was that which would be predicted by the Gibbs free energies of the individual degradation reactions and would be controlled by the reduction in interstitial hydrogen partial pressure with time. Biodegradable solids were solubilized rapidly during the first three weeks of benthal retention, but subsequent solubilization occurred much more slowly. The benthal simulation effectively predicted the dynamics of consolidating, organic deposits. Simulation of organic loading rates up to 250 g BVSS/(m2 day) indicated that the stabilization capacity of benthal deposits was far above the range of organic loading rates currently used in lagoon design.

The Strength of Liquid Bridges Between Dissimilar Materials

Abstract The strength of the liquid bridge between a sphere and a plate of dissimilar materials was studied. An equation was derived using the surface energy approach. For small amounts of liquid, the force of adhesion f was f=2πRy(cosθ1 + cosθ2 where R is the sphere radius, y is the surface tension, and θ2 , θ2 the contact angles. In the derivationn, major simplications about the meniscus shape were possible. The equation was experimentally tested with water, ethyl alcohol, aniline and iodobenzene using factorial combinations with different solids. Force of adhesion measurements were carried out using a tensile testing machine at controlled loading rates. Excellent agreement was obtained in the experimental and predicted adhesion values. The McFarlane-Tabor equation was identified as correct only for small amounts of liquids and similarly wet solids.