Complete Stress-deformation Research Articles

Combining Thermal Loading System with Acoustic Emission Technology to Acquire the Complete Stress-Deformation Response of Plain Concrete in Direct Tension.

The tensile properties of plain concrete are very important for the concrete structural design, and the complete tensile stress-strain curve is essential for creating accurate and reliable designs, especially when considering special load cases such as earthquakes and impacts. To study the complete tensile stress-deformation response of plain concrete, the direct tension tests were conducted on a novel thermal tensile testing machine (TTTM), which was reformed from a hydraulic universal testing machine (UTM). Acoustic emission (AE) technology was applied to monitor the damage process of plain concrete in tests. The TTTM was powered by the thermal expansion of loading columns, and had a stiffness similar to the specimen, thus eliminating the potential AE noises in the UTM, and simulating the rapid fracture process in real concrete structures. A static-dynamic acquisition system was established to obtain the complete tensile stress-strain curves, of which the data before and at the fracture moment were respectively acquired by the static acquisition system and the dynamic acquisition system. The AE technology is a useful approach to analyze the damage process of concrete, and makes it feasible to determine the damage state and the fracture location of the specimen in real time.

Investigation of Hydraulic Fracture Propagation Using a Post-Peak Control System Coupled with Acoustic Emission

This study investigates the fracture mechanism of fluid coupled with a solid resulting from hydraulic fracture. A new loading machine was designed to improve upon conventional laboratory hydraulic fracture testing and to provide a means of better understanding fracture behavior of solid media. Test specimens were made of cement mortar. An extensometer and acoustic emission (AE) monitoring system recorded the circumferential deformation and crack growth location/number during the test. To control the crack growth at the post-peak stage the input fluid rate can be adjusted automatically according to feedback from the extensometer. The complete stress–deformation curve, including pre- and post-peak stages, was therefore obtained. The crack extension/growth developed intensively after the applied stress reached the breakdown pressure. The number of cracks recorded by the AE monitoring system was in good agreement with the amount of deformation (expansion) recorded by the extensometer. The results obtained in this paper provide a better understanding of the hydraulic fracture mechanism which is useful for underground injection projects.

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ϕb, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.

Experimental study on complete stress-deformation curves of larger-size concrete specimens subjected to uniaxial tension

In order to provide parameters for numerical analyses of the huge Three-Gorge concrete dam (2309 m long by 175 m height), complete tensile stress-deformation curves for large-size plain concrete specimens were measured and studied by performing uniaxial tensile tests on large-size unnotched specimens (250 mm × 250 mm × 1400 mm). The specimens were prepared with the three-graded-aggregate materials provided by the client of the Three-Gorge project. To prevent a failure occurring near the ends of the unnotched specimens, both the ends of each specimen (450 mm in length) were cast using a higher-strength concrete than the middle part (i.e., active part). Tensile tests were completed on a specially-designed tensile testing machine, which can be easily re-assembled to accommodate different-size specimens. To make the specimens fail stably, a cyclic loading scheme was adopted after the peak strength was reached. Four of five tests in this study were successful, and four complete tensile stress-deformation curves were obtained. It was found that the post-peak curve of the large-size specimens used in this study is more gradual than those for the small-size specimens reported in the literature.

New test method for obtaining softening response of unnotched concrete specimen under uniaxial tension

A new method was developed to obtain the complete stress-deformation response of an unnotched concrete specimen. This method employs a digitally controlled closed-loop testing machine and five control channels: stroke LVDT and four LVDTs mounted on the specimen. The test portion of the specimen was fully spanned by these four LVDTs. The outputs of these control channels and the load cell were monitored during the test by a computer equipped with LabTech's Notebook software. A ‘C’ language program was written to enable quick switching of the mode of control from one LVDT to another. The problem of uncertainty in the location of the major crack was tackled by the said LVDT arrangement and the computer program. It was demonstrated that it is always possible to obtain stable post-peak response provided one ensures that at any time during the test, the feedback used is the LVDT that exhibits, at that time, the largest slope of the response-time curve. An acoustic emission (AE) measurement system of six channels was also used in the experiments. Monitoring of signals from the AE transducers provided valuable information which helped in making the decision to switch control.

Direct tension tests on notched and un-notched plain concrete specimens

Direct tension tests were conducted on plain concrete specimens in a stiff loading frame which allowed complete stress-deformation relations to be obtained. The suitability of the specimen shape and the testing apparatus was verified by finite-element analysis. The behaviours of notched and un-notched specimens with different water-cement ratios were examined and compared. Piecewise linear and exponential stress-deformation relations are proposed and interpreted physically. Relationships between fracture parameters fracture energy and brittleness indices) and basic strength properties are also presented.

Investigation of Complete Stress-Deformation Curves for Concrete in Tension

Investigation of Complete Stress-Deformation Curves for Concrete in Tension

Tensile Tests and Failure Analysis of Concrete

The tensile strength of concrete is receiving an increasing amount of attention since the loading capacity and durability of structures are being studied more thoroughly, and since nonlinear fracture mechanics and numerical methods require a complete stress deformation relation. The theoretical background of the application of the tensile strength of concrete in fracture analysis is given. Test results are presented that were obtained from deformation‐controlled uniaxial tensile tests under static and cyclic loading of lightweight and normalweight concrete. The results have been used to establish material models for finite element calculations. Numerical examples show the usefulness of these models for static and cyclic loading conditions.

Post-peak cyclic behaviour of concrete in uniaxial tensile and alternating tensile and compressive loading

Deformation controlled uniaxial tests have been carried out on cylindrical concrete specimens in order to establish the complete stress-deformation curve in tensile loading. Furthermore, cyclic tests have been performed under three types of loading: cycling from a tensile low stress, cycling from a compressive lower stress, and cycling between a lower and an upper stress to the envelope curve, respectively. The results seem to show that the tensile envelope curve is unique and the residual strength is the more affected the lower the stress at unloading is.

Analysis of Load-Bearing Fills over Soft Subsoils

The protection afforded soft subsoils by the stress-distributing characteristics of load-bearing fills is examined. Using recently developed finite element techniques of analysis, including nonlinear soil properties, a complete stress-deformation solution of a fill-subsoil system is obtained. The proposed method is first examined by applying it to three problems, one hypothetical and two actual load test case histories. The method is then used to study the stress distributing characteristics of granular fills overlying clay subsoils. As would be anticipated, the stiffer the fill relative to the foundation soil, the greater the reduction in stress transmitted to the underlying soil. Although the complexity of the load-bearing fill problem is such that simple charts cannot be prepared to indicate fill-subsoil interface stresses for different soil conditions and geometries, it is shown that approximate values of maximum vertical and shear stress can be obtained by use of an easily applied formula for modulus characterization and a simple geometrical function.