Papers by Keyword: Strain Measurement

Abstract: This article presents a study of the mechanical behavior of tuffeau, a porous building limestone. Previous studies were focused on using strain gauges for mechanical monitoring with some limited success due to the size and the local nature of the measurement. The Digital Image Correlation (DIC) method has proven to be a valuable tool for noncontact, full-field strain measurements in various materials, including rocks which are natural and heterogeneous materials. After a prior phase of optimization involving texture acquisition and lighting conditions, this paper compares several DIC software programs to achieve consistent results on soft limestone specimens. Once the DIC program is chosen, a focus is made on detecting heterogeneities in the stone specimens. The occurrence of such heterogeneities explains why strain gauge measurement sometimes fails when applied to soft and natural materials.

Abstract: Conventional strain gauges made of constantan or CuCr for instance have a low value for structural health monitoring issues in plastic composites. These strain sensor materials exhibit small elastic regions and show fatigue when dynamically loaded with strain levels over 0.3 percent. For this reason, these sensors would break or fail before the composite life-time and thus cannot be integrated into this kind of composite materials. Pseudoelastic thermal shape memory alloys are therefore used as strain sensors and integrated into composites in order to allow piezoresistive strain measurement and structural health monitoring in such materials. Thermal treatments are used to create sensor structures out of shape memory alloy wires. Pseudoelastic shape memory wires can be strained up to 8 percent repeatedly. Their gauge factor is higher than 5. Shape memory strain sensors are successfully embedded into glass fibre reinforced plastics and show a significant and reproducible resistance change when the composite is strained. The dynamic strength is magnificently higher compared to conventional strain gauges. Shape memory strain sensors are an efficient alternative to fiber-bragg-grating sensors and can potentially be used for strain measurements in different plastics and textile materials. Shape memory sensor structures can be embedded or applied and are good candidates for structural characterisation and monitoring applications.

Abstract: For strain-controlled fatigue tests, especially at elevated temperature, implementation of knife-edges and punching dimples within gauge length of specimen may cause premature failure on the test section. In this paper, an alternative approach is introduced to control and measure the strains in fatigue tests. Two dimples outside the gauge length section are punched and used to mount the extensometer. The correlation between the controlled (measured) strain and the strain within the gauge length section is established. By comparing the hysteresis loops obtained through the correlation and the one directly measured within the gauge length, it is found that the established correlations are in good agreement with the latter result.

Abstract: For a metal forming process, the uniaxial tensile properties of a material are the most fundamental and important properties to investigate. Solution heat treatment, forming and in-die quenching (HFQ^®) is a patented process to form complex shape panel components using aluminium alloys at high efficiency and low cost. A Gleeble materials thermo-mechanical simulator was used to conduct uniaxial tensile testing of AA6082 under HFQ^® conditions. A set of grips were specially designed to reduce the heat loss of specimen during testing in a Gleeble and allow the strain measurement by using digital image correlation (DIC) system. A large dog-bone specimen with parallel length of 80mm was designed to minimise the temperature gradient along the gauge section. Temperature gradient was measured and uniaxial tensile tests were conducted at the range of deformation temperature of 350-535 °C and the range of strain rate of 0.1-4 /s. The uniaxial tensile properties of AA6082 at different temperatures and strain rates under HFQ^® conditions were summarised and the viscoplastic response of the material was discussed.

Abstract: The paper deals with the experimental analysis focused on the determination of strain in wooden building structures using optical fibers with FBG sensors. Firstly, optical fibers were used to measure displacements of layers and surface strain of the lower surface of the ceiling panels made from mechanically jointed cross laminated timber (CLT). The measured displacement values of the optical fibers were compared with the results obtained from the inductive displacement sensors. Secondly, optical fibers were used for long-term monitoring of the roof structure of the sports hall. The fibers were mounted on chords of timber trusses and their correct function was verified by a load test.

Abstract: In this paper we present a calibration method, which enables a combining of 3D Digital Image Correlation and termographic systems for simultaneous measurements of displacements, strains and temperature. It concerns a model of a calibration target, that works for visible and infrared light spectrum and calibration method. The working system was applied for three interesting cases: monitoring of a tank filled with liquid, a heated plexi board and a human arm during muscle contraction.

Abstract: Ring-shaped specimen is commonly used in tests for determination of mechanical properties for arteries in hoop direction, especially for small mammals such as rats or mice. Although ring test is a lot more convenient than the tests concerning strip specimens, interpretation of the experimental data might be inconclusive – it is difficult distinguish whether it's still straightening up or the actual tension begins. The basic problem is to properly define initial length of specimen, which is essential for strain calculation. The purpose of this study was to evaluate various methods for strain evaluation. Ten Wistar Albino Glaxo male rats (3 months old, body weight about 200g) were sacrificed by decapitation and arteries were immediately removed. Three specimens, each approximately 2–3mm long, from each rat were excised adjacent to the aortic arch. Couple different initial lengths, as well as Digital Image Correlation based extensometer results were used for strain-stress response calculations. In addition a 3D Finite Element model, with an Ogden constitutive model based material, was made for deeper investigation of specimen behavior. This study has shown that the testing procedure for determination of mechanical properties of arteries, based on ring-shaped specimen, is prone to errors. Stiffness calculations are very sensitive to the choice of initial length of the specimen. As the use of DIC proved to give very good correlation with experimental data this method will be used in further studies.

Abstract: In fatigue crack growth experiment and fracture toughness test, the crack length measurement is very important. Here, this paper proposes a new method for measuring crack length, namely in the case of a known load, according to the strain on the front end face of the three point bending specimen would be to get crack length.

Abstract: Owing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale, i.e. phase and grain scale. In situ synchrotron diffraction (transmission mode) during tensile tests and modified self-consistent elastoplastic model were used to study elastic and plastic phenomena occurring in polycrystalline specimens during deformation. The evolution of stress for grains which belong to different phases of duplex stainless steel and pearlitic steel was analyzed.

Abstract: The strain measurement is a most important and absolutely necessary task on the probing into the constitution of the materials. The ways of strain measurement are theoretically divided as the mechanical type, vibrating-string type, inductive type and bonded resistance type, and the last one is widely used for its high precision and convenience. The geogrid, which has been widespread in the reinforcement structures, is more and more popular for its high strength, good toughness, corrosion resistant, anti-fatigue and effective interaction with soil. The geogrid is made of polymer, its ultimate strain can reach above 10%, and it is sensitive to the time, furthermore it usually has significant relative displacement with soil in the tests. Then some measures have to be taken to make the measuring range wider and make the all gauges achieving synchronization, measures also should be conducted to prevent the strain gauges separated from the geogrid when it has slightly bending, prevent the wires break when the gauges going forward and reduce the influences from the forward preventing measures. Some researchers have showed their techniques to avoiding part of the above problems. Based on a series of pullout tests of geogrid, this paper gives the contrast and analysis on those techniques above, and lists the relative measures that used in these tests, especially for the gauge sticking techniques and preventive measures.