Papers by Keyword: Digital Image Correlation (DIC)

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: In this paper, tests were performed by monitoring changes of strain amounts and their distribution by digital image correlation during static tensile testing of steel. The influence of different combination of speckles application on the sample surface (white speckles on the black sample surface and black speckles on the white sample surface) during cold deformation of steel was tested with the non-contacting digital image correlation. Strain amounts obtained by digital image correlation analysis of tested samples, with white on black and black on white painted speckle patterns, were compared. Different analysis for determining and monitoring strain amounts, provided by digital image correlation software, were used in this paper. The strain amounts were compared during cold deformation until the occurrence of sample fracture in both combinations of speckles application on the sample surface. Research has shown the importance of proper sample preparation for determination strain amounts and insignificant minor differences of strain amounts, between different sample preparations for digital image correlation, during cold deformation of tested steel.

Abstract: In the present work, uniaxial tension tests have been conducted to study the mechanical response of a commercial Al4.5wt%Mg alloy (AA5182). The measurements range from the strain rate of 10^-4 to 10⁰ s^-1 at room temperature. Digital image correlation (DIC) is also utilized to characterize the heterogeneous deformation of PLC localization. An extremely high camera frame rate (1000Hz) is applied to accurately measure the critical strain of instabilities, especially for tests at high strain rates (>10^-1 s^-1). The objective of this work has been to evaluate currently existing constitutive and physically based models to see to which extent they can match the experimental findings related to dynamic strain aging, i.e. stress-strain curves, work hardening behavior and critical strain behavior.

Abstract: We have developed a new testing device which is capable of detecting hydrogen gas release during slow strain rate tensile testing (SSRT) under ordinary pressure. The device is composed of an SSRT machine equipped with a closed chamber with an inspection window that is connected to gas chromatography with a semiconductor hydrogen sensor. Local strain distribution in the specimen during the SSRT is monitored dynamically with a digital image correlation (DIC) method. Hydrogen was pre-charged to aluminum alloys by means of friction in water process. Using the device, it was shown that hydrogen was released particularly in the stage of plastic deformation and fracture. In addition, the hydrogen gas release at the moment of fracture was clearly increased when the alloys were hydrogen-charged and tested at a slow strain rate. When we calculated hydrogen gas release from the fracture surface in Al-Zn-Mg base alloys tested at 3.3×10^-6 s^-1, the hydrogen amount was estimated to be 6.24×10^-10 mol /mm² in a hydrogen-uncharged alloy, and 1.30×10^-9 mol / mm² in a hydrogen-charged alloy.

Abstract: Earthen constructions are spread worldwide, not only as architectural heritage but also as new buildings, in which a total of around one fourth of the global population is estimated to live in. Among the different raw earth techniques, rammed earth was widely used to build vernacular dwellings in different regions. Despite the raising awareness for the high seismic hazard, recognized seismic vulnerability and high seismic exposure associated with earthen structures, the reduction of their seismic risk has been a topic insufficiently addressed. In general, the seismic vulnerability of rammed earth structures is due to poor connections between structural elements (walls and floors), high self-weight and low strength of the material. Hence, a TRM-based strengthening technique is proposed to improve their seismic capacity. To estimate the enhancement of the in-plane performance achieved with the TRM, an experimental program was conducted. Rammed earth wallets were tested under diagonal compression considering their unstrengthened and strengthened condition. The TRM strengthening was performed by embedding a glass fibre mesh (GRE) or a nylon mesh (NRE) in a compatible earth-based mortar. In general, the proposed TRM-based strengthening seems to improve the shear behaviour of rammed earth by enhancing the shear strength and promoting stress distribution after cracking, while no significant influence on the shear modulus of rammed earth seems to occur.

Abstract: Fiber reinforced cementitious matrix (FRCM) composites are increasingly adopted as a strengthening technique for existing masonry structures. Among the different fibers that can be employed in the reinforcing open-mesh textiles, which are embedded within cement- and lime-based matrices, glass fibers are gaining popularity due to their low price and promising performances observed so far. However, the stress redistribution between the glass fiber filaments within the textile is often uneven, which strongly affects the performance of the FRCM when subjected to external forces. In this paper, the stress redistribution between the glass fiber filaments is studied on the basis of tensile tests on a glass fiber textile left bare and impregnated with organic and inorganic matrices. The parameters studied are the fiber textile tensile strength and elastic modulus. Different systems, including the digital image correlation (DIC) technique, were employed to measure the specimen tensile strain. The results obtained shed light on the tensile strength of the glass textile, which is a key parameter in the design of glass FRCM strengthening.

Abstract: Fibre Reinforced Cementitious Matrix (FRCM) composites are becoming largely adopted for retrofitting masonry structures. These materials offer several advantages in comparison to Fibre Reinforced Polymer (FRP) composites, such as good resistance to fire and high temperatures, vapour permeability, possibility to be applied on wet surfaces, higher compatibility with the masonry substrate. However, the tensile behavior of FRCM materials is more complex compared to FRP composites, due to the limited tensile strength of the cement-based matrix. For this reason, FRCM materials require appropriate tensile characterization and, in this context, the use of non-conventional measurement systems, such as the Digital Image Correlation (DIC), can offer numerous advantages. This work presents an experimental study on the application of the DIC technique for the tensile characterization of Basalt Fibre Reinforced Cementitious Matrix (BFRCM) strips. Tensile tests were carried out on three series of specimens reinforced with one, two or three layers of basalt grid in order to investigate the effect of the reinforcement ratio on the tensile response of the composite strips. The test setup and the calibration of the DIC analyses are discussed. It is shown as the DIC allows obtaining detailed information on the tensile response, including the evaluation of the full strain field on the surface of the BFRCM strips and the location of cracks. Results are discussed also in terms of stress-strain curves and failure modes.

Abstract: In the present paper, two novel methods for determining the fatigue limit are presented. Despite the fact that these methods are different in principle, both represent a new approach to testing where the main benefit is reduced consumption of material. The first method is based on small round specimens and can be considered as one of semi-destructive testing methods. The second method is based on infrared thermographic analysis and requires only one specimen. Results obtained with these techniques were compared with those obtained from standard high-cycle force-controlled fatigue tests under constant loading until failure.

Abstract: This paper deals with an evaluation of long-term experimental work carried out in cooperation with concrete suppliers and a specific company (named Sobriety). The key part of this work is focused on the experimental determination of secant modulus of elasticity and Poisson's Ratio of special concrete (self-compacting concrete, steel fibre reinforced concrete, polymer fibre reinforced concrete, specific high-strength concrete). Two different real-time approaches were used to detect modulus of elasticity. In parallel, both approaches - the (European) standard approach and the DIC (2D DIC; 3D DIC with varying lengths in the vertical direction) - were applied.

Abstract: A novel 3-D edge detection methodology is developed to resolve the edge ambiguity problem encountered in 3-D optical surface profilometry employing digital image correlation (DIC). DIC has been surged as a full-field measurement technique for in-plane and out-of-plane dynamic mechanical structure analyses. However, up to date, one of the key issues in DIC is still remained in boundary edge detection since a surface edge is not detectable between two discrete neighboring height jumps due to optical diffraction. Generally, it is common to observe undesirable noisy measured data along surface edges in traditional DIC-based surface 3-D profilometry. To resolve this, a novel random speckle images processing method is established by proposing a new algorithm by employing the multiple symmetric partial template model to determine best edge location with accurate height reconstruction. A theoretical simulation on a pre-calibrated circle target was performed to verify the feasibility of the methodology. Some experiments on real industrial objects having various surface reflective characteristics were implemented to verify its capability on accurate detection of industrial objects having discrete surface edges. From its preliminary evaluation on measurement accuracy, it is found that the maximum measured error on critical dimension can be controlled within less than 6.0% of the overall measuring range while one standard deviation can be kept within less than 1.2%.