Electronic Speckle Pattern Interferometry Technique Research Articles

Advancement on optical methods in stress dead-zone characterisation and SIF evaluation

The work described in this paper is a study of the stress dead zone (SDZ) concept with advanced full-field optical methods. The SDZ is a region located around the fatigue crack flanks, adding a negligible contribution to internal stresses on the global deformation energy calculation. In the analytical modelling, this area can be removed from the structural member, resulting in a geometry simplification, and in the possibility of using analytical formulations in the Stress Intensity Factor (SIF) calculation associated to the existing fatigue crack. In a fatigue-testing program, aluminium middle tension (MT) specimens were prepared and then subjected to a cyclic fatigue loading, in order to generate different crack lengths for each specimen. Then, the fatigued specimens are monotonically tested under a uniaxial tensile loading to assess the SDZ concept.The experimental data is collected and analysed through two full-field high-resolution optical techniques with different system resolutions, Digital Image Correlation (DIC), and Electronic Speckle Pattern Interferometry (ESPI). As a result, the SDZ is geometrically characterized through a set of mathematical formulations based on the compliance function for the studied specimens, and is experimentally identified through deformation fields obtained from both DIC and ESPI techniques. The obtained results from both high-resolution methods verify the validity of the concept and demonstrate that it can be used to simplify the analysis.

The effect of radiolytic oxidation on the elastic properties of nuclear graphite measured by electronic speckle pattern interferometry

• Three-point bend measurement data characterised by ESPI and inverse modelling. • Static Young's modulus , Poisson's ratio and strength determined. • Measurements performed on 218 irradiated graphite beams taken from UK AGR stations. • Graphite samples (6 × 6 × 19 mm) cover dose range to 21.1 DPA and weight loss to 46%. • Modulus and strength correlate well with radiolytic weight loss. The Young's modulus, Poisson's ratio and flexural strength of 218 beams of radiolytically oxidised nuclear graphite have been determined by combining electronic speckle pattern interferometry, ESPI, and inverse finite element modelling techniques. The graphite was extracted from three operational advanced gas cooled reactor stations where it had been subjected to fast neutron irradiation and radiolytic oxidation resulting in mass loss. The 6 × 6 × 19 mm beam samples had densities ranging from 1063 – 1840 kg.m −3 , equivalent to weight losses up to 46%, with cumulative neutron doses of up to 161 × 10 20 n.cm −2 equivalent Dido nickel dose, or 21.1 displacements per atom, significantly extending the datasets available in the open literature to higher levels of both dose and oxidation. In situ ESPI measurements were collected during three-point bend tests, creating full field displacement maps of the evolving deformation to which simulations were matched by chi-squared minimisation of the elastic properties. Potential loading abnormalities were accounted for in the fitting process to ensure the simulations were representative, with total uncertainties in the determined Young's modulus and Poisson's ratio values of ± 4.5% and ± 0.08, respectively. Consistent trends were observed in the behaviour of the material from the different stations, with strong positive dependences on density for both the flexural strength and Young's modulus. The strength and Young's modulus remained well correlated even to high levels of oxidation mass loss. Conversely, the Poisson's ratio was practically invariant with oxidation with mean values of 0.21 ± 0.03, 0.24 ± 0.05 and 0.25 ± 0.05 for the three stations. The combined ESPI-finite element measurement approach is demonstrated as robust for irradiated small specimens, making it suitable for monitoring graphite in future reactors. The collected data support the safety assessments of the current reactors, enabling predictions of the responses to progressively higher mean core mass loss.

Size effects on the characteristics of fracture process zone of plain concrete under three-point bending

This paper studies the size effects on the characteristics of fracture process zone (FPZ) of plain concrete, including its physical features and softening behavior. Three-point bending tests were performed on pre-notched beams with similar geometry and different dimensions. Electronic speckle pattern interferometry (ESPI) technique was employed to monitor the crack evolution and full-field deformation of the beams. The development of FPZ was evaluated qualitatively based on the strain fields and quantitatively based on the displacement fields obtained by the ESPI technique. By adopting the incremental displacement collocation method, the tension softening curves (TSCs) of specimens with different sizes were estimated and approximated as bilinear curves. The features identified from the TSCs were correlated to the physical features in the development of FPZ. Finally, an empirical tension softening model and corresponding size-independent parameters were obtained based on the estimated TSCs of different sizes specimens.

Fracture properties of concrete under cyclic loading

Three-point bending tests were performed on pre-notched beams to investigate the fracture properties of concrete under cycling loading. Electronic speckle pattern Interferometry (ESPI) technique was used to measure the full-field displacement of the beams. The influence of loading history on the fracture properties of concrete were analyzed. Results measured by ESPI were compared with that measured by a clip gauge and good agreement was observed, which validates the reliability of using ESPI to measure the deformation of fracture process zone during crack extension. The experimental results indicate that load–displacement curves under monotonic loading are an envelope line for that under cyclic loading. The load–displacement curves of each loading cycle can be divided into five stages and in both loading and unloading process, elastic deformation responses earlier than inelastic deformation. The critical crack mouth opening displacement (CMODc) increases with the loading cycles and reaches about 0.090 mm while the critical crack tip opening displacement (CTODc) is almost constant about 0.034 mm which can be used as a fracture characteristic of the material. The cumulative dissipative energy tends to reach about 21 N/m as the loading cycles increase and can be used as a fracture parameter under cyclic loading.

Experimental and numerical analysis of the residual stress distribution in a three-point bending test of a TRIP sheet by using ESPI

In sheet metal forming, residual stresses are related to springback and material failure after the deformation process or during service due to load history. Thus, being able to either calculate or measure such stresses is of great importance. This paper aims to evaluate the use of the Electronic Speckle Pattern Interferometry (ESPI) technique to measure residual stresses on TRIP steel. Residual stresses are measured after springback on specimens used in three-point bending tests; where three bending angles are considered. Experiments are compared with Finite Element calculations in terms of punch force, springback angle and residual stresses. Work-hardening and anisotropy parameters, used in simulations, are experimentally determined by the uniaxial tensile test. Results indicate that advanced hardening models are necessary to increase the accuracy of springback predictions. Nevertheless, residual stress calculations show a good correlation with experimental values. Also, it was proved that ESPI is a powerful technique to measure the residual stress on complex surfaces, as the ones typically encountered in sheet forming processes. Future work includes residual stress predictions for different forming processes.

An investigation of fracture properties and size effects of concrete using the ESPI technique

In order to study the size effect in concrete fracture, three-point bending tests are performed on five groups of geometrically similar concrete beams. The electronic speckle pattern interferometry (ESPI) technique is used to measure the full-field deformation of concrete beams. Crack opening displacement data, which is crucial for understanding crack initiation and the evolution mechanism, but which is not readily available through conventional measurement techniques, is obtained. Fracture parameters of five groups of concrete beams, including the critical crack opening displacements (i.e. CMODc and CTODc) and crack growth lengths (i.e. ac and ae) at the peak, fracture toughness (KIc), fracture energy (GF) and nominal strength (σN), are determined. The influences of beam depth on the fracture parameters of concrete are evaluated and discussed. The results show that the CMODc, CTODc, ac, ae, KIc and GF increase with the increase in beam depth, while σN has a decreasing trend that follows the size effect law. Size-independent fracture toughness and fracture energy are estimated and proposed.

CHARACTERIZATIONS ON FRACTURE PROCESS ZONE OF PLAIN CONCRETE

The fracture property of concrete is essential for the safety and durability analysis of concrete structures. Investigating the characteristics of the fracture process zone (FPZ) is of great significance to clarify the nonlinear fracture behaviour of concrete. Experimental and numerical investigations on the FPZ of plain concrete in pre-notched beams subjected to three-point bending were carried out. Electronic speckle pattern interferometry (ESPI) technique was used to observe crack evolution and measure the full-field deformation of the beams. The development of the FPZ were evaluated qualitatively and quantitatively based on the in-plane strain contours and displacement field measured by ESPI, respectively. By integrating the cohesive crack model and finite element (FE) model, various tension softening curves (TSCs) were employed to simulate the fracture response of concrete beams. By comparing the deformation obtained by FE simulation and experiments, the TSCs of plain concrete were evaluated and most suitable TSCs of concrete were recommended.

Characterization on tensile behaviors of fracture process zone of nuclear graphite using a hybrid numerical and experimental approach

This paper reports an experimental and numerical study on the fracture properties of nuclear graphite NG-CT-01 produced in China. Three-point bending tests were performed on center-notch beams and electronic speckle pattern interferometry (ESPI) technique was employed to measure the full-field deformation of the beams. Besides, finite element analysis was performed by using extended finite element method to simulate the fracture behavior of graphite. Numerical results were compared with the experimental results and excellent agreement was obtained, validating the reliability and accuracy of the numerical model. Based on the strain field measured by ESPI technique, the formation and evolution of the fracture process zone (FPZ) in graphite were observed. The tension softening curve (TSC) of the graphite was determined by using a hybrid numerical and experimental approach. The influences of initial crack length to beam depth ratio on the fracture responses of graphite beam, including the load-displacement curves, double-K fracture parameters, TSCs and the length of the FPZ, were evaluated. In order to explain the correlation between the tension softening behaviors and the fracture mechanism of the FPZ, a tri-linear model was proposed. Furthermore, double-K fracture criterion was employed to predict the crack propagation in graphite.

Impact Damage Investigation on Glass Fiber-Reinforced Plate Laminates at Room and Lower Temperatures through Ultrasound Testing and Electronic Speckle Pattern Interferometry

In this work, the evaluation of damage area due to low-velocity impact tests at different impact energy values and different temperatures on glass fiber-reinforced plate laminates was investigated by two nondestructive evaluation (NDE) techniques, electronic speckle pattern interferometry (ESPI) and ultrasound testing (UT). Composites are characterized by several interacting failure modes such as matrix breakage, fiber failure and delaminations, which can be simultaneously induced by low-velocity impacts and can be different depending on the temperature. It can be complicated to detect by visual inspections of the structures for such phenomena. This paper aims to investigate the dependence of the damaged area with respect to temperature variation on glass fiber composite laminates. GF composite laminates were impacted with three energy levels (5, 10, 20 J) and at different temperatures (room temperature, − 25, − 50 °C) by a drop weight impact machine. The results show that at decreasing temperature, a decrease in the damage extension was observed, which is more evident at increasing impact energy. Moreover, as expected, the results confirm UT and ESPI techniques are able to identify the barely visible low-velocity impact damage. However, some limitations for detection were found in ESPI. Nonetheless, the ESPI technique can be considered as one of the useful NDE methods if the calibration and the post-processing methods are improved.

Characterization on crack propagation of nuclear graphite under three-point bending

Abstract The fracture properties of graphite produced in China, named as CLG, were studied by performing three-point bending tests on center-notched beams. The load-deflection (P-δ) curves and load-crack mouth opening displacement (P-CMOD) curves of the beams in the entire loading process were obtained. Electronic speckle pattern interferometry (ESPI) technique was used to measure the full-field displacement of the beams. Through analysis on the relationship between the δ and CMOD, the δ-CMOD curves were divided into nonlinear and linear parts, and its turning point corresponded approximately to the post-peak load of 55% Pmax. By curve fitting, empirical formulas were established for the δ-CMOD relationships. In addition, crack evolution rules were evaluated based on the relationship of crack opening displacement (COD) versus crack length a of specimen CLG-1 at 14 loading levels. Through piecewise analysis, it could be found that linear elastic fracture mechanics were applicable to determine the COD along the whole crack path before the peak load. Therefore, new formulas were proposed to estimate the COD-a relationship for the entire loading process. Furthermore, the COD-a curves of other specimens in the same batch with CLG-1 were calculated by using the present formulas and compared with the experimental results. Excellent agreement was observed, which validates the reliability of the present formulas.

To the problem of the subsurface defects detection: theory and experiment

We solve the problem of normal waves diffraction in an elastic layer rigidly connected with interface on an interface crack under the action of antiplane dynamic loading. The Wiener–Hopf technique is applied for the solution. The resonance vibrations of the dynamic system on the main mode are investigated. The subtractive synchronized electronic speckle pattern interferometry technique and the approach based on estimation of the dynamic speckle motion of surface areas of multilayer composites were developed for detection of subsurface defects. Experimental breadboards realizing these techniques are created and experiments to identify test and real subsurface defects are fulfilled.

Impact behaviour of hybrid basalt/flax twill laminates

The purpose of hybridization is to obtain a new material preserving advantages from all of its constituents. Hybridization offers intermediate properties respect to the original materials, by creating a balance effect within the fibres incorporated in the composite materials and leading to a composite with more tailored behaviour The increasing need to mitigate the environmental impact of synthetic fibres and polymers is promoting the use and application of natural materials orienting the research toward the development of biodegradable systems.In this framework, hybrid reinforced laminates with flax and basalt twill layers alternatively stacked, were manufactured by resin infusion fabrication technology and impacted at low velocity to investigate their dynamic behaviour, in an attempt to couple the impact resistance of basalt fibres with the environmentally friendly nature of flax fibres. For comparison purposes, the same experimental characterization has been performed on laminates reinforced with only basalt or flax fibres. The experimental results confirmed the positive role played by fibre hybridization in terms of damage.The Electronic Speckle Pattern Interferometry technique was adopted to analyze the internal damage and to provide information on the shape and the extent of the delamination, that was found concentrated under the impactor-material contact point for the basalt and flax/basalt laminates.

Strain Field Analysis in Electronic Components by ESPI: Bad Thermal Contact and Damage Evaluation

Designing electronic components and devices is a complex task that must include also considerations about thermo-mechanical behavior of the component itself. This aspect, in fact, can affect the reliability of the device as a consequence of the thermal stresses that are introduced in the component while it is working. Stress concentration can arise, in fact, due to different thermal expansion coefficients of the materials involved. Determining the thermo-mechanical response by numerically methods is not simple and not always possible so that experimental methods are preferable. In this paper, specifically, the electronic speckle pattern interferometry technique was adopted to measure full-field strain of the component during exercise. By using an optical technique it is possible to get information about the behavior of the component without being in contact with the component itself so that the dissipation coefficient is not altered. The system was able to detect gradients of deformation in the component itself connected to the different distribution of internal current in the component. Moreover, it was observed, that the presence of bad thermal contact between the package and the heat sink can be revealed because it results in a different thermos-mechanical behavior of the component. Finally, a critical defect was introduced in the component and it was observed the way the presence of damage affects the in-plane displacement of the component. Very huge differences were observed leading to the consideration that this kind of approach could be conveniently adopted also as a damage detection tool.

Evaluation of the Bending Behaviour of Laminated Glass Beams Via Electronic Speckle Pattern Interferometry

The paper is devoted to the experimental analysis of the kinematical and mechanical behaviour of laminated glass beam structures. In particular, the utilized laminated glass specimens are composed of two glass layers bonded by a polymer layer constituted by Ethylene-vinyl acetate whose thickness has been nominally considered as constant for all the specimens. The experimental behaviour of the analyzed specimens is deduced by applying Electronic Speckle-Pattern Interferometry technique; actually, among optical methods this technique (handled by phase-stepping technique) is very effective to obtain a full-field displacement map and to numerically achieve the longitudinal strain. In particular, this technique has been used to analyse the pure bending behaviour of two-layers specimens characterized by different thicknesses of glass layers. The longitudinal strain, numerically obtained, is the basis for the subsequent analysis of the stress acting on the specimen. The longitudinal stress shows the role of the stacking sequence in the structural behaviour of the analysed specimens opening new approaches to the optimization of such elements. The principal aim of the present paper is the evaluation of the reliability of the methods suggested by the international standards by means of the comparison of the obtained experimental results with those arising from the effective monolithic thickness approach reported in international standards. The effected analyses allow to observe that the numerical results obtainable by using the methods provided by international standards are not very effective especially for thin layers.

Detection of Crack Evolution in Plain Concrete by Electronic Speckle Pattern Interferometry

In order to study crack evolution in concrete, Electronic Speckle Pattern Interferometry (ESPI) technique was applied to measure full-field displacement of concrete beam subjected to three-point bending. Basic principles of ESPI technique were introduced. Mid-span deflection and crack mouth opening displacement were measured by linear variable differential transformers (LVDTs) and clip gauge, respectively. Typical load-displacement curves measured by different methods were compared and analyzed. Analysis results indicated that ESPI results were in good agreement with that measured by LVDT (clip gauge), verifying the validity and accuracy of ESPI measurement. From displacement contours, crack evolution including its initiation and propagation was observed. Furthermore, strain profiles near the crack at different loading levels were determined. Strain profile was nearly linear before crack initiation and became nonlinear with crack growth.

Study on Fracture Properties of Mortar Based on Electronic Speckle Pattern Interferometry

With high resolution, electronic speckle pattern interferometry (ESPI) technique can be useful in measuring full-filed deformation of laboratory specimens. In this study, ESPI was applied to measure the full-field displacements of mortar beam subjected to three-point bend. Load-displacement curves measured by ESPI were compared to that measured by clip gauge and LVDTs. Satisfactory agreement was observed in all the comparisons, demonstrating the effectiveness and accuracy of ESPI measurement. Furthermore, load versus crack tip opening displacement curve was obtained from ESPI results. Finally, extent of the fracture process zone and neutral axis of the mortar beam were analyzed based on full-filed displacement contour measured by ESPI.

Electronic speckle-pattern interferometry: Techniques for non-contact vibration measurements of musical percussion instruments

Electronic speckle-pattern interferometry (ESPI) provides a non-contact optical system that can be used to record images showing operating deflection shapes of structures being driven sinusoidally at resonant frequencies. In high-Q systems, these shapes can approximate the normal modes of the test object. Due to the sensitivity of the optical measurement system, non-contact excitation can be achieved acoustically using a speaker. The use of ESPI is described in relation to measurements of musical bars, plates, and membranes under various boundary conditions, though the methods described can be applied to many other types of structures. In addition, a dual-ESPI system that provides simultaneous measurements of two surfaces of an object is illustrated (e.g., two heads of a musical drum). These measurements provide information regarding the degree of coupling and the relative phase of the two vibrating surfaces. Finally, the ESPI techniques are applied to the fluid-structure interaction of an orchestral crotale (a thick metal disk) at various degrees of submersion in water.

Vibrational mode and sound radiation of electrostatic speakers using circular and annular diaphragms

This study modeled two diaphragms comprising a pair of indium tin oxide (ITO) transparent plates sandwiching a vibrating diaphragm to create circular (30mm radius) and annular (30mm outer and 3mm inner radius) push–pull electrostatic speakers. We then measured the displacement amplitudes and mode shapes produced by the devices. Vibration characteristics were used to predict sound pressure levels (SPLs) using the lumped parameter method (LPM) and distributed parameter method (DPM). The two measurement results obtained using a laser system were compared to the SPLs obtained using traditional acoustic measurement (AM) from 20Hz to 20kHz in order to verify our predictions. When using LPM and DPM, the SPL prediction results in the first three symmetric modes were in good agreement with the AM results. Under the assumption of linear operations, the DPM and amplitude-fluctuation electronic speckle pattern interferometry (ESPI) techniques proved effective in determining the visualization of mode shape (0,1)–(0,3). The use of ITO plates is a practical technique for the prediction of SPL, as well as measurement of mode shapes. The four evaluation methods, i.e. LPM, DPM, ESPI and AM, present a high degree of consistency with regard to vibrational mode and sound radiation characteristics.

3-D ESPI Measurements Applied to Selected Engineering Problems

The presented paper introduces the application of the Electronic Speckle Pattern Interferometry (ESPI) technique for measurements of contours of strains and stresses on 3-D surfaces. The paper focuses on three selected applications. Firstly, the methodology for determining the Poisson ratio of a material from tensile tests is described. Secondly, results of calibration verification of a biaxial extensometer used for measurements of shear strain in comparison with the ESPI measurements are presented. Finally, the experimental analysis of stresses in welds is discussed. As mentioned in the paper, the method is very useful for material testing, as well as for measurements on structural parts in service.

In-liquid characterization of in-plane and high order out-of-plane modes of AlN-based square microplates

We report on the characterization of in-plane and out-of-plane modes of vibration for various piezoelectric, aluminium nitride-actuated, square microplates with optimized excitation electrodes. We combined electrical and optical techniques to fully characterize the modes of vibration in different media, with special attention to the modes with the highest quality factor in liquid. An electronic speckle pattern interferometry technique was used for a full 3D detection of the movement of the microplates, allowing a precise identification of the modal shape, even under liquid immersion. The electrical characterization was carried out with an impedance analyser. Quality factors as high as 15,000 were obtained in vacuum, for an in-plane mode at 3.93 MHz, and as high as 144 in isopropanol, for a high order out-of-plane mode at 3.63 MHz.