Full-field Measurement Technique Research Articles

Exploring the response of a thin, flexible panel to shock-turbulent boundary-layer interactions

The use of nonlinear, dynamic methods for the simulation of aerospace structures has increased dramatically in recent years [1]; however, very little relevant experimental data exists to properly guide these developments. An experimental campaign was initiated by the AFRL Structural Sciences Center (SSC) for three reasons: (1) to observe and measure the effect of turbulence, shock boundary-layer interactions (SBLI) and heated flow on an aircraft-like panel; (2) to explore severe structural events (dynamic instabilities and material failure); and (3) to refine full-field and non-contacting experimental measurement techniques necessary to characterize the flow environment and structural response. All of the objectives were achieved. The panel response to turbulent, heated flow and sensitivity to panel back-pressure modulation was studied, with large-deformation limit cycle behavior leading to panel failure, observed and measured. For the first time, the 3D Digital Image Correlation (DIC) technique was also used to record the panel behavior while filming through the flow and SBLI environment. Finally, fast reacting pressure sensitive paint (PSP) was used, concurrently with 3D DIC, to record the dynamic pressure across the panel surface.

Thermoelastic Couplings and Interparticle Friction Evidenced by Infrared Thermography in Granular Materials

Infrared (IR) thermography was used to analyze the thermomechanical response of a two-dimensional non-cohesive granular assembly. Two constitutive materials with different types of thermoelasticity were chosen: thermoplastic polyurethane (TPU) and polyoxymethylene (POM), which feature entropic and isentropic elasticity respectively. Cylinders of each material were mixed together. Analysis was performed under confined compression at two observation scales. Thermoelastic couplings and interparticle friction were separately evidenced. First, the strong thermal effect of entropic coupling was revealed at the contacts, in the stress concentration zones. Second, image processing enabled us to clearly extract the thermal signature of the interparticle friction zone, a quantity that cannot be identified by the other full-field measurement techniques available today. It can thus be claimed that IR thermography provides two distinct routes for the analysis of granular materials by distinguishing the reversible and irreversible parts from the global thermomechanical response. The study also opens prospects for the experimental analysis of “soft” granular media.

Investigating wood under thermo-hydromechanical loading at the ring scale using full-field measurements

The thermo-visco-hydromechanical properties of Abies alba Mills (white fir of Massif Central) are investigated in this paper. These properties are generally complex to study and model because of various couplings that exist between underlying phenomena. In addition, wood is heterogeneous by species, but these properties are generally studied at the macroscopic scale, thus without having the possibility of discerning the contribution of the various levels of organization which exist in wood. In this context, the main contribution of this paper is to experimentally evidence that earlywood and latewood exhibit each a different visco-hydromechanical response and, more specifically, that humidity directly governs their viscoelastic properties. Three suitable experiments were carried out on three different specimens to highlight this phenomenon. These experiments consist of mechanically testing the specimens under natural drying conditions. The difference in response was observed at the scale of the annual ring by using a full-field measurement technique, namely the grid method. This technique provides the strain fields occurring on the surface of the specimens during the different steps of their hydromechanical loading. Its spatial resolution is such that the strain level in latewood and earlywood can be accurately measured, thus evidencing the significant difference in response of the rings. This measurement technique as well as the three experiments carried out during this study is first briefly described in this paper. The obtained results are then presented and discussed, with a special emphasis on the different responses observed in earlywood and latewood.

A Phase Compensation Method for Phase Extraction of Abrupt Surfaces in Projection Moiré

Projection moire method is a high-resolution, non-contacting and full-field optical measurement technique. But the 2π-ambiguity problem in phase unwrapping limits the phase difference between neighbouring pixels under π which makes projection moire method ineffective at measuring abrupt surfaces or steps. This paper aims at putting forward a phase compensation method to solve this 2π-ambiguity problem in projection moire. Since the phase of moire pattern is the variance between the phase of specimen grating on object’s surface and the phase of the reference grating, our method is to calculate the phases of specimen grating and the reference grating respectively, regard their difference as the phase of the moire pattern in theory and correct the wrong unwrapped moire fringe phase with integral multiple of 2π. In order to acquire the phase of specimen grating which also has the 2π-ambiguity problem, using multiple fringe pictures with different pitches. The experiments successfully reconstruct two objects with discontinuities in their surfaces. This phase compensation technique no longer requires us to count fringe orders or find a reference point to mark the zero-order moire fringe during experiments.

Determination of vibration amplitudes from binary phase patterns obtained by phase-shifting time-averaged speckle shearing interferometry.

Speckle shearing interferometry (shearography) is a full-field strain measurement technique that can be used in vibration analysis. In our case, we apply a method that combines the time-averaging and phase-shifting techniques. It produces binary phase patterns, where the phase changes are related to the zeroes of a Bessel J0 function, typical of time-averaging. However, the contrast and resolution are better compared to traditional time-averaging. In a previous paper, we have shown that this is particularly useful in vibration testing performed under industrial conditions, because fringe patterns are noisier than in quiet laboratory environments. This paper goes a step further in proposing a processing method for estimating the vibration amplitude, for helping non-experts to identify vibration modes. Since shearography measures the spatial derivative of displacement, spatial integration is required. Prior to that, different processes like denoising, binarization, automated nodal line detection, and amplitude assignment are applied. We analyze the performance of the method on synthetic and experimental data, in the function of noise level and fringes density. Results on data acquired in an industrial environment illustrate the good performances of the proposed method.

Deflectometry, a full field slope measurement technique: General perspectives and application to loading identification using the virtual fields method

The ability of photographic techniques to quickly record high volumes of scientific data has been understood since the early 1900s. It is only following the advent of digital imaging and data processing systems that so-called full-field optical measurement techniques became sufficiently reliable for performing high spatial density structural vibration measurements. In particular, the deflectometry technique directly provides a spatially and temporally resolved measurement of slope fields on plane structures using a single high-speed camera. The presentation first demonstrates the principles of this technique and explores some direct applications of such full-field measurements. The advantages and drawbacks of deflectometry are compared with other optical techniques like digital image correlation or scanning laser Doppler vibrometry. A key aspect of several engineering domains consists in the identification of dynamic loads acting on structures by inverse methods. It is shown that coupling deflectometry measurements with the virtual fields method enable the reconstruction of stationary and transient excitations without any specific regularization. Experimental reconstruction results on an aluminum panel are presented for two different transient mechanical loadings: instrumented impact hammer and impacting metal marbles (multiple unknown excitations). Finally, the identification of random excitations is considered using a plate and a membrane.The ability of photographic techniques to quickly record high volumes of scientific data has been understood since the early 1900s. It is only following the advent of digital imaging and data processing systems that so-called full-field optical measurement techniques became sufficiently reliable for performing high spatial density structural vibration measurements. In particular, the deflectometry technique directly provides a spatially and temporally resolved measurement of slope fields on plane structures using a single high-speed camera. The presentation first demonstrates the principles of this technique and explores some direct applications of such full-field measurements. The advantages and drawbacks of deflectometry are compared with other optical techniques like digital image correlation or scanning laser Doppler vibrometry. A key aspect of several engineering domains consists in the identification of dynamic loads acting on structures by inverse methods. It is shown that coupling deflectometry me...

Drop on Demand Colloidal Suspension Inkjet Patterning for DIC

Digital image correlation (DIC) is a non-contact, optical, full-field displacement measurement technique used to map deformations on a body under an applied load by tracking surface features. It is a widely used method in experimental mechanics, owed in part to its ease of setup, and applicability across length scales and material systems. Most commonly, DIC surface patterns consist of contrasting black and white speckles applied by spray paint or an airbrush, which can lack control and consistency of the distribution of speckle sizes. In this paper, drop on demand (DOD) inkjet printing of colloidal suspensions is introduced as a means to apply a speckle pattern with precision on the micro- to mesoscale. A dot array and pseudo-random DOD pattern are applied to high-density polyethylene (HDPE) specimens, tested in compression following ASTM D695-15, and compared with a traditional airbrush speckle method. The results show that the DOD pseudo-random pattern provides a more consistent strain measurement than the other two methods. The array DOD and airbrush patterns introduce erroneous signals which can be explained on the basis of two different principles. A directional pattern, such as the array DOD, is known to introduce errors in the displacement vector identification. For the random airbrush pattern, a quantification of the speckle sizes revealed a large fraction of speckles that had a characteristic length scale of around 1 pixel, which is known to introduce bias errors, since the sampling of the speckle pattern does not satisfy the Nyquist-Shannon sampling theorem. These findings illustrate the utility of DOD speckling for certain DIC applications.

Material Testing in Support of the Development and Calibration of Material Models for Forming Simulations

Material testing at various strain rates, temperatures and loading conditions provides data that is used for the development and calibration of constitutive equations (material models) that are utilized in numerical simulations of sheet metal forming. In general, the testing can be divided into characterization tests and validation tests. In characterization tests basic material properties (e.g. yield stress, ultimate stress, failure strain) are determined from a test in which a material coupon is loaded under a well-defined condition (stress, strain rate, temperature, etc.). The data is used for determining the values of parameters in plasticity and failure models. In validation tests a material specimen or a small component is loaded with a more complicated, but well defined, loadings. The test is numerically simulated and the calculated quantities (forces, deformation, failure, temperature, etc.) are compared with measurements. The recent development of the Digital Image Correlation (DIC) technique for full-field measurement of deformation has extended the useful data that can be extracted from traditional characterization tests and provide means for developing new experiments that can be used for obtaining more accurate material models. This paper reviews the advantages of using DIC in traditional tests and presents several new recently developed testing configurations for material coupons and small components. Many of these tests have been used during in the development of a failure surface that gives the equivalent plastic strain to failure as a function of stress triaxiality and the Lode parameter.

Enhancing the hydraulic bulge-test using full-field DIC data

The bulge test is an important experimental technique to identify the properties of materials and, in particular, metals. One of the main advantages of such technique, in testing metals, is the possibility of reaching high levels of deformation before fracture. For instance, a tensile test allows to evaluate the hardening curve only up the occurrence of necking, i.e. at the maximum force in the load vs. displacement curve. A bulge test can be performed using mechanical punches or the hydraulic pressure of a fluid. When hydraulic pressure is used, the test is not affected by friction. The hydraulic bulge test (HBT) is therefore a very efficient method to evaluate the properties of metals at large strains. Usually, the outcome of the HBT is the hardening curve in the equi-biaxial stress state that occurs at the top of the dome, during the expansion. In order to obtain the stress value is important to measure the curvature of the dome, this involve a second derivative of the vertical displacement field that can lead to measurement errors, especially if an optical technique is used to obtain the shape of the bulge. If DIC is employed as full-field measurement technique to evaluate the displacement and the strain field of the sheet metal during the HBT, only few measurement points in the center of the dome are actually used in the evaluation of the stress-strain curve. Accordingly, there is a large part of information that is not used in the identification procedure In this paper, the whole full-field measurement is exploited to derive the hardening behavior using an adaptation of the Virtual Fields Method (VFM). Suitable virtual fields are used to write the equilibrium equation in a large zone of the sheet metal during the test. All the strain data measured with DIC in such zone are then used in the identification procedure. The method is illustrated through simulated experiments on stainless steel sheet metals.

Measurement of anisotropic coefficients of thermal expansion of SAC305 solder using surface strains of single grain with arbitrary orientation

The anisotropic coefficient of thermal expansions (CTEs) of SAC305 grain are measured using a full-field in-plane displacement measurement technique. Theoretical relationships among (1) the transversely-isotropic CTEs, (2) the surface strains of a specimen containing a single grain with arbitrary orientation, and (3) the direction of a grain orientation are derived first. Cube shape specimens that contain a single SAC305 grain are fabricated by controlling cooling rates. Thermally-induced displacements fields with a sub-micron resolution are documented on two perpendicular surfaces of the specimen as a function of temperature, and the engineering strains are calculated from the displacement fields. Two directional CTE values are determined from the theoretical relationships. The direction of the c-axis is also obtained during CTE calculations. The validity of the measurement is corroborated by comparing the c-axis direction obtained from the experiment with a grain orientation measured by the electron backscatter diffraction (EBSD) method.

A q-Factor-Based Digital Image Correlation Algorithm (qDIC) for Resolving Finite Deformations with Degenerate Speckle Patterns

Digital image correlation (DIC) has become a widely utilized non-contact, full-field displacement measurement technique for obtaining accurate material kinematics. Despite the significant advances made to date, high resolution reconstruction of finite deformations for images with intrinsically low quality speckle patterns or poor signal-to-noise content has not been fully addressed. In particular, large image distortions imposed by materials undergoing finite deformations create significant challenges for most classical DIC approaches. To address this issue, this paper describes a new open source DIC algorithm (qDIC) that incorporates cross-correlation quality factors (q-factors), which are specifically designed to assess the quality of the reconstructed displacement estimate during the motion reconstruction process. A q-factor provides a robust assessment of the uniqueness and sharpness of the cross-correlation peak, and thus a quantitative estimate of the subset-based displacement measure per given image subset and level of applied deformation. We show that the incorporation of energy- and entropy-based q-factor metrics leads to substantially improved displacement predictions, lower noise floor, and reduced decorrelation even at significant levels of image distortion or poor speckle quality. Furthermore, we show that q-factors can be utilized as a quantitative metric for constructing a hybrid incremental-cumulative displacement correlation scheme for accurately resolving very large homogeneous and inhomogeneous deformations, even in the presence of significant image data loss.

Application of Moiré Interferometry to the Characterization of Orthotropic Materials in the Antisymmetric Configuration using the Virtual Fields Method

Moire interferometry is an effective full-field deformation measurement technique and has been utilized for mechanical behavior analysis of materials and structures. For isotropic materials, Moire patterns can be obtained by performing standard tests, such as, tensile and bending tests, to calculate the displacement and strain. Then, the mechanical properties can be characterized. However, standard tests are not sufficient to characterize the mechanical parameters of anisotropic materials due to the complexity of their material properties. Thus, in this work, Moire interferometry was combined with the Virtual Fields Method to obtain the four in-plane elastic constants (Q11, Q22, Q12, and Q66) of orthotropic materials in the form of a diametrically compressed disk. Firstly, according to finite element method simulation results, optimized parameters can be achieved when the principal direction of the material does not coincide with the loading direction, making the loading configuration antisymmetric. Therefore, Moire interferometry experiment was simulated to demonstrate the feasibility of measurement in the antisymmetric configuration. Finally, the Q11, Q22, Q12 and Q66 values of a unidirectional carbon fiber composite were measured in a real Moire interferometry experiment using the proposed method, yielding results that agreed closely with those obtained using the strain gauges.

Mode I fracture of tropical woods using grid method

The cracking properties in opening mode of three tropicals species from the Gabonese forest, namely Iroko (Milicia Excelsa), Okume (Aucoumea klaineana Pierre), and Padouk (Pterocarpus soyauxii), were investigated in this study. Various tests were performed at room temperature and for two different thicknesses. A full-field measurement technique, namely the grid method, was used in order to obtain the displacement and strain maps near the crack during the tests. The mechanical parameters, the specificities of the wood species, the Arcan system and the grid transfer method are first described. For all the specimens, the initial crack length was oriented along the fiber direction (RL). The instantaneous failure tests were performed using two types of specimens: the modified Compact Tension Shear (CTS) and the Mixed Mode Crack Growth (MMCG) specimens. The images of the grid were recorded and analyzed to obtain the displacement maps. The crack opening, the location of the crack tip as well as the strain fields on the surface of the specimens were deduced from these displacement maps. The experimental critical energy release rate Gc was evaluated by the compliance method in imposed displacement. In the case of instantaneous crack with CTS specimens, the comparison of the initial value of Gc for all the samples show that this quantity strongly depends on the thickness and density of the species under study. Crack propagation was observed for a thickness equal to 20 mm only. The results of the crack propagation with the MMCG specimens show the same dependence, and confirm the quasi-brittle fracture behavior of Iroko. The experimental results are scattered, but clear trends can be observed concerning the fracture parameters of the in previous studies described in the literature on wood species with wood species under study. The results are finally compared with those given densities similar to those studied here.

Experimental investigation of mixed mode fracture of tropical wood material

The process of mixed-mode crack growth of three tropical species of the Gabonese forest, namely iroko (Milicia Excelsa), okume (Pierre Aucoumea klaineana), and padouk (Pterocarpus soyauxii) is studied in this paper. Different tests were carried out at room temperature for different thicknesses. A full-field measurement technique, namely the grid method, was used to obtain both the displacement and deformation maps near the crack tip. The mechanical parameters, the specificities of the wood species, the Arcan system and the grid transfer method are described in the paper. For all the samples, the initial crack was oriented along the fiber direction (RL). Fracture tests were performed using modified Mixed Mode Crack Growth (MMCG) specimen. A thickness of the specimens is 20 mm, and a mixed mode ratio corresponding to 150 were studied. The force-displacement curves, the crack growth process as well as the opening crack were deduced from grid images treated with the grid method. The experimental critical energy release rate Gc was evaluated by the imposed displacement compliance method. The different results show the predominance of mode 1 compared to mode 2. The proportionality of the initial value of the energy release rate with respect to the density of these species is also highlighted.

Inertial Impact Tests to Identify the Plastic Properties of Metals

The behavior and failure mechanisms of materials often change at high strain rates (> 100 1/s) when compared with their quasi-static response. These differences are critical when designing structures or components that will be subjected to impact or blast loads. The recent progress in ultra-high speed imaging and full-field measurement techniques provides a unique opportunity to improve the quality of high strain rate test data. The objective of the current work is to design and validate an experimental technique to identify the elastoplastic material properties of metals. The methodology uses an ultra-high speed camera and the grid method to obtain time-resolved full-field deformation data as impact induced stress waves propagate in a sample. The virtual fields method is then used to inversely identify the plastic properties of the specimen. The results for five aluminum 6082-T6 impacted at 50 m/s are presented.

Review of single-camera stereo-digital image correlation techniques for full-field 3D shape and deformation measurement

Single-camera stereo-digital image correlation (stereo-DIC) techniques have gained increasing attentions and demonstrated excellent prospects in the experimental mechanics community owing to their prominent advantages of cost-effectiveness, compactness, and the avoidance of the complicated camera synchronization. Using additional optical devices, e.g. a diffraction grating, a bi-prism or a set of planar mirrors, pseudo stereo images of a test sample surface can be recorded with a single camera. By correlating these stereo images using DIC, full-field three-dimensional (3D) shape and deformation can be retrieved. This review comprehensively summarizes the historical development, methodologies, strengths and weaknesses of the diffraction grating-based, prism-based, four-mirror-adaptor-based single-camera stereo-DIC techniques, and the recently proposed novel full-frame single color camera-based stereo-DIC technique for full-field 3D shape and deformation measurement. The optical arrangements, principles and calibration procedures of these single-camera stereo-DIC techniques are described in detail. Since high-speed deformation measurement is efficiently achieved by combining the single-camera stereo-DIC with one high-speed camera, single-camera stereo-DIC techniques show great potential in impact engineering, vibration and other dynamic tests.

Accurate 3-D Surface Profilometry Using Novel Boundary Edge Detection on Digital Image Correlation

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%.

Distributed optical sensing in composite laminates

An optical fiber–based full-field strain measurement technique was used to investigate delamination growth in laminated composites. An experimental setup to load the test samples under idealized modes of delamination was used to investigate the ability to capture the shape and location of the delamination front. It is envisioned that the demonstrated approach has significant field applications in controlled laboratory settings where delaminations have to be located accurately. Furthermore, the ability of this measurement system to provide full-field strain measurements at any given pre-implanted location through the thickness overcomes the surface strain measurements obtained by digital image correlation. In order to demonstrate the technique, distributed fiber optic sensing is used to monitor the propagation of delaminations under pure mode I and II loading. Optical fibers were embedded one ply from the crack plane of both double cantilever beam and end notch flexure specimens. To establish a repeatable fabrication methodology, manufacturing techniques for embedding the optical fibers during the laminate layup process were established. Specimens with and without embedded fibers were tested to verify the fibers did not affect measured fracture toughness values. Crack lengths measured with the optical fibers compared well with true crack lengths, and measured strain distributions compared well with results from finite element analysis.

Validation and modeling of aeronautical composite structures subjected to combined loadings: The VERTEX project. Part 1: Experimental setup, FE-DIC instrumentation and procedures

The development and certification of aeronautical composite structures is still largely based on the pyramid of tests. This approach is extremely costly in terms of number of tests and design loops. Moreover, it is based on uniaxial tests whereas the real structures are mostly subjected to combined loadings. The aim of the collaborative research program “VERTEX” is to progress towards Predictive Virtual Testing and to significantly reduce the development costs of aeronautical and space programs. In this first part, the specific methodology for multiaxial tests of aeronautical structures is presented. The concept of the technological specimen and its size are justified. Then, the development of a specific test rig is presented, on which compression/tension, shear, internal pressure and combinations are possible. Since structural tests are complex to instrument, a specific full-field measurement technique is developed. It is based on a multi-camera instrumentation and an original Finite Element approach to Stereo Digital Image Correlation (FE-SDIC). Within such a framework, given that the very same mesh can be used for the simulation and the measurement, the corresponding displacements can be compared directly. In addition, a mechanical regularization of the FE-SDIC measurements allows mechanically consistent fields to be evaluated, such as displacement and rotation fields that could be used as boundary conditions in the simulations. The experimental procedure, the measurement methodologies and the calculation/test dialogue are validated on isotropic metal plates in this paper.

Thermomechanical failure response of notched NiTi coupons

Emerging applications of shape memory alloys (SMAs) functioning as actuators in areas such as the aeronautic and automotive industries, require better understanding of their failure mechanisms and mechanics under combined thermomechanical loading. In this paper we examine the thermomechanically–assisted fracture of double notched NiTi coupons during thermal cycling sweeps under isobaric loading conditions. This loading path is an idealization of typical loading paths that utilize these alloys as actuators. A new high performance full-field measurement technique known as Direct Strain Imaging is used to produce full-field strain measurements that are of both higher accuracy and higher spatial resolution than previously achievable. Failure by the formation of an unstable crack during cooling is observed in the experiments for bias load levels that are a fraction of the isothermal load needed for failure at the beginning of cooling (at a temperature well above austenite–finish temperature). This is an intriguing response, characteristic of SMAs, that from an energetic point of view is in disagreement with the general view of dissipative processes, such as phase transformation, resulting in fracture toughness enhancement.