Full-field Measurement Technique Research Articles

Strain Determination Using a Global Interpolation Concept Based on Coherence Scanning Interferometry Measurements

Abstract Background The experimental detection of small and large strains requires special approaches of full-field measurement techniques and their evaluation on 3D curved surfaces of components. Objectives Since classical digital image correlation methods have difficulties with the application of paints in some applications, one aim is to use a method in which the surface roughness is used to apply the strain calculation. Methods In this paper, 2D digital image correlation is applied to 2D intensity maps extracted from a coherence scanning interferometer together with height information. Height information are used to reconstruct the 3D motion of tracked material points. Surface interpolation and strain calculation are performed using globally formulated radial basis functions. Results The entire procedure leads to an appropriate technique for determining the in-plane strains in curved surfaces of parts, whereas the expected accuracy for various levels of the radial basis functions are discussed in detail. Conclusions Particularly, coherence scanning interferometry yields highly accurate height information. To smooth the surface motion, it turns out that in particular a regression analysis is required, where we apply radial basis functions with various approximation levels. This is an alternative procedure for surface strain determination.

A Novel Simulation Method for 3D Digital-Image Correlation: Combining Virtual Stereo Vision and Image Super-Resolution Reconstruction.

3D digital-image correlation (3D-DIC) is a non-contact optical technique for full-field shape, displacement, and deformation measurement. Given the high experimental hardware costs associated with 3D-DIC, the development of high-fidelity 3D-DIC simulations holds significant value. However, existing research on 3D-DIC simulation was mainly carried out through the generation of random speckle images. This study innovatively proposes a complete 3D-DIC simulation method involving optical simulation and mechanical simulation and integrating 3D-DIC, virtual stereo vision, and image super-resolution reconstruction technology. Virtual stereo vision can reduce hardware costs and eliminate camera-synchronization errors. Image super-resolution reconstruction can compensate for the decrease in precision caused by image-resolution loss. An array of software tools such as ANSYS SPEOS 2024R1, ZEMAX 2024R1, MECHANICAL 2024R1, and MULTIDIC v1.1.0 are used to implement this simulation. Measurement systems based on stereo vision and virtual stereo vision were built and tested for use in 3D-DIC. The results of the simulation experiment show that when the synchronization error of the basic stereo-vision system (BSS) is within 10-3 time steps, the reconstruction error is within 0.005 mm and the accuracy of the virtual stereo-vision system is between the BSS's synchronization error of 10-7 and 10-6 time steps. In addition, after image super-resolution reconstruction technology is applied, the reconstruction error will be reduced to within 0.002 mm. The simulation method proposed in this study can provide a novel research path for existing researchers in the field while also offering the opportunity for researchers without access to costly hardware to participate in related research.

A new approach for fracture process zone evaluation

The study presents a novel method for identifying and monitoring the fracture process zone and its impact on cracking parameters, such as damage rate and fracture energy. Concrete samples subjected to a Wedge Splitting Test were investigated. Digital Image Correlation, a full-field measurement technique, was utilized to measure the experimental displacement fields at various scales. The experimental measurements, optimized through a Newton-Raphson iterative adjustment procedure, revealed the presence of a damaged zone accompanying the cracking process. This analysis also enabled the estimation of the material damage rate and provided a more realistic assessment of cracking parameters by integrating the entire process of damage and macro-cracking into the calculation of fracture energy. The results suggest that the appearance of a fracture process zone correlates with a reduction in the material’s hardness downstream of the crack propagation front.

Structural intensity in thin plates and beams under transient and stationary forces using time-resolved, full-field optical slope measurements

Structural intensity, or structure-borne intensity or power flow, was introduced in the 1970s and implemented at that time using contact sensors like accelerometers. Point and contactless measurement methods removed added mass and sensor size constraints. Time-resolved and full-field vibration measurement techniques now open new perspectives for the estimation of structural intensity, but these prospects also come with many challenges. In this work, we study the applicability of optical deflectometry measurements to calculate the structural intensity in thin plates or beams under transient and stationary loadings. Since deflectometry directly provides slope fields, a slope formulation of structural intensity is proposed and evaluated. The effect of various parameters like frame rate (sampling frequency) and spatial resolution is studied from a numerical point of view on a simply-supported plate. Experimental results are then presented on a simply-supported plate and on a cantilever beam. Numerical and experimental results allow identifying the main challenges related to the implementation of structure intensity estimation using full-field and time-resolved data.

Mapping the defect acceptance for dynamically loaded components by exploiting DIC-based full-field receptances

Potential defect acceptance can be seen dependable upon the mapping of effective strains on the whole surface, due to dynamic loading of the components as they are mounted. With proper constitutive models and loading spectra, the experiment-based mapping of the equivalent stresses can be achieved from optical full-field receptances in an extremely dense grid, also for lightweight parts, without inertia distortions thanks to contactless measurements. Fatigue spectral methods turn this mapping into components’ life distributions, for a clear assessment of the material’s utilisation: a risk grading mapping for potential defects can be formulated in the area of inquiry in order to discriminate between safe and dangerous locations. By following this experiment-based approach, potential defects in exercise and production might be tolerated in safer locations, under the chosen dynamic task, with great savings in costs and maintenance. Among the image-based full-field measurement techniques, Hi-Speed DIC has proved to work in many environments, to be able to estimate full-field receptances of real components in their effective assembling and loading conditions, in different cases of industrial interest. The quality achieved by DIC in the receptance maps helps in numerically deriving the strain FRFs on the sensed surface. Extended scenarios – with newly modelled coloured noises for an advanced excitation definition from two shakers – and a vibrating rectangular plate, as real mounted component, are highlighted in details to prove the effectiveness of DIC-based risk index mapping under defect acceptance criteria.

TSA and FEM Analysis Applied to CAD/CAM Titanium All-on-Four Prosthesis

Patient with fully edentulous maxillaries often suffer of several disease as an alteration of facial appearance, difficulties in eating, speech, and chews. Fixed and removable prosthesis can be a solution to rehabilitate this issue. The All-on-Four protocol is one of the possible treatment modalities for implant-supported prosthetics. It consists of placing four implants in an intermorainal position, two perpendicular to the occlusal plane and two tilted distally at a 45° angle. This paper aims to assess the stress distribution on an All-on-Four titanium prosthesis by means of a Thermoelastic Stress Analysis (TSA). This full-field non-contact measurement technique allows, through a thermal imaging camera, to observe how the surface temperature of an object changes under the effect of cyclic load. The principle on which thermoelastic analysis is based is the existence of a relationship between deformation, hence applied stress, and temperature change, called the thermoelastic effect. To perform the tests, the prosthesis was fixed to a resin jaw cast that has similar characteristics to the human bone, so that the same damping that would be present under normal working conditions could be accounted for. The distal artificial molar tooth was kept in contact to a narrow screw, which was attached to a Medium-Force (M-Series) Air-Cooled Shakers made by Santek. This shaker generates a sinusoidal load of 800N with a load cell at 10 Hz frequency, which can maintain the temperature variation of the prosthesis constant. A pre-load was applied before starting the tests. To see the change in temperature a Flir A6751SC thermal camera was used, and videos were acquired with a frame rate of 125 Hz and a resolution of 640x512 pixels. From the experimental TSA tests, the trend of the experimental stress concentration on the titanium specimen was detected. The outcomes of the experimental tests were compared to a 3D prosthesis model by Finite Element Analysis (FEM).

Measuring 3D shape and deformation in the presence of extremely strong ambient light and thermal radiation with a single time-gated camera.

Monochromatic light-illuminated active-imaging stereo-digital image correlation (stereo-DIC) has been extensively used for measuring the surface deformation of materials and structures at elevated temperatures. Despite the improvements in the image acquisition techniques or devices, it is still challenging to measure the 3D deformation of materials and structures in the presence of strong, time-varying ambient light and thermal radiation. In this study, we present what we believe to be a novel dual-filtering single-camera stereo-DIC technique for full-field 3D high-temperature deformation measurement, even in the case of extremely intense ambient light and thermal radiation. In contrast to conventional active-imaging stereo-DIC that only suppresses the thermal radiations in the spectral domain, the proposed technique utilized a dual-filtering strategy (i.e., narrow bandpass optical filtering and ultrashort exposing) to suppress the strong ambient light and thermal radiation in both time and spectral domains. Besides, a four-mirror adapter is adopted to realize 3D shape and deformation measurement using a compact single time-gated camera. Experimental verifications, including assessments with laboratory experiments and validations on real thermal deformation tests under transient aerodynamic heating and direct ohmic heating, convincingly demonstrated that the proposed single-camera dual-filtering stereo-DIC method can achieve accurate 3D shape, motion and deformation measurement, even with strong light and thermal radiation from the quartz lamps and the heated sample.

Equivalent distortion imaging model-based calibration method for a projection moiré system using combined lenses

The projection moiré method is a noncontact full-field optical topography measurement technique, which requires a precise model and corresponding calibration method; however, the existing models and calibration methods are either incompatible with system distortion or involve complex experimental operations. This study presents an equivalent distortion imaging model coupled with a flexible calibration method that overcomes such shortcomings. The proposed model is constructed by considering the stacking effect of the distortions of the two lenses and the misalignment between the optical axes. The corresponding calibration method leverages the existing camera calibration method to avoid complex operations. The proposed model has high precision owing to its ability to describe the complex imaging of the system. Moreover, the calibration is flexible and requires far fewer operations than with the two-stage calibration method. Both simulations and experiments verify the effectiveness of the proposed method.

Review of research progress and development trend of digital image correlation

PurposeThe purpose of this paper is to provide a comprehensive review of a non-contact full-field optical measurement technique known as digital image correlation (DIC).Design/methodology/approachThe approach of this review paper is to introduce the research pertaining to DIC. It comprehensively covers crucial facets including its principles, historical development, core challenges, current research status and practical applications. Additionally, it delves into unresolved issues and outlines future research objectives.FindingsThe findings of this review encompass essential aspects of DIC, including core issues like the subpixel registration algorithm, camera calibration, measurement of surface deformation in 3D complex structures and applications in ultra-high-temperature settings. Additionally, the review presents the prevailing strategies for addressing these challenges, the most recent advancements in DIC applications across quasi-static, dynamic, ultra-high-temperature, large-scale and micro-scale engineering domains, along with key directions for future research endeavors.Originality/valueThis review holds a substantial value as it furnishes a comprehensive and in-depth introduction to DIC, while also spotlighting its prospective applications.

Experimental assessment of the functional fatigue in biocompatible Ti67Zr19Nb11.5Sn2.5 shape memory alloy in the vicinity of drilled holes

Shape memory alloys (SMA) exhibit desirable and unique functionalities that are suitable for a plethora of applications in the automotive, aerospace, and biomedical industries. The TiNbZrSn SMA has been specifically developed for biomedical applications owing to its enhanced biocompatibility compared to the widely available and heavily investigated NiTi-based SMA. In this work, the effects of mechanical rolling, heat treatment, and grain size on the functional fatigue properties of TiNbZrSn are investigated under cyclic loading conditions. Superelasticity was observed for samples rolled to reduction levels exceeding 90 %. However, the recovery magnitudes improved significantly following 96.6 % rolling due to the optimization of the grain size. The reduction in grain size, from about 400 μm in the homogenized conditions to 50 μm for the 96.6 % rolled state, increases slip resistance which consequently promotes superelasticity. The functional superelastic properties were also evaluated for samples with geometric stress concentration (drilled hole). Using full-field measurement techniques, the applied, recoverable, irreversible strains, and their evolution under cyclic loading conditions were carefully measured. The use of virtual extensometers located in multiple locations around the geometric stress concentration provides means to not only evaluate the functional degradation of local superelastic properties, but also the transition to structural damage through the initiation and propagation of fatigue cracks.

Key performance indicators for heterogeneous mechanical tests

Characterizing sheet metal behavior is crucial for accurately simulating sheet metal forming processes. Typically, material parameters are obtained through quasi-homogeneous mechanical tests, which makes the mechanical testing task expensive and time-consuming. However, newly proposed mechanical testing approaches, known as heterogeneous tests, are outperforming quasi-homogeneous mechanical tests. Full-field measurement techniques, such as Digital Image Correlation, are used to extract this information. Despite several proposed test designs, selecting the best one to characterize a specific complex material behavior remains challenging. This work aims to apply Key Performance Indicators (KPIs) in a robust and straightforward way to rank mechanical tests, based on their potential to improve the material behavior characterization procedure. Scalar indicators are suggested to quantitatively evaluate important features of each test, from the diversity of strain states to the test’s sensitivity to the material model parameters. Additionally, measurement errors associated with Digital Image Correlation are taken into account via a numerical procedure. For validation purposes, five recently developed advanced mechanical tests are chosen to be analyzed. The performance of each test is evaluated numerically, leading to an overall ranking of these heterogeneous mechanical tests.

Mechanical behavior analysis using small-size graphite disc compression testing with digital image correlation methods

The ASTM InternationalStandard Test Method for Tensile Stress-Strain of Carbon and Graphite (ASTM C749) requires dog-bone shaped specimens. Unfortunately, specimen sizes required in ASTM C749 are much too large for irradiation within a test reactor and are incompatible with irradiation capsule volumes, molten salt degradation facilities, or oxidation apparatus. The ASTM InternationalStandard Test Method for Tensile Strength Estimate by Disc Compression of Manufactured Graphite (ASTM D8289) was developed as an alternative that provides a means for testing tensile properties using smaller specimens when geometric constraints are present. However, the important stress–strain relation throughout the test loading history that could be recorded using ASTM C749 is lost by using ASTM D8289 because of the difficulties involved with using traditional strain gauges or extensometers on such a small specimen. In this work, a digital image correlation (DIC) method, which uses a full-field noncontact surface displacement measurement technique, was applied along with the ASTM D8289 splitting tensile test on eight small samples of graphite grade Mersen 2114 to track their deformation during compressive loading. Initial results showed that the DIC technique can measure the surface displacement/strain on these small (Ø6 × 3 mm) graphite specimens with high accuracy and good repeatability, indicating that the DIC technique, combined with the small-disc splitting tensile test in ASTM D8289, can capture the strain evolution in the test. Further comparison of measured strains with analytical and numerical simulation results provides additional understanding of the mechanical behaviour of this nuclear graphite material in complex stress states.

Smooth mode decomposition: Theory and its applications in full-field output-only modal analysis

Full-field output-only modal analysis (OMA) became popular due to its superior spatiotemporal resolution over traditional sensing methods. It provides rich spatial dynamic information, which is essential for high-fidelity modal models, model updating, and non-intrusive health monitoring. However, the full-field sensory techniques implementation in OMA is hurdled by the excessive storage and computational requirements and the temporal aliasing effect. This paper develops a novel modal decomposition method, called smooth mode decomposition (SMD), to tackle these hurdles by exploiting the spatial dynamics and smoothness of the observed field measurements. SMD imposes the smoothness constraint on the identified mode shapes to resolve a set of orthogonal modes, called the smooth orthogonal modes (SOMs), that maximally represent the original field while also keeping the mode shape as smooth as possible. Since the SMD seeks spatially-smooth modal structures, the requirements on the temporal sampling can be relaxed. Hence, the SMD can be applied to nonuniformly and sub-Nyquist sampled full-field measurement data. Moreover, the SOMs converge to linear normal modes of uniformly distributed parameter systems, which provides a physically meaningful decomposition of the measurement data. Through a series of numerical examples, the SMD is compared to other popular covariance-based OMA methods. These examples demonstrate the merit of the proposed method when the vibration system has nonuniform system parameters and when the data is undersampled. Further, a full-field strain measurement technique, the three-dimensional digital image correlation, is utilized where spatiotemporal displacement measurements are obtained from a vibrating cantilever beam. The SMD is shown to provide accurate mode estimates with undersampled experimental data. Statistically, SMD shows its flexibility and quality in modal identification using temporally randomly captured images, which opens an avenue for alleviating storage and processing requirements for image-based full-field OMA.

Creep damage parameter extraction from ex-service 12% Cr steel using digital image correlation computed strain data

Several continuum damage mechanics (CDM) modelling approaches for predicting creep deformation of tempered ferritic steels have been developed in the literature, which have evolved from efforts to extend the operability of power plant components. Few of these models, however, focus on damage assessment of ex-service states of power plant steels through the extraction of damage parameters. Furthermore, few CDM approaches leverage the high density of creep curve data available through full-field strain measurement techniques such as digital image correlation (DIC). This work uses multiple creep curves obtained from DIC computed strain data at several stresses and temperatures from individual specimens of X20CrMoV12-1 (X20) piping steel. These curves serve as input data to a modified Oruganti continuum damage mechanics (CDM) model whereby microstructural-specific damage parameters can be extracted. Good agreement is noted between CDM-extracted parameters and microstructural, creep cavity density and hardness damage indicators. Damage parameters based on subgrain growth are particularly sensitive to the ex-service state of the X20 steel. The proposed CDM approach using DIC computed creep curves is shown to be a material efficient alternative to traditional damage assessment methods of ex-service material.

Comparison of three full-field optical measurement techniques applied to vibration analysis

Digital image correlation, deflectometry and digital holography are some of the full-field optical measurement techniques that have matured in recent years. Their use in vibroacoustic applications is gaining attention and there is a need for cataloging their performance in order to provide, to a broad community of users and potential future users, quantitative and qualitative evaluations of these three approaches. This paper presents an experimental comparison of the three optical methods in the context of vibration measurements, along with classical reference measurements provided by an accelerometer and a laser Doppler vibrometer. The study is carried out on two mechanical structures exhibiting various vibration responses when submitted to an impact.

Constitutive behaviour of masonry prisms using a full-field measurement technique

To obtain analytical models for the compressive behaviour of masonry structures, it is crucial to accurately measure the compressive strengths of its constituents, i.e., brick and mortar. The mechanical properties and the stress-strain relationship under uniaxial compression are essential for thoroughly investigating and evaluating masonry structures. The present work aims to validate a non-contact measuring technique for obtaining full-scale strain and failure patterns and investigate joint mortar composition's effect on masonry behaviour. Forty masonry prisms were manufactured in this study using two distinct types of brick and five different types of mortar varying according to cement and lime contents. The results showed that 2D Digital Image Correlation (DIC) could generate full-strain assessments of masonry prisms, resulting in a full-scale stress-strain analysis. It was also found that, in the post-peak region, prisms with lower cement content exhibit a more ductile behaviour with strain localisation at the joint mortar level. Based on current experimental results and data from the literature, new analytical relationships were presented to predict masonry mechanical properties and masonry complete stress-strain compressive behaviour.

On the design of mechanical heterogeneous specimens using multilevel topology optimization

Sheet metal forming processes are of upmost importance in many industrial applications. Their virtualization has been addressed by numerical simulation software. However, realistic results can only be obtained when an accurate reproduction of the material behavior is achieved. This is where material characterization and model calibration procedures play a key role. The conventional procedure is time-consuming and expensive and needs to be overcome. To improve the efficiency of this process, heterogeneous mechanical tests have been proposed. Due to their rich mechanical fields and the emergence of full-field measurement techniques, a high diversity of valuable information about the material behavior can be extracted with a single test. A systematic design approach has not been found yet since the majority of the newly proposed mechanical tests have been developed based on the empirical knowledge or trial and error. However, this work aims at filling this gap by proposing a new methodology to design optimal mechanical tests. A bi-level design optimization problem is established to find out the optimal geometry for the specimen that presents the highest heterogeneity of stress states. The specimen design process gathers the design by topology optimization and the compliant mechanisms’ theory, which are responsible for the highly heterogeneous displacement fields. The test setup is also optimized, considering the use of universal testing machines. Several numerical designs are analyzed based on their ability to induce several stress states and their feasibility. The most suitable test configuration is chosen and analyzed with Digital Image Correlation (DIC) using synthetic images. An evaluation of the test in an elastoplasticity framework is also performed. The effectiveness of the methodology in designing tests with valuable information for the material characterization process is validated.

Experimental characterization and numerical modelling of the translaminar fracture of woven-ply hybrid fibers reinforced thermoplastic laminates

The understanding of fracture mechanisms and the assessment of fracture toughness are key factors to the design of high-performance composite structures to be used in aeronautics. Thus, this work addresses the influence of initial notch orientation on the translaminar fracture of woven-ply hybrid fibers reinforced thermoplastic polyether ether ketone (PEEK) laminates. The translaminar fracture of Single-edge-notch bending (SENB) specimens is experimentally characterized depending on two initial notches (0° and 45°). Such geometry results in a complex stress state within the laminates plies as well as simultaneous tension/compression failures. A full-field measurement technique has been implemented to monitor the crack initiation and growth on the specimen surface during mechanical loading. To better understand the role played by the initial notch orientation as well as the plies orientation contribution to fracture behavior, a specific Finite Element mesoscale model was built to account for the deformation mechanisms (namely local plasticity) and the different damage behaviours (fiber breakage in tension and compression, kinking/crushing in compression, delamination) occurring within the plies of quasi-isotropic laminates. Linear elastic fracture mechanics concepts have been applied to quantify the critical translaminar fracture toughness (about 40 kJ/m2 in both cases). Finally, this study provides useful information on the fracture toughness values for engineers willing to design thermoplastic-based composite parts with stress concentrators.

A full-field DIC analysis of the mechanical-deformation behavior of polyethylene terephthalate glycol (PET-G) aligners

ObjectivesThe aim was to investigate the full-field mechanical-deformation behavior of clear aligners made by polyethylene terephthalate glycol (PET-G) subjected to cyclic compression tests. MethodsDigital Image Correlation (DIC) (Chu et al., 1985; Schreier et al., 2009), a contactless full-field measurement technique, and Optical Microscope (OM) analysis were applied to study two PET-G aligners thermoformed from discs of a thickness of 0.75 mm and 0.88 mm. The clear aligners were placed on dental shape resin casts and were subjected to cyclic compression up to 13000 load cycles from 0 to 50 N at room temperature. The chosen number of load cycles simulates the average load history to which an aligner is subjected for one week. Local displacements and strains were measured for each test at 2, 6, 10, 20, 1000, 5000 and 13000 loading cycles. ResultsBoth aligners showed greater displacements in the early stages of loading, more pronounced for the 0.88 mm one. Local displacement and strain maps are derived both along the load direction and to the transverse one (never done from other researchers). Load-displacement cyclic curves allowed to evaluate the evolution of the stored energy and the stiffness during the test. The OM analyses showed significant morphological variations on the aligners’ surface, such as wear and tear, high depressions and cracks, especially for the 0.75 mm specimen. SignificanceFull-field analysis allowed to understand the mechanical behavior of device with complex geometry and complex load distribution, like invisible aligners. The knowledge of the stiffness and the direction of the total displacement helps the orthodontist to implement the best strategy to improve the patient's comfort and the treatment time.

Passive control of 3D adaptive shock control bumps using a sealed cavity

This paper presents a Fluid–Structure-Interaction study of a novel passive adaptive shock control bump concept. A flexible plate, clamped on all sides and placed above a sealed cavity, was tested beneath a Mach 1.4 normal shock in the Imperial College London supersonic wind tunnel. The plate was actuated into the shape of a 3D shock control bump by passively controlling the cavity pressure through an array of breather holes. Preliminary experiments were performed with active control of cavity pressure (via a vacuum tank) at Mach 1.4 and 2 to illustrate the potential of this concept. Full-field surface measurement techniques, namely photogrammetry and pressure sensitive paint, were employed in addition to static pressure tappings and schlieren photography. Results confirmed that cavity pressure plays a key role in determining the aerostructural behaviour of the flexible plate. In addition, it was found that carefully placed breather holes allowed the plate to deform into a 3D shock control bump when a shock was on the flexible region and remain flat otherwise. This shows significant potential for improving the off-design behaviour of adaptive shock control bumps.