F.A. Díaz

Integrating fringe projection and digital image correlation for high-quality measurements of shape changes

Abstract. An approach for the measurement of surface displacement fields in three dimensions is presented based on the combination of two-dimensional digital image correlation with fringe projection. Only a single RGB image is required at each deformation state, thereby allowing real-time data acquisition, which is achieved using red speckle and projected blue fringes that are captured in the single image and separated using a Bayer filter. The approach allows both a perpendicular alignment relative to a flat reference surface and self-calibration, i.e., no calibration object is employed. The minimum measurement uncertainty of such a system is found to be 0.0083±0.00239 and 0.0238±0.0068  mm, respectively, for the in-plane and out-of-plane displacements. The potential of the approach is demonstrated for an elastic membrane undergoing large (5 to 20 mm) applied out-of-plane displacements, and the results show no significant difference (<1%) in the measured in-plane displacement fields compared with a commercially available system for stereoscopic digital image correlation.

Application of thermoelastic stress analysis for the experimental evaluation of the effective stress intensity factor

In recent years, the advent of staring array detectors has made Thermoelastic Stress Analysis (TSA) a technique with considerable potential for fatigue and fracture mechanics applications. The technique is noncontacting and provides full field stress maps from the surface of cyclically loaded components. In addition, the technique appears to have a great potential in the evaluation of the effective stress intensity factor range during fatigue since fracture mechanics parameters are derived directly from the temperature changes in the vicinity of the crack tip rather than from remote data. In the current work TSA is presented as a novel methodology for measuring the effective stress intensity factor from the analysis of thermoelastic images. ?K values inferred using TSA have been employed to estimate an equivalent opening/closing load at different R-ratios in a cracked aluminium 2024 CT specimen. Results have been compared with those obtained using the strain-offset technique showing a good level of agreement.

Assessment of effective stress intensity factors using thermoelastic stress analysis

It has been demonstrated that thermoelastic stress analysis (TSA) can provide accurate information about the real crack-driving force for fatigue crack growth. Experiments were conducted using aluminium 2024 compact tension specimens which were initially pre-cracked to different crack lengths at a constant R (R = 0). Subsequently, thermoelastic images were captured at increasing R values from 0 to 0.5 and ΔK values from 3 to 6 MPa m0.5 were calculated. The images showed a dramatic change in the stress pattern ahead of the crack tip as R decreased which was always associated with an increase in phase difference (loss of adiabatic conditions) ahead of the crack tip. This indicates that the technique is able to account for the change in conditions that arise from contact between the crack faces. To support this observation, thermoelastic results have been compared with those obtained using compliance methods. The results showed a very good level of agreement, illustrating the ability of TSA to infer successfully the effective ΔK.

Observations on Photo-Emission and the Process Zone of a Fatigue Crack

Cyclic loading of a solid material induces a cyclic temperature fluctuation. The thermal cycle is asynchronous with the loading cycle when the strains are elastic. This is commonly known as the thermoelastic effect. In differential thermography, a sensitive infrared camera is used to measure this temperature variation; typically of the order of a few tens of milliKelvin. Since the magnitude of the temperature variation is proportional to the dilational strain, values of the surface elastic stress can be derived. Recent advances in infrared camera technology and data processing algorithms have enabled values of effective stress intensity factor ranges and the location of fatigue cracks to be determined from temperature fluctuations around the crack tip on the specimen surface. Careful observation in the region near a crack tip reveals deviations from the asynchronous behavior of the photon signal relative to the load cycle. Phase shifts up to π/8 are commonly observed. The spatial distribution of these phase shifts exhibits characteristic features which are described and discussed. The features in this distribution appear to be associated with high elastic strain gradients, regions of plasticity and contact between the crack faces. Interpretation of the phase shift and its spatial distribution may lead to a better understanding of the mechanics of fatigue crack growth.

Optical low-cost and portable arrangement for full field 3D displacement measurement using a single camera

In the current paper, an optical low-cost system for 3D displacement measurement based on a single camera and 3D digital image correlation is presented. The conventional 3D-DIC set-up based on a two-synchronized-cameras system is compared with a proposed pseudo-stereo portable system that employs a mirror system integrated in a device for a straightforward application achieving a novel handle and flexible device for its use in many scenarios. The proposed optical system splits the image by the camera into two stereo images of the object. In order to validate this new approach and quantify its uncertainty compared to traditional 3D-DIC systems, solid rigid in and out-of-plane displacements experiments have been performed and analyzed. The differences between both systems have been studied employing an image decomposition technique which performs a full image comparison. Therefore, results of all field of view are compared with those using a stereoscopy system and 3D-DIC, discussing the accurate results obtained with the proposed device not having influence any distortion or aberration produced by the mirrors. Finally, the adaptability of the proposed system and its accuracy has been tested performing quasi-static and dynamic experiments using a silicon specimen under high deformation. Results have been compared and validated with those obtained from a conventional stereoscopy system showing an excellent level of agreement.

Experimental evaluation of plasticity-induced crack shielding from isochromatic data

Abstract. The plasticity-induced crack shielding effect is evaluated during fatigue crack growth using transmission photoelasticity. The proposed methodology is based on the evaluation of the stress intensity factors calculated from the analysis of the isochromatic fringe patterns observed at the vicinity of a crack tip. Four different mathematical models describing the crack tip stress fields (namely models based on Westergaard’s, Williams’s, and Muskhelishvili’s equations and a new model called Christopher–James–Patterson) have been employed. Thus, a comparative study to evaluate which of the models is more suitable for fatigue crack shielding evaluation has been performed. A set of fatigue experiments on polycarbonate middle-tension specimens at different R-ratios have been conducted. Experimental results reveal the presence of plasticity-induced crack shielding on growing fatigue cracks for specimens tested at a low R-ratio. In addition, a retardation effect on the fatigue growth rate has been observed due to the shielding effect induced by the plasticity generated both at the crack tip and along the crack flanks. All these results highlight the enormous potential of transmission photoelasticity for the evaluation of plasticity-induced crack shielding on growing fatigue cracks.

A Validation Approach for Quasistatic Numerical/Experimental Indentation Analysis in Soft Materials Using 3D Digital Image Correlation

A quasistatic indentation numerical analysis in a round section specimen made of soft material has been performed and validated with a full field experimental technique, i.e., Digital Image Correlation 3D. The contact experiment specifically consisted of loading a 25 mm diameter rubber cylinder of up to a 5 mm indentation and then unloading. Experimental strains fields measured at the surface of the specimen during the experiment were compared with those obtained by performing two numerical analyses employing two different hyperplastic material models. The comparison was performed using an Image Decomposition new methodology that makes a direct comparison of full-field data independently of their scale or orientation possible. Numerical results show a good level of agreement with those measured during the experiments. However, since image decomposition allows for the differences to be quantified, it was observed that one of the adopted material models reproduces lower differences compared to experimental results.

Experimental evaluation of plasticity-induced crack shielding from crack tip displacements fields

In this work it is proposed a methodology for the evaluation of plasticity-induced crack shielding from the analysis of the crack tip displacements fields measured by digital image correlation. This methodology is based on the evaluation of the stress intensity factors determined from the displacements fields measured at the vicinity of the tip of a growing fatigue crack. For the characterisation of the crack tip displacements field, CJP model has been implemented. This model considers the shielding effects due to plasticity generated during fatigue crack growth. For the purpose of the current work, several fatigue experiments at different R-ratios have been conducted on Al2024-T3 compact tension specimens. In addition, compliance based methods have been adopted to perform a comparison of the results with those obtained by DIC. Results show a good level of agreement, illustrating the enormous potential of DIC technique for the study of fracture mechanics problems.

Modal Identification in an Automotive Multi-Component System Using HS 3D-DIC

The modal characterization of automotive lighting systems becomes difficult using sensors due to the light weight of the elements which compose the component as well as the intricate access to allocate them. In experimental modal analysis, high speed 3D digital image correlation (HS 3D-DIC) is attracting the attention since it provides full-field contactless measurements of 3D displacements as main advantage over other techniques. Different methodologies have been published that perform modal identification, i.e., natural frequencies, damping ratios, and mode shapes using the full-field information. In this work, experimental modal analysis has been performed in a multi-component automotive lighting system using HS 3D-DIC. Base motion excitation was applied to simulate operating conditions. A recently validated methodology has been employed for modal identification using transmissibility functions, i.e., the transfer functions from base motion tests. Results make it possible to identify local and global behavior of the different elements of injected polymeric and metallic materials.

Modal Parameters Evaluation in a Full-Scale Aircraft Demonstrator under Different Environmental Conditions Using HS 3D-DIC

In real aircraft structures the comfort and the occupational performance of crewmembers and passengers are affected by the presence of noise. In this sense, special attention is focused on mechanical and material design for isolation and vibration control. Experimental characterization and, in particular, experimental modal analysis, provides information for adequate cabin noise control. Traditional sensors employed in the aircraft industry for this purpose are invasive and provide a low spatial resolution. This paper presents a methodology for experimental modal characterization of a front fuselage full-scale demonstrator using high-speed 3D digital image correlation, which is non-invasive, ensuring that the structural response is unperturbed by the instrumentation mass. Specifically, full-field measurements on the passenger window area were conducted when the structure was excited using an electrodynamic shaker. The spectral analysis of the measured time-domain displacements made it possible to identify natural frequencies and full-field operational deflection shapes. Changes in the modal parameters due to cabin pressurization and the behavior of different local structural modifications were assessed using this methodology. The proposed full-field methodology allowed the characterization of relevant dynamic response patterns, complementing the capabilities provided by accelerometers.