Showing papers in "Strain in 2018"

DVC‐based image subtraction to detect microcracking in lightweight concrete

Abstract: This paper presents an image subtraction technique based on Digital Volume Correlation (DVC) to detect and extract the complex network of microcracks that progressively developed in a lightweight concrete sample submitted in situ to uniaxial compression and imaged by X-ray computed tomography (XRCT). From local DVC measurements, performed only on positions with sufficient image contrast, the mechanical transformation is estimated at all voxels within the whole sample using an adjusted interpolation procedure that computes an affine approximation of the local transformation. The deformed image (containing cracks) is thus transformed back to the same frame as the reference image (without cracks) to compute the difference between both images, taking into account possible brightness and contrast adjustments. The resulting subtracted image reveals the path of cracks which is clearly visible without the underlying heterogeneous microstructure of the concrete. The detection accuracy is here estimated to one-tenth of a voxel, allowing early age cracks to be detected while they would barely have been noticed on the XRCT images. Segmentation of the crack network is also made much easier. To overcome a low signal-to-noise ratio for the tiniest cracks, a Hessian based filter is used to extract the complex crack network. The cracks can be directly located in the microstructure segmented in the reference image and compared for all loading steps to characterise their initiation and propagation.

Strain‐induced crystallization in rubber: A new measurement technique

Abstract: The crystallinity of stretched crystallizable rubbers is classically investigated using X-ray diffraction. In this study, we propose a new method based on temperature measurement and quantitative calorimetry to determine rubber crystallinity during mechanical tests. For that purpose, heat power density are first determined from temperature variation measurements and the heat diffusion equation. The increase in temperature due to strain-induced crystallization is then deduced from the heat power density by subtracting the part due to elastic couplings. The heat capacity, the density, and the enthalpy of fusion are finally used to calculate the crystallinity from the temperature variations due to strain-induced crystallization. The characterization of the stress–strain relationship and the non-entropic contributions to rubber elasticity is not required. This alternative crystallinity measurement method is therefore a user-friendly measurement technique, which is well adapted in most of the mechanical tests carried out with conventional testing machines. It opens numerous perspectives in terms of high speed and full crystallinity field measurements.

Damage detection in a laboratory wind turbine blade using techniques of ultrasonic NDT and SHM

Abstract: This paper presents a case study in the use of ultrasonic NDE/SHM techniques for detecting and locating damage in a real (but small) wind turbine blade. Two techniques are considered: (1) non-linear acoustics, and (2) guided-wave “pitch-catch” SHM. While the non-linear acoustics approach proved disappointingly insensitive to damage induced experimentally in the blade, the guided-wave approach not only detected the damage but also proved capable of locating it, using a “network of novelty detectors” methodology. A first, slightly ill-conceived, programme of guided-wave tests actually provided valuable insight into attenuation of waves in the structure of interest and supported the idea that actuator-sensor networks of a feasible density could be used for wind turbine blade SHM.

Damage detection in beams from modal and wavelet analysis using a stationary roving mass and noise estimation

Abstract: Consejeria de Economia, Innovacion, Ciencia y Empleo, Junta de Andalucia. Grant Number: P12-TEP-2546

Investigating the static and dynamic tensile mechanical behaviour of polymer‐bonded explosives

Abstract: The tensile properties of a polymer-bonded explosive (PBX) were systematically studied by using quasi-static and dynamic experiments. A non-linear constitutive relation was developed to describe the tensile behaviour of the PBX. The tensile properties of the PBX under different strain rates and temperatures were measured in quasi-static tests. The tensile behaviour of the PBX was found to exhibit high strain rate and strong temperature dependence, attributable to the large fraction of the polymer binder. To obtain the rational dynamic tensile results, a modified split Hopkinson tensile bar (SHTB) setup was designed such that the specimens were in dynamic stress equilibrium and deformed homogeneously at nearly constant strain rates. To characterise the viscoelastic behaviour, the master modulus curve was derived from the tensile stress relaxation tests at different temperatures. The non-linear constitutive model was implemented in ABAQUS to predict the tensile behaviour of the PBX. The computational results were found to be in good agreement with the experimental results.

Validation of a multimodal set‐up for the study of zirconium alloys claddings' behaviour under simulated LOCA conditions

Abstract: In a previous paper, Campello et al. presented a combined experimental/numerical approach to identify the creep behaviour of as‐fabricated Zircaloy‐4 claddings under simulated LOCA conditions. The current paper deals with the uncertainties and errors estimation of the two key methods used to measure the thermal and kinematic full fields during the creep tests: near infrared thermography (NIRT) and two‐dimensional digital image correlation (2D‐DIC).The NIRT uncertainties are evaluated as 0.7% of the actual temperature. They are mainly due to the thermocouple measurements used to calibrate the radiometric model of the NIRT.A combined 2D‐DIC/edge detection approach is proposed to quantify the error related to 2D‐DIC when measuring the ballooning of the tubular specimen. The 2D‐DIC error is evaluated as 0.1% of the actual equivalent strain even for ballooning inducing a radius increase of 20%.

A parametric study of DIC measurement uncertainties on cracked metals

Abstract: Displacements and strains in the presence of a crack in welded steels have been studied using digital image correlation (DIC) under zero load and selected loading conditions. A parametric study has been carried out to assess the effects of some of the key parameters, including subset size, step size and the size of the measurement window, on the measurement uncertainties for systematic and random errors. The assessments were carried out a region around the crack tip, where high strain gradients and inhomogeneous deformation present significant challenges for DIC measurements.

Strain measurements on compliant knitted mesh used in space antennas, by means of 2D Fourier analysis

Abstract: The paper presents a strain measurement technique to be applied on repetitive knitted nets. The application of the procedure is focused on large deployable reflectors used in space applications. Manufacturing of these structures involves knitting of thin metallic wires. Besides space antennas, the method herein developed is advantageous for many other types of knitted tissues such as elastic coatings, elastic bandages for medical applications, or any other woven fabric requiring a defined elastic behaviour. All of them display an almost regular repetitive pattern whose variations due to imposed loads are the core of the method. Deformation of such kind of structural elements must be analysed in two dimensions, because the knitting causes an accentuated orthotropic behaviour. Strain measurement is tricky and requires special computational techniques. Indeed, the application of standard experimental methods and indirect strain determination, through well-established image analysis algorithm, can be totally unsuccessful or unsuitable because of the lack of continuity of these structures. The paper presents a method based on two-dimensional Fourier analysis. Numerical examples, supported by some experimental evidences, confirm the feasibility of the technique. Furthermore, a newly proposed frequency zooming algorithm, based on fast Fourier transform, makes the computational costs of the indirect strain measurement affordable.

Visualisation of deformation gradients in structural steel by macroscopic magnetic domain distribution imaging (Bitter technique)

Abstract: While classically used to visualise the magnetic microstructure of functional materials (e.g., for magnetic applications), in this study, the Bitter technique was applied for the first time to visualise macroscopic deformation gradients in a polycrystalline low-carbon steel. Spherical indentation was chosen to produce a multiaxial elastic–plastic deformation state. After removing the residual imprint, the Bitter technique was applied, and macroscopic contrast differences were captured in optical microscopy. To verify this novel characterisation technique, characteristic “hemispherical” deformation zones evolving during indentation were identified using an analytical model from the field of contact mechanics. In addition, near-surface residual stresses were determined experimentally using synchrotron radiation diffraction. It is established that the magnetic domain distribution contrast provides deformation-related information: regions of different domain wall densities correspond to different “hemispherical” deformation zones (i.e., to hydrostatic core, plastic zone and elastic zone, respectively). Moreover, the transitions between these three zones correlate with characteristic features of the residual stress profiles (sign changes in the radial and local extrema in the hoop stress). These results indicate the potential of magnetic domain distribution imaging: visualising macroscopic deformation gradients in fine-grained ferromagnetic material with a significantly improved spatial resolution as compared to integral, mean value-based measurement methods.

The effect of short range order on the thermal output and gage factor of Ni3FeCr strain gages

Abstract: The ability of alloy 70Ni-27.5Fe-2.5Cr to form an ordered crystalline structure upon application of elevated temperature, and the resulting effects on a variety of physical properties such as magnetism, strength, electrical resistance, and specific heat are well known. This paper demonstrates that strain gages made of 70Ni-27.5Fe-2.5Cr foil with an ordered crystalline structure have both high gage factors and high thermal outputs. The thermal output for strain gages made of 70Ni-27.5Fe-2.5Cr foil is demonstrated to be about 763 ppm/oF. The gage factor is nonlinear ranging in magnitude from about 5.2 to 3.0 for applied strains of 300 and 1300 με, respectively. The magnitude of gage factor and thermal output correspond with transformation of the crystalline structure from a state of disorder to a state of order. Practical application for 70Ni-27.5Fe-2.5Cr strain gages is demonstrated by dead-weight loading of shear-beam load cells at low applied strain levels to minimize the effect of nonlinear gage factor; and Wheatstone bridge cancellation of high thermal output.

Use of digital image correlation to measure strain recovery in particle toughened thermoset resins

Abstract: A methodology for finding the yield point of epoxy resins, both neat and particulate toughened, is described. Trends of the effect that particulate filling has on the time dependent response of these materials were constructed from observations made with stereo-based digital image correlation (3D-DIC), namely, creep and stress relaxation at constant load. The use of 3D-DIC also enabled the observation of differences in deformation mechanisms resulting from the particle addition. The focus is put on the technique’s potential to characterize materials and produce clear relationships between composition and mechanical strain response of any composition. The methodology proposed herein allows the observation and study of multiple deformation mechanisms from a single test, and thus can potentially minimize the number of specimens needed for a comprehensive test campaign.