Surface Displacement Measurements Research Articles

3Pa1-3 Direct measurement of surface displacement of metal thin films in picosecond laser ultrasonics(Poster Session)

3Pa1-3 Direct measurement of surface displacement of metal thin films in picosecond laser ultrasonics(Poster Session)

Characterization of a Low-Cost Optical Flow Sensor When Using an External Laser as a Direct Illumination Source

In this paper, a low cost optical flow sensor is combined with an external laser device to measure surface displacements and mechanical oscillations. The measurement system is based on applying coherent light to a diffuser surface and using an optical flow sensor to analyze the reflected and transferred light to estimate the displacement of the surface or the laser spot. This work is focused on the characterization of this measurement system, which can have the optical flow sensor placed at different angles and distances from the diffuser surface. The results have shown that the displacement of the diffuser surface is badly estimated when the optical mouse sensor is placed in front of the diffuser surface (angular orientation >150°) while the highest sensitivity is obtained when the sensor is located behind the diffuser surface and on the axis of the laser source (angular orientation 0°). In this case, the coefficient of determination of the measured displacement, R2, was very high (>0.99) with a relative error of less than 1.29%. Increasing the distance between the surface and the sensor also increased the sensitivity which increases linearly, R2 = 0.99. Finally, this measurement setup was proposed to measure very low frequency mechanical oscillations applied to the laser device, up to 0.01 Hz in this work. The results have shown that increasing the distance between the surface and the optical flow sensor also increases the sensitivity and the measurement range.

Experimental Analysis of Shear Zone Patterns in Cohesionless for Earth Pressure Problems Using Particle Image Velocimetry

Abstract: The evolution of shear zones in cohesionless sand for earth pressure problem of a retaining wall is experimentally investigated using a non‐invasive method called particle image velocimetry. It is an optical technique for measuring surface displacements from successive digital images. Small scale laboratory tests are performed for active and passive cases of a rigid retaining wall subjected to horizontal translation, rotation about its toe and rotation about its top. Attention is focused on the effect of initial sand density on distribution of volumetric and deviatoric strain. The results for initially dense sand are qualitatively compared with corresponding ones obtained with X‐rays at Cambridge University.

Surface Decoration for Improving the Accuracy of Displacement Measurements by Digital Image Correlation in SEM

In-situ straining experiments and residual stress evaluations by micromachining require accurate measurement of surface displacements. Such measurements can be conveniently done using Digital Image Correlation (DIC). Three surface decoration techniques are presented to enhance surface deformation and residual stress measurement capabilities on micron-scale samples within a Scanning Electron Microscope—Focused Ion Beam (SEM-FIB) instrument. They involve the use of Yttria-stabilized-zirconia nano particles applied chemically, nano platinum dots applied using FIB, and Focused Electron Beam (FEB) assisted deposition. The three decoration techniques create distinctive, random surface features that can be used with Digital Image Correlation to provide full field maps of surface displacements at high magnifications. A series of experiments using a FEGSEM-FIB demonstrated the effectiveness of the three surface decoration techniques for FEGSEM imaging at magnifications from 2,000× to 60,000×. The precision of the image correlation is substantially enhanced by the surface decoration, with displacement standard deviations reduced to the 0.005–0.03 pixel range, depending on the patch size used. By means of an example application, the use of surface decoration for microscopic hole-drilling residual stress measurements within a FIB-SEM is presented. The same trends in DIC uncertainty observed in the analysis of the surface decoration patterns carried through to the example application. Guidelines are given for appropriate choice of decoration method to suit various practical applications.

Characterization of mesoscale instabilities in localized granular shear using digital image correlation

Within shear bands in sands, deformation is largely non-affine, stemming primarily from buckling of well-known force chains and also from vortex-like structures. In the spirit of current trends toward multiscale modeling, understanding the links between these mesoscale deformational entities and corresponding macroscale response will form the basis for the next generation of sand behavioral models and may also aid in efforts to understand jamming–unjamming transitions in dense granular flows in general. Experimental methods to quantify and characterize such subscale kinematics, in particular in real sands, will play critical roles in these efforts. Digital Image Correlation (DIC) is a fast growing experimental technique to nondestructively measure surface displacements from digital images. Here, DIC has been employed to identify and characterize the development of vortex structures inside shear bands formed in dense sands during plane strain compression. A rigorous assessment of the DIC method has been performed, in particular for subscale behavioral characterization in unbonded granular solids, and guidelines are offered for accurate implementation. While DIC systematically overestimates shear band thickness, a methodology has been devised to compensate for this overestimation. Shear band thickness for four different uniform sands were found to range between 6 and 9 grain diameters, and for a well-graded sand between 8 and 9.5 grain diameters. These determinations agree with visual inspections of grain kinematics from the image data, as well as recent theoretical predictions.

Digital Image Correlation Search Method Based on Particle Swarm Algorithm

Digital image correlation method is an important optical technique for surface displacement and strain measurement. An approach based on Particle Swarm Optimization algorithm for sub-pixel correlation search is described in this paper. The new Algorithm does not involve reasonable guess of displacement and deformation gradient and the calculation of second-order derivatives of the digital images. Benefiting from the abilities of global optimum and parallelism searching, and compared with genetic algorithm, the new approach can complete the sub-pixel correlation search with high accuracy and less computational consumption. Computer-simulated images are then used to verify this method. The experimental results show that the new approach is a practicable sub-pixel searching method.

CTOD- R curve construction from surface displacement measurements

The construction of a fracture resistance δ– R (or J– R) curve requires the appropriate measurement of crack-tip opening displacement ( CTOD) as a function of crack extension. This can be made by different procedures following ASTM E1820, BS7448 or other standards and procedures (e.g., GTP-02, ESIS-P2, etc.) for the measurement of fracture toughness. However, all of these procedures require standard specimens, displacement gauges, and calibration curves to get intrinsic material properties. This paper deals with some analysis and aspects related to the measurement of fracture toughness by observing the surface of the specimen. Tests were performed using three-dimensional surface displacement measurements to determine the fracture parameters and the crack extension values. These tests can be conducted without using a crack mouth opening displacement- CMOD or load-line displacement gauge, because CMOD can be calculated by using the displacement of the surface points. The presented method offers a significant advantage for fracture toughness testing in cases where a clip gauge is not easy to use, for example, on structural components. Simple analysis of stereo-metrical surface displacements gives a load vs. crack opening displacement curve. Results show that the initiation of stable crack propagation can be easy estimated as the point of the curve’s deviation. It is possible to determine the deviation point if the crack opening displacement measurements are close to crack tip in the plastic zone area. The resistance curve, CTOD- R, is developed by the local measurement of crack opening displacement ( COD) in rigid body area of specimen. COD values are used for the recalculation with the CMOD parameter as a remote crack opening displacement, according to the ASTM standard.

Damage, crack growth and fracture characteristics of nuclear grade graphite using the Double Torsion technique

The crack initiation and propagation characteristics of two medium grained polygranular graphites, nuclear block graphite (NBG10) and Gilsocarbon (GCMB grade) graphite, have been studied using the Double Torsion (DT) technique. The DT technique allows stable crack propagation and easy crack tip observation of such brittle materials. The linear elastic fracture mechanics (LEFM) methodology of the DT technique was adapted for elastic–plastic fracture mechanics (EPFM) in conjunction with a methodology for directly calculating the J-integral from in-plane displacement fields (JMAN) to account for the non-linearity of graphite deformation. The full field surface displacement measurement techniques of electronic speckle pattern interferometry (ESPI) and digital image correlation (DIC) were used to observe and measure crack initiation and propagation. Significant micro-cracking in the fracture process zone (FPZ) was observed as well as crack bridging in the wake of the crack tip. The R-curve behaviour was measured to determine the critical J-integral for crack propagation in both materials. Micro-cracks tended to nucleate at pores, causing deflection of the crack path. Rising R-curve behaviour was observed, which is attributed to the formation of the FPZ, while crack bridging and distributed micro-cracks are responsible for the increase in fracture resistance. Each contributes around 50% of the irreversible energy dissipation in both graphites.

The Effects of Magnetic Resonance Imaging on Surgical Staples: An Experimental Analysis

Surgical staples are commonplace in repairing surgical incisions. Staples allow for expeditious closure and removal compared with suture materials. However, there are clinical concerns when obtaining a magnetic resonance imaging (MRI) scan with staples present. This study examined two issues related to MRI scanning in the presence of surgical staples: skin surface temperature change and staple displacement. Thirty pig feet had 3-cm surgical incisions repaired with five surgical staples. Once placed, each skin staple position was marked for later referencing. A surface temperature laser device recorded prescan skin surface temperature. A 35-minute MRI scan was performed with a 1.5-Tesla magnet and standard knee coil for each pig foot. Immediately afterward, the skin surface temperature and displacement measurements were recorded. The paired t test was used to analyze temperature change from prescan to postscan. The prescan mean temperature was 16.45°C (standard deviation: 0.70°C), and the range was 14.60°C to 18.20°C. After scanning, the mean temperature was 16.02°C (standard deviation: 0.63°C), and the range was 15.00°C to 17.60°C. The decrease of 0.43°C in skin surface temperature was statistically significant (p=0.001). No change in staple position was measurable or evident by visual inspection for any of the pig feet. This study found no increase in skin surface temperature or displacement of staple position after a standard extremity MRI scan. Based on our findings, MRI scanning in the presence of stainless steel surgical staples seems safe.

Surface Pressure and Shear Stress Fields within a Frictional Contact on Rubber

This paper addresses the issue of the determination of the frictional stress distribution from the inversion of the measured surface displacement field for sliding interfaces between a glass lens and a rubber (poly(dimethylsiloxane)) substrate. Experimental results show that high lateral strains are achieved at the periphery of the sliding contacts. As a consequence, an accurate inversion of the displacement field requires that finite strains and non-linear response of the rubber substrate are taken into account. For that purpose, a Finite Element (FE) inversion procedure is implemented where the measured displacement field is applied as a boundary condition at the upper surface of a meshed body representing the rubber substrate. Normal pressure is also determined in the same way, if non-diverging values are assumed at the contact edge. This procedure is applied to linearly sliding contacts as well as on twisting contacts.

Micro-relief development in polygonal patterned ground in the Dry Valleys of Antarctica

Polygonal patterned ground in polar regions of both Earth and Mars has received considerable attention. In comparison with the size, shape, and arrangement of the polygons, the diverse micro-relief and topography (termed here simply “relief”) of polygonal patterned ground have been understudied. And yet, the relief reflects important conditions and processes occurring directly below the ground surface, and it can be observed readily in the field and through remote sensing. Herein, we describe the relief characteristic of the simplest and relatively young form of patterned ground in the Dry Valleys of Antarctic. We also develop a numerical model to examine the generation of relief due to subsurface material being shouldered aside contraction cracks by incremental sand wedges growth, and to down-slope creep of loose granular material on the surface. We model the longterm subsurface deformation of ice-cemented permafrost as a non-linear viscous material. Our modeling is guided and validated using decades of field measurements of surface displacements of the permafrost and relief. This work has implications for assessing the activity of surfaces on Earth and Mars, and much larger scale potential manifestations of incremental wedging in icy material, namely the distinct paired ridges on Europa.

Identification of cohesive zone model and elastic parameters of fiber-reinforced cementitious composites using digital image correlation and a hybrid inverse technique

Traditional methods for the inverse identification of elastic properties and local cohesive zone model (CZM) of solids utilize only global experimental data. In contrast, this paper addresses the inverse identification of elastic properties and CZM of a range of materials, using full-field displacement through an optimization technique in a finite element (FE) framework. The new experimental–numerical hybrid approach has been applied to fiber-reinforced cementitious composites (FRCC). PVA microfibers are used at four volume fractions: 0.5%, 1%, 2% and 3%. Digital image correlation (DIC) technique is used to measure surface displacement fields of the test specimens. Four-point bend tests are carried out for the measurement of the modulus of elasticity, E, and the Poisson’s ratio, ν, while single edge-notched beams (SENB) are used for measurement of mode-I CZM parameters. A finite element update inverse formulation, which is based on minimization of the difference between measured and computed displacement field, is used for both identification problems. For the identification of E and ν, linearized form of the Hooke’s tensor in plane stress condition has been derived for two-dimensional linear elasticity in FE frame, and Newton–Raphson solver is employed for the inverse problem. For the identification of the CZM, generic spline curves have been used for the parameterization of any CZM thus avoiding the need of an assumption of the CZM shape, while derivative-free Nelder–Mead optimization with CZM shape regularization is employed as the solution method, which reduces the complexity of numerical implementation and improves robustness. The computed E and ν are consistent with published results. The computed CZMs of the FRCCs with different fiber volume fractions reveal a strain-hardening characteristic. The computed CZM is used in direct problem simulation, the results of which are consistent with the experimental global response.

Quantitative shearography: error reduction by using more than three measurement channels

Shearography is a noncontact optical technique used to measure surface displacement derivatives. Full surface strain characterization can be achieved using shearography configurations employing at least three measurement channels. Each measurement channel is sensitive to a single displacement gradient component defined by its sensitivity vector. A matrix transformation is then required to convert the measured components to the orthogonal displacement gradients required for quantitative strain measurement. This transformation, conventionally performed using three measurement channels, amplifies any errors present in the measurement. This paper investigates the use of additional measurement channels using the results of a computer model and an experimental shearography system. Results are presented showing that the addition of a fourth channel can reduce the errors in the computed orthogonal components by up to 33% and that, by using 10 channels, reductions of around 45% should be possible.

Shear strain determination of the polymer polydimethysiloxane (PMDS) using digital image correlation in different temperatures

In the present work a digital image correlation (DIC) method is used in order to analyze the adhesive shear modulus of poly-dimethylsiloxane (PDMS) submitted to different loads and temperatures. This is an optical-numerical full-field surface displacement measurement method. It is based on a comparison between two images of a specimen coated by a random speckled pattern in the undeformed and in the deformed states. A single lap joint testing is performed. This is a standard test specimen for characterizing adhesive properties and it is considered the simplest form of adhesive joints. For the single lap joint specimen, steel adherends are bonded using a flexible rubber elastic polymer (PDMS), which is a commercially available silicone elastic rubber

An ultrasonic device to measure surface displacements of water and floating bodies

Non-intrusive measuring instruments are desirable tools to investigate the response of vessels or marine structures subjected to water waves, whether in a laboratory or in the field. An ultrasonic sensing device is designed and built to measure surface displacements of a floating body. An ultrasonic sensor can sense any surface that lies in front of it, whether it is a solid or a liquid surface. In this study, a specialized electronic circuit board is designed to measure a wide range of surface displacements. The proposed system not only can be used to measure heave, surge, and pitch of a floating body continuously but also it can act as a water waveform recording instrument. The proposed ultrasonic measuring device is far more accurate than the current standard instruments (e.g. potentiometers) based on the simple fact that the measured surface is not disturbed. Furthermore, this device is of low manufacturing cost and has a practical size that renders it suitable for a broad range of applications. Hence, it can be used in a laboratory setup as well as in conducting field tests.

Coupling limit equilibrium analyses and real-time monitoring to refine a landslide surveillance system in Calabria (southern Italy)

Abstract. On 28 January 2009, a large debris slide was triggered by prolonged rainfalls at the southern suburbs of San Benedetto Ullano (Northern Calabria). The slope movement affected fractured and weathered migmatitic gneiss and biotitic schist, and included a pre-existing landslide. A detailed geomorphologic field survey, carried out during the whole phase of mobilization, allowed to recognize the evolution of the phenomenon. A set of datum points was located along the borders of the landslide and frequent hand-made measurements of surface displacements were performed. Since 11 February, a basic real-time monitoring system of meteoric parameters and of surface displacements, measured by means of high-precision extensometers, was also implemented. Based on the data gained through the monitoring system, and on field surveying, a basic support system for emergency management could be defined since the first phases of activation of the phenomenon. The evolution of the landslide was monitored during the following months: as a consequence, evidence of retrogressive distribution could be recognized, with initial activation in the middle sector of the slope, where new temporary springs were observed. During early May, the activity reduced to displacements of a few millimetres per month and the geo-hydrological crisis seemed to be concluded. Afterwards, the geological scheme of the slope was refined based on the data collected through a set of explorative boreholes, equipped with inclinometers and piezometers: according to the stratigraphic and inclinometric data, the depth of the mobilized body resulted in varying between 15 and 35 m along a longitudinal section. A parametric limit equilibrium analysis was carried out to explore the stability conditions of the slope affected by the landslide as well as to quantify the role of the water table in destabilizing the slope. The interpretation of the process based on field observations was confirmed by the limit equilibrium analysis: the first activation of the landslide was, in fact, to be expected in the middle portion of the slope, provided that the groundwater levels approximate the ground surface in the same sector. On 1 February 2010, another remarkable phase of landslide mobilization began, following a new period of exceptional and prolonged rainfalls. On 11 February, an abrupt stage of slope acceleration was observed, after further extraordinary rainfalls. The slope movement essentially replicated the phases of mobilization observed on 28 January 2009, thus confirming the results of the limit equilibrium analysis. Based on the outcomes of the parametric analysis, the support system for emergency management could then be tentatively refined on a more physical basis.

Postseismic deformation induced by brittle rock damage of aftershocks

Large earthquakes are commonly followed by abundant aftershocks that are densely located around the coseismic rupture zone. Laboratory experiments indicate that “microscopic” brittle rock failures (acoustic emission) are associated collectively with a “macroscopic” damage‐related inelastic relaxation. Utilizing basic relations between local brittle failures and gradual inelastic strain in a viscoelastic damage rheology model, we develop connections between aftershock decay rates and the aftershocks‐induced component of geodetic deformation. The discussed mechanism is relevant for postseismic relaxation produced by sources located within the seismogenic zone, and especially in regions that overlap locations of high aftershocks activity. Assuming the Omori‐Utsu decay rate for aftershocks, we find that the temporal decay of the damage‐related postseismic relaxation follows a generalized power‐law relation with the standard Omori‐Utsu law as a limit case. The results provide a way for estimating the separate contributions to observed postseismic displacements that stem from brittle failures in the seismogenic zone (aftershocks) and other (aseismic) processes. Using the obtained theoretical expectations, we analyze postseismic displacements measured by GPS stations around the North Anatolian fault ∼3 months following the 1999 M7.4 İzmit earthquake. We find that the observed postseismic displacements decay slower than the aftershock seismicity. Based on our theoretical results, we conclude that up to 50% of the measured surface displacements at near‐fault sites can be attributed to aftershock‐induced inelastic deformation in the seismogenic zone. The remainder postseismic deformation can generally be explained by relaxation in the deeper ductile substrate.

Sub-pixel precision image matching for measuring surface displacements on mass movements using normalized cross-correlation

This study evaluates the performance of two fundamentally different approaches to achieve sub-pixel precision of normalised cross-correlation when measuring surface displacements on mass movements from repeat optical images. In the first approach, image intensities are interpolated to a desired sub-pixel resolution using a bi-cubic interpolation scheme prior to the actual displacement matching. In the second approach, the image pairs are correlated at the original image resolution and the peaks of the correlation coefficient surface are then located at the desired sub-pixel resolution using three techniques, namely bi-cubic interpolation, parabola fitting and Gaussian fitting. Both principal approaches are applied to three typical mass movement types: rockglacier creep, glacier flow and land sliding. In addition, the influence of pixel resolution on the accuracies of displacement measurement using image matching is evaluated using repeat images resampled to different spatial resolutions. Our results show that bi-cubic interpolation of image intensity performs best followed by bi-cubic interpolation of the correlation surface. Both Gaussian and parabolic peak locating turn out less accurate. By increasing the spatial resolution (i.e. reducing the ground pixel size) of the matched images by 2 to 16 times using intensity interpolation, 40% to 80% reduction in mean error in reference to the same resolution original image could be achieved. The study also quantifies how the mean error, the random error, the proportion of mismatches and the proportion of undetected movements increase with increasing pixel size (i.e. decreasing spatial resolution) for all of the three mass movement examples investigated.

Improved Constraints on Models of Glacial Isostatic Adjustment: A Review of the Contribution of Ground-Based Geodetic Observations

The provision of accurate models of Glacial Isostatic Adjustment (GIA) is presently a priority need in climate studies, largely due to the potential of the Gravity Recovery and Climate Experiment (GRACE) data to be used to determine accurate and continent-wide assessments of ice mass change and hydrology. However, modelled GIA is uncertain due to insufficient constraints on our knowledge of past glacial changes and to large simplifications in the underlying Earth models. Consequently, we show differences between models that exceed several mm/year in terms of surface displacement for the two major ice sheets: Greenland and Antarctica. Geodetic measurements of surface displacement offer the potential for new constraints to be made on GIA models, especially when they are used to improve structural features of the Earth’s interior as to allow for a more realistic reconstruction of the glaciation history. We present the distribution of presently available campaign and continuous geodetic measurements in Greenland and Antarctica and summarise surface velocities published to date, showing substantial disagreement between techniques and GIA models alike. We review the current state-of-the-art in ground-based geodesy (GPS, VLBI, DORIS, SLR) in determining accurate and precise surface velocities. In particular, we focus on known areas of need in GPS observation level models and the terrestrial reference frame in order to advance geodetic observation precision/accuracy toward 0.1 mm/year and therefore further constrain models of GIA and subsequent present-day ice mass change estimates.

Surface displacement of the Mw 7 Machaze earthquake (Mozambique): Complementary use of multiband InSAR and radar amplitude image correlation with elastic modelling

In this paper we investigate the surface displacement related to the 2006 Machaze earthquake using Synthetic Aperture Radar Interferometry (InSAR) and sub-pixel correlation (SPC) of radar amplitude images. We focus on surface displacement measurement during three stages of the seismic cycle. First, we examined the co-seismic stage, using an Advanced SAR (ASAR) sensor onboard the Envisat satellite. Then we investigated the post-seismic stage using the Phase Array L-band SAR sensor (PALSAR) onboard the ALOS satellite. Lastly, we focussed on the inter-seismic stage, prior to the earthquake by analysing the L-band JERS-1 SAR data. The high degree of signal decorrelation in the C-band co-seismic interferogram hinders a correct positioning of the surface rupture and correct phase unwrapping. The post-seismic L-band interferograms reveal a time-constant surface displacement, causing subsidence of the surface at a ∼5cm/yr rate. This phenomenon continued to affect the close rupture field for at least two years following the earthquake and intrinsically reveals a candidate seismogenic fault trace that we use as a proxy for an inversion against an elastic dislocation model. Prior to the earthquake, the JERS interferograms do not indicate any traces of pre-seismic slip on the seismogenic fault. Therefore, slip after the earthquake is post-seismic, and it was triggered by the Machaze earthquake. This feature represents a prominent post-seismic slip event rarely observed in such a geodynamic context.