Non-contact Strain Measurement Research Articles

Physical modeling of the controlled shaft deformation law during the solid backfill mining of ultra-close coal seams

Solid backfill mining has gradually become the key technology to control shaft deformation during industrial square coal pillar recovery. The industrial square area of the Nantun coal mine was studied in this paper. Based on similarity criteria of the physical modeling, an experimental model was designed to study the shaft deformation law under caving mining and backfill mining. A non-contact full-field strain measurement system and a resistance strain gauge were adopted to monitor the model. The mining-induced deformations, failures and stress distributions of shaft-surrounding rock masses during the recovery of industrial square coal pillars by caving mining and backfill mining were explicitly compared and analyzed. The results showed that the deformation and failure of shafts was caused by strata movements during the mining of the coal seams. Different lithology, stiffnesses, and thicknesses of adjacent rock strata result in asynchronous stratum movements, which thus cause shaft failure. The key to controlling mining-induced deformation of shafts during solid backfill mining is to control strata movement as follows: solid backfilling → control of the strata movement → control of the shaft deformation → decrease of the tension and shear force experienced by the shaft → decrease of shaft damage. Finally, the design method of the backfill body’s compression ratio based on shaft deformation control during the solid backfill mining process is proposed, thus laying the foundation for the safe recovery of an unexploited coal industrial square in resource-exhausted coal mines.

Using digital image correlation to evaluate plastic shrinkage cracking in cement-based materials

Fresh concrete exposed to a drying environment is susceptible to plastic shrinkage cracking, which can result in negative impacts on concrete durability. In the current ASTM standard, plastic shrinkage cracking is evaluated by the average crack width measured at 24 h after concrete placement, considering that the traditional crack measurement tools cannot be used on fresh concrete. In this work, a non-contact strain measurement technique based on digital image correlation (DIC) was applied to study the behavior of plastic shrinkage cracking. The crack areas were also determined by image analysis using MATLAB functions. It was found that DIC can provide a series of strain contour maps that help to understand the process of plastic shrinkage cracking. The effects of air temperature, w/c, and substrate roughness were well explained by both DIC analysis results and crack areas computed in MATLAB.

Low-stress creep deformation in two opto-electronic glass-epoxy joints: Part I – adhesive creep data

This two-part paper presents measurements and modelling of creep deformation at relatively low stress of two epoxy adhesives used in opto-electronic applications, one having a relatively high modulus and the other a low modulus. Part I describes the bulk tensile creep tests at various temperatures (75–95 °C) and stress levels representative of those in opto-electronic devices. A novel laser-based method of non-contact creep strain measurement was used to eliminate errors associated with adhesive specimen contact. The high modulus epoxy, tested at stress levels between 5 and 20 MPa, displayed non-linear behavior at high stress levels but was linear over the range of interest. The low modulus adhesive, tested between 0.25 and 0.5 MPa, was linearly viscoelastic over the full range of tested stress levels. Part II assesses the accuracy of finite element models to predict the creep deformation in two different glass-epoxy joints designed to represent stress states in some opto-electronic applications.

Poisson's ratio of PDMS thin films

Despite the fact that a small uncertainty in PDMS Poisson's ratio leads to significant errors in traction force microscopy, there is a clear lack of data for PDMS films at the scale of 100 μm, a relevant size scale frequently employed in cell mechanics studies. Equally important is the need for consideration of the viscoelastic nature of PDMS, as no mechanical property - including Poisson's ratio - can be taken as a time-independent constant. The foremost challenge for addressing these issues is the difficulty of carrying out stress relaxation tests on miniature PDMS samples accompanied by non-contact strain measurement with a very high spatiotemporal resolution. This study introduces such a stress relaxation platform incorporating i) the proper means for the application of necessary boundary conditions, ii) a high-precision in load measurement, and iii) a non-contact, local strain measurement technique based on single particle tracking. During stretching, images were recorded at a rate of 18 Hz with a 40 μm spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 μm were prepared to study the Poisson's ratio by a local strain microscope. After tracing the displacement of microspheres by a single particle tracking method and using a strain mapping, Poisson's ratio for 155-μm-thick PDMS was found to decrease from 0.483 ± 0.034 to 0.473 ± 0.040 over a period of 20 min. For 125-μm-thick PDMS, this reduction took place from 0.482 ± 0.041 to 0.468 ± 0.038. Moreover, a non-monotonic reduction was observed in both cases. This negative correlation between Poisson's ratio and relaxation time was found to be statistically significant for both thicknesses with p < 0.001. The viscoelastic behavior was further characterized through the Burgers model. With a measurement field of 597 × 550 μm2, this study emphasizes the importance of the local investigation of mechanical properties. Furthermore, the dependence of transverse strain on a film thickness difference of 30 μm was measured to determine the sensitivity of local strain tracking. The inherent high resolution of the proposed approach enables one to measure deformations more precisely and to observe the temporal evolution of the Poisson's ratio that has not been observed before. In addition to the high-precision determination of PDMS Poisson's ratio, this work also offers a promising pathway for the accurate and time-dependent determination of the mechanical properties of other soft materials, where similar ambiguities exist regarding the mechanical behavior.

A Practical Procedure for Measuring the Stiffness of Foam like Materials

Polymer foams are used extensively in everyday life, from disposable packaging and soft furnishings through to engineering applications such as core structural materials in the marine industry or bone analogue materials for orthopaedic device testing. In the engineering field it is important that the mechanical behaviour of these materials is characterised correctly, as computationally based predictions of structural performance rely heavily on accurate input data. Mechanical property data from standard physical tests such as uniaxial compression are subject to artefacts including non-uniformity of applied loading, test fixture-sample contact conditions, and test machine compliance. These are well-documented problems, which techniques such as extensometry and point tracking of marker pairs attempt to resolve. In particular, in addition to being non-contact, the use of individual marker pairs can reveal non-linear behaviours because of alignment issues. In the current work, a practical, accurate experimental methodology is introduced to investigate this issue. Uniaxial compression tests were conducted on cellular polyurethane foam blocks. Both faces of the foam specimens were monitored using point tracking on multiple marker pairs to account for misalignment. Sample deformation was simultaneously measured by test-machine crosshead displacement. The Young’s modulus and Poisson’s ratio were calculated in both cases. To verify the measurements, digital volume correlation (DVC) was applied. DVC is a full-field non-contact strain measurement method that interrogates the interior structure of the foam to determine the physical response. Results demonstrated that misalignment effects could easily be followed during testing, which averaged out on both front and back surfaces to produce a single modulus value. Considerable differences were evident between crosshead displacement calculated modulus and point tracking, indicating that artefacts can lead to substantial errors, as evidenced in the published literature.

Mechanical properties of reinforced composite materials under uniaxial and planar tension loading regimes measured using a non-contact optical method

This study reports the hyperelastic material behaviour of a commercially available PVC/nitrile elastomer with woven continuous nylon reinforcement composite sheet under loading cases of uniaxial extension and pure shear achieved via wide strip tension testing. Through the novel use of an advanced non-contact optical strain measurement technique, the hyperelastic material behaviour of the composite is investigated, and materials parameters are reported for both the warp and the weft directions of reinforcement fibre alignment. The non-contact technique is used to acquire normal and shear strains at the surface of the composite sheet material when loaded to tensile strains (stretches) of up to 0.25. Directly measured shear strains are compared to those derived from the normal strain outputs of an optical rectangular strain rosette array, with both measures showing close agreement. The measured mechanical behaviour under loading is used to determine an approximate strain energy function for the composite via ABAQUS software hyperelastic materials modelling curve fitting, with the Ogden and Yeoh hyperelastic models showing reasonable agreement to experimental data.

Free, partial, and fully constrained recovery analysis of cold-programmed shape memory epoxy/carbon nanotube nanocomposites: Experiments and predictions

Thermally activated shape memory polymers are typically programmed by initially heating the material above the glass transition temperature ( Tg), deforming to the desired shape, cooling below Tg, and unloading to fix the temporary shape. This process of deforming at high temperatures becomes a time-, labor-, and energy-expensive process while applying to large structures. Alternatively, materials with reversible plasticity shape memory property can be programmed at temperatures well below the glass transition temperature which offers several advantages over conventional programming. Here, the free, partial, and fully constrained recovery analysis of cold-programmed multi-walled carbon nanotube–reinforced epoxy nanocomposites is presented. The free recovery analysis involves heating the temporary shape above Tg without any constraints (zero stress), and for fully constrained recovery analysis, the temporary shape is held constant while heating. The partially constrained recovery behavior is studied by applying a constant stress of 10%, 25%, and 50% of the maximum recovery stress obtained from the completely constrained recovery analysis. The samples are also characterized for their thermal, morphological, and mechanical properties. A non-contact optical strain measurement method is used to measure the strains during cold-programming and shape recovery. The different recovery behaviors are analyzed by using a thermo-viscoelastic–viscoplastic model, and the predictions are compared with the experimental results.

Effect of engineered cementitious composite on the bond behavior between fiber-reinforced polymer and concrete

Externally bonded fiber-reinforced polymer (FRP) to a degraded reinforced concrete structure is currently a very popular strengthening technology; however, premature debonding mainly caused by concrete crack propagation leads to the underutilization of FRP. Engineered cementitious composite (ECC) has an excellent crack control capability. When used in combination with FRP, ECC can effectively delay the debonding of FRP and enhance the bearing capacity and ductility of FRP-strengthened structures. For the safe and economic strengthening design, the bond-slip relationship of FRP-ECC-concrete needs to be properly understood. This paper investigated the bond behavior of FRP–ECC–concrete interface through a series of simple shear tests. The effects of ECC thickness, concrete surface treatment, and construction method on the bond behavior were evaluated. The deformation and strain data of FRP were collected and analyzed using a noncontact full-field strain measurement system VIC-3D (three-dimensional video image correlation system). The experimental results show that the addition of an ECC layer delayed the debonding of FRP and significantly increased the maximum strain of FRP. The bearing capacity, the ultimate slip, and the interface energy consumption capacity also improved significantly.

Experimental and numerical analysis on the forming and shearing quality of C-channel steel

The method of cold roll forming, without using the press brake prior, is a kind of plastic forming process which produces the specified cross sections by a series of forming rollers. To measure the forming strain, the experiments adopted a non-contact strain measurement method and compared with the numerical model. A number of process parameters such as forming angle increment (F.A.I), forming velocity and station distance are studied to find the effects on the forming quality. The FEM of 45° die shear system is set up and compared with the experimental results. The effects of shear parameters such as the shear angle and the shear velocity are also discussed. The consequences demonstrate that the 45° die shear system has shear blind zones which have bad shearing quality. The shear velocity has a significant influence on the shear zone length.

Study of rate dependent behavior of glass/epoxy composites with nanofillers using non-contact strain measurement

The present research work discusses the effect of strain rate and clay on the dynamic tensile behavior of glass/epoxy composites. Nanocomposites of different clay content in terms of 1.5, 3 and 5wt% are chosen for the study. Characterization of the nanoclay dispersed in epoxy system is investigated by X-ray Diffraction (XRD) and Fourier Transform Infra-red Spectroscopy (FT-IR), respectively. The non-contact Digital Image Correlation (DIC) technique is used for full-field strain measurement during dynamic loading using high speed CMOS camera. Stress-strain measurements are reported for glass/epoxy/clay nanocomposites over a wide range of strain rates from 0.001 to 450s−1 and the variation of modulus, strength and strain to failure with strain rate are studied. A non-linear regression function is used to predict the tensile properties of glass/epoxy and its clay nanocomposites. The results show that the tensile strength and stiffness increase with increase in strain rate for neat glass/epoxy and its clay nanocomposites. An improvement in tensile modulus and strength is achieved with the addition of nanoclay. The fractography of tensile specimens is investigated using scanning electron microscopy (SEM) to discern the surface features and dispersion state of clay.

Deformation Behavior of Hard Roofs in Solid Backfill Coal Mining Using Physical Models

Solid backfill coal mining technology has been widely applied in coal seams that are at risk of hard roof. Using actual measured strain–stress curves of the backfill body and the similarity theory, this study designed and employed four experimental models for physical simulation, corresponding to roof-controlled backfilling ratios of 0%, 40%, 82.5% and 97% using the geological conditions of Face No. 6304 in the Jining No. 3 coal mine—a solid backfill coal mining face under a hard roof. A non-contact strain measurement system and pressure sensors were used to monitor the deformation of the overlying strata and changes in abutment stress ahead of the face during mining of the models for varying roof-controlled backfilling ratios. The results indicated that the solid backfill body was able to support the roof. As the roof-controlled backfilling ratio was increased, the maximum subsidence of the roof and the maximum height of the cracks decreased. When the roof-controlled backfilling ratio was 82.5% or higher, the working face did not display any obvious initial fractures or periodic fractures, and both the value and the impact range of the abutment stress ahead of the face decreased.

High-resolution spatiotemporal strain mapping reveals non-uniform deformation in micropatterned elastomers

Micropatterns are generated on a vast selection of polymeric substrates for various applications ranging from stretchable electronics to cellular mechanobiological systems. When these patterned substrates are exposed to external loading, strain field is primarily affected by the presence of microfabricated structures and similarly by fabrication-related defects. The capturing of such nonhomogeneous strain fields is of utmost importance in cases where study of the mechanical behavior with a high spatial resolution is necessary. Image-based non-contact strain measurement techniques are favorable and have recently been extended to scanning tunneling microscope and scanning electron microscope images for the characterization of mechanical properties of metallic materials, e.g. steel and aluminum, at the microscale. A similar real-time analysis of strain heterogeneity in elastomers is yet to be achieved during the entire loading sequence. The available measurement methods for polymeric materials mostly depend on cross-head displacement or precalibrated strain values. Thus, they suffer either from the lack of any real-time analysis, spatiotemporal distribution or high resolution in addition to a combination of these factors. In this work, these challenges are addressed by integrating a tensile stretcher with an inverted optical microscope and developing a subpixel particle tracking algorithm. As a proof of concept, the patterns with a critical dimension of 200 µm are generated on polydimethylsiloxane substrates and strain distribution in the vicinity of the patterns is captured with a high spatiotemporal resolution. In the field of strain measurement, there is always a tradeoff between minimum measurable strain value and spatial resolution. Current noncontact techniques on elastomers can deliver a strain resolution of 0.001% over a minimum length of 5 cm. More importantly, inhomogeneities within this quite large region cannot be captured. The proposed technique can overcome this challenge and provides a displacement measurement resolution of 116 nm and a strain resolution of 0.04% over a gage length of 300 µm. Similarly, the ability to capture inhomogeneities is demonstrated by mapping strain around a thru-hole. The robustness of the technique is also evaluated, where no appreciable change in strain measurement is observed despite the significant variations imposed on the measurement mesh. The proposed approach introduces critical improvements for the determination of displacement and strain gradients in elastomers regarding the real-time nature of strain mapping with a microscale spatial resolution.

Full-Field High-Strain Gradient Evaluation from Wrapped ESPI Data Using Phasors

Electronic Speckle Pattern Interferometry (ESPI) is a sensitive optical method commonly used for making full-field measurements of surface displacements. It would be very desirable to be able to extend the technique also to determine surface strains. This would provide a full-field, non-contact strain measurement method that avoids the substantial installation burden of strain gauges. A mathematical approach is described where the ESPI data from an in-plane interferometer are numerically differentiated to determine surface strains. This is a challenging process because numerical differentiation is very sensitive to the presence of noise and ESPI data are inherently noisy. In addition, the phase information from ESPI data are wrapped modulo-2π. The resulting phase discontinuities make it difficult to use local averaging to smooth the data. A technique is described here where phasors are used to avoid the need for phase unwrapping. The effect of noise is reduced by a localized multiple smoothing technique that is effective in preserving spatial resolution, even near very high strain concentrations. Example measurements are shown and the effectiveness of the proposed method is illustrated.

Comparison of strain measurement in the mouse forearm using subject-specific finite element models, strain gaging, and digital image correlation.

Mechanical loading in bone leads to the activation of bone-forming pathways that are most likely associated with a minimum strain threshold being experienced by the osteocyte. To investigate the correlation between cellular response and mechanical stimuli, researchers must develop accurate ways to measure/compute strain both externally on the bone surface and internally at the osteocyte level. This study investigates the use of finite element (FE) models to compute bone surface strains on the mouse forearm. Strains from three FE models were compared to data collected experimentally through strain gaging and digital image correlation (DIC). Each FE model was assigned subject-specific bone properties and consisted of one-dimensional springs representing the interosseous membrane. After three-point bending was performed on the ulnae and radii, moment of inertia was determined from microCT analysis of the bone region between the supports and then used along with standard beam analyses to calculate the Young's modulus. Non-contact strain measurements from DIC were determined to be more suitable for validating numerical results than experimental data obtained through conventional strain gaging. When comparing strain responses in the three ulnae, we observed a 3-14% difference between numerical and DIC strains while the strain gage values were 37-56% lower than numerical values. This study demonstrates a computational approach for capturing bone surface strains in the mouse forearm. Ultimately, strains from these macroscale models can be used as inputs for microscale and nanoscale FE models designed to analyze strains directly in the osteocyte lacunae.

How are adjacent spinal levels affected by vertebral fracture and by vertebroplasty? A biomechanical study on cadaveric spines

Background ContextSpinal injuries and surgery may have important effects on neighboring spinal levels, but previous investigations of adjacent-level biomechanics have produced conflicting results. We use “stress profilometry” and noncontact strain measurements to investigate thoroughly this long-standing problem. PurposeThis study aimed to determine how vertebral fracture and vertebroplasty affect compressive load-sharing and vertebral deformations at adjacent spinal levels. Study DesignWe conducted mechanical experiments on cadaver spines. MethodsTwenty-eight cadaveric spine specimens, comprising three thoracolumbar vertebrae and the intervening discs and ligaments, were dissected from fourteen cadavers aged 67–92 years. A needle-mounted pressure transducer was used to measure the distribution of compressive stress across the anteroposterior diameter of both intervertebral discs. “Stress profiles” were analyzed to quantify intradiscal pressure (IDP) and concentrations of compressive stress in the anterior and posterior annulus. Summation of stresses over discrete areas yielded the compressive force acting on the anterior and posterior halves of each vertebral body, and the compressive force resisted by the neural arch. Creep deformations of vertebral bodies under load were measured using an optical MacReflex system. All measurements were repeated following compressive injury to one of the three vertebrae, and again after the injury had been treated by vertebroplasty. The study was funded by a grant from Action Medical Research, UK ($143,230). Authors of this study have no conflicts of interest to disclose. ResultsInjury usually involved endplate fracture, often combined with deformation of the anterior cortex, so that the affected vertebral body developed slight anterior wedging. Injury reduced IDP at the affected level, to an average 47% of pre-fracture values (p<.001), and transferred compressive load-bearing from nucleus to annulus, and also from disc to neural arch. Similar but reduced effects were seen at adjacent (non-fractured) levels, where mean IDP was reduced to 73% of baseline values (p<.001). Vertebroplasty partially reversed these changes, increasing mean IDP to 76% and 81% of baseline values at fractured and adjacent levels, respectively. Injury also increased creep deformation of the vertebral body under load, especially in the anterior region where a 14-fold increase was observed at the fractured level and a threefold increase was observed at the adjacent level. Vertebroplasty also reversed these changes, reducing deformation of the anterior vertebral body (compared with post-fracture values) by 62% at the fractured level, and by 52% at the adjacent level. ConclusionsVertebral fracture adversely affects compressive load-sharing and increases vertebral deformations at both fractured and adjacent levels. All effects can be partially reversed by vertebroplasty.

Strain controlled cyclic tests on miniaturized specimens

The paper is dealing with strain controlled cyclic tests using a non-contact strain measurement based on digital image correlation techniques on proportional sizes of conventional specimens. The cyclic behaviour of 34CrNiMo6 high-strength steel was investigated on miniaturized round specimens with diameter of 2mm that were compared with specimens in accordance with ASTM E606 standards. The cycle asymmetry coefficient was R= -1. This application is intended to be used for life time assessment of in service components in future work which enables to carried out a group of mechanical tests from a limited amount of the experimental material. The attention was paid to confirm the suitability of the proposed size miniaturization geometry, testing set up and procedure. The test results obtained enabled to construct Manson-Coffin curves and assess fatigue parameters. The purpose of this study is to present differences between cyclic curves and cyclic parameters which have been evaluated based on conventional and miniaturized specimens.

Self-calibration single-lens 3D video extensometer for high-accuracy and real-time strain measurement.

The accuracy of strain measurement using a common optical extensometer with two-dimensional (2D) digital image correlation (DIC) is not sufficient for experimental applications due to the effect of out-of-plane motion. Although three-dimensional (3D) DIC can measure all three components of displacement without introducing in-plane displacement errors, 3D-DIC requires the stringent synchronization between two digital cameras and requires complicated system calibration of binocular stereovision, which makes the measurement rather inconvenient. To solve the problems described above, this paper proposes a self-calibration single-lens 3D video extensometer for non-contact, non-destructive and high-accuracy strain measurement. In the established video extensometer, a single-lens 3D imaging system with a prism and two mirrors is constructed to acquire stereo images of the test sample surface, so the problems of synchronization and out-of-plane displacement can be solved easily. Moreover, a speckle-based self-calibration method which calibrates the single-lens stereo system using the reference speckle image of the specimen instead of the calibration targets is proposed, which will make the system more convenient to be used without complicated calibration. Furthermore, an efficient and robust inverse compositional Gauss-Newton algorithm combined with a robust stereo matching stage is employed to achieve high-accuracy and real-time subset-based stereo matching. Tensile tests of an Al-alloy specimen were performed to demonstrate the feasibility and effectiveness of the proposed self-calibration single-lens 3D video extensometer.

A Review of Surface Deformation and Strain Measurement Using Two-Dimensional Digital Image Correlation

AbstractAmong the full-field optical measurement methods, the Digital Image Correlation (DIC) is one of the techniques which has been given particular attention. Technically, the DIC technique refers to a non-contact strain measurement method that mathematically compares the grey intensity changes of the images captured at two different states: before and after deformation. The measurement can be performed by numerically calculating the displacement of speckles which are deposited on the top of object’s surface. In this paper, the Two-Dimensional Digital Image Correlation (2D-DIC) is presented and its fundamental concepts are discussed. Next, the development of the 2D-DIC algorithms in the past 33 years is reviewed systematically. The improvement of 2DDIC algorithms is presented with respect to two distinct aspects: their computation efficiency and measurement accuracy. Furthermore, analysis of the 2D-DIC accuracy is included, followed by a review of the DIC applications for two-dimensional measurements.

A Non-Contact Varying Temperature Strain Measuring System Based on Digital Image Correlation

An effective varying temperature digital image correlation (DIC) measurement is established for non-contact full-field deformation and strain measurement at varying temperatures. The test planar specimen can be heated from room temperature to 200 °C accurately using the temperature-controlled chamber. The digital images of the test specimen surface at various temperatures are recorded using an ordinary optical imaging system. In order to avoid the influence of variations in refractive index of the air at different temperatures, image averaging method was used to obtain high quality images of the specimen surface. The gradient-based DIC technique is used to extract full-field inplane thermal deformation from the corrected images. For verification, the thermal deformation fields and the values of coefficient of thermal expansion (CTEs) of 45 steel specimen are measured and compared with the literature values, confirming the effectiveness and accuracy of the proposed technique and method. The experimental results reveal that the proposed technique with image averaging method is able to accurately extract full-field thermal deformation at the temperature range of room temperature up to 200 °C.

Biaxial tensile testing of cruciform slim superalloy at elevated temperatures

The stress–strain curves and yield loci of metallic materials under biaxial tensile tests can be precisely predicted using a yield criterion. In this study, biaxial tensile testing equipment with high-accuracy displacement, temperature control, and digital speckle correlation method of non-contact optical strain measurement is developed to study the thermo-mechanical deformation behavior of a GH738 nickel-based superalloy, which is widely used for important components of aeronautic engines, to exploit its excellent mechanical properties at high temperatures. Additionally, the used specimen shape is discussed. The results of this study show that the special sample shape size solves the problem in which an extremely thin sheet metal bulges when subjected to tension. The equivalent stress–strain curves and yield surfaces, which were fitted according to the Hill'48 yield criterion, exhibit deformation characteristics that are more precise than those according to the von Mises criterion. Eventually, the efficacy of the related test methods is corroborated.