Non-contact Strain Measurement Research Articles

Application of strain-controlled fatigue testing methods to polymer matrix composites

This paper presents a short investigation of the benefits of non-contact strain measurement for monitoring and control of fatigue tests on composites. Recent developments in measurement technology offer the means to effectively measure both axial and transverse strain, instantly, throughout cyclic and highly dynamic tests. Several test scenarios are examined which demonstrate potential benefits of current state-of-the-art video extensometry, for strain controlled fatigue tests on thermoplastic composites.Live extensometry enables continuous monitoring and live calculations throughout the test and provides the option to automatically collect additional data for loading cycles with anomalous behaviour. It also allows a good control system to safely apply accurately strain-controlled loading of specimens. Digital image correlation techniques can offer complimentary information on the full-field strain behaviour of a specimen, but the data processing and storage requirements are considerably too large for live or continuous measurements during fatigue tests.Composites fatigue investigations, to date, have generally been focused on high cycle fatigue, typically under sinusoidal stress-controlled conditions. The authors propose that there is a growing need to understand composite behaviour, at well-defined strain-rates and under conditions of occasional, non-catastrophic, overload; furthermore that non-contact extensometry is an important enabling technology for such investigations.

High strain rate sensitivity of epoxy/clay nanocomposites using non-contact strain measurement

As polymers are rate sensitive to mechanical properties, dynamic tensile tests are carried out on drop mass setup, to obtain medium strain rate stress-strain response which fills the gap between quasi-static strain rate (<10 s−1) and split Hopkinson pressure bar (SHPB) technique (>1000 s−1). The present research work is to study the effect of medium strain rate on tensile behavior of epoxy/clay nanocomposites. Neat epoxy and nanocomposites containing 1.5, 3 and 5 wt% clay content are fabricated using mechanical stirrer. The dispersion of nanoclay in epoxy is examined using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The digital image correlation (DIC) technique is employed for evaluating full-field strain and strain rate using high speed CCD camera, which captures about 100,000 frames per second. Stress-strain measurements are reported for epoxy/clay nanocomposites for strain rates ranging from 0.008 to 450 s−1. The results reveal that the tensile strength and modulus increase with increase in strain rate for epoxy and its clay nanocomposites. The fracture surfaces of tensile specimens are investigated using scanning electron microscopy (SEM).

Modified foundation modelling of dowel embedment in glulam connections

This paper examines the embedment behaviour of single-dowel connections in Scandinavian Spruce Glulam by means of experimental and numerical investigations. First, the experimental results of a series of single-dowel tests on samples of different geometry and grain directions are presented. The evolution of local strain concentrations around the fastener at increasing levels of bearing deformation, is reported in detail by means of non-contact field strain measurements and its implications are discussed. Detailed Finite Element simulations are also carried out and subsequently employed to highlight the main features of the response of doweled connections in glulam. A foundation model, initially developed for Douglas-fir (Pseudotsuga menziesii) timber, is upgraded and adapted for Scandinavian Spruce Glulam (Picea abies) elements subjected to loads acting perpendicular and parallel to the grain direction. The proposed model is based on the definition of equivalent material parameters for the crushing region around the dowel hole. To this end, relationships for the estimation of material characteristics as a function of the crushing volume are suggested. The validity and accuracy of the proposed modified foundation models are examined against the experimental results. It is shown the improved foundation model is able to simulate the embedment stiffness, capacity and inelastic behaviour of single-dowel connections on glulam with reasonable accuracy for strains of up to 8%, and can therefore be used for design and assessment purposes.

Non-contact strain measurement in the mouse forearm loading model using digital image correlation (DIC)

This study investigates the use of a non-contact method known as digital image correlation (DIC) to measure strains in the mouse forearm during axial compressive loading. A two camera system was adapted to analyze the medial and lateral forearm displacements simultaneously, and the derived DIC strain measurements were compared to strain gage readings from both the ulna and radius. Factors such as region-of-interest (ROI) location, lens magnification, noise, and out-of-plane motion were examined to determine their influence on the DIC strain measurements. We confirmed that our DIC system can differentiate ROI locations since it detected higher average strains in the ulna compared to the radius and detected compressive strains on medial bone surfaces vs. tensile strains on lateral bone surfaces. Interestingly, the DIC method also captured heterogeneity in surface strain fields which are not detectable by strain gage based methods. A separate analysis of the noise intrinsic to the DIC system also revealed that the noise constituted less than 4.5% of all DIC strain measurements. Furthermore, finite element (FE) simulations of the forearm showed that out-of-plane motion was not a significant factor that influenced DIC measurements. Finally, we observed that average DIC strain measurements can be up to 1.5–2 times greater than average strain gage readings on the medial bone surfaces. These findings suggest that strain experienced in the mouse forearm model by loading is better captured through DIC as opposed to strain gages, which as a result of being glued to the bone surface artificially stiffen the bone and lead to an underestimation of the strain response.

Carbon nanotubes as non-contact optical strain sensors in smart skins

Progress is reported in the development of a novel non-contact strain measurement technology in which the sensors are single-walled carbon nanotubes. This approach exploits the characteristic short-wave infrared fluorescence signatures of semiconducting single-walled carbon nanotubes and the systematic shifts of their fluorescence wavelengths when the nanotubes are axially strained. A strain-sensing smart skin is prepared by coating the surface to be monitored with a thin film of a composite containing well-dispersed single-walled carbon nanotubes embedded in a urethane varnish host. Strain in the surface transfers through the host polymer to the embedded nanotubes. The nanotube strains are then quantitatively monitored by exciting the region of interest with a visible laser beam and capturing and analyzing the resulting single-walled carbon nanotube fluorescence spectrum. High-quality spectra are shown for strain-sensing smart skin films in which the nanotubes were pre-processed by selective extraction with the organic polymer poly(9,9-dioctylfluorenyl-2,7-diyl) and purification by centrifugation. The design of a compact, field-portable optical system for exciting, collecting, and interpreting strain-sensing smart skin spectral data is also described. This system is used to validate the linear dependence of peak shifts with strain in test specimens subjected to multiple tension cycles inducing strains from 0 to 5000 µε. Adequate linearity is found up to 1500 µε; however, minor hysteresis is observed above this level. The gage length for these measurements can be as small as several micrometers, and the strain resolution is currently between 10 and 100 µε (depending on strain magnitude). Cyclic reversible strain measurements are demonstrated, with minor hysteresis being attributed to imperfect adhesion between nanotube sidewalls and the surrounding polymer host.

(Invited) Progress Towards a Novel Technology for Non-Contact Strain Measurements Based on SWCNT Fluorescence Spectroscopy

One of the most distinctive properties of semiconducting single-walled carbon nanotubes (SWCNT) is their emission of structure-specific fluorescence (band gap photoluminescence) in the short-wave infrared (SWIR) spectral region. The fluorescence peak from each (n,m) structural species shifts in wavelength as the nanotube’s band gap is altered by axial strain. These shifts are known from theory and experiment to depend on the nanotube’s chiral angle and on its mod (n-m,3) value, and they are linear with strain for realistic axial deformations. We have previously shown that this effect may lead to a new strain measurement technology (called S4, for strain-sensing smart skin) in which individualized SWCNTs embedded in a polymeric film serve as strain sensors and readout is achieved by non-contact optical methods.1 Recent research progress aimed at enabling such an implementation will be described. Focus areas include processing and dispersing SWCNTs for effective use in S4; comparing suitability of different polymer formulations as S4 hosts; applying and stabilizing the S4 films; understanding factors that influence interfacial adhesion between SWCNTs and host molecules; miniaturizing the optical hardware to allow excitation and capture of SWCNT fluorescence spectra with hand-held devices; refinement of data analysis methods for deducing strains from spectral data; tests of sensitivity and reproducibility for S4 measurements with plastic, aluminum, and steel substrates; and construction of two-dimensional strain maps from sets of single-point measurements. 1 P. A. Withey, V. S. M. Vemuru, S. M. Bachilo, S. Nagarajaiah, and R. B. Weisman, Nano Lett. 12, 3497 (2012).

Viscoplastic tearing of polyethylene thin film

Recent advances in noncontact strain measurement techniques and large-strain constitutive modeling of the linear low-density polyethylene film used in NASA superpressure balloons StratoFilm 420 are combined to provide a novel measurement technique for the tear propagation critical value of the J-integral. Previously these measurements required complex test configurations and procedures. It is found that the critical value of the J-integral increases by approximately 50 % when the strain rate is decreased from 1.33×10−4 s−1 to 1.33×10−5 s−1. It is shown that there is good correlation between measurements made on uniaxially loaded dogbone samples and circular diaphragms loaded by pressure, both with a 2-mm-wide slit in the middle. This result indicates that more extensive studies of strain-rate dependence may be made with the simpler, uniaxial test configuration.

Evaluation and improvement of tire rolling resistance and grip performance based on test and simulation

In order to clarify the contradictory mechanism between tire rolling resistance and grip performance, ten (10) 205/55 R16 radial tires with different tread patterns were selected as the research objects. Using VIC-3D non-contact strain measurement system, the pattern deformation in the contact area under vertical load was tested and the relevant deformation parameters of the contact area were extracted. Correlation analysis was used to establish the relationship between the identified deformation parameters and tire performance indicators. Then the contradiction mechanism between tire rolling resistance and grip performance was identified. The mechanism is such that, in order to improve the grip performance of the tire, it is necessary to reduce the transverse tensile strain of the tread in the contact area and increase the longitudinal tensile strain of the tread, but with the increase of the longitudinal tensile strain, the rolling resistance of the tire will also increase, which leads to the contradiction between tire rolling resistance and grip performance. In order to better understand and solve this contradiction, a finite element model of 205/55R16 tire with complex pattern was established. The influence of the number and width of transverse grooves in outer shoulder area on tire rolling resistance and grip performance was analyzed by numerical simulation, where it was identified that, the longitudinal tensile deformation of the tread is the main cause of contradiction between the two performances. An optimized design of concave transverse groove with narrow groove in the middle and wide groove at both ends was proposed in the outer shoulder area to resolve the contradiction. Compared with the original scheme, the rolling resistance of the optimized scheme was reduced by 2.112 N, and the grip force saw an increase of 10.196 N, and thus delivering a cooperative improvement of tire rolling resistance and grip performance.

Fatigue crack initiation and damage mechanisms during ultrasonic fatigue testing of cast aluminium alloy AlSi7Mg

During the last years, an increased interest in in situ-damage characterization during ultrasonic fatigue testing evolved. Different non-contact in situ-surveillance systems have been developed or applied to the present ultrasonic fatigue testing systems, e.g. analysis of the nonlinearity parameter and the resonant frequency [1, 2] as well as thermal measurements [3–5]. In addition, Heinz et al. [6] performed noncontact strain measurements during ultrasonic fatigue testing. The aim of the present investigations is to evaluate the fatigue crack initiation in the cast aluminium alloy AlSi7Mg (A356) in sand and die cast condition under high cycle and very high cycle fatigue loading conditions. Therefore, a combined analysis of the development of the specimen temperature, the resonant frequency fres and the nonlinearity parameter rel was performed. For such investigations it is essential to keep the ambient temperature strictly constant and the increase of temperature due to the resonant conditions as low as possible. Therefore, a spot cooler using a vortex tube was applied to the ultrasonic fatigue testing system. With this special cooling device it was possible to keep the ambient temperature constant. An infrared thermo-camera system was used for the in situ-examination of the specimen temperature and the surveillance of the ambient temperature during ultrasonic fatigue testing. The determination of the resonant frequency and the nonlinearity parameter was realized online during fatigue tests according to Mayer et al. [2]. After failure, the fracture surfaces of the specimens were investigated intensively by scanning electron microscopy (SEM). According to Krewerth et al. [5], the correlation of the results from thermographic and fractographic investigations can shed some light on the crack initiation point. Figure 1a shows the S-N curves of the fatigue tests on the aluminum specimens produced by sand casting (batch 1) and die-casting (batch 2) in hot isostatically pressed condition, respectively. Specimens of both batches failed due to casting porosity. However, the casting porosity in the sand cast material was significantly higher leading to a lower fatigue strength. Several specimens were investigated by thermographic measurements accompanied by simultaneous analysis of the resonant frequency as well as the nonlinearity parameter. Exemplarily, specimen 1 (sand casting, see Fig. 1a) is explained here in more detail. The development of the resonant frequency (fres), the nonlinearity parameter ( rel), the ambient temperature Tamb, the maximum temperature Tmax and the temperature variation T is shown in fig. 1b. This specimen failed due to an internal defect at Nf = 2.16 × 106 cycles. The dashed black

The shear properties and deformation mechanisms of porous metal fiber sintered sheets

Porous metal fiber sintered sheets (MFSSs) are a type of layered transversely isotropic open cell materials with low relative density (i.e., volume fraction of fibers), high specific stiffness and strength, and controllable precision for functional and structural applications. Based on a non-contact optical full field strain measurement system, the in-plane and transverse shear properties of SMFFs with relative densities ranging from 15% to 34% are investigated. For the in-plane shear, the modulus and strength are found to depend linearly upon the relative density. The associated deformation is mainly due to fiber stretching, accompanied by the direction change of metal fibers. When the shear loading is applied in the transverse direction, the deformation of the material is mainly owing to fiber bending, followed by the separation failure of the fiber joints. Measured results show that the transverse shear modulus and strength have quartic and cubic dependence upon the relative density respectively and are much lower than their in-plane counterparts. Simple micromechanics models are proposed for the in-plane and transverse moduli and strengths of MFSSs in shear. The predicted relationships between the shear mechanical properties of MFSSs and their relative density are obtained and are in good agreement with the measured ones.

Measurements of the deformation zone around a split-axis snow micropenetrometer tip

Current physical models for the snow micropenetrometer (SMP) make the assumption that the tip of the SMP probe only induces stress or fractures in the ice grains in direct contact. However, past research indicates the existence of an extended deformation zone around the tip which contains many strained or fractured ice grains. A noncontact optical strain measurement system directly observed snow deformation within well rounded snow caused by a split-axis SMP probe simulator. Results confirm the existence of a deformation zone ahead of and normal to the surfaces of the SMP probe tip. By assuming axial symmetry, the estimated volume of the deformation zone was on the order of 1000mm3. Approximately 40%–50% of this volume was due to deformation in front of the tip, which is currently not considered in micromechanical penetration resistance theories for snow.

In situ identification of elastic–plastic strain distribution in a microalloyed transformation induced plasticity steel using digital image correlation

A non-contact strain measurement technique, based on an in-situ digital image correlation (DIC) method in association with magnetic martensite point measurement (Feritoscopy testing) was applied to study inhomogeneous deformation corresponding to martensitic transformation of a microalloyed low carbon transformation induced plasticity steel during tensile straining. The progress of inhomogeneous deformation is traced by the strain maps. The microstructural observation is used to validate the DIC results. The experimental steel shows continuous yielding with a high true fracture strength of 1410±10MPa at 25°C along with the lack of tensile necking. The DIC results show that the yield point is controlled by stress-assisted martensite transformation, which in turn induces the strain inhomogeneity. The latter starts prior to the yield point after straining to 0.016. The microstructural evolution reveals the ε-martensite is obtained through stress-assisted martensite formation. After yielding, thanks to the strain-induced martensite transformation, the deformation inhomogeneity in strain maps is increased with strain, corresponding to increasing the volume fraction of martensite. The results suggest that the continuous yielding and initial strain hardening is controlled by stress-assisted martensite formation while the higher total elongation to fracture (80%) and the tensile necking behavior is mainly influenced by the strain-induced martensite transformation.

High-temperature strain measurement using active imaging digital image correlation and infrared radiation heating

A technique for non-contact and full-field high-temperature strain measurement of a sample subjected to radiation heating using active imaging digital image correlation is described in this work. A high-performance quartz lamp heater system was designed to reproduce transient thermal environments experienced by hypersonic vehicles. The digital images of the test sample surface at various temperatures are captured using a novel active imaging optical system based on a combination of monochromatic light illumination and bandpass filter imaging. Subsequently, the captured images are processed by a robust reliability-guided displacement tracking algorithm with an automatic reference image updating scheme to extract full-field thermal deformation. With the improvements made in both the imaging system and correlation algorithm, the de-correlation problem of speckle patterns caused by the thermal radiation and surface oxidation of the heated test object are effectively addressed, enabling reliable deformation measurement in extremely high temperature environments. The performance of the proposed active imaging digital image correlation technique is verified by two experiments: (1) measurement of the uniform thermal strains of a chromium–nickel austenite stainless steel sample which is heated from room temperature to 1300 °C, and (2) measurement of the non-uniform thermal strain fields of a woven C/SiC composite at 1550 °C. The test results show that the active imaging digital image correlation is an easy-to-implement yet effective optical technique for high-temperature strain measurement, and has great potential in characterizing thermo-mechanical behaviour of materials and structures for hypersonic vehicles. Limitations and potential improvements of the present technique are also discussed.

The effect of film thickness on polypyrrole actuation assessed using novel non-contact strain measurements

Micro-actuators have been developed that exploit the electrochemically induced volume change of the electro-active polymer polypyrrole. The strain regime is inherently complex at a physical level and whilst volume change can be estimated indirectly using, for instance, bending beam theory, such methods become unreliable for large deflections owing to limitations in the mathematical model. A new non-contact measuring technique based on laser micrometry is presented to characterize the time-dependent expansion of electro-active films such as polypyrrole. Measurements have been made which demonstrate that the observed strain is dependent on film thickness. The new measurement technique is straightforward to perform and it is anticipated that it can be used for future materials development and performance assessment, including long-term stability evaluations and operational failure studies of the films.

Application of DIC System in the Tensile Test of Medium and Heavy Plate

Digital image correlation (DIC) is a kind of non-contact strain measurement method. It could provide deformation information of a specimen by processing two digital images that are captured before and after the deformation. the application of DIC method in performance testing and evaluation of new materials has great superiority over the traditional methods in the aspects that help to explore the features of new materials, such as mechanical performance, deformation mechanism and dynamic and static performance[1-. Tensile test of 8mm steel plate based on DIC is introduced in this paper, this test is easy operated and high accuracy. It can not only get all the data that traditional method done, but also reflect the full range strain, strain history and strain rate. It can also transform the result among different type strain. The experiments based on DIC method of medium and heavy plate can help to learn the properties of material like this kind much more comprehensively, reduce testing time and promote the wide application of new material in manufacturing.

Analysis of fatigue properties and failure mechanisms of Ti6Al4V in the very high cycle fatigue regime using ultrasonic technology and 3D laser scanning vibrometry

Accelerated fatigue tests with Ti6Al4V were carried out using a 20kHz ultrasonic testing facility to investigate the cyclic deformation behavior in the Very High Cycle Fatigue (VHCF) regime in detail. Beside parameters like the ultrasonic generator power and the displacement of the specimen, a 3D laser scanning vibrometer was used to characterize the oscillation and fatigue behavior of the Ti-alloy. The course of the S–Nf curve at the stress ratio R=−1 shows a significant decrease of the bearable stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 107 cycles. Microscopic investigations of the distribution of the α- and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. High resolution vibrometry was used to visualize the eigenmode of the designed VHCF-specimen at 20kHz in the initial state and to indicate local changes in the eigenmodes as a result of progressing fatigue damage. Non-contact strain measurements were realized and used to determine the stress amplitude. The determined stress amplitudes were correlated with strain gauge measurements and finite element analysis.

Monitoring the Wall Mechanics During Stent Deployment in a Vessel

Clinical trials have reported different restenosis rates for various stent designs1. It is speculated that stent-induced strain concentrations on the arterial wall lead to tissue injury, which initiates restenosis2-7. This hypothesis needs further investigations including better quantifications of non-uniform strain distribution on the artery following stent implantation. A non-contact surface strain measurement method for the stented artery is presented in this work. ARAMIS stereo optical surface strain measurement system uses two optical high speed cameras to capture the motion of each reference point, and resolve three dimensional strains over the deforming surface8,9. As a mesh stent is deployed into a latex vessel with a random contrasting pattern sprayed or drawn on its outer surface, the surface strain is recorded at every instant of the deformation. The calculated strain distributions can then be used to understand the local lesion response, validate the computational models, and formulate hypotheses for further in vivo study.

Monitoring the Wall Mechanics During Stent Deployment in a Vessel

Clinical trials have reported different restenosis rates for various stent designs. It is speculated that stent-induced strain concentrations on the arterial wall lead to tissue injury, which initiates restenosis. This hypothesis needs further investigations including better quantifications of non-uniform strain distribution on the artery following stent implantation. A non-contact surface strain measurement method for the stented artery is presented in this work. ARAMIS stereo optical surface strain measurement system uses two optical high speed cameras to capture the motion of each reference point, and resolve three dimensional strains over the deforming surface. As a mesh stent is deployed into a latex vessel with a random contrasting pattern sprayed or drawn on its outer surface, the surface strain is recorded at every instant of the deformation. The calculated strain distributions can then be used to understand the local lesion response, validate the computational models, and formulate hypotheses for further in vivo study.

Active noise thin-plate spline smoothing method for full-f ield strain measurement by digital image correlation

Digital image correlation has become an important and effective non-contact optical full-field strain measurement technique. The strain field obtained directly by image correlation algorithm is full of noise. In this letter, we explore a novel way of actively adding small amount of Gaussian random noise to original displacement field, subsequently utilizing the well-known thin-plate spline smoothing (TPSS) technique to smooth the noised displacement field, and finally differentiating smoothed displacement field to get reliable strain field. The resultant method, named as active noise thin-plate spline smoothing (ANTPSS), outperforms the conventional TPSS and spline least-squares approximation. Moreover, ANTPSS successfully smooths the displacement filed obtained from three-point bending experiment of foam block and generates a reliable inhomogeneous strain field.

High-temperature materials testing with full-field strain measurement: Experimental design and practice

Experimental characterization of the thermomechanical response of ceramic composites at very high temperatures is plagued by challenges associated with imaging and strain measurement. The problems involve illumination, heat haze, and surface contrast. Techniques that address these challenges have been developed and implemented in a laser heating facility, enabling non-contact strain measurement via digital image correlation. The thermomechanical characterization of both a Ni-based superalloy and a C/SiC composite are used to demonstrate the efficacy of experimental practices in realizing such measurements at temperatures up to 1500 °C.