Direct Strain Measurement Research Articles

Direct noninvasive measurement and numerical modeling of depth-dependent strains in layered agarose constructs

Biomechanical factors play an important role in the growth, regulation, and maintenance of engineered biomaterials and tissues. While physical factors (e.g. applied mechanical strain) can accelerate regeneration, and knowledge of tissue properties often guide the design of custom materials with tailored functionality, the distribution of mechanical quantities (e.g. strain) throughout native and repair tissues is largely unknown. Here, we directly quantify distributions of strain using noninvasive magnetic resonance imaging (MRI) throughout layered agarose constructs, a model system for articular cartilage regeneration. Bulk mechanical testing, giving both instantaneous and equilibrium moduli, was incapable of differentiating between the layered constructs with defined amounts of 2% and 4% agarose. In contrast, MRI revealed complex distributions of strain, with strain transfer to softer (2%) agarose regions, resulting in amplified magnitudes. Comparative studies using finite element simulations and mixture (biphasic) theory confirmed strain distributions in the layered agarose. The results indicate that strain transfer to soft regions is possible in vivo as the biomaterial and tissue changes during regeneration and maturity. It is also possible to modulate locally the strain field that is applied to construct-embedded cells (e.g. chondrocytes) using stratified agarose constructs.

Research and Implementation of Blade Tip Timing Vibration Monitoring Method Based on Fiber Bragg Magnetic Coupling Sensors

Fiber Bragg grating sensor is a kind of optical fiber sensor, the sensing process based on fiber grating is through the external physical parameters on fiber Bragg wavelength modulation to obtain sensing information. It is a kind of wavelength modulation type optical fiber sensor. Fiber Bragg grating sensor can realize the direct measurement of temperature, strain and other physical quantities. This paper have the research and analysis on blade tip of mine by using fiber Bragg magnetic coupling sensor, in order to realize long distance and non-contact real-time safety monitoring of blade vibration. The paper first describes the theoretical method of the detection of blade tip timing vibration, then does the dynamic characteristic test. By means of the compare between normal and damaged blade, the method is proved feasible.

Pulse width modulation-based temperature tracking for feedback control of a shape memory alloy actuator.

This work presents a temperature-feedback approach to control the radius of curvature of an arc-shaped shape memory alloy (SMA) wire. The nonlinear properties of the SMA such as phase transformation and its dependence on temperature and stress make SMA actuators difficult to control. Tracking a desired trajectory is more challenging than controlling just the position of the SMA actuator since the desired path is continuously changing. Consequently, tracking the desired strain directly or tracking the parameters such as temperature and electrical resistance that are related to strain with a model is a challenging task. Temperature-feedback is an attractive approach when direct measurement of strain is not practical. Pulse width modulation (PWM) is an effective method for SMA actuation and it can be used along with a compensator to control the temperature of the SMA. Using the constitutive model of the SMA, the desired temperature profile can be obtained for a given strain trajectory. A PWM-based nonlinear PID controller with a feed-forward heat transfer model is proposed to use temperature-feedback for tracking a desired temperature trajectory. The proposed controller is used during the heating phase of the SMA actuator. The controller proves to be effective in tracking step-wise and continuous trajectories.

Direct measurement and modelling of internal strains in ion-implanted diamond

We present a phenomenological model and finite element simulations to describe the depth variation of mass density and strain of ion-implanted single-crystal diamond. Several experiments are employed to validate the approach: firstly, samples implanted with 180 keV B ions at relatively low fluences are characterized using high-resolution x-ray diffraction; secondly, the mass density variation of a sample implanted with 500 keV He ions, well above its amorphization threshold, is characterized with electron energy loss spectroscopy. At high damage densities, the experimental depth profiles of strain and density display a saturation effect with increasing damage and a shift of the damage density peak towards greater depth values with respect to those predicted by TRIM simulations, which are well accounted for in the model presented here. The model is then further validated by comparing transmission electron microscopy-measured and simulated thickness values of a buried amorphous carbon layer formed at different depths by implantation of 500 keV He ions through a variable-thickness mask to simulate the simultaneous implantation of ions at different energies.

On the Assessment of Fatigue Damage in Railway Bridges

This paper discusses the benefits to using direct strain measurements when undertaking a fatigue assessment of a railway bridge. The paper gives a brief overview of the assessment methodology for assessing fatigue in a bridge carrying a single track before outlining a method suitable for assessing multi-track bridges. The accuracy of a fatigue assessment depends on the level of knowledge of many different parameters including: an accurate loading history comprising axle loads and train frequencies over its service life; details of changes to the track form, including changes in the depth of ballast; and changes to the structure including damage or repairs. Many of these parameters are not known with certainty. However, using monitoring systems to measure the strain cycles in a bridge helps to address some of these uncertainties and hence improves the accuracy of the assessment.

Real-time monitoring of phase maps of digital shearography

Digital shearography has demonstrated great potential in direct strain measurement and, thus, has become an industrial tool for nondestructive testing (NDT), especially for NDT of delaminations and detection of impact damage in composite materials such as carbon fiber reinforced plastics and honeycomb structures. The increasing demand for high measurement sensitivity has led to the need for real-time monitoring of a digital shearographic phase map. Phase maps can be generated by applying a temporal, or spatial, phase shift technique. The temporal phase shift technique is simpler and more reliable for industry applications and, thus, has widely been utilized in practical shearographic inspection systems. This paper presents a review of the temporal phase shift digital shearography method with different algorithms and the possibility for real-time monitoring of phase maps for NDT. Quantitative and real-time monitoring of full-field strain information, using different algorithms, is presented. The potentials and limitations for each algorithm are discussed and demonstrated through examples of shearographic testing.

Direct measurements of field-induced strain in magnetoelectric composites by X-ray diffraction studies of forbidden reflections

Static and magnetic field induced strains at a magnetoelectric (ME) buried interface, consisting of a ZnO(001) piezoelectric substrate and an amorphous Fe58Tb42 magnetostrictive layer, were determined by measuring the intensity of the forbidden ZnO(3–31) Bragg reflection. The comparison with an uncoated ZnO substrate shows that a permanent strain is induced by the growth process. For FeTb coated ZnO, an additional static strain of (1.6 ± 0.2) × 10−4 relative to the pure ZnO substrate is found near the interface, which is close to that determined from the peak position shift of the allowed ZnO(4–40) Bragg reflection. Upon applying a magnetic field along the [1–10] direction, the intensity of the ZnO(3–31) forbidden Bragg reflection increases with increasing field magnitude as predicted. The induced strain, transferred to the piezoelectric ZnO substrate due to the external magnetic field acting on the magnetostriction FeTb layer, approaches a saturation value of εH = (3.7 ± 0.2) × 10−4 at B ≥ 0.05 T, in good agreement with the results from cantilever bending measurements. These results indicate an excellent coupling at this ME interface.

Large Deformation and Adhesive Contact Studies of Axisymmetric Membranes

A model membrane contact system consisting of an acrylic copolymer membrane and a PDMS substrate was utilized to evaluate a recently developed nonlinear large-deformation adhesive contact analysis. Direct measurements of the local membrane apex strain during noncontact inflation indicated that the neo-Hookean model provides an accurate measure of membrane strain and supports its use as the strain energy function for the analysis. Two membrane contact geometries, exhibiting significantly different strain distributions during withdrawal, were investigated. The first examines the wet contact of an air pressurized membrane. The second looks at the dry contact of a fluid deformed membrane in which a stepper motor controls membrane-substrate separation. A time-dependent modulus emerges from the analysis, with principal tensions obtained from a comparison of predicted and experimental membrane profiles. The applicability of this numerical analysis for determining membrane tension, however, is limited by wrinkling instabilities and viscoelasticity. For this reason, a conceptually simpler method, based on the direct measurement of the membrane tension and contact angle, was also utilized. The traditional peel energy defined with this direct measurement accurately described the membrane/substrate adhesive interactions, giving well-defined peel energies that were independent of the detailed strain state of the membrane.

Direct and Indirect Strain Measurement of Flexible Printed Circuit Boards - fPCBs

This work is related to reliability of strain measurement in flexible printed circuit boards (fPCBs) made with polyimide substrate. It was observed that the fPCBs are very sensitive to strain mounting stiffness. The indirect measurement method will be done employing High Speed Camera (HSP). The direct method will be formulated in two ways: 1) conventional strain gauge glued in an fPCBs; 2) printed strain gauge in a polyimide substrate. This paper will point out mistakes and show advantages when using different method to extract the deformation field of the selected area in a flexible thin film.

Direct measurements of field-induced strain at magnetoelectric interfaces by grazing incidence x-ray diffraction

The magnetic field induced strain at the interface of a magnetoelectric composite, consisting of a ZnO(001) piezoelectric substrate and an amorphous (Fe90Co10)78Si12B10 magnetostrictive layer, was directly determined by grazing incidence X-ray diffraction. Upon applying a magnetic field along the [110] direction, the ZnO single crystal is under tensile strain in [110] direction and compressive strain in [1-10] direction, in agreement with the magnetostriction in the (Fe90Co10)78Si12B10 layer. At room temperature, the saturation strain along [1-10] of about 3 × 10−5 is close to the saturation magnetostriction of the film measured with the cantilever bending technique.

Fatigue Testing and Structural Health Monitoring of Retrofitted Web Stiffeners on Steel Highway Bridges

Numerous steel highway bridges, still in use today, were built during the construction boom between the late 1950s and the late 1970s. Fatigue cracking can be considered a main source of deterioration for these bridges. The largest category of observed fatigue cracks is caused by out-of-plane distortion. The most susceptible locations are those at which transverse structural components (such as diaphragms or cross frames) are framed into longitudinal girders through web stiffeners that are not attached to the flanges. In the current study, a web stiffener detail is fatigue tested under different cyclic loading conditions. As-welded specimens are tested, along with specimens retrofitted by grinding and rewelding, needle peening, or the adhesive bonding of fiber-reinforced polymer attachments. Direct strain and deflection measurements are compared with finite element analysis predictions, and local (hot-spot) stresses are compared with hot-spot stress design curves. A time series–based method for damage detection is also explored for the prediction of fatigue crack depth with strain data. The method is validated through the use of small- and large-scale specimen strain data. It is found that damage measures based on strains in the vicinity of the critical hot spot are closely correlated with the true crack depth.

A Novel Fiber Bragg Grating Based Sensing Methodology for Direct Measurement of Surface Strain on Body Muscles during Physical Exercises

The present work proposes a new sensing methodology, which uses Fiber Bragg Gratings (FBGs) to measure in vivo the surface strain and strain rate on calf muscles while performing certain exercises. Two simple exercises, namely ankle dorsi-flexion and ankle plantar-flexion, have been considered and the strain induced on the medial head of the gastrocnemius muscle while performing these exercises has been monitored. The real time strain generated has been recorded and the results are compared with those obtained using a commercial Color Doppler Ultrasound (CDU) system. It is found that the proposed sensing methodology is promising for surface strain measurements in biomechanical applications.

Fault rock injections record paleo-earthquakes

Fault rocks such as pseudotachylyte melt or granular gouge are sometimes injected into cracks off a fault surface. The width to height ratio of the injection veins is a direct measure of the shear strain in the wall rock required to accommodate the injection, so the aspect ratios preserve a record of the overpressure opening the cracks and injecting melt or fluidized granular material from the fault zone. We measured the aspect ratios for 201 pseudotachylyte injections and 29 granular injections. They have aspect ratios of ∼0.2 (0.17±0.025 and 0.22±0.077, respectively, within 99% confidence). These are significantly proportionally wider than the aspect ratios of ordinary dikes (from 10−4 to 10−3). The injection aspect ratios exceed the usual limit of elastic strain in the wallrock (∼1%) so we infer that the injection veins were accommodated by permanent deformation by microcracking and slip on foliation surfaces in the host rock, which is difficult to detect observationally. The fully elastic model therefore offers an upper bound on the stress required to open these injection veins (∼108–1010Pa, with higher pressures in stiffer host rock lithologies.) These overpressures cannot be achieved by expansion of the fault rock fluids during melting or gouge fluidization mechanisms, as the pressure along the fault is limited to ∼107Pa by the wall rock compliance. Dynamic tension parallel to the fault is required to explain the large apparent overpressures in the injection veins. Therefore, both pseudotachylyte and gouge injections of this aspect ratio must be considered seismic signatures.

분산제 농도에 따른 MWCNT 전도성 유연필름의 전기-기계적 특성

탄소나노튜브는 유연 전도체로서 투명전극, 유연 히터, 투명 스피커 분야에 활발히 응용되고 있다. 본 연구에서는 PET 유연 모재의 전면적에 다중벽 탄소나노튜브(MWCNT)를 도포하는 스프레이법 이용하여, 투명하고 전도성 있는 MWCNT/PET 복합 박막을 제조하였다. 분산제로 사용된 SDS (sodium dodecyl sulfate)의 농도를 조절함으로써 MWCNT의 분산도를 조절하였으며, 분산도가 조절된 시험편의 인장실험을 통해 기계적인 변형 하에서 MWCNT/PET 전도성 박막의 전기-기계적인 거동과 분산제의 효과를 평가하였다.

In vivo dynamics of the internal fibrous structure in smooth adhesive pads of insects

Many insects with smooth adhesive pads can rapidly enlarge their contact area by centripetal pulls on the legs, allowing them to cope with sudden mechanical perturbations such as gusts of wind or raindrops. The short time scale of this reaction excludes any neuromuscular control; it is thus more likely to be caused by mechanical properties of the pad’s specialized cuticle. This soft cuticle contains numerous branched fibrils oriented almost perpendicularly to the surface. Assuming a fixed volume of the water-filled cuticle, we hypothesized that pulls could decrease the fibril angle, thereby helping the contact area to expand laterally and longitudinally. Three-dimensional fluorescence microscopy on the cuticle of smooth stick insect pads confirmed that pulls significantly reduced the fibril angle. However, the fibril angle variation appeared insufficient to explain the observed increase in contact area. Direct strain measurements in the contact zone demonstrated that pulls not only expand the cuticle laterally, but also add new contact area at the pad’s outer edge.

Vision-based force sensing for nanomanipulation

In this paper, a vision-based algorithm for estimating tip interaction forces on a deflected atomic orce microscope (AFM) cantilever is described. Specifically, we propose that the algorithm can estimate forces acting on an AFM cantilever being used as a nanomanipulator inside a scanning electron microscope (SEM). The vision-based force sensor can provide force feedback in real time, a feature absent in many SEMs. A methodology based on cantilever slope detection is used to estimate the forces acting on the cantilever tip. The technique was tested on a scaled model of the nanoscale AFM cantilever and verified using theoretical estimates as well as direct strain measurements. Artificial SEM noise was introduced in the macroscale images to characterize our sensor under varying levels of noise and other SEM effects. Prior knowledge about the cantilever is not required, and the algorithm runs independent of human input. The method is shown to be effective under varying noise levels, and demonstrates improving performance as magnification levels are decreased. Therefore, we conclude that the vision-based force sensing algorithm is best suited for continuous operation of the SEM, fast scanning rates, and large fields-of-view associated with low magnification levels.

Direct measurement of radial strain in the inner-half layer of the left ventricular wall in hypertensive patients

Two-dimensional speckle tracking echocardiography (2D-STE) is a novel technology that directly measures regional left ventricular (LV) wall contraction. This study aimed to directly measure inner-layer thickening (radial strain) of the LV using 2D-STE, and to examine the relationship between radial strain and the degree of hypertrophy. The study enrolled 63 untreated hypertensive patients with normal geometry (N group, n=32) or concentric hypertrophy (CH group, n=31), classified according to LV mass index (LVMI) and relative wall thickness (RWT). Thirty normotensive subjects (C group, n=30) served as controls. Radial strain (ɛ) in the inner half (ɛi) and all layers of the LV wall (ɛa) were calculated from the LV short-axis view by 2D-STE. LV ejection fraction did not differ significantly among the groups. However, ɛi and ɛa were significantly lower in the CH group compared with the C and N groups (p<0.01). A ratio of ɛi to ɛa was significantly lower in the CH group compared with the C and N groups (p<0.01). A multivariate regression model that included midwall fractional shortening, E/e', LVMI, RWT, and LV ejection fraction showed that LVMI (p=0.002) and RWT (p=0.014) were independent predictors (R(2)=0.59) of ɛi. Radial strain in the inner half layer of the LV wall decreases in parallel with the degrees of LV concentricity and hypertrophy in hypertensive patients. Radial strain in the inner half layer may identify subtle systolic dysfunction even in hypertensive patients with preserved LV chamber function.

Direct measurement of intrinsic critical strain and internal strain in barrier films

Resistance measurements during uniaxial tensile deformation of very thin (10 nm) conducting oxide films deposited on 150 nm SiN films on polyethylene naphthalate are discussed. It is first shown that certain characteristics of resistance versus strain curves are representative for the fracture behavior of the SiN film and not for that of the thin conducting oxide film. Subsequently, it is shown that the hysteresis in curves of resistance as a function of strain offers a way to directly measure the intrinsic critical strain of the SiN film without the need to determine internal strains from independent (curvature) measurements that rely on knowledge of moduli and geometry. The method should be applicable, in general, to measure intrinsic critical strain and residual strains of thin brittle films on polymers. Advantages and limitations of the method are discussed.

Determination of displacement distributions in bolted steel tension elements using digital image techniques

Digital imaging methods have found a great interest in various engineering fields to study stress-deformation characteristics of materials. Recent enhancements in visual instrumentation with the availability of cost-effective hardware and software products make the digital imaging techniques a viable tool to replace direct strain or displacement measurement methods in engineering applications. In this study, deformation characteristics of bolted steel connections are investigated by calculating in-plane displacement distributions using digital image correlation method (DIC). Validation of the method is presented by comparing the strains measured in standard tension specimens using electrical resistance strain gages and the DIC method. Finite element analysis of the connection specimen is also performed to compare the in-plane displacement distributions calculated from both methods. Results from the validation process indicate that the strains obtained from the DIC method compare well with the results of strain gages. The findings also indicate that the displacement distributions calculated from the finite element method may differ from those of the DIC method in terms of distribution pattern, and the location and magnitude of the extreme values of displacements. It is suggested that the proposed method can be used to determine the in-plane displacement distributions for the bolted connections, hence to evaluate their deformation characteristics under loading.

Experimental study of dynamic compressive properties of fibre reinforced concrete material with different fibres

This research conducts drop weight impact tests to study the dynamic compressive properties of fibre reinforced concrete (FRC) material with different types of fibres. The impact tests are conducted with an instrumented drop-weight impact system consisting of a hard steel drop weight, two 180 t fast response loadcells, a high-speed video camera, and a fast response data acquisition system. Seven fibre types with different shapes and material properties are considered in the study. They are synthetic fibres, undulated, cold rolled, flattened, hooked end, and two new spiral shape steel fibres developed in this study. A volume fraction of 1% fibre is used in all specimens. The concrete matrix for all FRC specimens is mixed to obtain a compressive strength of 35 MPa. The drop-weight impact experiments are conducted with two different drop heights in order to study the dynamic material properties at different strain rates. The impact forces on top and bottom of specimens are measured to investigate the axial inertia effects and the stress wave propagation effect. The high-speed video camera is used to capture the failure process, displacement and velocity responses of specimens, which are used to estimate the strain and strain rates of the specimen under impact loading. Strain gages are also used for direct strain measurements. The dynamic stress–strain relations and impact resistance of the tested specimens are compared. The influence of fibre shapes on the failure modes, strength and energy absorbing capability of FRC is discussed. The rate sensitivities of the compressive strength, Young’s modulus and toughness of FRC are also examined. The testing results demonstrate that the new spiral steel fibre proposed in this study provides better confinement to concrete matrix and thus better bonding to concrete material, therefore increases the dynamic resistance and energy absorption capacity (toughness) of FRC.