Optical Strain Measurement Techniques Research Articles

Optical strain measurement technique for estimating degradation of the properties of carbon fiber reinforced polymer composites under cyclic loading

The goal of the study is the fatigue process in polyimide-based composites reinforced with short carbon fibers. Parameters of mechanical hysteresis loops such as the loop area, secant and dynamic moduli were used in the study. Hysteresis loops were constructed using the developed hardware and software system based on the optical method of strain measurements using a digital image correlation (DIC) technique. Methods for calculating the moduli and the parameters of mechanical hysteresis loops is considered. The results of their evaluation and the experimental data on the fatigue behavior of polyimide-based composites reinforced with short carbon fibers are presented. It is shown that an important quantitative measure of the differences in the fatigue behavior of the studied composites is the hysteresis induced energy loss. For a composite with carbonized fibers, the energy loss level per cycle is 35 kJ/m3, whereas for a composite with graphitized fibers it is 34% lower (23 kJ/m3). At the same time, the fatigue durability of the latter is - 40 times lower. A decrease both in the secant modulus (up to 11%) and the dynamic modulus (up to 3.5%) was observed in cyclic tests. However, the reduction was twice as much in a composite with carbonized fibers possessing a longer durability. Thus, the DIC-based estimation of mechanical hysteresis loops by the parameters of the secant and dynamic moduli, as well as the loop area can be successfully used to interpret the difference in the fatigue characteristics at the stage of scattered damage accumulation, whereas an unambiguous prediction of the residual life appeared impossible. The problem requires further systematic studying using approaches of the fracture mechanics.

Preliminary study to investigate the applicability of optical strain measurement technique for the detection of hot cracks in laser metal deposited layers

Laser metal deposition (LMD) as an additive manufacturing technique became increasingly important in recent years and thus the demand for component safety. This is the reason, for the need for reliable in-situ defect detection techniques. For laser beam weld seams an optical measurement technique based on an optical flow algorithm was successfully used to define the critical straining conditions that lead to hot cracking. This algorithm was adapted for bead-on-plate weld seams on LMD deposited layers of IN718 alloy while performing external strain on the specimen in an externally loaded hot cacking test facility. The resulting transversal hot cracks along the weld seam were localized via X-Ray inspection and the type of cracking confirmed by Scanning Electron Microscopy (SEM). The strain distribution was measured in the vicinity of the solidification front and correlated to the detected hot cracks. Based on the results this technique could be adopted for LMD experiments.

Damage growth and failure detection in hybrid fiber composites using experimental in-situ optical strain measurements and smoothing element analysis

In previous study the failure initiation and development in hybrid fiber laminates was successfully monitored and determined. In current investigation a novel damage monitoring approach is proposed for hybrid laminates by combining different optical strain measurement techniques namely digital image correlation (DIC), fiber Bragg grating sensors (FBG) and infrared thermography (IRT) with smoothing element analysis (SEA). This viable experimental procedure eliminates the effects of global/local nature of optical strain measurement systems on heterogeneous damage accumulation and is a two-step approach. First, all optical sensing systems together with conventional strain gauges are utilized concurrently to indicate the differences in the measured strains and monitor damage accumulation under tensile loading. This demonstrates how failure events disturb the measurement capabilities of optical systems, which can cause a miscalculation of hybrid effect in hybrid-fiber laminates. The second step involves the utilization of SEA algorithm for discretely measured DIC displacements to predict a realistic continuous displacement/strain map and rigorously mitigate the inherent noise of the full field optical system. Remarkably, for large deformation states in hybrid composites, the combination of SEA/DIC enables early prediction of susceptible damage zones at stress levels 30% below material strength.

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.

On the Indentation Resistance of a PC Layer on PVC Foam Substrate

The indentation response of a polycarbonate face sheet bonded to a PVC foam substrate is investigated experimentally and numerically. The deformation mode involves foam crushing, and membrane stretching of the PC layer at large indenter displacements; this is quantified using optical strain measurement techniques. The bottom corners of the foam substrate lifts off its underlying support when the foam layer is sufficiently thin. Peak load is dictated by tensile failure of the foam on the bottom face. Finite element simulations suggest that a deep foam core prevents this lift‐off and results in a greater load carrying and energy absorption capacity.

The fatigue life of notched magnesium sheet metals with emphasis on the effect of bands of twinned grains

Notched AM50 sheet metal specimens with four different stress concentration factors are tested under quasi-static and cyclic uniaxial loading, and the results are analyzed and compared with those of unnotched specimens. Special attention is given to the formation of bands of twinned grains and the inhomogeneous local strain field in the vicinity of notches, which is measured by an in situ optical strain measurement technique. It is shown that the notch sensitivity is reduced, if bands of twinned grains occur. In the range between 500 and 1000 load cycles to fracture, the endurable load is only slightly reduced by the presence of a notch for completely reversed fatigue tests. In contrast, specimens are notch sensitive under loading conditions, where no twin formation occurs.

Edge crack sensitivity of lightweight materials under different load conditions

This study addresses the analysis of edge crack sensitivity of DP800 steel and AA5182 aluminum alloy in dependency of punching and machining operation as well as load case of subsequent forming. The inserting of a round hole by punching with defined punch-to- die-clearance, milling and drilling is compared. Subsequent forming is performed by standardized hole expansion test and by Nakajima-tests with three different specimen geometries. Local strain distribution at the surface for Nakajima-tests is measured by optical strain measurement technique and investigated in order to evaluate local deformation before failure. Additionally, resulting hole expansion ratio λ is determined. Significant higher X as well as local strain values εmax are achieved by machined holes. This is directly coupled to higher local formability and stretchability for both materials. Furthermore, the load condition has a strong impact on the edge crack sensitivity of the material. Prior failure is observed with changing stress conditions using different specimen geometries also influencing the reachable maximum failure strain. Higher edge crack sensitivity is observed for DP800, which is in good accordance to the material properties in terms of ductility and strength. These data in dependency of the process parameter can be used for the design of automotive components.

Mechanical characterization between room temperature and 1000 °C of SiC free-standing thin films by a novel high-temperature micro-tensile setup.

A novel high-temperature micro-tensile setup allows the characterization of the elastic and plastic as well as creep behavior of free-standing thin films at temperatures of up to 1000 °C. Correspondingly, a new layout for free-standing thin film tensile test structures has been developed, enabling accurate self-alignment upon loading. Furthermore, a differential optical strain measurement technique as well as optimizations of the optical path has been implemented, providing a strain resolution of well below 1 × 10(-4) at 1000 °C. Two different polycrystalline SiC free-standing thin films have been investigated in tension to acquire stress-strain data and corresponding Young's modulus at up to 1000 °C. The high sensitivity of the strain measurement technique makes it also possible to identify creep strains in the high-temperature regime.

A camera-based strain measurement technique for elastomer tensile testing: Simulation and practical application to understand the strain dependent accuracy characteristics

The present work investigates the precision characteristics of two optical strain measurement techniques applied to elastomers subjected to large deformations. The measurement approach is based on generating intensity profiles by using several horizontal image lines in the region of an optical marker to be detected. For detectability and accuracy reasons, these lines are combined using a rank (maximum) value filter and a moving average filter, while the calculation of the marker centers is carried out either geo- or gravimetrically. Based on simulated profiles, the first part of this work investigates the influence of method related parameters on the measurement precision obtained, expressed in terms of the root mean square error (rms). Further, it establishes model relations between most important image/profile parameters and rms. In the second part, experimental image data obtained during tensile testing of an elastomer sample is analyzed by i) applying the strain measurement technique, ii) determining experimental rms-values and iii) discussing them in comparison to values predicted by the rms-model of part one. It was found that, for the gravimetrical center calculation, rms strongly depends on the number of profile pixels which is caused by image noise. In the present approach, image noise was reduced by multiple image line fusion, which can be assumed to be in terms of computation effort more effective than averaging multiple images. The developed rms-models were found to represent the strain dependent decrease of accuracy efficiently up to high strains (e.g. 900%) under practical conditions. To obtain optimal measurement precision with the presented methods in practice, appropriate low marker detection threshold intensities of about 0.3gs (gs –signal intensity) and application of a single application cycle of the moving average filter were proved to yield optimal results. At high strains, the application of the rank filter in combination with a geometric center calculation results in best measurement precision, while the differences to the gravimetric method are less but its trend is comparable to the simulation.

Assessment of strain measurement techniques to characterise mechanical properties of structural steel

Strain measurement is important in mechanical testing. A wide variety of techniques exists for measuring strain in the tensile test; namely the strain gauge, extensometer, stress and strain determined by machine crosshead motion, Geometric Moire technique, optical strain measurement techniques and others. Each technique has its own advantages and disadvantages. The purpose of this study is to quantitatively compare the strain measurement techniques. To carry out the tensile test experiments for S 235, sixty samples were cut from the web of the I-profile in longitudinal and transverse directions in four different dimensions. The geometry of samples are analysed by 3D scanner and vernier caliper. In addition, the strain values were determined by using strain gauge, extensometer and machine crosshead motion. Three techniques of strain measurement are compared in quantitative manner based on the calculation of mechanical properties (modulus of elasticity, yield strength, tensile strength, percentage elongation at maximum force) of structural steel. A statistical information was used for evaluating the results. It is seen that the extensometer and strain gauge provided reliable data, however the extensometer offers several advantages over the strain gauge and crosshead motion for testing structural steel in tension. Furthermore, estimation of measurement uncertainty is presented for the basic material parameters extracted through strain measurement.

Calibration of a Digital Image Correlation System

The use and results of the procedure published by Standardisation Project for Optical Techniques of Strain measurement (SPOTS) for a successful calibration of a digital image correlation (DIC) system are described. The details of the calibration specimen used are discussed together with procedure and criteria that must be met to achieve an acceptable calibration. The DIC system was evaluated over a strain range of 289 to 2110 μstrain, with a resulting calibration uncertainty ranging from 14 to 28.7 μstrain. The optical strain measurements were obtained from images taken directly from the bare metal surface, which had been prepared with grit paper, as opposed to generating a speckle pattern by painting the surface.

Dependence of elastic properties of argillaceous rocks on moisture content investigated with optical full-field strain measurement techniques

The mechanical behaviour of the argillaceous rock considered in France as a potential host rock for nuclear waste repository strongly depends on water content. In order to quantify the dependence of the mechanical properties on humidity, an experimental setup that combines hydromechanical loadings with optical observations and digital image correlation (DIC) has been developed. It allows investigation of the hydromechanical behaviour of a material from the sample scale (cm) down to micrometric scale (100μm). Using this system, the shrinkage and swelling properties of the argillaceous rock at various constant uniaxial stresses, and the linear mechanical behaviour of the material at different moisture levels, were experimentally studied. A quasi-linear relation between the relative humidity, ranging from 39% to 85%, and the deformation during dehydration and rehydration is found at both the sample scale (cm), and a mesoscopic scale of a few hundred micrometres. This relation depends on the applied stress with much less swelling at higher axial compressive stress. In addition, a linear decrease of the axial Young's modulus (normal to bedding plane) with an increase of the relative humidity is observed.

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.

SPOTS Calibration Example

The results are presented using the procedure outlined by the Standardisation Project for Optical Techniques of Strain measurement to calibrate a digital image correlation system. The process involves comparing the experimental data obtained with the optical measurement system to the theoretical values for a specially designed specimen. The standard states the criteria which must be met in order to achieve successful calibration, in addition to quantifying the measurement uncertainty in the system. The system was evaluated at three different displacement load levels, generating strain ranges from 289 µstrain to 2110 µstrain. At the 289 µstrain range, the calibration uncertainty was found to be 14.1 µstrain, and at the 2110 µstrain range it was found to be 28.9 µstrain. This calibration procedure was performed without painting a speckle pattern on the surface of the metal. Instead, the specimen surface was prepared using different grades of grit paper to produce the desired texture.

SPOTS project and development of a standard and reference material

The ‘Standardization Project for Optical Techniques of Strain measurement’ (SPOTS) was focused on the development of reference materials for calibrating optical systems for static strain measurement and standardized tests for evaluating the capabilities of such systems. The reference material consists of a beam in four-point bending within a monolithic frame that ensures reproducibility of the boundary and loading conditions. A procedure for the use of the reference material has been developed and leads to the evaluation of measurement uncertainties that allow the construction of confidence limits for the data obtained with the calibrated optical system. The design and methodology have been developed into a proposed draft standard which is being endorsed by VAMAS (Versailles Agreement on Materials And Standards) for review by ISO (International Standards Organisation). An overview of philosophy underpinning the proposed draft standard is presented and serves as an introduction to the reference material and standardized test and their use in providing higher confidence in optical measurements of strain.

Electrical ageing of polymers. The role of micro-domains

Polymer materials are disordered systems and their structure is continuously undergoing conformational changes called physical ageing. Furthermore, polymers exhibit a spatially varying morphology and a distribution of their properties and characteristics. In this article, the existence of micro-domains into polymer materials is proved, using an optical strain measurement technique. Then after, the role of these micro-domains is taken into consideration for a phenomenological description of their electrical ageing process. The different parameters affecting the ageing process, such as the local structure evolution and the local field value, are presented in a distributed way. In a first analysis the ageing process is described in a local scale. Afterwards, a statistical description of the totality of processes at play is necessary to present the global effect of the applied stress over the material structure which leads to the failure mechanism.

Effect of microscopic damage events on static and ballistic impact strength of triaxial braid composites

The reliability of impact simulations for aircraft components made with triaxial braided carbon fiber composites is currently limited by inadequate material property data and lack of validated material models for analysis. Methods to characterize the material properties used in the analytical models from a systematically obtained set of test data are also lacking. A macroscopic finite element based analytical model to analyze the impact response of these materials has been developed. The stiffness and strength properties utilized in the material model are obtained from a set of quasi-static in-plane tension, compression and shear coupon level tests. Full-field optical strain measurement techniques are applied in the testing, and the results are used to help in characterizing the model. The unit cell of the braided composite is modeled as a series of shell elements, where each element is modeled as a laminated composite. The braided architecture can thus be approximated within the analytical model. The transient dynamic finite element code LS-DYNA is utilized to conduct the finite element simulations, and an internal LS-DYNA constitutive model is utilized in the analysis. Methods to obtain the stiffness and strength properties required by the constitutive model from the available test data are developed. Simulations of quasi-static coupon tests and impact tests of a represented braided composite are conducted. Overall, the developed method shows promise, but improvements that are needed in test and analysis methods for better predictive capability are examined.

Optical Strain Measurement Techniques to Assist in Life Monitoring of Power Plant Components

Sensors for monitoring creep strain in high-pressure steam pipes and other power plant components are subjected to very demanding environmental and operational conditions. It is important that the sensors are of a rugged design and that measurement can be made that only relates to creep movements in power plant components. The E.ON UK auto-reference creep management and control (ARCMAC) optical strain gauges have been designed to have this capability. These optical strain gauges are installed across sections of welded steam pipe and other plant components in locations that provide the best monitoring points to reveal the early onset of failure processes. Reported in this paper are recent developments to improve optical creep strain measurement to achieve a 65 microstrain accuracy level with an error of less than 10%. Also reported are trials of combining optical strain gauges with digital image correlation (DIC) to obtain detailed information of the creep strain distribution around the gauges. The DIC data for known defect geometries have been validated with finite element analysis.

Visualization and quantification of the mechanical deformation induced by an electrical field in poly(ethylene naphthalene 2,6‐dicarboxylate) (PEN) films

AbstractAn optical nondestructive strain measurement technique was performed to analyze the mechanical deformation induced by an electrical field within the insulating materials. Poly(ethylene naphthalene 2,6‐dicarboxylate) (PEN) films were then subjected to constant electrical fields right up to their electrical breakdown. The experimental technique made it possible to follow the various stages of the mechanical behavior of PEN in real time. The final breakdown occurred in the observation zone and the related mechanical deformation was captured. A “margarita” structure was observed with a hole at the center. The experimental results indicated that the level of the induced‐mechanical deformations depended on the local environment. We defined two different zones representing the inside and the outside of the damaged area. The induced‐deformations were larger in the damaged zone. It was also observed that deformations increased when the sample had a lower degree of crystallinity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Calibration of a Speckle Interferometry Full‐Field Strain Measurement System

Abstract: This study describes the calibration of a full‐field speckle interferometry strain measurement system using the calibration specimen and protocol defined in the Standardisation Project for Optical Techniques of Strain measurement (SPOTS) standard. The specimen was based on the monolithic embodiment of a four‐point bending test and was manufactured from aluminium following the SPOTS design. Strain‐gauge rosettes attached to the upper and lower faces of the beam were used to derive two correction factors of an analytical expression that predicted the strains generated in the gauge section of the beam. Following the SPOTS protocol, the comparison of measured and predicted strains yielded two fit parameters and their associated uncertainties for each of three displacement‐load steps which indicated the closeness of the data sets. An acceptable calibration was achieved for the single normal component of in‐plane strain considered in this study, for each load step employed. For the highest load range, which generated a maximum strain of approximately 810 μstrain in the gauge section, the overall calibration uncertainty was found to be 35.3 μstrain, which in relative terms can be expressed as 2.2% of the strain measurement range for which the instrument was calibrated.