Full-field Surface Deformation Research Articles

Non-contact technique for characterizing full-field surface deformation of shape memory polymers at elevated and room temperatures

Thermally activated shape memory polymers (SMPs) can display modulus changes of approximately three orders of magnitude in transitioning from the high modulus, “glassy” state below the glass transition temperature ( T g) to the low modulus, “rubbery” state above the T g. In the high temperature region, SMPs can achieve strain levels well above 100%. Their complex behavior includes large modulus changes to as low as ∼1 MPa, extremely high strain levels, and path dependent properties, thus precluding the use of traditional strain gages and low-contact force extensometers. The present study presents a comparison of thermomechanical testing techniques developed to characterize the material behavior of SMPs. Specifically, the performance of strain measurements using contact methods (clip-on extensometers and adhesive strain gages) are compared to non-contact methods (laser extensometer and digital image correlation). An MTS environmental chamber with an observation window allows for non-contact optical measurements during testing. A series of tensile tests are performed on a commercial SMP (with a T g of ∼105 °C) at 25 °C and at 130 °C. It is observed that the clip-on extensometer significantly affects the SMP behavior even in the low temperature, high modulus state. Overall, the laser extensometer provides a robust method for controlling the axial strain in the gage section of the samples at moderate strain rates. The digital image correlation allows for full field measurement of both axial and transverse strains of SMPs over a range of temperatures and strain rates.

Measurement of full-field deformation induced by a dc electrical field in organic insulator films

Digital image correlation method (DIC) using the correlation coefficient curve-fitting for full-field surface deformation measurements of organic insulator films is investigated in this work. First the validation of the technique was undertaken. The computer-generated speckle images and the measurement of coefficient of thermal expansion (CTE) of aluminium are used to evaluate the measurement accuracy of the technique. In a second part the technique is applied to measure the mechanical deformation induced by electrical field application to organic insulators. For that Poly(ethylene naphthalene 2,6-dicarboxylate) (PEN) thin films were subjected to DC voltage stress and DIC provides the full-field induced deformations of the test films. The obtained results show that the DIC is a practical and robust tool for better comprehension of mechanical behaviour of the organic insulator films under electrical stress.

Compressive behavior of 64% porosity NiTi alloy: An experimental study

An experimental investigation of the compressive behavior of 64% porosity NiTi alloy was undertaken in this study. Both quasi-static and low-frequency fatigue compressive tests were carried out to obtain the compressive stress–strain curves of specimens. Special attention was paid to the loading–unloading history effect on the stress–strain curves. Full-field surface deformation measurement was carried out via moiré interferometry when specimens were compressed. Inhomogeneous deformation property of the material caused by the porous structure was clearly presented. The loading–unloading history effect was also studied using the moiré interferometry method. Finally real time metallographic analysis during compression tests explained the causes of the loading–unloading history effect of the porous material from microscopic point of view. These findings are believed to be essential elements in the theoretical modeling of compressive behavior especially loading–unloading history effect of the high porosity NiTi alloy, and which still need further systematic examination in the future.

A new mark shearing technique for strain measurement using digital image correlation method

The digital image correlation (DIC) method is a noncontact, full-field surface deformation measuring method, which is widely used in experimental mechanics. Although the DIC method has a high precision in displacement measurement, its precision in strain determination is a bit low. In this paper, a new mark shearing technique is proposed to improve the accuracy of the strain measurement of the DIC method. A wedge mirror is used to introduce a shearing distance of the marks, which are made on the specimen to calculate the strain. The measurement principle of the mark shearing technique is described in detail. From the analysis, it can be concluded that this method is suitable for both large-scaled and small-scaled specimens, and thus has a wider adaptability than the common DIC method. Using this method, the maximum gauge length is 80 mm, and the accuracy of strain measurement can reach 4 microm strains. A tensile experiment with aluminum sample was conducted, and the successful results demonstrated the feasibility of this method.

Measurement of thermal expansion of ceramic matrix composite tubes at temperatures up to 1400 °C

A newly developed method using a computer-vision technique for digital image correlation which can measure full-field three-dimensional surface deformations at various temperatures was used for measuring the thermal expansion behaviour of C/SiC (carbon-reinforced silicon carbide) tubes. Samples were subjected to thermal expansion only and investigated at temperatures from ambient temperatures up to 1400 °C. Results are presented which show that the digital image correlation technique remains fully capable within this temperature range, providing accurate measurement of the displacements and strains of the cylinders.

The Effects of Motion on Dynamic Moiré Interferometry

Experimental techniques (e.g. laser interferometry, photoelasticity) that generate fringe data are currently being utilized for analyzing full-field surface deformations in a variety of dynamic problems in experimental mechanics. As opposed to static problems, the transience of surface deformations in dynamic problems poses a unique problem in accurately resolving the fringe data that is acquired. Neumann determined that for time-lapse, double-exposure holographic interferometry the fringe contrast depends on the data acquisition time, wavelength of the light source, and transience of the dynamic phenomena. Dally performed a similar analysis for dynamic photoelasticity, where the dependence was on the stress gradients instead of the wavelength of the light source. This paper attempts to extend the analysis of Neumann and Dally to the analysis of transient fringe intensity data obtained from using dynamic moiré interferometry. It is found that the fringe contrast can depend on the frequency of the fringe field rather than the wavelength of the light source or stress gradients, and the fringe contrast will decay more rapidly for Gaussian light sources than constant intensity light sources. © 1997 Elsevier Science Ltd. All rights reserved.

High-temperature deformation measurements using digital-image correlation

The ability of the computer-vision technique of digital-image correlation to measure full-field in-plane surface deformations at elevated temperatures was evaluated by a series of experiments. Samples were subjected to pure translation, free thermal expansion and uniform tensile loads. Results are presented which show that the digital-image-correlation technique remains fully capable of accurate measurement of the displacements and strains on the surface of a planar object at temperatures up to 650°C.