Digital Image Correlation For Measurement Research Articles

The use of digital image correlation for measurement of strain fields in a novel wireless intraspinal microstimulation interface.

Intraspinal microstimulation (ISMS) has emerged as a promising neuromodulation technique for restoring standing and overground walking in individuals with spinal cord injury. Current and emerging ISMS implant designs connect the electrodes to the stimulator through lead wires that cross the dura mater. To reduce possible complications associated with transdural leads such as tethering and leakage of cerebrospinal fluid, we aim to develop a wireless, fully intradural ISMS implant based on our prior work in the cortex with the Wireless Floating Microelectrode Array (WFMA). Although we have extensive data about WFMA cortical stability, its mechanical and electrical stability in the spinal cord remains unknown. One of the quantifiable metrics to assess long-term implant stability is mechanical strain. The aim of the current work is to develop a method to assess implant stability by measuring strain fields in a surrogate of the human spinal cord. A physical model of the spinal cord was studied using an electromechanical testing apparatus, simulating typical spinal cord motion. Strain fields were digitally analyzed using an optical method known as digital image correlation (DIC). Displacement and strain were visualized using contour plots. The strain values in the vicinity of each WFMA device were significantly different from the strain values in the same locations in the control surrogate spinal cord. The results demonstrate that DIC can be used for in-vitro screening of intraspinal implants. Accurate optical strain measurements will enable researchers to optimize implant design over a wide range of motion conditions.

Strain Rate Effect on Martensitic Transformation in a TRIP Steel Containing Carbide-Free Bainite

<div class="section abstract"><div class="htmlview paragraph">Adiabatic heating during plastic straining can slow the diffusionless shear transformation of austenite to martensite in steels that exhibit transformation induced plasticity (TRIP). However, the extent to which the transformation is affected over a strain rate range of relevance to automotive stamping and vehicle impact events is unclear for most third-generation advanced high strength TRIP steels. In this study, an 1180MPa minimum tensile strength TRIP steel with carbide-free bainite is evaluated by measuring the variation of retained austenite volume fraction (RAVF) in fractured tensile specimens with position and strain. This requires a combination of servo-hydraulic load frame instrumented with high speed stereo digital image correlation for measurement of strains and ex-situ synchrotron x-ray diffraction for determination of RAVF in fractured tensile specimens. Specifically, the potentially competing effects of strain rate on austenite transformation to martensite were investigated to determine which predominate at nominal strain rates of 0.5 s<sup>-1</sup>, 5 s<sup>-1</sup>, 50 s<sup>-1</sup> and 500 s<sup>-1</sup>. A corresponding decrease in austenite volume fraction at a fixed true strain with strain rate suggests that austenite transformation to martensite with strain is accelerated with increased strain rate despite potential energetic inhibition of the transformation due to adiabatic heating. Increased transformation rate to martensite results in increased work hardening rates, strengths, and elongations with strain rate increases to 500 s<sup>-1</sup>. Observations are discussed in the context of contributing mechanisms both favoring and inhibiting the strain-assisted transformation of austenite to martensite.</div></div>

Development of a measurement system for laparoendoscopic single-site surgery: reliability and repeatability of digital image correlation for measurement of surface deformations in SILS port.

Objective:Analysis of mechanical measurements in laparoendoscopic single-site surgery (LESS) is important for instrument design and surgical simulators. The aim of this study was to develop a measuring system for different instruments and manipulations in LESS using a single-incision laparoscopic surgery (SILS) port.Methods:The loads on the SILS port were applied and recorded by the universal material testing machine by the following method. The handle of the forceps inserted in the SILS port was connected with the machine by a fishing wire and pulled at a constant rate. The surface deformations (displacements and strains) of the SILS port were recorded with digital image correlation (DIC) simultaneously. The correlation between deformation measurements and loads were analyzed. This experiment was repeated 8 times.Results:Strong correlations existed between deformation measurements calculated by DIC and objective criteria “loads” applied and recorded by the universal material testing machine (r > 0.98). The correlation coefficients were statistically significant (P < .001). A high repeatability of the results appeared in all repetitions of the experiment.Conclusions:A DIC measurement system has been developed for LESS, and comprehensive mechanical parameters of a SILS port can be obtained precisely by using this system. It is reliable and repeatable for evaluation of instruments and manipulations in LESS.

Repeatability of digital image correlation for measurement of surface strains in composite long bones

Digital image correlation (DIC) can measure full-field surface strains during mechanical testing of hard and soft tissues. When compared to traditional methods, such as strain gauges, DIC offers larger validation data (∼50,000 points) for, e.g., finite element models. Our main aim was to evaluate the repeatability of surface strain measurements with DIC during compressive testing of composite femurs mimicking human bones. We also studied the similarity of the composite femur samples using CT. Composite femurs were chosen as test material to minimize the uncertainties associated with the use of cadaveric tissues and to understand the variability of the DIC measurement itself. Six medium-sized fourth generation composite human proximal femora (Sawbones) were CT imaged and mechanically tested in stance configuration. The force–displacement curves were recorded and the 3D surface strains were measured with DIC on the anterior surface of the femurs. Five femurs fractured at the neck–trochanter junction and one at the site below the minor trochanter. CT image of this bone showed an air cavity at the initial fracture site. All femurs fractured through a sudden brittle crack. The fracture force for the composite bones was 5751±650N (mean±SD). The maximum von Mises strain during the fractures was 2.4±0.8%. Noise in one experiment was 5–30µε. When applied loads were equalized the variation in strains between the bones was 20–25%, and when the maximum strains were equalized, variation in the other regions was 5–10%. DIC showed that the ability of nominally identical composite bones to bear high strains and loads before fracturing may vary between the samples.

Full-field analysis of notch effects of 3D carbon/carbon composites

Carbon fiber reinforced carbon matrix composites (C/Cs), as important candidate aviation and aerospace materials, have good prospects for application in extreme environments. However, notch effects of C/Cs have not been studied adequately. In this work, the notch effects of 3D-C/C composites were investigated by the full-field measurement digital image correlation method. Six kinds of specimens with different notch patterns including one central hole, multi-holes, and double edge notches, were examined to analyze their mechanical properties. The tests were carried out on MTS 809 testing machine and the deformations of specimens were monitored by a CCD camera. Using the full-field measurement, both the local mechanical properties (stress concentration, shear damage, and fracture behavior) and the globe properties (notch sensitivity and tensile behavior) were studied. The results indicated that the fracture stress on the net cross section was constant and it was independent on the material heterogeneity, notch patterns, and notch depth.

In-plane strain measurement by digital image correlation

This paper presents a fast and simple (FAS) detection algorithm based on the digital image correlation for measurement of the surface deformation of planar objects. The proposed algorithm uses only fine search at the pixel level resolution and surface fitting for sub-pixel level. Two different specimens are investigated to explore the feasibility of this proposed algorithm. The displacements calculated by the FAS algorithm are compared with the ones obtained from Newton-Raphson method (N-R) and Enhanced Sequential Similarity Detection Algorithm (ESSDA). The results show that the experimental data are in good agreement with the theoretical solution. The proposed algorithm is found to be much faster than Newton-Raphson method with inferior, yet reasonable, accuracy for displacement and strain evaluation in the cases of uniaxial tension and disk under diametrical compression tests.