Measurement of mechanical strains

• Project number: SC18-006
• Project management: Dieter Pahr, Karl Landsteiner University for Health Sciences / Department of Biomechanics
• Project duration: 26 months from 01 July 2019

Background

Bone is a fascinating, living and intelligent load-bearing tissue. It supports the body, facilitates locomotion and protects internal organs. Understanding the mechanical properties of bone helps in the development of treatments and clinical applications that lend themselves to more complex and personalised solutions.
Biological tissues are generally inhomogeneous and anisotropic. Understanding the mechanical behaviour of biological tissues requires a complete description of this behaviour over the entire geometry and shape of the sample. The mechanical properties of bones and soft tissues have been extensively studied using different approaches, such as in vitro experiments and numerical modelling. Strain gauges (SGs) are considered the gold standard for strain measurements on the bone surface due to their high accuracy. However, measurements with SGs only allow the evaluation of discrete points and do not provide a distribution of full-field stress on the surface of the specimen. Furthermore, SGs require detailed surface preparation. Poor preparation can lead to highly inaccurate results. Transducers and extensometers have also been used to measure global strain in bone. All three of these previous strain measurement techniques introduce a bias into the results due to their contribution to load-bearing capacity and lead to a systematic underestimation of the true strain distribution.
In recent years, optical measurement techniques based on digital image correlation (DIC) and computational power have enabled non-contact measurement of entire surfaces. They thus overcome the limitation of contacting the surface and the availability of single measurement points.

DIC depends on tracking the displacement of detected features (patches) on the sample surface. DIC tracks the displacement between deformed and undeformed digital images of the surface. Based on the digital images, a full-field displacement map is calculated, from which a full-field strain map is derived. The accuracy of DIC depends on the quality of the patches, the measurement conditions (size and distribution of light and patches) as well as different software (facet and grid size) and hardware parameters (optics and camera resolution) that need to be optimised. Despite the versatile advantages of the DIC approach in obtaining full-field strain measurements on the surface of interest, DIC has not yet been fully exploited for measurements on biological samples and in particular on bone.
The objectives of this study are: (i) to investigate in depth the accuracy and precision of the DIC technique based on standardised metallic and polymeric specimens under zero load by evaluating the size and distribution of the patch pattern, (ii) to validate the accuracy and precision of DIC DIC measurement system against an accurate extensometer; (iii) to evaluate the distribution of 3D full-field strain on the surface of biological tissues such as bone and tendon specimens; (iv) to provide practical guidelines on how to take advantage of the DIC application to measure strain fields on biological hard and soft tissues.