A novel quasi-static compression test set-up with micron order accuracy for small specimens

Abstract

Accurate displacement measurements during compression tests on small specimens using large standard universal testing machines are adversely affected by the compliance of the apparatus. The influence of compliance becomes more acute when conducting tests on quasi-brittle materials, such as cortical bone, that exhibit low failure strains and for which accurate modulus values are required. This paper presents a custom compression test set-up (i.e., a subpress) that facilitates routine quasi-static compression tests on small specimens by eliminating the effect of testing machine compliance on the results. The displacement of the compression test set-up was recorded using a combination of Hall effect sensors and multipole magnetic strips, which offers a resolution of less than one micron. The results of quasi-static compression tests on small polymer specimens are reported where the displacements were measured using the custom compression test set-up, the integrated displacement measurement system of a universal testing machine and a visual extensometer. Additional tests on cortical bone specimens demonstrate the effectiveness of the compression test set-up. Based on the results, the compression test set-up appeared to yield more consistent and accurate measurements compared to both the integrated measurement system of a universal testing machine and a visual extensometer. Novel data regarding the incipient fracture of cortical bone were obtained. The compression test set-up allows for the routine testing of a large number of bone specimens with micron accuracy in a short time frame, thus reducing the effects of degradation on biological specimens.