Video extensometer for structural steel using silhouette image from telecentric illumination system

Steel structures experience plastic deformation and damage during large earthquakes. The stress–strain (S–S) curve of structural steel can be used to develop a constitutive model to simulate its structural behavior. The S–S curve can be obtained by video extensometers, the efficiency of which with a telecentric lens and telecentric illumination remains unclear. The applicability of a telecentric lens and telecentric illumination system to video extensometers were investigated. Rigid-body motion and tensile testing were performed and the carrier of displacement of six gauge lengths was traced. Further, N images were obtained under a constant displacement/force and averaged to reduce the pseudo strain. Experimental results showed that the pseudo strain increases with the out-of-plane rigid displacement but remains unaffected by the in-plane rigid-body displacement. Tensile testing results showed an error difference of <10 % for the Young’s modulus obtained from the video extensometer and strain gauge.