METHOD OF ESTIMATING STRESS-STRAIN CURVE IN DIRECT TENSILE TEST UNDER STRAIN GRADIENT USING INVERSE ESTIMATION THEORY

Abstract

 There are various direct tensile tests to estimate tensile stress-strain curve of concrete. However, in direct tensile test, bending moment which is hard to erase generates strain gradient at cross section of specimen. Present direct tensile test estimates stress in the way of dividing resultant force by the area of cross section. If strain gradient occurs, this calculation method can t estimate accurate stress-strain curve. Inverse analysis using bending test apparatus is proposed which can estimate tensile stress-strain curve from force-deformation curve measured at loading point. However, this inverse analysis has some problem. For example, reproducing detail test condition is required. I propose method to estimate tensile stress-strain curve in direct tensile test under large strain gradient. This method is characterized by measuring system and equations. The measuring system is as follows. Indentation marks are established at the both narrow side of tabular test piece to stabilize fracture location. I define cross section between these two indentation marks as estimation section. Strain measuring apparatuses are established at the unilateral long side to measure strain distribution in estimation section. Give tensile force to tabular test piece and measure resultant force vertical to estimation section. The equations are based on inverse estimation method and can calculate stress corresponding to strain from the time series data of resultant force and strain distribution in estimation section by linking the time series data. I certificate that this method can estimate accurate stress-strain curve under strain gradient by executing numerical analysis. This method gives variance to the shape of test piece around the estimation section. So test equipment can be simplified.