Deformation characteristics of brick masonry due to partial unloading

Experimental investigation into the behaviour of half-scale brick masonry panels were<br />conducted under cyclic loading normal to the bed joint and parallel to the bed joint. For each cycle, full<br />reloading was performed with the cycle peaks coinciding approximately with the envelope curve.<br />Unloading, however, was carried out fully to zero stress level and partially to two different stress levels of<br />25 percent and 50 percent of peak stress. Stability point limit exhibits a unique stress-strain curve for full<br />unloading but it could not be established for partial unloading. Common point limit was established for all<br />unloading-reloading patterns considered, but its location depends on the stress level at which unloading is<br />carried to. Common point curves were found to follow an exponential formula, while residual strains<br />versus envelope strains can be expressed by a polynomial function of a single term. The relation between<br />residual strain and envelope strain can be used to determine the stress level at which deterioration due to<br />cyclic loading began.