The acoustic emission Kaiser effect is used to evaluate the quantitative state of stress of leaf springs used in the automotive industry. Changes in curvature radii or in yield stress caused by thermal or mechanical treatment of the leaves are accurately assessed by a bend test procedure. The computation of the assembly stresses of the springs can be made knowing the actual yield stress of each leaf, thus enabling optimal design. The technique may be applicable to the reuse of discarded leaf springs subjected to mechanical deformation.