  Photoelasticity is employed to investigate the stress state near stiff
rectangular and rhombohedral inclusions embedded in a 'soft' elastic plate.
Results show that the singular stress field predicted by the linear elastic
solution for the rigid inclusion model can be generated in reality, with great
accuracy, within a material. In particular, experiments: (i.) agree with the
fact that the singularity is lower for obtuse than for acute inclusion angles;
(ii.) show that the singularity is stronger in Mode II than in Mode I
(differently from a notch); (iii.) validate the model of rigid quadrilateral
inclusion; (iv.) for thin inclusions, show the presence of boundary layers
deeply influencing the stress field, so that the limit case of rigid line
inclusion is obtained in strong dependence on the inclusion's shape. The
introduced experimental methodology opens the possibility of enhancing the
design of thin reinforcements and of analyzing complex situations involving
interaction between inclusions and defects.
