The invention relates to a collar for measuring the lateral deformation of a test piece during uniaxial or triaxial compression tests. According to the invention, the collar includes a metal hoop or a hoop that is made from composite materials, which can clasp the test piece and which is open. The free ends thereof are separated by a distance .DELTA.. The inventive collar also includes a mechanism for directly or indirectly measuring the distance .DELTA.. Between the free ends of the hoop, the mechanism includes at least one strain gauge.