Tensile and tear-type fracture toughness of gypsum material: Direct and indirect testing methods

Abstract

In this context, four specimens, i.e. (i) circumferentially notched cylindrical torsion (CNCT), (ii) circumferentially notched cylindrical direct tension (CNCDT), (iii) edge notch disc bend (ENDB) and (iv) three-point bend beam (3PBB), were utilized to measure the modes I and III fracture toughness values of gypsum. While the CNCT specimen provides pure mode III loading in a direct manner, this pure mode condition is indirectly produced by the ENDB specimen. The ENDB specimen provided lower KIIIc and a non-coplanar (i.e. twisted) fracture surface compared with the CNCT specimen, which showed a planar mode III fracture surface. The ENDB specimen is also employed for conducting pure mode I (with different crack depths) and mixed mode I/III tests. KIc value was independent of the notch depth, and it was consistent with the RILEM and ASTM standard methods. But the mode III fracture results were highly sensitive to the notch depth. While the fracture resistance against mode III was significantly lower than that of mode I, the greater work of fracture under mode III was noticeable.

Keywords

• Gypsum
• Pure modes I and III fracture toughness
• Mixed mode I/III
• Circumferentially notched cylindrical torsion (CNCT) specimen
• Circumferentially notched cylindrical direct tension (CNCDT) specimen
• Edge notch disc bend (ENDB) specimen