Investigations into the fracture mechanics of acetylsalicylic acid and lactose monohydrate

The fracture mechanics of acetylsalicylic acid (ASS) and lactose monohydrat e (LM) were studied using three-point beam bending experiments and compared with conventional tabletting performance. ASS was found to have an unusual behaviour in terms of its Young's modulus and tensile strength when determ ined with beams of different porosities. The Young's modulus as a function of beam porosity showed two exponential parts separated by a constant regio n and the tensile strength as a function of the porosity followed a non-exp onential law. Tabletting experiments revealed that ASS undergoes different deformation mechanisms at the different compaction pressures associated wit h the porosity ranges covering the different regions. The different deforma tion mechanisms might have caused different crack and flaw patterns or diff erent crack lengths, in particular at the beam surfaces, which are under ma ximum tensile stress during the tests. The unusual findings were, however, not reflected in experiments to determine the critical stress intensity fac tor as a function of beam porosity, because here crack propagation is contr olled via a notch introduced into the beams. In contrast to ASS, LM behaved like the majority of materials i.e. Young's modulus, tensile strength and critical stress intensity factor were found to relate to the beam porosity exponentially. (C) 2001 Kluwer Academic Publishers.