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Title: An assessment of the compaction behaviour of pharmaceutical compacts by means of complementary mechanical tests
Author: Brown, David Roger
ISNI:       0000 0001 3488 9045
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1995
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Mechanical strength measurements have been widely performed on pharmaceutical compacts which often involve only one or two techniques that are not utilised to their full potential. Four approaches to the tensile strength measurement of cylindrical compacts were used (diametral compression, modified diametral compression, three-point flexure and equi-biaxial fracture stress testing). A change in the mechanism of failure was demonstrated following diametral compression which was explained using a crack propagation model. The mechanical strength for all tests was affected by specimen geometry changes and explained by considering the loading configuration, imposed stress state and site at which crack propagation is initiated during testing. The degree of specimen inhomogeneity and inherent flaws were also considered. Diametral compression and three-point flexure testing were least influenced by specimen volume changes for Avicel PH102 and Methocel E4M 12mm diameter compacts prepared by slow compaction. Thin, porous Methocel E4M specimens were unable to support the compressive load during the former test thereby questioning the validity of using materials possessing poor compaction properties. The tests were also applied to bevelled-edge larger diameter Avicel PH102 specimens prepared by fast compaction. This demonstrated the time-dependent nature and strain rate sensitivity of the material which sometimes dominated specimen volume effects. With all specimens the equi-biaxial fracture stress and modified diametral compression tests were particularly sensitive towards volume changes, although the equation for tensile strength for the latter may require modification. All of the tests appeared to infer a non-consistent change in the specimen physical structure or volume reduction characteristics with specimen thickness. Compression testing on 12mm diameter equi-dimensional compacts illustrated a complex mechanism of failure. The compressive to tensile (diametral compression) strength ratio was constant with specimen porosity. A clear linear relationship between the natural logarithm of the compressive strength and porosity was shown. Larger diameter specimens with length/diameter ratios of 0.5 were shown to have invalid geometries for compression testing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Tensile strength; Compression tests; Fracture