Papers by Author: Ho Sung Kim

Abstract: Flatwise compression and flexural behaviour of perlite/sodium silicate composites made of expanded perlite and sodium silicate is studied using a new manufacturing method for applications of sandwich foam core materials. Sodium silicate content in composites (a perlite particle size of 2-3mm) was varied for a range of 0.1 - 0.3 g/ml and compaction ratio for moulding for a range of 2.0 - 3.0. Specimens under flatwise compression were found to be more capable for energy absorption than those previously reported for lengthwise compression, ascribing to appearance of densification stage following the initial cracking. Also, under flexural loading, energy absorption in composites after the initial cracking was found to be in operation, supporting their candidacy for wide applications where gypsum boards are dominant. It was deduced from both flexural testing results and fracture mechanism that compressive strength is higher than tensile strength, suggesting the future directions of mechanical performance improvement.

Abstract: A theory for mathematical framework is developed with axioms for fatigue damage, quantification and relativity concept, boundary conditions, and compatibility, allowing us to evaluate the validity of candidate damage equations/quantities on S-N plane for prediction of fatigue life. Manifestation points for accumulated damage were defined for boundary conditions by differentiating between damage accumulated before failure, and failure caused by damage at quantised fatigue loading cycles. A selected damage equation leading to a theoretical S-N curve was validated as an example.

Abstract: The mixing required for post-mould gelatinisation of syntactic foams using starch as binder was studied. It was found that starch particles tend to adhere to hollow microspheres during mixing, forming agglomerates. A transition in the buoyancy of microsphere-starch phases was found at a particular volume fraction of starch. The transitional point was close to a volume fraction of starch at which a calculated relative density for a system consisting of multiple starch particles per microsphere was unity. Starch-microsphere inter-distance appears to be an important parameter affecting starch content in an agglomeration.

Abstract: Various manufacturing parameters involved in the ‘buoyancy method’ were inter-related. An equation based on unit cell models for a relation between volume expansion ratio (VER) of bulk microspheres in aqueous starch and microsphere size was derived. A good agreement between the equation and experimental data was found. The inter-microsphere distance (MID) concept was introduced and it was demonstrated that the MID can be calculated numerically for microspheres with known statistical data.

Abstract: Two dimensional statistical characteristics of inter particle/void distance (ID) for various particle/void and dispersion types are studied in relation with toughening of plastics using computer generated three dimensional models. Particle/void size groups adopted were of log-normal distribution. Particles/voids were dispersed at uniform-random. It was found that IDs are (a) of approximately Gaussian distribution but; (b) not of Gaussian distribution for particle/void sizes of bimodal log-normal distribution (created by mixing of two groups of articles/voids). It was also found that the degree of ID uniformity, which can be represented by the inverse of the coefficient of variation, for a single group of log-normally sized particles/voids is not sensitive to standard deviation of particle/void size. Mixing effect on ID characteristics using two groups of log-normally distributed particles/voids with similar mean particle/void diameters was simulated. It was found that, when a significant amount (36 vol %) of particles/voids of a small mean and standard deviation of ID, was mixed with a group of particles/voids of a large mean and standard deviation of ID, mean and standard deviation of ID for the mixture were not substantially lower than those of the group of particles/voids of the large mean and standard deviation of ID. It was also found that the degree of ID uniformity for the mixture of the two groups were lower than those of individual groups, indicating that the mixing has deleterious effect on toughening.

Abstract: Toughening of thermosets by creation of residual compressive stresses around microspheres is studied. Expandable hollow micro-spheres containing liquefied gas were used for the creation of residual compressive stresses. Microscopic compressive residual stresses around the micro-spheres in the vicinity of the crack tip were graphically analysed and related to macroscopic mechanical behaviour for mode I fracture. It was confirmed that toughening was due to residual compressive stresses rather post-cure effect.