published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

This thesis presents experimental and numerical investigations on the fracture properties of quasi-brittle materials, including mortar, concrete and graphite. Fracture toughness in terms of the critical stress intensity factor K_IC and fracture energy G_F of the materials were determined through three-point bending tests on centre-notched beams. Furthermore, full-field displacement of the beams subjected to bend was obtained using Electronic Speckle Pattern Interferometry (ESPI) technique. In order to verify the accuracy of the displacement data measured using the ESPI technique and to obtain reliable deformation information, the displacement and strain errors induced by the rigid-body motions of the specimen were quantified. This study found that the displacement errors were negligible whereas the strain errors were notable and must be eliminated. The influence of different rigid-body motions was analyzed. It was found that the out-of-plane movement of the specimen was critical and affected considerably the accuracy of strain data. Thus the experimental setup was improved accordingly to eliminate the influence of critical rigid-body motions. Quasi-brittle materials have a finite stress region near the crack tip, known as the fracture process zone (FPZ). The materials exhibit nonlinear fracture behaviour in the FPZ. The cohesive crack model (CCM) is widely used to characterize the nonlinear fracture behaviour of quasi-brittle materials. According to the CCM, all the nonlinear behaviours in the FPZ can be represented by a cohesive crack, and the crack propagation is controlled by the relationship between the cohesive stress and crack opening, namely, the tension softening curve (TSC). Thus an accurate estimation of the TSC is essential. In order to determine the TSC of quasi-brittle materials, an incremental displacement collocation method (IDCM) was originally developed in this study. In the IDCM, the deformation data measured by the ESPI sensor was analyzed to obtain the crack opening displacement (COD) of the notched specimens. The experimental COD profiles together with the CCM were then integrated into a finite element model to simulate the nonlinear fracture response of the specimen. By minimizing the difference between the computed and measured displacements at selected collocation points, the cohesive stress corresponding to certain crack opening was determined. The entire TSC was constructed in a step-by-step manner following the loading steps. The IDCM was first applied to estimate the TSC of mortar. By using the estimated TSC, the displacements of the specimen under certain loading levels were computed. By comparing the computed displacements with the experimental data, the reliability of the IDCM and the accuracy of the estimated TSC were verified. The application of the IDCM was further extended to the determination of the TSCs of concrete and graphite. The parameters used to define the shape of the TSC of the materials were determined using regression analysis. The applicability of those parameters was verified by comparing the TSCs estimated in the present study with those derived by other researchers. Recommendations were put forward to choose appropriate tensile properties of quasi-brittle materials in the numerical simulations. Furthermore, by using the ESPI technique, fracture phenomena of quasi-brittle materials were observed and reported. Such records can greatly enhance the understanding of crack initiation, growth and arrest in quasi-brittle materials, and lead to improvements to the existing fracture models. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

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Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 155-167). Available also in a digital version from Dissertation Abstracts.

Includes bibliographical references (p. 77-80). / "Metals, Ceramics, and Materials, UC-37" -t.p. "TID-4500 (37th Ed.)" -t.p.

Thesis (Ph. D.)--University of Hong Kong, 2005.

Thesis (doctoral)--Helsinki University of Technology, 2007. / Includes bibliographical references (p. 181-195). Also available on the World Wide Web.

The effect of two additions of molybdenum (2.5% and 5%) on the brittleness of an alloy based on Fe-8%Mn has been investigated. The intergranular brittleness exhibited by the as-quenched base alloy was eliminated by the addition of 2.5Mo possibly due to the interaction of molybdenum with embrittling agents Mn and N and the subsequent increase in solid solubility of these elements. A further addition of molybdenum did not significantly increase the toughness of the as-quenched Fe-8Mn-2.5Mo alloy in accordance with the theory of a limiting amount of molybdenum that may combat embrittlement. Aging quenched alloys at 450°C gave rise to an increase in DBTT and a change in brittle fracture mode to intergranular. The effect was most severe in the alloy with higher molybdenum (5%) due to precipitation of second phase particles both increasing the hardness of the matrix dramatically and denuding the matrix of molybdenum that would otherwise be free to combat embrittlement. A buildup of Mn and N at prior austenite grain boundaries was detected by AES, another factor contributing to embrittlement on aging. Aging quenched alloys at 525°C led to a more rapid change from cleavage to intergranular fracture for both molybdenum additions but toughening of the alloy containing the lower molybdenum content (2.5%). Although grain boundary segregation in this alloy meant that the preferred crack path for brittle fracture changed from transgranular to intergranular on aging, the buildup of reverted austenite at prior austenite grain boundaries acted as a crack blunter, slowing down crack propagation and thus increasing overall toughness. Embrittlement in the alloy containing 5% Mo was very severe due to more rapid precipitation. An investigation was also carried out on the aging of a commercial alloy based on 0.25C, 1.4Mn, 0.5Mo, 0.6Ni, 0.3Si doped with 0.4%Cu in one instance and 0.04%P in another. Phosphorus was seen to characteristically induce embrittlement on aging at 450°C and 525°C with an increase in the DBTT and a change in the brittle fracture mode from cleavage to intergranular. Aging of the base alloy at 450°C was also seen to be an embrittling treatment but the effects were not as severe as in the alloy containing phosphorus. The addition of copper is seen to retard this embrittlement on aging due to its grain refining ability.

This investigation was initiated by IBM to determine the types of cracks formed in hot-isostatic-pressed (HIP) Ni-Zn ferrite under impact and static loading conditions. A 1/8-inch tungsten carbide (WC) ball was used to apply the load in both cases. The impact loading condition was accomplished by dropping the WC ball from various heights between 40 and 200 cm. The static loading condition was accomplished by applying loads of 10 kg, 15 kg, and 30 kg on a Rockwell Hardness Tester. The response of HIP Ni-Zn ferrite to impact and static loading was elastic/plastic yielding permanent indentations. "Dimple" impressions, Hertzian-ring cracks, and radial cracks were formed upon applying increasing loads to the HIP Ni-Zn ferrite. As the grain size of the material was increased, both the Hertzian-ring and radial cracks were formed at lower loads compared to the as-received samples. The "dimple" impressions were not observed to follow this trend.