Papers by Keyword: Weibull Parameter

Abstract: Properties of composite materials are often predicted from properties of its component materials. In the case of green composites that are typically filled with natural fibers however, a large deviation from predictions is observed due to the large property variation in natural fibers. In this study, techniques have been developed to minimize the effect of the said variations, which included the determination of a fiber useful length and critical length, and the utilization of controlled chemical treatment to remove unwanted fiber components that interfere in fiber-matrix interfacial bonding. The abaca fiber was determined to have a diameter of 190 + 2 mm in about two-thirds of the fiber length in the middle section. A large variation in fiber diameter was observed at the root and tip sections such that the diameter could be as high as 200 mm at the root while the tip tapers to 110 to 165 mm. The useful length with constant diameter was determined to be about 2000 mm at the middle section. The critical length of this useful length was found to be 3.15 mm. The tensile strength was also determined to have an average of 970 MPa when measured at 15 mm gauge lengths but is found to decrease up to 796 MPa with increasing gauge lengths up to 35 mm. This superior tensile strength of abaca is also associated to the 2-3^o microfibril misorientation from the axis of the fiber. Use of the fibers in composite as continuous and unidirectional filler at 5% loading to unsaturated polyester (tensile strength of 40 MPa) resulted to a tensile strength of 48 MPa. The tensile strength increased to 71 MPa when chemically treated continuous fiber was employed. Alkali treatment at relatively high temperature improved the surface morphology of the fiber, with waxes and lignin removed from the surface and activating the surface with hydroxyl functional groups, that essentially improved the wettability of the polymer to the fiber, and densified the fiber with the closure of its lumens.

Abstract: Reliable prediction of fracture conditions is a major concern in the integrity assessment of structural components. This is specifically critical within the transition regime where there is a significant scatter in fracture test data. In recent years local stress based approaches that use a "Weibull distribution function" have been examined to predict probability of cleavage fracture at lower shelf temperature. Furthermore the role of constraint in toughness prediction has been noted. An extensive experimental programme known as "Euro fracture dataset" aimed at characterisation of the "Ductile-to-Brittle" transition (DBT) behaviour of ferritic steels. Recently this data set was used by authors to propose a set of "Global" equations for determination of temperature and thickness dependence of Weibull distribution parameters. In this paper finite element simulations of fracture tests are carried out firstly to verify the experimental findings and secondly to examine and validate the proposed "Global" equations. This objective has been achieved through the comparison between the experimental data, predictions of "Global" curves and the results of performed finite element simulations.

Abstract: The paper deals with the determination of the characteristic strength and the Weibull modulus m of Si3N4 and SiC ceramic materials using conventional four-point bending and unconventional contact tests between opposite rollers and opposite spheres. Ceramographic and fractographic methods were used for the characterization of strength degrading defects represented by processing flaws and by cracks of different types arising during the loading. The processing flaws influenced the Weibull parameters mainly in the bending mode, and the strength and its scatter in contact modes was influenced by lateral, median and contact end cracks, originated during the contact test using rollers, and by cone cracks originated during the contact test using spheres.

Abstract: Vitreous cylinders with compositions Bi2-xPbxSr2CaCu2Oy, (x = 0 and 0.4) were prepared and used as precursors to fabricate textured bars through a Laser Floating Zone melting method (LFZ). The resulting textured cylindrical bars were annealed and were mechanically characterized through mechanical strength, σ, Young modulus, E, Vickers hardness, H, and Weibull parameters, S0 and m. The study of the mechanisms controlling the fracture process was made by means of the fractographical analysis using Laser Scanning Confocal Microscopy (LSCM). Finally, the microstructure was determined and correlated with the mechanical properties.