Effects of adhesive thickness on global and local Mode-I interfacial fracture of bonded joints

AbstractThe interfacial fracture of adhesively bonded structures is a critical issue for the extensive applications to a variety of modern industries. In the recent two decades, cohesive zone models (CZMs) have been receiving intensive attentions for fracture problems of adhesively bonded joints. Numerous global tests have been conducted to measure the interfacial toughness of adhesive joints. Limited local tests have also been conducted to determine the interface traction-separation laws in adhesive joints. However, very few studies focused on the local test of effects of adhesive thickness on the interfacial traction-separation laws. Interfacial toughness and interfacial strength, as two critical parameters in an interfacial traction-separation law, have important effect on the fracture behaviors of bonded joints. In this work, the global and local tests are employed to investigate the effect of adhesive thickness on interfacial energy release rate, interfacial strength, and shapes of the interfacial traction-separation laws. Basically, the measured laws in this work reflect the equivalent and lumped interfacial fracture behaviors which include the cohesive fracture, damage and plasticity. The experimentally determined interfacial traction-separation laws may provide valuable baseline data for the parameter calibrations in numerical models. The current experimental results may also facilitate the understanding of adhesive thickness-dependent interface fracture of bonded joints

Increasing Access for Economically Disadvantaged Students: The NSF/CSEM & S-STEM Programs at Louisiana State University

Increasing college degree attainment for students from disadvantaged backgrounds is a prominent component of numerous state and federal legislation focused on higher education. In 1999, the National Science Foundation (NSF) instituted the Computer Science, Engineering, and Mathematics Scholarships (CSEMS) program; this initiative was designed to provide greater access and support to academically talented students from economically disadvantaged backgrounds. Originally intended to provide financial support to lower income students, this NSF program also advocated that additional professional development and advising would be strategies to increase undergraduate persistence to graduation. This innovative program for economically disadvantaged students was extended in 2004 to include students from other disciplines including the physical and life sciences as well as the technology fields, and the new name of the program was Scholarships for Science, Technology, Engineering and Mathematics (S-STEM). The implementation of these two programs in Louisiana State University (LSU) has shown significant and measurable success since 2000, making LSU a Model University in providing support to economically disadvantaged students within the STEM disciplines. The achievement of these programs is evidenced by the graduation rates of its participants. This report provides details on the educational model employed through the CSEMS/S-STEM projects at LSU and provides a path to success for increasing student retention rates in STEM disciplines. While the LSU\u27s experience is presented as a case study, the potential relevance of this innovative mentoring program in conjunction with the financial support system is discussed in detail. © 2011 The Author(s)

ETCE2002/OT-29154 RHEOLOGICAL ANALYSIS OF CURING PROCESS OF EPOXY PREPREG USED AS COMPOSITE PIPE JOINTS

ABSTRACT The rheological properties of curing process of epoxy prepreg were measured by Bohlin Rheometer. The variations of storage modulus, loss modulus and viscosity are monitored vs. the cure time and temperature. Viscosity profiles were described by different models. Except the first order viscosity models, new viscosity models based on Boltzmann function were proposed. In the new models, a parameter called critical time was introduced. Critical time is a function of temperature and also meets an Arrhenius law. The activation energy calculated by critical time closes to that obtained by initial viscosity. The kinetic rate constants in the old and new models are comparable at each temperature, and the kinetic activation energies calculated from rate constants in the old and new models are very close. The fitting results show that the proposed new viscosity models are better than the old models for both isothermal and dynamic cure processes

Association of maternal Vitamin D status with glucose tolerance and caesarean section in a multi-ethnic Asian cohort: the growing up in Singapore towards healthy outcomes study

10.1371/journal.pone.0142239PLoS ONE10111-16GUSTO (Growing up towards Healthy Outcomes

Four-phase sphere modeling of effective bulk modulus of concrete

A four-phase sphere model, extended from Christensen and Lo\u27s three-phase sphere model for two-phase composite materials, was proposed to estimate the effective bulk modulus of three-phase concrete. The formulations were developed by reducing the four-phase sphere model to an equivalent three-phase sphere model and an equivalent two-phase sphere model. A distinctive characteristic of the proposed model is that, in addition to considering other physical-mechanical parameters, it is able to evaluate the effect of the maximum aggregate size and aggregate gradation on the effective bulk modulus of concrete. Reasonable agreement was found between the calculated effective Young\u27s modulus and the experimental results from the literature. This suggests that the proposed four-phase sphere model is suitable for estimating the effective elastic modulus of concrete. It is found that the maximum aggregate size, aggregate gradation, and the interfacial transition zone have a significant effect on the effective modulus of concrete

Analytical modeling of particle size and cluster effects on particulate-filled composite

A three-layer built-in model, extended from Christensen and Lo\u27s three-phase sphere model for particulate-filled composites (PFC) containing no clusters [R.M. Christensen and K.H. Lo, J. Mech. Phys. Solids, 27 (1979) 315.], is proposed to evaluate the particle size and cluster effect on the mechanical properties of PFC. Different from the self-consistent model used by Corbin and Wilkinson [S.F. Corbin and D.S. Wilkinson, Acta Metall. Mater., 42 (1994) 1311], which gives only average stress-strain distribution, this model can be used to estimate the point-by-point stress-strain distribution induced by either external force or temperature variation. Particles that are harder and softer than matrix are studied. It is found from the calculated results that reducing cluster and particle size, using less scattered particles, reinforcing the bonding strength at the interface of particles and matrix, enhancing the deformability of matrix, and employing particles with a coefficient of thermal expansion smaller than that of matrix are efficient methods to resist damages of PFC. In addition, reducing cluster concentrations and increasing particle contents are preferred for PFC containing hard particles and have negative effect for PFC involving soft particles. The selection of particle rigidities should be based on a balanced comparison between strength and rigidity requirements of PFC. © 1999 Elsevier Science S.A. All rights reserved

Micromechanical modeling of polymer modified asphalt at low temperatures

Polymer modified asphalt (PMA) is treated as a two-phase composite material with polymer particles dispersed in an asphalt matrix. Christensen and Lo\u27s three-phase sphere model is thus extended to a two-layer built-in model to evaluate parameters affecting the low temperature cracking resistance of a PMA mixture. A distinctive characteristic of the proposed model is that it can consider the effect of the PMA layer thickness on the mechanical properties when a PMA is used in an asphalt pavement. In addition, it can consider the effect of the polymer particle size distributions on the mechanical properties of the PMA. Approaches which are beneficial to enhance the low temperature cracking resistance of a PMA mixture are suggested based on the calculated results