Bentang Arief Budiman

Modeling of stress transfer behavior in fiber-matrix composite under axial and transverse loadings

Abstract Interface between fiber and matrix as a stress transfer medium determines composite performances in load-bearing structures. For instance, failures in composite are most likely initiated by an accumulation of interfacial cracks allowing little or no stress transfer from the matrix to the fiber and vice versa. This paper studies stress transfer behaviors at the interface subject to axial and transverse loadings using the finite element method. Single fiber surrounded by matrix was modeled by introducing a cohesive zone model (CZM) at the interface taking into account the bonding mechanism. By the proposed technique, plastic deformation in the matrix and exerted friction at the interface was verified to govern the role of stress transfer at the interface. Further, the influence of other fibers in matrix surrounding the model was also discussed.

A new method of evaluating interfacial properties of a fiber/matrix composite

Interfacial debonding frequently initiates composite failure in a fiber/matrix composite. A single-fiber fragmentation test and its modifications can be used to evaluate interfacial properties. However, they still have accuracy problems due to fiber impurities and friction work. This paper presents a new method of evaluating interfacial properties using a stress contour of composite matrix. A single-fiber fragmentation test model was developed to simulate the stress contour. The interface was modeled as a cohesive zone model. Four characteristic lengths on the stress contour were found after conducting simulations with many interfacial properties values. The stress contour was then captured from the single-fiber fragmentation test employing a photo-elasticity technique and the four characteristic lengths were measured. Iteration in simulation involved changing interfacial properties until corresponding characteristic lengths from experiment and simulation were obtained. The results were compared with those obtained with existing methods and found to be reasonable.

The Role of Interfacial Rigidity to Crack Propagation Path in Fiber Reinforced Polymer Composite

This paper reveals the role of interfacial rigidity in a fiber reinforced polymer (FRP) composite to crack propagation path and its reinforcement, investigated by simulating a plane-strain model – containing the matrix with a fiber inclusion – loaded transversely. Two fibers, i.e. carbon fiber and glass fiber, were examined. The interface between fiber and matrix was pre-defined using the spring model characterized by a stiffness parameter. A range of interfacial stiffness values from infinity (rigid bonding) to low stiffness were carefully investigated. The results suggested that direction of crack propagations inclined to approach the fiber when the interfacial stiffness was low, even if the fiber had much higher elastic modulus than the matrix – this finding is in contrast to the inclusion theory that merely considers rigid bonding interface. It also suggests that there existed a transition region in which the crack propagation path was altered, from approaching to avoiding the fiber. The region occurred when the interfacial stiffness was in the range between 105 and 106 MPa/mm, within which mechanical properties of the model were recorded to vary notably

Fail-safe design and analysis for the guide vane of a hydro turbine

A design for the fail-safe mechanism of a guide vane in a Francis-type hydro turbine is proposed and analyzed. The mechanism that is based on a shear pin as a sacrificial component was designed to remain simple. Unlike the requirements of conventional designs, a shear pin must be able to withstand static and dynamic loads but must fail under a certain overload that could damage a guide vane. An accurate load determination and selection of the shear pin material were demonstrated. The static load for various opening angles of the guide vane were calculated using the computational fluid dynamics Fluent and finite element method Ansys programs. Furthermore, simulations for overload and dynamic load due to the waterhammer phenomenon were also conducted. The results of load calculations were used to select an appropriate shear pin material. Quasi-static shear tests were performed for two shear pins of aluminum alloy Al2024 subjected to different aging treatments (i.e. artificial and natural aging). The test results indicated that the Al2024 treated by natural aging is an appropriate material for a shear pin designed to function as a fail-safe mechanism for the guide vanes of a Francis-type hydro turbine.