Ignatius Pulung Nurprasetio

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.

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

Spectral Error Analysis of Windowed Discrete Sinusoidal Signals

This paper presents an analysis of spectral error analysis of windowed discrete sinusoidal signals. The objective of the work is to understand how window functions affect the discrete frequency spectrum and how to minimize spectrum errors. The spectral error analysis, in which the mathematical expression of the spectral error is derived, is conducted to a discrete sinusoidal signal with single frequency. The analysis reveals the fact that the applied window functions will produce peak spectral error regardless of the sampling rate and recording time. This result has been confirmed through numerical simulations. In addition, an experiment is conducted to investigate another type of spectral error, which is frequency error. The frequency error can be minimized by collecting more samples in signal measurement process, which means the recording time is increased, even though this effort does not improve the amplitude accuracy.

Development of Chatter Threshold Determination Methodology in Milling Process by Using Inclined Workpiece

Machined product quality depends on its dimension and surface quality. The dimension quality depends on machine tool accuracy while the surface quality depends on machining system stiffness. A low machining system stiffness will shift spindle shaft-tool resonance frequencies to low frequencies. When one of these frequencies coincides with spindle rotational speed or its harmonics, chatter will be generated which in turn worsen the workpiece surface roughness. In addition to increasing machining system stiffness, chatter can be eliminated by decreasing the axial depth of cut as well. Maximum axial depth at certain spindle rotational speed which will not generate chatter is called as chatter threshold. A diagram describing chatter thresholds for certain range of spindle rotational speed is called as a SLD (stability lobe diagram). The diagram is very useful for selecting a maximum depth of cut at certain rotational speed in order to obtain chatter-free machining process. The SLD can be generated theoretically or experimentally. The theoretical one is fast and cheap but it is not guaranteed to be correct. On the other hand, although the experimental one will produce exact values but it is long, cumbersome and expensive, because for certain rotational speed many machining with different axial depth of cut must be conducted until chatter threshold is reached. The same process is then repeated for other rotational speeds. This paper deals with a new method in determining the chatter threshold or SLD experimentally, by using inclined workpiece, by which it only needs one time machining-test for each rotational speed. In this method, during machining process, chatter occurrence is detected by using accelerometer and validated by its surface roughness afterwards. It is shown in this paper that the new method works well for machining aluminium workpiece in vertical machining center.