L.S. Ong

Cure monitoring of smart composites using Fiber Bragg Grating based embedded sensors

A sensor embedded in the composite laminate can act as a temperature transducer during the composite cure mechanism. Once the composite is cured, the same sensor can be used to provide the information about the mechanical changes that influence the performance of the material. Fiber Bragg Grating (FBG) sensor is one such sensor which one can use for the composite cure monitoring. We present here the results obtained with an associated FBG sensor system for the cure monitoring of smart composites. The performance of the embedded FBG sensor smart composite specimens under 3- and 4-point bending conditions are also being investigated. Finally, the performance analysis has been extended to cantilever specimens.

Vibration of pretwisted cantilever shallow conical shells

Abstract This paper presents a mathematical model to investigate the effects of initial twist on the vibratory characteristics of cantilever shallow conical shells. The energy functional is minimized according to the Ritz procedure to arrive at the governing eigenvalue equation. A set of orthogonally generated two-dimensional polynomials associated with a basic function, which accounts for the boundary expressions and constraints, is introduced to approximate the in-plane and transverse displacement amplitude functions. The complete procedure has been automated to compute the vibration frequencies and mode shapes for exemplary problems. In the numerical experiments, the convergence of eigenvalues is confirmed by increasing the degrees of polynomials employed in the admissible shape functions. To enhance the existing literature, a set of first known frequency parameters is presented. The paper highlights the important effects of angle of twist on the vibration frequencies and mode shapes of conical shells. The fundamental physical frequency ω decreases monotonically for a longer conical shell. The result shows that an increase in the angle of twist does not ensure higher torsional stiffness for a conical shell, which is in contradiction with previous observation for a pretwisted beam or plate. The symmetry of modes is absent when the angle of twist is non-zero.

An experimental study on tearing energy in splitting square metal tubes

Abstract An experiment was performed to study the tearing energy in splitting square aluminium and mild steel tubes of thicknesses ranging from 0.47 to 1.67 mm. It was carried out by driving four rollers each attached to the side wall of the tube, leading to the bending of the wall to a constant curvature and, at the same time, the tearing along the four corners. By pre-cutting some corners to a different length, the tearing energy involved was determined. It was found that the tearing energy per unit torn area (R) may be related to the ultimate stress of the material (σu) and the fracture strain (ef) as R = 8.8σuef for mild steel and R = 37.2σuef for aluminium tubes; here the two coefficients have length dimensions in mm.

Efficient implementation of a spatial light modulator as a diffractive optical microlens array in a digital Shack-Hartmann wavefront sensor

A traditional Shack-Hartmann wavefront sensor (SHWS) uses a physical microlens array to sample the incoming wavefront into a number of segments and to measure the phase profile over the cross section of a given light beam. We customized a digital SHWS by encoding a spatial light modulator (SLM) with a diffractive optical lens (DOL) pattern to function as a diffractive optical microlens array. This SHWS can offer great flexibility for various applications. Through fast-Fourier-transform (FFT) analysis and experimental investigation, we studied three sampling methods to generate the digitized DOL pattern, and we compared the results. By analyzing the diffraction efficiency of the DOL and the microstructure of the SLM, we proposed three important strategies for the proper implementation of DOLs and DOL arrays with a SLM. Experiments demonstrated that these design rules were necessary and sufficient for generating an efficient DOL and DOL array with a SLM.

Large-scale optical traps on a chip for optical sorting

The authors present a power-efficient large-scale lensless optical traps on a chip (OTOCs) as an optofluidic element for optical sorting of microparticles. Based on the well-known Talbot self-imaging effect in the Fresnel region, the OTOC makes use of a two-dimensional microfabricated chessboardlike structure to create an optical lattice near its emergent plane. Simultaneous trapping of hundreds of microparticles in a regular array (>200×200μm2) is proved experimentally without adopting an external optical projection lens configuration. Furthermore, the authors demonstrate experimental results for large-scale sorting of microparticles by sizes using the OTOC.

Investigation of residual stress and its effects on the vibrational characteristics of piezoelectric-based multilayered microdiaphragms

In this study, residual stress influences on the vibrational behavior of piezoelectric circular microdiaphragm-based biosensors are investigated theoretically and experimentally. The piezoelectric microdiaphragm was first fabricated by combining sol–gel PZT thin film and MEMS technology. The stress measurements by the wafer curvature method and the micro-Raman technique demonstrate that high tensile stresses are generated in the upper films (Pt/PZT/Pt), while the silicon oxide layer experiences a compressive stress. After backside etching of the microdiaphragm, the suspended membrane method was used to measure the average stress and equivalent Youngs modulus of the diaphragm, which were 96 MPa and 106.4 GPa, respectively. The dynamic behavior of the fabricated sensor under this stress was then investigated. A comprehensive mechanics model based on vibration modes is presented and the natural frequencies of the diaphragm are obtained. A nondimensional tension parameter is defined, and effects of this parameter on the resonant frequency of the diaphragm are presented. It was concluded that both flexural rigidity and tension contribute to the resonant frequency of the diaphragm sensor. Finally, the resonant frequencies of the fabricated sensor were measured by impedance analysis and laser vibrometry techniques. These frequencies were compared with their theoretical counterparts and a good agreement was observed.

Medium damping influences on the resonant frequency and quality factor of piezoelectric circular microdiaphragm sensors

Medium damping influences on the resonant frequency and quality factor of piezoelectric circular microdiaphragm sensors (PCMSs) are investigated theoretically and experimentally in this paper. The acoustic radiation and viscosity damping as the two main sources of energy dissipation in a medium virtually added the mass of the diaphragm and therefore decrease the frequency and Q-factor of the diaphragm. The magnitude of medium damping inversely depends on the radius-to-thickness ratio. An increase in this ratio is the trend in the fabrication of thin microdiaphragms by MEMS fabrication processes, which implies the higher influence of medium damping on the dynamic behavior of microdiaphragms. The fabricated PCMSs were tested in vacuum, air, and ethanol. The Q-factor and the resonant frequency of the device increase by almost seven times, 4.7% from air to 0.05 atmpressure, respectively. The Q-value drops from 111.195 in air to 23.908 in ethanol. Throughout this work, theoretical and experimental values were compared and a fairly good correlation was observed.

Composite-microlens-array-enabled microfluidic sorting

We propose a simple, reliable and cost-effective method for microfluidic sorting of microparticles using an optical potential landscape projected by a composite microlens array (MLA). The MLA enables a high power efficient approach to forming composite shape and size of the projected pattern. Sorting particles by size is demonstrated both theoretically and experimentally.

Simultaneous optical trapping of microparticles in multiple planes by a modified self-imaging effect on a chip

The authors propose a three-dimensional (3D) optical trapping of microparticles in multiple planes simultaneously based on a modified self-imaging effect. Similar to the Talbot self-imaging effect, the modified self-imaging effect is induced by a layer of trapped particles and it is subsequently used as a periodic grating structure to generate its own self-imaging pattern in 3D. Based on this secondary layer-by-layer self-imaging effect, optical trapping of silica and polystyrene colloidal particles at different planes in a microchamber are demonstrated experimentally.

Relaxation of Peening Residual Stresses Due to Cyclic Thermo-Mechanical Overload

Shot-peening induced residual stresses can be relaxed due to cyclic loading. This relaxation plays an important role in determining the fatigue life of the peened components. It is therefore the purpose of this study to conduct comprehensive three-dimensional dynamic elasto-plastic finite element analysis of the joint peening treatment and relaxation process. In this regard, a novel symmetry cell is developed and used to model the multiple impact indentations resulting from multiple impingements of a cluster of shots. The model was further extended to integrate the relaxation resulting from cyclic loading at stresses above the yield strength of the material. This integrated model accounts for the main features of both stages by considering strain-rate effects, shot and target inertia and the dependence of the mechanical properties of the target material on temperature. A wide spectra of cyclic mechanical and thermal loads as well as their combinations is considered and the resulting relaxed residual stress field is determined. As an application, the model was used to predict the residual stress relaxation in a high-strength steel target made from AISI 4340 under different peening and thermomechanical cyclic overload.