Kyung Seop Han

Optimum placement of piezoelectric sensor/actuator for vibration control of laminated beams

The optimum placement of a collocated piezoelectric sensor/actuator is investigated numerically and verified experimentally for vibration control of laminated composite beams. The finite element method is used for the analysis of dynamic characteristics of the laminated composite beams with the piezoceramic sensor/actuator. The damping and the stiffness of the adhesive layer and the piezoceramics are taken into account in the process of finite element modeling. Tailoring that varies the stiffness and the damping properties of the composite material is used. The stacking sequence of the laminated composite beam is [θ 4 /0 2 /90 2 ] s , where θ = 0, 15, 30, 45, 60, 75, and 90 deg. The sensor/actuator attached to a structure changes the mass, the damping, and the stiffness of the entire structure. Thus, interaction between sensor/actuator and structure is very important in the vibration control of a flexible structure. Modal damping (2ζω) is chosen as a more appropriate performance index, because it is directly related to the settling time of the vibration. The structural damping index (SDI) is defined from the modal damping. Weights for each vibrational mode are taken into account in the SDI calculation. The optimum location of the sensor/actuator is determined as the point where the SDI is maximum. Numerical simulation and experimental results show that the SDI depends on outer-layer fiber orientations of the host structure, the location, and the size of the sensor/actuator.

Acoustic Emission Behavior during Fatigue Crack Propagation in 304 Stainless Steel

This study investigated acoustic emission behavior during fatigue crack growth test under constant and variable amplitude loading in 304 stainless steel. To describe the acoustic emission behavior, counts rate(dη/dn) was related with stress intensity factor range (SIFR, ΔK) in log-log plot. As a result of test, the relationship was represented a curve, which forms rise and fall behavior in counts rate as the SIFR increases. AE response to a single overload was sudden drop and slow recovery in counts rate, which was similar to crack growth retardation behavior. Under block loading, counts rate of each loading block was same as that of constant amplitude loading. Overall experimental results indicated that stress intensity factor controls the counts rate (dη/dn) as well as crack growth rate (da/dn) regardless of load range or crack length.

Optimum stacking sequence of composite laminates for maximum strength

A method is presented for maximum strength optimum design of symmetric composite laminates subjected to in-plane and transverse loadings. The finite element method based on shear deformation theory is used for the analysis of composite laminates. Ply orientation angles are chosen as design variables. The quadratic failure criterion which is meant to predict fracture, is used as an object function for optimum stacking sequence design of a laminated plate. The Broydon-Fletcher-Goldfarb-Shanno optimization technique is employed to solve the optimization problem effectively. Numerical results are given for various loading conditions, boundary conditions, and aspect ratios. The results show that the quadratic failure criterion such as Tsai-Hill theory is effective for the optimum structural design of composite laminates.

Squeeze-casting conditions of Al/Al2O3 metal matrix composites with variations of the preform drying process

The characteristics of the preform play a role in determining the final properties of MMCs. Effects of organic binder and microwave drying on preform microstructure have been examined by SEM. In the preform with organic binder, flocking processes are observed during drying. The preform has a uniform distribution of binder and dries quickly with microwave drying owing to its internal and volumetric heating patterns. The fundamental manufacturing process and controlling parameters of squeeze casting, including preform temperature, mould temperature, applied pressure and molten metal temperature, have been studied in Al/Al2O3 composites. MMCs have poor mechanical properties with too high temperatures of preform and molten metal due to thermal shocking of the preform, oxidation of the matrix and thermal damage to the fibers. Mould temperature barely affects the tensile strength of MMCs. High applied pressure reduces voids and solidifies the matrix faster. Conditions for squeeze casting to achieve optimal processing, are suggested. The tensile strength of MMCs can be improved by up to about 20% compared with the unreinforced matrix alloy.

Prediction and measurement of modal damping of laminated composite beams with piezoceramic sensor/actuator

Damping factor and modal damping of carbon/epoxy laminated composite beams with the piezoceramic sensor and actuator are predicted theoretically and measured experimentally. Finite element method is used for the analysis of dynamic characteristics of the laminated composite beams with and without the piezoceramics. The impulse technique is applied to measure the fundamental frequency, the damping ratio and the modal damping for the first bending mode of the beams. When a pair of piezoceramics is attached to the clamping side of the beam as a sensor and an actuator, the damping and the stiffness of the beam are changed. Taking into account the damping and the stiffness of the adhesive layer and the piezoceramics in the finite element modeling, damping ratio, fundamental frequency and modal damping are in good agreement with those of the measured values. To investigate the effects of sensor/actuator dynamics, vibration analysis of the beam without piezoceramics is also carried out. Damping ratio, fundamental frequency and modal damping of the beams without piezoceramics agree very well with those of measured values, as for the beams with piezoceramics. It is suggested that the modal damping (2w) is a more appropriate performance index rather than the damping ratio (O) for the vibration suppression in the structure, since one of the goals of structural vibration control is to suppress vibrational amplitude as fast as possible and the modal damping is directly related to the settling time of the vibration.

On the failure indices of quadratic failure criteria for optimal stacking sequence design of laminated plate

The quadratic failure criterion, which is intended to predict fracture, may be used as an object function for optimal stacking sequence design of laminated plate. However, calculations using a symmetric laminated plate demonstrate that Tsai-Wu theory may give incorrect optimum predictions under uniaxial loading.

Acoustic Emission as a Tool of Fatigue Assessment

A series of laboratory investigations concerned about fatigue assessment with acoustic emission method was presented. Fatigue aspects including cumulative fatigue damage, fatigue crack growth and creep-fatigue interaction were considered. As a basic approach, residual strength and acoustic emission characteristics of fatigue damaged materials were considered from the nominal stress-life (S-N) viewpoint. Acquired signal indicated that counts emission quantity can be a good measure of cumulated fatigue damage. In the fatigue crack growth approach, interrelationship between acoustic emission parameter and stress intensity factor was examined with different stress level and crack length. Experimental results were somewhat scattered since sensitive characteristics of acoustic emission method. However, their empirical relation indicated that counts rate correlated with fracture mechanics parameter. Finally, acoustic emission application was extended to the creep-fatigue interaction at elevated temperature. Emission response under each damage mode was compared and characterized. Based on these characteristics, creep-fatigue interaction was evaluated by use of acoustic emission parameter. Overall investigations concluded acoustic emission is very effective tool of fatigue assessment.

Fatigue life modeling of short fiber reinforced metal matrix composites using mechanical and acoustic emission responses

The cyclic fatigue behavior of short fiber reinforced metal matrix composites was studied. Three fatigue life prediction models were developed by monitoring the resultant maximum strain and counts of acoustic emission during cyclic fatigue. Their increasing trends with the number of fatigue cycles were studied using the assumption that they can be expressed as a power-law function of the fatigue cycle number. Post-fatigue static tension tests were conducted to examine the degradation of the damaged materials. The acoustic emission count accumulated during tension testing decreased as the applied fatigue cycles increased; this change was also used to formulate life prediction models. All the new models show a better agreement to experimental data than do the existing equations.

Characterization of Expanded Graphite/Flake-Type Graphite Filled Conductive Polymer Composites

Conductive polymer composites (CPCs) consisting of expanded graphite (EG), flake-type graphite (FG) and thermalsetting resin were fabricated by means of a preform molding technique. Conductive fillers, EG and FG, were mechanically mixed with the phenol resin to provide an electrical property to composites. The filler loadings were fixed at 75wt.% to obtain a high electrical conductivity. The mechanical and electrical properties of CPCs were optimized according to the weight ratio and the particle size of FG. As the weight ratio increased, the flexural strength increased, however, the electrical conductivity decreased for both cases of CPCs using different sizes of FG. The particle size was an important parameter to change the mechanical and electrical behaviors. The flexural strength was sensitive to the particle size due to the different level of densification. The electrical conductivity also showed size-dependent behavior because of the different contribution to the conductive networking.

Damage and Failure Monitoring of Fiber-Metal Laminates Using Optical Fiber Sensors

Optical fiber vibrations sensors(OFVS) and extrinsic Fabry-Perot interferometer(EFPI) were used in damage monitoring of fiber-metal laminates(FML). The OFVS and EFPI were applied in order to detect and evaluate the strain, damage and failure of FML. Indentation tests were performed with the measurement of optical signal and acoustic emission(AE). Robust signal processing methods based on time-frequency analysis were proposed to monitor damage signals of the structure with removing the influence of environmental and sensor noise. The signals of the OFVS due to damages were quantitatively evaluated by wavelet transform(WT). Relative position and condition between the specimen and indenter were able to estimate by observing the fiber-optic interference fringes of EFPI. It was found that damage information of comparable in quality to acoustic emission data could be obtained from the OFVS signals. The OFVS proved to be effective for monitoring the damage process of the material studied here.