Key Engineering Materials Vols. 321-323

Abstract: Recently, the use of composite material to railway vehicles is gradually increasing due to its light weight. The concept of light weight design will be very helpful for the tilting train to travel around a curve at a faster speed because the light weight car body makes the center of gravity lower. The impact characteristics of composite materials should be investigated because the impact property of composites is very important for the design and repair of a tilting train. This paper investigates the low-velocity impact and damage characteristics conducted on woven carbon/epoxy laminates for a tilting train. Low-velocity impact tests were carried out at different energy levels on woven type of laminates using a drop-weight apparatus. The impact-induced delaminations are examined by ultrasonic technique such as C-Scan. Those delaminations are also examined by laser technique such as ESPI (Electronic Speckle Pattern Interferometry). Experimental results show that the type of damage is dependent on the impact energy level and the delamination area becomes larger as the impact energy increases.

Abstract: Several nondestructive evaluation (NDE) techniques, including ultrasonic C-scan, X-ray computed tomography (CT), and infrared (IR) thermography, were employed on ceramic matrix composites (CMCs) to illustrate defect information that might effect mechanical behavior and to analyze structural performance of CMCs. Prior to tensile testing, through C-scan and CT analyses results, the qualitative relationship between the relative ultrasonic transmitted amplitude and porosity based on CT was exhibited. An IR camera was used for in-situ monitoring of progressive damages and to determine temperature changes during tensile testing. Moreover, scanning-electron microscopy characterization was used to perform microstructural failure analyses. This paper describes the use of nondestructive evaluation (NDE) techniques to facilitate the understanding of tension behavior of CMCs.

Abstract: To analyze the stress distribution and the crack initiation due to the location of the defects, artificial defects were made on the different locations of the high strength monolithic Al and GLAss fiber REinforced laminate (GLARE). Experimental study shows that the defect location in the vicinity of the circular hole was changed from ° = 90 4 θ to ° = 0 1 θ , the stress concentration was increased. The stress concentration of GLARE was about 15% higher than that of the monolithic Al. When the defect was at ° = 30 2 θ , Multi Site Damage (MSD) crack was found in the monolithic Al but not in GLARE. Instead of MSD crack, a delamination was made in GLARE and that it resulted in the prevention against the second crack initiation.

Abstract: Bolted joints are widely used for composite structures. As is well known, excessive bearing load gives rise to bearing failure at hole boundaries. Detecting bearing failure is important for assuring integrity of composite structures. Since conventional nondestructive inspection methods are expensive, cumbersome, time-consuming, and not suitable for health monitoring, a simple, low-cost inspection method for bearing failure must be developed. Authors have demonstrated the feasibility of detecting bearing failure by using an electric resistance change method. In this study, more detailed analyses were carried out to investigate the detectability in terms of the damage size and the distance between damage and electrodes. The results show that bearing failure of less than 10mm square causes the electric resistance change of a few hundred ppm and thus can be easily detected, and that the electrodes can be mounted more than 10 mm far from a bolt hole.

Abstract: In this paper, a smart skin, i.e. a conformal load-bearing antenna structure, which is a multi-layer sandwich structure composed of carbon/epoxy, glass/epoxy and dielectric material, designs, analyses, fabrications and tests are conducted. Mechanical properties of each structural layer of the designed smart skin are obtained from experimental tests. Tests and analyses are conducted to study the deformation behavior of the smart skin under compressive loads. The measured data are compared with the numerical results from geometrically linear/nonlinear finite element analyses. Numerical prediction for the buckling load of the smart skin agreed well with the experimental data.

Abstract: For improving quality of a carbon fiber reinforced composite material (CFRP) by preventing defects such as delamination and void, it should be inspected in fabrication process. Novel non-contacting evaluation technique is required because the transducer should be contacted on the CFRP in conventional ultrasonic technique during the non-destructive evaluation and these conventional contact techniques can not be applied in a novel fiber placement system. For the non-destructive evaluation of delamination in CFRP, various methods for the generation and reception of laser-generated ultrasound are applied using piezoelectric transducer, air-coupled transducer, wavelet transform technique etc. The high frequency component of laser-generated guided wave received with piezoelectric sensor disappeared after propagating through delamination region. Air-coupled transducer was tried to be adopted in reception of laser-generated guided wave generated by using linear slit array in order to generate high frequency guided wave with a frequency of 1.1 MHz. Nevertheless, it was failed to receive high frequency guided wave in using air-coupled transducer and linear slit array. Transmitted laser-generated ultrasonic wave was received on back-wall and its frequency was analyzed to establish inspecting technique to detect delamination by non-contact ultrasonic method. In a frequency spectrum analysis, intensity ratio of low frequency and center frequency was approvable parameter to detect delamination.

Abstract: Ultrasound techniques have recently been widely used for the characterization of articular cartilage (artC) in vitro and in vivo. The sound speed in artC plays an important role either as an indicator for the artC degeneration or for the calculation of other parameters such as tissue thickness or stiffness. Previous studies on artC have been carried out in bathing saline solutions with different concentrations to investigate swelling behaviors of artC. There is a lack of information in the literature about the effect of bathing saline concentration on the sound speed of artC. In the present study we measured sound speed in artC under different saline concentrations ranging from 0M to 2.5M at the room temperature temperature (21±1°C). ArtC specimens from bovine patellar models (n=20) were used in these in-vitro studies. Results demonstrated that the sound speed in artC ranged from 1681±50 m/s to 1816±54 m/s when the saline-concentration varied from 0M to 2.5M, while the sound speed in saline changed from 1521± 3 m/s to 1674 ± 3 m/s. The sound speed linearly (r2 = 0.99, p <0.001) increased with the increase of the saline concentration at a rate of 55 m/s per mole change. It is concluded that the variations of the bathing saline concentration significantly affect the sound speed in artC and should be well documented in the ultrasonic studies of artC.

Abstract: PMN-PT, a piezoelectric single crystal, has been known to be a better material for transducer arrays due to its high electromechanical coupling coefficient (k33) and high dielectric and piezoelectric constants. It may also be good even for high frequency single element transducers using relatively high kt and low attenuation and velocity dispersion. However, it’s challenging to fabricate high frequency transducers using PMN-PT since it is easily breakable and requires small area and thickness of the transducer. A KLM model was used to simulate a 40 MHz single element transducer including 2 matching layers and a conductive backing. The simulation showed that the PMN-PT transducer turned out to be better in sensitivity and bandwidth than a 40 MHz LiNbO3 transducer. A 40 MHz PMN-29%PT transducer was fabricated and the pulse echo signals were obtained and analyzed. Its sensitivity was found to be –48 dB and –6dB bandwidth was about 48 %.

Abstract: Pressure distributions on the buttocks and thighs by the functional electrical stimulation on the gluteus maximus, sartorius and hamstring in the seating posture were analyzed for ten healthy young volunteers in order to determine which muscle can be stimulated for pressure ulcer prevention. Muscles were stimulated every 5 seconds over 30 seconds by the MP150 stimulator. Pressure distribution and ground reaction forces on the stimulated buttocks and thighs increased when the gluteus maximus was stimulated. Pressure on the stimulated thigh slightly decreased when stimulating sartorius and hamstring individually. With a simultaneous stimulation of the gluteus maximus and sartorius, pressures on the stimulated buttocks and thighs increased significantly, but pressure on the opposite buttock decreased significantly. With a simultaneous stimulation of the sartorius and hamstring, both pressure on the stimulated thigh and the ground reaction force significantly decreased.

Abstract: The purpose of this study is to quantitatively evaluate foot motions in the normal and hallux valgus(HV) patients using a proposed multi-segment foot model. Results showed that HV patients exhibited relatively smaller flexion/extension, especially in terminal stance. An early abduction at the ankle during terminal stance was also noted in HV patients, which resulted in an excessive abduction in subtalar joint. In HV patients, medial MP joints exhibited excessive amount of adduction during terminal stance. In HV patients, hallux MP joint and talocrural joint exhibited excessive external rotation over the whole period of gait cycle. Understanding the detailed foot motions by the present multi-segment foot model would be very useful to diagnose and to treat patients with various foot diseases.