Papers by Author: David K. Hsu

Abstract: Recently, terahertz ray imaging has emerged as one of the most promising new powerful nondestructive evaluation (NDE) techniques for the area applications. In this study, a new time-domain spectroscopy system was utilized for measuring the coating thickness on CFRP composite laminates. Extensive experimental measurements in reflection mode were made to map out the T-ray images. Also, the refractive index was estimated based on the electromagnetic properties. The CFRP composite laminates were observed in reflection mode and limitations will be discussed in the T-ray processing. By using these characterized material properties, the characteristics was successfully demonstrated for T-ray behavior propagating through the Shim Stock films for acquiring the refractive index. The T-ray technique has been developed for the measurement of the thickness of the Shim Stock films and the coating thickness on CFRP composites. Good results have been obtained in tests made on the thickness of the standard film samples with the coating thickness ranging from around hundreds of μm.

Abstract: Recently, terahertz ray imaging has emerged as one of the most promising new powerful nondestructive evaluation (NDE) techniques, and new application systems are under processing development for the area applications. In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating layup effect and flaw in FRP composite laminates. Extensive experimental measurements in reflection mode were made to map out the T-ray images. Especially in this characterization procedure, we estimated the electromagnetic properties such as the refractive index. Estimates of properties are in good agreement with known data. Furthermore layup effect and flaw of FRP composite laminates were observed in reflection mode and limitations will be discussed in the T-ray processing.

Abstract: A nondestructive technique would be very useful for evaluating the CF/Epoxy composite laminates. It is found that a pitch-catch signal was more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composites (damages, fiber orientation, low level porosity, ply waviness, and cracks). The depth of the sampling volume where the pitch-catch signal came from was relatively shallow with the head-to-head miniature Rayleigh probes, but the depth can be increased by increasing the separation distance of the transmitting and receiving probes. Also, a method was utilized to determine the porosity content of a composite lay-up by processing micrograph images of the laminate. The porosity content of a composite structure is critical to the overall strength and performance of the structure. The image processing method developed utilizes a free software package to process micrograph images of the test sample. The results from the image processing method are compared with existing data. Beam profile was characterized in unidirectional CFRP (Carbon fiber reinforced plastics) with using pitch-catch Rayleigh probes and the one-sided and two-side pitch-catch technique was utilized to produce C-scan images with the aid of the automatic scanner. Also, it is confirmed that the pitch-catch ultrasonic signal was corresponding with simulated results assuming in unidirectional CFRP composites.

Abstract: Carbon/carbon(C/C) composite materials have obvious advantages over conventional materials, which consist of carbon fibers embedded in a carbon matrix. It’s low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for aerospace applications especially aircraft brake disks. Because of permeation of coupling medium such as water, it is desirable to perform contact-less nondestructive evaluation to assess material properties and part homogeneity. In this work, a C/C composite material was characterized with non-contact and contact ultrasonic methods using automated acquisition scanner. . Due to the acoustic impedance mismatch found between most materials and air, a major limitation for air-coupled transducers, through-transmission mode was performed. Especially ultrasonic images and velocities for C/C composite disk brake were measured and found to be consistent to some degree with the non-contact and contact ultrasonic measurement methods. 400 kHz frequency through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the inhomogeneity in material property. Non-contact measured results were compared with those obtained by the motorized system using contact drycoupling ultrasonics and through transmission method in immersion. Results using a proposed peak-delay measurement non-contact method corresponded well to the ultrasonic velocities of the contact pulse overlap method.

Abstract: In particular, CFRP (carbon fiber reinforced plastics) composite materials have found wide applicability because of their inherent design flexibility and improved material properties. CFRP composites were manufactured from uni-direction prepreg sheet in this paper. However, impacted composite structures have 50-75% less strength than undamaged structures. It is desirable to perform nondestructive evaluation to assess material properties and part defect in order to ensure product quality and structural integrity of CFRP. In this work, a CFRP composite material was nondestructively characterized and a pitch-catch technique was developed to measure impacteddamaged area using an automated-data acquisition system. Also one-sided mode was performed to scan defect both manual contact measurement and an immersion tank. It is found a pitch-catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composite, including fiber orientation, low level porosity, ply waviness, and cracks. The paper describes the depth of the sampling volume with the head-to-head miniature Rayleigh probes and also ultrasonic C-scan images are acquired experimentally using one-sided measurement and a conventional scanner.

Abstract: Owing to the advantages associated with their very large strength-to-weight and stiffnessto- weight ratios, composite materials are attractive for a wide range of applications. Increasingly, high performance engineering structures are being built with critical structural components made from composite materials. In particular, the importance of carbon-fiber reinforced plastics (CFRP) has been generally recognized in both space and civil aircraft industries, and CFRP composite laminates are widely used. It is very important to detect fiber orientation error in orthotropic composite laminates because the layup of a CFRP composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. In this study, a new approach was investigated on detection of fiber orientation with using two longitudinal and a shear wave ultrasonic transducers for the orthotropic composite laminates. During testing, the most significant problem is that the couplant conditions do not remain the same because of changing the viscosity of the couplant. Therefore, making a design for generating shear wave with longitudinal transducers would greatly aid in alleviating the couplant problem. A pyramid with an isosceles triangle was made of aluminum in order to generate shear waves using two longitudinal transducers based on ultrasonic-polarized mechanism. It is found that the shear wave was very sensitive to fiber of CFRP composite. Finally, a CFRP composite material was nondestructively characterized in order to measure fiber orientation error area using automated data acquisition C-scan system.

Abstract: A nondestructive technique would be very beneficial, which could be used to test the CF/Epoxy composite laminates. A new method for nondestructively determining the ply layup in a composite laminate is presented. A one-sided pitch-catch setup was used in the detection and evaluation of flaws and material anomalies in the composite laminates. Two Rayleigh wave transducers were joined head-to-head and used in the pitch-catch mode on the surface of the composites. The pitch-catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composite. Also the other method employs a normal-incidence longitudinal ultrasound to perform C-scan of ply interfaces of the laminate, and extracts fiber orientation information from the ultrasonic reflection in the laminate. Using two-dimensional spatial Fourier transform, interface C-scan images were transformed into quantitatively angular distribution plots to show the fiber orientation information therein and to determine the orientation of the ply. Therefore, it is found that the efficiency of developed system shows between the one-sided pitch-catch testing and C-scan images in characterizing ply-layup orientation of the laminates.

Abstract: Composite laminates help to lighten components because they have the advantage of higher strength-to-weight ratio. So, they have come to be utilized as parts of advanced areas. An NDE technique for stacking orientation determination would be very beneficial because of layup orientation influence to the laminate stiffness. In this study, an investigation of shear wave ultrasonic technique was carried out in order to detect stacking orientation error for quasi-isotropy composite laminates. During testing, the most significant problem is that the couplant conditions do not remain the same because of changing the viscosity of the couplant. Therefore, the design and use of a shear wave transducers would greatly aid in alleviating the couplant problem. A pyramid with an isosceles triangle with two 45o was made of aluminum to generate shear waves using two longitudinal transducers based on ultrasonic-polarized mechanism. Also, the signal splitter was connected to the pulser jack on the pulser/receiver and to the longitudinal transducers. The longitudinal transducers were mounted with mineral oil and the shear transducer was mounted on the bottom as a receiver with burnt honey. It is found that the shear wave was generated at a maximum and a minimum based on the ultrasonic-polarized mechanism. To apply this method into a practical inspection tool, the quasi-isotropy composite laminates were nondestructively evaluated for detecting error of the laminates.

Abstract: Carbon/phenolic composite (CPC) materials have obvious advantages over conventional materials, which are consisting of carbon fibers embedded in a carbon matrix. The CPCs have become to be utilized as parts of aerospace applications and its low density, high thermal conductivity and excellent mechanical properties at elevated temperatures make it an ideal material for aircraft brake disks. Because of permeation of coupling medium such as water, it is desirable to perform contact-less nondestructive evaluation to assess material properties and part homogeneity. In this work, a C/P(Carbon/phenolic) composite material was characterized with non-contact and contact ultrasonic methods using automated acquisition scanner. Also through transmission mode was performed because of the main limitation for air-coupled transducers, which is the acoustic impedance mismatch between most materials and air. Especially ultrasonic images and velocities for C/C(Carbon/Carbon) composite disk brake was measured and found to be consistent to some degree with the non-contact and contact ultrasonic measurement methods. Low frequency throughtransmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Measured results were compared with those obtained by the motorized system with using dry-coupling ultrasonics and through transmission method in immersion. Finally, results using a proposed peak-delay measurement method well corresponded to ultrasonic velocities of the pulse overlap method.

Abstract: Air-coupled ultrasound is a non-contact technique and has obvious advantages over water-coupled experimentation. Especially, wood materials are very sensitive to water and inspection without any coupling medium of a liquid is really needed to wood materials due to the permeation of coupling medium such as water. In this work, it is desirable to perform contact-less nondestructive evaluation to assess wood material homogeneity. A wood material was nondestructively characterized with non-contact and contact modes to measure ultrasonic velocity using automated data acquisition software. We have utilized a proposed peak-delay measurement method. Also through transmission mode was performed because of the main limitation for aircoupled transducers, which is the acoustic impedance mismatch between most materials and air. The variation of ultrasonic velocity was found to be consistent to some degree with those in infiltrated area for air-coupled and conventional scan images. Through-transmission scans for aircoupled and conventional UT were used based on both amplitude and time-of-flight images. Finally, results using a peak-delay measurement method well somewhat corresponded to ultrasonic velocities of the pulse overlap method.