Sun Kyu Kim

On Feasibility of Ply-Layup Orientation in CF/Epoxy Composite Laminates Using Rayleigh Probes

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.

Ultrasonic Application to Scan the Layup of CFRP Composite Laminates

Layup sequence of carbon-fiber reinforced plastics (CFRP) composite laminates greatly influences its properties in a composite laminate. A nondestructive technique would be very beneficial, which could be used to test the part after curing and requires less time than the optical test. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. Firstly, a motorized scanner was built for making transmission measurements using a pair of normal-incidence shear wave transducers to find the effect of fiber misorientation of composite laminates. Also a method for nondestructively determining the p1y layup in a composite laminate is presented. The method employs a normalincidence longitudinal ultrasound to perform C-scan of ply interfaces or full-waveform B-scan of the laminate; therefore a 2-D fast Fourier transformation had been adopted to compare the results of composite layup. And a ply-by-ply vector decomposition model has been developed, simplified, and implemented for composite laminates fabricated from unidirectional plies. It is found that high probability shows between tests and the model developed in characterizing fiber orientation of the laminates. _______________________________________________________________________________ Email: khim@woosuk.ac.kr Materials Science Forum Online: 2004-09-15 ISSN: 1662-9752, Vols. 465-466, pp 241-246 doi:10.4028/www.scientific.net/MSF.465-466.241

Axial Impact Collapse Analysis of Spot Welded Hat and Double-hat Shaped Section Members Using an Explicit Finite Element Code

The purpose of this study is to analyze the collapse characteristics of widely used spot welded section members (hat and double hat section members of vehicles) which possess the greatest energy absorbing capacity in an axial impact collapse. This study also suggests how the collapse load and deformation mode are obtained under impact. In the program system presented in this study, an explicit finite element code, LS-DYNA3D, is adopted for simulating complicated collapse behavior of the hat and double hat shaped section members with respect to section dimensions and spot weld pitches. Comparing the results with experiments, the simulation has been verified under a velocity of 7.19 m/sec (impact energy of 1034J).

Coating Thickness Characterization of Composite Materials Using Terahertz Waves

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.

A Nondestructive Study on Fiber Layup and Porosity Level in CFRP Composite Laminates Using Ultrasonic Pitch-Catch Method

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.

Feasibility and Application on Ultrasonic-Polarized Shear Wave of Fiber Orientation Detection in Orthotropic Composite Laminates

It is very important to detect fiber orientation error in orthotropic composite laminates because the layup of a CFRP (carbon-fiber reinforced plastics) composite laminates affects the properties of the laminate, including stiffness, strength and thermal behavior. In this study, an investigation of shear wave ultrasonic technique was carried out in order to detect stacking orientation error 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, 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 ultrasonicpolarized mechanism. Also, the signal splitter was connected to the pulser jack on the pulser/receiver and to the longitudinal transducers which were mounted with mineral oil. 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. Finally, test results with model data were compared for a fiber orientation of the laminates.

Ultrasonic Inspection of Carbon/ Phenolic Composites Using a Peak-Delay Measurement Method

Carbon/phenolic composite (CPC) materials are unique which consist of carbon fibers embedded in a carbon matrix. The CPCs are originally developed for 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. The properties of the CPC are dependent on the manufacturing methods used for production and fiber arrangement. It is desirable to perform nondestructive evaluation to assess material properties and part homogeneity in order to ensure product quality and structural integrity of CPC brake disks. In this work, a CPC material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in C/P composites using automated data acquisition software. Also a motorized system was adopted to measure ultrasonic velocity on the point of CPC materials under the same coupling conditions. Manual results were compared with those obtained by the motorized system with using drycoupling ultrasonics and through transmission method in immersion. A peak-delay measurement method well corresponded to ultrasonic velocities of the pulse overlap method and throughtransmission mode and C-scan image signal based on peak-to-peak amplitude.

Influence of Ultrasonic Shear Wave on Defect Angles of Stacking Using CFRP Composite Laminates

Because the layup of composite laminates influences there properties, the strength of composites depends on layup sequence of CFRP laminates. It is very important to detect ply error before the laminate is cured for both manual procedure and fiber placement procedure. An ultrasonic technique would be very beneficial, which could be used to test the part after and before curing laminates and requires less time than the optical test. Scanners were set out for different measurement modalities for acquiring ultrasonic signals as a function of in-plane azimuthal angle. The first motorized scanner was utilized for making transmission measurements using a pair of normal-incidence shear wave transducers. A scanner was built for the acousto-ultrasonic configuration using contact transducers. And a ply-by-ply vector decomposition model has been utilized for evaluating layup errors in composite laminates fabricated from unidirectional plies. We have compared the test results with model data. It is found that high probability shows between experimentations and the decomposition model in characterizing cured and uncured laminates with defect angles.

Effect of Air-Coupled/Conventional Ultrasound Waves on Resin-Infiltrated Time of Wood Natural Materials

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.

Effect of Ultrasound Waves on Resin-infiltrated Time of Wood Materials Using a Peak-Delay Measurement Method

A nondestructive evaluation (NDE) technique would be very beneficial to assess resininfiltrated absorption effect of wood when wood was resin-treated. In this work, a wood material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in wood using automated data acquisition software. We have proposed a peak-delay measurement method based on the pulse overlap measurement method. Also through transmission mode was performed to compare ultrasonic velocity with the above peak-delay measurement method in an immersion tank. The variation of ultrasonic velocity was measured and found to be somewhat consistent with those in infiltrated area in the course of nature absorption. For mapping out the material property inhomogeneity, through-transmission scans were used based on both amplitude and time-of-flight of the ultrasonic pulse. A peak-delay measurement method well somewhat corresponded to ultrasonic velocities of the pulse overlap method and through-transmission mode and C-scan image signal based on peak-to-peak amplitude.