Jae-Ki Sim

A Study on Ultrasonic Evaluation of Material Defects in Carbon/Carbon Composites

It is desirable to perform nondestructive evaluation to assess material properties and part homogeneity because manufacturing of carbon/carbon (C/C) composites requires complicated and costly processes. In this work several ultrasonic techniques were applied to carbon/carbon composites for the evaluation of spatial variations in material properties that are attributable to the manufacturing process. In a large carbon/carbon composite manufactured by chemical vapor infiltration (CVI) method, the spatial variation of ultrasonic velocity was measured and found to be consistent with the densification behavior in CVI process in order to increase the density of C/C composites. Ultrasonic velocity and attenuation depend on a density variation of materials. Low frequency through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. These results were compared with that obtained by dry-coupling ultrasonics. Pulse-echo C-scans was used to image near-surface material property anomalies such as the placement of spacers between disks during CVI. Also, optical micrograph had been examined on the surface of C/C composites using a destructive way.

Ultrasonic characterization on sequences of CFRP composites based on modeling and motorized system

Composites are a material class for which nondestructive material property characterization is as important as flaw detection. Laminates of fiber reinforced composites often possess strong in-plane elastic anisotropy attributable to the specific fiber orientation and layup sequence when waves are propagating in the thickness direction of composite laminates. So the layup orientation greatly influences its properties in a composite laminate. It could result in the part being rejected and discarded if the layup orientation of a ply is misaligned. 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. Therefore a ply-by-ply vector decomposition model has been developed, simplified, and implemented for composite laminates fabricated from unidirectional plies. This model decomposes the transmission of a linearly polarized ultrasound wave into orthogonal components through each ply of a laminate. Also in order to develop these methods into practical inspection tools, motorized system have been developed for different measurement modalities for acquiring ultrasonic signals as a function of in-plane angle. It is found that high probability shows between the model and tests developed in characterizing cured layups of the laminates.

Feasibility on Ultrasonic Velocity using Contact and Non‐Contact Nondestructive Techniques for Carbon/Carbon Composites

Advanced materials are to be required to have specific functions associated with extremely environments. One of them is carbon/carbon(C/C) composite material, which has obvious advantages over conventional materials. The C/Cs 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/C composite material was characterized with non‐contact and contact ultrasonic methods using a scanner with automatic‐data acquisition function. 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 compo...

Axial collapse characteristics of CFRP composites with stacking conditions under the hygrothermal

CFRP composite material has superior specific strength and rigidity compared to metallic material, and is widely adopted in the various fields. Exceptional corrosion resistance enables the acceptance in maritime structural members such as ship and oildrilling machineries. However, CFRP composite material has the weakness in hygrothermal environment and crash environment. Especially, moisture ingress into composite material under hygrothermal environment can change molecule arrangement and chemical properties. In addition, interface characteristics and component material properties can be degraded. An experimental investigation was carried out to study the crash evaluations of CFRP composites to dynamic crushing by impact loading. We have made a collapse experiment to research into the difference of absorbed energy and deformation mode between moisture absorbed specimen and non-moisture absorbed specimen. As a result, the effect of moisture absorption and impact loads of approximately 30~50% reduction in strength are shown.

Energy Absorption Characteristics of Side Member for Light-weight Having Various Stacking Condition and Shape of Section

Front-side members of automobile, such as the hat shaped section members, are structures with the greatest energy absorbing capability in a front-end collision of vehicle. This paper was performed to analyze energy absorption characteristics of the hat shaped section members, which are basic shape of side member. The hat shaped section members consisted of the spot welded side member which was utilized to an actual vehicle and CFRP side member for lightweight of vehicle structural member. The members were tested under static axial loading by universal testing machine. Currently, stacking condition related to the collapse characteristics of composite materials is being considered as an issue fer the structural efficiency and safety of automobiles, aerospace vehicles, trains, ships even elevators during collision. So, energy absorption characteristics were analyzed according to stacking condition and shape of section and compared the results of spot welded side member with those of CFRP side member.

An Ultrasonic Nondestructive Method for Evaluating Carbon/Carbon Composites

In this work, especially carbon/carbon (C/C) composites are one of the few materials that are suitable for structural applications at high temperature environments. Characterization and integrity of carbon/carbon(C/C) composite materials should be evaluated because of its inhomogeneity and composite. A C/C composite material was nondestructively characterized and a technique was developed to measure ultrasonic velocity in C/C composites 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 consistent with those in a large C/C composite manufactured by chemical vapor infiltration (CVI) method. These results were compared with that obtained by dry‐coupling ultrasonics. A peak‐delay measurement method well corresponded ...

Axial Collapse Characteristics of Aluminum CFRP Compound Square Members for Vehicle Structural Members

An aluminum or CFRP (Carbon Fiber ReinfDrced Plastics)is representative one of light-weight materials but its axial collapse mechanism is different from each other. The aluminum member absorbs energy by stable plastic deformation, while the CFRP member absorbs energy by unstable brittle failure with higher specific strength and stiffness than those in the aluminum member. In an attempt to achieve a synergy effect by combining the two members, aluminum CFRP compound square members were manufactured, which are composed of aluminum members wrapped with CFRP outside aluminum square members with different fiber orientation angle and thickness of CFRP, and axial collapse tests were performed fur the members. The axial collapse characteristics of the compound members were analyzed and compared with those of the respective aluminum members and CFRP members. Test results showed that the collapse of the aluminum CFRP compound member complemented unstable brittle failure of the CFRP member due to ductile characteristics of the inner aluminum member. The collapse modes were categorized into four modes under the iuluence of the fiber orientation angle and thickness of CFRP. The absorbed energy Per unit mass, which is in the light-weight aspect was higher in the aluminum CFRP compound member than that in the aluminum member and the CFRP member alone.

A Study on Ultrasonic Evaluation of Material Homogeneity in Wood

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. It is desirable to perform nondestructive evaluation (NDE) to assess material properties and absorption of wood because wood is a natural composite and often inhomogeneity. Several ultrasonic techniques were applied to wood for the evaluation of variations material properties. The variation of ultrasonic velocity was measured and found to be somewhat consistent with those in infiltrated area in the course of nature absorption. Low frequency (1MHz) through‐transmission scans based on both amplitude and time‐of‐flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. These results were compared with those obtained by dry‐coupling ultrasonics. A good correlation was found between ultrasonic velocity (a pulse overlap and peak‐delay measurement method) and C‐scan image signal based on peak‐to‐peak am...