Kwang-Hee Im

Effects of temperature on impact damages in CFRP composite laminates

Abstract In this paper, the effect of temperature variations (low and high temperatures) was studied experimentally on impact damage to CFRP laminates. The composite laminates used in this experiment were CF/EPOXY orthotropic laminated plates with lay-up [0 6 /90 6 ] s and [0 4 /90 4 ] s , and CF/PEEK orthotropic laminated plates with a lay-up of [0 6 /90 6 ] s . A steel ball launched by the air gun was used to generate the CFRP laminate impact damage. For impact-damaged specimens, nondestructive evaluation (NDE), such as a scanning acoustic microscopy (SAM) was performed on the delamination-damaged samples to characterize damage growth at different temperatures. Therefore, this study was undertaken to experimentally determine the interrelations between impact energy and impact damage (i.e. the delamination area and matrix) of CFRP laminates (CF/EPOXY and CF/PEEK) subjected to foreign object damages (FOD) at low and high temperatures.

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.

Experimental approach on the collapse mechanism of CFRP composite tubes

This study is to investigate the energy absorption characteristics of CFRP(Carbon-Fiber Reinforced Plastics) tubes on static and dynamic tests. Axial static compression tests have been carried out using the static testing machine(Shin-gang buckling testing machine) and dynamic compression tests have been utilized using an vertical crushing testing machine. When such tubes are subjected to crushing loads, the response is complex and depends on the interaction between the different mechanisms that could control the crushing process. The collapse characteristics and energy absorption have been examined for various tubes. Energy absorption of the tubes are increased as changes in the lay-up which may increase the modulus of tubes. The results have been varied significantly as a function of ply orientation and interlaminar number. In general, the stacking sequence [0/90] CFRP tubes absorbed more energy than the [15/−15] CFRP tubes.

Experimental Approach on the Behavior of Composite Laminated Shell under Transverse Impact Loading

Composites are to be considered for many structural applications structural weight. These materials have high strength‐to‐weight and stiffness‐to‐weight ratios. However, they are susceptible to impact loading because they are laminar systems with weak interfaces. Matrix cracking and delamination are the most common damage mechanisms of low velocity impact and are dependent on each other. This paper is to study the behavior of composite shell under transverse impact loading. In this study, carbon‐epoxy composite laminates with various curvatures was used. Low velocity impact tests were performed using a drop weight testing machine. The 100mm×100mm shells were clamped in order to produce a central circular area (φ=80mm). An hemispherical impactor (m=0.1kg and φ=10mm) was used and the tests were done with velocities ranging from 2.8 to 4.8 m/s. The real curve force/time was registered in order to obtain the maximum contact force and contact time. And then, we know that contact force and delamination area of ...

Ultrasonic Inspection of Solid Composite Laminates Using a One‐Sided Pitch‐Catch Contact Mode

Ultrasonic NDE of composite laminates is often based on the backwall echoes of the sample. In the inspection of actual composite hardware, the back surface of the part may be curved or contain a complex geometry that does not provide a usable backwall echo. In this work, a one‐sided pitch‐catch setup was used in the detection and evaluation of flaws and material anomalies in solid composite laminates. Two miniature potted Rayleigh wave transducers (for generating surface waves in contact mode on steel) were joined head‐to‐head and used in the pitch‐catch mode on the surface of composites. The pitch‐catch signal was found to be more sensitive than a normal incidence backwall echo of a longitudinal wave to detect subtle flaw conditions in the composite, including low level porosity, ply waviness, and cracks. The depth of the sampling volume that the pitch‐catch signal came from was relatively shallow to the head‐to‐head miniature Rayleigh probes, but the depth can be increased by increasing the separation d...

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...

A Study on the Splitting of the Valance Band for a CdIn2te4 Single Crystal Using Photocurrent Measurement

The p‐CdIn2Te4 single crystal has been grown by using Bridgman method without a seed crystal in a three‐stage vertical electric furnace. From the photocurrent measurement, it was found that the three peaks of A, B, and C corresponded to the intrinsic transition due to the band‐to‐band transition from the valence band state Γ 7(A), Γ 6(B), and Γ 7(C) to the conduction band state Γ 6, respectively. Also, the valence band splitting of the CdIn2Te4 crystal has been first confirmed through the photocurrent spectroscopy. The crystal field splitting and the spin orbit splitting were obtained to be 0.2360 and 0.1119 eV, respectively. Also, the temperature dependence of the band gap energy of the CdIn2Te4 crystal has been driven as the following equation of Eg(T) = Eg(0) − (9.43 × 10−3)T2/(2676 + T). In this equation, the Eg(0) was estimated to be 1.4750, 1.7110, and 1.8229 eV at the valence band state A, B, and C, respectively. The band gap energy of the p‐CdIn2Te4 at room temperature was determined to be 1.2023 eV.

Characteristic Evaluation of Impact Responses and Damages in Composite Shell with Various Curvatures

Composites are considered for many structural (aerospace vehicles, automobiles, trains and ships) applications structural weight. However, impacted composite structures have 50–75% less strength than undamaged structures. The present work experimentally characterizes and explores the response of composite structures with a wide range of structural configurations to both impact and quasi‐static loadings. In this study, CF/Epoxy(Carbon Fiber/Epoxy Resin) composite laminates with various curvatures was used. The objective of this study is to find a method for changing the radius of the curvature and to determine how the impact velocity affects the contact force‐deflection, damage area and absorbed energy under low‐velocity impact loadings. By experimenting with composite structures, we found that there were substantial differences between the responses of a flat‐plate and shell to transverse loading. And contact force, damage area, and absorbed energy of the composite shells were a function of curvature radi...

An Experimental Study on the Impact Collapse Characteristics of CF/Epoxy Circular Tubes

This study is to investigate the energy absorption characteristics of CF/Epoxy (Carbon‐Fiber/Epoxy Resin) circular tubes in static and impact tests. The experimental results varied significantly as a function of interlaminar number, orientation angle of outer and trigger. When a CFRP composite tube is crushed, static/impact energy is consumed by friction between the loading plate and the splayed fronds of the tube, by fracture of the fibers, matrix and their interface, and the response is complex and depends on the interaction among the different mechanisms, such as transverse shearing, laminar bending and local buckling. The collapse mode depended upon orientation angle of outer of CFRP tubes and loading status(static/impact). Typical collapse modes of CFRP tubes are wedge collapse mode, splaying collapse mode and fragmentation collapse mode.