Seung-Moon Jun

Modeling of Composite Laminates with Multiple Delaminations under Compressive Loading

Multiple delaminations are more realistic damage types in the laminated composite structures. In this study, buckling and postbuckling analysis was conducted for the composite laminates with multiple delaminations under compressive loading. In a nonlinear finite element formulation, the updated Lagrangian description and modified arc-length method were adopted. For a finite element modeling of composite laminates, the eight-node degenerated shell element was used. To avoid the overlapping between delaminated layers, which is a physically inadmissible buckling mode, the contact node pair was defined by use of virtual beam element. It employed constraints at the overlapped region in the load incremental procedure. Using this modeling of multiple delaminations, accurate results were obtained for the buckling and postbuckling behavior of composite laminates. Numerical results showed that multiple delaminations lower buckling loads and load carrying capacities in the postbuckling region.

Stabilized interrogation and multiplexing techniques for fibre Bragg grating vibration sensors

We demonstrated a simple interrogation system for multiplexed fibre Bragg grating (FBG) sensors in a high-frequency range. A tunable fibre Fabry–Perot (FFP) filter with narrow free spectral range (FSR) was used to simplify the multiplexing demodulator for FBG vibration sensors. A stabilization-controlling unit was also developed for the maintenance of maximum sensitivity of the sensors. In order to verify the performance of the stabilization control unit, we measured the sensitivity of the FBG sensor by changing environmental temperature, and the system showed an average sensitivity of 2.5 neRMS Hz−1/2 for a stabilization-controlled case. Finally, multi-point vibration tests using in-line FBG sensors were conducted to validate the multiplexing performance of the FBG system.

Experimental and numerical study on the failure of sandwich T-joints under pull-off loading

In this study, the failure mechanism of sandwich-to-laminate T-joints under pull-off loading was investigated by experiment and the finite element method. A total of 26 T-joint specimens were manufactured and tested in order to investigate the effects of both adhesive thickness (0.4, 2.0, and 4.0 ㎜) and environmental conditions on the failure of the joints. The results showed that failure occurred mainly as intralaminar failure in the first layer of the sandwich face, which was contacted to the paste adhesive. The failure load did not significantly change with increasing adhesive thickness in both RTD (Room Temperature and Dry) and ETW (Elevated Temperature and Wet) conditions. In the case of ETW conditions, however, the failure load increased slightly with an increase in adhesive thickness. The joints tested in ETW conditions had higher failure loads than those tested in RTD conditions. In addition to the experiment, a finite element analysis was also conducted to investigate the failure of the joint. The stress inside the first ply of the sandwich face was of interest because during the experiment, failure always occurred there. The analysis results showed good agreement with the trend of experimental results, except for the case of the smallest adhesive thickness. The highest stress was predicted in the regions where initial failure was observed in the experiment. The maximum stress was almost constant when the adhesive thickness was beyond 2 ㎜.

Localizations and force reconstruction of low-velocity impact in a composite panel using optical fiber sensors

A new approach of monitoring low-velocity impact events on composite structures was presented and experimentally evaluated. In this approach, impact strain histories of a composite plate were measured with optical fiber sensors. The recorded signals were used to estimate locations and forces of low-velocity impact events. For experimental validation, four-sensor heads were surface mounted at the four corners of a composite panel. The optical sensors caught dynamic strain due to impacts on the panel, and the optical cables transmitted wavelength changes of the sensors. The multiplexed wavelength changes were measured using a high-speed fiber Bragg grating interrogator for the determination of impact location. The locations of impact events were estimated using a neural network program and measured signals. The estimated locations and converted strain signals were taken as the inputs to the process of reconstructing impact forces. Even though the current study is an initial investigation on a simple impact problem, the result shows the possibility that the proposed technique can be applied to low-velocity impact monitoring of general composite structures.

Flight Test Measurement and Assessment of a Flapping Micro Air Vehicle

Flight test of flapping micro air vehicles (FMAVs) is carried out using an instrumented measurement system to obtain various engineering parameters and hence to assess the flight performance of the vehicles through the data investigation. An indoor flight test facility equipped with a motion capture system and tracking cameras is used for the work presented in this paper. Maneuvers including straight-level flight, ground flapping, takeoff and landing are tested. Spatial position and orientation data are obtained from the retro-reflective tracking markers attached to the vehicles. Subsequent test analysis is carried out by generating performance parameters from raw data and then assessing the flight performance by comparison of the vehicles. The main findings of this work confirm that the test method and procedures presented here enable the systematic numerical data measurement and assessment of the flying performances of these vehicles, and show the applicability for the test and evaluation of general flapping MAVs.

Analysis and Test of Hydrodynamic Ram in Welded Metallic Water Tanks

Analysis and test of hydrodynamic ram in welded metallic tanks containing water were performed to investigate the phenomena and to understand the effects on the resulting structural behavior. Arbitrary Lagrange-Euler coupling method was used for the analysis of the fluid-structure interaction occurring in the hydrodynamic ram, where the projectile, tank, and water are exchanging load, momentum, and energy during the traveling of the projectile through the water of the tank. For a better representation of the physical phenomena, modeling of the welded edges is added to the analysis to simulate the earlier weld line fracture and its influence on the resulting hydrodynamic ram behavior. Corresponding hydrodynamic tests were performed in a modified gas gun facility, and the following panel-based examinations of various parameters, such as displacement, velocity, stress, and energy, as well as hydrodynamic ram pressure show that the analysis and test are well correlated, and thus the results of the study reasonably explain the characteristics of the hydrodynamic ram. The methodology and procedures of the present study are applicable to the hydrodynamic ram assessment of airframe survivability design concepts.

Ground Test & Evaluation of Conformal Load-bearing Antenna Structure for Communication and Navigation

This paper suggests a test and evaluation procedure of conformal load-bearing antenna structure(CLAS) for high speed military jet application. A log periodic patch type antenna was designed for multi-band communication and navigation antenna. Carbon/Glass fiber reinforced polymer was used as a structure supporting aerodynamic loads and honeycomb layer was used to improve antenna performance. Multi-layers were stacked and cured in a hot temperature oven. Gain, VSWR and polarization pattern of CLAS were measured using anechoic chamber within 0.15~2.0 GHz frequency range. Tension, shear, fatigue and impact load test were performed to evaluate structural strength of CLAS. Antenna performance test after every structural strength test was conducted to check the effect of structural test to antenna performance. After the application of new test and evaluation procedure to validate a new CLAS, a design improvement was found.

Study of Analysis Technique of Flight Compatibility Test Data using Instrumented Pod for External Store

Analysis technique of flight compatibility test results using instrumented pod for external store is studied. Raw data of sensors and MIL-STD-1553B acquired from instrumented pod are analyzed separately. Besides, tuning and synchronization of sensor data enable the analysis of those two type of data simultaneously. Evaluation of analysis result shows that the analyzed data represent maneuvers of the aircraft successfully and agree the values of the real flight tests.