Khanh-Hung Nguyen

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 ㎜.

Optimization of Composite Laminates Subjected to High Velocity Impact Using a Genetic Algorithm

In this study, a genetic algorithm was utilized to optimize the stacking sequence of a composite plate subjected to a high velocity impact. The aim is to minimize the maximum backplane displacement of the plate. In the finite element model, we idealized the impactor using solid elements and modeled the composite plate by shell elements to reduce the analysis time. Various tests were carried out to investigate the effect of parameters in the genetic algorithm such as the type of variables, population size, number of discrete variables, and mutation probability.

Failure Load Prediction by Damage Zone Method for Single-lap Bonded Joints of Carbon Composite and Aluminum

A damage zone method based on 3D finite element analysis was proposed to predict the failure loads of single-lap bonded joints with dissimilar composite-aluminum materials. To simulate delamination failure, interply resin layers between any two adjacent orthotropic laminas of composite adherend were assumed with a thickness of one-tenth of a composite lamina. Geometrically nonlinear effects due to the large rotation of the single-lap joint were included in the analysis. Analysis also considered the material nonlinearity of the aluminum adherend due to the stress exceeding yield level. Based on the experimental observation that the failure modes of the specimens were dominated by delamination and debonding, the Ye-criterion was applied to account for the out-of-plane failure of composite adherend and the Von Mises strain criterion was applied for the adhesive layer. The failure indices were multiplied to the predicted damage zone as a weight factor and the calculated damage zones were divided by an area or volume considering the joint geometry. Predicted failure loads show deviation within 18% from experimental results for nine different bonding lengths or adherend thicknesses.

Structural Analysis of a Composite Target-drone

A finite element analysis for the wing and landing gear of a composite target-drone air vehicle was performed. For the wing analysis, two load cases were considered: a 5g symmetric pull-up and a -1.5g symmetric push-over. For the landing gear analysis, a sinking velocity of 1.4 m/s at a 2g level landing condition was taken into account. MSC/NASTRAN and LS-DYNA were utilized for the static and dynamic analyses, respectively. Finite element results were verified by the static test of a prototype wing under a 6g symmetric pull-up condition. The test showed a 17% larger wing tip deflection than the finite element analysis. This difference is believed to come from the material and geometrical imperfections incurred during the manufacturing process.

A Numerical Study of the High-Velocity Impact Response of a Composite Laminate Using LS-DYNA

The failure of a Kevlar29/Phenolic composite plate under high-velocity impact from an fragment simulation projectile was investigated using the nonlinear explicit finite element code, LS-DYNA. The composite laminate and the impactor were idealized by solid elements, and the interface between the laminas was modeled as a tiebreak type in LS-DYNA. The interaction between the impactor and laminate was simulated using a surface-to-surface eroding contact algorithm. When the stress level meets the given failure criteria, the layer in the element is eroded. Numerical results were verified through existing test results and showed good agreement.

Characterization of composite material properties using eigenstrain method

A technique to solve the periodic homogenization problem is described systematically in this work. The method is to solve the cell problems by imposing eigenstrains in terms of thermal or piezoelectric strain to the representative volume element. Homogenized coefficients are then calculated from stress solutions of those cell problems. Benefit of the proposed technique is that it is readily applicable for common finite element softwares regardless of using user subroutines. Several numerical examples are examined. The obtained results show good agreements with the published data.