• Proceedings of the Korean Society For Composite Materials Conference
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• 2001.10a
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• pp.111-115
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• 2001

Thermo-acoustic emission (AE) from composite laminates under the repetitive thermal cyclic loads have been quantitatively analyzed in consideration of AE source mechanisms. The repetitive thermal load brought about a large reduction, i.e. an exponential decrease in AE total ringdown counts and AE amplitudes. It was thought that generation of thermo-AE during the first thermal cycle was not caused by crack propagation but by secondary microfracturing due to abrasive contact between crack surfaces.

• International Journal of Aeronautical and Space Sciences
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• v.18 no.2
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• pp.236-244
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• 2017

In this study, the high-velocity impact penetration behavior of $[45/0/-45/90]_{ns}$ carbon/epoxy composite laminates was studied. The considered configuration includes a spherical steel ball impacting clamped circular laminates with various thicknesses and diameters. First, the impact experiment was performed to measure residual velocity and extent of damage. Next, the impact experiment was numerically simulated through finite element analysis using LS-dyna. Three-dimensional solid elements were used to model each ply of the laminates discretely, and progressive material failure was modeled using MAT162. The result indicated that the finite element simulation yielded residual velocities and damage modes well-matched with those obtained from the experiment. It was found that fiber damage was localized near the impactor penetration path, while matrix and delamination damage were much more spread out with the damage mode showing a dependency on the orientation angles and ply locations. The ballistic-limit velocities obtained by fitting the residual velocities increased almost linearly versus the laminate diameter, but the amount of increase was small, showing that the impact energy was absorbed mostly by the localized impact damage and that the influence of the laminate size was not significant at high-velocity impact.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.11a
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• pp.39-44
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• 2005

The effects of aging of PSF/AS4 laminates on fatigue was studied using the new energy release rate analysis. The analysis by the variational mechanics has been useful in providing fracture mechanics interpretation of matrix microcracking in cross-ply laminates. This paper describes the changes of the critical energy release rate (microcracking toughness) according to the aging period under fatigue loading. The master plot by modified Paris-law gives a characterization of a material system's resistance to microcrack formation. PSF $[0/90_{s}]_{s}$ laminates were aged at four different temperature based on the glass transition temperature for 0 to 60 days. At all temperatures, the toughness decreased with aging time. The decrease of the toughness at higher temperature was faster than at lower temperature. To assess the effects of aging on fatigue, the unaged laminates were compared with the laminates which had been aged for 60 days at 170$^{\circ}C$ near 180 $^{\circ}C$ t$_g$. The slope of dD/dN versus ${\Delta}G_m$. of the aged laminates was lower than that of the unaged laminates. There was a significant shift of the aged data to formation of microcracks at the lower values of ${\Delta}G_m$.

• Structural Engineering and Mechanics
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• v.73 no.6
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• pp.631-640
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• 2020

Using backup plate is one of the most commonly used methods to decrease drilling-induced delamination of composite laminates. It has been shown that, the size of the delamination zone is related to the vertical element of cutting force named as thrust force. Also, direct control of thrust force is not a routine task, because, it depends on both drilling parameters and mechanical properties of the composite laminate. In this research, critical feed rate and thrust force are predicted analytically for delamination initiation in drilling of composite laminates with backup plate. Three common theories, linear elastic fracture mechanics, classical laminated plate and mechanics of oblique cutting, are used to model the problem. Based on the proposed analytical model, the effect of drill radius, chisel edge size, and backup plate size on the critical thrust force and feed rate are investigated. Experimental tests were carried out to prove analytical model.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.05a
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• pp.203-206
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• 2002

The present study proposes a theoretical model for predicting the matrix crack density growth of each layer in composite laminates subjected to thermo-mechanical loads. Each layer with matrix cracks is treated as an equivalent continuum of degraded elastic stiffnesses which are functions of the matrix crack density in each slyer. The energy release rate as a function of the degraded elastic stiffnesses is then calculated for each layer as functions of thermo-mechanical loads externally applied to the laminate. The matrix crack densities of each layer in general laminates are predicted as functions of the thermo-mechanical loads applied to a number of laminates. Comparisons of the present study with experimental data in the open literatures are also provided.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2003.10a
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• pp.150-153
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• 2003

Longitudinal strains({$varepsilon}_x$) of the core and skin layers in glass fiber reinforced plastic(GFRP) cross-ply composite laminates have been studied using the embedded optical fiber sensor of totally-reflected extrinsic Fabry-Perot interferometer(TR-EFPI). Foil-type strain gauges bonded on both the upper and lower surfaces were used for the measurement of the surface strains. Both TR-EFPI sensor and strain gauge bonded on the specimen surface showed excellent agreement within -0.0086 ~ +0.0302% strain. It was shown that values of {$varepsilon}_x$ in the interior of the surface layer and the core layer measured by embedded TR-EFPI sensor was significantly higher than that of the specimen surface measured by strain gauges. The experimental results were ascertained with finite element analysis. Embedded TR-EFPI optical fiber sensor could measure accurately the internal strains which were different from the surface.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2003.04a
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• pp.37-41
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• 2003

In this study the mechanical behaviors of fiber metal laminates (FML) such as ARALL, GLARE and CARE which are recently developed as new structural materials and known to have excellent fatigue resistant characteristics while with relatively low densities compared to the conventional aluminum materials, are considered through the classical lamination theory. The mechanical properties such as elastic moduli, thermal expansion coefficients and hygro-thermally induced residual stresses in the fiber metal laminates are obtained and compared each other. Also load carrying mechanism between metal sheets and composite layers in the FML are considered.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2000.11a
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• pp.54-57
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• 2000

Thermal deformation behavior has been investigated for unsymmetric laminates composed of various kinds of material layers, such as stainless steel, aluminum, carbon/epoxy or glass/epoxy. The thermal deformations of unsymmetric laminates were predicted using the classical lamination theory and compared with those obtained from experimental measurement. In the case of unsymmetric laminate composed of stainless steel and aluminum layer, the experimental results were agreed well with the values predicted. But in the case of unsymmetric laminate composed of fiber composite layers, there was a considerable difference of thermal deformation between the prediction and experimental measurement, which may be from the change of material properties of fiber composite layers for temperature variation.

• Composites Research
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• v.32 no.6
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• pp.327-334
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• 2019

The delamination is a special mode of failure occurring in composite laminates. Several numerical studies with finite element analysis have been carried out on the delamination behavior of unidirectional composite laminates. On the other hand, the fracture for the multi-directional composite laminates may occur not only along the resin-fiber interface between plies known as interply or interlaminar fracture but also within a ply known as interyarn or intralaminar fracture accompanied by matrix cracking and fiber bridging. In addition, interlaminar and intralaminar cracks appear at irregular proportions and intralaminar cracks proceeded at arbitrary angle. The probabilistic analysis method for the prediction of crack growth behavior within a layer is more advantageous than the deterministic analysis method. In this paper, we analyze the crack path when the mode I load is applied to the cross-ply carbon/epoxy composite laminates and collect and analyze the probability data to be used as the basis of the probabilistic analysis in the future. Two criteria for the theoretical analysis of the crack growth direction were proposed by analyzing the stress field at the crack tip of orthotropic materials. Using the proposed method, the crack growth directions of the cross-ply carbon/epoxy laminates were analyzed qualitatively and quantitatively and compared with experimental results.

• Composites Research
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• v.25 no.6
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• pp.191-197
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• 2012

In this paper, the penetration behavior of carbon/epoxy composite laminates subjected to high velocity projectile impact was studied by numerical simulation. The composite laminates made of carbon/epoxy with $[45/0/-45/90]_{ns}$ stacking sequence and the spherical steel impactor were three-dimensionally modeled. The ply numbers of 16 and 24 and the impact velocities in the range of 140-250 m/s were considered. The analysis was performed using an explicit finite element code LS-DYNA. The residual velocity and the amount of damage were predicted and compared to the experimental results.