• Journal of the Korean Society for Precision Engineering
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• v.10 no.4
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• pp.152-162
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• 1993

The purpose of this study is to investigate problems of residual bending strength and the impact damage experimentally when CFRP composite laminates are subjected to Foreign object damage. The specimens composed of four types of CR/EPOXY and a CF/PEEK composite laminates which involved difference of fiber stracking orientation and matrics. The result were summariged as follows : 1) It is found that both orthotropic and guasi-isotropic composite laminates are increasimg lineally between impact energy and damage delamination area. 2) Delamination devel- opment energy(mm$^{2}$J) OF cf/epoxy composite aminates is less than that of CF/PEEK. 3) When impact energy is applied to specimens within 3J, the residual strength of orthotropic is greater than guasi-isotropic composite laminates. On the other hand, it is predicted that residual bending strength of orthotropic composite laminates is less than that of quasi-isotropic when impact energy is more thaen 3J. 4) It is found in CF/PEEK that for the impact side compression, residual of bending strength versus impact energy is almost constant, while in case of impact side tension, residual bending strength is decreased rapidly near 1.2J. of impact energy due to the effect of delamination buckling.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.8 no.2
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• pp.80-85
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• 1999

The purpose of this work is to investigate fatigue damage of quasi-isotropic laminates under tensile loading in different directions. Low cycle fatigue tests of [0/-60/+60]a laminates and [+30/-30/90]s lamina tes were carried out. Material systems used are AS4/Epoxy and AS4/PEEK. The fatigue damage of [+30/-30/90]s is very different from that of [0/-60/+60]s. The position of delamination generated at AS4/Epoxy and AS$/PEEK laminates were differentiated by the matrix difference that is, we suppose, the value of both GIcr(critical energy release rate of mode-I) and GIIIcr(critical energy release rate of mode-III) difference.

• Journal of Mechanical Science and Technology
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• v.14 no.5
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• pp.483-489
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• 2000

The purpose of this work is to investigate fatigue damage of quasi-isotropic laminates under tensile loading in different directions. Low cycle fatigue tests of $[0/-60/60]_s$ laminates and $[30/-30/90]_s$ laminates were carried out. Material systems used are AS4/Epoxy and AS4/PEEK. The fatigue damage of $[30/-30/90]_s$ is very different from that of $[0/-60/60]_s$. The experimental results are compared with the result obtained from the method for determining strain energy release rate components proposed by the authors. The analytical results were in good agreement with the experimental results. It is proved that the failure criterion based on the strain energy release rate is an appropriate approach to predict the initiation and growth of delaminations under cyclic loading.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1991.10a
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• pp.121-126
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• 1991

In his recent book, D.H. Kim proposes to use the quasi-isotropic constants by Tsai for the preliminary design of the composite primary structures for the civil construction. Such structures generally require a large number of laminae layers. Simple equations which can predict "exact" values of the buckling strength, the natural frequency of vibration, and the deflection for the special orthotropic laminates are presented. Many laminates with certain orientations lave decreasing values of B$\_$16/ and B$\_$26/ as the number of plies increases. Such laminates, with D$\_$16/=D$\_$26/\longrightarrow0, including the laminates with anti-symmetric configurations can be solved by the same equation for the special orthotropic laminates. If the quasi-isotropic constants are used, the equations for the Isotropic plates can be used. Use of some coefficients can produce "exact" value for laminates with such configuration.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2001.04a
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• pp.104-107
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• 2001

The purpose of this work is to investigate the static strength, the stress distribution, and the failure process of quasi-isotropic composite laminates made of two different matrices when loading directions are changed. We carried out static tests of $[0/-60/+60]_s$ and $[+30/-30/90]_s$ laminates. Two types of matrices used are AS4/epoxy and AS4/PEEK. The damage mechanisms of the quasi-isotropic laminate, $[0/-60/+60]_s$, strongly depend on the load direction applied to the laminate.

• Journal of the Korean Society for Precision Engineering
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• v.15 no.1
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• pp.130-136
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• 1998

A procedure of stress and strength analysis has been proposed for the centrifuge rotor of composite materials of quasi-isotropic laminates. The goal in this study is to maximize the allowable rotating speed, that is, to minimize maximum strength ratio with the given path length by changing the geometric parameter-outer radius and ply angles in quasi-isotropic laminates. Optimum values of the geometric parameter-outer radius and ply angles are obtained by multilevel optimization. All the geometric dimensions and stresses are normalized such that the result can be extended to a general case. Two dimensional analysis at each cross section with an elliptic tube hole subjected to internal hydrostatic pressures by samples as well as the centrifugal body forces has been performed along the height to calculate the stress distribution with the plane stress assumption, and Tsai-Wu failure criterion is used to calculate the strength ratio. The maximum allowable rotating speed can be increased by changing the radii of the outer surface along the height with the maximum strength ratio under the unit value : The optimal number of ply angles maximizing the allowable rotating speed in quasi-isotropic laminates is found to be the half number of tube hole, and the optimal laminate rotation angle is the half of $[{\pi}/m]$. A $[{\pi}/3]$ laminate, for instance, is stronger than a $[{\pi}/4]$ laminate for the centrifuge rotor of 6 tube hole number even though they have the same stiffness.

• Composites Research
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• v.12 no.4
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• pp.8-16
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• 1999

In this study, when the applied directions of tensile loading is changed fatigue damage of quasi-isotropic composite laminates was discussed. Low cycle fatigue tests of $[0/-60/+60]_s$ laminates and $[+30/-30/90]_s$ laminates were carried out. Material systems used were AS4/Epoxy and AS4/PEEK. The fatigue damage of $[+30/-30/90]_s$ laminates differed from that of $[0/-60/+60]_s$ laminates. The position of delamination generated at AS4/Epoxy and AS4/PEEK $[+30/-30/90]_s$ laminates appeared differently according to the kind of matrix. Critical values of strain energy release rate were obtained by using the strain measured at the initiation of delamination. The experimental results agreed well with the results obtained by the proposed method for determining strain energy release rate.

• Journal of the Korean Society for Precision Engineering
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• v.14 no.6
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• pp.135-141
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• 1997

Low velocity impact test and compressive residual strength test after impact were performed by using Hercules AS4/3501-6[45/0/-45/90]$_{2s}$ laminated plate to investigate the low velocity impact damage behavior and the post-impact strength degradation on orthotropic composite laminate plate. Due to the lateral impact losd, the load path showed "" shape according to the laminate central deflection. Damage in a laminate occurs by inclined matrix crack at the damage initiation load stage and vertical matrix crack, occurs on the outer surface. Evaluating the compressive residual strength after the low velocty impact test, it could be found that there is a transient range where the compressive residual strength drop suddenly in the initial damage which is in the matrix crack range and the initial delamination area. is in the matrix crack range and the initial delamination area.

• Journal of the Korean Society of Safety
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• v.12 no.2
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• pp.27-36
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• 1997

This paper evaluates the static and fatigue bending strengths of CFRP (carbon fiber reinforced plastic) laminates subjected to hygrothermals. The specimens which had different stacking composition, orthotropic and quasi-isotropic laminated plates, were prepared for this experiment. A steel ball launched by the air gun collides against CFRP laminates to generate impact damages, and the 3-point fatigue bending test is carried out by using the impacted laminates to investigate the influence of hygrothermals on the effect on the residual bending fatigue strength of CFRP laminates.

• Journal of the Korean Society of Propulsion Engineers
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• v.5 no.2
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• pp.59-64
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• 2001

The purpose of this work is to investigate the static strength, the stress distribution, and the failure process of quasi-isotropic composite laminates made of two different matrices when loading directions are changed. We carried out static tests of $[0/-60/+60]_s$ and $[+30/-30/90]_s$ laminates. Two types of matrices used are AS4/epoxy and AS4/PEEK. The damage mechanisms of the quasi-isotropic laminate, $[0/-60/+60]_s$, strongly depend on the load direction applied to the laminate.