Huei-Jeng Lin

Impact-Induced Fracture in Laminated Plates and Shells

A study of the impact damage of laminated plates and cylindrical shells was conducted by experimental and numerical analysis. The curvature effect on the impact damage of composites was investigated. Laminating sequences of [0°5/90°5/0°5] and [90°5/0° 5/90°5 were considered. Detailed presentation of impact damage was provided through photographs and schematic diagrams. It is found that there are matrix cracks in the lower layer and delamination in the lower interface in plates, while the fiber crack in the upper layer, shear crack in the middle layer and delamination in the upper and lower interfaces are observed in cylindrical shells. A numerical analysis to predict and explain the crack initiation of laminated plates and cylindrical shells was also established.

Modeling and Analysis of Composite Laminates with Continuous Fiber around a Circular Hole

The stress distribution of glass woven roving composites containing a cir cular hole was examined. Two types of circular hole, drilled and moulded-in, were con sidered. A finite element model which used the panel method to simulate the fiber con figuration was used in the stress analysis of laminates with moulded-in circular hole. Experimental verification of strain distribution to check the accuracy of the model is also conducted. The stress distributions of [0°], [90°], [0°/90°] s and [±45°]s laminates were investigated. Numerical results show that the stress distribution σy on laminates with a moulded-in hole is smaller than that with a drilled hole when loading in the fiber direc tion, but it is reversed when loading in the perpendicular direction. The fiber configura tions and strain distributions of numerical and experimental were compared. The results agree well.

Methods to Reduce the Print-through Phenomenon on the Surface of FRP

The reflected image on the surface of the fiber-reinforced plastic (FRP) often presents twists and wrinkles. This phenomenon is called print-through phenomenon (PTP) in this investigation and the lines present on the material surface are called print-through lines (PTL). Earlier studies and experimental observations indicate that PTP is related to the non-uniform inner stress in the gel coating layer of the FRP. The non-uniform inner stress is caused by the various shrinkages of the plastic matrix and fibers of the fiber layers during the laminating process. So the main idea to reduce PTP is to reduce or uniform the non-uniform inner stress in the gel coating layer of the FRP. Several methods can be used to reduce PTP and two of them are investigated in this paper. One is removing atmospheric pressure before the plastic matrix of the fiber layers hardens. The other method is inserting a core-Mat layer into the Mat layers during laminating the FRP. Removing atmospheric pressure before the plastic matrix hardens can remove the influence of atmospheric pressure on causing the inner stress of the gel coating layer. Once the inner stress of the gel coating layer is reduced, PTP on the surface of the FRP can be eased off. Inserting a core-Mat layer into the Mat layers will form a buffer zone which can uniform the inner stress in the gel coating layer of the FRP. Therefore, PTP on the surface of the FRP can be eased off. To verify the present contentions, some experiments and qualitative analysis were performed. Both experimental and analysis results show that the PTP on the surface of FRP can be reduced by the present methods.

Combined Analytical and Finite Element Beam Model for Wind Turbine Blades

Finite element method (FEM) has been widely used in recent years to analyze fiber-reinforced polymer (FRP) beams such as wind turbine blades. Although it is generic and convenient, FEM does little to shed light on the mechanical behavior of structures. Moreover, it is inefficient in calculation. This study presents a combined analytical and finite element beam (CAFB) model with an inclination angle correction to solve the non-prismatic, blade-like FRP beam problem. It not only substantially reduces computational time, but also helps us comprehend the mechanical behavior. Also, the inclination angle correction is supposed to correct the overestimated axial stiffness due to the topology of beams. In the presented cases, the CAFB model can accurately analyze the FRP thin-walled beams with all results being certified by finite element analysis software, and the correction is especially effective in small fiber orientation angles.

A Study of Elastic Coupling to the Wind Turbine Blade by Combined Analytical and Finite Element Beam Model

Elastic coupling is a feature specific to fiber-reinforced plastic materials, and it is often used in structures to improve stabilities or for other purposes. In the investigation of the elastic coupling properties, the commonly used finite element analysis (FEA) software package in structures does not usually provide a theoretical understanding. Thus, this study adopted a combined analytical and finite element beam model to address this problem. The model can provide better understanding on structures, and the computation time is far less than the FEA. This study examined several factors that may influence the couplings of wind turbine blades, such as material selection, coupling positions, and the wind force distribution. The results showed that mixed-use of carbon and glass fibers can provide better coupling properties, and the blade developers should arrange the off-axis fibers to adapt to the operating conditions in order to reach better performance.

Discussion on the Cause of Print-through Phenomenon of FRP and Several Improvement Methods

A print-through phenomenon (PTP) often occurs on the surface of the gelcoat layer of fiber-reinforced plastic (FRP). The fiber prints are visible with the naked eye. According to the scanning profile measured by instruments, the wrinkles indeed exist at the microlevel on the surface. In our previous works, it was concluded that PTP is induced by the nonuniform residual stress that is yielded while the resin hardens. This study further developed this argument regarding the cause of PTP and proved it by experiments. Moreover, several improvement methods to PTP were also tested by changing the configurations of core-mat, gelcoat, and resins to mitigate the tiny-scale deformations on the gelcoat surface. The simulation and experimental results showed that the increase in the thickness of core-mat and gelcoat layers is an effective way of reducing PTP. In addition, from simulation, it was found that searching for a substitution for core-mat, may possibly be another effective way of improving PTP.

Print-through Phenomenon on the Surface of GFRP: Pilot Study

Unsaturated polyester filled with coloring agent is commonly used as the surface material of a GFRP yacht and is called a gel-coating layer. The reflection on the gel-coating layer surface will be imperfect if twists and wrinkles exist on the gel-coating surface. This phenomenon is called print-through phenomenon (PTP) in this investigation. The PTP seriously reduces the beauty of a yacht and therefore limits the application of GFRP to the yacht. Therefore, it is urgent to solve the PTP problem. The first goal of this study is to objectively define and quantitatively measure the existence of PTP and its level. The surface of the gel-coating layer of GFRP is scanned by using the portable high resolution surface roughness and form measurement instrument. Based on a large number of experiments, the average parameter, arithmetic mean deviation (Ra), and the peak parameter, the altitude of the surface profile (Rt), are proposed to determine the existence of PTP and its level. The second goal of this study is to consider the causes of PTP happening and find out the factors that can influence it. Through experimental observations and qualitative analysis, it is believed that PTP is related to the non-uniform residual stress in the gel-coating layer of GFRP. So, any factors which can produce the non-uniform residual stress in the gel-coating layer can influence PTP, such as the shrinkage of the resin during its hardening chemical reaction and the atmospheric pressure during the Seemann composite resin infusion molding process (SCRIMP). After ascertaining what causes PTP, research methods that can be used to reduce PTP will then be studied.

Effects of Stacking Sequence on Nonlinear Hydroelastic Behavior of Composite Propeller Blade

A coupled structural and fluid flow analysis was performed to assess the hydroelastic behavior of a composite marine propeller. The finite element method and lifting surface theory were applied in the structural and fluid analyses, respectively. A coupled equation derived from the Bernoulli equation and the hydrostatic pressure in terms of the blade deflection and strength of the vortex was solved by Newton-Raphson procedure. A MAU 3–60 propeller was analyzed with different stacking sequences of composite layup. The hydroelastic behaviors of the propeller with balanced and unbalanced stacking sequences were investigated and discussed. The unbalanced Stacking sequences were shown to influence the performance of the composite propeller, especially in the region in which the advanced coefficient is low.

Study of the decrease in print-through phenomenon on fiber-reinforced plastic material

Fiber-reinforced plastic (FRP) is a general kind of composite material used in many industries. This study examines the print-through phenomenon (PTP) via quantitative measurements on the gel-coated surface of FRP specimens. Moreover, finite element analysis is also adopted in this investigation. Based on a large number of experiments and computations, an average parameter, the arithmetic mean deviation is utilized to determine the degree of the PTP. Further, the reflections of a light image on a FRP surface were observed in photographs. Experimental results show that insulating residual stress can reduce PTP. Adding a core-mat layer to the FRP stacking scheme is a good solution for minimizing the PTP. In addition to using the core-mat materials, substitute materials used to prevent the PTP are also investigated.

Estimating the Elastic Modulus through the Thickness Direction of a Uni-direction Lamina which Possesses Transverse Isotropic Property:

This investigation presents three formulae of the material constants. A simple method is applied to determine the material properties of a unidirectional lamina (UD) in the through-thickness direction. The mechanical characteristics of an orthotropic composite material generally require nine parameters: three Youngs moduli; three shear moduli; and three Poissons ratios. Most studies concentrate on two orthogonal axial moduli, a shear modulus and a Poissons ratio in the plane of the lamina. Four material parameters were adopted in those works instead of nine. Some cases in many studies are based on three-dimension modeling, making the nine material constants necessary, as the four material parameters cannot satisfy such cases. However, the material constants through the thickness direction cannot easily be measured experimentally. In this work, the relationships between the nine mechanical properties are revealed by the wave equations of an orthotropic material. Two restrictions in the study are considered: first, the suitable cases are limited to UD possessing the transverse isotropic property; second, the distribution of the fibers in the cross-section of UD laminate must be uniform. On the base, the shear modulus and the Poisson ratio through the thickness direction of the UD material can be computed from the simple relationships.