W. S. Chan

Improvement of Edge Delamination Strength of Composite Laminates Using Adhesive Layers

A thin but tough adhesive inner layer placed in laminates at the proper interface has been found to give improved delamination strength. Static tension tests of laminates with (′30 2 /90 3 /∓30 2 ) T and (′35/0/90) s layupsof graphite/epoxy show that the added adhesive inner layer effectively suppresses delamination until final failure. The effects of adhesive layers on interlaminar stresses and strain-energy release rate have been studied analytically using a finite-element method. It was found that the introduction of adhesive layers reduced the interlaminar stresses. Although there was no significant change in total strain-energy release rate, there was a significant decrease in the G 1 (tension mode) percentage of the total strain-energy release rate.

Frequency Effect on the Fatigue Life of a Laminated Composite

Effects of load frequency on the fatigue behavior of notched (+ or - 45 deg)2s laminated graphite/epoxy composites are investigated. Experimental results are obtained using four frequencies at three load levels. Temperature rises near the center hole are also measured. It is found that when temperature increase during the fatigue test is small, fatigue life increases as the load frequency increases. However, an increase in the near hole temperature results in a reduction in fatigue life. A theoretical model based upon crack propagation in viscoelastic media is presented. The model is modified to account for the temperature effect.

Analysis of Composite Bonded/Bolted Joints Used in Repairing

The performance of composite repairing using single lapped joints is sensitive to factors such as ply stacking sequence of laminate, bonding surface preparation, anisotropic nature of adherend, as well as bearing and load transfer from one adherend to the other. This research is focused to investigate the stress distributions in laminated composite joints with multiple pin loads using finite element analysis. The structural response of various configurations of single lap joints, namely bonded, bolted and bonded-bolted joints, are analyzed by three-dimensional finite element method. A comparison of the results obtained using finite element analysis is conducted with experimental results and the results are validated. Parametric study is conducted to investigate the effect of stress distribution and load transfer due to ply stacking sequence of laminate.

Influence of Fiber Waviness on the Structural Response of Composite Laminates

Fiber waviness is often found to be a failure initiation site in a structural part. The objective of this research is to evaluate the influence of in-plane fiber waviness on structural response and its sensitivity on a composite beam. Explicit forms of lamina and laminate stiffness and their sensitivity analysis due to fiber waviness are formulated. The change of deflection and natural frequency of a symmetrically laminated composite beam due to fiber waviness was also investigated.

Static structural response of plates with piezoceramic layers

A finite element formulation is developed to analyze laminated plates with arbitrarily placed piezoceramic patches. However, only isotropic layered plates are analyzed as illustrative examples because of the primary emphasis on the effect of piezoceramic patch shapes. The technique is applied to obtain static response and stress fields due to the application of electric field to the piezoceramic patches. Different shapes of piezoceramic patches are inserted in a square aluminium plate. The actuation stress fields due to the piezoceramic patches are discussed in detail. It is observed that the shape of the patches has a direct effect on the stress field within the patch and in the surrounding material. The choice of the shape of the patches depends on the desired stress field, however, sharp corners should be avoided. The circular patch is subjected to the in-plane hydrostatic stress field. The stresses are homogeneous in the elliptical patch. The stresses are higher near the edges of the square and rectangular patches.

Design Approaches for Edge Delamination Resistance in Laminated Composites

This paper reviews the material and structural approaches to increase resistance of delamination initiation and growth. In the material approach, it was found that currently available toughened thermosets and thermoplastic composites can significantly improve delamination resistance for static, but not fatigue, loading. In the structural approach, it was concluded that stitching and interleafing are effective ways to resist delamination as a result of impact. However, terminating and discretizing the critical ply, as well as selective interleafing, are the most effective ways to increase delamination resistance for both static and fatigue in-plane loadings. 25 refs.

A Simple Closed-Form Solution of Bending Stiffness for Laminated Composite Tubes

Accurate evaluation of bending stiffness is important for better prediction of deflection, buckling load, and vibration response of structures. This paper presents two new approaches based upon laminated plate and shell theories for calculating the bending stiffness of circular tube made of fiber reinforced composites. Both new approaches account for the ply orientation due to radius curvature. The conventional approach using the laminate smear property is also included for purpose of comparison. Finite element analysis was conducted and the results were used for comparison.

Interfacial stress estimation using least-square extrapolation and local stress smoothing in laminated composites

Abstract This paper applies the finite element least-square extrapolation and smoothing technique to demonstrate its advantages in the evaluation of interfacial stress distributions in composite laminates. The analysis uses the quasi-three-dimensional finite element modeling technique and complete three-dimensional analysis using ABAQUS to investigate the stress distributions in graphite/epoxy laminates. Linear (two points integration) and quadratic (three point integration) least square fits in eight-node quadrilateral and 20-node solid isoparametric elements are demonstrated. The evaluation of transformation matrix from Gaussian stresses to nodal stresses was performed using symbolic mathematics on “Mathematica”. The results show that use of extrapolation and smoothing offer better estimates of stress distributions and the interfacial stresses in composite laminates.

Effects of delamination and ply fiber waviness on effective axial and bending stiffnesses in composite laminates

Abstract An analytical method based upon a sublaminate modeling approach was developed to quantify the loss of the effective axial and bending stiffnesses due to defects in laminates. Defects considered were single and multiple delaminations as well as ply fiber waviness. Laminates with (45 °, −45 °, 0 °, 90 °, 0 °, 90 °) s layup of AS4/35016 graphite/epoxy were used to investigate the stiffness loss due to defects at various interfaces and with various percentages of overlap in the multiple delaminations. It was found that the reductions of the stiffness in a given direction is more pronounced if the thinner sublaminate of the delaminated laminate contains a ply which contributes more stiffness in that direction. For multiple delaminations, reduction of stiffness increases as the overlap percentage of two delaminations decreases. The loss of axial and bending stiffness is significant if a ply contributing more stiffness in the laminate contains a fiber waviness.

Delamination characterization of laminates under tension, bending and torsion loads

A two-dimensional finite-element model has been developed to calculate interlaminar stresses and strain energy release rates for the study of delamination in composite laminates subjected to tension, bending and torsion loads. This paper addresses the formulation, implementation, and verification of this element to investigate the variation of the interlaminar stresses and strain energy release rates for composite laminates. The study concentrates on establishing relationships of GI, GII, and GIII as functions of crack length and stacking sequence in laminates due to beding and torsion.