Delamination and debonding

failure of composite T-joint

Background

During composite structure design and certification

process, the building block approach (BBA) is usually

used. The benefits from the interaction between

experiments and analysis strongly rely on the

predictive ability and accuracy of analyze methods.

The increase in computational capabilities and

advance in failure simulation methods has made it

feasible to analyze the fracture mode and loading

capacity of engineering structures. This project aims

to improve the knowledge of the debonding and

delamination failure mechanisms, and to develop

accurate methods to predict the mechanical response

of composite T-joints. In the last three years, several

subtopics has been investigated, which are present

as follows.

Based on the present theory, a new method has been

developed to measure the adhesive shear stiffness

and strength

Shear strain distribution in SLJ and SBS test

4. Constitutive law of ductile adhesive layer

The failure mechanism of adhesive material (particle

toughened epoxy ) under tension/shear load was

investigated with in situ SEM. The local constitutive

response of ductile layer was obtained on the base of

J-integral. The dependence of adhesive loading

capacity on tension-shear displacement path was

discussed.

Deformation and failure of adhesive layer

Mixed mode local constitutive law

Summary

New numerical model and experiment method have

been developed by the author to explore the fracture

behavior of composite materials at meso/micro scale.

Base on the improved knowledge of fracture

properties, corresponding analysis methods have

been present to analyze practical composite

structures, i.e. T-joint in this topic.

The present work does not only contributed to the

analysis of composite structure. Some potential

applications include the failure of rivet joints, new

method for biaxial test of polymer materials.

Publications Hao Cui, Yulong Li, Yuanyong Liu, Jiaping Guo, Qiulian Xu, (2010)”Numerical Simulation of composites joints failure based on cohesive zone model”, Acta Mate Comp Sinica, pp161-168 H. Cui, Yulong Li, S. Koussios, L. Zu, A. Beukers, (2011) “Bridging micro mechanisms of Z-pin in mixed mode delamination” , Composite Structures, pp 2685-2695 L. Zhu, H. Cui, Y-L.Li, W.Sun, (2012) “Numerical Simulation of the Failure of Composite T-joints with Defects”, Acta Aeronautica et Astronautica Sinica, pp 287-296 H. Cui, Y-L. Li, (2013) “Failure of composite T-joints in bending with through-the-thickness reinforcement: stitching vs Z-pinning”, Key Engineering Materials, pp 233-236 Hao Cui, Sotiris Koussios, Yulong Li, Adriaan Beukers,(2013), “Measurement of adhesive shear properties by short beam shear test based on higher order beam theory” International Journal of Adhesion and Adhesives, pp 19-30. Hao Cui, Yulong Li, Sotiris Koussios, Adriaan Beukers,(2013) “Mixed mode cohesive law for Z-pinned composite analyses”, accepted by Computational Materials Science. Hao Cui , Yulong Li, S. Koussios, A. Beukers , (2010)” Parametric evaluation on the curved part of composite T-joints base on numerical simulation”, ICAS 2010 H. Cui, S. Koussios, Y. Li, A. Beukers, (2012) ” Mixed mode cohesive law for Z-pinned composite delamination” CST 2012

Building block approach for composite aircraft structures

1. Pull-off failure of co-cured T-joints

Cohesive elements are inserted between every two

bulk elements to predict the random crack initiation

in the filler at the root of the composite joint. The

composite joint is simulated under pull-off load, and

the numerical result is in good agreement with

experimental one.

Bilinear cohesive zone model

Pull-off failure of T-joint

2. Multi-scale simulation of Z-pinned composites

Failure mechanisms of Z-pin

A meso-scale FEM for analyzing the Z-pin failure

during mixed mode delamination was developed,

which is able to predict the Z-pin bridging response

in UD and cross-ply laminates. An enhanced

frictional zone with significant plastic deformation

was located near the delamination surface.

Coupled CZM for Z-pinned composites

A new method for analysing mixed mode failure of

Z-pinned composite laminates. A novel cohesive law

to account for mixed mode response of Z-pins is first

presented. The computational model is validated by

mode I/II and mixed mode delamination test results.

The path-dependence of cohesive law in mixed mode

simulation is discussed.

3. Higher order beam theory A novel higher-order sandwich beam theory has

been developed to analyze the beam consists of a

single layer of adhesive and two adhered laminates.

This theory is based on a special form of a cubic

expansion for the in-plane displacement

components through the thickness

Displacement profile through the beam thickness.

PhD Candidate: Hao Cui Department: LR Section: composites Supervisor: S,Koussios Promoter: A. Beukers Start date: 22-10-2009 Funding: CSC&TUD

Aero

space

Engin

eering