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Modelling of mechanical and creep behaviour of honeycomb core sandwich structuresCompleted

Project leaders: Yan, Ning
 

Objective:

It is known that moisture ingression in honeycomb core sandwich structures always occurs on some
level and causes damage in industry products such as aircraft etc. (Cise and Lakes 1997). The moisture
ingression rate depends on direction and position of core, core structure, and material properties of core
and face (Shafizadeh et al 1999). Fogarty (2009) summarized that the proper design of honeycomb core
structure and the choice of face and core materials can reduce or even prevent moisture ingression.
However, for the sandwich panels containing Kraft paper honeycomb cores and wood composite skins,
the moisture does not only affect the core structure, but also the material properties of core paper and
wood skin. Fibres in paper are bonded to each other, and the difference between longitudinal and
FPI 11-08E UT: Prediction of the Behaviour and
Performance of Light Weight Hollow Core Panels
Project No.: FPI 11-08E- Last Update - N/A
2
transversal swelling can cause large stresses at the bonds when moisture content is changed (Van Den
Akker 1962). These stresses can accelerate creep of paper. Using mechanical models, Habeger and
Coffin (2000),explained how creep of paper could be accelerated by moisture cycling with exclusion of
any special mechanisms for mechanosorptive creep. Study of Alfthan et al. (2002) indicated that stress
created at the bonds due to anisotropic swelling during absorption and desorption of moisture caused
mechanosorptive creep.
Some researchers reported that moisture diffusivity increases with relative humidity after an
approximate value of 50%. In machine made paper, possible weak dependence on the in-plane fiber
orientation may give rise to higher diffusion in the machine direction (MD) as compared to the cross
machine direction (CD). Moisture diffusion in paper is Non-Fickian due to the material characteristics
such as: a) internal relaxation which implies formation of hydrogen bonds with water and swelling, and
b) vapor and bound water mechanisms of diffusion which occur in the pores and in the fibre walls,
respectively (Massoquete 2005).

There are many publications about moisture effects on mechanical and creep behaviours of timber and
wood composites. Among these researches, some FE models have been developed for analyzing
mechanism of moisture influence (Ormarsson 1999, Sjodin 2004, Vidal-Salle and Chassagne 2007
Hanhijarvi and Mackenzie-Helnwein 2003). The typical model among these developments is a 3D
moisture-stress FEM model for time dependent problems in timber structures, based on the rheological
constitutive model and developed by Fortino et al. (2009). However, there is still little information
available regarding the relationships among moisture transfer, core structure and mechanical behaviour
of sandwich panels containing Kraft paper honeycomb core and wood composite skins. Of particular
concern are the moisture effects on the creep of panels. In order to understand the influence of humid
environment on the mechanical properties of sandwich panels, a series of sandwich panels with various
thicknesses and skins and various relative humidity (65% and 95%RH) were tested under short and
long term loadings. Based on the test results and the early developed FE models, relative models for
predicting creep behaviour of sandwich panels under different humidity were developed.

Start date: 2011-04-08 End date: 2012-04-08
 

Reports:

 

For more information on this project, please contact:

Yan, Ning
University of Toronto
Email: ning.yan@utoronto.ca
Telephone: 416-946 8070
Fax: 416-978 3834
 
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