influence of the growth ring angle on transverse elastic...
TRANSCRIPT
October 6th, 2010 Daniel Keunecke ([email protected])
Influence of the growth ring angle on transverse elastic properties of softwoods
Daniel Keunecke1, Jozsef Garab2, Stefan Hering1, Jozsef Szalai2, Peter Niemz1
1 Institute for Building Materials (Wood Physics), ETH Zurich, Switzerland2 Faculty of Wood Sciences, Inst. for Applied Mechanics and Structures, University of West Hungary
Cost Action FP0802 Workshop “Wood Structure/Function-Relationships”, October 5-8, Hamburg, Germany
October 6th, 2010 Daniel Keunecke ([email protected])
Load-directional dependence of compliance
Three principal planes
Tensor transformation approach to analyze compliance for “off-axis” load directions
Complete set of elastic engineering parameters required
Compliance of Norway spruce and Common yew visualized in polar diagrams
Introduction • Materials and methods • Results and discussion • Conclusions
Previous study: Keunecke et al. 2008, Wood Sci Technol
October 6th, 2010 Daniel Keunecke ([email protected])
Load-directional dependence of compliance
LR and LT planes
only slight divergence between both species
similar general course of curves but at a different scale
Introduction • Materials and methods • Results and discussion • Conclusions
Previous study: Keunecke et al. 2008, Wood Sci Technol
October 6th, 2010 Daniel Keunecke ([email protected])
Load-directional dependence of compliance
RT plane
curves completely different
deformation of spruce highly anisotropic: compliance highest at α= 40-50°
Yew behaves clearly less anisotropic: compliance minimum near α = 45°
Introduction • Materials and methods • Results and discussion • Conclusions
Previous study: Keunecke et al. 2008, Wood Sci Technol
October 6th, 2010 Daniel Keunecke ([email protected])
Goals of this study
To experimentally determine MOE values in R and T direction, and for several off-axis angles between R and T (with L always being perpendicular to the load direction)
To determine Poisson’s ratios for the same load axes
Wood species: Norway spruce, Common yew
Static compression test on miniature specimens with contemporary testing equipment
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
(1) Planing slats with a cross section of 30mm x 30mm
Specimen preparation
(2) Cutting 7 disks, each 4mm thick
(3) Cutting specimens with 10mm x 10mm in the RT plane
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Specimen orientation
Growth ring orientation in the specimens and load orientation
0° corresponds to the radial load direction, 90° to the tangential one
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Testing setup: compression stage and specimen
Load cell (maximum capacity: 300N)
Specimen with high-contrast random dot texture (airbrush)
LVDT
Clamps
20 mm
Teflon sheets
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Load limit and MOE calculation
Introduction • Materials and methods • Results and discussion • Conclusions
Segment for MOE calculation
Compression test stopped at 300N (load cell limit)
E
stresscorresponding strain
σε
October 6th, 2010 Daniel Keunecke ([email protected])
Full-field surface tracking of in-plane sample deformation
Specimen surface (10mm x 10mm) resolved with 950 x 950 pixels, capture frequency: 4 Hz
Computing 2-dimensional strain distribution based on grey value cross-correlations of the speckle pattern (Vic 2D)
Calculation of average strain in the load direction and transverse to it ( Poisson’s ratios)
CCD camera & digital image correlation
Introduction • Materials and methods • Results and discussion • Conclusions
k
km
m
direction of lateral extensionload-directional compression
k
m
October 6th, 2010 Daniel Keunecke ([email protected])
Experimentally determined parameters
Introduction • Materials and methods • Results and discussion • Conclusions
Yew mean values higher (due to 30-40% higher density)
Range between minima and maxima wider for spruce
October 6th, 2010 Daniel Keunecke ([email protected])
Experimentally determined parameters
Introduction • Materials and methods • Results and discussion • Conclusions
Low MOE values:
size effect? geometry effect?
(length-to-width ratio, panel vs. bar-shaped geometry, …)
October 6th, 2010 Daniel Keunecke ([email protected])
Compliances of spruce and yew wood
The compliances are normalized with s(α)/s(0) = 1 for a growth ring angle α = 0°
corresponding to the radial load direction.
“approximated” represents the results of our prior study (Keunecke et al. 2008)
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Experimentally determined parameters
Introduction • Materials and methods • Results and discussion • Conclusions
Smaller range for yew than for spruce values
Differences between spruce and yew less pronounced than for the elastic moduli
October 6th, 2010 Daniel Keunecke ([email protected])
Poisson’s ratios of spruce and yew wood
A growth ring angle α = 0° corresponds to the radial, α = 90° to the tangential load direction.
i refers to the load direction and j to the direction of lateral extension.
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Load-directional strain in spruce and yew samples
Spruce
Yew
Load direction
Images show strain situation at the end of the linear-elastic range, immediately before plastic deformation started
0° 90°
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Load-directional strain in spruce and yew samples
Spruce
Yew
Load direction
Strain differences between EW and LW more distinct for spruce wood
0° 90°
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Density differences between EW zones of spruce and yew
Density differences between EW and LW clearly smaller for yew than for spruce
spruce
yew
yew spruce
Exemplary measurements in a previous study: Keunecke et al. 2009, IAWA J
Introduction • Materials and methods • Results and discussion • Conclusions
October 6th, 2010 Daniel Keunecke ([email protected])
Conclusions
Introduction • Materials and methods • Results and discussion • Conclusions
Transverse compression test: principle tendencies for off-axis MOE values for spruce (tensor transformation, older publications) experimentally confirmed
Not a general behavior of softwoods: different behavior and lower degree of anisotropy for yew wood
Good agreement between experimental results and compliances previously calculated on basis on tensor transformations
spruce and yew can be treated as orthotropic materials!
October 6th, 2010 Daniel Keunecke ([email protected])
Conclusions
Introduction • Materials and methods • Results and discussion • Conclusions
Future investigations (modeling approaches): how is elastic behavior influenced by density and density distribution?
