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Adaptive Neutron Radiography Correlation for Simultaneous Imaging of Moisture Transport and Deformation in Hygroscopic Materials

01 Feb 2015-Experimental Mechanics (Springer US)-Vol. 55, Iss: 2, pp 403-415

AbstractNeutron radiography is a key non-destructive testing technology for the investigation of moisture transport in materials. However, quantitative moisture measurements in swelling materials are currently challenging due to the lack of referencing between moist and dry state radiographs. A novel adaptive texture correlation algorithm is presented to simultaneously image inhomogeneous moisture distributions and moisture-induced strain fields. The proposed method provides a valuable tool for the study of time- and position-dependent hygromechanical interactions. Moreover, it requires no modification of existing neutron installations. The method was validated against gravimetric moisture content and optic surface deformation measurements. Its applicability was demonstrated for two actual topics in wood science, the investigation of moisture gradients within the growth ring microstructure and the study of moisture transport processes in wood-fiber composites. The algorithm can be widely used to characterize hygroscopic materials with heterogeneous texture, as frequently found in wood constructions, food industry, engineering and soil science.

Topics: Moisture (57%)

Summary (2 min read)

Introduction

  • Neutron imaging (NI) is an established method to nondestructively study the dynamics of hydrogenous fluids, such as bound or free water molecules, in geomaterials (rocks, soils), engineered media (ceramics, polymers, metals, concrete, fuel cells, heat pipes, engines…) and biological composites (wood, food, natural fiber-reinforced composites..), among others.
  • Wood is a classical example, volumetric swelling strains >10 % are not rare within its large hygroscopic range, which goes up to 30 % moisture content from oven-dry state until the fiber saturation point (FSP), due to the hydrophilic behavior of the lignocellulosic cell walls.
  • Quantitative moisture measurements with neutrons also require hygro-expansion data.
  • These shortcomings have so far been alleviated with a global pre-registering of neutron radiographs based on sample edge detection [18–21], which reduces edge misalignment, but does not account for inner deformation gradients.
  • NI was recently combined with optical deformation measurements [15].

Adaptive Neutron Radiography Correlation (ANRC)

  • The starting point for the ANRC algorithm is a set of neutron radiographies of the test samples, which are experimentally acquired at well-controlled dry and moist states.
  • Subsets of the reference image Iref (dry state) are automatically searched at specific positions r of the deformed image Itest (moist state), resulting in local estimates of the deformation vector u(r), from which the strains fields εij are differentiated.
  • In order to minimize false correlation hits, the search region is adaptively adjusted.

Experiments

  • Moisture and Swelling Gradients in Softwood Growth Rings at Hygroscopic Equilibrium Optical surface deformation measurements.
  • The goal was to identify swelling strain gradients between the alternating bands of low-density earlywood (EW)– thin-wall wood cells with large internal lumens grown in spring – and high-density latewood (LW) – thick-wall cells with small-sized lumens grown in summer (Fig. 2a).
  • The neutron radiographs (Fig. 3b) show a pixel size of 145 μm and a lateral resolution of ~0.7 mm (minimum detectable line spacing).
  • The deformation vectors u and strain fields εij at each RH step were computed with ANRC and a subset sizeΔ of 21×21 pixels2 (Fig. 4).
  • Following the experimental setup of [3, 17], six samples were ovendried, sealed with aluminum tape on the four lateral surfaces and placed in a climatic box (RH=84 %, T=31 °C) above a desiccating silica gel.

Results and Discussion

  • The comparison of ANRC correlation statistics for specific test scenarios shows distinctive trends (Table 1).
  • Unique subset correspondence was only satisfied along one image direction, which led to a minimum of 16 % error values, even with the highest correlation coefficients r=0.98.
  • Due the resolution loss, the peak value of εRR in the ANRC profiles was displaced towards the center of the growth ring (Fig. 5b).
  • A profile in thickness direction (Y) clearly reveals the moisture-induced deformation, and shows a strong contrast between the dense outer layers and the lighter middle layer (Table 1).

Conclusions and Outlook

  • Neutron imaging together with ANRC processing is able to simultaneously image moisture and moisture-induced deformation fields with high spatial and temporal resolution, with opens new horizons for the hygromechanical characterization of highly swelling materials, as found in wood constructions, but as well in food and agricultural industry, engineering and soil science [6].
  • The ANRC algorithm is readily extensible to three-dimensional stacks as provided by neutron computed tomography.
  • Above saturation, liquid water may lead to stronger contrast differences [19], which can be removed by iteratively subtracting the moisture fields from the radiographs.
  • Applied to the study of moisture transport in highly hygromorphic wood-fiber composites, the ANRC correction enabled the otherwise not possible optimization of moisture diffusion coefficients, which showed a strong correlation (R2=0.978) with steady state values.
  • Experimental studies of the rheology of moisture-induced deformation fields have been so far rare, due to the practical challenges in simultaneously mapping time-dependent moisture and strain fields.

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Adaptive Neutron Radiography Correlation
for Simultaneous Imaging of Moisture Transport
and Deformation in Hygroscopic Materials
S.J. Sanabria & C. Lanvermann & F. Michel & D. Mannes &
P. Niemz
Received: 13 March 2014 /Accepted: 18 September 2014 /Published online: 21 October 2014
#
Society for Experimental Mechanics 2014
Abstract Neutron radiography is a key non-destructive test-
ing technology for the investigation of moisture transport in
materials. However, quantitative moisture measurements in
swelling materials are currently challenging due to the lack
of referencing between moist and dry state radiographs. A
novel adaptive texture correlation algorithm is presented to
simultaneously image inhomogeneous moisture distributions
and moisture-induced strain fields. The proposed method
provides a valuable tool for the study of time- and position-
dependent hygromechanical interactions. Moreover, it re-
quires no modification of existing neutron installations. The
method was validated against gravimetric moisture content
and optic surface deformation measurements. Its applicability
was demonstrated for two actual topics in wood science, the
investigation of moisture gradients within the growth ring
microstructure and the study of moisture transport processes
in wood-fiber composites. The algorithm can be widely used
to characterize hygroscopic materials with heterogeneous
texture, as frequently found in wood constructions, food in-
dustry, engineering and soil science.
Keywords Non-destructive neutron imaging
.
Heterogeneous
texture correlation
.
Moisture-induced deformation strains
.
Wood composite materials
.
Mechano-sorptive rheology
Introduction
Neutronimaging(NI)isanestablishedmethodtonon-
destructively study the dynamics of hydrogenous fluids, such
as bound (hygroscopic) or free water molecules, in geomate-
rials (rocks, soils), engineered media (ceramics, polymers,
metals, concrete, fuel cells, heat pipes, engines) and biolog-
ical composites (wood, food, natural fiber-reinforced compos-
ites..), among others. Compared to X-ray and Nuclear
Magnetic Resonance (NMR) imaging, NI is highly sensitivity
to hydrogen while being transparent to most solid-state mate-
rials, including heavy metals. Moreover, it allows dynamic
full-field imaging in two (radiography) and three
(tomography) dimensions with tenth-of-millimeter spatial res-
olution and sub-second measurement times [15].
Moisture intake in porous media is often accompanied by
swelling, which is especially relevant for biological compos-
ites [6]. Wood is a classical example, volumetric swelling
strains >10 % are not rare within its large hygroscopic range,
which goes up to 30 % moisture content from oven-dry state
until the fiber saturation point (FSP), due to the hydrophilic
behavior of the lignocellulosic cell walls. If not accounted for,
swelling strain gradients can impair dimensional stability and
induce eigenstresses, leading to cracking and delamination,
which reduce the serviceability of wooden constructions [7].
Natural fiber-reinforced composites have drawn widespread
attention in the last decades due to their low cost and biode-
gradability, accompanied by high strength and insulation
Electronic supplementary material The online version of this article
(doi:10.1007/s11340-014-9955-2) contains supplementary material,
which is available to authorized users.
S.J. Sanabria (*)
:
C. Lanvermann
:
F. Michel
:
P. Niemz
Institute for Building Materials ETH Zurich, Stefano-Franscini-Platz
6, CH-8093 Zurich, Switzerland
e-mail: ssanabria@ethz.ch
C. Lanvermann
e-mail: lanvermannchr@ethz.ch
F. Michel
e-mail: frmichel@ethz.ch
P. Niemz
e-mail: niemzp@ethz.ch
D. Mannes
Neutron Imaging and Activation Group, Paul Scherrer Institute,
CH-5232 PSI, Villigen, Switzerland
e-mail: david.mannes@psi.ch
Experimental Mechanics (2015) 55:403415
DOI 10.1007/s11340-014-9955-2

properties [8]. However, their highly hygromorphic behavior
is challenging. For example, the inner stresses induced in the
hot pressing of wood-based fiber composites lead to larger
thickness swelling than in raw wood material [9]. The cou-
pling between moisture transport and deformation is also a
key element to describe food drying, clay consolidation and
sorption in polymer electrolyte fuel cells. Although sophisti-
cated hygromechanical models are available, they often lack
of accompanying experimental data. In this context, the si-
multaneous full-f ield im aging of moist ure and moist ure-
induced deformation is necessary [6, 1014].
Quantitative moisture measurements with neutrons also
require hygro-expansion data. The gravimetric moisture con-
tent ω=m
h
/m
o
is defined in terms of the mass m
h
of water with
respect to the mass of the dry solid phase m
o
, which requires
the local referencing of the neutron radiographs in moist T
state to the dry state T
o
. Moreover, the density of the solid
phase is lower in moist than in dry stat e due to sample
swelling. Thus the in-plane swelling strain fields ε
ii
with
respect to the reference dry state (Lagrangian descrip-
tion) need to be included in the calculation of ω
(Equaton 1)[11, 15]:
ω ¼ 1 þ ε
XX
ðÞ1 þ ε
YY
ðÞρ
h
z
h
ρ
o
l
z
ðÞ
1
ð1Þ
with l
z
the sample thickness, ρ
h
and ρ
o
the densities of water
(10
3
kg m
-3
) and dry solid phase substance, respectively, and
z
h
the effective water column thickness (Equaton 2):
Z
h
¼ Σ=ρðÞ
h
ρ
h

1
lnTlnT
o
1 þ ε
XX
ðÞ
1
1 þ ε
YY
ðÞ
1
hi
ρ
o
¼ lnT
o
Σ=ρðÞ
o
l
z

1
ð2Þ
(Σ/ρ)
h
and (Σ/ρ)
o
are respectively the mass-attenuation coef-
ficients in water and solid phases. For the energy spectrum of
the thermal neutron beamline NEUTRA at the Paul Scherrer
Institute (Villigen, Switzerland) [41], where the presented
investigations were carried out, (Σ/ρ)
o
=0.18 m
2
kg
1
for
wood (Table 1,[42]) and (Σ/ρ)
h
=0.35 m
2
kg
1
for water
[43]. Operating Equation 1 shows that neglecting hygroscopic
strains of 10 % in wood leads to unacceptable quantitative
moisture errors over 5 % (absolute error), as shown in
Equation 3:
ω ¼ ω
jε¼0
1 þ ε
XX
þ ε
YY
½
þ Σ=ρðÞ
o
Σ=ρðÞ
h

1
ε
XX
þ ε
YY
þ ε
XX
ε
YY
½ω
jε¼0
þ 0:52 ε
XX
þ ε
YY
½ ð3Þ
Finally, the edge misalignment induced by hygro-
expansion leads to unbounded errors and non-physical mois-
ture values at image discontinuities, which impedes or makes
the study of moisture transport through multi-layered com-
posites and surface coatings challenging [16, 17].
These shortcomings have so far been alleviated with a
global pre-registering of neutron radiographs based on sample
edge detection [1821], which reduces edge misalignment,
but does not account for inner deformation gradients. Full-
field strain measurements with neutrons have been achieved
in some crystalline materials by using a tunable monochro-
matic beam, for which Bragg cut-off wavelengths provide
lattice spacing [1]. Yet a combination with moisture measure-
ments has not been reported. NI was recently combined with
optical deformation measurements [15]. Although accurate
high-resolution ε
ii
measurements were obtained, the setup
requires sample speckling and an involved synchronized cam-
era installation. Moreover, only surface deformation is mea-
sured, which leads to significant edge misalignment artifacts
in volumetric samples.
In this work, a novel Adaptive Neutron Radiography
Correlation (ANRC) algorithm is presented, which uses the
texture information in neutron images to locally estimate the
strain fields. This m ethod is exclusively based on the
postprocessing of radiographs, therefore requiring no modifi-
cation of existing neutron installations, and providing average
deformation estimates over the sample thickness. The new
approach is validated with gravimetric and optic methods. Its
potential and applicability are demonstrated for two actual
research topics in wood science:
a) Gradients in moisture and hygroscopic swelling in soft-
wood growth rings [15, 16, 2225]
b) Investigation of moisture transport through swelling
wood-fiber composites [3, 17, 18, 26]
Adaptive Neutron Radiography Correlation (ANRC)
The starting point for the ANRC algorithm is a set of neutron
radiographies of the test samples, which are experimentally
acquired at well-controlled dry and moist states. State of the
art corrections are first applied to compensate for source and
detector inhomogeneities (CCD dark current, median filter,
intensity normalization, flat field correction, spectral effects),
background scattering (black body calibration) and sample
scattering (Monte-Carlo simulations) [43].
Figure 1 summarizes the building blocks of the ANRC
algorithm. The inner core performs Digital Image
Correlation (DIC) processing. Subsets of the reference image
I
ref
(dry state) are automatically searched at specific positions r
of the deformed image I
test
(moist state), resulting in local
estima tes of the deformation vector u(r), from which the
strains fields ε
ij
are differentiated. The DIC method is well-
known and performs best for artificial random speckle
404 Exp Mech (2015) 55:403415

patterns, where unique subset correspondence is satisfied.
Typical DIC parameters are subset size, search function
and region and deformation order [27]. Many heteroge-
neous materials show enough natural texture to track u
without need of artific ial sample s peckling, the DIC
process is then specifically termed Texture Correlation
(TC) [28]. Neutron radiographs of wood composites are
assumedtoshowenoughtextureforTC,whichis
empirically confirmed by the high correlation statistics
observed in the investigated test samples (Table 1).
The proposed ANRC processing is thus a TC algorithm,
which is optimized and expanded to robustly extract defor-
mation information from NI radiographs while ignoring
shadow regions, where only poor correlation statistics are
available. With this purpose, the DIC core implements a zero-
order search (rigid subsets). A zero-normalized cross-correla-
tion function I
corr
(Equation 4) is calculated at each integer
pixel position within a search window W for a subset size Δ.
The peak maximum r provides the displacement vector u,
which is refined by applying bicubic interpolation to I
corr
:
I
corr
b
x;
b
y

¼
X
x;yΔ
Tx
b
x; y
b
y

T
hi
T
o
x; yðÞT
o
hi
X
x;yΔ
Tx
b
x; y
b
y

T
hi
2
X
x;yΔ
T
0
x; yðÞT
0
hi
2
0
B
B
B
@
1
C
C
C
A
1
u ¼ arg max I
corr
b
x;
b
yW
ð4Þ
In order to minimize false correlation hits, the search region
is adaptively adjusted. The algorithm starts searching in a
large reset window W
R
. Subsequent search positions r
i
are
ordered in a raster scan of the reference image. For each of
them, the search is reduced to a shorter window W
S
cen-
tered at an initial deformation estimate û, which is
calculated based on the linear ex trapolation of N
û
neighboring pixels. An error contro l block ensures the
well-posing of the estimated u by controlling its: a)
correlation: the co rrelation coefficient r is above a
minimum threshold r
min
,b)uniqueness: u is within
image bou nds, is not an e dge pixel of I
corr
, an d does
not exceed a maximum deformation value u
max
,c)
continuity: u does not increase by more than Δu
max
in each iteration. In the c ase of an error of type b), a
second trial is admitted with a larger window W
L
before discarding u. Only non-error u values (e=0)
are used for the adaptive search and the continui ty
error check. Ill-posed u (e 0) increment the error
count c
e
up to a threshold N
e
. Then the reset control
block starts operation, deactivating the adaptive search
and resetting the search window to W
R
. The algorithm
manages errors along individual dimensions of t he de-
formation vector u
j
by only storing well-posed u
j
.The
loss of continuity upon reset is additionally avoided by
managing an additional reset count c
R
and not
accepting u values at first reset positions (c
R
=1) for
updating the deformation estimate û, by using at reset
positions û instead of u for c ontinuity control, and by
filtering out isolated u pixels.
Table 1 Summary of neutron attenuation properties, ANRC settings and correlation statistics for the investigated test materials
ρ
o
(kg m
-3
)(Σ/ρ)
o
(m
-2
kg) d (m) Δ (mm)
b
s
ρ
/ρ (%)
c
CNR (a.u.)
c
r (a.u.) e=1 (%) u
max
(mm)
I: Optical, softwood growth
rings
a
(Fig. 2,4.1.1)
n.a. (not applicable) n.a. n.a. [0.2, 0.2] n.a. 4.6 0.96 [0.7, 0.7] [1.5, 1.0]
II: Neutron, softwood growth
rings
a
(Fig. 36,4.1.2)
362 0.18 0.005 [2.9, 2.9] 34 5.1 0.98 [0.5, 16] [0.8, 1.7]
III: Neutron, wood-fiber
composites
a
(Fig. 78,4.2)
Isonat chanvre 44 0.27 0.04 [4.5, 9] 15 2.5 0.92 [0, 0] [0.2, 0.04]
Pavaflex 48 0.23 0.04 [4.5, 9] 16 2.5 0.91 [0, 0] [0.2, 0.08]
Pavatherm 135 0.19 0.04 [4.5, 9] 6 1.2 0.76 [0, 0] [0.1, 0.04]
Aerogel Vliesmatte 147 0.10 0.04 [4.5, 9] 9 1.6 0.88 [0, 0] [0.3, 0.2]
Isoroof-natur-KN 225 0.18 0.04 [4.5, 9] 6 1.1 0.64 [1.6, 1.4] [0.6, 0.2]
a
The pixel sizes for I, II and III are 23, 77 and 145 μm, respectively
b
Fixed algorithm settings (px): W
S
=[3, 3], W
L
=[5, 5], Δu
max
=[3, 3], N
e
=5, N
û
=[10, 100]
c
s
ρ
is the standard deviation of the dry material density ρ
o
within the correlated subsets. CNR=s
Δ
/s
n
,withs
Δ
and s
n
respectively for correlation subset
and background image noise. s
Δ
=s
ρ
(Σ/ρ)
o
l
Z
exp[ρ
o
(Σ/ρ)
o
l
Z
] for the neutron images T, following from Equation 2.s
n
=0.02 for all radiographs
Exp Mech (2015) 55:403415 405

Continuous deformation fields are calculated by fitting the
estimated deformation vectors u to cubic smoothing splines
f(r
i
) (Equation 5), which trade-off between data fidelity and
smoothness [29]:
X
i¼1
N
r
i
ju
i
j
f r
i

j
2
þ 1pðÞ
Z
D
2
jftðÞj
2
dt p ¼ 1 þ h
3
=6

1
ð5Þ
The weights of the error function are adjusted to the peak
correlation coefficients r. The resolution h, which con-
trols the smoothness, is adjusted with the subset size in
the range h=Δ/3Δ. The fitted spline functions are
finally differentiated to compute the strain fields ε
ij
.In
order to minimize noise sensitivity, only first order
derivatives are included in ε
ij
=0.5(u
i
/x
j
+u
j
/x
i
).
The settings of ANRC for each case study are summarized
in Table 1. The subset size Δ is a trade-off between well-
posing and lateral resolution of u. A threshold Δ is typically
found, above which correlation statistics do not significantly
improve. The ANRC algorithm was implemented in Matlab®
(R2011b, The Mathworks Inc., Natick, MA, USA) and is
provided in the supplementary materials for free use and
development (Supp. Mat.).
Experiments
Moisture and Swelling Gradients in Softwood Growth Rings
at Hygroscopic Equilibrium
Optical surface deformation measurements
The strain field ε
ij
calculation with ANRC was first quantita-
tively validated against a commercial DIC tool (VIC 2D 2009,
Correlation Solutions Inc., USA) for high-resolution optical
images of speckled softwood (Fig. 2). The DIC measurements
were previously used to investigate wood cross-grain hygro-
expansion at the growth ring structural level [15, 24]. The goal
was to identify swelling strain gradients between the alternating
bands of low-density earlywood (EW) thin-wall wood cells
with large internal lumens grown in spring and high-density
latewood (LW) thick-wall cells with small-sized lumens grown
in summer (Fig. 2a). Four cuboid samples (40×40×5 mm
3
)
were cut fr om a Norwa y spruce (Picea abies Karst.) stem, their
edges were well-aligned with respe ct to the main wood material
axes (radial R×tangential T×longitudinal L). The samples were
sprayed with a black/white speckle pattern by means of an
airbrush gun with a nozzle size of 0.2 mm. They were then
subjected in a portable climatic chamber to a moisture sorption
desorption cycle with relative humidity (RH) adjusted in 15 %
steps between 0 and 95 %. For each nominal RH state, the
samples were conditioned to equilibrium moisture content and
optical images were acquired with a CCD camera (Fig. 2b). The
Fig. 1 Flow diagram of ANRC algorithm
406 Exp Mech (2015) 55:403415

oven-dry state (RH=0 %) was used as reference. Next, the in-
plane swelling strains were calculated with ANRC and com-
pared to the commercial DIC results (Fig. 2c). The subset size Δ
was 9x9 pixels
2
for both methods, for a pixel size of 23 μm.
Finally, the strains were transformed with respect to the material
axes (R, T) and plotted in function of the relative growth ring
position φ=01, with 0 and 1 respectively defining EW and
LW regions (Fig. 2d) . The coordinate transformation method is
further detailed in [15, 24].
Simultaneous neutron imaging of moisture content
and moisture-induced deformation
The moisture and strain gradients within the growth rings
were investigated at equilibrium conditions with NI at the
thermal neutron beam line NEUTRA at the Paul Scherrer
Institute in Villigen, Switzerland. The experimental setup
consists of a neutron beam collimator, a neutron detector and
a portable climate chamber (Fig. 3a) and has been detailed in
[15]. The neutron radiographs (Fig. 3b) show a pixel size of
145 μm and a lateral resolution of ~0.7 mm (minimum de-
tectable line spacing). The deformation vectors u and strain
fields ε
ij
at each RH step were computed with ANRC and a
subset size Δ of 21×21 pixels
2
(Fig. 4). The moisture content
distribution ω was calculated with Equation 1 and plotted in
function of the growth ring position (Fig. 5).
The average ω values were validated gravimetrically (Fig. 6).
At each RH state the weight of the sample was recorded with
0.1 mg accuracy with a precision scale. The local moisture and
strain fields were validated with the combined NI and optical
surface deformation measurement setup of [15]. With this pur-
pose, a stereo-vision installation (Fig. 3a) consisting of two
radiation-shielded CCD cameras and cold-light illumination
captured optical images from the sample laterals without
perturbing the neutron line of sight. The surface deformation
vectors u were then calculated with commercial DIC (VIC3D,
Correlated Solutions). As in 3.1.1, a subset size Δ of 9×9 pixels
2
was used, the resolution was here lower due to the optical
constraints (pixel size 77 μm). The optical images (Fig. 3c)
and the neutron radiographs were referenced with fiducial
markers. The calculated deformation vectors were then used to
aligntheneutronradiographsandtocomputeω.
Transient moisture diffusion and moisture-induced swelling
in wood-fiber composites
The applicability of the ANRC algorithm to simultaneously
investigate dynamic moisture transport and the large coupled
deformations in biological composites was demonstrated for a
selection of multi-layer building insulation materials. Three
commercial wood fiberboards (Pavaflex, Pavatherm and
Isoroof-natur-KN from Pavatex SA, Fribourg, Switzerland),
a hemp fiber composite (Isonat chanvre from Valnaturel SA,
Saxon, Switzerland) and a nanoporous aerogel (Aerogel
Vliesmatte from Aspen Aerogels Inc., Northborough, MA,
USA) were tested (Table 1). The material properties are de-
tailed in [26, 30]. The test objects (40×40 mm
2
section) were
typical three-layer insulation combinations, with two 20 mm
thick Isoroof outer layers and a 35 mm (Isonat, Pavatherm,
Pavaflex) or a 5 mm thick (Aerogel) middle layer. Following
Fig. 2 Quantitative validation of
ANRC processing on high-reso-
lution CCD camera images. a)
Test Norway spruce samples. The
growth ring microstructure is il-
lustrated at cellular scale with LM
and at subcellular scale with
ESEM. b) Optical surface images
at dry (RH=0 %) and moist
(RH=95 %) states, correlation
subsets I
ref
, I
test
andsearchfunc-
tions I
corr
are highlighted. c)
shows hygroscopic strain fields
calculated from b) with both
ANRC and a commercial DIC
tool, which in d) are averaged and
plotted in function of the growth
ring position φ
Exp Mech (2015) 55:403415 407

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Journal ArticleDOI
TL;DR: It is concluded that ultrasonic velocity coupled with specific gravity can be employed as predicting parameters to evaluate the dimensional stability of oak and chestnut wood during manufacturing process.
Abstract: The objective of this study was to examine the potential of ultrasonic velocity as a rapid and nondestructive method to predict the dimensional stability of oak (Quercus petraea (Mattuschka) Lieblein) and chestnut (Castanea sativa Mill.) that are commonly used in flooring industry. Ultrasonic velocity, specific gravity, and radial, tangential and volumetric shrinkages were measured on seventy-four 20×20×30-mm(3) specimens obtained from freshly cut oak and chestnut stems. The ultrasonic velocities of the specimens decreased with increasing moisture content (MC). We found that specific gravity was not a good predictor of the transverse shrinkages as indicated by relatively weak correlations. Ultrasonic velocity, on the other hand, was found to be a significant predictor of the transverse shrinkages for both oak and chestnut. The best results for prediction of shrinkages of oak and chestnut were obtained when the ultrasonic velocity and specific gravity were used together. The multiple regression models we developed in this study explained 77% of volumetric shrinkages in oak and 72% of volumetric shrinkages in chestnut. It is concluded that ultrasonic velocity coupled with specific gravity can be employed as predicting parameters to evaluate the dimensional stability of oak and chestnut wood during manufacturing process.

12 citations


Cites background from "Adaptive Neutron Radiography Correl..."

  • ...[17] reported that applied to the investigation of hygroscopic gradients in softwood growth rings, Adaptive Neutron Radiography Correlation (ANRC) provided an excellent agreement with gravimetric moisture content (0....

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  • ...Neutron imaging is able to simultaneously image moisture and moisture-induced deformation fields with high spatial and temporal resolution [7,15,17,12]....

    [...]


Journal ArticleDOI
TL;DR: A novel Individual Cell Tracking approach for in-situ quantification of nanometer-scale deformations of individual wood cells during mechanical loading of macroscopic millimeter-scale wood samples, which provides cellular insight into macroscopy relations, such as anisotropic Poisson effects, and allow direct observation of previously suspected wood ray reinforcing effects.
Abstract: Wood-based composites hold the promise of sustainable construction. Understanding the influence on wood cellular microstructure in the macroscopic mechanical behavior is key for engineering high-performance composites. In this work, we report a novel Individual Cell Tracking (ICT) approach for in-situ quantification of nanometer-scale deformations of individual wood cells during mechanical loading of macroscopic millimeter-scale wood samples. Softwood samples containing > 104 cells were subjected to controlled radial tensile and longitudinal compressive load in a synchrotron radiation micro-computed tomography (SRµCT) setup. Tracheid and wood ray cells were automatically segmented, and their geometric variations were tracked during load. Finally, interactions between microstructure deformations (lumen geometry, cell wall thickness), cellular arrangement (annual growth rings, anisotropy, wood ray presence) with the macroscopic deformation response were investigated. The results provide cellular insight into macroscopic relations, such as anisotropic Poisson effects, and allow direct observation of previously suspected wood ray reinforcing effects. The method is also appropriate for investigation of non-linear deformation effects, such as buckling and deformation recovery after failure, and gives insight into less studied aspects, such as changes in lumen diameter and cell wall thickness during uniaxial load. ICT provides an experimental tool for direct validation of hierarchical mechanical models on real biological composites.

10 citations


Journal ArticleDOI
Abstract: Easel paintings on canvas are subjected to alteration mechanisms triggered or accelerated by moisture. For the study of the spatial distribution and kinetics of such interactions, a moisture exposure chamber was designed and built to perform neutron radiography experiments. Multilayered sized and primed canvas samples were prepared for time-resolved experiments in the ICON cold neutron beamline. The first results show that the set-up gives a good contrast and sufficient resolution to visualise the water uptake in the layers of canvas, size and priming. The results allow, for the first time, real-time visualisation of the interaction of water vapour with such layered systems. This offers important new opportunities for relevant, spatially and time-resolved material behaviour studies and opens the way towards numerical modelling of the process. These first results show that cellulose fibres and glue sizing have a much stronger water uptake than the chalk–glue ground. Additionally, it shows that the uptake rate is not uniform throughout the thickness of the sized canvas. With prolonged moisture exposure, a higher amount of water is accumulating at the lower edge of the canvas weave suggesting a decrease in permeability in the sized canvas with increased water content.

9 citations


Journal ArticleDOI
Abstract: The study and identification of the water diffusion properties of Abies alba Mill at the growth ring scale are investigated in this paper. A full-field measurement technique, the grid method, is used in the experimental tests in order to observe heterogeneities and to measure strain fields in the annual growth rings. In this case, the mechanical behaviour of latewood (LW) and earlywood (EW) is studied under the effect of water stress. This technique is coupled to an inverse method for parameter identification. It enables us to assess the diffusivity of EW and LW in the tangential diffusion direction (T) and their expansion coefficients in the radial direction (R) from strain maps obtained using the grid method. Thus the water diffusion phenomenon and its effect on the deformation of wood specimens are studied. A suitable experimental device is designed to provide direct contact with water on one side of the specimen, and strain maps are obtained over time on the other side. During the experiments,...

8 citations


Cites methods from "Adaptive Neutron Radiography Correl..."

  • ...In a more recent work, Sanabria et al. (2015) obtained hydric strain maps of Picea Abies Karst under moisture equilibrium using DIC....

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TL;DR: Though it incorporates much new material, this new edition preserves the general character of the book in providing a collection of solutions of the equations of diffusion and describing how these solutions may be obtained.
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Abstract: This book is based on the author's experience with calculations involving polynomial splines. It presents those parts of the theory which are especially useful in calculations and stresses the representation of splines as linear combinations of B-splines. After two chapters summarizing polynomial approximation, a rigorous discussion of elementary spline theory is given involving linear, cubic and parabolic splines. The computational handling of piecewise polynomial functions (of one variable) of arbitrary order is the subject of chapters VII and VIII, while chapters IX, X, and XI are devoted to B-splines. The distances from splines with fixed and with variable knots is discussed in chapter XII. The remaining five chapters concern specific approximation methods, interpolation, smoothing and least-squares approximation, the solution of an ordinary differential equation by collocation, curve fitting, and surface fitting. The present text version differs from the original in several respects. The book is now typeset (in plain TeX), the Fortran programs now make use of Fortran 77 features. The figures have been redrawn with the aid of Matlab, various errors have been corrected, and many more formal statements have been provided with proofs. Further, all formal statements and equations have been numbered by the same numbering system, to make it easier to find any particular item. A major change has occured in Chapters IX-XI where the B-spline theory is now developed directly from the recurrence relations without recourse to divided differences. This has brought in knot insertion as a powerful tool for providing simple proofs concerning the shape-preserving properties of the B-spline series.

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Abstract: This review article concerning natural and man-made cellulose fibre reinforced plastics, introduces possible applications of this material group. The physical properties of natural fibres are mainly determined by the chemical and physical composition, such as the structure of fibres, cellulose content, angle of fibrils, cross-section, and by the degree of polymerization. Only a few characteristic values, but especially the specific mechanical properties, can reach comparable values of traditional reinforcing fibres. This physical structure can be modified by using alkali treatment and acetylation processes. The application of natural fibres as reinforcements in composite materials requires, just as for glass-fibre reinforced composites, a strong adhesion between the fibre and the matrix, regardless of whether a traditional polymer (thermoplastics or thermosets) matrix, a biodegradable polymer matrix or cement is used. Further this article gives a survey about physical and chemical treatment methods which improve the fibre matrix adhesion, their results and effects on the physical properties of composites. These different treatments change among others the hydrophilic character of the natural fibres, so that moisture effects in the composite are reduced. To bring about hydrophobic properties to natural fibres, a special treatment, termed acetylation, can be used. The effectiveness of this method is strongly influenced by the treatment conditions used. The mechanical and other physical properties of the composite are generally dependent on the fibre content, which also determines the possible amount of coupling agents in the composite. The influence of such treatments by taking into account fibre content on the creep, quasi-static, cyclic dynamic and impact behaviour of natural fibre reinforced plastics are discussed in detail. For special performance requirements, hybrid composites made of natural and conventional fibres can be prepared with desired properties. The processing conditions play, next to the mechanical properties of natural fibres, an important role for the industrial use of these materials. The results presented in this paper show, that natural fibres can be processed with the already commonly applied methods: glass mat thermoplastic matrix (GMT), sheet moulding compound (SMC) or bulk moulding compound (BMC). For the processing of thermoplastics reinforced with natural fibres, new methods (e.g. the “EXPRESS” processing) are of increasing importance. Natural fibres seem to have little resistance towards environmental influences. This can be recognized in the composite and can be advantageously utilized for the development of biological degradable composites with good physical properties.

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Book
26 Mar 2009
Abstract: Image Correlation for Shape, Motion and Deformation Measurements provides a comprehensive overview of data extraction through image analysis. Readers will find and in-depth look into various single- and multi-camera models (2D-DIC and 3D-DIC), two- and three-dimensional computer vision, and volumetric digital image correlation (VDIC). Fundamentals of accurate image matching are described, along with presentations of both new methods for quantitative error estimates in correlation-based motion measurements, and the effect of out-of-plane motion on 2D measurements. Thorough appendices offer descriptions of continuum mechanics formulations, methods for local surface strain estimation and non-linear optimization, as well as terminology in statistics and probability. With equal treatment of computer vision fundamentals and techniques for practical applications, this volume is both a reference for academic and industry-based researchers and engineers, as well as a valuable companion text for appropriate vision-based educational offerings.

2,454 citations


Frequently Asked Questions (1)
Q1. What are the contributions in "Adaptive neutron radiography correlation for simultaneous imaging of moisture transport and deformation in hygroscopic materials" ?

A novel adaptive texture correlation algorithm is presented to simultaneously image inhomogeneous moisture distributions and moisture-induced strain fields. The proposed method provides a valuable tool for the study of timeand positiondependent hygromechanical interactions. Its applicability was demonstrated for two actual topics in wood science, the investigation of moisture gradients within the growth ring microstructure and the study of moisture transport processes in wood-fiber composites.