Stress relaxation and failure behaviour under swelling and shrinkage
loads in transverse directions
Simo KOPONEN
Licentiate in Technology (Eng.)
Senior Research Scientist
Helsinki University of
Technology
Tekniikantie 6A, Espoo Finland
[email protected]
Jari VIRTA
Licentiate in Technology (Eng.)
Senior Research Scientist
Summary
Stresses and cracking of wood due to restricted swelling and shrinkage have been studied. Better
understanding of the transverse shrinkage and swelling behaviour, relaxation of stresses and
mechano-sorptive creep is needed to avoid cracks and deformations and to increase service life of
wood based materials, wooden structures and claddings boards.
Keywords: Creep, mechano-sorption, moisture, relaxation, shrinkage, stress, swelling, wood
1.
Introduction
The aim is to study wood's moisture technical behaviour; shrinkage and swelling, swelling stresses,
creep, stress relaxation, and cracking due to moisture content changes. Obtained results can be
utilised in the analysis of cupping of cladding boards due to driving rain moisture load and cracking
of wooden surfaces due to drying of wood products.
The modulus of elasticity (MOE) of spruce (density 450 kg/m3) is 690 MPa in the radial and 390
MPa in the tangential direction at dry conditions. The radial MOE is 1.5…2 times the tangential
value. At green state the MOE is reduced 50 %…70 %. According to F.E.Siimes the tangential
MOE at green condition is 130 MPa [2].
The tangential compression strength of spruce is 3.2 MPa. The compression strength of conifers is
50 % higher in the tangential direction than in the radial direction [1]. At green conditions the
compression strength in tangential direction is reduced to 1.1 MPa [2]. The transverse compression
strength and MOE degreases to minimum when the annual ring orientation is 45° to the direction of
stress.
The shrinkage of spruce and pine from green to oven dry is about 4 % in the radial direction and 8
% in the tangential direction. The moisture content at 65 % RH is about 12.5 % and the fibre
saturation point is 24 %…28 %. According to the theory of elasticity, 4 % tangential shrinkage
(drying from green to 12.5 % MC) causes about 16 MPa tensile stress, when specimen’s
deformations are fixed. However, the tangential tensile strength is only 3.5 MPa at 10% moisture
content and 2.2 MPa at green condition. Also in the case of wetting, the theoretical stresses
calculated according to the theory of elasticity are far above transverse compression strength.
The true stresses are less than the theoretical ones due to the creep and damaging of wood.
Shrinkage causes small cracks and swelling local plastic compression. Deformations and creep at
varying environment humidity can be analysed dividing deformations to elastic, plastic, viscous,
mechano-sorptive and shrinkage parts:
'H
'H E 'H plast 'H visc 'H ms 'H shr
(1)
The changes of moisture content of wood cause much faster and larger creep (mechano-sorptive
creep) than pure viscoelastic creep at constant environment conditions. According to HelinskaRaczkowzka, 1979 [3] the creep in the tangential direction at 20 % moisture content is 0.4 %/MPa
during 24 hours at the stress level 1…3 MPa. According to Molinski 1986 [3], the creep at the
moisture cycle 3%….27%…3% is 4…5 %/MPa at the stress level 1…2 MPa. If the deformations
of the specimen are fixed (relaxation test), the total strain is zero and the total stresses can be
expressed as
(2)
'V
'V plast 'V ms 'V shr
2.
Materials and methods
2.1 Materials
Stress relaxation and failure behaviour was studied using small clear specimens. Swelling tests
were performed on spruce (32 specimens, fig. 1.a) and shrinkage cracking tests on pine (45
specimens). The specimen size in the swelling test was 10x15x15 mm3. The cross section of the
shrinkage cracking test specimen was 2x10…15mm2 (fig. 1.c) and the free length between clamps
was 20 mm.
2.2 Swelling test and micro-testing machine
The micro-testing device is suitable for testing small samples in tension and compression loading.
The 5 kN load gauge is assembled on it and the loading heads can be moved accurately with the
stepper motor. Displacements are measured using two inductive gauges. Figure 1.b shows the
experimental device. Before the swelling test, specimen was either oven dried or conditioned to
65% RH. After conditioning or oven drying, specimen was put into a small plastic bag, assembled
to the apparatus, small initial load was applied, and then water was added to immerse specimen.
Swelling stresses were recorded during 24 hours. In the end of the test the wet specimen was
compression loaded to failure.
b)
a)
c)
Fig. 1. a) Swelling test specimens. b) Micro testing apparatus. c) Shrinkage cracking device
with clamped end specimens.
2.3 Shrinkage cracking test
Specimen were first conditioned to initial RH (90%, 85% or 65%) in weather chamber and then
ends were clamped to the grips. Then the specimens were let to dry at lower RH conditions (65%,
50%, 40% or 10%) in the weather chamber until equilibrium moisture content was obtained. Crack
propagation and the number of cracked specimens were observed.
2.4 Test results
The relaxation of swelling stresses of the tangentially fixed specimens are shown in figure 2.a. Due
to the mechano-sorptive creep and stress relaxation maximum stresses obtained in the swelling test
were 60% or less of the ultimate compression strength in wet condition. The specimen direction
(radial, tangential or intermediate) affected strongly swelling stress, the ultimate strength and
relaxation behaviour. Intermediate direction gave the lowest maximum swelling stresses (fig2.b),
residual short-term strength (fig. 3.a) and residual modulus of elasticity values (fig. 3.b).
2
2
36T,oven
42T, oven
41T, oven
40T, oven
39T
4T
35T
10T
Swelling stress [MPa]
1.6
1.4
1.2
1
From RH65% to green
Oven dry to green
1.8
1.6
Swelling stress [MPa]
1.8
0.8
0.6
1.4
1.2
1
0.8
0.6
0.4
0.4
0.2
0.2
0
0
0
2
4
6
8
10 12 14 16 18 20 22 24
0
15
Time [h]
30
45
60
75
90
Orientation [degree]
3
120
2.5
100
2
80
E [MPa]
Compression strength [MPa]
a)
b)
Fig. 2. a) Increase and relaxation of swelling stresses in tangential tests and b) the maximum
swelling stress as a function of the orientation of specimen (0q=tangential, 90q=radial direction)
1.5
1
40
0.5
20
0
0
a)
60
15
30
45
60
Orientation [degree]
75
0
90
0
15
30
45
60
75
90
Orientation [degree]
b)
Fig. 3. a) Residual compressive strength and b) residual MOE after swelling test as a function of
orientation of the specimen (0q=tangential, 90q=radial direction)
1003
The percentages of cracked specimens in shrinkage tests are presented in table 1. Crack propagated
in radial direction starting from the sapwood side of the specimen. Often crack propagation stopped
at the latewood-early wood boarder and there was a side step or initiation of new crack or opening
of a small existing crack caused by lumber drying. In the other cases there was straight crack
propagation indicating the existence of ray cells. 12…14 % degrease of the MC caused cracking in
all of the specimens. There is a risk of cracking when wood dries 8 % or more.
Table 1. The number of cracked specimens in fixed shrinkage tests
Initial RH Final RH
[%]
[%]
65
40
90
65
85
50
65
10
90
40
85
10
3.
Estimated MC Estimated free Number of cracked
change [%]
shrinkage [%] specimens [%]
4.1
1.2
0
7.8
2.3
0
8.3
2.5
25
8.6
2.6
71
11.9
3.6
100
14.3
4.3
100
Discussion and Conclusions
The maximum swelling stress obtained during the 24 hours immersion was the highest in the
tangential direction. The lowest values were obtained in the diagonal direction. The maximum
swelling stress was obtained typically after 1…4 hours water immersion. The maximum was
obtained after 1-2 hours when the specimen was fixed in tangential direction. In the radial specimen
the maximum swelling stress was obtained after 2…4 hours immersion. The stress level at the end
of the 24 hours immersion was typically 10…20 % lower than the maximum swelling stress.
Theoretically swelling stress can be 5…10 MPa in tangential direction. However, due to the
damaging of wood material and mechano-sorptive creep, maximum swelling stresses are limited to
the level of 1 MPa in tangential direction. Reduction in the residual MOE from 130 MPa to 50 MPa
indicates that swelling and high stress level causes compression failures and plastic deformations
and nonlinear material behaviour. However swelling does not cause reduction of the compression
strength of the wet specimen. In tangential specimens swelling from oven dry to green caused
higher swelling stresses than swelling from 65% RH to green. Test results obtained in the swelling
stress tests help to understand deformations and cupping of cladding. Fundamental knowledge on
the material behaviour for the modelling was obtained.
In shrinkage tests cracks propagated in a radial direction, and the initiation point was at the bark
side specimen edge. When drying of wood moisture content was less than 8 % (based on the
average sorption isotherm) no cracking occurred. Drying of 8…9 % caused cracking of 25…70 %
of the specimens. Drying of 12 % or more caused cracking of all the specimens. Results can be
utilised on the analysis of the cracking of wooden surfaces (solid wood and glulam products)
subjected to dry wintertime indoor environment during commissioning the building after building
process completion. During winter time the equilibrium moisture content of wood surface is less
than 7 %. If the initial moisture content of glulam is 15 % or more there is a risk of surface
cracking. It is recommended to produce glulam using lumber which MC is 12 % instead of 15 %
when glulam is used in dry indoor environment. Proper weather protection during construction is
also important.
4.
References
[1]
Kennedy R.K., "Wood in Transverse Compression, Influence of Some Anatomical Variables
and Density on Behavior", Forest Products Journal, Vol 18. No. 3, 1968, pp. 36-40
Siimes F.E., Suomalaisen mäntypuun rakenteellisista ja fysikaalisista ominaisuuksista,
Doctoral dissertation, Finnish University of Technology , Helsinki 1938, 206 pp.
Kangas J., Ranta-Maunus A., Havupuun viruminen syitä vastaan kohtisuorassa suunnassa,
Technical Research Centre of Finland, Research notes 969, Espoo, 1989, 60+15 pp.
[2]
[3]