THE MECHANICAL
PROPERTIES OF WOOD
Various strength properties of wood in its natural
state,
Tests carried out to determine the strength
properties,
Factors influencing the strength properties
Relation between the strength properties and the
uses of wood.
Strength indicators for stress grading timber
THE MECHANICAL
PROPERTIES OF WOOD
Anisotropic structure of wood
Different strength characteristics in different directions.
The prime strength modes: tension, compression and shear.
Compressive stresses compress the wood; tensile stresses
stretch or pull the wood apart.
Shearing stresses tend to cause two portions of the wood to
slide over each other.
Bending is a secondary strength mode. In practice wood is
often subjected to a combination of the 3 primary stresses.
Applied in directions: longitudinal, radial or tangential.
SERVICE AND LABARATORY TESTS
• Service tests: under actual service conditions
• data available only after a prolonged period,
• external influences which may affect the strength properties are
more difficult to control
• costs are higher as a result of decentralisation + time.
• Laboratory tests : carried out on standard test specimens, or on
structural timber.
• standard test specimens are prepared from defect free timber.
• A reduction (correction) factor must be applied if the results are
to be used for the calculation of safe working stresses for
structural timber.
• When structural timbers are tested, representative samples are
used, so that the conditions are closer to actual service
conditions.
• Standard strength tests have already been carried out on most
timbers in common use, and in spite of their limitations the data
obtained is of considerable practical value.
DEFINITIONS
• Strength refers to its ability to resist external forces which
tend to change its shape or size.
• The factors which change an object’s shape or size are
known as stresses and (caused by a force)
• the physical change in shape or size expressed as strain
(deformation).
• Stress is measured as force / area, expressed on SI units as
N/m2 or Pa. Often as kN/m2 (kPa), MN/m2 (MPa)
• Physical change is usually measured in millimetres, and is
expressed as strain by change/ original length unit
– A dimensionless figure: mm/mm.
• See examples
• Every stress causes a strain (a
change in dimension).
• Stress slight => strain is usually
also slight
• If stress is released, the wood will
return to it's original shape, either
partly or completely, depending
on its elasticity.
• Up to a certain point, the strain is
directly proportional to the stress
=> the proportional limit.
• When the proportional limit is
exceeded the strain increases
faster than the stress.
• When further stress will cause the
wood to take on a permanent set
=> elastic limit (often = prop limit)
• Loads are normally determined at
the proportional limit.
• If stress > fibre strength, failure
will occur. This load = maximum
or breaking load = MOR.
Flexibility
• The ability to bend freely, and to regain it's
original shape.
• Stiffness indicates wood's ability to withstand
deformation, and is applicable only within the
proportional limit. It is a convenient way of
expressing the stiffness or flexibility of wood:
the higher the modulus of elasticity, the more
stiff the wood, and the lower the modulus of
elasticity, the more flexible the wood. For each
type of stress there is a separate modulus of
elasticity.
Brittle / brash
• The wood shows only slight deformation
before failing. Brashness does not
necessarily denote weakness. Both cast
iron and chalk are brittle, but the stress
required to cause them to fail differs
considerably.
Lack of splints when at a compression wood
failure
Toughness
• In a wood technological sense it indicates
shock resistance, i.e. the wood's ability to
withstand sudden and brief loads which
exceed the proportional limit.
Hardness
• More than one meaning, eg. resistance to
cutting, resistance to wearing, and
resistance to indentation. The two latter
properties are interrelated, but for practical
reasons only resistance to indentation is
determined when the strength tests are
carried out.
DIFFERENT TYPES OF LOADS
• (I) The manner of application;
• (ii) The distribution of the load (in the case
of beams);
• (iii)The nature of the deformation caused
by the load.
ACCORDING TO THE MANNER
OF APPLICATION
• (a) Static loads
Constant loads, or loads which increase very gradually, are known
as static loads. Floor joists are for instance normally subjected
to static loads.
• (b) Sudden loads
The load is applied quickly, and for a limited period, but without
momentum. A short bridge on a level road is for instance
subjected to a sudden load when a vehicle crosses the bridge.
(Should the bridge be situated in a dip however, the load will
also be applied with momentum).
• (c) Shock or Impact loads
The load is applied suddenly with speed, and thus also with
momentum. Tool handles, wheel spokes and handles of bats,
rackets, clubs etc, used in ball games, are examples of objects
which are subjected to shock loads.
ACCORDING TO THE
DISTRIBUTION OF THE LOAD
• (a) Uniform distribution
The load is evenly distributed over the bearing
surface.
• (b) Third point loading
The load is applied at two points, each a third of
the distance between the two points of support.
• (c) Centre loading
The load is applied halfway between the two points
of support.
ACCORDING TO THE NATURE
OF THE DEFORMATION
CAUSED BY THE LOAD
• Anisotropic structure of wood, it has different
strength characteristics in different directions.
• The prime strength modes are tension,
compression and shear which must be qualified
as to the longitudinal, radial or tangential
direction. Bending is a secondary strength mode
which is a composite of the above three modes.
• (a) Tensile load
Tensile loads are equal and opposite, parallel acting forces which
cause tensile stresses, and thus tend to elongate or pull the
wood apart.
• (b) Compression loads
Compression loads are equal and opposite, parallel acting forces
which cause compressive stresses, and thus tend to compress
the wood.
• (c) Shearing loads
Shearing loads cause shearing stresses, and thus tend to cause
one portion of the wood to slide over the other portion adjacent
to it.
• (d) Bending loads
Bending loads tend to bend the wood, and are thus a combination
of tensile, compression and shearing loads. The side of the
beam on which the load is applied, i.e. the side which becomes
concave, is subjected to compression stresses, the opposite
side, i.e. the convex side, is subjected to tensile stresses and
the portion midway between these sides, and is subjected to
shearing stresses.
Bending load
• As long as the proportional limit is not exceeded, the
shortening of the fibres on the concave side, and the
lengthening of the fibres on the convex side, is directly
proportional to the intensity of the load.
• Compressive stresses are most severe on the surface of
the concave side, while tensile stresses are most severe
on the surface of the convex side, and both gradually
decrease towards the inside until a neutral plane is
obtained in the centre.
• Here, i.e. in the middle of the beam's depth, the shearing
stresses are most severe, and they again gradually
decrease in the direction of the concave and convex
sides.
Compression break
Order of strength from weakest
• Tension perpendicular (across) the grain is low due to a weak tensile
strength of the lignin in the middle lamella
• Tension perpendicular to grain in the tangential direction, being in the order
of 3 MPa.
– ray bundles and axial tissue interface: reduced contact area due to intercellular
air pockets and the large number of perforations due to the crossfield pitting.
– reduced contact area between the (less abundant) ray bundles and the
longitudinal cells.
– Due to intercellular air pockets and large number of perforations of the cross-field
pitting.
• Tension perpendicular to grain in the radial direction is only slightly stronger,
but still low due to weak tensile strength of the lignin in the middle lamella
weakness. Contact areas are bigger, but still low.
• Shear in the radial longitudinal plane, also due to the ray tissue, being in the
order of 5 MPa.
• Compression in the longitudinal direction due to cell wall buckling to 50 Mpa.
• Compression in the tangential and radial direction can go up to 80 MPa
• Bending across longitudinal direction can go up to 100 MPa.
• Tension parallel to the longitudinal axis can go higher as 100 MPa
SAFE LOADS
• Any load which can applied to the object
with the desired degree of safety, is a safe
load. Safe loads are always less than the
load at the proportional limit (except
sometimes in the case of compression
loads), and are usually calculated as a
portion of the ultimate breaking load.
• SF = break load / safe load
• Before breaking, a beam will obviously bend (much,
if the wood has a low modulus of elasticity, and less
if the depth of the beam is great).
• The stiffness of a beam is also adversely affected
by prolonged loading, and the same load will cause
such a beam to bend more than a beam which is
loaded briefly.
• Where bending is undesired, a greater safety factor
than the one prescribed by the strength
requirements will have to be applied.
• Greater stiffness may sometimes be obtained
without incurring additional cost by using larger
(and thus deeper) beams of a cheaper grade or
species of wood.
STRENGTH TESTS
• The moisture content of dry samples must be as near as
possible to 12 %, (air dried and then kept in a constant
moisture chamber under conditions (temperature 21.1˚C,
relative humidity 66 %) which represent an equilibrium
moisture content of 12 %.
• When EMC has more or less been reached, the samples
are finally planed to the desired dimensions, and placed
in the constant moisture chamber again for a period of
approximately three weeks, after which period they are
ready for testing.
Strength tests - SEE TEXT
• Static bending -; (https://www.youtube.com/watch?v=zijlwXjhkH0),
– Modulus of rupture - Fibre stress at failure
– Fibre stress at the proportional limit
– Modulus of Elasticity (MoE)
•
•
•
•
•
•
•
Compression parallel to grain (crush strength)
Compression perpendicular to grain;
Hardness;
Shearing strength parallel to grain;
Toughness. (Shock resistance)
Tension parallel and perpendicular to grain
Cleavage
modulus of rupture
• the maximum fibre stress in the top and bottom
surface fibres at maximum load, i.e. breaking load
• For centre loading
MoR =
1.5 P1
bh
2
P = breaking load in kN
l = span in metre
b = breath in metre
h = height (depth) in metre
The Young's Modulus of Elasticity (MoE) can easily be
determined by measuring the deflection in the centre point
loading by the formula:
• MoE =
PL3
4dwh 3
MPa.
• P= Centre point force (N)
L= Span between supports (mm)
w= horizontal width of beam (mm)
h= vertical height of beam (mm)
d= deflection at centre point (mm), under load P.
• The MoE of SA pine structural timber can vary between
3000 and 25 000 MPa depending on its stress grade.
Destructive testing
Bending test
Tensile test
STRENGTH PROPERTIES VARIOUS
TIMBER SPECIES, AT 12 % MC
SPECIES
SOUTH AFRICA
Pinus patula
Pinus teada
Pinus radiata
Pinus canariensis
MOR
(kN/m2)
MOE
(MN/m2)
COMPRES
II TO GRAIN
65 800
72 600
84 700
129 500
10 900
9 200
12 900
19 300
28 600
35 900
37 000
65 000
CANADA & USA
Pseudotsuga menziesii (Douglas Fir)
Coast and Mountain
Tsuga heterophylla (West Hemlock)
80 700
66 200
76 500
13 200
9 700
11 700
51 200
41 800
44 100
SY PINE
Pinus rigida (Pitch pine)
Pinus palustris (Long leaf Pitch pine)
Pinus taeda (Loblolly pine)
Pinus elliottii (Slash pine)
74 500
101 400
88 300
109 600
9 900
13 700
12 400
14 200
41 000
58 200
48 800
62 700
NORTHERN EUROPE
Pinus sylvestris (European redwood)
77 200
8 900
43 900
FACTORS INFLUENCING THE
STRENTH PROPERTIES OF
LOADS
• Defects (knots, reaction wood, juvenile
wood, cross grain, splits and shakes, decay,
insect holes, warp; compression breaks,
defect size)
• Density
• MC
• Temperature
• Preservation
• Seasoning
• Duration of the Load.
A. DEFECTS
1.
2.
3.
4.
5.
6.
7.
8.
9.
Knots
Splits and ring shakes
Cross grain and diagonal grain
Decay
Compression wood
Insect holes
Pitch pockets
Warping
Compression breaks
1. Knots
• Branches
surrounded by the
growing stem.
• Alive branch: tight
knot is formed
• Dead branch: no
connection
=>
loose knot
• Both knots cause
localised
cross
grain.
Sound / live / tight knot
Loose knot
1. Knots
• Both knots cause
localised
cross
grain.
• From previously:
• Tension perpendicular
(across) the grain is low
due to a weak tensile
strength of the lignin in
the middle lamella… as
low as 3 Mpa in
tangential direction.
Radial little bigger (3-4
MPa).
• Tension parallel to the
longitudinal axis can go
higher as 100 MPa
Knots influence on strength
modes:1. Tensile strength: Parallel to the grain
decrease due to cross grain
2. Compression strength: Small knots
usually little influence, but larger knot
holes weaken considerably.
3. Shearing strength: Not great effect
4. Bending strength: Mainly on the tensile,
side - degree of weakening depends
mainly on the location of the knot in the
beam.
The magnitude of weakening
caused by knots depend on:
• a) Relative size in comparison with the dimensions of the
wood in which they occur.
• b) Location: Weakening are biggest if in the centre of the
convex side, i.e. on the side which is subjected to tensile
tresses; less if they are situated in the centre of the concave
side, (compressive stresses); and least if they are situated
in the centre or neutral plane, i.e. the portion which is
subjected to maximum shearing stresses, or at the ends of
the beam.
• c) Nature: Sound, tight knots weaken the wood less than
dead, loose knots, particularly as regards tensile stresses.
• d) Distribution: If the knots occur in the form of a knot
cluster, their weakening effect is obviously concentrated,
and thus more severe.
Size of knot
Knot cluster
Determination of Knot
Circumference Ratio (KCR)
• KCR is determined by measuring all the
knot diameters at right angles to the axial
direction of the board over the worst
150mm length of board, adding them and
dividing them by the total circumference of
the board
Knot Circumference Ratio
KCR = (a+b+c+d+e) / 2 (t+w)
• KCR is determined by measuring all the knot diameters at right
angles to the axial direction of the board over the worst 150mm
length of board, adding them and dividing them by the total
circumference of the board
KCR
• The strengthening effect of density
becomes completely overshadowed when
knots reach sizes above 0,25 Knot
Circumference Ratio (KCR) .
5’th percentile bending strength response
surface for 38 x 114 mm SA Pine
2. Splits and ring shakes
1. Separations of the wood fibres in the
direction of the grain, and occur mainly as
seasoning defects.
2. Effect of splits and ring shakes on strength
1. Tensile strength: very great if it is applied
perpendicular to the grain.
2. Compression strength: only slightly
diminished,
3. Shearing strength: considerably with plane.
4. Bending strength: Largely dependant on the
closeness of the split or separation to the
neutral plane,
Ring shake (cup shake, arc shake)
DEFECTS:
Cross
grain
Different grain
types in wood
DEFECTS: Cross grain
• Deviation from the longitudinal axis of the wood in the direction
of the grain.
– Spirality andDiagonal grain
• The safety factor usually allows for some degree of X grain
• Deviation of 1:25 reduces the shock resistance of wood by as
much as 9 %, (tool handles, sporting goods etc)
• Bending strength reduced by about 4 % if the inclination is 1:25,
and by approximately 45 % if the inclination is 1:5. Stiffness is
also reduced, but to a lesser extent.
• Cross grain is not always easily recognizable on the wood's
surface. In soft wood species the annual rings give a clear
indication of the presence of diagonal grain, but this is
frequently not so in the case of hardwoods. Spiral grain is
usually difficult to recognize.
Grain deviation
Fig.12.1: Some grain orientations
(Haygreen & Bowyer)
Defects: Decay
• Fungi destroy the wood substance, thus
weakening the wood.
• Decay diminishes the shock resistance of wood
at an early stage, and as it is difficult to
determine the degree of damage, wood showing
signs of decay should not be used where
strength in any form is essential.
• Blue stain fungi hardly weaken wood at all, but
their presence indicates suitable conditions for
the development of more harmful fungi.
Decay
Defects: Reaction wood
• See 11.1 “Reaction wood”
• The higher density of compression wood is
not accompanied by increased strength
properties, and particularly bending
strength and toughness (or shock
resistance) usually suffers.
Fig.: 10.1: Compression wood in Pinus radiata showing rounded tracheids with helical cavities in
their cell walls. Note the intercellular spaces. (Enlarged by 3500 times). (Butterfield & Meylan, p45)
Fig.10.2: Normal poplar in cross section (x 800). (Haygreen
& Bowyer, p.115)
Fig.10.3: Tension wood fibres and a vessel in xylem of poplar
(x1600) (Haygreen & Bowyer, p115)
Fig 10.4: Fuzzy (or wooly) grain develops when cutting through tension
wood. (Haygreen & Bowyer, p.113)
Defects: Insect holes
• Odd insect holes have little influence on
the strength of wood. Numerous holes
have the same sort of effect as knots,
except that cross grain does not play a
part. The reduction in strength can be
severe without it being evident on the
surface of the wood.
Insect holes
Defects: Pitch pockets
• Pitch pockets are clearly defined pockets
or gaps which occur parallel to the annual
rings. The pockets are filled with resin,
and develop only in certain softwoods,
including pine species. The pitch pockets
have more or less the same effect on the
strength properties as splits and ring
shakes.
Pitch pocket – like shakes
Defects: Warping
• All forms of warping lower the strength
properties of wood, for the load does not
then act at right angles or parallel to the
grain. The weakening effect is particularly
severe when a long pole or pillar is
subjected to a compression load parallel to
the grain. A deflection of as little as
1:1.000 can then reduce the strength by
20 %.
deviation flatwise due to T vs. R
shrinkage differences – pressure will
keep them flat
deviation along the fiber due to
juvenile or reaction wood shrinkage
differences – impossible to control
same as crook
control with pressure
due to spiral, wavy or diagonal
grain - control with pressure
due to R & T shrinkage differences
in square timbers
Defects: Compression breaks
• Compression breaks often occur in
hardwood species, and are particularly
troublesome in Euc. saligna. The theory is
that stresses which occur naturally in the
tree, cause the heart portion to be
subjected to compression stresses, which
then lead to the development of these
breaks. They are sometimes invisible, and
cause the wood to break suddenly and
bluntly when loaded.
Compression break
The effect of defect size on timber
strength
The effect of defect size on timber strength
All strength modes decrease with increase in defect size,
compression decreases at a slower rate (Fig 13.5).
The strength distribution curve of structural timber (SA Pine) is
strongly influenced by silvicultural and sawmilling policies
DENSITY (not a defect)
B. DENSITY
1. The density is the best single indication of strength,
2. The relation is however disturbed by factors:
1. The presence of non-woody substances, such as resin,
gum or infiltrates, which increase the density of wood
without improving it's strength properties.
2. Possible weak planes in the wood, e.g. as caused by wide
modullary rays, or by the large, thin walled spring wood
cells of some species.
3. The presence of abnormal wood cells, such as those of
compression wood. (The cells of compression wood have
a higher percentage lignin, and a lower percentage
cellulose than normal wood cells).
DENSITY
•
•
•
The density is the best single indication of strength
Cell wall material has a density of ± 1500 kg/m3
MoR. (in lbs./sq in) = 28 000 x SG x 1.25
=
The following factors have an effect
on the density
a Natural variations
b Location of the wood in the tree
– i)
– ii)
Vertical:
Horizontal
c Percentage summerwood
d Rate of Growth
certain optimum rate. Variable
e Site factors
f Infiltrates etc.
MOISTURE CONTENT
•
•
The strength properties of wood do not alter
appreciably while the wood dries from wet to
fibre saturation point. Once further drying
takes place, the strength properties begin to
increase (with the exception of flexibility, and
usually also shock resistance).
In practice the samples are dried to a moisture
content of as close to 12% as possible, slight
differences being accounted for by theoretical
adjustment.
http://www.km.fgg.uni-lj.si/coste24/data/CoimbraDocuments/Coimbra-larsen.PDF
TEMPERATURE
•
•
•
•
Exposure to mild temperatures for extended
periods, or to high temperatures for brief
periods, reduces the strength
However so slight that it is generally ignored.
If the heat is accompanied by humid
conditions, or if the wood itself is wet, the
weakening effect is very much greater.
Hot, humid wood is very flexible, and if cooled
and a dried in the bent position, will largely
retain the bent shape.
PRESERVATION
•
•
Very little effect on the strength.
Excessive temperatures and pressures could
permanently weaken the wood.
SEASONING
•
•
•
•
all strength properties of wood increase when it is
dried beyond the fibre saturation point.
Provided that the wood is not damaged during
seasoning, the method of seasoning has little
influence on the strength of the seasoned wood.
As great differences of temperature and humidity
are possible in the case of kiln seasoning, a greater
possibility of defects developing exists, but on the
other hand a capable kiln operator is able to adjust
the seasoning factors to the requirements of the
timber far better than would be the case with air
seasoning.
greater strength, can be obtained with kiln seasoning
due to drying it to alower MC
DURATION OF THE LOAD
•
•
•
As with most other materials, wood is subject to
fatigue, or as it is termed in the case of wood, creep.
If the working load is applied over a long period, the
deflection or bending may, in the case of green
wood, be twice to four times more than when the
wood is subjected to the same load for a short
period only. If the load is applied for a brief moment
only, i.e. in the form of a shock load, wood can
withstand a load that would normally cause it to fail.
Research into "creep" in wood done overseas has
shown that the bending strength of timber loaded
continuously for a 50 year period will be 56 % of the
short-term strength of the timber as determined from
standard laboratory tests
WORKING STRESS
• Basic Stress (characteristic stress): The stress that a defectfree structural member can withstand permanently and safely if
an equilibrium moisture content of 12 % is maintained.
• Working Stress (grade stress): The stress that a structural
member of a particular grade can withstand permanently and
safely. To derive working stresses from basic stresses, a grade
factor (the strength ratio of the particular grade) and a moisture
content correction factor is simply applied. E.g S5.
• Design Stress: The stress that a structural member of a
particular grade can withstand permanently and safely under
particular service and loading conditions (overloading, wind,
duration, sudden applied loads, snow)
• Strength Ratio: The ratio of the strength of a structural member
containing the maximum defects permitted in the grade, to the
strength it would have had if it had been free from defects.
THE DERIVATION OF WORKING
STRESSES
• The average mechanical strength properties of a species are
determined from the tests carried out on defect-free specimens.
• Basic Stress (Charac stress): Average values reduced to allow:
– Variability in the strength of defect-free timber, and
– the effect of duration of loading on the strength of timber.
• Basic stress are reduced to Working Stress (stress grade) with a
strength ratio factor (grade factor )
– Duration of the load
– Safety factor for permissible defects
• Timber used under particular service and loading conditions, the
working stress is reduced by a safety factor to get the design
stress.
• SF = break load / safe load
Characteristic stresses / 2.22 = grade stress
Permissible stress
STRESS GRADING
• The aim of stress grading is to ensure that all
timber of a particular grade complies with certain
minimum strength requirements.
• Stress grading by visual inspection needs
grading rules.
• The working stress allocated to each grade
borders on the low side of the strength ranges
included in that grade, and the strength of a
proportion of the graded material is thus rated
below its actual strength.
• The grade stress allocated to a grade is calculated as
the lower 5th percentile value of that grade's strength
distribution (or basic strength) / 2.22
– 5th percentile means that 95% of the boards are stronger
than the indicated value
– 5% of the boards are weaker than the indicated value
• To determine the 5th percentile, a big representative
sample (200) of each grade is drawn, tested to
destruction, the strength results / 2.22 = grade stress
ranked from lowest to highest and the strength of the
5%th percentile (10th board out of 200) from the
bottom taken.
• Depending on the efficiency of the non-destructive
strength predictor used, the post-grading strength
distributions of the grades overlap to a greater or
lesser extent.
• Stress grading of
timber attempts to
divide the
strength
distribution up
into three or more
grades of
increasing
strength
• The grades are numbered in terms of the stress grade
in bending.
• By definition this is the numerical value of the working
stress in bending (in MPa) that can be safely
sustained, with a 95% confidence limit, by the piece
under long-term loading conditions.
• It is thus simply a measure of the strength – the higher
the number, the greater the strength.
• Bending strength is chosen, because it contains
components of the primary stresses: Tension,
compression and shear.
• It is indicated as S5, S7, S10, meaning 95 % of the
timber from the batch will respectively have a greater
strength of 5, 7 or 10 MPa on bending.
• It was previously indicated as V4, V6, V8 and V10 or
M4-M10 or P4 – P10. V = Visual grading, M =
Mechanical grading, P = Proof grading.
SANS1783-1
STRENGTH PREDICTORS
• The strength predictors which have a strong
correlation with strength and which have
successfully been implemented in practical
stress grading systems, have been:
• a) The Visual stress grading based on the KCR
and Density relationship discussed above.
• b) The Modulus of Elasticity (MoE) ("mechanical
"stress grading) method,
• c) Other methods
Visual stress grading
•
•
•
•
Stress grading according to visual inspection
Not very precise, rules are very conservatively.
Based on the KCR and density relationship
KCR should act as primary grading criterion while
density can only be used as a secondary criterion on
the clear and semi clear timber.
Visual grading
biased defect location
Set of grading rules based on
visual inspection of defects, like
knots
Knot Circumference Ratio
KCR = (a+b+c+d+e) / 2 (t+w)
• KCR is determined by measuring all the knot diameters at right
angles to the axial direction of the board over the worst 150mm
length of board, adding them and dividing them by the total
circumference of the board
5’th percentile bending strength response
surface for 38 x 114 mm SA Pine
KCR
ELASTICITY
• Elasticity is not a strength characteristic but is
important in structural design work as deflections can
only be allowed up to a certain limit.
• non-destructive methods lead to the development of
stress grading machines
• each piece of timber can be used at loads closely
related to the timber's actual bearing capacity.
• Takes into account factors such as the natural
variation in the strength of defect free timber, and the
effect of knots on strength.
Mechanical grading
The Young's Modulus of Elasticity (MoE) can easily be
determined by measuring the deflection in the centre point
loading by the formula:
• MoE =
PL3
4dwh 3
MPa.
• P= Centre point force (N)
L= Span between supports (mm)
w= horizontal width of beam (mm)
h= vertical height of beam (mm)
d= deflection at centre point (mm), under load P.
• The MoE of SA pine structural timber can vary between
3000 and 25 000 MPa depending on its stress grade.
Strengths along the length of the board can thus be determined.
For each timber specie, there is a direct relationship between the
MOE and the Modulus of Rupture (MOR). See fig 13.9,
Elasticity in wood is strongly correlated with
microfibril angle in the S2 cell wall layer (Fig 13.8).
Other methods
• Other methods which have been tried, are
1. stress wave
2. vibration methods to determine MOE
(Acoustic Emission method)
3. piezo-electric response.
4. X-rays,
5. Microwaves,
6. Optical scanning
Stress wave
• Board is placed under a gradually
increasing tension proof load while the
energy of the micro-fractures taking place
is accumulated.
• When this accumulated energy reach a
certain predetermined level, the proof load
is released.
• Strong boards take longer to reach this
level than weak boards.
Vibration methods = Acoustic grading
• Energy dissipation properties are measured by the rate of
decrease in free vibration or attenuation (decrease) in stress
waves measurements. The impact of a hammer, or ultrasonic
pulses are sometimes used to generate pulses for these
measurements
• The dynamic stiffness in a the board can be calculated by the of
the speed of sound by measuring the frequency of vibration by
means of a hammer blow at the one end
• Vibration content is detected using a microphone or a laser
vibrometer and fast Fourier transformation is used for
calculation of eigenfrequencies (also called resonance
frequencies or natural frequencies
•
The modulus of elasticity is calculated with the equation (Bacher, 2008):
MoEdyn = ρ·(2·L·f)2 where:
•
MoEdyn is the modulus of elasticity calculated from frequency tests, in MPa;
•
ρ is the density of the test specimen at the time of testing, in kg/m3;
•
L is the length of the test specimen in meters; and
•
f is the frequency of the wave, in Hertz.
Acoustic grading
Determining strength with acoustic vibrations
X-Ray scanners
• Measure a combination of dimensions, density, natural
vibration frequency, knots, micro-fibril angle, moisture
content, and other defects in timber.
• Use cameras, optical lasers, radiation and ultrasound
or frequency measurement units.
• A few Golden Eye scanning systems were introduced
in SA
• Structural timber will be used more sparingly and with
more confidence once stress grading machines are in
common use.
Microtec Goldeneye X-ray scanner
Strength prediction of end-products from
standing trees
1m