Note: Descriptions are shown in the official language in which they were submitted.
~L~763~3~
PENETRATION BODY FOR IMPACT STRENGTH ~5EASUREMENT OF WOOD.
The appllcation relates to a method of measuring the strength of
a body, such as a wooden material, and an apparatus for perfor-
mance of the method.
Various methods for testing the strength of a body, such as a
wooden material, by mechanical stressing of the material, are
known. A pointed object can be pressed into it with a constant
power and the penetration be judgedr Furthermore drill samples
can be taken, the compressive strength of which can be then mea-
sured. These two methods, however, have proved to be too unde-
pendable. Furthermore the rigidity of the material can be mea-
sured, where practically feasible, by exposing the material to
a given load and the strength be determined on a basis of de-
flection. This method is, however, very difficult to carry out
in practice.
Finally the impact strength of the material can be determined.
This method is the preferred one, as the impact strength has
proved to be an indicator of the mechanical properties of the
material, which i.a. depends on the degree of disintegration
of the material and its density. The impact strength thus
largely depends on the general condition of the material
Disintegration by biological or baterial means e.g. means a
marked fall of the impact strength. Furthermore the impact
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strength strongly depends on the density of the material. And
finally the impact strength is practically independent on the
contents of water of the material.
Hitherto known measurings of the impact strength have been made in
laboratories on small prisms, taken from the material and placed
in a special testing machine, in which the prism is exposed to a
combined dynamical bending and tensile stress. This method is
often inexpedient as it partly demands that one or several samples
are taken, and paxtly that the measuring must be made on a machine
in a laboratory. Furthermore it weakens the material and it is a
costly and time-consuming method.
An object of the present invention is to provide an improved
method for the non-destructive testing of wood to determine the
impact strength thereof essentially independently of the moisture
content. The method involves driving a piercing plug having a
blunt head into a body of wood substantially transverse to the
grain thereo~ by imparting to said piercing plug a predetermined
amount of kinetic energy to cause the plug to penetrate the
wood and rupture fibers thereof to produce surfaces of frac-
ture as a result of dynamic bending stress and dynamic tensile
stress of the wood fibers, and determining the extent of pene-
tration of the plug into the wood, whereby said extent of
penetration is proportional to the impact strength of the wood.
An advantage of the method of the invention is that the testing
can be conducted in situ. Furthermore a direct determination
of the impact strength of the wood immediately after the im-
pact is achieved, as the penetration of the piercing plug
depends on all the factors, which are of importance to the
mechanical properties of the material. There is, therefore,
a direct connection between the strength and the degree of
penetration. As the penetration of the piercing plug can be
read off directly after the driving in, is in addition achieved,
that the operators can make supplementary measurings, where zones
appear, which may require a further examination. Furthermore, the
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material is not spoiled by this method as no samples ha~e tD be taken.
The method may be performed using an impac~ hammer, which has
the advantage that the testing operation can be performed in-
dependently of the operators, as the impact hammer contains
a well defined kinetic ener~y, which is uniform at each impact.
The impact hammer may have a drive arrangement which is simple
and steady, and may include a trigger which is operable in re-
sponse to a given pressure on the impact hammer. This enables
a constant pre-pressure to be achieved for the driving in be-
fore the impact. This will secure uniform conditions o~ the
driving in.
To be able to read off the penetration o the piercing plug into
the material it is suitable to the purpose to provide the casing
with an opening opposite to the inertia body and to place a scale
in the opening so that the penetration depth and with that, the
strength can be determined.
To facilitate the determination of the strength of e.g. a burried
post the ront end of the impact hammer casing may be obliquely
cut, so that testings of the strength of the post in the area
immediately under the surface of the ground can be made without
having first to remove laxge quantities of earth.
The invention will be explained in further details below with
reference to the drawings in which:
Fig. 1 shows an impact hammer in side elevation in section;
Fig. 2 shows the impact hammex in its advanced position; and,
Fig. 3 shows the impact hammer in process of being driven into a
wooden material.
The apparatus for use in performance of the method can be designed
as shown in Fig. 1 and 2. It consists of an impact hammer 4 which
has uttermost a casing 6, which in front is provided with a front
piece 9 and to the rear with a trigger arrangement. The casing 6
is suitably shaped like a cylindrical pipe, into which the impact
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arrangement can be built in. This impact arrangement consists
of an inertia body 5 which in front is provided with a re-
tainin~ member, to which a penetration body or piercing plug
2 can be attached. The forward motion of the inertia body
5 causes that the piercing plug 2 is led out through an open-
ing in the front piece 9, max. until the retaining member
rests against the inner side of the front piece, as shown in
fig. 2.
To the front of the inertia body 5 is attached a tension
spring 7, the opposite end of which is attached to the front
end of the casing at the front piece 9. In the compressed
position of the spring 7, i.e. in its relaxed position, as
shown in fig. 2, the piercing plug 2 is most advanced. By
ca~rying the inertia body 5 back into the casing 6, the
spring 7 is loaded. In order to keep the impact arrangement
in its loaded position, as shown in fig. 1, the inertia body
is at the back provided with a retainer pin 16, with a head,
which can be caught by a rocker arm 15 at its one end. The
rocker arm 15, which turns round a pin in the casing, is
spring loaded for introduction into the path of the retainer
pin by means of a compression spring 14 which loads the
other end of the arm so that the inertia body 5 is kept by the
rocker arm 15.
Release of the impact hammer takes place by compressive
stress of the rocker arm 15 against the spring 14. This
stress takes place on a trigger part 13 forming the rear end
of the tool 4. This trigger part is mounted axially dis-
placeable in the casing 6 against the power of one Or several
compression springs 17. When the part 13 is pressed in-
wardly, the spring power from the springs 17 and 14 must be
overcome before the rocker arm 15 swings out of its mesh
with the pin 16 thereby releasing the inertia body which is
driven forward by the spring 7 and thereby leads the piercing
plug 2 out of the casing 6.
The piercing plug 2 is in the shown example a cylindrical
body with a blunt front end 3, as shown in fig. 2. During
its penetration surfaces of fracture are produced in the
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~L~763~3~
material, for which reason the size of the piercing plug and
the shape of the front end can be found out by way of experi-
ments, dependent on the use of the tool.
The casing is provided with an opening 8, as shown in fig. 1.
This opening is located in such a n~anner that the penetra-
tion of the ~iercing plug into the material can be read off.
The opening can suitably be provided with a scale lo, which
scale can be replaceable dependent on the use of the tool
and the nature of the material.
The retaining member can be stopped at the front end 9 of the
tool, by means of an insertable pin 12, which can be manually
inserted into the member. Hereby it is partly prevented from
turning itself by replacement of the piercing plug by turning
of the bushing which keeps the piercing plug 2 and partly to
keep the member in the advanced position when using the tool
as a awllike tool for informatory testing of the nature of the
surface of the material when inserting the piercing plug into
the material.
The front end 9 has been obliquely cut for formation of an
oblique contact face 11. Two opposite oblique contact faces
11 have been shown and on the parts of the front end 9
having not been cut, one or several pins have been mounted,
as can be seen in fig. 2. These pins facilitate the inser-
tion work, as they can get into the material and keep it
during the triggering. If it is desired to drive in, in an
inclined direction, i.e. not at right angles to the surface
of the material, the contact faces are used for guiding the
tool. This is particularly suitable to the purpose when
exàmining buried piles, as hereby is avoided to have to re-
move unnecessarily large quanties of earth.
To describe the method reference is made below to fig. 3,
which shows the tool in use. The trigger is loaded by a
manual pressure under overcoming of the said spring powers.
This spring load is of a definite size in order to secure
that the front end 9 of the tool possesses a f inely tuned
starting pressure against the material 1. This is a
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condition for comparability of the readings. It ensures that
the piercin~ plug is always released at the correct point of
time, viz. when the spring load has been overcome. During
the penetration of the piercing plug into the material sur-
faces of fracture are produced in the material dependent
on its condition and nature. As the degree of penetration
depends on all the factors being of importance to the
mechanical properties of the material the penetration is,
therefore, a measure of the strength of the material. This
strength can be read off directly after the impact, by
observing the position of the inertia body through the open-
ing 8 in the tool. In determining now the proportion by a
given penetration and the strength of a body by way of experi-
ment and mark it out on the scale, the tool can be used as a
strength measurer.
The strength of a material, e.g. a wooden material can be
stressed by many sorts of disintegration. It is consequently
extraordinarily important, that people in situ and immediately
after the driving in can determine whether the material
possesses the necessary strength. This applies to wiring
work in wooden masts, foundations, pile structures, etc. These
measurings can be made by people without any special qualifi-
cations, and as the material is not interfered with, which
could possibly weaken it, the determination of the strength
is enormously suitable to the purpose.
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