| Pages:
1
2 |
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
So, forgetting about comparison with Janka hardness values for now let's finally turn to our mechanically simple (but otherwise not so
straightforward!) 90° cone tester.
The hardness value of this tester is given by 1/D² where D is the measured indent diameter (see previously posted calculations).
So plotting Hardness Value (1/D²) against measured indent diameter (D) gives the attached calibration curve for this tester.
Note that for harder materials accurate measurement of the indent diameter is a lot more critical for harder materials than it is for softer materials
in order to obtain consistent and comparable hardness results.
This may go some way to explaining the wider deviations of the harder woods (like African Ebony) from the ideal straight line curve seen in the #2
trial results previously posted.
This tester is designed to objectively compare the 'before and after' situation of chemically impregnated wood. So the calibration curve may be used
to determine approximate % hardness increases from the indent diameter measurement - as shown in the attached example.
Ready to go!
|
|
|
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
Wood hardness testing by indentation suggests the mechanism that leads to observed string 'wear' on fingerboards - primarily associated with wire
wound strings.
Indentation hardness testers work by stressing the wood fibres (under pressure) beyond their elastic limit (yield point) to produce a permanent
deformation (indent) that can then be measured to give an indication of relative hardness of a wood.
Pressure is defined as force per unit contact area. For a ball or cone shaped tester tip the initial pressure applied is (theoretically) infinitely
high - the contact area being zero. For a given load the tip will sink into the wood until an equilibrium state is established where the pressure
exerted on the wood by the tip (force per surface area of the indent) is balanced by the reacting forces of the wood fibres.
For a low level repetitive (but briefly applied) force, the equilibrium state (dependent upon the size of the force and 'hardness' of the wood) may
not be achieved for many force cycles - perhaps never if a wood is hard enough not to yield significantly under the applied load.
This situation applies to a fingerboard where the finger applies a relatively low force on a string - pressing it in firm contact with the wood
surface. This force - usually of short duration but repeated many times in the course of playing an instrument may be sufficient to permanently and
significantly indent softer woods. This will be particularly noticeable with wire wound strings where there is point contact with the windings and
where the work-hardened windings of copper are significantly harder than the fingerboard wood.
Two quick trials were undertaken to test if simply pressing a wire wound string under finger pressure onto a wood surface might replicate the
indentation 'wear' observed in some instrument fingerboards.
The force usually applied by the fingers of the left hand in playing a stringed instrument is difficult to measure without specialised equipment
(small load cells) so a rough idea was obtained by pressing down with a finger on a piece of string set on a digital scale and judging the force by
feel. I play lutes with relatively low tension strings (2.5 Kg to 3.8 Kg - bass to treble) and judged finger force to be around 1.2 Kg - but the might
be higher than this from time to time dependant on the complexity of a chord shape being held. The force applied would likely vary quite a bit between
individual players and depend to some extent on string tension, higher tension = higher force.
The wood chosen for the trials was a piece of yellow poplar - measured hardness similar to mahogany or black ash with a Janka value of around 1000 lb
force (so relatively soft). The test surface was coated with black Indian ink (to make any indentation more visible). Indian ink contains shellac.
For the first trial a short piece of wound string (0.82 mm outside diameter) was taped to the test piece and pressed into the wood for 100 cycles -
the applied force being held for about a second for each cycle. The test was repeated and in each case a clear impression of the string windings was
observed indented into the surface of the wood (note that no string tension is involved).
For the second trial I used one of my metalworking tools (for convenience) to apply a fixed load of 1.2 Kg to the string. The force was applied
through a draughtsmans eraser to replicate a finger tip. The hinged arm of the metalworking tool was then gently raised and lowered on to the string
for 100 cycles the force again being applied for about a second for each cycle. This trial was repeated twice and again clear string indentations were
observed.
To take this further, a custom designed, motorised test rig would provide more accurate information by testing wood samples of varying hardness over a
greater number of force application cycles. Subject of a future topic.
|
|
|
jdowning
Oud Junkie
Posts: 3485
Registered: 8-2-2006
Location: Ontario, Canada
Member Is Offline
Mood: No Mood
|
|
This paper by Hiroshi Miyajima, Experimental Forestry Dept of Hokkaido University, may be of general interest concerning wood hardness testing.
http://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/20732/1/17%2...
Issued in 1955 it predates by 18 years the initial work by researchers of the US Forestry Products Laboratory investigating a modified Janka hardness
test procedure where depth of penetration of a spherical Janka tip for a specified load is used as a measure of hardness (Hardness Modulus).
The Japanese standard wood hardness measure - like the Janka test - was (is?) a modification of the Brinell hardness test for metals (i.e. a ball
indentation test). As the paper rightly points out microscopic measurement of the indentation diameter cannot be precise (as it can be with metals) on
account of distortions of the boundaries of the indent due to the anisotropic nature of wood - a difficulty previously observed in this thread. The
solution was to measure indent depth under different loading conditions.
The research paper - although tests on only one species of wood were undertaken including end grain measurements - makes useful comparisons with other
properties of the wood.
This confirms that the conical tip hardness tester - subject of this topic - where average indent diameters are measured to determine a relative wood
hardness number, cannot be a consistently precise method (by laboratory standards). However, it should be good enough, as a low cost simple tool, for
comparative 'before and after' hardness testing of chemically treated wood - which is its purpose.
|
|
|
| Pages:
1
2 |
![[*]](images/xmbprobrown/default_icon.gif){kind=icon}
