Prof. J. Burkitt Webb, C.E., now in Europe, writes as follows:
On the invitation of Prof. Spangenberg we visited the
"Versuchsstation," at the Gewerbe-Akademie, where the important
experiments upon materials for engineering purposes are being made.
These tests are of two kinds--trials of strength and trials of
endurance. The first are made by means of very heavy and accurate
machinery, mostly new within the last two or three years; the latter
are the celebrated "Dauer-Versuche," a description of which we will
reserve for another letter.
The main machine, of which there are three or four duplicates at work
at various points in Germany, is housed in a special building in the
interior court of the academy. It consists of heavy iron "ways," some
fifty feet long, accurately planed and secured to a stone foundation,
with a hydraulic pump and scales at one end, and a number of massive
attachments for subjecting the piece of iron or other material to
various kinds of strains. There are also other instruments which
belong to the machine as delicate as it is heavy, and which are used
for adjusting the parts of the apparatus, reading the results of a
test, or making calculations. This machine differs from others in the
way of measuring the force used. It has been the custom to take
the pressure on the liquid in the hydraulic cylinder, as shown by a
manometer, as the basis of calculation. This introduces an inaccuracy,
as part of this is due to the friction on the piston packing, and the
true pressure is less than that shown by this irregular quantity.
To avoid this difficulty a massive lever is introduced between the
hydraulic press and the point where its pressure is applied. One arm
of this lever is one-eighth inch long, and the other five hundred
times as long, so that to measure a pressure of one hundred tons, four
hundred pounds must be placed on the scale pan which hangs from
the end of the long end of the lever. The fulcrum rests against the
piston, and the short end of the lever is connected by heavy
links with the apparatus by means of which the strain is applied.
Technically speaking the fulcra of scales are "knife edges," but to
convey a pressure of one hundred tons and remain free to move, these
edges must be very obtuse, perhaps 160 deg. to 170 deg.; they must be as
long as possible, some fifteen inches, of the best hardened steel,
accurately ground, and must rest against a hardened plate of steel.
Made with the greatest care the sharp edge under such a pressure will
sometimes make a dent in the plate and the scales are clogged. As
it is very difficult to measure the one-eighth inch with accuracy,
another lever is provided with a ratio of one to ten, and with a
short arm long enough to be made of a certain length with but a small
percentage of error. To test the main lever this occupies essentially
the same place as a sample of iron to be stretched; it is loaded with,
say, two hundred pounds, which it multiplies to a ton; this pressure
is then weighed by placing four pounds upon the main scale pan,
and the fulcrum of the main lever is adjusted until the two weights
balance.
The attachments consist of: I. Jaws for holding round, square, and
flat bars to be submitted to tension. II. Arrangements for holding
beams and columns in various ways at their ends, and compressing them
until they are crushed or "buckle." III. Two massive graduated iron
beams, which are placed crosswise on the "ways," and used for twisting
shafts, railroad axles, etc. IV. A face plate, about four feet square,
for holding plates of boiler iron nearly as large by the perimeter,
and crushing in the middle by forcing various shaped pieces against
it. V. Apparatus for bending a beam by crushing an angular piece into
it; and in the same connection, VI. Shears for cutting off bars of
metal and measuring the force required.
In connection with this main machine were some, quite old, which had
been used in the infancy of the subject by a former professor, and a
new special machine for the same purpose as attachment V., and which
seemed to "kink" a piece of railroad iron as if it were only lead. In
this the pressure was obtained by screws.
Among the instruments used for the adjustment of the parts of the
main machine we saw the finest cathetometer we had ever seen. This
instrument, by Breithaupt, in Cassel, has two telescopes, with
micrometer screws with more than one hundred and twenty-five threads
per inch, and scales graduated on glass with more than six hundred and
twenty five divisions to the inch. Another instrument for measuring
the deflection, in two directions at once, of a column under pressure,
has micrometer screws with more than two hundred and fifty threads per
inch. We saw also a planimeter, which not only calculated mechanically
the area of a figure, but gave also its center of gravity, moment
of inertia, etc. We saw also a French calculating machine; the other
apparatus is, we believe, all German. If one is, however, critical, it
will be found in many lines of business that all the fine goods here
are imported, though naturally the Germans are slow to acknowledge it.
We witnessed the experiments on a sample of round iron over an inch
in diameter, and on a piece of iron plate three inches wide by half an
inch thick. It is perhaps needless to say that they seemed to stretch
like putty and to break like thread. The pressure is put on a few
hundred pounds at a time, and the elongation is read by two telescopes
and a scale, which multiply the distance five hundred times. At the
same time the first "elastic limit" is watched for. Before this is
arrived at the piece will return to its original length when the
tension is removed; after this the stretching is in part permanent.
One of the facts brought out is that there are _several_ elastic
limits, in copper seven or eight. The appearance of the surface after
the elastic limit is passed and the iron stretches is peculiar. A wavy
appearance is seen, and longitudinal ridges begin to form, due to the
changes going on in the crystals, by which they adapt themselves to
the increased length. After a further general adaptation of structure
becomes impossible, these appearances culminate in the weakest part.
The apparatus for measuring the increase in length has long since been
removed, and the places where it was attached have been filed smooth
to avoid introducing the weak point artificially. The diameter of this
part now reduces rapidly, and the surface becomes rough and the iron
hot--you can see it stretch. When it has reduced twenty-five or more
per cent. it gives way suddenly with a sharp crack. The percentage of
reduction before breaking is now recorded with the observations on the
elasticity and the breaking strain, and the experiment is at an end.
It suggested itself to see if the work done in pulling the iron
apart was fully accounted for by the heat generated. We could easily
calculate the work up to the point of maximum tension, but after this
the force required was not measured; however, a rough calculation
showed that the iron was as hot as required, or at least that the data
would require to be quite complete if any residual was to be found.
Berlin, May 13, 1880.
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