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Determination of the penetration depth of the blade in
stab protection measurements according to VPAM
Andrea Ehrmann, Susanne Aumann, Andrea Brücken, Marcus
O. Weber, Faculty Textile and Clothing Technology,
Niederrhein University of Applied Sciences,
Mönchengladbach, Germany
Thorsten Bache, Bache Innovative, Rheinberg, Germany
The stab protection effect of a textile can be examined
according to different standards or test instructions. In
the VPAM test instruction “Stich- und Schlagschutz”
(“stab and impact protection”), e.g., which is widely used
in German-speaking countries, the textile is placed upon
a box filled with plasticine, in which the penetration depth
of a defined blade is measured after the test. Since the
evaluation of this test has shown to be error-prone, an
alternative evaluation method is suggested in this article.
Evaluation of stab resistance measurements according to
VPAM
According to the test instruction “Stich- und Schlagschutz“ of
the VPAM (Vereinigung der Prüfstellen für angriffshemmende
Materialien und Konstruktionen – union of testing laboratories
for attack-blocking materials and constructions), stab resistant
textiles are tested by letting a blade with a defined weight fall
from a defined height through the test sample into a box with
ballistic plasticine (opposite to the British HOSDB using foam
rubber under the textile). For the evaluation of the penetration
depth, the VPAM test instruction only describes: “After each
test, the penetration depth in the plasticine has to be
measured.” More detailed instructions, how this measurement
has to be performed, are not available here.
Own experience as well as discussions with companies
performing tests according to the VPAM test instruction have
shown that for this purpose the penetration channel has to be
excavated piece by piece. Other depth measurement
methods, e.g. by introduction of a fine, long, stiff caliper,
cannot be used due to the easy deformability of the plasticine;
filling the channel with a hardening material would be too time-
consuming. Excavation of the penetration channel, however,
has shown to be complicated, protracted and error-prone due
to the mechanical impact on the plasticine near the
penetration channel.
Fig. 1: The test blade is ground on one side only (here at the
right bottom part of the picture) and chamfered on both sides;
this beveling stops at different heights on both sides.
Correlation between penetration depth and cutting width
Since the test blade has a defined form (Fig. 1), each
penetration depth (up to a depth of 50.75 mm, where the point
of the blade transitions into the straight stem) corresponds to
a defined cutting width.
Thus, we have examined whether penetration depth and
cutting width are uniquely correlated. After verifying this
assumption, we have tested the standard deviations on same
test conditions. For this, for 4, 6, and 10 layers of an aramide
nonwoven, the penetration depth and cutting width have been
measured ten times each. Comparing the values measured on
identical conditions shows that the deviations of both values
are not correlated, i.e. a larger penetration depth is not
necessarily correlated with a larger cutting width. Apparently,
these deviations can be attributed to evaluation inaccuracies,
instead of erroneous changes in the measurement situation,
e.g. by thickness variations of the nonwovens.
Accuracy of both measurement methods
Thus, the standard deviations of the experimental values of
penetration depth and cutting width can be used as a measure
of the accuracy of the respective method. As shown in Fig. 2
(left panel), the relative standard deviation, related to the
mean value of the measurements, is always larger for the
penetration depth than for the cutting width. This means that
measurements of the cutting width give more reliable results
than evaluations of the penetration depths.
This finding can be attributed to the technical difficulties in
excavating the penetration channel. Although the penetration
depth always takes larger values than the cutting width which
should thus be easier to evaluate, this advantage is
apparently compensated by experimental error sources.
Fig. 2 (right panel) shows the same results as absolute
values. Here it becomes visible that measuring the penetration
depth is more and more inaccurate for more textile layers, i.e.
smaller penetration depths, while measuring the cutting width
always results in similar errors.
Fig. 2: Relative (left panel) and absolute (right panel) standard
deviations of cutting widths and penetration depths for 4, 6
and 10 layers of an aramide nonwoven.
Conclusion
The depicted measurement results show that especially in
comparative studies of new stab resistance materials
according to the VPAM test instruction, more reliable results
can be afforded if the cutting width is measured instead of the
penetration depth. Additionally, measurements of the cutting
width are much faster than evaluations of the penetration
depth, allowing for more measurements in the same time
period, which enhances the statistical significance of the
results.
Therefore, the cutting width should at least be measured
additionally to the penetration depth to enhance the reliability
of the test.