Standardization of textile testing
When a textile material is tested certain things are
expected from the results. Some of these are explicit but other requirements
are implicit. The explicit requirements from the results are either that they
will give an indication of how the material will perform in service or that
they will show that it meets its specification.
The implicit requirement from a test is that it is
reproducible, that is if the same material is tested either at another time, or
by another operator or in a different laboratory the same values will be
obtained. In other words, the test measures some 'true' or correct value of the
property being assessed. If the test results vary from laboratory to laboratory
then the test is not measuring anything real and it is pointless carrying it
out. However, the values that are obtained from testing textile materials are
not expected to be exactly the same, so that appropriate statistical criteria
should be applied to the results to see whether they fall within the accepted
spread of values.
The lack of reproducibility of test results can be due to a
number of causes.
Variation in the
material: Most textile materials are variable, natural fibres having the
most variation in their properties. The variation decreases as the production
progresses from fibres to yarns to fabrics, since the assembly of small
variable units into larger units helps to smooth out the variation in
properties. The problem of variable material can be dealt with by the proper
selection of representative samples and the use of suitable statistical methods
to analyse the results.
Variation caused by
the test method:
It is important that any variations due to the test itself
are kept to the minimum. Variability from this source can be due to a number of
causes:
- The influence of the
operator on the test results. This can be due to differences in adherence
to the test procedures, care in the mounting of specimens, precision in
the adjustment of the machine such as the zero setting and in the taking
of readings
- The influence of specimen
size on the test results, for instance the effect of specimen length on
measured strength.
- The temperature and
humidity conditions under which the test is carried out. A number of
fibres such as wool, viscose and cotton change their properties as the
atmospheric moisture content changes.
- The type and make of
equipment used in the test. For instance pilling tests can be carried out
using a pilling box or on the Martindale abrasion machine. The results
from these two tests are not necessarily comparable
- The conditions under which
the test is carried out such as the speed, pressure or duration of any of
the factors.
It is therefore necessary even within a single organisation
to lay down test procedures that minimise operator variability and set the
conditions of test and the dimensions of the specimen. Very often in such
cases, factors such as temperature, humidity and make of equipment are
determined by what is available.
However, when material is bought or sold outside the factory
there are then two parties to the transaction, both of whom may wish to test
the material. It therefore becomes important in such cases that they both get
the same result from testing the same material. Otherwise disputes would arise
which could not be resolved because each party was essentially testing a
different property.
This requires that any test procedures used by more than one
organisation have to be more carefully specified, including, for instance, the
temperature and humidity levels at which the test takes place. The details in
the procedure have to be sufficient so that equipment from different
manufacturers will produce the same results as one another. This need for
standard written test methods leads to the setting up of national standards for
test procedures so making easier the buying and selling of textiles within that
country. Even so certain large organisations, such as IWS or Marks and Spencer,
have produced their own test procedures to which suppliers have to conform if
they wish to carry the wool mark label or to sell to Marks and Spencer.
Most countries have their own standards organisations for
example: BS (Britain), ASTM (USA) and DIN (Germany) standards. The same
arguments that are used to justify national standards can also be applied to the
need for international standards to assist world-wide trade, hence the
existence of International Organization for Standardization (ISO) test methods
and, within the European Union, the drive to European standards
