Civil Engineering Reference
In-Depth Information
is mechanical characteristics is possible only
through a biaxial test. A biaxial rig is a testing machine able to pull both warp and
An adequate analysis of the fabric
'
fill directions at the same time. It should hold the sample without causing any
damage to the fabric and allow elongation in both directions in order to create a
central area, characterised by a uniform stress state, to measure the elongation.
As already mentioned, the absence of international standards or designing codes
referring to biaxial procedures led to the proliferation of several approaches for the
mechanical characterization of fabrics and foils. As described in Sect.
2.2
, the
possible application of technical fabrics and foils is extremely wide, this paragraph
is mainly focused on membrane materials for architectural application, and thus, the
testing apparatus and procedures here considered are strictly related to this area of
application.
3.2.1 Uniaxial Tests
In the absence of standards on biaxial testing, the mechanical behaviour of coated
fabrics is mostly based on uniaxial tests. The most common data provided in the
product data sheet are the tensile strength, the tear resistance and the adherence of
the coating. The basic principle is that the test piece is extended at a constant rate of
force or extension until it breaks measuring the properties of resistance and elon-
gation. Uniaxial tests for fabrics are very similar to those carried out for other
materials, such as steel, with the only exception that the stress-strain curve is not
linear and is generally elastic only after several cycles of loading and unloading.
Because of the structure of the fabric and its orthotropic behaviour, uniaxial tests
only roughly approximate the real conditions in the
field and remain only an
approximation of how an architectural fabric works.
By means of uniaxial tests it is possible to determine the maximum force and
elongation recorded in extending the test piece to breaking point, the force and the
elongation at break. In addition, with similar equipment and using different sample
geometries it is possible to determine the tear strength, the tear force required to
propagate a tear initiated under the speci
ed conditions and the coating adhesion
strength (EN ISO 2411:
2000
).
The uniaxial tests are mainly based on the strip method (EN ISO 1421:
2000
) (EN
ISO 13934-1:
1999
), the grab method (EN ISO 13934-2:
2000
), the trapezoidal
method (EN 1875-3:
1997
), the single tear method (trouser-shaped and wing-shaped
test specimens) (EN ISO 13937-2:
2000
) (EN ISO 13937-3:
2000
) (DIN 53363:
2003
)
(EN ISO 4674-1:
2003
) and the double tear test (tongue-shaped test specimens) (EN
ISO 13937-4:
2000
). The ballistic pendulum method (EN ISO 13937-1:
2000
)
(EN ISO 4674-2:
1998
) is an alternative to uniaxial tests for the investigation of the
tear resistance.
