This grab test is used for geotextile fabrics to determine the breaking load
(grab strength) and elongation
(grab elongation). The grab test is a tensile test
where the central part of the specimen's width is tested in the grips, which establishes the “effective strength” of the fabric. The effective strength is the strength of the material in a specific width, together with the additional strength contributed by adjacent material.
The specimen sizes are cut to 101.6 mm x 203.2 mm (4 in x 8in), with attention to the fabrication machine direction. The longer dimension in the machine direction is not the same as the machine cross-direction. The specimen is loaded onto the grips leaving a 75 mm (3 in) separation between the jaw faces. The pull test speed requirement is 300 mm/min (12 in/min). The required test results are the breaking load and apparent elongation.
Geotextile materials tend to have coatings that make it difficult to test without slippage or jaw face breaks due to over- tightening. We suggest using pneumatic side action grips that provides ease of use, productivity, and enhanced repeatability. Pneumatic action grips allow the user to set a clamping pressure, while manually operated grips depend on the operator's strength (which may not be so repeatable). This grab specifies 25.4 mm x 50.8 mm (1 in x 2 in) jaw faces with the longer dimension being parallel to the direction of the load. The opposing jaw face has to be at least as big and the combination must correspond in the second grip.
We find that gripping pressure and specimen alignment are very important in these tests. Too much gripping pressure can produce premature breaks, while not enough gripping pressure can lead to specimen slippage or breaks at or near the jaw faces. Specimen centering devices are crucial for repeatability and increase productivity during the specimen loading process. Materials testing software helps tremendously with grab tests since the software can correct for preload or pretension once the grips close or for slack correction if the specimen is held too loosely in the grips at the start of the test.