A tensile test determines various tensile properties on a defined specimen under defined environmental conditions. The main objectives of this test are to record a stress-strain diagram, and to determine the tensile strength and strain at break.
The method is used to compare materials and for quality control purposes.
- The specimens for these tests are normally taken from finished parts or a sheet. ZwickRoell offers sheet cutting presses with standardized blades. It is important that the blades are equipped with an ejector to avoid the risk of operator injury during specimen removal.
- The cross-section of the specimen must be determined before the test. The width of the specimen is normally adopted from the dimensions of the cutting die, while the thickness is measured using a defined surface pressing, for example, to ISO 1923. ZwickRoell offers special measuring instruments for this.
- There are a variety of pneumatic or mechanical specimen grips available for gripping the specimen. The specimen must be gripped so that it is held securely. No specimen fractures may occur at the fixing point. ZwickRoell's testing lab is here to help you select a suitable specimen grip and grip inserts.
- To measure the strain at break, ZwickRoell offers mechanical and optical extensometers, which capture the extension of the specimen in a specified gauge length range. These extensometers function largely automatically. When using a mechanical extensometer, you must ensure that the weight and frictional forces of the extensometer remain low in comparison to the measured forces. If optical sensors are used, the specimen may not be damage by the marking.
A tear growth test determines the tear growth of foam. When subjected to tensile load, a stress peak forms at the end of the crack in which material failure occurs. Tear resistance is specified in N/cm in relation to the width.
Trouser and angle specimens are defined by the standard, whereby the trouser specimen is addressed only in the ISO standard. As opposed to rubber, it is loaded to crack opening (mode I).
For trouser specimens, crack prorogation takes place in the longitudinal axis of the specimen over a distance of approximately 25 mm, whereby a large surface is involved. This increases the probability of encountering an area with a closer cellular structure, which is then responsible for a higher maximum force. Since the crack is introduced by an incision, this method evaluates the force required to propagate a crack.
The angle specimen is loaded without an incision. As a result, a stress peak forms, however not before a sharp incision. In this method, the crack is propagated transverse to the longitudinal axis of the specimen.
The results of the trouser specimen are significantly lower and scatter much more as compared to those of the angle specimen. In a round robin test performed to ISO 8067, the degree of reproducibility for trouser specimens are, at 16 to 36%, three times that of angle specimens.
The test method can be performed with an electro mechanical testing machine. Since the forces are relatively low, a load cell that achieves Class 1 measurement accuracy should be used in the force range used. Displacement measurement is not required, since tear resistance is calculated from the maximum force that occurs during the test. In practice, the tear growth is often depicted as a graph that follows the travel of the tightening clamp.
The testXpert III testing software controls the test sequence and evaluates the tear growth to the standard.