Innovative Testing Solutions for Static and Dynamic Tests on Pre-Stressing Steel and Stranded Wires

Pre-stressing steel is a high-strength steel that is primarily used for pretensioning in pre-stressed concrete construction. The steel may take one of three different forms: bar steel (15 to 36 mm), wire (5 to 16 mm), or strands made of either three or seven individual wires twisted around each other (the nominal diameter of the wires for three-wire strand is 5.2 to 7.5 mm, for seven-wire strand the diameter is 7 to 18 mm). These various forms place high demands on materials testing. ZwickRoell offers a complete portfolio for static and dynamic testing of pre-stressing steel for testing stranded wires in particular.

Dynamic tests on stranded wires Manufacturers are required by product standards to perform these tests. These tests are often outsourced to a service provider in order to save equipment costs. Additionally, the manufacturers are externally monitored by independent institutes.

During fatigue tests (e.g. according to ISO 15630-3, XP A 35-045-2011, FprEN 10138-3:2009) on wire strands, the specimen must endure two million test cycles at a frequency at max. 20 Hz without breaking. If the specimen breaks nearby or during gripping, the test is invalid and must be repeated. The test lasts several days, therefore making invalid tests very costly. Since stranded wires are extremely stiff, they are also very sensitive to notches. The gripping force causes a load to be superimposed on the dynamic test load in the clamping area, which can lead to premature specimen failure. Therefore, for classic tensile specimens made of solid material, the ends often have larger cross-sections to ensure that the specimen breaks in the free portion between the specimen grips. With wire strands it is not so simple. To obtain correct test results, extensive specimen preparation, such as sealing, is required.

With specimen grips developed specifically for wire strand testing, ZwickRoell offers the option to test strands without having to use specially attached gripping elements. This development substantially reduces the required handling, time, and expense for the customer. The grips feature hydro-mechanical and straight mechanical clamping. The latter comprises a conical sleeve into which each end of the strand is fed together with a counter-cone and clamped. Most of the gripping force is applied in this manner.

The remaining portion still needed for the respective test is supplied by classical hydro-mechanical clamping via jaws. The combination of these two gripping principles enables the gripping force to be applied in such a manner that jaw breaks are eliminated and wire strands can be tested without being pretreated. The specimen grips (Fmax 500 kN) are extremely flexible and can be used for a wide range of applications. The variably adjustable hydraulic gripping force can be easily changed from test to test by regulating the oil pressure accordingly. Tests are typically performed with a servohydraulic testing machine from the HA series. Static tensile tests on stranded wires The primary quality assurance checks include tensile testing with strain measurement, for which ZwickRoell offers a tensile testing machine for loads up to 600 kN as a standard configuration, in accordance with established standards (ISO 6892-1, ISO 15630, ISO 12076). Since stranded wire unwinds under tension, ZwickRoell uses special specimen-grip inserts to prevent premature failure at undesirable points. The testing machine is equipped with monobloc specimen grips with special inserts for stranded wire to ensure secure gripping of the specimen. It can test stranded wires with diameters of 3 to 20 mm during tensile tests. Deflected tensile tests, e.g according to EN ISO 15630-3, can be performed with the testing machine as well. The moving crosshead of the testing machine is provided with a through-hole for these tests. If a stranded wire breaks, it often leads to the deflection of individual wires and as a result there is a risk of damage to a contact extensometer. ZwickRoell therefore employs optical long-travel extensometers for safe, reliable, and accurate measurements on highly extensible and elastic materials. The lightXtens extensometer achieves a resolution of 5 µm over the entire measurement travel of up to 900 mm and satisfies the requirements of Accuracy Class 1 (ISO 9513) from 3mm. Other advantages of this extensometer include automatic identification of gauge marks on the specimen plus the initial gauge length and transmission of all data to the testXpert III software.