ISO 15630-3 / ASTM A416 / ASTM A1061: Testing of Steel Strands
The ISO 15630-3, ASTM A416 and ASTM A1061 test standards provide information on how steel strands must be tested. Manufacturers of steel strands are obligated via product standards to test their products under static and dynamic conditions according to these standards. These tests are often outsourced to a service provider in order to save equipment costs. Additionally, the manufacturers are externally monitored by independent institutes.
Static tensile testing with strain measurement is one of the primary quality assurance tests. For this purpose, ZwickRoell offers a standard configuration tensile testing machine for forces up to 600 kN. Since steel strands unwind under tension, ZwickRoell uses special specimen grip inserts to prevent premature failure at undesirable locations.
In the high cycle fatigue test, the steel strand must withstand two million test cycles at a max. frequency of 20 Hz. Specimen clamping presents a special challenge in this case, because clamping fractures can occur relatively easily.
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Static tensile tests on steel strands to ISO 15630-3 / ASTM A416 / ASTM A1061
In static tensile tests to ISO 15630-3, ASTM A416 and ASTM A1061 the steel strand is elongated until it breaks. Steel strands consist of several wires that are twisted together, which try to untwist when subjected to tensile loads. During the tensile test, the force and elongation of the specimen are measured. Since prestressing steel is a high-strength steel, testing machines for high test loads are primarily used.
Tensile tests can be performed on steel strands with a diameter of 3 to 20 mm. Depending on the product or test standard, the gauge length L0 can, for example be 500 mm or 610 mm. The overall elongation can reach values up to 20%.
Deflected tensile tests, e.g to EN ISO 15630-3, can also be performed. For these tests, the moving crosshead of the testing machine is equipped with a through-hole.
Key characteristic values from tensile testing on steel strands to ISO 15630-3 and ASTM A416
- The yield point as offset yield (Rp0.2), which is generally determined as replacement yield point at 0.2 % plastic elongation
- The yield point elongation; more accurately yield point extensometer elongation, because it can only be determined with the use of an extensometer (Ae)
- Tensile strength (Rm)
- Uniform elongation (Ag)
- (A), whereby the normative specifications with regard to the gauge length are of significant importance
Dynamic tests on prestressing steel strands
In the high cycle fatigue test (e.g. to ISO 15630-3, XP A 35-045-2011, FprEN 10138-3:2009) on steel strands, the specimen must endure two million test cycles at a max. frequency of 20 Hz without breaking.
If the specimen breaks near or in the grip, the test is invalid and must be repeated. Because the fatigue test lasts several days, invalid tests are very costly. Since stranded wires are extremely stiff, they are very sensitive to notching and clamping in the specimen grips can be a tremendous challenge. ZwickRoell has developed a special specimen grip for steel strands, with which high cycle fatigue tests can be carried out without having to use specially attached gripping elements.
High cycle fatigue tests are typically performed with an HA series servohydraulic testing machine.
Specimen clamping in dynamic testing
Due to the high tensile strength of the individual steel strand wires—up to 2000 Mpa—and their smooth surface, it is particularly important to ensure that the specimen is not damaged by clamping (for example by a pyramid pattern on the jaw inserts). Therefore hydraulic, parallel-closing specimen grips with adjustable clamping pressure are required. In addition, the geometry of the jaws should be designed in such a way that reliable and homogeneous gripping of the specimen is possible, so that the specimen does not start slipping during the test.
To avoid specimen breaks outside the gauge length L0, the product quality of the steel strand specimen itself is also important. During a tensile test, only specimen tears/breaks within the gauge length can be accepted as a reliable test result. A damaged specimen surface or strongly inhomogeneous tensile strength of the individual wires often lead to specimen breaks outside L0, near the jaws.
Clamping the specimen in a high cycle fatigue test on steel strands according to ISO 15630-3 represents a particular challenge, as clamping fractures can occur relatively easily. 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 ungripped section between the specimen grips. With wire strands this is not so simple. To obtain correct test results, extensive specimen preparation, such as sealing, is required.
With specimen grips specially developed for steel 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 user.
The grips feature hydro-mechanical and purely mechanical gripping. 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 clamping fractures are eliminated and steel strands can be tested without being pretreated.
The specimen grips 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.
Strain measurement
A steel strand fracture often leads to the deflection of individual wires, and as a result there is risk of damage to a sensor arm extensometer. ZwickRoell therefore relies on optical extensometers for safe, reliable, and accurate measurements on highly extensible and elastic materials.
With a field of view of 680 mm and a resolution of 0.6 µm, the videoXtens 6-680 extensometer allows you to perform precise strain measurements, even in the case of long gauge lengths such as those encountered in tests on steel strands. Features at a glance:
- Measurement of the yield strength to ASTM A1061
- No damage caused to the extensometer at the point of break, since the extensometer measures without making contact
- Automatic determination of strain at break
- Through connection to the crosshead, the gauge marks are always in the center of the field of view (FOV), making optimal use of the measurement range
Other advantages of this extensometer include automatic identification of gauge marks on the specimen and the initial gauge length, and transmission of all data to the testXpert testing software.
What is prestressing steel?
Prestressing steel is a high-strength steel that is primarily used in prestressed concrete construction. The use of prestressing steel makes it possible to significantly increase the properties of concrete components for static and dynamic loads.
Prestressing steel is manufactured from a long steel product and its surface shape varies according to the manufacturing standard. An individual wire, for example, is designed according to the manufacturer standard DIN EN 10138-1, while a steel strand can be designed according to DIN EN 10138-3. The individual wire has strength values up to 2000 N/mm² and is designed in a round shape, whereby there are also trapezoidal cross-sections, which are then be referred to as compact strands.
Under the general heading of prestressing steel, there are three forms that it may take depending on the product standard:
- Bar steel (diameter 15 to 36 mm)
- Wire (diameter 5 to 16 mm)
- Steel strands made up of 3, 7 or more twisted individual wires (three-wire strand: 5.2 to 7.5 mm, seven-wire strand: 7 to 18 mm)
These different forms impose high demands on materials testing and therefore a wide variety of test standards (e.g. ISO, ASTM, etc.) are used to ensure that the required standard of quality is met.
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