The Importance of Rope Tensile Testing
Concept and Key Metrics of Rope Tensile Testing
Cable rope tensile testing primarily involves the measurement of linear density, twist distance, braid pitch, elongation, and breaking strength. Among these, the measurement of elongation and breaking strength is the most critical. Elongation refers to the increase in length when the tension on the rope rises from its initial value (pre-tension) to 50% of the rope’s rated minimum breaking strength.
Breaking strength refers to the maximum load required to gradually apply force until the rope breaks. When the rope reaches its maximum load, the severe consequence of cable breakage occurs. Therefore, users must be clear about the type, length, dimensions, safe working load, and breaking strength of the rope they are using.
Main Steps of Rope Tensile Testing
Under no significant tension (not exceeding 20% of the pre-tension), straighten the sample rope on a flat surface, measure its length, and mark two "w" points (symmetrical about the midpoint of the sample rope, with a distance of 500 mm between them) as shown in the figure above.
Depending on the type of tensile testing machine, secure the eyesplices at both ends of the sample rope and mark two "r" points (defining a test section with a distance of 2 m between them). If breakage occurs within this section, it is considered valid. The distance from each "r" mark to the closed end should be 200–250 mm.
Based on the type of rope, apply the specified pre-tension to the sample rope and measure the distance between the two "w" marks (the pre-tension is determined according to national standards). This distance is denoted as L2, in millimeters.
Elongation Measurement: Gradually increase the tension at a uniform rate using the tensile testing machine. When the tension reaches 50% of the rope’s minimum breaking strength, measure the distance between the two "w" marks. This distance is denoted as L3, in millimeters.
Elongation (E) calculation formula:
E = [(L3 - L2) / L2] * 100%
Breaking Strength Measurement: Temporarily set the breaking strength as "N" based on the material and dimensions of the rope.
Pre-tension once at 10% * N load.
Pre-tension three times at 20% * N load.
Pre-tension twice at 30% * N load.
There are two scenarios for rope breakage:
If breakage occurs between the "r" and "r" marks, the breaking strength is the value displayed on the tensile testing machine, denoted as Fc.
If breakage occurs between the "r" mark and the closed end, the breaking strength is Fc / 0.9.
Tips:
The eyesplice should be level with the rope.
During breaking strength testing, the rope should break between the "r" and "r" marks. If breakage occurs elsewhere, the eyesplice may not have been spliced correctly.
The above highlights the importance of cable rope tensile testing. We hope this provides valuable insights for professionals in the rope and cable industry. For more information about cable ropes, feel free to follow Qingdao Huakai Ocean Technology Co., Ltd.