Crack problem of large section armoured PE outer sheath cable

Polyethylene (PE) is widely used in the insulation and sheath of power cables and telephone cables because of its good mechanical strength, toughness, heat resistance, insulation and chemical stability. However, due to the structure of PE itself, its resistance to environmental stress cracking is poor, especially when PE is used as the outer sheath of armored cable with large section, the cracking problem is particularly prominent.

1.Mechanism of cracking of PE sheath

PE sheath cracking mainly has the following two situations: one is environmental stress cracking, refers to the cable in the installation and operation, the sheath in the combination of stress or environmental medium contact, from the surface of the brittle cracking phenomenon.

This cracking is generally caused by two factors: one is the existence of internal stress in the sheath, the other is the cable sheath for a long time in contact with polar liquid. This kind of cracking mainly depends on the material itself resistance to environmental stress cracking performance, through many years of material modification research this situation has been fundamentally solved.

The other is mechanical stress cracking, because the cable has shortcomings in the structure or the sheath extrusion process is not appropriate, there is a large stress in the structure of the sheath, and it is easy to produce stress concentration, so that the cable deformation and cracking during the construction of cable release. This kind of cracking is more obvious in the outer sheath of large section steel tape armored layer.

2.Causes of cracking of PE sheath and improvement measures

2.a. Influence of cable steel strip structure

When the outer diameter of the cable is large, the armored layer is generally made of double layers of steel belt gap wrapping. Depending on the outer diameter of the cable, the thickness of the steel strip is 0.2mm, 0.5mm and 0.8mm. The greater the thickness of the armored steel strip, the stronger the rigidity, the worse the plasticity, the greater the distance between the lower layers of the steel strip. 

In the process of extrusion and stretching, the thickness difference between the upper and lower steel bands on the surface of the armored layer is very large. The part of sheath at the edge of outer steel strip has the thinnest thickness and the most concentrated internal stress, which is the main location of cracking in the future. In order to avoid the influence of armored steel belt outer sheath, the buffer layer with a certain thickness should be wrapped or extruded between the steel belt and PE outer sheath, and the buffer layer should be tightly uniform, no wrinkles, no bumps. 

The addition of buffer layer, improve the flatness between the two layers of steel belt, so that the thickness of PE sheath material is uniform, in addition to the contraction of PE sheath, so that the sheath will not appear loose bag phenomenon, also will not pack too tight, thus reducing the internal stress.

2.b. Influence of cable production process

The main problems existing in the extrusion process of large diameter armored cable sheath are insufficient cooling, unreasonable mold configuration, excessive tensile ratio and excessive internal stress in the sheath. Due to the thick sheath and large outer diameter, the length and volume of the water tank in the general extrusion production line are limited. It is difficult to cool the cable from the high temperature of more than 200 degrees to normal temperature when the sheath is extruded.

If the cooling of the sheath is not sufficient after extrusion, the part of the sheath close to the armored layer will be soft, and it is easy to cause the cut mark on the surface of the sheath caused by the steel belt when the finished cable plate is bent, resulting in the cracking of the outer sheath under greater external force during the construction of cable release.

On the other hand, insufficient cooling of the sheath will cause a larger internal contraction force after further cooling of the cable into a disk, so that the probability of cracking of the sheath increases under the action of a larger external force. In order to ensure sufficient cooling of the cable, the length or volume of the tank can be appropriately increased, and the extrusion speed can be appropriately reduced on the basis of good plasticization of the sheath, so as to ensure that the inner and outer layers of the cable sheath have been fully cooled when the cable is put on the coil.

At the same time, considering that polyethylene is a crystalline polymer, it is advisable to adopt the warm water cooling mode of segmental cooling to reduce the internal stress generated during cooling. Generally, it is cooled from 70-75℃ to 50-55℃, and finally cooled to room temperature.

2.c. Influence of the bending radius of cable

When the cable is plied, the cable manufacturer shall select the appropriate delivery tray according to the industrial standard JB/T 8137.1-2013. However, when the delivery length required by the user is long, it is very difficult to select the appropriate coil for the finished cable with large outer diameter and large length.

Some manufacturers in order to guarantee the delivery length, had to cut with small tube diameter, caused by the bending radius is not enough, armoured layer due to the bending is too large displacement, large shear force on the sheath, serious when armored steel belt burrs will prick the buffer layer directly embedded within the sheath, the sheath along the strip edge cracks or cracks. During the construction of cable release, the cable is subjected to great transverse bending force and tension force, resulting in cracking along the crack direction of the sheath after the finished cable is unfolded from the tray, and the cable close to the shell layer is more prone to cracking.

2.d. The influence of site construction and laying environment

The cable construction should be standardized and carried out in strict accordance with the standard requirements. It is recommended to reduce the speed of cable release as much as possible to avoid excessive side pressure, bending force and tensile force on the cable, and avoid cable surface collision to ensure safe construction.

At the same time, ensure that the minimum installation bending radius of the cable meets the design requirements during construction. The bending radius of the single-core armored cable is ≥15D, and the bending radius of the three-core armored cable is ≥12D (D is the cable outer diameter).

Before laying the cable, it is best to place it at 50-60℃ for a period of time to release the internal stress in the sheath. At the same time, the cable should not be exposed to the sun for a long time, because the temperature of different sides of the cable is not consistent during the exposure, which is prone to stress concentration, which increases the risk of sheath cracking during cable construction and disconnecting.

Conclusion

The cracking of large section armoured PE cable sheath is a difficult problem that cable manufacturers must face. In order to improve the cracking resistance of PE sheath of cable, it should be controlled from many aspects, such as sheath material itself, cable structure, production technology and laying environment, so as to prolong the service life of cable and ensure the quality of cable.