Steel wire ropes degrade through multiple mechanisms simultaneously. ISO 4309 categorises the visible patterns and gives discard thresholds for each. This piece walks through the main ones.
1. Broken wires (per lay length)
The classic indicator. ISO 4309 defines a lay length as the axial distance the rope's outer strands travel in one full helical turn — typically 6-7 times the rope diameter. The standard counts the number of broken wires within one lay length and one strand and applies thresholds based on rope construction and equipment type.
Visible broken wires are the easy case. Internal broken wires need MRT.
2. Corrosion
Surface corrosion is detectable visually — rust, pitting, discoloration. Internal corrosion is not, and it is far more dangerous because the rope's load-carrying core is what's degrading.
Marine environments, chemical plants, and outdoor uncovered ropes are most prone. Lubrication intervals matter — under-lubricated ropes corrode faster.
3. Wear and abrasion
Outer wires lose diameter from contact with sheaves, drums, and adjacent rope wraps. ISO 4309 gives a discard threshold based on rope-diameter reduction.
Visual is sufficient for the wear pattern itself, but it interacts with broken-wire counts — a worn rope with broken wires is in worse shape than the broken-wire count alone suggests.
4. Deformation
Kinks, bird-cages, waviness, dog-legs, and protruding cores indicate mishandling — usually a one-time event rather than gradual wear. ISO 4309 takes these seriously: most deformations are immediate-discard conditions.
5. Fatigue (loss of metallic area)
Cumulative loading and unloading cycles fatigue the steel. Wires fail in fatigue not by gross breakage but by gradual cracking. The summed cross-sectional area lost across the rope tells you how much load-carrying capacity is gone.
MRT measures this directly — the magnetic flux signal is proportional to the cross-sectional steel.
Why combine visual + MRT
Patterns 1, 2, 3, and 4 are partly visible. Pattern 5 is invisible. Patterns 1 and 2 have a hidden internal component visual cannot reach.
ISO 4309 expects both inspection methods because no single method catches everything. The MRT trace + the visual log are the complete picture.