Experimental Dynamic Fingerprints Associated with Progressive Damage Caused by Wire Breakage in Seven-Wire Helical Strands

Wire breakage alters strand continuity, interwire contact, load redistribution, and modal response. This study evaluates normalized frequency changes as a laboratory fingerprint of persistent mechanical changes in unloaded seven-wire helical strands. Six 12.7 mm, 1 + 6 ASTM A586-18 strands with free lengths of 200 mm (S20) and 400 mm (S40) were tested in tension. Dynamic measurements comprised five repeated impacts on one strand per length in the intact state, after the elastoplastic transition, and during progressive wire breakage. Tensile records and optical observations documented nonsimultaneous fracture and intermediate load-bearing configurations. After unloading from the elastoplastic transition, the fundamental flexural frequency reductions were 5.6% and 6.3%, compared with a maximum repeat-impact coefficient of variation of 0.3%. A maximum reduction of 18% was reported during the wire-break sequence. A comparison using the same experimental summaries shows that direct frequency shift and the squared-frequency index yield identical detection decisions under corresponding thresholds. Sensitivity calculations quantify the effect of the threshold and modal-mass assumption, and published strand experiments provide an external comparison. The contribution is an experimentally grounded interpretation of residual frequency changes as a global state indicator. The results support a laboratory proof-of-concept for the two tested strands, with equivalent stiffness loss interpreted under explicit modal assumptions.