Study On Characteristics Of Fatigue Behavior And Failure Mechanisms Of Steel Wire Ropes

Mechanisms of fatigue failure of wire rope working with sheave are related to safety, reliability and economy of Wire rope transmission. Designer, manufacturer and user of crane are urgently demanded to understand characteristics of fatigue behavior, law of failure and mechanisms of failure of wire rope due to ceaseless appearance of new type and new specifications of wire rope and sheave, in order to design and choose to match wire rope to sheave properly by scientific method.For this reason, test assessment of fatigue property of wire rope winding sheaves, influence of groove material of sheave on fatigue life of wire rope, fatigue failure analysis of wire rope and microstructure of wire rope have been studied by a new type electro hydraulic servo automatic wire rope-sheave synthetic tester, scanning electron microscopy, micro-hardness measurement, imagine processing and so on.Based on massive fatigue tests, dynamic curve of development of fatigue broken wire of wire rope is proposed. It takes number of broken wire inside 6d and 30d (d is rope diameter) as X-axis, and takes cycle of bending fatigue of rope as Y-axis, which reveals development and distribution of broken wire with increasing of cycle of bending fatigue. The curve shows dynamic procedure of development of broken wire induced by accumulation of fatigue failure, which is contributed both by wearing and bending fatigue of wire rope. It can be not only used to study macro law of fatigue of wire rope, but also to provide theoretical basis to engineering application of wire rope.For wear of outside wires as predominated form of rope fatigue, a nondestructive method of measurement of wear of outside wires is put forward on the base of imagine processing. It possesses theoretical basis, and it is easy to operate in practice. Wear of outside wires coincides basically with loss of rope breaking strength . by experimental measurement. Thus the method is rather accurate for wear of outside wires.Influence of groove hardness of steel sheave on fatigue life of wire rope from HB170 to HRC60 is investigated. The test result presents a curve of fatigue life with two peaks (like an inverted W), thus too hard and too soft of groove both are detrimental to fatigue life of wire rope. Influence of soft and hard groove of steel sheave on fatigue life of wire rope is studied by contrast test. Sensitive indexαof influence of groove hardness of sheaves on fatigue life of a wire rope is proposed, which is a ratio of fatigue life of wire rope winding soft sheave group to one of hard sheave group. It is effective to study influence of groove hardness of sheave on fatigue life of wire rope. And it is an important basis for evaluating a matching of a wire rope and sheaves. Value of a is related to strength grade of wire rope when the other condition is same by test. The higher the strength grade is, the smaller the value of a is.Amount of wear of the groove of the sheave with polymide groove is much more than one of the steel grooves in the beginning of working, and then the rate of wear of the polyamide groove goes down rapidly and tends to stable gradually due to the running-in. The life of wire rope working around the sheaves with nylon groove is at least twice of working around the steel sheaves by test. The mechanisms are: wear of their outer wires is less considerably compared with winding steel sheaves. And their fracture surfaces have feature of a smooth crack initiation zone, a wide crack propagation zone with many second cracks, and a narrow tearing zone.In order to study characteristics and mechanisms of fatigue of crane wire rope, failure analysis of wire ropes in bank container handling bridge is studied. The result shows: The fatigue crack initiation of rope strand is located in surface layer of the broken wire with smooth fracture surface. Its crack propagation zone is wide and there are small second cracks in the region. Its tearing zone is narrow and there are considerable tearing ridges in the region. While the fatigue crack initiation of rope core is also located in surface layer of the broken wire with smooth fracture surface. But the crack initiation is induced by fretting between core wires. Not only has a wear plane presented in the side of core wire due to fretting wear. But also there are regular micro striations with approximately 3μm interval in the plane. So displacement of the fretting is determined to be micron order. These micro striations provide important basis on research of mechanisms of fretting wear.By analysis of microstructure of fatigue wires, primary reason for secondary cracks in crack propagation zone of fracture surface is: on one hand, the fatigue wires are bearing superimposed stress of complex external stress and microstructure internal stress during crack propagation. On the other hand, texture in fiber microstructure of wires leads to its anisotropic property.According to characteristic of fatigue fracture surface of wires of rope, a method of secondary crack quantitative fracture surface analysis is proposed on the base of imagine processing. It is mainly applied to those cases there are many large secondary cracks in fracture surface. Secondary crack area ratio is regarded as a parameter of quantitative analysis in evaluating fatigue life of component. The preliminary result shows that there is correlation between secondary crack area ratio and fatigue life of component.In order to improve anti-fatigue property of wire rope, poval solution sorbitized treatment by water quenching replacing by lead quenching has been studied in laboratory, and a heat treatment process to obtain fine and homogeneous sorbite and possess high micro-hardness has been found. The process has practice value to improve mechanical property and reduce lead pollution of heat treatment.