The Case Studies

1.   FAILURE OF A WIRE ROPE ON A CRANE

Second Section of Case Study

Tensile Testing  

Before performing tensile testing of the rope, it was necessary to establish the original grade and size of the wire rope.  This would indicate what degradation of properties had taken place over the service life, and provide an indicator of the severity of service and quality of maintenance.  The only information that the operator could supply, was that the rope was a 1770 MPa grade.  Thus it was necessary to measure the diameter of wires near to the break (average approximately 1.5 mm) and the rope diameter (approximately 21.5 mm).

Use the information contained in the wire rope manufacturer's table of properties (Activity 2) to find the most likely original rope diameter and breaking force.

Tensile testing was performed on a 1.5 m length sample using a wire rope testing machine, giving a measured failure load of 232 kN.  It should be noted that this load did not represent complete failure of the rope, but rather fracture of 11 strands (66 wires) out of a total of 18 strands (108 wires).  Two inner and five outer strands remained unbroken.  Examination of the 66 wires that broke indicated that only 10 of them had a flat fracture surface, which would be indicative of the presence of an initial fatigue crack, with the rest showing ductile failure modes.

Referring back to the information on original rope diameter and breaking force, use Activity 3 to compare this load with the observed value for breaking load in the tensile test.  What conclusions can be drawn regarding the likely influence of pre-existing fatigue cracks?

Fractography

A number of individual broken wires were cut off the fractured ends and examined at low magnification using stereo binoculars, and at high magnification in a scanning electron microscope (SEM).  The total number of wires in all strands was 108, and 20 wires were selected from the outer strands and 11 from the inner strands.  The wires were de-rusted and ultrasonically cleaned in a de-greasing agent.

Typical SEM observations of the fracture surfaces are given below at both a low and a high magnification, together with information on the number of wires in the sample which were similar.

Type 1:   Tensile cup-and-cone fracture - 3 occurrences - 1 wire in outer strands, 2 wires in inner strands.

Low magnification fractograph of cup-and-cone.                           High magnification fractograph from the central region of the cup-and-cone fracture.