SecMet (Pty) Ltd was requested to investigate the failure of a 7 strand wire rope. The investigation concentrated on analysing the fracture surfaces in order to elucidate the failure mechanism. The fracture surfaces of wires were examined using stereo microscopy and Scanning Electron Microscope (SEM).

Elements of the 7 strand wire rope

The overall condition of the cable bore evidence of poor maintenance as dust, sand and regions with a lack of lubrication were noticed. Areas with substantial external corrosion were also found suggesting that for a prolonged time insufficient lubrication prevailed or that a corrosive medium was in contact with the cable. Although most of the cable length showed little evidence of mechanical damage, the region in the vicinity of the failure showed significant localised damage.

Frayed individual wires following failure

The fracture surface of most wires appeared to be contaminated with foreign matter. The residue was analysed by means of Energy Dispersive Spectroscopy (EDS) and the results of one of the contaminated regions is shown in Table 1. The source of the contaminant warrant further investigation as the chlorine and sulphur could cause significant corrosion and premature failure of any replacement wire rope.

Table 1: EDS analysis of the contaminated fracture surface

SEM image showing indentations on individual wires following hardness testing of a sectioned strand mounted in resin.

After dissolving the surface contaminants in an ultrasonic bath with hot water and subsequently rinsing with acetone, the fracture surfaces were scrutinised.

While brittle, intergranular failure was observed for some of the outer strand wires, ductile failure and significant necking of most wires was confirmed using both electron and stereo microscopy. This finding would suggest that the majority of wires failed in overload. Such overload could be due to loads exceeding the rated capacity or due to normal load exceeding the remaining strength after intergranular failure of the wires/strand. Some individual wires from various strands were corroded, worn and mechanically damaged insofar that the cable’s load bearing capacity had been reduced.

Scanning electron image of a single wire showing intergranular failure.

SecMet (Pty) Ltd was requested to assist with the design, material selection and drafting of the minimum requirements that would be necessary to ensure sufficient protection in combination with improved manoeuvrability for suits used during attack dog training by the police.

The proposed material layer(s) and material type selection was facilitated by establishing a minimum test criteria that will allow sufficient protection of the wearer and via performance comparison to existing suits. The minimum tearing strength of the alternative suits was benchmarked to the existing suits in use.

The maximum biting and biting-pulling force that could be exerted on the suit was established by taunting the training dog(s) to bite on multiple 10cm diameter Pine wood rods. The indentations made during the biting were measured and characterised and compared to artificial indentation (mimicking a canine teeth) made with a calibrated force on the same rod.

Various materials and combination of materials and/or number of layers were tested for compliance with the attack suit specification, specifically in terms of penetration resistance and spread of load. The materials included Leather, nylon, wool, felt, sponge and high density foam.

The optimised material combination which consisted of a leather layer, quilted ballistic nylon and wool material, was evaluated to comply and exceed the minimum requirements. The latter provided satisfactory protection in field testing and surpassed the existing training suits in terms of manoeuvrability due to the significantly reduced thickness.

Screening Assessment (level 1)

A Level 1 assessment is based on a screening assessment that relies on published creep material behaviour and is limited in its applicability.

An applicable example is that of a localized hotspot that was prevalent for 1 month (744 hours) on a processing unit for which locally the internal refractory lining had deteriorated. The plant had the unit shutdown and the lining repaired and wanted to know whether the shell was still fit-for-service given the creep degradation that could have been incurred.

Thermography image indicating hot spot

The thermography survey showed maximum temperature of 536°C on the outer surface of the unit during the excursion period.

The material of construction was reportedly carbon manganese steel and the unit had no prior temperature excursion.

An Finite Elemental Analyses (FEA) based model indicated stress at the affected region to be approximately 40 MPa.

Screening chart for carbon steel

Using the screening data published in API 579-1 (2007), it was possible to conclude that the conditions (i.e. the combination of endured temperature and stress) was within the allowable range for carbon steel. The screening curve indicates that for carbon steels operating at 536°C and exposed to a stress of 40 MPa, a minimum creep service life of 2500 hours can be expected. Due to the excursion being shorter than the permissible hours the unit was considered fit for continued use. No repair of the shell was required, and the unit was returned to service.

Creep FFS and RLA assessment (level 2)

For a level 2 FFS assessment reference stress solutions can be applied. Occasionally, FEA models are made, specifically for components exposed in the creep range where discontinuities exist close to the regions of interest. When FEA models are used, the extraction of the peak stress values can be considered as a conservative approach, however, due to creep strain stress redistribution that may follow, membrane stresses are normally preferred and provide more realistic results.

When the creep life consumed to date, and the expected creep life to be consumed is calculated provision is made for the tri-axiality of stresses. Conversely, the Omega properties determined via uni-axial testing is converted to multi-axial values for this reason. Should the actual material condition have been characterised or Omega properties ascertained for service exposed material, representing the worst-case damage, then the past operation can be ignored during a desktop review of the creep remaining life study. It is important to incorporate a future corrosion allowance and/or the prevailing corrosion rate as the stress is likely affected by the latter and this, in turn, drives additional creep degradation.

Schematic of the exposure periods of the assessed component

The creep fitness-for-service and remnant life calculations considers creep damage fractions over various load or temperature periods as indicated by the schematic timeline.

The plant data received from the client indicated that some temperature excursions above the design temperature were sporadically experienced as detailed by the metal skin temperature history.

Historic metal skin temperature data indicating thermal excursions above design.

By incorporating the data above, with the reported long-term corrosion rate and operating pressure over the service life of the component, it was possible to calculate the creep life fraction deemed consumed till date and to predict the remnant life in accordance with paragraph 10.5.2 of API 579-1/ASME FFS-1. It is customary to include a safety margin with all remnant life studies and therefore a creep life fraction of 0.8 is typically considered end of life (albeit that actual creep failure may not yet occur).

Various scenarios were considered for the client during the desktop review, since the original material condition was not known, and since in-situ replication results indicated that the material condition was more favourable than initially anticipated from scenarios 1 to 3, which incorporated conservative assumptions. The outcome of the various scenarios as contemplated by the equipment owner is given in the figure below. There were a couple of scenarios that rendered the component safe in terms of creep degradation till 2022, provided the scenario operating conditions were met.

An example of desktop review of the creep life based on various scenarios and sporadic thermal excursions

A sabotage attempt on the Mtamvuna River Bridge caused extensive cracking of the steel channel sections. Crack-like and corrosion indications were also visible. A condition assessment was carried out on the steel sections with a view to the planned temporary and permanent repairs.

The bridge has developed cracking in areas adjacent to welds, and the root cause of cracking was investigated.

The results displayed a number of characteristic features of fatigue failure.

The materials, microstructures and properties associated with the pipe bridge were normal for this type of application, while the quality associated with the welding process showed quality non-conformances in certain cases.

Mozal required a liquid pitch storage facility to receive and stock high quality pitch imported from Japan for the fabrication of anodes.

The facility consists of four storage tanks each with:

MegChem designed a special welding and installation procedure to repair a cracked elbow on a pipeline at an operating pressure and temperature of 4000 kPa and 420 °C respectively.

The weld pool temperature was monitored throughout the repair to ensure that it did not exceed the calculated safe value at which the pipeline material is capable of withstanding the operating pressure.

A potential shutdown of the Sasol Secunda plant was avoided, resulting in a production saving of approximately R20 million.