by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : March 2005
careless assembly , e.g. mismatch of mating parts, parts omitted, damage, over- or under- tightened threads, incorrect fitting, entrained dirt or grease, etc. In considering possible material/technical reasons for failure it is vital not to overlook the possible effects of "human factors". Correct design must recognize human capabilities and fallibility and all engineers must have an appreciation of the causes of human errors. Over the years many lessons have been learnt from public inquiries into disasters. Such events have shown how the interactions between people and materials/equipment/systems can produce catastrophic results  and have highlighted the importance of ergonomic considerations in the design process. In design, risk assessment and failure analysis it must be appreciated that human performance is influenced by the inter-actions between the following factors : the person -- their behaviour, suitability & competence in carrying out their work the job -- including the equipment, plant, premises & substances in use the organization in which the working environment is placed. Behavioural factors are discussed in detail in standard texts  but it is useful at this stage to list some of the main causes of human errors as: Inadequate design Poor communication Inadequate information Pre-occupation and/or lapses of attention Mistaken actions Mis-perceptions Mistaken priorities Intentional wayward behaviour (short cuts in procedures). Looking back at earlier times a Parliamentary Committee was set up in Great Britain in 1839 to report on the high number of steamship disasters that had occurred in British waters in the previous 20 or so years . The report states ". . . even masters have frequently been caught sitting ,or even standing, on the safety valves or hanging weights on the levers in order to raise the pressure of steam at the moment of starting . . .". In more recent years the sinking of "M.V. Herald of Free Enterprise" ,a roll on-roll off car ferry, provides another example of the human factor in procedures. The disaster resulted from the common practice for such ferries to leave port with the bow vehicle doors open! The prevailing sea conditions and uneven ballast and loading causing sea water to enter through the closing doors. Although human behaviour must be recognised as a possible cause contributing to a failure we must ensure that the "pilot error" cause is not a cover up for the lack of ergonomics in a design or for the lack of understanding of the performance of the materials used in its construction. MECHANISMS OF FAILURE The mechanisms of failure are often complex and failure analysis requires an understanding of the structure and properties of the materials used in the failed component or plant, the stress systems encountered in service and the nature of the operating environment. The modes of failure are many and varied and are quite often inter-related; for example in a failure which has occurred during service at elevated temperatures the interaction between creep, fatigue loading, thermal shock, oxidation, high temperature corrosion, and microstructural instability, etc. may all need to be considered. Failure mechanisms can be investigated in a number of ways using a variety of materials characterisation techniques, e.g. optical/electron microscopy, NDT, etc.,  and property testing (e.g. mechanical, wear, corrosion, etc.). During the last 30 years progressive developments in Fracture Mechanics and the examination of fracture surfaces, wear and corrosion damage, etc. by Scanning Electron Microscopy (SEM) in conjunction with a "toolkit" of other methods of examination have improved our understanding of the ways in which cracks can nucleate and grow and of the importance of surface-environment inter-actions in the damage and deterioration of engineering materials. The subject of Fractography is now very well developed and there are a number of published collections of standard fractographs which can act as useful 16 METAL Casting Technologies March 2005 Above: Ductile failure in a low C steel. The dimpled fracture surface results from nucleation and coalescence of microvoids. Each void is nucleated by a second phase particle of carbide or non-metallic inclusion. Below: Brittle failure in the same low C steel which has occurred below the fracture transition temperature of the steel. Fracture is via transgranular cleavage of the ferrite grains. Figure 1. Use of fractography to differentiate between ductile and brittle fracture in low Carbon steel [SEM 1000x]. TECHNICAL FEATURE 1