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
management /workers/ users. Obtain plans, drawings etc. Get the full history of components or plant -- manufacture and processing, maintenance records, service history, operating logs, replacement parts, repairs, previous incidents/accidents, etc Obtain all relevant specifications, standards, codes of practice, quality & safety procedures. It is very important to photograph the site so that the scene can be accurately described. 2. Visual Examination. Simple examination with the naked eye and with low power magnification can reveal information about likely fracture initiation sites & fracture paths, presence of macroscopic defects, surface damage by wear or corrosion, scale, contamination, heating effects, signs of abuse, workmanship in the original part, signs of modification or repair, etc. A complete photographic record should be kept and a dimensional analysis should also be performed to enable comparison with design drawings. Any corrosion pits, areas of damage, extent of any wall thickness thinning etc should be measured. Precautions to prevent damage of fractures or other important surfaces must be taken as soon as possible in the investigation to prevent any damage during transport, handling & storage before examination. 3. Non-destructive Testing. NDT can be used to reveal manufacturing or other defects (that should have been found before service) and the position and extent of any secondary cracking. Secondary crack surfaces will tend to be less damaged /contaminated than the main fracture and hence can give additional information about the failure. Such secondary cracks can be opened, taking suitable precautions for examination after observation of the main fracture surfaces . 4. Selection of Specimens . The positions and number of test specimens will depend on the nature of the failure and the requirements of the examination/analytical techniques which are to be used such as: fractographic examination using SEM +EDS metallographic examination of the structure at and away from the failure zone using optical and SEM techniques +EDS determination of relevant mechanical properties examination of corroded/worn areas and non-corroded/ unworn areas chemical analysis (wet, combustion, spectrographic, XRF, etc) Additional specimens may be needed for other techniques such as : X ray diffraction for identification and quantitative phase analysis, determination of texture, measurement of residual stress. TEM examination to study fine scale microstructures ,obtain phase identification and analysis, determine crystallographic relationships surface analysis via Auger, XPS, SIMS, EELS, etc gas content analysis e.g. hydrogen , nitrogen determination in steels The characterisation techniques listed above may also be applicable to loose corrosion products, wear or other debris, foreign matter, paint or other coatings, etc. Samples of any lubricants, solutions, or other environments that have been in contact with the failed item may also be needed. In making use of instrumental characterisation techniques it is vital that there is good communication between the failure team and the instrument specialist so that suitable samples can be prepared and so that the nature of the information is fully understood, i.e. accuracy and limitations in sensitivity . In all cases of taking samples we must take suitable precautions to avoid any damage, changes in the condition, and contamination of the specimen .e.g. heating or mechanical damage during sectioning. The positions from which samples are taken must be accurately recorded using photographs, sketches etc. All samples and remaining pieces of component must be correctly identified and preserved carefully for additional work which may be needed. It must be remembered that uunsuitable and incorrectly identified samples will give misleading information. 5. Review Progress. This is an on-going process involving constant appraisal and corroboration of the evidence, consultation of literature/ materials & product specs., and decisions about the next stage and further work. Where do we go from here? What other information can be obtained? What additional techniques can be used? What are the missing pieces of the jigsaw puzzle? 6. Additional Techniques. Laboratory tests to study aqueous or high temp. corrosion, wear behaviour , thermal cycling, etc. may be required. Accelerated or simulated service testing on models to reproduce the conditions of failure and the failure itself can also be attempted. Computer modeling of stress distributions ,fatigue behaviour, etc. is also used, together with fracture toughness data in the solution of fracture problems. 7. Analysis of Evidence/Data. The penultimate stage before reporting the findings of the investigation is to evaluate and cross check all the evidence paying special attention to any contradictions and to any gaps in the information collected. In this way the failure team can find out if there is a need for any further work or for any of the previous work to be re-examined and repeated. The aim is to develop a coherent explanation of the root causes of the failure and hence recommend preventative measures for future operations. 8. Failure Report. The report on the failure investigation and any recommendations for prevention should be as clear and concise as possible. Many failure reports have to be understood by non-technical people hence technical jargon must be minimised. 18 METAL Casting Technologies March 2005