Preserve your fracture surfaces or lose information
When a component fractures, the atomically clean fracture surfaces contain many features indicative of how and why the fracture occurred. Preservation of these fracture surfaces is one of the most important aspects of any failure investigation and precautions should be taken to provide the failure analyst with a sample in the best possible condition for accurate assessment of the failure scenario.
When a component fractures, the exposed surfaces are atomically clean and contain both macroscopic and microscopic features that can give indications of the failure mechanisms and origins of failure. Damage caused by, inter alia, attempts to put the surfaces back together, poor or lack of proper protection against mechanical damage and corrosion, can lead to loss of this evidence and potentially inconclusive results. Surprisingly even touching the fracture surfaces can lead to a loss of information as at the time of failure the surfaces are atomically clean and oils/sweat from finger tips can lead to corrosion of the rapid surface which may need to be removed prior to analysis.
For example, fatigue striations are typically in the region of 2 to 5 microns in size and can only be truly identified using electron microscopy - but can give details as to direction of crack propagation; the number of cycles to failure; the magnitude and cyclic nature of the stresses causing crack growth and these striations are often lost by mechanical damage during failure, attempts to place surface back together, corrosion, and damage associated with poor packaging during transport.
Adhering to the following simple steps when preserving a fractured component can largely prevent the loss of vital information:
Sketches, operating logs, manuals and reports from operators/witnesses to the incident should be gathered as soon as possible after the failure. The failure analysis sheet available on the Origen website provides useful outline to background information that should be collected and helps identify the conditions at the time of failure. Photographs of the fractured component, illustrating the component’s intended use, the fracture surfaces (or failed region if fracture surfaces are not visible), as well as the relative location of failed remnants, should be taken as soon after failure as possible. Although detailed photos are important, generally more global photographs often give a failure analyst valuable information as to the component’s operation and possible loading conditions. A steel rule or other item of known size should also be included in the focal plane of the item photographed to provide scale. Ideally photographs should be taken perpendicular to the item to prevent distortion (unless a particular feature is being highlighted).
The photographs should obviously be of decent quality and resolution. Not surprisingly ‘out of focus’ photographs, are not good enough!
Fractured parts, especially steel components, can corrode and degrade from both atmospheric conditions and/or improper handling. Excessive handling may remove surface deposits such as scales and oxides that may be relevant to the investigation. Rinsing in clean water (or other appropriate non corrosive solvent) to remove contaminants and then drying the component is generally advisable. A coating of a light oil can help protect the fracture surface and component and should be applied except where the underlying cause of the failure is corrosion related or there are oxides on the surfaces.
Protecting the surfaces from mechanical damage during transport/shipping is extremely important. Wrapping the component/fracture surfaces in clean bubble wrap or layered polyethylene film are suitable choices to ensure that component surfaces are protected. The use of lightly oiled rags or grease impregnated tapes (such as Denso tape) can also be very useful (except in cases with corrosion products/oxides on the surface). The mating of opposing fracture surfaces or the placing of component fracture surfaces next to hard objects during shipping should be avoided at all costs. Contact, especially from hard objects, can smear, gouge and scratch fracture surfaces. Ensuring the packing is water tight is also important - on a number of occasions failed components have been received in a highly corroded state due to saturation of the packing with sea water. In other cases the ‘failed scrap’ has been left outside to corrode. It should also be noted that using cardboard to provide mechanical protection can be problematic as corrugated cardboard is often alkaline and it can lead to corrosion if additives in the paper react with oil/residues on the component.
Removal/grinding of the sharp edges of a component to protect the analyst is not a good idea no matter how well intended!
It is not always practical to ship large components to a lab for examination. Proper sectioning of a component, after proper documentation, is critical in ensuring that fractographic features representative of the failure are preserved. Any distinctive colour features, indicative of heat exposure (such as temper colours) should be recorded before any sectioning. Monitoring of the width of the cut, or related deflection/distortion, can be very useful in identifying the presence of residual stress in the component.
Heat generated from the cutting process can easily change the mechanical properties and microstructure of the component being investigated. Thought should be given to the cutting method needed to obtain the desired section, with the aim of minimising the heat generated - particularly in the area of the fracture. Water jet cutting is often a good choice. Simple cooling provided by placing wet rags on the surface between the cut and area of interest during cutting can be very effective in ensuring the thermal changes do not influence the result of the investigation - particularly when the temperatures are also monitored.
Simply cooling the component after it is heated is obviously not suitable!
The message is simple document, protect and preserve to get the best out of the investigation.
This is the last Tech Tip for the year - we trust you have found the tips informative and they have been useful in helping improve integrity/prevent failure. Please note we will be closed from 15 December 2016 and will be back on 3 January 2016.
The Origen team wishes you and yours a stress free festive season and trust the New Year is all it is cracked up to be!
Published in Technical Tips by Origen Engineering Solutions on 1 December 2016