All you need to know about failure analysis – the causes and effects, prevention of failure, benefits of undertaking a failure analysis and more.
What Are Some Of The Effects Of Failure?
Failures have a wide range of effects – through the loss of profit and in some cases injury, to even the loss of life. If a detailed failure analysis is carried out and appropriate ameliorating action taken, repetition of such failure can often be avoided. Often some of these effects, specifically the loss of profit, may be reversed and in nearly all cases recurring failures prevented.
What are Some Of The Causes Of Failure?
Failure may be caused by many factors which can include inter alia: poor design, manufacturing and/or material defects, improper use – overloading; as well as service and environmentally induced failures.
Often some of these act in conjunction and cause premature failure that would not have occurred had each factor been acting independently. For example, for stress corrosion cracking (SCC) if either the stress or the corrosive environment is removed, the failure may be prevented.
Furthermore, some failure mechanisms are insidious in nature in that a component, that has been operating in an apparently satisfactory manner for years, may suddenly fail catastrophically. Fatigue cracking, for example, may initiate and propagate under normal working loads, albeit over a long time. This in turn will lead to catastrophic failure when the crack has grown to critical dimensions.
Residual stresses, that may be introduced through either thermal or mechanical manufacturing processes, can also contribute to failure. Such residual stresses may increase the mean stress levels to unacceptable levels, and lead to fatigue crack initiation or SCC. (Such residual stresses can be alleviated – see component stabilisation/stress relief page).
What Can Be Done To Prevent Failure?
An understanding of possible causes of failure, allows the risk of premature failure to be minimised.
Specifically, the stress levels in components need to be kept acceptably low, materials need to be chosen to be able to withstand the rigours of the environment; quality control standards need to be imposed so that the manufactured components meet the requirements of the design; and the system needs to be operated as designed, without abuse or overload.
If, using appropriate techniques, a potential failure and its cause can be timeously identified ameliorating action to prevent complete/final failure may, in some cases, be taken. In other cases, if accurate loading data is available and the material properties are known, the potential for catastrophic failure may be assessed using fracture mechanics principles, such as life prediction.
What Savings Can Be Achieved By Undertaking A Failure Analysis?
If you don’t learn from your mistakes, you are bound to repeat them (E Waugh). This truism about life is also very applicable to engineering components – it is vital to understand the true cause of a failure to prevent further failure. Only through such understanding can appropriate ameliorating action be taken. Corrective action that prevents further failures prevents losses associated with ineffectual or incorrect repairs, downtime, loss of morale/direction and prevents injury and loss of life.
Failure analyses that identify the cause of failure can also be used to apportion blame (where this is appropriate) and help in the recovery of losses and/or associated costs from responsible parties.
A large percentage of Origens clients have been able to recover costs, either directly or indirectly, well in excess of those associated with undertaking the failure analysis.
What Is The Viability Of Carrying Out A Failure Analysis?
The costs associated with undertaking a Failure Analysis need to be offset against the possible rewards. It is normally well worth- while undertaking a failure analysis in cases involving production items; those where similar failures have occurred; where production losses are significant; where costs associated with the failure may be recovered or where injury/loss of life was caused.
What Should Be Done Immediately After Failure To Prevent Valuable Information As To The Cause Of Failure Being Lost?
Obviously the most important aspect is to make the area of failure safe and to prevent further injury or loss of equipment. After the site has been made safe and any injuries have been treated the site should be documented. If possible professional failure analysts should be retained as soon as possible.
If such personnel are not immediately available, the documentation of the failure should be carried out, by a responsible person, as soon as possible before the site of failure has been disturbed. Sketches/drawings of the area highlighting the positions of the fracture debris should be made, photographs complementing these sketches and of the failed items (and specifically the fracture surfaces of these items) should be taken. It is important that both global and close up photographs be taken – often the macro information is lost as to much importance is placed on the failure/failed components themselves. After the site has been fully documented all the failure debris should be carefully retrieved, labelled and stored.
No matter how enticing, the fracture surfaces of failed components should never be placed together or handled more than necessary – such action can cause the loss of valuable information such as fatigue striations and may initiate corrosion. If medium term storage is necessary the components should be protected from mechanical damage and corrosion – in general, the fracture surfaces should be protected with a light oil (except where the underlying cause of failure is corrosion related) and protected against mechanical damage with a lint free rag and the individually labelled items sealed in dry plastic bags. However, in specific circumstances other action may be required and professional advice should be sought.
To assist with interviews and allow the events preceding failure to be interrogated, operational data logs and maintenance records should be collected/copied and lists of witnesses/operational personnel should be made. Often a significant degree of damage is caused during the transportation of the failure debris to the laboratory – to prevent this the components should be well packed and transported in a suitably robust, well labelled container.
The downloadable Origen Failure Information Sheet will aid the process of gathering the necessary information that will enable a failure analyst to draw meaningful conclusions from the fractographic and material information. The relevant sections of this sheet should be completed in as much detail as possible. Often seemingly irrelevant information can supply the clue needed to identify the root cause of failure – as such, more information than necessary is preferable to insufficient detail.