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Originally Compiled by Pieter Van Dijk, Air Safety Investigator (Engineer),

Bureau of Air Safety Investigation, January 1997 for use by TC on his R22 Safety programs

Amended and updated by TC March 2004

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Metal Has Memory!

An Introduction to Metal Fatigue

Introduction

Metal Fatigue is the most common cause of structural failure in machinery.

What makes metal fatigue such an important issue for helicopters is that, if it is not properly understood by pilots and maintenance engineers, it can bite you hard - very hard.

R22 Main Rotor Blade – time exceedence - fatal

 

Our understanding of metal fatigue is only a recent phenomenon i.e. post World War II as a result of the quest for greater strength from lighter structures.

Before this time, some metal bridges, liberty ships and aircraft seemed to crack, collapse or fall apart for what seemed at the time inexplicable reasons.

Fortunately, our understanding of this type of metal failure mode has evolved from this early baffling state into a sound engineering tool, based on metallurgical, mathematical and statistical principles.

What is especially significant is that metal fatigue can now be controlled and predicted.

The airworthiness of a helicopter can almost be guaranteed throughout its entire lifetime, providing everyone follows certain golden rules.

 

What is Metal Fatigue?

Metal fatigue is the initiation and propagation of microscopic cracks, due to the slippage of atomic planes within a metal component. This occurs as a result of the repeated application of stresses and can be caused by

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flight loads

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landing loads

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vibrations

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or simply by just having the engine running.

Metal fatigue is a dynamic phenomenon. A helicopter in storage will not develop fatigue cracks!

Generally speaking, in order to occur it requires moving parts, of which a helicopter has plenty in comparison to a fixed wing aircraft.

These moving parts represent a mechanism for the application, and subsequently removal, of a force or a load from one component to another i.e. On..Off...On..Off... On..Off.. On..Off..etc. Each one of these “On..Off’s” represents one fatigue cycle. It is these fatigue cycles that are responsible for the slippage occurring in the atomic planes that we call metal fatigue.

What is particularly noteworthy is the relatively small magnitude of applied load required for a fatigue crack to occur.

This load, and hence the accompanying stress applied to the component, need only be around 30% of the ultimate load or stress the component is physically able to withstand.

This load, however, is generally required to be applied many thousands of times (fatigue cycles) in order for fatigue cracking to take place.

The fatigue crack, once developed, will continue to grow, seemingly with a life of its own, up until a certain point. This crack growth rate can be anywhere between 0.0000001 - 0.001 mm per fatigue cycle.

Eventually, the crack becomes so large that the component is no longer physically able to carry the load for which it was designed. The remaining cross-section of the component i.e. the area as yet unaffected by the fatigue crack, subsequently fractures, due to overload, and the component fails. Such a failure can have significant safety of flight implications.

Enter the term coined by TC – Metal has Memory”.

All the different metal components that make-up a helicopter “remember”

·         every single load and stress (fatigue cycle) that they have ever been subjected to

·         for every single hour, minute and second of operation

·         for every one of its previous operators – including you.

The net effect of all these fatigue cycles is cumulative and non-reversible throughout the life of the component.

It is for this reason that the helicopter’s total number of hours in service, as opposed to time since new, is so important. Recording In-service (Maintenance Release) hours is one of the most effective ways of measuring the fatigue life of metal components.

What Causes Metal Fatigue? 

Many different factors contribute to the particular fatigue crack propagation rate in any given component. Each different size of every different type of metal or alloy has its own unique and individual susceptibility to fatigue.

The most significant factors affecting fatigue include

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the type of operations (straight and level flying versus mustering operations)

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the magnitude and frequency of loads (all day everyday operations versus the occasional flight)

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as well as the quality of the materials concerned (the presence of flaws or damage to the metal materials leads to reduced fatigue tolerance).

Mustering operations are likely to subject the helicopter airframe to higher stress levels that that of normal flying operations.

For instance, even one particularly steep (high G) maneuver could have the equivalent fatigue effect on the helicopter’s structure or rotors as a week’s - or even a month’s - worth of normal landings and takeoffs.

A given component’s fatigue life, and hence time to failure, is in this way reduced by a proportionate amount of time to what it otherwise might have been if the sever maneuver had not occurred.

Similarly, conducting flying operations on a regular basis will subject the airframe to more stress and hence cumulative fatigue cycles, than would be the case if the helicopter was only flown on an infrequent basis.

It is for these reasons that the use of bogus or life-expired airframe components should never be contemplated. Although from external inspection, the component may appear to be in reasonable condition, only detailed metallurgical analysis can accurately determine the true state of its structural integrity.

The installation of a component of this nature also brings with it more than you may have bargained for: its memory. Such a component could be the weak link in the safety chain. By installing this component, you also get its entire collective history of accumulated fatigue cycles and any damage, despite the fact that it may “look all right” from the outside. 

An easier way to assist with understanding the nature of metal fatigue is to think of it as a modest inheritance in your bank account.

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Normal flying operations result in fatigue damage corresponding to withdrawals of only a couple of dollars per hour.

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High G maneuvers, on the other hand, cost you hundreds each time they occur.

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Overstress; sets you back thousands and thousands.

Unfortunately, there is no mechanism for making deposits and topping up your account.

There is only so much money in the bank. When it’s gone ….. it’s gone.

 

  

How the Manufacturer Fits In

Nobody knows better than the manufacturer where its product’s potential fatigue “hot spots” might be.

The manufacturer is obliged to go to a great deal of trouble, from an early stage, to conduct simulated fatigue crack tests on the helicopter’s flight critical components. This is not only done in order to verify that the design data used is correct, but also to try and establish whether or not any potential problem areas exist.

This process commences long before the in-service fleet approaches the equivalent number of fatigue cycles and is undertaken by installing the components on test rigs.

Most of us are familiar with the term “Certification”. In essence this means that a manufacture has proven the safety of his product – including fatigue data - to a Regulatory body such as the FAA or CASA.

Should a particular component on test develop a fatigue crack, it then becomes the subject of close scrutiny. Such details as the number of fatigue cycles required before the crack first appears, as well as the number of fatigue cycles to when the cracks size threatens the structural integrity of the component are accurately recorded.

Accordingly, an appropriate Service Bulletin, servicing schedule amendment or maintenance instruction is promptly developed and issued to industry.

These instructions will typically specify that at a certain number of in-service hours, which is equivalent to the number of fatigue cycles the test piece was subjected to on the test rig (including a safety factor), certain inspection, maintenance or replacement action should take place.

The importance of conforming to this critical fatigue management information is absolutely paramount from a safety point of view. Similarly, this kind engineering feedback can also be gained by the manufacturer from accidents and incidents in the field as well as from the advice of operators with high time, in-service helicopters.

 

Conclusion

This paper was intended as a brief introduction into the nature of metal fatigue.

From an operational point of view, it is not necessary to have an in-depth knowledge and understanding of the subject - that’s for the rocket scientists.

However, it is extremely important that every one involved in the operation of all helicopters complies fully with everything these rockets scientists have to say.

To do otherwise is to act at your own peril, as well as those who may operate the helicopter after you - inheriting every one of your fatigue cycles.

 

  

The Golden Rules of Fatigue Management

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Maintain accurate records of total airframe, engine and component hours to allow for the safe management of fatigue in helicopters.

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Adhere closely to the manufacturer’s operational and engineering advice i.e. service bulletins, servicing schedules, newsletters etc to ensure the continued safe operation of the helicopter.

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Don’t use bogus, time-expired or accident damaged components even though they may “look all right”.

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Always operate the helicopter within the manufacturer’s approved performance envelope. Excursions from these parameters will compromise the airworthiness of fatigue critical components.

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Report all/any exceedence.

 

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