Fatigue Failure

[MolaKule]

This question is open to all and follows on the heels of our first Fatigue question.

The process of Fatigue Failure is characterized by three distinct Steps.
Name these three distinct steps.
A.

B.

C.

 

[FlyNavyP3]

Initiation Propagation Rupture

 

[TheLawnRanger]

Initiation, propagation, final rupture

 

[CT8]

It starts
then it progresses
then failure occurs.
Fly Navy nailed it.

 

[MolaKule]

Thanks for your responses and participation.

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses.


The process of Fatigue Failure is characterized by three distinct Steps.

A. Crack initiation

where a small crack forms at some point of high stress concentration.

B. Crack propagation

The crack advances incrementally with each stress cycle.


C. Final failure

This can occur very rapidly once the advancing crack has reached a critical size.

So you guys pretty much nailed it and congratulations!

 

[Cujet]

Early Aircraft cylinder and cylinder head hold down bolts are a great example of fatigue failure.

Short bolts were used and over a period of time they would loosen or break despite being locked in place. . Larger bolts were tried with the same results.

The solution was to use screw on cylinder heads and affix the cylinders with long studs. Those studs would stretch considerably and the tension produced was far more than the combustion loads. Problem solved.

Even today, lycoming aircraft engine cylinders use 3 long studs and one short (for packaging reasons) The short one is prone to failure. Both the stud and the case. Fatigue rears it's ugly head!

 

[MolaKule]

You bring up a good point. There are both static stresses and cyclic stresses.

Cylinder head bolts are definately subjected to both types of stresses.

 

[Shannow]

The head studs is a good one.

There's the static clamping stresses, the low cycle (once per start) fatigue that occurs every time the engine is warmed up and parts expand (ameliorated by giving it a longer elastic member to work with), and the cyclic stresses due to cylinder firing.

Longer stud means that there's less variance in overall strain in each of these cyclic events.

Steel is a good medium to work with, as most typical steels there's a "fatigue" limit which allows infinite life if cyclic stresses are kept below that limit. Most non ferrous metals do not have this, and will eventually fail if cycled for long enough.

 

[JimPghPA]

An additional fact about cylinder head bolts is that it very important to tighten them to a specified torque with a torque wrench.

I remember reading somewhere about someone over tightening cylinder head bolts without a torque wrench on an airplane motor and the motor failed in flight.

 

[CR94]

Originally Posted by Shannow
... Steel is a good medium to work with, as most typical steels there's a "fatigue" limit which allows infinite life if cyclic stresses are kept below that limit. Most non ferrous metals do not have this, and will eventually fail if cycled for long enough.
Yes, and that's why I initially was skeptical of the aluminum rocker arms of my Mazda. However, they survived roughly a billion cycles without fracturing.

 

[Kestas]

I'm amazed how the auto industry introduced aluminum into suspension components with no wholesale reports of high cycle fatigue fracture or fracture from corrosion.