is done in the ground vehicle industry.

NOTE: This is ONLY a rough guide. Use at your own risk.

There are three things you need to get to solve the problem:

**Material (fatigue data)****Geometry of component****Load on component**

Is it aluminum? What type? (e.g.: AA 2024-T4) What heat treat (i.e. T6 etc).

If its a steel what is the Carbon wt%, heat treatment, hardness or ultimate tensile strength.

Go to into a database for materials and find the your material.

Example dbIf no exact match exists you can try something similar for a rough guess at

a prediction. Match the carbon level, and hardness/Ultimate for steel.

Many steels for example, if of medium carbon (about 0.40 to 0.55 wt.%), can be

"represented" by an SAE1045 of similar hardness or ultimate tensile strength.

If you want best possible answer get the material fatigue tested (ASTM E606).

"pass through" the component? Where is the critical failure location and

what is the elastic stress at that location?

E.g.: for a simple gear (Sprocket? :) the tooth may break, or something at the

shaft attachment. Lets assume we are looking at the tooth. It can

be analyzed as a cantilever beam. Apply a Unit Load of some sort (eg. 1000 lbs)

at the contact point. Find the elastic stress at the tooth root. Use text

books, Peterson's Stress Concentration factors or FEA analysis.

Elastic analysis is fine. Note that a gear tooth can also get hit from

"behind" -as you back-up a hill, or engine break etc. These reversed loads

have a huge influence on the critical fatigue "cycle".

component. Specifically the loads that you considered in the

Geometry section above. The gear tooth load, for example, arises from a

torque on the gear. The gear is rotating at a certain RPM. You can

calculate how often a tooth gets hit at each torque level. It has been

done for the complete torque history of a vehicle on the proving grounds

for each tooth of a gear, but this takes some effort. One could

do a worst case analysis. Note that reversed loading is very

important. A material "remembers" a hit in one direction, and then

"links-up" with a hit in the opposite direction. A simple gear

tooth at constant torque would perhaps see a tooth root stress

as A, B, C below

A B C (tension) ^ ^ ^ / \ / \ / \ / \ / \ / \ 0___/ \______/ \______/ \_____ ______ _____> t \ / \ / \ / \/ D (compression)A back-up load (D) would cause a compressive stress at the hot-spot.

This compressive stress actually would link-up with the largest of

A, B, or C to form the largest cycle. Thus lets assume that

load A is slightly bigger than B or C (non-constant torque on gear)

Then the above load history would be counted as

cycle: Max= A Min=D number= 1 cycle: Max= B Min=0 number= 2 (assuming B=C)This is called "Cycle Counting". The best automated way is to use the

Rainflow cycle counting program. -if you have a long messy (variable amplitude)

history. The best easy way to understand the counting method is to watch the

stress-strain behaviour of a fatigue test, but if that is not possible, try

the description of the "Reservoir Method" given in the British Standards.

It can be applied "by hand" to short load histories.

elastic hot-spot

for your unit load of X newtons (or lbs, or whatever) then ratio your cycle

history Max, Mins according to the actual load history. Plug these stresses

into one of the "Calculators"

example for SAE4130in the material database and click

(Note: remove the example elastic stresses entered on the page and put in your own)

Many of the entries in the material data base have three items for each material:

- Raw test data
- Fitted curve (sort of a least sq. fit to raw)
- Calculator (uses the Fitted to calculate life) Screen shot

Calculate will bring back a page that has a table at the top that looks like this:

#xcalc2 Loop Smax Smin N Sigmax Sigmin Delta Epsmax Epsmin DeltaEps %Eps %SWaT %Sts %Morr %Goodm #xcalc2 1 1200.0 -600.0 1.0 679. -550. 1229. 0.00962 -.00234 0.01196 99.1 95.1 99.1 98.0 96.6 #xcalc2 2 600.0 -100.0 20.0 475. -214. 689. 0.00403 0.00081 0.00323 0.9 4.9 0.9 2.0 3.4 #xcalc3 StrainLife_Reps SWaT_Life_Reps StressLife_Reps Morrow_Reps Goodman_Reps (Reps= Repetions) #xcalc3 3664.3 2829.9 3664.3 2076.4 1517.0 Smax, Smin and N are what you put into the table for calculation. Sigmax, Sigmin, Delta, Epsmax, Epsmin, DeltaEps are the hysteresis loop tips shown in the first graph below the table. %Eps, %SWat %Sts %Morr %Goodm are the fatigue damage percentage that each of the entered cycle sets caused (hopefully). (some folks have their favorites). StrainLife_Reps, .... etc are the number of times that your entered history can be repeated before predicted failure.In my opinion if you have tensile mean stresses at your hot spot where

Smean =(Smax-Smin)/2use the SmithWatsonTopper estimate SWaT_Life_Reps. If your mean is in

compression (negative) use the Morrow_Reps.

The graphs below the table are the stress-strain paths that your elastic

stresses caused at the critical hot spot. They are shown to give you an

idea of how much plasticity is going on. For more on how this is done see

the page on

Plasticity Correction

The second graph is a plot of each of you cycle sets and re-ordered with

largest range(max-min) on the left and smallest on the right. Also shown is

how much damage each Rainflow counted cycle set has caused.

Thats it. Have fun.

**
Note that if you are designing something critical you should
not rely on this rough outline without talking to an expert person.
**