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Garnet

## Curve fitting units

Hello,

What is the best way to handle units in a curve fitting equation situation.

On line 4 if I leave the ksi in then line 6 does not work, I had to create a unitless line 5 to get line 6 to work. Also I had to text in the word "cycles"  because MC does not take cycles unit in lines 6 & 8.

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25 REPLIES 25

## Re: Curve fitting units

1) To be able to calculate "sigma -23" obviously both must be of same dimension. Itz can't work if sigma has unit ksi and 23 is unitless

2) The arguments of functions like exp, log, sin, etc. must be unitless. Thats not a limitation of Mathcad, though 😉

3) If this is an old empiric formula, you may assign unit ksi to sigma and then use log(sigma/ksi-23) to make it work. No need to create a unitless variable manually.

4) Concerning cycles you may define your own "unit" (its not a unit anyway) for this. Assign cycles:=1 and label the word cycles as "unit".

## Re: Curve fitting units

@Werner_E wrote:

4) Concerning cycles you may define your own "unit" (its not a unit anyway) for this. Assign cycles:=1 and label the word cycles as "unit".

cycles: = 1

Pi + cycles=4.142 ???

Better use cycles: = mole

## Re: Curve fitting units

wrote:

Pi + cycles=4.142 ???

Garbage in - garbage out 😉

Better use cycles: = mole

Is it really better?

## Re: Curve fitting units

why not cycles = 2*pi*rad?  or cycles := rev

"rev" is a built-in unit!

Pure Genius!

AWWWW

shucks!

## Re: Curve fitting units

Hmmm! where do you create this unit?

## Re: Curve fitting units

on your top row, select cycles and under math/labels select unit.  The font and color will change to match other units on the sheet.

## Re: Curve fitting units

Be careful with this.  Depending on your usage, this may incorrectly change the value of the result.

From the equations provided, I did not interpret "cycles" to be equal to revolutions.  Instead, I interpreted "cycles" to be the number of times an object has been subjected to a design event.  For example, in fatigue design you may be required to design a member for 2M cycles of a given fatigue loading.

I believe OP calculated that a member can undergo the specified stress 3,500 times.  After accounting for a service factor of 6, the member would be rated for only 590 design events.

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