## HOWTO Calculate CRASH3 A & B Coefficients, AKV (for SMAC)

Questions/Topics related to Damage and Crush Analysis
MSI
Posts: 1765
Joined: Thu Jun 18, 2009 12:37 pm

### HOWTO Calculate CRASH3 A & B Coefficients, AKV (for SMAC)

This is a very preliminary writeup
In the tools menu of msmac3D we have the Lsqfit - A&B Coefficient Calculator (same program as discussed below)
and we have now added a link to it to the Ver 3.3 3DGraphics HELP Button with a direct link to the NHTSA CASH Test database
lsqfit and crash test databse on Help menu.jpg (19.38 KiB) Viewed 208 times
The Crash test Database brings up the test selection menu (on Left see below) which allows you to select Make, then Model, Then Year
and all Tests are then listed:
Select a Test from the list of tests for the make/mode/yr/etc and then:
• From the datasheet take the average crush, test weight,
From the data on the sheet Record the following (numbers listed in red):
1. the average crush
• C1,C2,C3,C4,C5,C6
Add these 6 crush measurements and divide by 6
2. the Veh Speed Test Impact Speed
3. The Weight aka Test Weight
4. The LENCNT aka Length of Contact
• for some reason they don't present in inches in () like other nums, so simply divide by 25.4
5. The Test# aka Test number
6. Note that if photos, videos, reports available these buttons will activate you can review them/collect them/etc
NHTSA Crash Test Database Dialog v3.3.jpg (254.44 KiB) Viewed 209 times
We also recommend you check the Sisters and Clones Database from Greg Anderson
• We have an older version of that Sisters Clones Database available if you pop out to Medit3D Grid: Database Menu->
Medit3D Grid Database Menu.jpg (28.45 KiB) Viewed 208 times
It is probably 10 or so years old so for vehicles made in past decade and for refinements of the database please consider a subscription to the Sisters and Clones database
Then select the LSQFIT Program which brings up a file selection menu (to load prior tests)
• An instructional video on the LSQFIT will be posted and linked here soon
The file menu will have an array of files (note some of these will be in the ver 3.31 update to be updated shortly
lsqfit file selection menu.jpg (73.54 KiB) Viewed 206 times
We selected the 1985 Ford Escort
• From the Prasad SAE 900412 Report A Study Using the Repeated Test Technique
• one of the datapoints was bad (50 mph/50 inches) or simply skewered the results (see below for details)
• We added 3 more tests from 81-84 which the sisters & Clones Database said are the same body
Here are the inputs we put into the program:
output of pressing enter.jpg (112.43 KiB) Viewed 207 times
And the chart created and A,B CRASH3 Coefficients and also AKV Stiffness were as follows:
prelim display of Ford Escort tests with 3 plus minus outlier.jpg (101.81 KiB) Viewed 207 times
CRASH was developed as a preprocessor for the SMAC simulation program
To Help NHTSA with their statistical NASS study get starting speeds for SMAC
NHTSA Later adopted CRASH since 'the errors will wash out in the large sample size' and that they were 'looking for trends'
However now CRASH and damage analysis have been used in individual crash reconstrctions
Reasons to be cautious when using CRASH or other damage analyses:
• Most CRASH or damage analysis techniques are based on a single data point and assumption of 'no-damage' intercept
• Be sure to check the source of the data for any crush coefficients you use
• below we present the data for a 1985 Ford Escort
and now we show in more practical format:
First a summary of what would be the Crash B coefficient (and corresponding AKV for SMAC) with the data as presented and then dissected:
1985 Ford Escort Summary.jpg (94.38 KiB) Viewed 205 times
Now here are the six items presented in graphical form so you can see that depending on the amount of crush, the results can have major differences.
And realize that most Crush Coefficients sold are based on a single Crash Test at approx 35 MPH and then an assumed 'no damage' intercept.
More write up on this will follow in the next week or so
this first collection from the Prasad SAE 900412 Report has an outlier (the green dot on chart behind data) which skewers the fit away from the other points:
1. Prasad 6 Ford Escort Tests.jpg (52.14 KiB) Viewed 193 times
Here is what the fit looks like if you remove that outlier:
2. Ford Escort Prasad less outlier.jpg (51.59 KiB) Viewed 193 times
Next we added the extra crash tests (sisters/clones of the test vehicle) but again with the outlier (green dot on chart behind data)
Note once again the fit is skewered
3. Ford escord Prasad plus 3.jpg (57.32 KiB) Viewed 193 times
And now with that outlier removed, note how much better the least square fit matches the data
4. Ford Escort Prasad plus 3 less outlier.jpg (55.1 KiB) Viewed 193 times
Now we simply go with 'what if only single tests available (we found 3):
5. Ford Escort 3 tests.jpg (49.3 KiB) Viewed 193 times
We do it again with a low speed test from that Prasad paper:
6. Ford Escort 3 tests plus low speed test.jpg (52.55 KiB) Viewed 193 times
These next 3 are what if only 1 test available for the Escort?
• And which test?
• and how much do the A/B fits change?
here are all 3 side by side and note the change in the A & B Values, depending on which single test the fit is made from?
• This demonstrates the sensitivity and possible changes in the A & B CRASH3 Coefficients depending on test results
• and why it is important to use MORE than a single test and an assumption for the 'no damage' intercept!
Below you see depending on which individual test is chosen (or if only one ran, which one?)
the CRASH3 A & B Coefficients would be:
If based on all 3 tests:
• NHTSA 3 Test: A = 213 lb/in^2 B = 58 Lb/in
If Based on all 3 tests with a low speed test:
• NHTSA 3 Test and Low speed Test: A = 207 lb/in^2 B = 64 Lb/in
If based on eachg individual test:
• NHTSA Test #407: A = 188 lb/in^2 B = 55 Lb/in
• NHTSA Test #452: A = 221 lb/in^2 B = 63 Lb/in
• NHTSA Test #509: A = 190 lb/in^2 B = 70 Lb/in
test 6 7 8 single point fits.jpg (49 KiB) Viewed 160 times
Below are the individual fits:
7. Ford Escort test num 407.jpg (44.86 KiB) Viewed 193 times
8. Ford escort Test num 452.jpg (46.85 KiB) Viewed 193 times
9. Ford Escort test num 509.jpg (44.84 KiB) Viewed 193 times
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MSI
Posts: 1765
Joined: Thu Jun 18, 2009 12:37 pm

### Re: HOWTO Calculate CRASH3 A & B Coefficients, AKV (for SMAC)

A recent posting on another forum included a vendor who sells CRASH3 A & B coefficients
• I have removed any vehicle specific or vendor references to make this more generalized information:

The following were posted up in response to a request for information on the A & B Coefficients for a specific vehicle:
• "Depending on how you calculate the A-B values:
• A= 441 B= 143
• or
• A=689 B=348"

Our response to the posting:
This obviously indicates a wild variability in at A & B Crash3 Crush Coefficients so one wonders HOW? WHY? WHAT data?
• They said "Depending on how you calculate the A-B values"
• Are there different ways to calculate A & B which can skewer the results?
It appears basically a 2.43 to 1 difference for the Crash3 "virtual" Linear Force-Deflection Rate?
• For B the Crash3 "virtual" Linear Force-Deflection Rate: 143 lb/in vs 348 lb/in??!!
• For A the CRASH3 Unit width Force Level for ‘no residual damage’: 441 lb/in^2 vs 689 lb/in^2??!!
• And so obviously any analysis will be skewered by which one is chosen and used? Or a wide range of results.
I looked at the test mentioned, The test had an average crush of approx 10" at 35 MPH!
and if A&B values calculated it indicates a quite high value for B.
But i also looked at the videos for the test!
• The 35" front overhang is crushed to wheels so more like 20" crush??
• is that how they got such wildly different results?
And also see elastic response of tires into walls which was originally posted to the NAPARS facebook page and i mentioned in my followup post: I also checked the sisters/clones and they seem to indicate that 2009-2017 share the same body (quick look, obviously double check)

These tests may all be a product of the 'SUV tire/wall interaction' elastic rebound which makes the frontal structure seem extremely stiff (once the tires encountered and if they don't deflate you get a large amount of elastic rebound and reduction in residual crush.

Only reason i post this up is that folks who use damage analysis need a explanations for wild variations in crush stiffnesses.

The main points:
• Below is our calculations of the A & B for the test vehicle, with adjustments of the average crush of 0,1,2,4,6,8,9,and 10 inches.
Best match of the two Vendor posted values is:
• use of 5 MPH assumption for 'no damage intercept'
• For the Higher stiffness values (A=689/B=348) the Vendor adjusted the measured average crush by 2.5 inches
• For the Lower Stiffness values (A=441/B=143) the Vendor adjusted the measured average crush by 9 inches
compare adjustments of test results with vendor A and B.png (86.61 KiB) Viewed 149 times
Here's the sanitized test results (VIN & Veh identification removed to keep this topic more generalized):
sanitized veh data.png (151.34 KiB) Viewed 95 times
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