Hole Expansion Ratio Test Procedure with

Design of Experiments

Study with Design of Experiments

Curt Horvath, General Motors

Eric Batt, ArcelorMittal USA

Team Members

Mentor: Dean Kanelos - Nucor Steel

Co-Principal Investigator: Eric Batt - ArcelorMittal

Co-Principal Investigator: Curt Horvath – General Motors Company

Project Manager: Michael Bzdok, Bart Clark

Statistical Consultant Pat Hammett – MVS Group, LLC

A/SP Team Members:

J. Bickham – ArcelorMittal

J. Cole – Ford Motor Company

R. J. Comstock – AK Steel

J. J. Coryell – General Motors Company

C. Matthew Enloe – General Motors Company

J. J. Fitzpatrick - ArcelorMittal

M. P. Hammerl – AK Steel

M. M. Huang – ArcelorMittal

P. Makrygiannis – AK Steel

A. D. Pearson – General Motors Company

R. Radzilowski – AK Steel

N. Ramisetti – ArcelorMittal

D. S. Ruhno – Ford Motor Company

J. Singh – FCA Group

P. Som -ArcelorMittal

J. Stachowski - Nucor

A. Thompson - Nucor

Y. Wang – AK Steel

S. Wolf - ArcelorMittal

W. Wu – AK Steel

Objective

• Identify Key Variables in Hole Expansion and

Perform a Design of Experiment (DOE) to Validate

A/SP Hole Expansion Test Procedure

Hole Expansion Test

Where:

Df – D0

D0

x 100HER =

Hole Expansion Test

Parameters

• Inputs

− Material – DP 590 MPa

− Gauge – 1.4 mm

• Output

− Hole Expansion Ratio (HER%)

• Analysis

− Mean, Standard Deviation

− N (sample size) – approximately 30

Variables Investigated

• Reading Style – Manual or Camera

• Hole Punch Location

• Test Speed –

• Slow (~0.2 - 0.3 mm/sec) or Fast (~1.0 mm/sec)

Additional Work Completed

• Round Robin

• Different Steel Grades

• Commonization of Sample Preparation

Reading Style: Camera vs. Visual (Manual)

• Throughout this study, two primary methods of data collection

were used;

• ‘Visual (or Manual)’ Method

− Rely on operator to stop test at observed breakpoint (TTC)

− Used calipers to measure hole diameter to compute HER

• ‘Camera’ Method

− Recorded video and reviewed to find breakpoint per TTC

definition

− Electronically calculated hole diameters to compute HER

TTC: Through-Thickness Crack

HER: Hole Expansion Ratio

Definition of Through-Thickness Crack (TTC)

From ArcelorMittal

• The TTC definitions may be

defined with the three criteria as

shown

• For visual test: Test is stopped at

the TTC of approximately 0.1 mm

― ‘Visual (Manual)’ Method

• Similarly, for semi-automatic video

HER testing, the definition of the

TTC is based on the 0.1 mm crack

width

― ‘Camera’ Method

Summary: Test Conditions

• The following represents a

summary of the general

test conditions used during

Round Robin Testing

− Note: Some of these were

purposely varied in Phases 1-

4

− Actual conditions vary slightly

by test source (see Test

Location Comparison Matrix

for further details by test

site)

A/SP Round Robin Hole Expanding Test (8-2015)

ASP

Specifications

Testing

Up / away from punch

Angle (degree) 60

Diameter (mm) 40 (approx)

Material Carbide

R (mm) 5 (approx)

Dia (mm) 52±3

Material D2/Tool steel

Punch centering

? 50kN

0.3mm/sec (visual)

1.0mm/sec (camera)

Not specified

Crack thru thickness

0.1±0.03mm A.I.M.

Others

Lubrication at test None

Lubrication vol None

Tester Not specified

Measuring

0.05mm unit

30

Ave of 0 and 90deg or

diameter of circle fitted

Measurement

N of test

Calculation

Cushion press

Form speed

Forming speed check

Crack definition

Crack Determination

Testing Method and Conditions

Burr dir at test

Punch spec

Die spec

Centering

Test Locations / Methods

• Supplier 1– location 1 – Visual

• Supplier 1 – location 2 – Visual

• Supplier 1 – location 1 - Camera

• Supplier 2 – location 1 - Visual

• Supplier 3 – location 1 - Visual

• Supplier 4 – location 1 – Visual

• Supplier 4 – location 1 – Camera

• Supplier 4 – location 2 – Camera

• Supplier 3 – location 2 - Camera

Variance Effects: Reading Style and Test Speed

• No significant difference in variance across 4 combinations

− Average St Dev (n=30) = ~7.6

− Camera-Fast had lowest St Dev but not statistically different

Assume alpha = 0.05

Combination St Dev

Manual-0.2 8.64

Camera-0.2 7.23

Manual-1 7.61

Camera-1 6.77

7.56Average

Implication: No effect on Std. Deviation attributed to study factors

Variance Effects: 6 Study Combinations

• Although mean differences were observed, standard deviation

was not significantly different across the 6 study combinations.

− Pooled Std Deviation ~ 7

Variation Consistency Across All Tests

• No significant difference in variance across test combination group

− Note: Based on tests of 30 or more observations

Pooled StDev = 7 Note: 95% CI (N=30): 5.6 – 9.4

Within a

lab/test

combination,

variation is very

consistent!

Variance Effects: Six Study Combinations

• Although mean differences were observed, standard deviation

was not significantly different across the 6 study combinations.

− Pooled Std Deviation ~ 7

Round Robin – Variation

• No significant difference in variance across test labs(St Dev = 6.5).

− Note: Based on tests of 30 observations

Note: 95% CI (N=30): 5.2 – 8.7

Standard

deviation was

consistent from

lab to lab

ANOVA Model Summary

Pooled S 6.5

Round Robin – All Lab Results

• Mean Differences observed from lab-lab with: − Overall Mean HER = 53.7; Low Mean HER = 44; High Mean HER = 67

− Individual Readings (Min/Max): Min = 34; Max = 78

Sigma Repeatability vs. Sample Mean HER

• Sigma Repeatability (St Dev) appears a function of the sample

mean (based on n=5-6) with significantly more variation as

mean HER increases (characteristic of material).

CV = ~0.15 – 0.17

CV (coefficient of variation)

was similar across materials

CV = Mean

Std. Dev.

Sigma Repeatability by Test Lab/Material

• When data are segregated

by material, there is no

significant difference in

sigma repeatability by test

location.

HDG HSLA 420 (1 mm)

Average 43.1 Overall S = 5.8

( mean +/- 3S = 25.7-60.5)

DP980 Bare (1 mm)

Average 18.6

Overall S = 2.6

Mean +/- 3S =10.8-26.4

HR590SF (2.5 mm)

Average 80.7

Overall S =13.4

Mean +/-3S =40.5-120.9

Summary and Recommendations

• Variability of hole expansion ratio results are a combination of:

− Microstructural differences within the material

− Slight variation in the quality of the sheared hole

− Inherent differences in specific testing equipment from site-site

− Inherent challenges in uniform interpretation of a through-thickness crack

• Phase 5 reinforced recommendation that testing to date

demonstrates:

− Current hole expansion test can be useful for qualification of a product

based on a representative sample (e.g., Sample Size N = 30).

− However, current test is not suitable for evaluating acceptance criteria for

shipped material because of the inherent variation in material/testing

results – particularly for material with higher average HER values.

#GDIS

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