Current Techniques ENGINEERING STRESS-STRAIN
Engineering Stress [MPa]
800 700 600 500 400 300
Some areas appear to have failed at t=0ms.
200 100 0 0.00
RD0
t=100ms 0.05
0.10
0.15
0.20
Engineering Strain [-]
Conduct tensile test & determine strain or elongation at fracture
0.25
Either; 1. Add failure strain to material card & observe failure in CAE or 2. Inspect CAE results & infer areas at risk of failure
Current Techniques: Issues
10mm Mesh
8mm Mesh
6mm Mesh
Some areas appear to have failed at t=0ms.
4mm Mesh
2mm Mesh
t=0ms
Material model set to fail at EPS=0.8 Plastic Strain at Failure is mesh size dependent !
When used in crash simulation, some elements appear to have failed at t=0ms!
Major Principal Strainε1()
General Fracture Theory: Possible Fracture Prediction Methods Thinning
Maximum Principal Strain
-2.0
Measured FLC
EPS =
Plastic Strain
-1.0
0.0
2 ε 12 + ε 22 + ε 1ε 2 3
1.0
2.0
Minor Principal Strain (ε2)
ISSUES • No theories agree, FLC is used extensively for forming simulation • Current EPS method agrees only at one point
Summary of Current Technique •
Current technique is
• • • •
inaccurate mesh dependent open to interpretation
Leading to…………
• •
increased risk mass inefficient design
Improved CAE Capability of Fracture Prediction in BIW Sheet Metals • • • •
Project Justification Project History Current Technique – Statement of Issues Requirements for CAE Technique • Predict relevant modes of fracture • Accurate model of flow curve, yield locus • Accurate implementation of fracture criteria • Accurate treatment of non linear strain paths & ‘damage’ transfer from stamping to crash CAE model
•
• • •
Mesh independence of prediction
Project Examples Limitations Summary & Risks
CAE Technique: A New Approach CrachFEM •
CrachFEM is similar to a FLC approach but also includes…
•
Non-linear plastic strain accumulation
•
A more comprehensive range of fracture modes than just necking
•
Improved yield locus model (kinematic-isotropic hardening)
•
A mesh independent solution
MATFEM Example of CrachFEM Fracture Prediction (Aluminium Box)
DP600 CrachFEM Fracture Curves
Material Model: Hardening & Yield Locus Evolution 1.2
NG5754 DP600 MAT 24
0.8
0.4
0.0 -1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
-0.4
-0.8
-1.2
Yield Loci for 2 materials & the Von Mises criteria as used in Dyna Mat24 FLOW CURVE 900 800
True Stress [MPa]
700 600 500
1000
400
250
500 100 10
300
1 0.1
200
0.01 0.008
100 0.0
0.2
0.4
0.6
True Plastic Strain [-]
0.8
1.0
Creation of Fracture Limit Curves
1.00 DNF
Ductile Normal Fracture
DSF FLC GENYLD
0.80
Ductile Shear Fracture 0.60 EPS
Source: W. Schatt, Einführung in die Werkstoffwissenschaften, 1991, ISBN 3-342-00521-1
0.40
Instability (necking) 0.20
-1.00
ductile fracture
Modelling Metals with MF GenYld + CrachFEM © MATFEM 2008 - Seminar
shear fracture
-0.75
-0.50
-0.25
0.00 0.00
α
0.25
0.50
0.75
1.00
Ductile Fracture Model: Strain Paths EPS =
2 ε 12 + ε 22 + ε 1ε 2 3
α = ε 2 ε1
Potential, improved criteria
Current, EPS failure criteria
Strain Paths: Single Element Test
PLANE STRAIN COMPRESSION
UNIAXIAL COMPRESSION
SHEAR (APPROX.)
UNIAXIAL TENSION
α=0
α=-2
α=-1
α=-0.5
TENSION X COMPRESSION Y
α=-1
PLANE STRAIN TENSION
α=0
BIAXIAL TENSION
α=+1
BIAXIAL COMPRESSION
α=+1
Strain Paths: FLD Domain • Gradient indicates strain path • Very difficult addition of non linear strain paths
Linear Strain Paths 1.00 DNF DSF FLC GENYLD
0.80
EPS
0.60
0.40
0.20
Shear
-1.00
Tension
-0.75
-0.50
Plane Strain
-0.25
0.00 0.00
Îą
Biaxial Tension
0.25
0.50
0.75
1.00
Non Linear Strain Paths: Damage Accumulation Material will start to fail when ‘Damage’ = 1. 0.60
Damage = 0.5 (0.25/0.5) Total Damage =1
0.40
EPS
0.50
0.30
Tension Damage = 0.5 (0.3/0.6) Total Damage =1
CRASH 1 Tension
CRASH 2 Biaxial
0.20
0.10
STAMPING Plane Strain
Damage = 0.1/0.2 = 0.5
-0.50
-0.25
0.00 0.00
0.25 α
0.50
0.75
1.00
FR5 Mesh Dependence • The CrachFEM instability risk (FR5) can be considered to be mesh independent for elements in the 2-10mm range.
3mm element deletion range 10mm Mesh
8mm Mesh
6mm Mesh
4mm Mesh
2mm Mesh
Error ~ 8%
Improved CAE Capability of Fracture Prediction in BIW Sheet Metals • • • •
Project Justification Project History Current Technique – Statement of Issues Requirements for CAE Technique • Predict relevant modes of fracture • Accurate model of flow curve, yield locus • Accurate implementation of fracture criteria • Accurate treatment of non linear strain paths & ‘damage’ transfer from stamping to crash CAE model
•
• • •
Mesh independence of prediction
Project Examples Limitations Summary & Risks
Example: L359 DP600 X-Member Drop Test X-Member with cutaway arches 60kg Impactor Drop height up to 5m
CAE Prediction 2.3m Drop
No failure predicted (c.f. EPS) -Fracture is predicted at the ∅15mm U slot -Note: CAE model includes damage from stamping simulation
Fracture Risk
Test 2.3m Drop
Fracture occurs at the apex of the U slot
Summary of 2.3m Drop • •
Tested part fractured CAE predicted the failure
But………..
•
2.3m drop had more than enough energy to create fracture
So………...
•
Can we predict the energy required to take part to limit without failure?
CAE Prediction 1.95m + 0.5m Drop 1.95m Drop
Subsequent 0.5m Drop
Fracture Risk
Fracture risk at the ∅15mm U slot is 0.88 – no fracture predicted
Fracture is predicted at the ∅15mm U slot – fracture predicted
Drop Test – 1.95m +0.5m Drop
In the 1.95m drop, no fractures occur.
In the subsequent 0.5m drop, fracture occurs from the apex of the ∅15mm U slot
Summary of Component Test •
New CAE technology for predicting fracture in thin sheet metals has been validated through component testing.
•
CAE accurately predicted the energy to fracture, at the correct location
•
CrachFEM is easy to use, providing a simple fracture risk without the need for extensive knowledge of methods.
•
CrachFEM technology shows significant improvement over current state of the art.
Limitations of Current FE Analysis • Discontinuities created by welds or rivets affect fracture performance.
SPR
RSW
5mm Finite Element
5mm Finite Element
• Difference in cut edge can’t be captured in shell elements • Capture of notches etc. is limited by mesh size e.g. SPR model requires 0.1mm solids.
5mm Finite Element
Cut Edges Thickness
=> Further work required to characterise increased risk from these features.
Phase II Project – Effect of SPR & Welds • • • •
Al:Al SPR & RSW, Steel:Steel RSW, Al:Steel SPR (Mixed Metal) Coupons manufactured by WMG & STC Coupons joined by WMG Testing at STC
JLR
NG5754 2mm SPR Direction = Patch to Sample
NG5754 2mm SPR Direction = Sample to Patch
NG5754 2mm RSW
TOTAL
Sample 1
3
3
3
9
Sample 2
3
3
3
9
Sample 3
3
3
3
9
Sample 4
3
3
3
9
No Patch
3
0
0
3
TOTAL
15
12
12
39
CORUS Spotwelded Steel Patch SPR Aluminium Patch No Patch TOTAL
Boron Steel Coupon (0.95mm) 12 12 3 27
DP800 Coupon DP600 Coupon (1.5mm) (1.5mm) 12 12 3 27
TOTAL
12 12 3 27
36 36 9 81
TOTAL
120
Phase II Coupon Tests – Effect of SPR/Weld Proximity to Edge Investigations into the effects of RSW & SPR on aluminium, steel & mixed metal coupons.
Sample 2, 3 almost identical
SPR Centre is 18mm from hole edge SPR Centre is 13mm from hole edge SPR centre is 8mm from hole edge (standard minimum). SPR Centre is 5mm from hole edge 10mm Dia. Hole Gauge is 2mm 160mm
Discretisation adequate to detect differences in test
Example 1: L538 Side Impact EPS
Max EPS 24%
Max FR 0.74
Plots @ 60ms IIHS(SC5)- 5dr Baseline (Run A504b)
BENTELER BORON CrachFEM MAT (A507a)
L538 Door Beam Example
Very high risk using EPS => large change in gauge to fix the issue
Very small area of risk using CrachFEM => small change in gauge to fix the issue
L538 SC4 Door Beam
Original Design
Improved Design
Plastic Strain @ 45ms
Plastic Strain @ 80ms
CrachFEM @ 50ms
CrachFEM @ 80ms
Plastic Strain @ 45ms
Plastic Strain @ 80ms
CrachFEM @ 50ms
CrachFEM @ 80ms
Example 2: L538 Side Impact Max FR 1.59
EPS
CrachFEM
Elements deletion starts @ 57.5ms
Max EPS 29%
Max FR 1.59
Europole(SC4)-5dr Baseline (Run A504e)
L538 5DR B Post: UPDWG SC5
Test?
Max. EPS = 0.25
Max. FR = 0.86
Failure Criteria = >0.06
Failure Criteria = >1
L538 3DR B Post: UPDWG SC7
Test?
Max. EPS = 0.19
Max. FR = 0.56
Failure Criteria = >0.06
Failure Criteria = >1
Extra •
New CAE technology for predicting fracture in thin sheet metals has been validated through component testing.
•
CrachFEM is easy to use, providing a simple fracture risk without the need for extensive knowledge of methods.
•
CrachFEM technology shows significant improvement over current state of the art.
•
‘Damage’ accumulated in stamping & crash needs to be included in the simulation.
•
Technology can be implemented on program for cold formed panels.
Strain Paths Practical 0.50 Example 0.45
Equivilent Plastic Strain
0.40 2. CRASH Damage to fracture = 0.3 (1-0.7) along uniaxial tension strain path.
0.35 0.30
X Fracture
0.25
e ur l i Fa
Cr
ria e it 6111T4 2mm
EPS Limit from Tensile Test
0.20 0.15 0.10
1. STAMPING Damage = 0.7 along plane strain tension path (Îą=0)
0.05 0.00 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Ratio of Principal Strains