Study on fracture mode of spot weld joint using continuum damage mechanics model
Osaka Prefecture University

Kazuki IKUSHIMA
Ryohei NATSUME
Mitsuru OHATA

Osaka University

Plug fracture＜Spot welding joint＞

HighPrediction of fracture mode is important
to investigate the mechanism of changing fracture mode

Simplified crack propagation analysis method

Apply to the analysis of cross tension test on spot weld
Plate thickness

Nugget diameter

Plate thickness：tPlate thickness

Crack
Crack

50mm
※Follow : JIS Z3137

Analysis flow
Set up analysis modelΣ𝑚𝜀𝑝 = 𝐴𝑒𝑥𝑝 𝐵Σ
Stress triaxiality
dependent ductility

Add external force
Elastic-plastic analysis

Determination of element fracture
NOStress triaxiality

Nugget diameter:
3 𝑡, 4 𝑡, 5 𝑡
Plate thickness:
1.2mm, 1.4mm, 1.6mm
2.2mm, 2.4mm, 2.6mm, 3.0mm

Ductilecracking

Applied strain 𝜀Crack【Zoomed view of welded part】
150mm

Submicron voids
generation
Micro voids
formation

Reference : M.Ohata, T.Fukahori, F.Minami: Damage Model for Predicting the Effect of Steel Properties on Ductile Crack Growth Resistance,
International Journal of Damage Mechanics, Vol. 19(2010), pp.441-459

・Proposal of analysis method to simulate ductile fracture
・Prediction of the fracture mode of the spot weld joints

Analysis model(Ⅱ) Micro voids(Ⅰ) Submicron(Ⅲ) Ductile
voids generation formation
cracking
Damage model based on the observed damage process

Equivalent plastic strain 𝜀𝑝Interface + Plug fracture

Stage (Ⅲ)

Stage (Ⅱ)

Stage (Ⅰ)

Strength of joint：LowObjectives

Progress of damage
Damage fraction

Strength of spot weld joint may decrease on high tensile steel
depending on the fracture mode

Fracture mode of spot weld jointInterface fracture

YANO
Masakazu SHIBAHARA

Proposal of analysis method

Background, Objectives of this study
Problems ofspot welding joint○ TakahiroΣ𝑚ΣΣ𝑚
𝐸𝑝 = 𝐴𝑒𝑥𝑝 𝐵ΣYESDeactivate element
𝐹 = 𝐹𝑡𝑜𝑡𝑎𝑙

Fracture is evaluated from only
stress triaxiality and equivalent plastic strain

YES
FinishDuctile fracture is predicted by performing elastic-plastic analysis

Influence of plate thickness on fracture mode
Analysis model and result

Influence of nugget diameter on fracture mode
Analysis model and result【Zoomed view of welded part】
Plate thickness : t【Distribution of equivalent plastic strain】
𝑑=5 𝑡
𝑑=3 𝑡

【Zoomed view of welded part】

Plate thickness : t
7.56mm

1.6mm【Distribution of equivalent plastic strain】
𝑡 = 1.2𝑚𝑚
𝑡 = 1.6mm

1.6mm
Crack initiation

Nugget diameter : dResult of experiment

On propagation

5 𝑡

Reference: SADASUE Teruki, TANIGUCHI Koichi, MORIKAGE Yasushi, IGI Satoshi, IKEDA
Rinsei and ENDO Shigeru : Numerical study on fracture behavior of resistance spot
welding for high strength steel sheets, Welding Society, Vol92, pp.166-167.

Factors of difference of fracture mode
3 𝑡（Interface fracture）
Interface fractureInterface fracture

ACBD

Plug fracture

Plug fracture

0.20

Stress triaxiality
dependentductility

B

0.10

0.05

A
0.00

0.0

1.0

2.0

3.0
4.0
5.0
Stress triaxiality

6.0

7.0

Large stress triaxiality ⇒ Interface fracture

D0.15

On propagation

On propagation

On propagation

Plug fracture

Plug fractureInterface fracture

Plate thickness : thin ⇒ Plug fracture, Plate thickness : thick ⇒ Interface fracture
Load-displacement curve in several plates thickness
14000
Stability
region

UnStable
region10000
8000

1.2mm (Plug fracture)
1.4mm (Plug fracture)
1.6mm (Plug fracture)
2.2mm (Plug fracture)
2.4mm (Plug fracture)
2.6mm (Interface fracture)
3.0mm (Interface fracture)

6000
4000

0.05

2000

C0.00

Crack initiation

12000

Stress triaxiality
dependent
ductility

0.10

Crack initiation

Force (N)

0.15

5 𝑡（Plug fracture）

Equivalent plastic strain

Equivalent plastic strain

0.20

Crack initiation
On propagation

Plug fractureInterface fracture
These results are in agreementwith result of the past experimentInterface fracture Plug fracture𝑡 = 2.6𝑚𝑚

Crack initiation( 𝑑 = 3 𝑡, 4 𝑡, 5 𝑡 )3 𝑡Plate thickness : t𝑡 = 1.2mm, 1.4mm, 1.6mm,2.2mm, 2.4mm, 2.6mm, 3.0mm

0.0

1.0

2.0

3.0
4.0
5.0
Stress triaxiality

6.0

7.0

Large equivalent plastic strain⇒Plug fracture

Change of fracture mode due to nugget diameter can be predicted

00.0

5.0

10.0
15.0
20.0
Displacement (mm)Change of fracture mode due to plate thickness can be predicted

Conclusions
In this study, a simplified ductile fracture evaluation method based on continuum damage mechanics was proposed. And the proposed method was applied to cross tension
test on spot weld joints to investigate fracture modes. The following results were obtained.
1) It was shown that the fracture mode of the cross tension test on spot welding can be predicted by using the proposed method.
2) It is found that size of nugget diameter affect fracture mode, in the case of small nugget diameter, fracture mode is interface fracture, and in the case of large nugget
diameter, fracture mode is plug fracture.
3) Through the comparison of the fracture behavior, it was found that crack propagates into the nugget due to large stress triaxiality in the case of interface
fracture. On the other hand, crack propagates along the outer periphery of the weld nugget due to the large equivalent plastic strain in case of plug fracture.