数字图像相关技术在熔融焊点材料中测定CTOA的研究

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Determination of CTOA in the molten material of spot welds using

the Digital Image Correlation technique

R. Lacroix

⇑

, A. Lens

, G. Kermouche

, J.M. Bergheau

, H. Klöcker

École Nationale Supérieure des Mines de Saint-Étienne, SMS, UMR CNRS 5146, 158, cours Fauriel 42023 SAINT-ÉTIENNE Cedex 2, France

Université de Lyon, ENISE, LTDS, UMR 5513 CNRS/ECL/ENISE, 58 rue J. Parot 42023 Saint-Etienne Cedex 2, France

ArcelorMittal Maiziéres Research SA Voie Romaine 57280 Maiziéres-lés-Metz, France

article info

Article history:

Received 19 July 2011

Received in revised form 6 February 2012

Accepted 16 February 2012

Keywords:

Stable crack growth

Crack Tip Opening Angle

Strain localization

Spot weld

abstract

In order to compare the strength of the molten material of different spot welds bonding

steel sheets, a speciﬁc wedge test has been developed. It produces stable crack growth at

the interface of sectioned spot welds. The deformation of the molten material at the pro-

gressing crack tip is observed in situ during wedge insertion. The Crack Tip Opening Angle

and the extend of the strain localization ahead of the crack tip are measured by Digital

Image Correlation. It is shown that the rather simple measurement of the Crack Tip Open-

ing Angle provides a good indication of the extend of the strain localization, which is

believed to be related to the toughness of the molten material.

1. Introduction

Due to environmental issues and cost reduction, the today’s challenge in the transport industry is weight reduction. Thin

(about 2 mm thickness) steel sheets play a signiﬁcant role in the automotive industry. These thin sheets are assembled by

spot welding. But, High Strength Steel welds may exhibit full interfacial failure (i.e. crack propagation through the molten

material at the interface between the welded sheets). Each vehicle contains several thousand welds. The safety analysis re-

quires thus the application of crack initiation and crack propagation criteria to structures of thin sheets assembled by

welding.

Several models for the analysis of crack propagation have been developed within the last three decades for homogeneous

materials. The critical Crack Tip Opening Angle (CTOA) was shown to be the most suited for modeling stable crack growth

and instability during the fracture process of homogeneous materials [1–6]. But, in thin sheet material, crack tunneling and

slanting will lead to non constant values of the CTOA [7,8]. Newman et al. [9] showed that the critical CTOA values should be

determined for various specimen conﬁgurations, crack lengths, and thicknesses. Unfortunately, spot welding of thin (about

2 mm thick) sheets leads to ‘‘small’’ volumes of the fusion zone, i.e. about 4  8  8 mm. Determining the critical CTOA values

for different specimen conﬁgurations and thicknesses is thus almost impossible.

Furthermore, the microstructure in the molten zone of a spot weld of steel is largely heterogeneous, as described by

Zhang and Senkara [10]. Indeed, dendritic growth of austenite occurs during the fast cooling steps of the successive pulses

of the spot welding process, ﬁnally resulting in micron-sized lenticular grains of martensite, along with a signiﬁcant segre-

gation of chemical elements between the periphery and the center of the molten zone. Using classical Linear Elastic Fracture

Mechanics tools to characterize the toughness of such a heterogeneous material and such a non-standard geometry would

raise concerns regarding the validity of the hypothesis associated with these approaches.

doi:10.1016/j.engfracmech.2012.02.012

⇑

Corresponding author. Tel.: +33 (0)1 48 39 57 62; fax: +33 (0)1 48 39 55 62.

E-mail address: remi.lacroix@mail.com (R. Lacroix).

Engineering Fracture Mechanics 86 (2012) 48–55

Contents lists available at SciVerse ScienceDirect

Engineering Fracture Mec hanics

journal homepage: www.elsevier.com/locate/engfracmech

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数字图像相关技术在熔融焊点材料中测定CTOA的研究

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