FRACTURE MECHANICS APPROACH
TO HETEROGENEOUS WELDED STRUCTURES
Dražan Kozak1, Nenad Gubeljak2, Pejo Konjatić3
1 Mechanical Engineering Faculty, Slavonski Brod, CROATIA, e-mail:
2 Faculty of Mechanical Eng., Maribor, SLOVENIA, e-mail:
3 Mechanical Engineering Faculty, Slavonski Brod, CROATIA, e-mail:
Summary: A large data bank of information related to welded structural steels has been used to assess the influence of weld metal strength mis-matching on the fracture resistance of these weldments, based on Charpy impact test results, crack tip opening displacement CTOD values and wide plate results, along with supporting welding details, metallography and mechanical properties. This paper discusses some aspects of fracture behaviour of welded structures with different levels of strength mis-match. However, generally accepted conditions for the strength mismatched welded joints of high strength steel are not yet clearly defined.
Keywords: fracture behaviour, welded structures, strength mismatch, weldment performance
1. INTRODUCTION
The effect of strength mis-match in steel weldments has received much international attention over the last years [1-5]. A main reason for this interest is the increased use of steels with higher strength and the difficulties in satisfying the specified weld metal toughness and high productivity with a demand of 10-15% weld metal strength overmatch with respect to the base material. These so-called high strength steels (HSS) can give yield strengths of 450-1000 MPa ensuring a good combination of toughness and weldability. They are seeing particular application in mobile installations, such as cranes and offshore platforms, and highly loaded applications such as roof supports in mines. However, allowable mismatch levels in weld metal strength are still the topic of investigations. Overmatching (OM) weld metal strength has been the normal condition for conventional structural steels for some years and can be achieved in these steels with normal welding procedure with suitable levels of weld metal toughness. However, the achievement of an overmatching weld strength condition while maintaining adequate weld metal toughness becomes by HSS more difficult. This raises the question whether is overmatched weld strength necessary for such steels and, if not, what level of undermatching can be tolerated without compromising a joint’s performance.
2. ASPECTS OF MIS-MATCH IN HIGH STRENGTH STEELS
In order to assess the significance of mis-match in HSS it is necessary to consider not only the relative yield strength levels of base metal, weld metal and heat affected zone (HAZ) but also their work-hardening characteristics, their fracture toughness, the geometry of the joint, location of defect, the welding procedure, the particular application of the structure and the philosophy of the design code to which it has been designed. Mis-match level is usually defined by strength mis-match factor M as the ratio of yield strength of the weld metal (WM) related to yield strength of the base metal (BM):
(1)
There are basically two different views with respect to weld metal yield strength matching. Since toughness is more readily attained at lower strength levels, some researchers suggest to consider undermatching (UM) weld metals as far as weldment performance and weld deposition rates are concerned [6].
The advantages of undermatching weld metal in HSS are:
-avoidance of cold cracking and other fabrication-related defects,
-improved weld metal toughness,
-reduction of welding residual stresses,
-reduction in HAZ constraint,
-relaxation of stress in the HAZ in the case of a softened HAZ etc.
However, the shortcoming of this UM approach is that weld metal as relatively small part of the structure may collapse while the base metal of the plate might be loaded elastically. In that case a carrying capacity of the whole structure is not fully used.
In the other hand, codex by welding practice orders to produce the welded joints with at least the same or greater strength than the yield strength of the base metal. Thus, plastic strains remain in the base metal and possible crack arrest in the weld metal preventing the structure from the crack propagation into base metal. Nevertheless, one should be reasonable by choosing of the filler material because OM greater than 20% on principle causes lower toughness e.g. for HS steels. Also, when welding HSS by the manual arc welding process, it is not easy to obtain weld deposits that overmatch the base metal in yield strength and toughness. A solution for that problem is to make hybrid welds i.e. the root area should be welded with matching whilst the filler and cap passes should be made with highly overmatching weld consumables.
Mis-match effect can be illustrated on the example of the butt welded plates which are tensile loaded either parallel (Fig. 1a) or transverse to the weld line (Fig. 1b). Weld metal in both cases could be OM or UM.
For butt welds loaded parallel to the weldment, both weld metal and base metal are strained equally and simultaneously. The weld metal, regardless of its yield strength level, will be forced to strain with the base metal. This implies that the differing stress-strain characteristics of the various weld zones have no direct effect on weld performance. Thus, a weld with its length parallel to the applied load is forced to yield and deform by equal amounts despite its OM (or UM) yield strength if the surrounding base metal yields.
Figure 1a: Parallel loaded weldment / Figure 1b: Transverse loaded weldmentWhen the welded plate is subjected to the nominal stress 0 transversally, the metal with the lowest yield strength determines its fracture behaviour. It is obviously from the Fig. 1b that UM weld metal is exposed to largest deformation. One can conclude that OM weld metal loaded transversally may significantly reduced the risk of the structure failure.
2.1. Overmatch vs. undermatch
Many parameters influenced fracture behaviour of the welded structure with cracks: weld groove geometry, weld strength mis-match, crack length/structure width (a/W) ratio, crack location, thickness of the construction, type of the loading etc. Here, the effect of mis-match has been investigated analysing the single edge notch bend specimen (SENB) containing a 45º single-V groove filled with 20% OM or 20% UM weld deposit [7]. Shallow crack (a/W=0,15) has been located in the middle of the weld metal. The aim of this analysis was to compare plastic zones in the specimen for two combinations of the weld metal. For this purpose, finite element modelling was performed. Each specimen has been subjected to such amount of the loading, which results with the same load line displacement. Fields of equivalent stresses are presented on the Figure 2.
It is evident that 20% UM condition increases the risk of the plastic collapse because yielding zones spread only in the weld metal. In that case, the width of the V-groove root is very important. Narrow groove may present the geometric constraint to the weld metal yielding. As a consequence appears the fracture by lower level of loading relating to the other specimens. Wider root reduces the constraint, but of course this is not beneficial from the economic point of view. Therefore, 10% of UM is usually taken as a lower bound of the strength mis-match with no significant influence on the fracture behaviour of the whole structure.
On the other side, yielding in the material of the specimen with 20% OM is better distributed. In that case yielding zone of the weld metal is smaller than the same in the specimen with 20% UM. Crack propagation from the OM weld metal toward softer BM is obstructed. Thus, more prevention to the stable/unstable failure is achieved. Also, carrying capacity of the construction is then more used.
Figure 2: Plastic zones for the SENB specimen with 20% UM and 20% OM3. CONCLUSION
When the weld metal yield strength is undermatching, yielding will start in the weld deposit, what in the case of UM>10% may be dangerous for the structure stability. The width of the weld groove is then very important. Overmatching with OM<20% is more desirable condition, because OM weld metal stops the crack growing into base metal. Therefore, carrying capacity of such OM welded structure is significantly better.
REFERENCES
[1]Thaulow C. and Toyoda M.: Strength mis-match effect on fracture behaviour of HAZ, IIW Doc. X-F-033-96, April 24-26, 1996, Reinstorf-Lüneburg, F.R.G.
[2]Spurrier J., Hancock P. and Chubb J. P.: An assessment of weld mis-matching, Engineering Fracture Mechanics, Vol. 53 (1996) No. 4, p. 581-592
[3]Gubeljak N.: Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints, International Journal of Fracture, Vol. 100 (1999) p. 155-167
[4]Gubeljak N., Kozak, D., Matejiček, F.: Prediction of strength mis-match welded joints failure by using of crack driving force curve and crack tip opening displacement (CTOD) resistance curve, Strojarstvo, Vol. 42 (2000) No. 3,4, p.85-100
[5]Denys, R.: Provisional definitive statement on the significance of over and undermatching weld metal strength, IIW Doc. X 1222-91, The Hague 1991
[6]Kirk M. T., Dodds R. H.: The effect of weld metal strength mismatch on the deformation and fracture behaviour of steel butt weldments, UILU-Eng-91-2002, Structural Engineering Studies, University of Illinois, 1991
[7]Kozak, D.: The contribution to numerical and experimental analysis of fracture behaviour of the heterogeneous structures, Dissertation (in Croatian), University of Zagreb, 2001
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