Advanced Technologies for Developing Countries
September 12-14, 2002
Slavonski Brod, Croatia /
SPREADING OF PLASTIC ZONES BY HETEROGENEOUS FRACTURE TOUGHNESS SPECIMENS CRACKED IN THE HAZ
Dražan Kozak, Nenad Gubeljak, Franjo Matejiček, Maks Oblak
Keywords: weldments, HAZ, fracture toughness, strength mismatch, FEA, plastic zones
1. Introduction
It is almost impossible to produce welded joint with homogeneous structure. This structure is composed from at least three different regions: base material, weld metal and heat affected zone (HAZ). Coarse-grained HAZ (CGHAZ) is in many welded structures considered as preferred location for the initiation and propagation of cracks. The situation becomes more complex if some of high strength low alloyed (HSLA) steels as base metal of the structure are considered [1]. A lot of work during last 10 years has been devoted to investigate an effect of strength mismatch on fracture mechanical behaviour of HAZ notched weld joint [2,3]. For HAZ crack position the effect of strength mis-match will depend on the detailed conditions of testing and calculations [4]:
- local vs. global mis-match,
- specimen geometry,
- crack depth,
- notch positioning,
- toughness level,
- mode of failure, e.g. brittle, ductile or plastic collaps,
- mode of loading etc.
-
This paper concentrates more stress and strain level adjacent to a HAZ crack tip in order to causes of fracture be better understood. Distribution of stresses and deformations could be calculated using some of commercial finite element analysis (FEA) software, but also f.e. crack path deviation (CPD) as well as possible geometric constraint to further yielding. Namely, brittle HAZ may appear as barrier to flowing of weld metal. Therefore, FE calculations of stress and strain distribution in the vicinity of the HAZ crack tip are the topic of the recent investigations [5,6].
The work presented in this paper shows spreading of plastic zones in different materials as well as maximum stress magnitude and location by increasing of loading.
2. Fracture toughness testing of weldments
HSLA steel with 700 MPa strength class was used as a base material (BM) of the weldment (Nionicral NN70A). The root of X-welded joint was produced with two passes of overmatched metal with strength mismatch factor M=1,13 (W.B.525 wire) and filler was produced with M=1,22 (W.B.800 wire). Three-point bending test specimens with 36 mm of thickness were extracted from the welded joints (Fig. 1). These bend specimens were notched from the surface to almost 1/3 of specimen width trying to locate notch tip in the HAZ region. All specimens were fatigued in accordance with BS 7448 [7]. During fracture toughness testing force, load line displacement LLD, crack mouth opening displacement CMOD, crack tip opening displacement CTOD (δ5) as well as crack extension Δa were measured. These parameters are necessary to describe fracture behaviour.
Figure 1. Single edge bend specimen cracked in the HAZ
3. Finite element modelling
Distribution of stress values could not be considered as a fracture toughness parameter, but the magnitude of the maximum principal stress may serve as a global parameter by assessment of material resistance with respect to brittle fracture [5]. Numerical model of the specimen is then necessary. But, in the case where crack tip is positioned in the HAZ, material and geometry may influence on the FE modelling. Some of input data is difficult to determinate i.e. shape and width of the HAZ, true mechanical properties of all HAZ regions, effective crack length etc. For the sake of simplicity, HAZ in this paper as one homogeneous X-shaped region with width of 1,5 mm is idealised. Yield strength as well as ultimate strength needed for the HAZ yielding law is determined from empirical relations based on the microhardness [8]:
(1)
(2)
where HV0,1 presents Vickers microhardness and n is material hardening coefficient.
It has been noticed that during fatigue of the specimens, a significant plastification appears in the vicinity of the crack. Because of that additional yielding it was necessary to correct initial crack length. Here plastic zone was calculated by using of known expression from Schwalbe:
(3)
where aeff is effective crack length, a0 is initial crack length, rp is plastic zone radius, K is stress intensity factor by maximal fatigue load and σy is yielding strength of the HAZ material. Adding of plastic zone to initial crack length in the FE model move the crack tip from the fusion line to CGHAZ region.
Figure 2. depicts finite element mesh consisted from 413 8-node isoparametric plane stress elements with 1278 nodes. The first row of elements around crack tip has the size of 50 μm, what is nearly the grain size.
Figure 2. Two-dimensional finite element model of the fracture specimen
Singularity in the crack tip is described by collapsing of three untied nodes in one node at the tip. These untied nodes form blunted crack from initially sharp shape under loading. ANSYS software [9] has the possibility to assign the stress concentration to crack tip key-point. From this reason free meshing technique has been used. Two key-points at CTOD (d5) locations before meshing have to be foreseen. Note that left d5 node is situated in base metal as well as right d5 node in weld metal. These d5 nodes will have different displacements one to other. Contact between rollers and the specimen is so modelled to allow moving of the supports in x-direction. This displacement is then disabled for the node where the force is applied. Each material law in input FE code is given by point to point using Ramberg-Osgood stress-strain formulation. Except base metal, three different materials have been considered, weld metal with M=1,13, weld metal with M=1,22 and coarse–grained HAZ metal. Incremental plasticity theory has been employed.
Figure 3. Enlarged detail of the FE mesh around the crack tip
4. Results and discussion
In practice it is often specified that the CTOD level must exceed 0,25 mm to be acceptable. Lower CTOD values make concern, especially when values in the range of 0,09 mm and lower occur. And this is just in the range of toughness where local mismatch is thought to be dominating. Therefore here, more attention to CTOD (5) displacement as a local parameter has been directed. The comparison between experimental and numerical values of CTOD (5) displacements shown good agreement (Fig. 4). But, CTOD (5) values are obviously low. This is in accordance with opinion that overmatch condition lowers the critical CTOD value of the embrittled HAZ.
Figure 4. Comparison of experimental and FE results for CTOD (d5) values
Such behaviour is attributed to the elevation of the local stress in the HAZ caused by constraint effect of the overmatch weld metal. Because of that effect, plastic zones around HAZ crack tip for three levels of loading are revealed (Fig. 5).
Figure 5. Plastic zones spreading around HAZ crack tip
As plastic zone we consider all elements where the equivalent stress (von Mises) is greater than yielding stress of appropriate material. The highest yielding region is observed in the soft BM and the smallest in the HAZ. This is the proof that the locally overmatched CGHAZ will restrict the deformation and locally rise the stresses.
Similar assymetric behaviour around crack tip shows the distribution of 0,2 % of equivalent plastic strains (Figure 6). The yielding is directed from the HAZ toward base material, but CGHAZ with its high strength restricts further plastic deformation.
Figure 6. Distribution of 0,2% of equivalent plastic strains
The results for principal stresses distribution shown that highest maximum principal stress is reached just under the HAZ crack tip in direction of BM (Figure 7). Higher stresses are observed in the hard HAZ than in the softer WM.
Figure 7. Distribution of s1 principal stress
Values in the legend are chosen depending on the yielding stresses of single materials. Finer distribution of highest stress field (from 1600 MPa to 1780 MPa) has shown that the stress will reach its peak value in the CGHAZ at a distance less than two times the local CTOD's. Picking up stress values through different paths drawn from the crack tip, we can conclude that highest stresses moved from the CGHAZ toward base metal. The plastic zone develops preferably in the softer material, which gives rise to asymmetrical crack opening. Conventional fracture mechanics parameters, such as the total CTOD, are not necessarily applicable to the HAZ-notched joints. Generally, here is demonstrated that the strength overmatch of the weld metal is not always beneficial, because the local stress in the CGHAZ is elevated by the constraint effect under the overmatching condition. Therefore, one can expect reduced cleavage resistance of the weld HAZ in the overmatched joints.
Acknowledgement
The authors gratefully acknowledge to Ministry of Science and Technology of both Croatia and Slovenia to support their investigations in bilateral Slovenian-Croatian project.
References
[1] N. Gubeljak, "Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ) of strength mis-matched welded joints", International Journal of Fracture 100, 1999, pp 155-167.
[2] N. Gubeljak, D. Kozak, F. Matejiček, "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, No 3, 4, 2000, pp 85-101.
[3] F. Minami, M. Toyoda, C. Thaulow and M. Hauge, "Effect og strength mismatch on fracture mechanical behaviour of HAZ notched weld joint", IIW Doc. X – F – 008 –94, Paris, 1994.
[4] C. Thaulow, M. Toyoda, "Strength mis-match effect on fracture behaviour of HAZ", IIW Doc. X – F – 033 -96, Reinstorf – Lüneburg, 1997.
[5] C. Thaulow, Ø. Ranestad, M. Hauge, Z. Zhang, M. Toyoda and F. Minami, "FE calculations of stress fileds from cracks located at the fusion line of weldments", Engineering Fracture Mechanics, Vol. 57, No. 6, 1997, pp. 637-651.
[6] G. Lin, X.-G. Meng, A. Cornec, K.-H. Schwalbe, "The effect of strength mis-match on mechanical performance of weld joints", International Journal of Fracture 96, 1999, pp 37-54.
[7] BS 7448, "Fracture mechanics toughness test, Part 2. Method for determination of KIC, critical CTOD and critical J-values of welds in metallic materials", TWI Abingdon Hall Cambridge, 1997.
[8] O. M. Akselsen and G. Rorvik, "Tensile properties of heat affected zone of medium strength low carbon C – Mn and 2.25Cr – 1Mo steels, Materials Science and Technology, Vol. 6, 1990, pp. 383-390.
[9] ANSYS User’s Manual, Version 5.6, 2000
[10] D. Kozak, "Contribution to numerical and experimental analysis of fracture behaviour of heterogeneous structures", Dissertation (in Croatian), University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture, 2001
Dr Drazan Kozak, Mechanical Engineering Faculty in Slavonski Brod, University of Osijek, Department of Mechanical Design, Trg Ivane Brlic-Mazuranic 18, HR-35000 Slavonski Brod, Croatia, Tel. +385 35 446 188, Fax: +385 35 446 446, E-mail:
Doc Dr Nenad Gubeljak, Faculty of Mechanical Engineering, University of Maribor, Department for Structures and Design, Smetanova 17, SI-2000 Maribor, Slovenia, Tel. +386 2 220 7661, Fax: +386 2 220 7661, E-mail:
Prof Dr Franjo Matejicek, Mechanical Engineering Faculty in Slavonski Brod, University of Osijek, Department of Mechanical Design, Trg Ivane Brlic-Mazuranic 18, HR-35000 Slavonski Brod, Croatia, Tel. +385 35 446 188, Fax: +385 35 446 446, E-mail:
Prof Dr Maks Oblak, Faculty of Mechanical Engineering, University of Maribor, Department for Structures and Design, Smetanova 17, SI-2000 Maribor, Slovenia, Tel. +386 2 220 7661, Fax: +386 2 220 7661, E-mail:
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