Traction behaviour simulation of spot welded joints.
Finite element method is a powerful and efficient instrument in design and scientific research with multiple and various applications, but also a main component of computer research and design.
The expansion of using the finite element method is due, on the one hand, to its general character of its fundamental concept formulation, fact that allowed its utilization in numerous domains of science and technology, and on the other hand, its capacity to improve studied physics phenomena comparable with other numeric computation methods, fact that allowed to be preferred by many specialists.
A high fidelity reflection of materials and structures behaviour is a major exigency required by designers and used calculus methods being the primary condition for a rigorous dimensioning and accomplishing for important material savings, energy and labour. From this point of view, finite element method (FEM) is superior to all the other calculus methods.
The possibility of a precise investigation of materials behaviour is also encouraged by study and spreading of replacing materials which is another economic advantage.
Developing of this method is reflected by increasing number of publications and worldwide scientific sessions.
Highly significant is the increasing number of FEM calculus software, the fact that software companies are preoccupied with supplying of these products together with calculus equipments as base soft, and as well, the development of dedicated computers for finite element calculus and its hardware components, with software included, which allow general use computers to be able to perform finite element calculus by a simple installation of the software.
2. THEORETICAL CONSIDERATIONS
FEM is an approximated solving procedure of a large variety of engineering issues by using the computer. Parts of these problems are those involving metallic materials welding.
[FIGURE 1 OMITTED]
In order to shape a geometric domain can be used dots, straight or curved lines, plane or curve surfaces. The geometrical modelling algorithm is generally based on subdomains, with a convenient choice for passing from one subdomain to another. For the finite elements with straight lines, digitised error can be reduced by increasing the number of nodes and therefore the number of finite elements. For finite elements with curve edges, digitised error can be reduced due to curve boundaries of finite elements (Catana, 2004).
For spot welding case (see figure 1) these dimensions and therefore the allowable stress depends on welding process parameters. Welding can be performed in a soft or hard regime.
In the case of hard regime, only the welded spot area is brought to a high temperature, welding being finished before the surface of elements to be welded in contact with electrodes to reach a high temperature (Iovanas, 2004). In case of soft regime, a larger metal volume is heated which exceeds spot welded area and the electrodes in contact with the metal in plastic state will leave deep marks in the components to be welded. By comparison the spot dimensions obtained with these two welding regimes we have:
--[d.sub.hr] > [d.sub.sr] which means that shear resistance of welded spot with hard regime is higher than that one obtained with soft regime welding (Novac, 1994);
--[h.sub.hr] < [h.sub.sr] which means that at soft regime a bigger volume of melted metal is obtained and therefore more dendrite structure, which is more fragile and less resistant; for hard regime [h.sub.hr] [congruent to] 0.3 H, and for soft regime [h.sub.sr] [congruent to] 0.9 H.
Therefore the traction behaviour of a spot welded joint will be tested (see figure 2) (four different spots will be arranged).
[FIGURE 2 OMITTED]
The welding points were arranged as follow:
Due to forces that stress the joining, within the material stress will be present and when these exceed material breaking strain will determine the damage of welded joint.
The study of welded spot arrangement influence has been done on [A.sub.3] (STAS 9485-80) metal sheet samples with the next chemical composition:
C--max 0.08 %;
Mn--0.2 ... 0.4 %;
Used parameters of the welding regime as well as the results are shown in table 1 (Catana & Machedon-Pisu, 2003).
The regime parameters were:
--regime I: I=3 kA, t=3 [per];
--regime II: I= 5 kA, t=5 [per];
--regime III: I= 6 kA t=5 [per].
The parts are warmed (due to Joule-Lentz effect) by connecting them to the electrical power supply. The amount of necessary heat to perform the welding was calculated with equation 1:
Q = [I.sup.2.sub.s] x ([R.sub.c] + 2 x [R.sub.p]) x t (1)
[R.sub.c]--resistant of contact;
[R.sub.p]--own resistant of the free heads.
For all regimes the traction speed was: v=6.5 mm/min and the dimensions of the electrodes were (10 mm diameter for body and 6 mm for peak). The thickness of metal sheets was of 0.4 mm, spot welding method being used to join two components of equal thickness and with the electrodes having the same constructive parameters. After having welding joint done, these have been tested at shearing traction according to the standard (Catana, 2005).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Mechanical characteristics of used material are:
--yield limit [R.sub.p0,2] (U--Max 240 MPa;
--breaking strength--270 ... 370 MPa;
--breaking elongation--[A.sub.5] min 34 %.
In figure 4 is presented the unwanted case of parallelogram arrangement welding points. In this case the stress has maximum value and asymmetrical distribution. The tests were confirmed by simulation trials for all types of welding points' arrangement.
Spot welding is used to produce body cars of all types in car industry, electro technical and home appliances. This method fits most welding operation automation. From the analysis of results obtained through simulation a conclusion derives that the most convenient arrangement of welded spots is the square one where developed stresses are symmetrical and have lower values. Based on tests, line and rhomb arrangements are not to be preferred taking into consideration the value of tensions and the required space. Test results showed that parallelogram arrangement is unwanted and should be avoided when spot welding procedure is used. Research plan will continue with studying the influence over traction resistance for spot welded joints as well for the distance on longitudinal and across direction for different spot welded arrangements.
The simulation can be improved through a realistic modelling closer as possible of the way in which forces strain the joint, more precisely by establishing the volume of welded spot and its position within the joint.
Catana, D. & Machedon-Pisu, T. (2003). The simulation of behaviour at traction of spots welded joints. Sudura, Vol. XIII, No. 2, 2003, 23-27, ISSN 1453-0384
Catana, D. (2004). The study through simulation of the stress developed in the spots welded joints solicit at traction, Proceedings of International Scientific Conference "Modern Technologies, Quality, Restructuring" TMCR 2004, Musca, G., pp. 461-464, ISSN 1011-2855, Jassy, 052004, Bulletin of Polytechnic Institute of Jassy, Jassy
Catana, D. (2005). Theoretic contributions for the plastic deformation simulation process, Proceedings of International Scientific Conference ,Modern Technologies, Quality, Restructuring" TMCR 2005, pp. 333-337, ISBN 9975-9875-4-0, Technical University of Moldova, 05-2005, University of Moldova, Chisinau
Iovanas, R., (2004). Spots pressure welding, Lux Libris, ISBN 973-9240-71-8, Brasov
Novac, G., (1994). Welded joints calculus, Lux Libris, ISBN 973-96308-7-1, Brasov
Tab 1. Breaking strength of spot welded joints. Spot Regime I Regime II Regime III arrangement o o o o 256 433 571 o o 258 497 596 o o o o 260 532 583 o o o o o 229 458 546 o