The influence of the weld toe grinding and wig remelting weld toe rehabilitation techniques, on variable stresses, in case of cross fillet welds, reinforced with additional welding rows

Variable stresses where the load value varies between a maximum and a minimum value, or varies the position in time, cause after accumulating a large number of load cycles in those structures, the emergence of drug fatigue. Fatigue is characterized by failure on values of the applied stress from the load cycles, below the material flow, values which in case of static stress would not have caused problems. Knowing that the variable stressed structures are sensitive to stress concentrators, the paper aims to highlight the influence of two techniques to reduce stress concentrator weld toe grinding and WIG remelting weld toe, on the behavior of variable tensile test of cross corner welded specimens, reinforced with additional welding rows.


Introduction
There are many types of welded structures like welded bridges, ship structures; aircraft structures; bodies of the vehicles that are stressed variable and in time, they are affected by the phenomenon of fatigue which causes failure to values below the flow limit [1][2].
Fatigue failure are specific to welded structures which have fillet welds, which shows the stress concentrators at the intersection between the base and the filler material. Reducing these stress concentrators is accomplished by applying two reconditioning techniques: weld toe grinding and WIG remelting weld toe on cross welded samples, from which the specimens are taken for the variable tests [3][4]. Both reconditioning techniques determine either by polishing or by WIG remelting the reduction of stress concentrators at the junction base material / filler, causing an increase fatigue lifetime of the structure on which it is applied.
We considered telling this research to be done on the cruciform fillet welded joints as a welded structure, this type of connection showing the biggest failure by fatigue susceptibility.
Research is extracted from a large study in which 5 cross welded samples were used denoted by A; B; C; D and E depending on the welding process used.
On all samples were applied the reconditioning techniques weld toe grinding and WIG remelting weld toe, then extracted samples for static and variable tests. [5][6].
The cruciform welded sample which is the subject of the sample with sign C, was welded by manual welding process with coated electrode in horizontal position on vertical wall that was reinforced with additional welding lines.

Experimental procedure
The shape and size of welded sample C on which were applied two reconditioning techniques weld toe grinding and WIG remelting weld toe in order to reduce the tensile concentrator and implicitly to enlarge the fatigue lifetime are shown in figure 1. Specimens C1; C4 and C7 will be subjected to static tensile testing static and are not subject to this paper, and specimens C2; C3; C5; C6; C8 and C9 will be subject to variable loads, the reconditioning techniques are applied before the test as follows: C2 and C3 without reconditioning; C5 and C6 with weld toe grinding and C8 and C9 with WIG remelting weld toe.
Fatigue testing specimens have been obtained by cutting the cross welded sample C yielding strips with width of 30 mm as shown Figure 1.c.
After mechanical cutting, vertical parts of the specimen attached to the horizontal plate will be milled to reduce the width from 30 mm to 10 mm in welding seams area, as shown in Figure 1.c. and Figure 2 a. This milling is done in order to, during the variable loads to be submitted specimens, the section being reduced and the tensile increased on its surface, conduct the breaking in the seam or in the thermos-mechanical influenced area of the seam.
It is noticed that section is not reduced sharply from 30 to 20 mm but milling is done with a radius R = 10 mm, in order to not introduce additional stress concentrations.

Fig. 2. Specimen shape after milling
The specimens are made of steel type S 235 JR. Welding regime parameters used to obtain sample C are shown in Breaking sections are presented below.
In Figure 3 are shown breaking sections for specimens C2 and C3 without reconditioning.  Table 2. where: a is the thickness of the welding seam; K1 and K2 are the seam legs and r is the radius between seam and base material.
Fatigue tests were carried out with LVF 1000-HM unit. All attempts were made at 10 Hz frequency, the loadd cycle being an symmetrical alternating one, with asymmetry coefficient R = -1, the application being tensile-compression. For each set of specimens, with reconditioning techniques applied or not, they were raised durability curves that they have been compared to each other. For raising the durability curves were applied the the following loads variations, as follows: for C2; C5 and C8 ±14 kN and for C3; C6 and C9 ±7,5 kN. The fatigue test samples are shown in figure 5.   3004 We observe that, with decreasing the solicitation applied force for samples C3 and C2, increase the number of cycles untill the fracture: 198640 cycles for C3 and 16030 cycles for C2.
Another two frames for fatigue tests in case of C5 and C6 samples with weld toe grinding applied technique are shown in figure 7.
We also aobserve that, with decreasing the solicitation applied force for samples C6 and C5, increase the number of cycles untill the fracture: 287992 cycles for C6 and 22400 cycles for C5.
The last two frames for fatigue tests for C8 and C9 samples with WIG remelting weld toe applied technique, are shown in figure 8.

Results and discussions
Forces applied to the specimens and results obtained from fatigue tests are presented centralized in  3004 We notice that, with decreasing tensile stress on the samples C2 and C3, increases the number of cycles up to fracture. We mention that on specimens without reconditioning we have the highest value of stress concentrator at the junction welding seam / base material, because of the smallest radius connecting seam to the base material, namely r = 2mm.
The sample C5 on which we apply ± 14kN, was broken after 22440 cycles and C6 on which we have applied ± 7.5 kKN broke after 287 992 load cycles. We found an increase in the number of cycles compared to homologous specimens C2 and C3. This is explained by the reduction of the stress concentrator at the top of the seam connection by increasing the radius of 2 to 2.5 mm as shown in table 2 We also observed that the sample C8 on which we apply ± 14kN, broke after 47 280 cycles and the sample C9 on which we have applied ± 7.5 KN, broke after 595 880 load cycles. We determinate an increase in the number of cycles compared to the homologous C5 and C6 specimens. This is also explained by the stress concentrators reduction at the top seam according to table 2, by increasing the radius from 2.5 to 8 mm.
Radii measurements were made with a specialized software by entering the crosssections pictures of the specimens, scale 1:1.

Conclusions
Conclusions can be drawn after raising the durability curves from figure 9, with Mathcad calculation program.
With Mathcad calculation program, for samples C2 and C3, without technical reconditioning, we found values p = 2 and r = 6.7, for which the chart function n c 1 V , approaching most of our points c1 = (16030; 40027; 198 649) and f representing force vector f = (14;9;6). The shape of this curve is show in figure 9 with dashed point line. For the third set of samples C8 and C9, on which we have applied WIG remelting weld toe, with the Mathcad software, were found the values p = 2 and r = 6.5 where the function chart is closest to our points c3 = ( 47281; 118080; 595888) and the force vector f = (14;9;6). The shape of this curve is shown in figure 9 with continuous line.