Analysis of Spot Weld Joints by Ultrasonic Inspection, Fem and Residual Stress Measurements

The present work seeks to determine the parameters for approval, through ultrasonic techniques, of spot welded joints of low carbon steel sheets. For such experience the mechanical behavior of a spotwelded joint was characterized under fatigue, in load cycles ranging from zero to 14 kN, on sheets of 1.5 mm thickness, joined by three spot welds of 7.5 mm of medium diameter, being the distance among spot weld centers 15 mm. The welding parameters were modified to generate indentations of up to 40% of the nominal thickness of the joints, measured by ultrasonic techniques. It was verified that fatigue cracks were nucleated in the areas of maximum equivalent von Mises stress, calculated by Finite Element Method (FEM) analysis, and their propagation occurred in the direction of the maximum main stress also calculated by FEM, together to the action of residual stresses, which were confirmed by X-Ray diffraction. The fatigue tests demonstrated that samples with indentation among 20 and 40% of the nominal thickness showed life equivalent to 77% of the life found in the joints where the indentation didn't overcome 20% of the nominal thickness. Besides, the fatigue life standard deviation of samples with indentation among 20 and 40% of the nominal thickness is equal to 35% of the medium value, while in the samples where the indentation did not overcome 20% of the nominal thickness, this deviation was not superior to 9%. The results proved the efficiency of FEM in the determination of stress acting in the joints, and validated the criterion of acceptance of the spot welds through the medium thickness measure of these joints through ultrasonic techniques. Introduction: The material flaw as a result of cyclic loading was first mentioned by ALBERT, in 1838[1]; however, the definition of fatigue as flaw of material under alternate loads appears in PONCELET's (1839) [1] work, and the study of this flaw cause was performed by WÖHLER, in 1860[1], where the alternate stress were generated using rotative bending loads. WÖHLER's work defines the S-N curves, Stress amplitudes applied in function of number of cycles for failure, where the medium stress applied in each cycle is null (in other words, the minimum stress is compressive and, in module, is equal to the maximum applied tension stress) [1]. However, it is necessary here the discussion of the mechanism of flaw formation for fatigue in metals [1,2], that begins with the formation of persistent slip bands, caused by dislocation movement in the crystalline structure over small distances, taking the formation of intrusions, extrusions and protrusions at the surface, or places for cracks nucleation, because of great plastic strain accumulation. These cracks grow from each stress cycle until the instability determinated by fracture mechanics. Being fatigue a process of highly located plastic strain, local stress concentration can rise the applied stress, releasing sessile dislocations, facilitating their movement and the crack nucleation and propagation. Sheets used in automotive bodies can be joined through spot welds, obtained from heat generated by electric current passed through the sheets by electrodes under a certain pressure. The heat causes a local melt of the sheets, causing the spot weld formation. One of the defects that can happen in spot welds is the deep penetration of the electrode, resulting in indentations in the sheet surfaces [3]; such indentations reduce the thickness of the welded joint locally and they can introduce stress concentration in these joints, and this could reduce the fatigue life of these joints. One way of verifying the geometric stress concentrations due to these indentations is the finite elements method (FEM). The FEM technique employed for spot weld joints simulation can be as simple as possible since the focus of the simulation is on the load transfer from one plate to another. In the present paper, metal sheets were simulated using Reissner-Mindlin plate elements without bending-torsion coupling. Normal and frictional contact between plates were simulated using simple adaptive gap elements. Loading transfer from one plate to another was simulated using a RBE-type approach where rigid and beam elements were combined. No effort in simulating residual thermal stresses resulting from thermal contraction was made since the RBEapproach is unreliable for detailed time-dependent stress and/or temperature analysis within the spot weld. Owed to the spot weld cooling is performed using fictitious temperature gradients based on experimental analysis or detailed simulations of the welding process such as in LINDGREEN [4]. However, in the present analysis such procedure cannot be used because of the geometric variation owed to the electrode indentation in the sheets cannot be made using plate elements. The indentation severity can be determined by ultrasonic techniques. In the Quality Technological Center (CTQ) at DaimlerChrysler, Brazil, the ultrasonic technique is used in spot weld joints to determine these indentations, through the measure of the medium thickness of the spots. This technique has the first experiments dated about 1986, in Germany, and it differs basically from the conventional technique by the use of a special transducer, that possess a water column between the piezoelectric crystal and the piece to be tested, eliminating the undesirable effect of the "dead zone", defined as the area of strong turbulences in the end of the transducers, disabling the measurement of small thickness. The technique consists basically on a non-destructive rehearsal, where the equipment generates an electric pulse to a piezoelectric crystal that receives the pulse and vibrates, generating sound waves in the range of 0.5 up to 30 MHz, inaudible for the human being. Those waves penetrate in the material to be tested, and they contemplate when finding the bottom of the piece or any other reflector, generating signs that are amplified properly and, after codified, exhibited in graph form or image, making possible to an operator properly trained the identification of which signs represent a eventually flaw in the material. Objectives: The present work seeks to determine the parameters for approval of low-carbon steel sheets spot weld joints through ultrasonic techniques, comparing the indentations previously measured by ultrasonic techniques to pre-stabilished acceptance parameters as the results of fatigue tests of sheets welded in three points. The results were analysed using residual stress measurements in both 3 points, and simulation by FEM of the joints. Experimental procedure: Starting from lower carbon steel sheets of 1.5 mm thickness, with the chemical composition showed in the table I and the nominal mechanical properties showed in the Table II, 3-point joints were obtained in specimens of 50 width mm and 110 mm length, as showed in Illustration 1. Table I. nominal chemical Composition of the studied steel (weight %). Element C Mn P S Al weight % 0,08 0,45 0,03 0,03 0,02 Table II. Mechanical properties of the studied steel. Modulus of elasticity (GPa) 207 Yield strength (MPa) 195 Tensile strength (MPa) 304 Total elongation in 50 mm (%) 49.2