Chon L. Tsai

Analysis and Development of A Real-Time Control Methodology in Resistance Spot Welding

The single-parameter, inprocess monitor and automatic control systems for the resistance spot welding process have been studied by many investigators. Some of these have already been commercialized and used by sheet metal fabricators. These control systems operate primarily on one of the three process parameters: maximum voltage or voltage drop, dynamic resistance, or thermal expansion between electrodes during nugget formation. Control systems based on voltage or dynamic resistance have been successfully implemented for industrial appl ications. A great amount of experience on these two control methods has been accumulated through trial-and-error approaches. The expansion-based control system is not commonly utilized due to lack of experience and understanding of the process. Since the expansion displacement between electrodes during welding responds directly to the weld nugget formation, this control parameter provides a better means to produce more precise spot welds. However, the control algorithm of this method is more complex than that for the other two methods. Fruitful development of such a system cannot be obtained by trial-and-error approaches. This paper presents a systematic approach to develop the expansion-based control algorithm for resistance spot welding. The finite element method was used to simulate the welding process and to determine the physical response of C. L. TSAI, W. L. DAI and D. W. DICKINSON are with the Department of Welding Engineering and J. C. PAPRITAN is with the Department of Agricultural Engineering, The Ohio State University, Columbus, Ohio. the joint material to the various welding conditions. Direct correlations between nugget formation and expansion displacement between electrodes were obtained. By systematic computer simulations, the weldability characteristic curves for resistance spot welding were developed. These weldabi l i ty curves show inadequate welding conditions that would cause nugget expulsion and current shunting. A welding duration curve, which shows appropriate time for squeeze, weld and hold cycles, was developed and used as a basis for in-process resistance spot welding control.

Numerical analysis of welding residual stress using heat source models for the multi-pass weldment

Numerical prediction of welding-induced residual stresses using the finite element method has been a common practice in the development or refinement of welded product designs. Various researchers have studied several thermal models associated with the welding process. Among these thermal models, ramp heat input and double-ellipsoid moving source have been investigated. These heat-source models predict the temperature fields and history with or without accuracy. However, these models can predict the thermal characteristics of the welding process that influence the formation of the inherent plastic strains, which ultimately determines the final state of residual stresses in the weldment. The magnitude and distribution of residual stresses are compared. Although the two models predict similar magnitude of the longitudinal stress, the double-ellipsoid moving source model predicts wider tensile stress zones than the other one. And, both the ramp heating and moving source models predict the stress results in reasonable agreement with the experimental data.

Strain-Based Assessment and Modeling for Low-Distortion Welding Procedure

In this article, the effect of gas metal arc welding (GMAW), pulsed gas metal arc welding (GMAW-P), and cold metal transfer (CMT) welding processes on angular distortion is investigated along with strain relaxation by removing weld deposits. Moreover, in order to investigate the behavior of the joint in terms of angular distortion, stress stretching (i.e., pressure model) is applied on welds via finite element analysis simulation to compensate weld shrinkage. In addition, a conceptual welding procedure using two different welding processes (i.e., deep-heat model) for filling the joint and strain relaxation of the weld, respectively, is investigated by a modeling approach. The joint type studied in all cases is a double-fillet welded Tee-joint.

Analysis of fatigue crack propagation behavior in fillet welded T-joint

Abstract In the fatigue analysis, the crack growth behavior in a two-dimensional welded T-joint was studied both numerically and experimentally. The geometric parameters were weld size, initial crack orientation and unsupported flange length. Crack growth direction was predicted using the minimum strain energy density factor theory. The results show that even though the fillet size and initial crack orientation affected the crack growth in the early stage, the cracks tended to converge when they entered the far-field stress region. This observation shows the initial period of crack growth could be distinguished from the rest of the propagation due to its crack growth behavior. There was reasonable agreement between the predicted and the experimentally observed crack growth paths. A correlation between the crack growth rate and the driving force parameter range was obtained for a T-joint constructed from hot-rolled AISI 1035 steel. A separate study showed that grain size had an insignificant effect on fatigue strength of the edge-cracked plate model.

Fundamental Studies on the Relationship Between Plastic Strains and Angular Distortion in Fillet Welded T‐Joints

The procedure of plasticity‐based distortion analysis (PDA) directly mapping cumulative plastic strains into elastic models using equivalent thermal strains was developed and applied to the investigation of the characteristic relationship between cumulative plastic strains and angular distortion in fillet welded thin plate T‐joints. The PDA successfully demonstrated the validity of the unique relationship between cumulative plastic strains and distortion and the applicability of a linear elastic model in welding‐induced distortion analysis. The PDA procedure also provided the quantitative relationship between six cumulative plastic strain components and angular distortion of fillet welded T‐joints, showing the importance of shear component in angular distortion.

Thermal and mechanical evolution of welding-induced buckling distortion

Abstract The evolution of the buckling phenomenon starts during the weld cooling cycle, caused by an onset inelastic strain incompatibility condition. This initial bifurcation phenomenon may continue to grow until the completion of the cooling cycle, which results in the final buckling distortion of the plate. With lower heat input and/or smaller plate dimensions this initial instability may stop during the cooling cycle due to diminishing strain incompatibility and recovering of the plate rigidity. The buckling evolution process is complex due to the highly nonlinear nature of the welding problem. This paper studies this buckling evolution process using an integrated experimental and numerical approach. Bead‐on‐plate welds of AH36 steel were experimentally studied. The welding process was numerically simulated and analyzed using a three‐dimensional, thermo‐elastic‐plastic, large deformation model. The transient stress bifurcation phenomenon and the displacement evolution process were analyzed to understand the critical weld conditions causing the final buckling distortion of the weldment. The critical weld conditions were evaluated on the longitudinal inherent shrinkage (plastic) strain distribution in the weldment.

Fatigue design and analysis for welded tubular joints: Results from the API Offshore Tubular Joint Research Center

The fatigue performance of tubular connections can limit the life of offshore constructions. Fatigue research for the American Petroleum Institute (API) Offshore Tubular Joints Research Centre focused in its first three projects on fatigue design. An examination of design rules in API RP2A and equivalent standards as well as the basic data has defined several areas where API RP2A can be updated to include new design curves and explicit means of determining hot spot stresses. In addition, several suggestions have been made that would introduce new provisions, such as inclusion of other weld improvements besides grinding and a different method of including the effects of cathodic protection in seawater.

Study of Acoustic Emission Characteristics for Fracture Assessment

Inspection for structural integrity of the Space Shuttle Solid Rocket Boosters (SRBs) is of paramount importance to mission safety. After every shuttle launch, the booster rockets are retrieved and an extensive inspection operation performed to detect any mission-related anomaly. If damage occurs to the structure during shuttle mission, Acoustic Emission (AE) from cracks could be monitored and used as a means for initial screening to identify potential damage locations for selective postlaunch inspection.