7.1 Introduction and Synopsis

A welded structure is usually designed by a structural engineer who designs the structure but only specifies the requirements of the welded joints. A welding engineer, not the structural engineer, specifies the weld procedure for each joint to meet the requirements specified by the structural engineer.

Next a production engineer specifies the sequence of welds, i. e., the order in which welds are made and where each pass in each weld joint starts and stops. Various inspection methods such as ultrasonic, x-ray and magnetic particle can be used to look for defects in the welds. Finally the structure may be stress relieved and/or pressure tested.

This is a very brief summery of the design and fabrication process for a welded structure.

To date this process relies primarily on experience and testing. Computational Mechanics has been used primarily to analyze short single pass welds in test coupons. It has rarely been applied to analyze welds in real welded structures. There are several reasons for this:

1. The Numerical methods for Computational Welding Mechanics (CWM) have only reached sufficient maturity to contemplate the analysis of welded structures in the last decade.

2. The numerical methods for CWM have tended to be too computational intensive to analyze more than a single pass weld longer than one meter.

3. The preparation of input data to do the analysis, particularly creating the mesh, has been so complex and hence difficult that it has discouraged attempts to analyze complex welded structures.

Analyses of single weld joints in real structures are beginning to appear [1]; however, the major obstacle for using the simulations in industrial practice is the need for material parameters and the lack of expertise in modeling and simulation. The Finite Element Method is the most important tool used in simulating the thermomechanical behavior of a structure during welding. It is a general tool but may be computer demanding, [32]

By giving designers the capability to predict distortion and residual stress in welds and welded structures, they will be able to create safer, more reliable and lower cost structures. This capability is expected to become available to industry in the near future.

Real-time CWM will become feasible. The reason is that the speed of computers increases by a factor of roughly 1.7 times each year. In the past 18 years the speed of computers has increased roughly 2,000 times. In the next 18 years the speed of computers is expected to increase at least as rapidly. Clearly even if one does nothing to improve software for CWM, someday real time CWM would become feasible simply due to the increasing power of available computers.

This chapter presents a methodology for the computational welding mechanics analysis of welded structures with complex geometry and many weld joints and such that each weld joint could have more than one weld pass. The methodology is intended to be used to analyze the fabrication of complex welded structures such as ships, automotive, heavy equipment and piping networks. This methodology promises to facilitate research into the effect of welding sequence on a structure, the effects of interaction between welds on a structure and the effect of welds on the behavior of a structure, particularly the life of a structure.

The methodology supports distributed design in that the structure to be welded, the weld joints, the weld procedure and the sequence of welding the weld joints can be specified independently by a structural, a welding and a production engineer. It also supports integration and team centered design.