Fabrication and Engineering Technology for Lightweight Ship Structures, Part 1: Distortions and Residual Stresses in Panel Fabrication

Abstract

An increase in shipboard applications of lightweight structures has been evident over the recent years in both military and commercial vessels. Ship panel distortions generated through various stages of production (e.g., material handling, blast and paint, panel fabrication, subassembly, assembly, outfitting, and erection) have emerged as a major obstacle to the cost-effective fabrication of these lightweight structures. This problem is particularly challenging for naval ships that are built with relatively thin plate and require fair surfaces to maximize hydrodynamic performance and minimize radar signature. With a recent major initiative funded by the U.S. Navy, a comprehensive assessment of the lightweight panel fabrication technology has been undertaken. This assessment took into account the residual stresses of thin plate conditions during the material handling, cutting, fitting, and welding processes. A series of test panels with varying degrees of complexity representing the typical shipboard applications were designed and used to quantify dimensional variations through the entire fabrication processes in a production environment. A light detection and ranging (LIDAR) measurement system was used to analyze panel distortion topography resulting from different processes. Welding attributes, stiffener assembly sequence, and material handling methods were systematically monitored and evaluated to identify areas for fabrication improvement. Advanced computational tools were further developed and used to establish the underlying distortion mechanisms and critical process parameters in these panel structures. Some of the major findings include the following:local buckling is the dominant distortion mechanisms in lightweight panel structures;special care must be exercised in material handling of lightweight structures in preventing long-range permanent deformation;dimensional accuracy from thermal cutting can have a significant impact on buckling distortions, particularly for different thickness combinations in complex panels;any effective mitigation techniques for minimizing buckling distortion should either reduce the buckling driving force (fabrication induced stresses) and/or increase the buckling resistance (e.g., panel geometric parameters and assembly procedures);butt welding of plates to make panels requires a low heat input narrow groove process to minimize distortion prior to fillet welding of stiffeners;precision fillet welding process with automatic seam tracking offers the potential to minimize overwelding.

Contemporary approaches to reducing weld induced residual stress

“…In addition to shrinkage and bending type distortion modes, welds in thin-walled structures often produce residual stresses large enough to cause buckling, which can complicate distortion analysis and prediction. 52 The implications of welding distortion for manufacturing (aside from those related to other effects of residual stress) are of great practical concern, 53,54 and have led to the development of a host of techniques specifically to prevent distortion, without aiming to remediate the underlying residual stress. These include bending back of distorted weldments, presetting, 55 control of mechanical constraint, 56,57,58 control of weld sequencing, 53,59,60 and flame straightening.…”

Materials selection for current and future UK naval requirements

“…One of the consequences of using such a high proportion of thin plate is that the issue of thin plate distortion during fabrication becomes a major focus of attention. 5,6 It is well established that as the plate thickness decreases then the level of distortion/buckling increases, with a significant change below 6 mm as shown in Fig. 2.…”

Review on finite element analysis of welding deformation and residual stress

“…Welding deformation and residual stress of some special joints (stiffened plates with brackets [33]) or structures (spiral welded pipe [34]) were studied by TEP-FEA and experiment verification. Taking into account the residual stress, the compressive assessment of the lightweight panel fabrication technology especially for naval ships was completed considering the material handling, cutting, fitting, and welding processes, while a low heat input narrow groove process in butt welding of plates was good to minimise distortion before fillet welding of stiffeners [35]. With TEP-FEA method, Cozzolino et al reduced the welding residual stress and distortion by using the post-weld rolling process [36].…”