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.