The increased use of thin section (i.e., less than 10-mm (3/8-in.) thick) steel in ship panel construction has aggravated distortion problems, adding to the costs for fitting and flame straightening. This paper describes the results of a study to evaluate techniques for control of distortion in thin panels used in ship construction. The objective of this project is to identify cost effective techniques for controlling distortion. Buckling is usually the dominant mode of distortion in thin panels, followed by angular distortion. Means of reducing both forms of distortion are discussed. During the design phase relatively small adjustments to the design, such as changing plate thickness or stiffener spacing, can significantly reduce distortion. Improving manufacturing techniques, including reducing weld size, implementing intermittent welding, and restraining the panel during welding, can also make a major impact on reducing distortion. New techniques such as egg-crate construction, laser welding, thermal tensioning, back-side line heating, back-bending, and weld quenching also show promise.

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Shipboard applications of lightweight structures have increased over recent years in both military and commercial vessels. Thin steel reduces topside weight, enhances mission capability, and improves performance and vessel stability, but the propensity of buckling distortion has increased significantly. At present, several US Navy construction programs are experiencing high rates of buckling distortion on thin steel structures. The standard shipyard practice of fabricating stiffened steel panels by arc welding is one of the major contributors to this distortion. Correcting the distortion is a necessary but time-consuming operation that adds no value and ultimately tends to degrade the quality of the ship structure. With a major initiative funded by the US Navy, Northrop Grumman Ship Systems (NGSS) has undertaken a comprehensive assessment of lightweight structure fabrication technology since 2002. Through collaborative research, significant progress has been achieved in the development of distortion-control techniques. Reverse arching, transient thermal tensioning (TTT), stiffener assembly sequencing, and other preferred manufacturing techniques were developed at NGSS to reduce distortion and eliminate the high rework costs associated with correcting that distortion. Complex lightweight panel structures, which are reinforced by long slender stiffeners along with numerous cutouts and inserts, pose a major challenge for distortion control. The geometric complexity yields a more complicated buckling behavior, which drives the need to develop a more fine-tuned finite element model to determine critical parameters and heating patterns for the TTT process. NGSS has recently teamed with Edison Welding Institute (EWI), Battelle Memorial Institute, and the University of New Orleans on a Navy project to further refine TTT procedures for complex lightweight ship structures. In this paper, functional requirements and the design of TTT process and production equipment are discussed. The refined TTT process will be benchmarked by the test panel observations, and a laser scanning device, LIDAR, will be used to analyze panel distortion topography.

The trend in both military and commercial shipbuilding is the increased use of thin steel to reduce weight and improve performance. Complex panel structures have thickness transitions for weight and structural optimization with multiple inserts ranging from 5 to 45 mm. Welding practices developed for thicker plate can result in significant out-of-plane distortion when applied to thin-plate structures. Buckling distortion of complex lightweight panels has resulted in a significant negative effect on manufacturing cost and production throughput, limiting the shipbuilders' ability to produce innovative ship designs. High fitting and welding costs are the consequence of this large welding distortion. This problem is exacerbated as the fairness requirements are tightened. New methods are needed to control distortion when welding thinner materials. To tackle the distortion problems, in 2002 Northrop Grumman Ship Systems initiated a multiyear program to develop distortion-control technology for complex panels. This paper reports the results of a study to develop "best practices" for welding of lightweight structures. Control of welding distortion for thin structures requires control of each welding operation from butt welding of plates through to unit assembly. A general philosophy was applied to minimize welding heat input while maximizing restraint during unit construction. To achieve this, the following techniques were evaluated: increasing restraint during each welding operation, improving fitting practice, weld sequencing, and minimizing welding heat input. Additionally, an active distortion mitigation approach, known as transient thermal tensioning, was investigated for reduction of buckling distortion during thin-panel longitudinal stiffener welding. A series of tests were performed to evaluate various distortion control approaches and to optimize production processes. The culmination of the project will involve demonstrating best practices in the production of thin-steel structures. A plan is also being developed for implementing the most advantageous approaches into production.

Manual visual inspection is by far the most widely used weld inspection method. A given weld may be visually examined multiple times as parts are joined and made into assemblies. Because visual inspection is somewhat subjective, and prone to error (a typical inspector only identifies about 85% of the visible defects); welds that pass one inspection may fail subsequent inspections, resulting in multiple inspection and repair cycles. This paper discusses a project that tested the benefits and limitations of a semiautomated weld inspection system. The goal of this semiautomated inspection approach is to provide quantitative, nonsubjective quality measurements of welded structures in order to: eliminate redundant inspections, reduce unnecessary multiple repair cycles, avoid repair of welds that meet minimum size requirements, and enable recording of weld size so that overwelding can be identified and reduced. The technology would also allow tracking of weld quality and statistical analysis of welding process capability to support lean/six-sigma continuous improvement initiatives. A prototype system was assembled and field tested by inspecting actual ship structures. The equipment evaluated has potential, but needs both hardware and software modifications before it can be used as a tool on a regular basis in a shipbuilding environment. As is, it will be useful as an audit tool to gauge the health of the visual inspection process and to further document the inherent variability of the visual inspection process. Recommendations were made for improvements to refine the prototype tool prior to broader deployment.