Corrosion Effects on the Ductile Fracture, Strength and Reliability of Membranes, Plates and Shells.

Abstract

Corrosion is a degradation of material properties and strength due to interactions with the environment. This dissertation details the analyses of corrosion effects on two failure modes: ductile fracture and bending strength of membranes, plates and shells. Furthermore, corrosion induced stress or strain concentration effects are incorporated in reliability studies.

Annular elastic membrane deformation is studied analytically. Large out-of-plane ship hull deformation in ship grounding or collision is often analyzed by using membrane theory. It has been found that older ships with corrosion are more susceptible to fracture in grounding or collision. Therefore, corrosion effects on ductile fracture initiation are studied next. Corrosion effects inducing strain localization are revealed by a 3D model based on the finite element method. The relationship between local and overall strain and model size effects are examined for a rectangular plate under uniaxial and biaxial tension. The ductile fracture of corroded plates is also studied under different pit intensities, distributions and loading conditions.

Corrosion also degrades the bending capacity of pipelines. Previous research focused mainly on burst capacity of corroded pipelines. Key points of finite element analyses of corroded pipelines in bending are addressed in detail. An experimental testing program is fully reported and validates the finite element analysis results.Bending strength reduction because of corrosion has been revealed. Secondary effects in the experimental testing are also discussed.

Finally, corrosion induced high local stress and strain levels are evaluated by means of stress and strain concentrations. Both analytical and numerical analyses are performed including the use of Neuber’s stress concentration theory. Such studies are further incorporated into the reliability analyses of corroded plates. Structural failure probability and partial safety factors are calculated by considering the randomness of corrosion geometry and of the applied nominal strain. A reliability study is applied on a corroded bottom shell of a double bottom oil tanker in ship grounding.