Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics
Authors: Ibrahim R. A. 
Published: January 2021

Structural life assessment periodically evaluates the state and condition of a structural system and provides recommendations for possible maintenance actions or the end of structural service life. It is a diversified field and relies on the theories of fracture mechanics, fatigue damage process, probability of failure, and reliability. With reference to naval ship structures, their life assessment is not only governed by the theory of fracture mechanics and fatigue damage process, but by other factors such as corrosion, grounding, and sudden collision. The purpose of this series of review articles is to provide different issues pertaining to structural life assessment of ships and ocean structures. Part I deals with the basic ingredients of the theory of fracture mechanics, which is classified into linear elastic fracture mechanics and elasto-plastic fracture mechanics. The amount of energy available for fracture is usually governed by the stress field around the crack, which is measured by the stress intensity factor. The value of the stress intensity factor, which depends on the loading mode, is evaluated by different methods developed by many researchers. The applications of the theory of fracture mechanics to metallic and composite structures are presented with an emphasis to those used in marine structures. When the inertia of relatively large pieces of a structure is large enough that the correct balancing of the energy of fracture requires the inclusion of kinetic energy, then the dynamic nature of fracture dominates the analysis. For a crack that is already propagating, the inertial effects are important when the crack tip speed is small compared with the stress wave velocities. This fact has been realized in the theory of fracture mechanics under the name of dynamic fracture and peridynamic. In essence, peridynamic replaces the partial differential equations of classic continuum theories with integro-differential equations as a tool to avoid singularities arising from the fact that partial derivatives do not exist on crack surfaces and other singularities. A brief overview of fracture dynamics and peridynamics together with damage mechanisms in composite structures is presented. The limitations of fracture mechanics criteria are also discussed. Life assessment of ship structures depends on the failure modes and the probabilistic description of failure, which are addressed in Part II. Life assessment of ship structures depends on the failure modes and the probabilistic description of failure. In view of structural parameter uncertainties, probabilistic analysis requires the use of reliability methods for assessing fatigue life by considering the crack propagation process and the first passage problem, which measures the probability of the exit time from a safe operating regime. The main results reported in the literature pertaining to ship structural damage assessments resulting from to slamming loads, liquid sloshing impact loads of liquefied natural gas in ship tankers, and ship grounding accidents, and collision with solid bodies are discussed in Part III. Under such extreme loadings, structural reliability will be the major issue in the design stage of ocean structures. The treatment of extreme loading on ship structures significantly differs from those approaches developed by dynamicists. Environmental effects on ship structures play a major factor in the life assessment of ocean systems. In particular, these effects include corrosion and hydrogen embrittlement. Part IV is devoted to a ship's life assessment resulting from corrosion and hydrogen embrittlement. Because structural components made from aluminum and its alloys are vital to the ship and aerospace industries, the influence of environment on aluminum structures and the means of corrosion control and monitoring in both aluminum and nonaluminum metals are presented. Hybrid ships consist of a stainless steel advanced double-hull center section, to which a composite material bow and/or stern is attached. Such structures require strong joints between the composite and the steel parts. Some of the difficulties with joining composites and metal are related to the large difference in mechanical properties such as stiffness, coefficient of thermal expansion, etc., between the adherents and the large anisotropy of composites. Such differences generally lead to large stress concentrations and weak joints. Fatigue crack growth, stress concentrations resulting from details, joints, and fasteners are addressed in Part V. Fatigue improvement in welded joints is considered one the major tasks of this part. Brittle fracture of hull structures causes serious structural damage and this motivated the ship structure community to develop some means to prevent brittle cracks from occurring. The basic principle behind the use of a crack arrester is to reduce the crack-driving force below the resisting force that must be overcome to extend a crack. The crack arrestor can be as simple as a thickened region of metal or may be constructed of a laminated or woven material that can withstand deformation without failure. Part VI provides different approaches of passive crack control in the form of crack arresters to stop crack propagation before it spreads over a structure component. Crack arresters used in ship structures and pipelines are described for both metal and composite materials. This six-part review article is by no means exhaustive and is based on over 1800 references. It does not address the structural health monitoring, which constitutes a major task in the structural diagnostic process.