## 2 Materials Characterization as a Basis of Structural Integrity Assessment  
To get engineering structures assessed regarding their integrity, a material's behavior has to be known. This becomes specifically essential when a material ages and when a material's condition changes gradually. Such a change is also often associated with damage, where damage has to be specified accordingly. A further expression in that regard is degradation, where the word itself already implies that the 'grade' of the material has been reduced. While the word 'damage' has a clear negative touch, the word 'degradation' is more quantifiable. The word 'degradation' may therefore be used for a phenomenon, which is generally observed when materials and structures are used over time. When a structure is exposed to loading in service, if the loading result from mechanical or environmental loads, it is generally accepted knowledge that a structural material degrades. From a point of view of design, this degradation is accepted to occur, as long as a minimum grade, being equivalent to a respective damage, is not undercut. Such a principle can also be considered as a damage tolerant design, although it may not be expressed as such explicitly. A 'tolerable damage'--or better 'tolerable degradation'--is therefore a structural condition at which the structure's performance is not going to face catastrophic failure. Material's degradation due to loading is a continuous accumulation process, that is difficult to describe. In its possibly most simplistic form, it is assumed to be a linear process, such as expressed with the Palmgren-Miner rule applied in the case of fatigue damage accumulation. However, it is endlessly proven through experimental evidence, that damage accumulation--or better degradation--is a non-linear process. Traditionally, damage in a material is only considered from the crack initiation onwards. The primary reasons are that it can mechanically clearly lead to a structure's catastrophic failure, that it can be principally monitored visually and that its propagation as a function of loading can be practically described based on modern fracture mechanics. This allows prognostics to be made in terms of a residual life assessment in case an initial condition of degradation is known. This principle has been extended to a structure's complete fatigue life assessment, where the crack propagation phase being comparatively easier to describe has been merged with the much less known crack-free phase, by simply linearizing the complete degradation process, bearing in mind, that the description of the crack-free phase is associated with a much higher amount of uncertainty, being the major reason why conventional fatigue life evaluations are based on a high degree of uncertainty. A deeper look into a material's degradation process during the crack free period elucidates, that the mechanisms are truly versatile. When considering a metallic material, this starts from dislocations in a material that cluster and that further lead to persistent slip bands, starting within grains and gradually growing beyond grain boundaries. For the slip bands on a material's surface intrusions and extrusions may occur, that become the incubators of cracks in the longer term. The same may apply also inside a material, however, the higher constraints inside the material will prevent cracks to be incubated rather at a later than an early stage. In excess, there may be transformations to occur within a metallic material, such as from austenite to martensite, which is globally observed as a material hardening and where the transformed martensite may build another source of stress concentration and cracking within the material's microstructure. Reflecting on those degradation processes along the crack-free period of a material's life endurance is, therefore, most likely far from being linear and is so far only described to a very limited extent. If the degree of degradation of a material needs to be known, the material needs to be tested.