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Strength as a Property Depending on Structure Size
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By general convention, the load capacity predicted by plastic limit analysis or any (deterministic)
theory in which the material failure criterion is expressed in terms of stress or strain (or both) are said to exhibit no
size effect. The size effect represents the deviation from such a prediction, i.e., the size effect on the structural
strength is the deviation, engendered by a change of structure size, of the actual load capacity of a structure
from the load capacity predicted by plastic limit analysis (or any theory based on critical stresses or strains).
The size effect is, for design engineers, the most compelling reason for adopting fracture
mechanics. There are six different size effects that may cause the nominal strength to depend on structure
size:
- Boundary layer effect;
- Diffusion phenomena, such as heat conduction or pore water transfer;
- Hydration heat or other phenomena associated with chemical reactions;
- Statistical size effect, which is caused
by the randomness of material strength and has traditionally been believed to explain most size effects in
structures;
- Fracture mechanics size effect, due to the release of stored energy of the structure into the
fracture front; and
- Fractal nature of crack surfaces.
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