I have been reading a few papers related with the non-linear finite element analysis of RC beam under static loading, in Abaqus using CDP model. So far, I have found that some researchers use damage parameters while others do not, claiming that damage parameters are only essential for cyclic loading.
According to Abaqus manual, when damage parameters are not considered, inelastic strain is equal to the plastic strain.
In such case, what difference does it yield in the response of the beam, failure planes and surfaces?
It seems that, without consideration of damage parameters, the stresses in concrete go beyond the maximum compressive and tensile strength, as given in the input. How does that justify the failure of the beam and damage patterns represented with criteria, such as PEEQ, PE-Max. Principal strain?
When a certain element fails (cracks or crushes), the FE software (Ansys, Abqqus, ...etc) reduces its stiffness and therefore the load is transferred to the adjacent elements in the subsequent iterations and increments. The strains and stresses of adjacent elements are increased and might be exceed the "damage parameters" or still in the safe range. If the stresses of adjacent elements exceeded the prescribed damage limits, they will fail too...
Without setting failure parameters, no element will fail and the strains (& stresses if hardening exists) will fictitiously propagate continuously beyond the failure limits.
Damage always exists in the RC structural members under any type of loading (Cyclic, monotonic or static loading). The damage is quantify by damage index and its value depends on the imposed strain or imposed displacement (or before peak loading may say depends on imposed force), but in different phases their values are different. In the phase of post-elastic its value is greater. Different phases of damaging to the RC structural members may be classified as: Tensile crack, compression crack, developing the compression cracks, forming plastic hinge by extending cracks in other fibers of concrete in a section and finally failure. But in the design directly the damage phases are not analyzed.
I think depending on the application; if damage is expected; for example in seismic application, then they consider it; whereas if damage is not a major issues, then some researchers tends to ignore it to simplify the analysis
Damage always exists in Concrete under any type of loading: monotonic, Cyclic or siesmic. So, damage parameters must be considered in RC structural members under any type of loading. As @Pramod rai said, without consideration of damage parameters the stresses in concrete go beyond the maximum compressive and tensile strength.
The damage is numerically taken as a decrease in young's modulus. If the damage is not considered the failure load will be higher then the real one (you can compare the load-displacement for both cases with and without damage).
Note that there is Tensile damage and compression damage. in cyclic lodaing we have to consider the interaction between the Tensile and compression damages.
Finally in static loading, it not the damage that can be neglected, however, the interaction between the Tensile and compression damages can be neglected.
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