I am modeling a concrete cylinder compression test (see attached). Loading is applied along z direction, and I am checking the sigma_33 stresses. I am having maximum signma_33 at the element centroid around 78 MPa (see attached graph). I don't get it why it is higher than the ultimate concrete strength of concrete (58 MPa).
Which stress does ABAQUS consider to check yielding and damage for concrete material?
The impurities in the concrete cylinder may yield higher stress values. The centroid position has uneven surfaces in the z direction, so the compression may be focused on more pronounced surfaces than the entire surface of the cylinder. The other problem could be a human error, where before taking stress measurements particular to the z axis is not reset to zero. An increased repetition of the experiment will also reduce the errors.
Dear Sushil.
What is the analysis procedure use in Abaqus?
I am think the explicit dynamic procedure was used in FE analysis. The term dynamic explicit analysis refers to the solution of the dynamic equilibrium equation by quasi-static analysis in which the loading is applied so that inertia effects become insignificant. A quasi-static problem solved with the explicit method would have much smaller time increments than an equivalent problem solved using the static method. To increase time increments (∆t), it is possible to increase the rate of loading or reduce the wave speed of the material by artificially increasing the material density. Quasi-static analysis with either option should have negligible inertia
effects. However, , the ratio of the kinetic energy to the external work energy must be insignificant.
In ABAQUS, the yielding and damage criteria for concrete materials are typically based on the principal stresses or the effective stress. ABAQUS uses the concept of effective stress for concrete materials, which takes into account the influence of tensile stresses on the material's behavior.
The effective stress, denoted as σ_eff, is calculated as:
σ_eff = σ - α * f_t
where σ is the total stress, α is the tension stiffening factor, and f_t is the tensile strength of concrete.
When checking yielding and damage in a concrete material in ABAQUS, the effective stress is compared to the material's yield strength and ultimate strength. The yield strength is typically used to determine the onset of plastic deformation, while the ultimate strength represents the maximum stress the material can sustain before failure.
In your case, where you are modeling a concrete cylinder compression test, ABAQUS considers the effective stress (σ_eff) to check yielding and damage for the concrete material. It is important to note that the effective stress can be different from the nominal stress (σ) due to the inclusion of the tension stiffening factor (α * f_t) in the calculation.
Therefore, when comparing the maximum σ_33 stress at the element centroid (78 MPa) to the ultimate concrete strength (58 MPa), it is essential to consider the effective stress (σ_eff) and the tensile strength of the concrete material. If the effective stress exceeds the yield or ultimate strength, it indicates that the concrete material may have reached or exceeded its capacity and may be subject to yielding or damage.