Crack option assists in predicting crack initiation and in simulating tension softening, plastic yielding and crushing.
Analytical procedures that accurately determine stress and deformation states in concrete structures are complex due to several factors. Two of which are the following:
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The low strength of concrete in tension that results in progressive cracking under increasing loads.
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The nonlinear load-deformation response of concrete under multiaxial compression.
A crack develops in a material perpendicular to the direction of the maximum principal stress if the maximum principal stress in the material exceeds a certain value.
After the formation of an initial crack at a point in a material, a second crack can form perpendicular to the first. Likewise, a third crack can form perpendicular to the first two.
The material loses all load-carrying capacity across the crack unless tension softening is included.
Tension softening
If tension softening is included, the stress in the direction of maximum stress does not go immediately to zero; instead the material softens until there is no stress across the crack.
At this point, no load-carrying capacity exists in tension.The softening behavior is characterized by a descending branch in the tensile stress-strain diagram, and it may be dependent upon the element size..
Crack closure
Even after a formation of a crack, the loading can be reversed; it is important that the opening distance of a crack is carefully considered. In this case, the crack can close again, and partial mending occurs. When mending occurs, it is assumed that the crack has full compressive stress-carrying capability and that shear stresses are transmitted over the crack surface, but with a reduced shear modulus.
Crushing
As the compressive stress level increases, the material eventually loses its integrity, as its resistance to deformation changes and all load-carrying capability is lost; this is referred to as crushing. Crushing behavior is best described in a multiaxial stress state with a crushing surface having the same shape as the yield surface. The failure criterion can be used for a two-dimensional stress state with reasonable accuracy. For many materials, experiments indicate that the crushing surface is roughly three times larger than the initial yield surface.
Shear Retention Factor
Shear Retention Factor is the reduction of the shear stiffness after cracking. The shear modulus is the product of initial shear modulus and shear retention factor. Therefore, the shear modulus reduces with increasing strain in the direction normal to the crack. This represents the reduction of the shear stiffness due to crack opening. This consideration helps to avoid convergence difficulties and physically unrealistic and distorted crack patterns. Especially in concrete, the shear retention factor allows for reduced decrease of shear modulus cased by the roughness of crack faces due to aggregate interlocking.
