Research On Damage Constitutive Model Expriement And Application For Compressed Concrete

Concrete is a kind of material widely used in civil engineering, which is a kind of heterogeneous composite matieral formed by cement,sands,stones and water setting and hardening with a mix proportion, and these components have different properties, initial microcracks exist, besides. From the perspective of stress-strain relationship (constitutive model), this material can't be totally described as elastic or plastic. Since different to metal materials ,concrete also can't be totally follow the classical elastoplastic theory. So finding a reasonable and well-working model is essentially important to structure analysis.Damage theory provides a good method to find this kind of constitutive model through combination elastoplastic theory. This elastoplastic damage model can well solve the problems of description of concrete irreversible deformation,reloading rigidity degradation and softening segment behind top stress. A condition that model is based on experiment results and abbey thermodynamics principles must be met to establish elastoplastic damage model. This paper based on others'previous work tries to propose a new elastoplastic damage constitutive model. My main work follows as bellow:By summarizing existing constitutive models the characteristics and application scope of several usual models are presented, and its advantages and disadvantages are compared each other. Totally speaking, these models can solve some specific problems but have scopes of application. Some models are too simply established and have too many subjective assumptions, also have unclear physical meanings. Some other models is too complicated and have too heavy workload to be applied in realistic engineering conditions, for too many parameters are referred.Referring and analyzing literatures about plastic and damage theory, this paper points out: it is a key to determine the state with no damage influence(ideal undamaged state in this paper) in order to set up the damage model on the basis of the hypothesis of strain equivalence. Deformation modulus degradation is equivalent to effective stress decreasing, which is starting point of measuring damage. It is proved that unloading modulus can be as the definition of damage variable of elastoplastic materials.Three-dimensionally monotonic experiments were made for different grades of compressive concrete, in which firstly some were made with different stress ratios (0.15,0.18,0.19,0.20,0.22) for C25 and gets the result that with a ratio 0.20 there are no cracks in concrete specimen and no deceasing segment at stress-strain curve when loading. So concrete can be thought undamaged. Secondly specimen of C35,C45 were also taken to experiment under a ratio of 0.20, and stress-strain curves were gotten, which provided a experiement data to establish a unified form for constitutive relationship of kinds of grades of concrete.Cyclic loading and unloading experiments were made at ratio of 0.20 for C25,C35. It shows that ideal stress-strain relationship can be used as a referenced curve for one-dimensional compressed state.Based on three-dimensional and one-demensional experiment result , an ideally undamaged stress-strain curve is proposed for usual grade concrete. Evolution formula is also proposed and its parameters are determined.Using the constitutive model, numerical calculation is implemented to simulate the capacity of reinforced concrete and its damage values at different stages. The results are compared with the code.