Shilang Xu

Flexural fatigue damage model of ultra-high toughness cementitious composites on base of continuum damage mechanics

Ultra-high toughness cementitious composite is a fiber reinforced cementitious material, with the characteristics of strain hardening and multiple cracking under uniaxial tension. It could be applied in some structures to sustain complicated loading conditions, such as fatigue loads. This paper presents an experimental and theoretical investigation on the flexural fatigue damage property of ultra-high toughness cementitious composites. Based on continuum damage mechanics and elastic-plastic isotropic damage model, a flexural fatigue damage model of ultra-high toughness cementitious composites is developed. The parameter of starting damage Ds is introduced, which equals to the damage amount happening in fatigue stage I. Meanwhile, the plastic strain ɛp on the bottom surface of the standard specimen is employed to calculate fatigue damage, D. The fitted parameters of the damage model are obtained with the experimental data. The research result shows that the starting damage amount and the cumulative damage degree drop with the decrease of stress levels. It is proved that this fatigue damage model is applicable to calculate the fatigue damage of ultra-high toughness cementitious composites.

Deformation calculation of ultra-high toughness cementitious composite-concrete beam under flexure fatigue with ultra-high toughness cementitious composite fatigue damage model

Ultra-high toughness cementitious composite is a construction material, with the characteristics of strain hardening and multiple cracking under uniaxial tension. It can be applied as a feasible repair material for structures subjected to fatigue load. In this paper, flexural fatigue tests on the composite beam constituted of an ultra-high toughness cementitious composite layer and a concrete layer with the same height (ultra-high toughness cementitious composite–concrete beam) are carried out. Then, on the basis of fatigue stress degradation models of concrete and ultra-high toughness cementitious composite, the fatigue process of ultra-high toughness cementitious composite–concrete beam is analyzed. It is shown that, the fatigue failure of ultra-high toughness cementitious composite–concrete beam in this paper is caused by the damage of ultra-high toughness cementitious composite layer. Therefore, with the flexural fatigue damage model of plain ultra-high toughness cementitious composite beam, the evolution curve of maximum tensile strain on the bottom surface of ultra-high toughness cementitious composite layer with the ratio of load cycles is calculated. The calculated strain is in good agreement with the experimental result. That is to say, the factual deformation, dependent on the fatigue stress level, can be predicted by the calculated result. As a result, the flexural fatigue damage model of ultra-high toughness cementitious composite can be applicable to evaluate the fatigue performance of ultra-high toughness cementitious composite–concrete beam.

Loading Rate Effect on the Double-K Fracture Parameters of Concrete

This work studies the effect of the loading rate on the double-K fracture parameters in concrete. These are the crack initiation toughness K! !! and the unstable toughness K! !, which mark the two main stages of crack propagation and are calculated from a load-crack mouth opening curve, P-wM. However, the original methodology does not take into account the influence of the load velocity, which may not be negligible for certain testing configurations adopted to avoid an excessive test duration. The double-K method includes a third parameter, the cohesive toughness K! ! , that relates K! !! and K! ! by integrating the cohesive stresses in the fracture process zone. In this paper, the influence of the loading rate is introduced in the method through K! ! by including in the softening law a viscous factor, function of the crack opening velocity. The idea arises from the viscous-cohesive approach implemented in a numerical model successfully used in a previous work (Engineering Fracture Mechanics 82:195-208, 2012). Rate-dependent double-K parameters are computed from P-wM curves obtained with the numerical model, analysing a wide range of specimens and crack opening rates. R-curves are also obtained. The results show that double-K parameters increase with the loading rate. The rate dependence is slightly greater for large specimens, which amplifies the size effect.

Fatigue damage propagation models for ductile fracture of ultrahigh toughness cementitious composites

Ultrahigh toughness cementitious composites are a kind of high-performance cementitious material with a characteristic of ductile fracture. Based on the continuum damage mechanics theory and flexural fatigue damage model, two damage propagation models of ultrahigh toughness cementitious composites are built. One is a linear bilogarithmic model with J-integral range as its independent variable, while the other one is a linear model on a semilogarithmic scale with fatigue stress level as its independent variable. However, according to former research, the J-integral depends strongly on specimens’ geometry, so the first damage propagation model is deeply influenced by material dimension. As a result, the second damage propagation model is more convenient in application, shows the material fatigue property in comparison with the first model. In order to prove these two models and obtain the parameters, a three-point flexural fatigue experiment on single-edge-notched fracture specimens is carried out. The results shows that the two models fit better with the experimental results, rather than the crack propagation law of ultrahigh toughness cementitious composites.