A new insight into the three phases of secant stiffness degradation of concrete under fatigue loading

Concrete’s adaptability, sturdiness, and affordability have made it one of the most widely used and popular building materials. Its crucial role in the development of modern societies cannot be overstated. China, for instance, demonstrates a high demand for concrete, with an astounding production of 2 m³ per individual per year. This statistic highlights the pervasive use of concrete, especially in social infrastructure projects. Road and railroad bridges, offshore gas and oil platforms, wind turbine foundations, and airport pavements are just a few examples of structures where concrete plays a vital role.

However, these infrastructures are not immune to the challenges of repeated loading and significant stress. Over time, concrete can experience fatigue, leading to eventual failure. Understanding the fatigue behavior of concrete has become a crucial area of scientific inquiry. Researchers have developed numerous theoretical models and conducted experimental tests to study the effect of fatigue loading on concrete’s failure behavior. These studies aim to shed light on the mechanisms underlying concrete fatigue and facilitate the development of strategies to increase its resistance to fatigue. By comprehending how concrete reacts to cyclic loading, engineers can design structures that are more resilient and capable of withstanding anticipated stresses and strains throughout their intended service life.

In a recent study published in the peer-reviewed International Journal of Fatigue, Dr. Yanpeng Wang from Guangdong University of Technology in China presented new insights into the nonlinearity observed in the secant stiffness degradation of concrete under fatigue loading. The author investigated the three-phase feature of secant stiffness deterioration in concrete and enhanced understanding of this experimental phenomenon. As we know, the secant stiffness of concrete degrades in three distinct phases with load cycles. Initially, there is a rapid decrease in secant stiffness, followed by a relatively steady decline in a fairly linear pattern. Finally, there is a sharp decrease, indicating impending failure. These three phases were attributed generally to the initiation, stable propagation, and unstable propagation of microcracks generated by fatigue stress. However, Dr. Wang found these explanations for the three-phase decline of secant rigidity to be phenomenological and not satisfactory.

To account for stochastic damage accumulation in concrete under fatigue loading, Dr. Wang introduced their own two-scale stochastic damage model (TSSDM). The model accurately predicts the experimentally observed decrease in secant stiffness through simulations. Analysis using the model shows clearly that the three-phase feature of the decrease is physically resulted from an acceleration mechanism as well as a deceleration mechanism. The acceleration mechanism arises from the growth of damage energy release rate due to stress redistribution, while the deceleration mechanism is associated with the randomness of a material parameter termed nano-to-micro comprehensive fracture energy. Based on test results, the author concluded that the acceleration mechanism predominates in the latter 60% of concrete’s fatigue life, whereas the deceleration mechanism predominates in the first 40%.

In summary, Dr. Yanpeng Wang’s research provides new insights into the three phases of secant stiffness degradation in concrete under fatigue loading. The nonlinearity observed in degradation is a result of competing deceleration and acceleration mechanisms, influenced by the randomness of fracture energy and stress redistribution. These findings contribute to a better understanding of concrete’s fatigue behavior, which is crucial for designing structures that can withstand the wear and tear over time. With continued research in this field, engineers can enhance the durability and longevity of concrete-based infrastructures, ultimately benefiting modern societies.