Predictive Power Of Crack Opening Rate For Flexural Fatigue Life Of Steel Fiber-Reinforced High And Ultra-High Performance Concrete

Abstract

Numerical modelling of deterioration in high-cycle fatigue is limited primarily by the computational time required to explicitly calculate large numbers of load cycles on a specific system. Therefore, the aim of this work is to develop an experimentally driven approach to predict the damage evolution of notched 3-point bending beams specimens made of fibre-reinforced high-and ultra-highperformance concrete (HPC/UHPC), which can be used to limit the necessity of extensive numerical calculations of such tests. One possible application is cycle-jump approaches, which aim to avoid the explicit calculation of a large part of the load cycles up to the fatigue failure of the system in the numerical calculations -without having to accept significant losses in the predictive capability of the model. In this contribution, the predictive capability of the crack mouth opening displacement rate ∆CMOD per load-cycle in the second phase of fatigue onto the overall fatigue damage progression is characterised for flexural beams. In this case, this is not explicitly linked only to final failure of the structure. The outlined approach can also be used to estimate the number of cycles until a specifically defined CMOD is reached. This can be important, for example, if crack widths may not be exceeded in cyclically loaded structures. In addition to reference specimens of extremely brittle UHPC and HPC without fibres, the test program consists of specimens with fibre contents of 0.75 % and 1.5 % by volume, which show strain-hardening behaviour. I.e., in preliminary tests, the bending tensile strength was only reached after the proportional limit was reached. The experimental programme includes numerous fatigue tests with constant lower and upper load levels as well as multi-level tests in which one or both load levels were gradually shifted in a block-wise manner. For each block, the cyclic loading was applied long enough to reach the second, linear phase of fatigue deterioration. Overall, it is shown that the secondary creep rate in terms of crack mouth opening provides a valid basis for predicting the failure time (or reaching a given CMOD), despite the well-known scatter of fatigue test series. In a brief outlook, the applicability of a similar use of the stiffness development of the test specimens as a means of prognosis is additionally discussed.