Experimental investigations of concrete at a very high number of load cycles as basis for modeling of concrete fatigue taking into account viscous and damage-induced strains

土木工程结构的使用寿命除了静力作用外，越来越多地受到循环作用的影响，循环作用会导致疲劳破坏。混凝土是土木工程和建筑工程中使用最多的建筑材料，因此具有相当重要的意义。关于混凝土在循环压缩荷载作用下可承受的荷载循环次数，目前仅限于N=10^5个荷载循环的所谓低周疲劳(LCF)范围，以及N=10^7个荷载循环的所谓高周疲劳(HCF)范围。具有N&gt；10^7个载荷循环的疲劳场景被归类为甚高循环疲劳(VHCF)范围，因为目前还没有重要的研究工作。该研究项目包括在考虑附加应变模型方法的情况下，对非常高的载荷循环的疲劳行为进行详细和基本的检查。弹性、粘性、损伤和热应变组分在VHCF范围内的循环载荷试验的基础上，结合特别协调的蠕变和收缩试验以及温度测量，被量化。应变分量被考虑到相互作用的影响，因为VHCF试验中记录的试件加热被明确地考虑为HCF以及蠕变和收缩试验中的外部环境温度。通过使用高频试验技术，以时间压缩的方式获得疲劳试验，从而可以实现载荷循环次数N&>10^7。对混凝土在三种不同应力水平下的疲劳行为进行了详细的研究，试验频率从f=1赫兹到f=150赫兹，得到了大量的试验结果。为了将试验结果与混凝土在低频应力下的疲劳过程联系起来，补充了在HCF范围内的具体调查。通过对两种不同尺寸的试件的平行调查，记录了标度效应的可能影响。广泛的试验首次使获得混凝土VHCF范围的基本知识成为可能，并实现了与时间相关的高频和低频疲劳试验的试验结果之间的联系。这是根据VHCF范围内的各个应变分量了解混凝土的疲劳行为、将其锚定在基于测试工程的模型中以及将来将其用于设计的基础。

The service life of civil engineering structures is dominated apart from static actions more and more by cyclic actions which can cause fatigue failure. Concrete is the most used building material in civil and building engineering and therefore of considerable importance. Well-founded knowledge concerning the number of load cycles endurable by concrete under cyclic compression loading is currently limited to the so called Low Cycle-Fatigue (LCF) range up to N = 10^5 load cycles as well as the so called High-Cycle-Fatigue (HCF) range up to N = 10^7 load cycles. Fatigue scenarios with a number of load cycles N > 10^7 are classified as belonging to the Very-High-Cycle-Fatigue (VHCF) range, for this fatigue range currently no significant research work exists.The research project includes a detailed and fundamental examination of fatigue behaviour for very high numbers of load cycles under consideration of an additive strain model approach. Elastic, viscous, damage-induced and thermal strain components are quantified on the basis of cyclic load tests in the VHCF range in combination with specifically coordinated creep and shrinkage tests as well as temperature measurements. The strain components are considered taking into account interaction effects, as the recorded specimen heating in the VHCF tests are explicitly taken into account as the external ambient temperature in the HCF and creep and shrinkage tests. By using a high-frequency testing technique, fatigue tests are obtained in a temporally condensed manner so that load cycle numbers N > 10^7 can be realised. A detailed investigation of the fatigue behaviour of concrete is carried out at three different stress levels with test frequencies from f = 1 Hz to f = 150 Hz with a significant number of test results. To connect the test results to fatigue processes in concrete subjected to low-frequency stress, specific investigations in the HCF range are supplemented. Possible influences of scaling effects are recorded by the parallel investigation of two different specimen sizes. For the first time extensive tests make it possible to obtain basic knowledge in the VHCF range of concrete and to realise the connection between test results from time-related, high-frequency and low-frequency fatigue tests. This is the basis for understanding the fatigue behaviour of concrete on the basis of the respective strain components in the VHCF range, for anchoring it in a modelling based on test engineering and for using it for design in the future.