Influence of stress rate and load frequency on the fatigue resistance of concrete

在过去的几十年里，混凝土的疲劳性能越来越多地被研究。疲劳性能的测试通常采用圆柱形混凝土试件进行动态加载。执行这些测试，直至发生失效或达到规定的载荷循环次数。在这种疲劳试验中，试样通常以fP = 1 Hz和fP = 10 Hz之间的频率加载。特别是高周疲劳试验（N > 10^3）和极高周疲劳试验（N > 10^7）的载荷频率尽可能高，以尽量减少试验时间和成本。然而，对于混凝土，施加的荷载频率对失效循环次数有影响。这种现象是已知的，一些研究人员提出了可以解释这种现象的假设，但根据新的调查，这些假设不能无限制地用于所有压力水平。目前，由于混凝土细观结构内部的损伤过程尚不清楚，频率影响的原因尚不清楚，本课题将对混凝土在不同加载频率下的疲劳性能进行试验研究，并采用材料模型进行分析和描述。其目的是确认以下假设：在高相关最大应力水平（Smax > 0.75）下的频率影响主要基于根据所施加的疲劳应力率的混凝土强度的较高值。另一方面，预计在较低的相关最大应力水平（Smax < 0.75）下，频率影响可以用温度对混凝土强度的影响来解释。在本拟议项目中，应通过使用温度相关试验控制，将试样的温度降至最低。因此，可以通过对试件施加不同频率的载荷来分析应力率的单独影响。与DFG项目中正在研究的温度影响一起，可以从这两个单独的影响中确定负载频率的影响。如果这一假设可以得到证实，这将是一个基本的步骤之间的实验室结果和混凝土的疲劳性能在真实的结构部件，通常是加载较低的负载频率。此外，将研究载荷频率对应变行为的影响。因此，计划将测量的疲劳应变的弹性、粘性、损伤诱导和热部分分开。这将能够分析载荷频率对每个应变分量的影响。最后，将开发一个基于力学的应变模型，该模型在混凝土应变行为的计算中考虑了荷载频率。

During the last decades the fatigue behavior of concrete was increasingly investigated. The fatigue behavior was usually tested by using cylindrical concrete specimens that were dynamically loaded. These tests were performed until failure occurred or a defined number of load cycles was reached. During such fatigue tests the specimens were usually loaded with frequencies between fP = 1 Hz and fP = 10 Hz. Especially high cycle fatigue tests (N > 10^3) and very high cycle fatigue tests (N > 10^7) were conducted with a load frequency as high as possible to minimize the test durations and –costs. However, for concrete the applied load frequency has an influence on the number of cycles to failure. This phenomenon is already known and some researchers developed hypotheses that can explain this phenomenon, but according to new investigations, these hypotheses cannot be used unlimited for all stress levels. So far a description of the true damage processes inside the concrete microstructure is missing, so that the reason of the frequency influence is not clarified yet.In this project, the fatigue behavior of concrete under different load frequencies will be experimentally investigated, analyzed and described with a material model. The aim is to confirm the hypothesis that the frequency influence at high related maximum stress levels (Smax > 0.75) is mostly based on the higher value of the concrete strength according to the applied fatigue stress rate. On the other hand it is expected that at lower related maximum stress levels (Smax < 0.75) the frequency influence can be explained with the temperature influence on the concrete strength. In this proposed project the temperature of the test specimens shall be minimized by using a temperature dependend test control. Thus, the separated influence of the stress rate can be analyzed by loading specimens with different frequencies. Together with the temperature influence that is being investigated in a DFG-project the influence of the load frequency can be determined out of these two separate influences. If this hypothesis can be confirmed, this will be a fundamental step for the transferability between laboratory results and the fatigue behavior of concrete in real structural components that are usually loaded with lower load frequencies. Furthermore, the influence of the load frequency on the strain behavior will be investigated. Therefore it is planned to separate the elastic, viscous, damage induced and thermal parts of the measured fatigue strains. This will enable the analysis of the load frequency influence on each strain component. Finally a mechanically based strain model will be developed that considers the load frequency in the calculation of the strain behavior of concrete.