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

近几十年来，混凝土的疲劳性能得到了越来越多的研究。疲劳性能的测试通常采用动加载的圆柱形混凝土试件。执行这些测试，直到发生故障或达到规定数量的负载循环为止。在这种疲劳试验中，试件的加载频率通常在fP = 1hz和fP = 10hz之间。特别是在尽可能高的载荷频率下进行高周疲劳试验（N > 10^3）和高周疲劳试验（N > 10^7），以尽量减少试验时间和成本。然而，对于混凝土，施加荷载的频率对破坏的循环次数有影响。这一现象已经为人所知，一些研究人员提出了可以解释这一现象的假设，但根据新的调查，这些假设不能无限地用于所有的压力水平。到目前为止，还没有对混凝土微观结构内部的真实损伤过程进行描述，因此频率影响的原因尚未明确。在本项目中，混凝土在不同荷载频率下的疲劳行为将通过材料模型进行实验研究、分析和描述。其目的是证实这样一个假设，即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施加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。在这个建议的项目中，应通过使用与温度相关的测试控制来最小化测试样品的温度。因此，可以通过不同频率加载试件来分析应力速率的分离影响。与dfg项目中正在研究的温度影响一起，可以从这两种单独的影响中确定负载频率的影响。如果这一假设能够得到证实，这将是实验室结果与实际结构构件中混凝土疲劳行为之间可转移性的基本步骤，这些构件通常加载较低的荷载频率。此外，还将研究载荷频率对应变行为的影响。因此，计划将所测疲劳应变的弹性、粘性、损伤诱发和热部分分开。这样就可以分析载荷频率对每个应变分量的影响。最后，将建立一个基于力学的应变模型，该模型在计算混凝土的应变行为时考虑了荷载频率。

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.