Specimen heating as damage indicator for fatigue tests of concrete

高强度可流动细粒混凝土通常用于风能行业，承受高循环应力。然而，他们的疲劳行为存在相当大的不确定性。初步测试显示，与普通高强度混凝土相比，其疲劳行为明显不便。然而，到目前为止，尚不清楚观察到的疲劳行为差异在多大程度上是由材料技术或主要技术测试影响引起的。作为初步测试的一部分，疲劳加载过程中测试样本的加热被认为是不同疲劳行为的可能指标。为了在延时实验中检查建筑物的生命周期，疲劳测试中迫切需要增加测试频率和增加应力水平，这对加热有显着影响。该研究项目有几个目标。这里的重点是进行疲劳测试，以便确定实际的材料行为。考虑到这一点，有必要消除技术测试干扰影响，尤其是实验过程中样品温度升高可能产生的干扰影响。同时，疲劳试验应以尽可能高的试验频率进行，以确保尽可能有效的试验次数。由于疲劳载荷而引起的测试样本的加热应用作不希望的额外可能损坏的指标，因此作为实验程序的控制参数。首先，应捕获样本加热的后果，应确定影响温度升高的参数，并应确定各个混凝土结构与温度发展之间可能关系的初步指示。新的测试方法应该基于这些研究并借助温度发展和可能产生的损伤力学的补充数值模拟而得到推广。因此，可以显着提高从精心疲劳测试中得出的陈述的能力和简洁性，因为只有这样才能科学地识别和追踪实际发生的材料特定对疲劳行为的影响。同时，该方法应阐明细粒混凝土与普通混凝土相比实际上是否具有偏离的疲劳行为，并据此确定细粒混凝土是否可以按照与普通混凝土相同的规则进行设计和应用，或者必须对每种材料进行详细的疲劳验证。这项研究工作对于未来普通混凝土以及进一步增加种类的特种高性能混凝土的疲劳研究也具有重要意义。此外，它们可以帮助消除各种文献结果之间现有的差异。

High-strength flowable fine-grain concretes are commonly used in the wind energy industry, where they are exposed to high cyclic stresses. However, there are considerable uncertainties regarding their fatigue behaviour. Preliminary tests show signs of a significantly inconvenient fatigue behaviour com-pared to common high-strength concrete. However, it is so far not clear to what extent the observed differences in the fatigue behaviour are caused by material-technological or primarily technical testing influences. As a part of the preliminary tests, the heating of the test specimen during the fatigue load-ing was identified as a possible indicator of different fatigue behaviour. Increased testing frequencies and increased stress levels, which are significantly influencing the heating, are imperatively required in fatigue tests in order to examine the building life cycle in time-lapse experiments. The research project has several objectives. The focus here is to carry out fatigue tests so that the actual material behaviour can be determined. Concerning this, it is necessary to eliminate technical testing disturbance impacts, which may particularly arise from the increase in temperature of the spec-imens during the experiment. At the same time the fatigue tests should be carried out with the highest possible testing frequency in order to ensure as effective as possible test times. The heating of the test specimen due to the fatigue loading should be used as an indicator for undesired additionally possible damage and therefore as a control parameter for the experimental procedure. Firstly, the consequences of the specimen heating should be captured, parameters influencing the increase in temperature should be identified and initial indications of possible relationships between the respective concrete structure and the temperature development should be determined. The new testing method should be generalizable derived based on these studies and with the help of complementary numerical simulations of the temperature development and possible resulting damage mechanics. Thus, the capacity and the conciseness of statements derived from elaborately fatigue tests can be significantly improved, since only this way the actual occurring material-specific influences on the fatigue behaviour could be identified and pursued scientifically. At the same time, this approach shall clarify if fine-grain concretes in fact have a deviating fatigue behaviour compared to common concrete or not and according to this, whether fine-grain concretes can be designed and applied to the same rules as common concrete or elaborately fatigue verification has to be performed for each material. The research work is also of fundamental importance for future fatigue investigations on both normal concrete as well as on the further increasing variety of special high-performance concretes. Furthermore, they can help clear up existing discrepancies between various literature results.