Causes and modelling of the warming of fatigue-loaded concrete

循环载荷在混凝土以及其他材料中引起变形和损坏过程，从而导致结构或试件失效。这种失效甚至是由远低于材料强度的载荷条件引起的。通常，这种材料特性被称为疲劳。混凝土对循环荷载的抵抗力在力控制疲劳试验中被量化。在这些试验中，混凝土试件表现出明显的升温。材料阻尼和摩擦过程在混凝土的微观水平上的意思是原因。疲劳试验的加载频率、应力范围和试验持续时间对试件的升温有显著影响。更高的应力范围和载荷频率导致更高的温度。实验研究表明，在较高的荷载频率和较高的混凝土温度下测试的疲劳加载混凝土试件比在较低的荷载频率和较低的温度下更早失效。因此，升温应该会影响混凝土试件的抗疲劳性。在本研究项目中，疲劳加载混凝土试件的热行为将进行实验和数值研究。预定的研究活动是基于一个充满活力的方法，确定在疲劳加载混凝土试样的热释放。将通过考虑混凝土试件内的热传递和向周围的热传递的有限元模型计算试件的温度行为。将比较疲劳荷载混凝土试件的实测温度和计算温度。这种比较将表明所建立的能量方法的准确性。此外，将评估混凝土试件升温的影响。这些问题涉及温度变形以及由于样品升温而降低的疲劳强度。

Cyclic loads evoke in concrete, as well as in other materials, deformation and damage processes, which lead to failure of the structure or specimen. This failure is even caused by loading conditions far below the material strength. Gernerally, this characteristic material behavior is referred to as fatigue.The resistance of concrete to cyclic loads is quantified in force-controlled fatigue tests. In these tests, concrete specimen exhibit significant warming. Material damping and friction processes at the micro level of the concrete are meant to be the causes. Load frequency, stress range and test duration of the fatigue tests affect the warming of the specimens significantly. Higher stress ranges and load frequencies result in higher temperatures. Experimental studies show that fatigue loaded concrete specimens tested at higher load frequencies and thus higher concrete temperatures fail earlier than at lower load frequencies with lower temperatures. Therefore, the warming is supposed to affect the fatigue resistance of concrete specimens.Within the research project, the thermal behavior of fatigue-loaded concrete specimens will be investigated experimentally and numerically. The scheduled research activities are based on an energetic approach identifying the heat release in fatigue-loaded concrete specimens. The temperature behavior of the specimens will be calculated by a finite element model considering heat transport within the concrete specimen and heat transfer to the surrounding. Measured and calculated temperatures of fatigue loaded concrete specimens will be compared. The comparison will indicate the accuracy of the established energy approach. Furthermore, the effects of the warming of concrete specimens will be evaluated. These concern the temperature deformations as well as the reduced fatigue strength due to the warming of the specimen.