Causes and modelling of the warming of fatigue-loaded concrete

循环荷载引起混凝土以及其他材料的变形和损伤过程，从而导致结构或试件的破坏。这种破坏甚至是由远低于材料强度的加载条件引起的。通常，这种材料特性被称为疲劳。在力控疲劳试验中，混凝土对循环荷载的阻力是量化的。在这些试验中，混凝土试样表现出明显的升温。混凝土微观层面的材料阻尼和摩擦过程被认为是原因。疲劳试验的载荷频率、应力范围和试验时间对试件升温有显著影响。更高的应力范围和负载频率导致更高的温度。试验研究表明，在较高的荷载频率和较高的混凝土温度下，疲劳混凝土试件比在较低的温度和较低的荷载频率下更早失效。因此，加温应影响混凝土试件的抗疲劳性能。在研究项目中，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种能量方法来识别疲劳加载混凝土试件的热释放。试件的温度行为将通过考虑混凝土试件内部的热量传递和向周围的热量传递的有限元模型计算。将疲劳荷载混凝土试件的实测温度与计算温度进行比较。比较将表明所建立的能量方法的准确性。此外，还对混凝土试件的升温效应进行了评价。这些涉及温度变形以及由于试样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.