Influence of load-induced temperature fields on the fatigue behaviour of UHPC subjected to high frequency compression loading

细（超）高强混凝土（UHPC）在循环荷载作用下的疲劳性能试验表明，高频压缩荷载引起的温度场对UHPC的损伤过程和疲劳强度有不可忽视的影响。这是因为与普通混凝土相比，UHPC的材料成分密度明显更大，这导致循环荷载下混凝土微观结构中相应加热的内部摩擦更大。除了混凝土成分外，荷载频率和状态（最小和最大应力）以及试样的几何形状也被认为是产生热量的决定性因素。由于混凝土摩擦材料的复杂力学行为，目前对混凝土材料（特别是超高性能混凝土）在疲劳荷载作用下温度场变化规律的研究尚缺乏系统的实验和数值模拟，而对混凝土材料更精确的材料模型的研究也尚未完成。没有材料模型可以表示载荷引起的温度应力，并考虑其对损伤过程和疲劳强度的影响。对于各种力学现象的现实考虑，基于热力学方法的Bazant微平面概念具有最大的潜力。因此，这一概念被用来作为一个新的材料模型的发展的基础上，以描述的疲劳行为的UHPC经受高频压缩loading.The本研究项目的中心目标是通过实验确定的影响，负载引起的温度场的疲劳行为的UHPC在高频荷载下，并模拟这些影响，通过使用一种新的材料模型的基础上热力学。首先，在静态荷载下的混凝土试件进行实验测试，以产生基本参数，如特征应变和强度，包括多轴性能和导热系数，用于模拟UHPC的基本结构性能。在大量的循环试验中，记录了循环荷载对应变、裂缝、混凝土微观结构和混凝土样品中产生的温度的影响。在这里，混凝土成分和年龄，样本大小，负载频率和制度以及试验机是不同的。通过对试验结果的分析，推导了相应的模型参数，并将其应用于一种新的材料模型中，以描述超高性能混凝土在高频荷载作用下的疲劳性能。实验研究将通过具有广泛参数研究的数值模拟来补充。研究结果是确定不同因素对UHPC在高频荷载下的疲劳强度的重要性，一个新开发的材料模型来测量这些因素和相关的材料模块，这是在标准软件中实现。

Newest experiments regarding the fatigue behaviour of fine-grained (ultra-)high-strength concrete (UHPC) under cyclic loading show that induced temperature fields due to high frequency compression loading have a non-negligible influence on the damage process and the fatigue strength of UHPC. This is because UHPC has a significantly denser material composition compared to normal concrete, which causes greater internal friction with corresponding heating in the concrete microstructure under cyclic loading. Beside the concrete composition, the load frequency and regime (minimal and maximal stress) and specimen's geometry are recognised so far as decisive factors for the resultant heat. Systematic experimental and numerical studies on the impact of load-induced temperature fields on the degradation of the concrete material by fatigue loading, particularly for UHPC, are still pending.Today, the research on more accurate material models for concrete is not finished due to the complex mechanical behaviour of cohesive friction material. There are no material models that can represent load-induced temperature stresses and take account of their impact on the damage process and the fatigue strength. For realistic consideration of various mechanical phenomena, the Microplane concept of Bazant, which is based on a thermodynamic approach, has the greatest potential. Therefore, this concept is used as a basis for the development of a new material model to describe the fatigue behaviour of UHPC subjected to high frequency compression loading.The central objective of this research project is to experimentally identify the effects of load-induced temperature fields on the fatigue behaviour of UHPC under high frequency loading and to model these influences by using a new material model based on thermodynamics. Firstly, experimental tests on concrete specimens under static load are conducted in order to generate the basic parameters, like characteristic strains and strengths including multiaxial behaviour and thermal conductivity, for simulation of the basic structural behaviour of UHPC. During numerous cyclic tests, the effects of cyclic loading on strains, cracks, concrete microstructure and resulting temperatures in concrete samples are recorded. Here, concrete composition and age, sample size, load frequency and regime as well as the testing machine are varied. Through analysing the development of damage on the basis of test results, corresponding model parameters are derived and implemented in a new material model to describe the fatigue behaviour of UHPC under high frequency loading. The experimental studies will be complemented by numerical simulations with extensive parametric studies. Research results are the determination of the importance of different factors on the fatigue strength of UHPC under high frequency loading, a newly developed material model to measure these factors and an associated material module, which is implemented in standard software.