Influence of loading speed and frequency on fatigue resistance
Table of contents
Project data
| Titel | Title Einfluss der Beanspruchungsgeschwindigkeit und der Belastungsfrequenz auf den Ermüdungswiderstand von Beton (EBBE-Beton) | Influence of loading velocity and loading frequency on the fatigue resistance of concrete (EBBE-Beton) Förderer | Funding Deutsche Forschungsgemeinschaft (DFG) Zeitraum | Period 08/2020 – 12/2024 Projektleiter | Project manager Prof. Dr.-Ing. Steffen Marx Team | Team Raúl Enrique Beltrán Gutiérrez Doreen Sonntag (Versuchsdurchführung | Test execution), Heiko Wachtel (Messtechnik | Measurement technology) |
Report from the year book 2024/25
What cyclically stressed concrete has in common with earthquake research
Ultrasonic transducer for monitoring damage evolution in a prestressed concrete beam
Bridges and wind turbines made of concrete are essential components of modern infrastructure. These structures belong to a unique category of constructions: They are subjected to an enormous number of load cycles over their service life. For example, when a truck or train crosses a bridge, its axles sequentially transfer loads to different sections of the bridge. Repeated loading and unloading occurs because these vehicles have multiple axles – a characteristic process for many concrete structures. For this reason, fatigue assessment plays a crucial role in predicting the lifespan of such constructions.
Until now, fatigue assessment has primarily relied on static material properties such as compressive strength. However, dynamic effects like the increase in concrete strength under fast loading or the partial recovery of material properties during unloading have not been adequately considered. Interestingly, such effects have long been observed in earthquake research. Geophysicists have discovered that rocks deep in the earth’s crust exhibit similar phenomena during earthquakes: material properties change depending on the stress level and the speed of dynamic loading. In this context, the increase in strength during loading and the recovery of the material during unloading are crucial – these phenomena are directly linked to the timing of the earthquake, the moment when the rock fails.
The research project EBBE-Beton (Fatigue and Fracture in Concrete under Loading) applies these insights to civil engineering. Its goal is to develop new models that can better predict the number of load cycles until failure – the “breaking point” – by incorporating dynamic phenomena observed during loading. The findings from geophysics serve as a foundation for improving the prediction of structural failure and for more accurately assessing the condition of fatigue-stressed concrete structures. This innovative approach could significantly enhance the safety and lifespan of modern infrastructure.
Report from the year book 2023
Playing with a concrete ball
Specimen in a concrete creep test set-up
Imagine a ball that on the one hand behaves like a hacky sack – without bouncing, but stretching very slowly but continuously – and on the other hand bounces incessantly like a space ball, as if it had a life of its own. There is a group of materials, including concrete, that exhibits this peculiar behavior. This means that the deformation behavior changes depending on the load conditions. The evolution of the deformation of concrete depends on the time scale in which the loading process takes place. The rate of loading therefore determines the deformation behavior. If the load is increased very slowly with, e.g.
10–3 N/mm²s at most, or remains almost constant, the deformation appears to be significantly influenced by concrete creep. On the other hand, when concrete is loaded very rapidly, its stiffness appears to increase and its deformation pattern changes significantly.
The investigation of these properties is particularly important for concrete structures that are exposed to fast cyclic loads, such as wind turbines or bridges. Both in these structures and in laboratory tests, the speed at which load cycles occur has an influence on the fatigue strength of the material or structure. To highlight these differences and to investigate the changes in the mechanical properties of concrete exposed to extreme loading rates, in the frame of the EBBE-Beton project, we carried out creep tests and compression tests at very high loading rates up to approximately 106 N/mm²s. Based on the experimental data, the influence of the loading rate on the development of fatigue damage is taken into account and models will be further developed.
The use of these models will improve the previously limited applicability of results from fatigue tests with high loading rates in the laboratory to real component situations with lower loading rates. Even Haaland or Messi would not dare to play with our quirky concrete ball without this knowledge and appropriate models.
Report from the year book 2021
Fast or slow to concrete fatigue?
Concrete specimen under cyclic loading
Concrete bridges are subjected to a large number of loading cycles caused by cars, trucks or trains during their service life. As a result, the concrete fatigues at the areas subjected to high stresses. In recent years, knowledge gaps, as well as discrepancies between the fatigue behaviour observed in the laboratory and the actual fatigue behaviour of concrete structures, have been discovered. For example, fatigue strengths calculated based on laboratory tests have a lower value than the actual fatigue strengths estimated in the real structures. The high cyclic loading frequency and the resulting internal temperature increase during the laboratory tests apparently cause additional damage to the concrete specimens.
Currently, concrete specimens are tested under laboratory conditions in fatigue tests at the highest possible cyclic loading rate until fatigue is reached. This is done to minimize cost and testing time. For example, to simulate the number of cyclic loads on a specimen that a bridge will be subjected to over 10 years, it would take about 23 days under normal laboratory conditions with a loading frequency of fp = 5 Hz. Moreover, one would need several many specimens to validate the results of the tests. Therefore, it seems logical to increase the cyclic loading rate. Only if the testing times for the fatigue tests could be shortened without compromising the quality of the results, it will be possible to realistically simulate the fatigue damage evolution over the expected lifetime of the bridge in the laboratory.
As part of this research project, numerous tests will be conducted at different cyclic loading rates and temperature conditions to relate the fatigue strength of concrete to the above factors. The experimental data will later be used to develop models that take into account the influence of the loading rate as well as the temperature increase on the development of fatigue damage. With the help of the models developed in this research project, the previously insufficient applicability of laboratory results from fatigue tests to real component situations will be made possible.