Promotion Raúl Enrique Beltrán Gutiérrez

On March 16, 2026, Raúl Enrique Beltrán Gutiérrez successfully defended his dissertation 👨‍🎓 entitled "Model development for the frequency-dependent fatigue behavior of concrete based on nonlinear ultrasonic methods". The event took place in the Beyer Building of TU Dresden.

Abstract:

This dissertation investigates the nonlinear elastic fatigue behavior of concrete under cyclic loading and links experimental observations with a physically based modeling framework. The focus is on the role of loading frequency, temperature, and dissipative mechanisms in governing damage evolution and their consistent representation within a material-physics-based approach.

Comprehensive quasi-static and dynamic experimental campaigns were conducted, enabling the simultaneous characterization of the material response from a macroscopic low-frequency and a microscale high-frequency perspective. The results show that fatigue behavior is predominantly governed by thermomechanical coupling: Increased loading frequencies lead to enhanced nonlinear, partly reversible dissipation, low macroscopic energy dissipation, and a significant rise in specimen temperature. This temperature evolution alters the effective material properties and accelerates damage progression.

The analysis of dissipation measures reveals that macroscopic and microscale contributions capture distinct physical mechanisms. While macroscopic dissipation represents irreversible viscoelastic losses, microscale measurements reflect reversible stiffness modulations and friction-driven processes within the material structure.

A strong correlation between the strain rate in phase II and fatigue life (R² ≥ 0.9) is identified as a robust, state-dependent indicator. Based on this finding, a stochastic degradation model is formulated, describing fatigue life as the result of a progressive, frequency-influenced damage process and enabling the prediction of failure probabilities as a function of stress level and loading frequency.

The results demonstrate that accelerated fatigue tests at elevated frequencies may lead to systematic misinterpretation if thermal effects are neglected. At the same time, the combination of dynamic acoustoelastic testing and macroscopic measurements provides experimental access to a quantitative description of damage evolution.

Overall, the study shows that the fatigue strength of concrete must be interpreted as a state-dependent material property governed by coupled thermomechanical and nonlinear processes. This work thus provides a basis for physically consistent modeling approaches and improves the transferability of laboratory results to real structural conditions.

Dear Raúl, we would like to take this opportunity to wish you every success in your scientific career and all the best for the future. 🥳 👍