Project Description
The scientific objective is to develop and validate a method for condition monitoring of bolted joints based on electromechanical impedance spectra, with a clear focus on the reliable detection of corrosion-induced damage.
The practical value of the project lies in providing a cost-effective, continuously operating, and automatable method for the early identification of corrosion conditions relevant to failure, particularly on components that are difficult to access or safety-critical. To facilitate practical implementation, concrete, application-oriented recommendations for action will also be developed.
The project builds directly on the findings of previous projects on EMI-based determination of preload force (IGF 20844) and on the detection of crack formation in bolted joints (IGF 22867). Both projects demonstrated the fundamental suitability of electromechanical impedance spectra for the highly sensitive detection of condition-dependent changes in structural behavior. The methodological approaches and evaluation procedures developed in those projects form the basis for now specifically expanding the range of detectable damage mechanisms to include corrosion-induced changes.
The overarching goal is to develop an “intelligent bolt” that enables automated and statistically validated monitoring. The results to date demonstrate the viability of this approach and provide a robust foundation for its application to corrosion-related damage.
Key Focus Areas of the Project
Development and validation of a method for monitoring preload force in bolts based on electromechanical impedance spectra / guided waves.
The method must be capable of:
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reliably detect failure-relevant damage while taking into account the “true positive rate” of a binary classifier (statistical significance) (Priority 1), even under
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varying environmental conditions (temperature) (Priority 2).
Everything at a Glance
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Duration
July 1, 2026 – June 30, 2028 -
Research Area
Structural Health Monitoring, Corrosion Detection, Bolts, Electromechanical Impedances, Ultrasound, Piezoelectric Sensors -
Funding
Federal Ministry for Economic Affairs and Climate Action (BMWK): €252,635 (Total: €495,221) -
Project Partners
Chair of Mechanics with a Focus on Damage Monitoring (MSHM)
Research Methods & Procedures
Preliminary FE Studies
- Development of FEM models for small- and large-scale experiments
- Simulation of corrosion processes using COMSOL Multiphysics (MEMS + Corrosion)
- Development of a concept for reproducible corrosion of the samples
- Identification of suitable frequency ranges for damage detection
- Conducting numerical EMI simulations
- Prediction of the evolution of impedance spectra at different corrosion levels
- Gradual calibration and model updating
- Provision of FE models as a basis for:
- Selection and optimization of sensor systems and measurement strategies
- Development of the corrosion concept
- Identification of corrosion parameters relevant to failure
- Development and validation of temperature compensation models
- Parameter studies and sensitivity analyses
Sensors | Measurement Methods
- Definition of Measurement Objectives (Localization, Quantification, and Characterization of Corrosion)
- Selection of suitable PWAS sensors (material, size, frequency range, sensitivity)
- Development of a sensor positioning strategy for small- and large-scale tests
- Consideration of corrosion hotspots and uniform sensor distribution
- Development of a measurement concept for EMI and guided-wave measurements
- Establishment of standardized measurement parameters and measurement protocols
- Frequency ranges
- Signal amplitudes
- Pulse duration
- Development of a concept for signal preprocessing and data filtering
- Development of a protection concept for sensors and cabling
- Investigation of long-term stability and robustness (e.g., HASS tests)
- Conducting a pilot study to optimize sensor systems and measurement settings
- Calibration and validation of measurement methods with regard to accuracy, reliability, and reproducibility
- Provision of the optimized measurement strategy as the basis for subsequent small-scale and large-scale trials
Concept: Corrosion
- Development of a concept for reproducible corrosion of the samples
- Development of a concept for the quantitative measurement of corrosion (corrosion grades according to ISO 8501-1+2, mass loss, 3D scanning, etc.)
- Control of corrosion
- Development of suitable methods for protecting the measurement equipment
Experimental Corrosion Measurement | FEM Simulation
- Identification of Failure-Relevant Corrosion Parameters Through Numerical Studies
- Correlation between corrosion condition and probability of detection
- Development of methods for the quantitative assessment of the corrosion state
- Visual assessment according to ISO 8501-1/-2 and DIN 55928
- Precise measurement of samples using a 3D scanner
- Development of a test rig for reproducible measurements at identical positions
- Investigation of cross-sectional changes across all corrosion stages
- Determination of mass loss due to corrosion using a high-precision balance (small-scale tests)
- Derivation of failure-relevant parameters as a basis for damage assessment and validation of detection methods
Experiments
- Conducting small-scale tests on screws (e.g., M16, M20, M24)
- Tests at two temperature levels (0 °C and 20 °C)
- Conducting measurements for several defined corrosion states
- Recording of:
- EMI spectra
- Guided-wave measurements
- Measurements in undamaged and corroded states
- Reproducible temperature control of the test specimens before each measurement
- Gradual increase in the degree of corrosion with measurements taken after each corrosion stage
- Conducting large-scale tests on preloaded high-strength bolts at room temperature
- Investigation of typical beam connections (IH 3) under various corrosion conditions
- Baseline recording and measurements after each defined corrosion stage
- Conducting EMI and guided-wave measurements using appropriate measurement techniques and MATLAB analysis
- Provision of a comprehensive dataset for the evaluation and validation of corrosion detection
Corrosion
- Development of Methods for Feature Extraction Using Machine Learning
- Creation and evaluation of damage indicators
- Analysis of measurement data from large- and small-scale experiments (MATLAB for small-scale experiments)
- Comparison of damage-sensitive features for:
- Corrosion detection
- Loss of prestress
- Crack detection
- EMI analysis:
- Comparison of impedance and admittance spectra
- Calculation of damage indicators (e.g., cross-correlation)
- Statistical evaluation according to Military Handbook 1823
- Application of temperature-compensated analysis methods from AP 2
- Guided-Waves Analysis:
- Analysis of wave transit time (time of flight)
- Analysis of signal energy
- Calculation of damage indicators
- Comparison of the methods’ performance in terms of corrosion detection
Temperature
- Methods for Temperature Compensation
FE analyses
- Parameter study (2 additional local corrosion mechanisms (crevice corrosion, pitting corrosion))
- Expansion of the test matrix (additional screw sizes M12, M36 according to EN 14399)
Measurement Guide
Based on the studies conducted, a guide will be prepared with the following content:
- Fundamentals for determining screw damage caused by corrosion based on electromechanical
- impedance spectra (EMI) / based on guided wave (GW) measurements
- Conducting EMI measurements / Guided Waves measurements
- Evaluation of the measurements (corrosion, temperature effects)
The guide includes a list of the required actuators, sensors, and equipment needed to perform the respective measurements and provides cost estimates (for installation and operation) for different numbers of sensors.
