High-Frequency Fatigue Crack Threshold Testing Without In-Situ Sensors
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Solution Overview
Problem
Existing methods for determining the fatigue crack non-propagation threshold of turbomachine blades are limited by quasi-static conditions and require in-situ instrumentation, which is not feasible at high frequencies, and suffer from inaccuracies due to frequency dependence and material sensitivity, especially for materials like Ti-6Al-4V and TiAl.
Innovation Solution
A high-frequency determination device with a specific geometry, utilizing an elliptical hole and notch, applies cyclic loads to a structural specimen, allowing crack propagation to stop spontaneously, enabling the determination of the non-propagation threshold without real-time crack length measurement, using a nomogram to calculate ΔKth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional quasi-static methods are used to determine crack propagation threshold, then measurement precision is improved, but frequency range is limited to 1-30 Hz and not representative of actual blade operating conditions
Solution Approach 1:
The patent applies mechanical vibration by subjecting the specimen to high-frequency cyclic loading (300-2000 Hz) to simulate actual blade operating conditions. The vibratory loading system generates controlled vibrations that propagate stress cycles through the specimen, enabling threshold determination at frequencies representative of real turbomachine blade operation rather than quasi-static conditions.
Solution Approach 2:
The patent changes the loading frequency parameter from traditional quasi-static (1-30 Hz) to high-frequency ranges (300-2000 Hz) to match actual service conditions. This parameter change allows the determination of frequency-dependent threshold values that accurately reflect material behavior under operational vibrations, resolving the contradiction between measurement precision and frequency representativeness.
2Measurement precision
If in-situ instrumentation is used to measure crack length in real-time, then crack propagation control is improved, but device complexity increases and is incompatible with high-frequency testing
Solution Approach 1:
The patent extracts the measurement function from the testing process itself. Instead of requiring complex in-situ instrumentation during high-frequency loading, the method uses the specimen's own geometry (elliptical hole) to create a predictable stress field, and determines threshold from post-test crack length measurements combined with nominal stress calculations, eliminating the need for real-time measurement systems.
Solution Approach 2:
The patent performs preliminary characterization of the specimen geometry and stress distribution before testing. The elliptical hole dimensions and specimen configuration are pre-designed to create a known stress field, allowing threshold determination from simple post-test measurements rather than requiring complex real-time monitoring during the actual high-frequency cycling.
3Force
If traditional hydraulic tensile testing machines are used, then load control is improved, but frequency range is limited to quasi-static conditions and cannot reach operational frequencies above 500 Hz
Solution Approach 1:
The patent replaces traditional hydraulic loading with a vibratory loading system that generates high-frequency cyclic stresses through mechanical vibration. This system can operate at frequencies between 300-2000 Hz, matching actual blade operating conditions, while maintaining sufficient load control capability through controlled vibration amplitude and frequency.
Solution Approach 2:
The patent uses periodic vibratory loading instead of continuous hydraulic pressure application. The cyclic nature of vibration naturally provides the periodic stress application needed for fatigue threshold determination, while enabling operation at high frequencies that hydraulic systems cannot achieve due to their inherent response time limitations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate determination of the fatigue crack non-propagation threshold at frequencies between 300 Hz and 2000 Hz, eliminating the need for in-situ instrumentation and providing precise ΔKth values through controlled cyclic loading and crack length recording post-test.
Implementation Method 1
a high-frequency determination device with a specific geometry, utilizing an elliptical hole and notch, applies cyclic loads to a structural specimen
Implementation Method 2
determining the fatigue crack non-propagation threshold of a turbomachine blade
Data Source
Figure 1A~2
Figure 3~4
AI summary
A high frequency method for determining the non-propagation threshold of fatigue cracks in which a cyclic load (32, 32A) is applied to at least one test piece comprising, in a test area (10A), an elliptical hole (12) having a notch (14) at one apex, and held between two rigid masses (24, 26), two rigid prestressing plates (20, 22) being disposed on either side of said test piece and being secured at each of the two ends (20A, 22A; 20B, 22B) of same to the two rigid masses, the frequency of the cyclic load (32, 32A) being chosen as equal to the natural frequency of the test piece/masses/prestressing plates assembly so as to generate a fatigue crack from the notch; then, when it is observed that the crack has stopped propagating, the final length of the crack is recorded, and said non-propagation threshold DeltaKth of the fatigue crack is determined using a table, the cyclic load being obtained by a vibrating electrodynamic pot integrally attached by means of rigid posts to a frame supporting the two rigid masses and comprising a push rod to transmit said cyclic load to the test piece/masses/prestressing plates assembly.