Application of High-Frequency Fatigue Testing Machine in Infinite Life Testing of Valve Springs

This article introduces the application of high-frequency fatigue testing machines in the infinite life testing of valve springs. The test simulates the long-term working stress of the spring through high-frequency cyclic loading, with parameters such as preload, amplitude, and number of cycles set according to international standards. During the test, it is necessary to calibrate sensor accuracy, control temperature, and record load and displacement data. The acceptance criterion is that the spring shows no cracks or deformation within the allowable range after 10^7 cycles. Key considerations include avoiding inertial effects and paying attention to the impact of surface strengthening treatments, with the importance of fixture precision and real-time monitoring emphasized.

Test Principle:

The high-frequency fatigue tester uses electromagnetic excitation or mechanical resonance to make the valve spring operate under a cyclic load much higher than the actual operating frequency (usually greater than 50Hz). The test is based on the international standard ISO 6270 and industry general specifications, and sets thresholds for preload, amplitude and cycle times to simulate the long-term alternating stress of the spring in the engine valve train. The infinite life of a valve spring is generally defined as no fracture or significant relaxation after 10^7 cycles, when the stress amplitude is below the material's durable limit.

Test parameter setting

Before testing, key parameters should be determined based on the geometric size and material properties of the spring. The following table sets out the main settings and descriptions:

Test load range: 0.3P~0.7P (P is the maximum working load of the spring)
Loading frequency: 60~120Hz, adjusted according to spring rate
Cycle number target: ≥1×10^7 times, corresponding to the infinite life criterion
Temperature control: room temperature (20±5°C) to avoid temperature rise affecting the result
Failure criterion: The test is stopped when cracks appear or the load drops by 10%

Test system verification

The high-frequency fatigue testing machine needs to calibrate the force sensor and displacement sensor regularly to ensure that the dynamic load error is less than ±1%. When installing the spring, a special fixture should be used to ensure the coaxiality, and the preload amount should be set at 5%~10% of the free length of the spring to eliminate the gap and simulate the actual assembly state. The system needs to have automatic shutdown protection to stop loading immediately when a resonant frequency drift or amplitude anomaly is detected.

Data collection and analysis

During the test, the number of cycles, load amplitude, displacement amplitude and frequency changes are continuously recorded. The fitting formula based on the S-N curve (stress-life curve) is as follows:

σ_a = σ_f×(2N_f)^b

Among them, there σ_ais the stress amplitude, σ_fis the fatigue intensity coefficient, N_fis the number of failure cycles, and b is the fatigue intensity index. When all the sample data points fall on 10⁷When there is no failure on the right side, it can be determined that the spring meets the requirements of infinite life. In the actual analysis, early failure data due to surface defects or loose fixtures should be excluded.

Criteria for determining results

Referring to ISO 6270 and material standards, the valve spring is at 1×10⁷If there are no macroscopic cracks after the second cycle, and the free length change is less than 0.5% of the original length, it is considered to have passed the infinite life test. For springs with a wire diameter ≤ 4mm, the maximum permanent deformation allowed is 0.3mm. If there is a fracture during the test, the fracture location, cross-sectional topography should be recorded and the fatigue safety factor should be calculated.

Application Notes

The inertia effect generated by the high-frequency test may lead to the distribution deviation of the stress in the spring, and it is recommended to use the finite element method to verify whether the stress concentration area is consistent with the actual working conditions before the test. For surface-strengthened springs (e.g. shot peening), additional attention should be paid to the relaxation behavior of residual stresses. After the test, it is recommended to perform a metallographic examination of the spring to observe whether the microscopic structure shows signs of slip bands or microcracks to assist in life assessment.

Summary

The high-frequency fatigue testing machine combines strict parameter setting and data criteria to efficiently complete the infinite life assessment of valve springs. This method shortens the test period by increasing the loading frequency and uses standardized failure criteria to ensure the comparability of results, which provides a reliable basis for spring selection and durability design in engineering applications. In actual operation, attention should be paid to the coaxiality of the fixture, ambient temperature control, and real-time data monitoring to ensure the effectiveness of the test.

References

1. ISO 6270 Standard for Fatigue Test Methods for Metallic Materials
2. SAE J 1237 Technical Specification for Valve Springs
3. Spring fatigue life test procedures and data analysis methods
4. High-frequency fatigue test technology and its application in auto parts