High-strength steels are the bainitic-martensitic steels that are used for coil spring products. Coil springs are particular components of the engine system that are widely used in automotive industries. Their main function is to absorb vibrations of the engine system, which imposes a high level of energy to these coil springs. Recent developments in automotive industries require the coil springs with high fatigue life and strength.
One of the objectives of this study is to assess the main reasons for fatigue life discrepancy of coil springs that broken in different numbers of cycles, varying in the range of 10 4 to 10 5 cycles. To that end, different material and microstructural characterization methods were used. These characterizations on coil springs, tested with controlled stress, revealed that the stress relaxation, and initial microstructure, was not the same in the coils with low life cycles (10 4) and high life cycles (10 5). Stress relaxation during fatigue test was more in springs failed at low numbers of cycles than the ones failed at high numbers of cycles. This work showed that microstructure of these low fatigue life springs have more martensite, which allows the harder material be formed. However, hard materials decreased the performance of the coil in service.
The other objective of this research work is to study the fatigue life of the parts as a function of the surface integrity conditions for different materials and process parameters. The fatigue testing was performed by rotating bending method (R=-1) with a stress of 1000 MPa. Fatigue life studies showed that failure for the steels without surface defects and residual stress came from inclusion particles. The chemical composition of these inclusions is rich in Aluminum-Calcium (Al-Ca). It was investigated that these inclusion particles do not control fatigue life after surface shot peening. It was shown that surface defects induced by shot peening cause failure. Surface defects after chemical polishing were removed and the best fatigue life obtained when specimens were shot peened and chemically polished.
| Date | 20 Dec 2016 |
|---|
| Original language | American English |
|---|
| Awarding Institution | - École de technologie supérieure
|
|---|
| Supervisor | Philippe Bocher (Supervisor) |
|---|
Heidari, K. (Author),
Bocher (Supervisor),
20 Dec 2016Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering