276Q actuator body spring failure analysis

The broken bolt fractured along the root of the thread at the lower end, with a distinct fracture surface. There was no visible deformation at the fracture site, and the outer diameter remained consistent with other areas of the bolt. The fracture surface was flat, without any cup-shaped features, indicating a brittle failure mode. The central region of the fracture displayed radiating fibers, while the outer edge had a porcelain-like appearance. No shear lip or slanting feature was observed, but there was a torsional shear step, confirming that the failure was due to torsional stress. A small dark brown mark was found near the porcelain-like area on one of the fractures, suggesting the presence of a pre-existing crack. A mechanical performance test was conducted on the bolt. Due to its length, samples for hardness and tensile testing were cut from the rod body and tested according to standard procedures. However, because of equipment limitations, the yield strength (Rs) was measured using a pointer method, which may introduce some inaccuracies. The results showed that the actual yield strength was likely lower than the measured value, as the test piece had already been deformed during installation. Additionally, the pointer method tends to lag behind the true yield point, leading to an underestimation of the force applied. Metallographic analysis was performed on all six failed bolts. Observations of both horizontal and longitudinal sections revealed uniform tempered sorbite structures, with no free ferrite or undissolved carbides detected. Minor non-metallic inclusions were present, but they were within acceptable limits. However, the precipitated ferrite blocks in the sorbite structure were slightly elongated compared to typical bolt microstructures, which could affect mechanical properties. Under a 500× magnification, microcracks were observed at the root of the thread on two of the broken bolts. The crack tips were sharp, with no signs of decarburization or plastic deformation at the open ends. These cracks were not present in the other failed bolts, suggesting that the cracks originated prior to failure and left characteristic traces on the fracture surfaces. Due to the deformation experienced by the test specimen during installation, a hardening effect was observed, meaning the actual yield strength was likely lower than the measured value. Moreover, the yield point measured via the pointer method exhibited hysteresis, resulting in a lower recorded force. As a result, the measured yield strength (Rs) was higher than the actual value. Most of the values fell below the minimum required material strength of 830 N/mm², and the hardness did not meet the specified requirements, indicating poor mechanical performance in the failed bolt. Although the chemical composition of the failed parts met the standard specifications, the levels of key alloying elements—carbon (C), chromium (Cr), and molybdenum (Mo)—were below the recommended ranges. This could have contributed to the reduced mechanical performance. Molybdenum plays a crucial role in enhancing yield strength, and its low concentration likely affected the overall strength of the bolt. The metallographic structure showed no free ferrite or undissolved carbides, indicating proper quenching and tempering processes. However, the morphology of the precipitated ferrite needles was somewhat more pronounced, which could reduce strength and hardness, aligning with the observed performance results. In normal operation, when a pre-tightening torque (M) is applied to a bolt, the resulting pre-tightening force (P₀) can be estimated as P₀ ≈ M / 0.2d. The pre-tightening stress (R₀) is approximately 20 MPa for a bolt with a diameter (d). The torsional shear stress (S) caused by the resistance moment during tightening is similar in magnitude. The total equivalent stress (R) on the bolt is roughly 1.3 times R₀, giving R ≈ 26 MPa. While the minimum yield strength of the material meets this requirement, the actual yield strength was lower, leading to premature yielding and relaxation of the bolt. Although the yield strength of the fractured parts met the required standards, the tensile strength did not. The presence of microcracks at the thread root significantly reduced the bolt’s strength. These cracks acted as stress concentrators, causing localized failure and increasing the stress on the remaining cross-sectional area. This led to progressive crack propagation and eventual fracture. The macroscopic fracture lacked plastic deformation, cup-shaped features, or buried regions, showing only a porcelain-like surface and radiating fibers. The crack tail was sharp, with no signs of decarburization or plastic deformation, and the surface showed a dark brown discoloration. These characteristics suggest that the cracks were not due to raw material defects or loading-induced damage, but rather formed during post-processing, likely due to quenching and tempering issues. **Conclusion:** 1. Bolt loosening occurred due to insufficient mechanical properties that could not withstand the normal pre-tightening torque. 2. Bolt failure was primarily caused by microcracks at the thread root and inadequate strength. 3. The bolt's mechanical properties did not meet technical requirements, partly due to low alloy element content and improper heat treatment. Microcracks also developed during processing, contributing to the failure.

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