Electronic Theses and Dissertation

Pegas ulir merupakan komponen penting pada sistem suspensi kendaraan yang berfungsi meredam getaran dan menahan beban. Dalam kondisi operasi, pegas ulir mengalami pembebanan berulang yang dapat menyebabkan kegagalan lelah dan menurunkan kinerja suspensi. Penelitian ini bertujuan untuk menganalisis mekanisme kegagalan pegas ulir suspensi depan kendaraan Multi Purpose Vehicle (MPV) serta mengevaluasi distribusi tegangan dan regangan menggunakan metode elemen hingga (Finite Element Method/FEM). Metode penelitian dilakukan melalui analisis eksperimental dan numerik. Analisis eksperimental meliputi pengamatan visual serta pengujian Scanning Electron Microscopy (SEM) dan Energy Dispersive Spectroscopy (EDS) untuk mengidentifikasi morfologi patahan dan komposisi kimia material. Hasil pengujian menunjukkan bahwa material pegas merupakan baja paduan SAE 5160 dan mekanisme kegagalan didominasi oleh kegagalan lelah yang ditandai dengan adanya beach marks dan perambatan retak bertahap. Analisis numerik dilakukan dengan pemodelan pegas ulir tiga dimensi menggunakan perangkat lunak FEMAP pada kondisi tanpa cacat dan dengan cacat awal, serta diberikan pembebanan statik dan dinamik. Hasil simulasi menunjukkan bahwa keberadaan cacat awal menyebabkan peningkatan signifikan distribusi tegangan dan regangan akibat konsentrasi tegangan di sekitar ujung cacat. Nilai faktor intensitas tegangan (KI) yang diperoleh melebihi nilai ketangguhan retak material (KIC), sehingga retak berpotensi merambat hingga terjadi kegagalan. Sebaliknya, pegas tanpa cacat awal masih berada dalam batas aman. Penelitian ini membuktikan bahwa metode elemen hingga efektif dalam memprediksi kegagalan pegas ulir dan mendukung evaluasi umur pakai serta perancangan sistem suspensi yang lebih andal.

Kata kunci: pegas ulir, kegagalan lelah, metode elemen hingga,faktor intensitas tegangan

Coil springs are critical components of automotive suspension systems, responsible for load support and vibration isolation during vehicle operation. In service conditions, these components are continuously subjected to cyclic loading, which can promote fatigue damage and ultimately lead to premature failure. This study investigates the failure mechanism of a front suspension coil spring used in a Multi Purpose Vehicle (MPV) and evaluates the associated stress–strain behavior through a combined experimental and numerical approach based on the Finite Element Method (FEM). Experimental investigations include macroscopic inspection, Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS) to characterize the fracture morphology and material composition. The results confirm that the coil spring is manufactured from SAE 5160 alloy steel and that the failure is governed predominantly by fatigue mechanisms, as evidenced by the presence of beach marks and stable crack propagation features. Subsequently, a three-dimensional finite element model of the coil spring is developed using FEMAP software, considering both defect-free and initial-defect configurations under static and dynamic loading conditions. Numerical simulations reveal that the introduction of an initial defect significantly amplifies local stress and strain concentrations in the vicinity of the crack tip. The calculated Mode I stress intensity factor (KI) exceeds the critical fracture toughness (KIC) of the material, indicating unstable crack propagation and a high likelihood of structural failure. Conversely, the defect-free model exhibits stress levels within the allowable limits of the material. The findings demonstrate the capability of FEM-based analysis to accurately predict fatigue-induced failure in coil springs and provide valuable insights for durability assessment, preventive maintenance strategies, and the design of more reliable automotive suspension systems. Keywords: coil spring, fatigue failure, finite element method, stress intensity factor