The present work investigates the failure of locker bolts from axles of off-highway tractor application that failed prematurely. Detailed failure analysis has been carried out involving visual inspection, magnetic particle inspection, fractography, microstructural analysis, and hardness test. The investigation leads to an understanding on a sudden and catastrophic fracture of bolts even before being put in service. The findings are strongly consistent with internal hydrogen embrittlement and incomplete hydrogen removal during post-plating processing is considered the most probable contributing factor based on the available evidence. Necessary design changes were initiated based on the investigation that eliminated this failure once and for all.
This study presents an integrated statistical and machine learning framework for vibration-based structural health monitoring (SHM) of friction stir spot welded (FSSW) aluminum joints in automotive applications. Using a 3 × 3 × 3 full factorial design of experiments, the effects of rotational speed (1400, 1700, and 2000 rpm), dwell time (15, 20, and 25 s), and material configuration (similar AA 2024-T3, similar AA 6061-T6, and dissimilar combinations) were evaluated for 27 parameter combinations, each tested with four specimens (108 welded specimens in total), covering tensile shear strength and natural frequency degradation under progressive static loading at 25, 50, and 75
Severe sticking occurred during the thermal assembly of a large-scale interference-fit shaft–drum system used in a vertical ship lift, making subsequent disassembly extremely difficult. This study presents a failure analysis of the sticking problem and develops a thermo-mechanical disassembly method as a corrective action. The failure was attributed to the combined effect of high contact pressure in the interference-fit region and geometric interlocking caused by ploughing damage in the clearance-fit region. Analytical calculations based on thick-walled cylinder theory and friction models showed that the required extraction force under non-heating conditions reached 2284 t, far exceeding the 1000 t hydraulic capacity available on site. Finite element analysis was then used to compare different heating-cooling schemes and determine an effective operating window. Based on the optimized scheme, thermo-mechanical disassembly was implemented through drum heating and localized shaft cooling. Field application achieved successful extraction with a maximum force of approximately 286 t. The results provide a practical corrective method and useful guidance for preventing similar disassembly failures in large interference-fit structures.
Radiographic testing (RT) of a semicryogenic rocket engine heat exchanger fabricated from a 67Ni–18Cr–4.5Mo–4.5W–2.5Ti–1.5Al nickel-based superalloy revealed low-density linear indications in the jacket ring adjacent to the circumferential weld. As the indications were confined to the parent material, a metallurgical investigation was conducted using repeat RT, computed tomography (CT), optical microscopy, field-emission scanning electron microscopy (FESEM), fractography, and energy-dispersive x-ray spectroscopy (EDS). CT confirmed that the discontinuities were located within the parent material and separated from the weld. Fractography revealed predominantly intergranular fracture associated with coarse second-phase particles. Optical microscopy showed intergranular cracks decorated by coarse grain-boundary particles in a recrystallized austenitic matrix. EDS mapping and point analysis identified the particles as Ti-rich primary MC carbides containing Mo and W, designated Ti(Mo,W)C. No detectable impurity enrichment or chemical segregation was observed near the cracks. The results indicate that the radiographic indications were primarily associated with carbide-related intergranular cracking originating from the alloy solidification microstructure rather than conventional weld defects. Mechanical stresses associated with subsequent shrink fitting and local section reduction around machined holes may have assisted to crack initiation and propagation. The findings emphasize the need for improved melting and solidification control and microstructural screening of incoming nickel-based superalloy.
Non-metallic inclusions critically influence the very high cycle fatigue (VHCF) behavior of high-strength steels. This study investigates the impact of plastic inclusion clusters on VHCF properties. Fatigue cracks are more readily initiated from plastic inclusion clusters compared to single brittle inclusions. Furthermore, at equivalent fatigue lives, the size of plastic inclusion clusters is notably smaller. Selected area diffraction (SAD) analysis reveals discontinuous diffraction rings in the vicinity of the clusters, suggesting the formation of nanograins. To address the varying effects of different inclusion types, a correction method for the size of plastic inclusion clusters is proposed. The corrected size improves the prediction of fatigue behavior and better distinguishes the influence of plastic versus brittle inclusions on fatigue. This approach provides a more unified framework for evaluating the role of inclusions in the fatigue performance of high-strength steels.
