Research On Welding Defect Tolerance In Key-parts Of High-speed Train

High-strength aluminum alloy is often used in manufacture of high-speed trains toachieve energy-conservation and efficient security goals. But due to ignorance of effectscaused by welding fabrication in train body design and strength analysis, that’s to say,introduction of welding defects and inhomogeneous microstructure, security issueshappen frequently in service periods of aluminum train. Critically insufficient structuralsecurity theories and research methods of high-speed train cause a big blindness in traindesigning and manufacturing, thus frequent maintenance must be operated to preventaccidences.In this paper, based on fracture mechanics and finite element analysis, combiningfracture parameters tests, service reliability evaluation system of key bearing parts inhigh-speed train was established, which is able to assess security of structure containingdefects and initial tolerance under given fatigue life. The main contents and results areas follows:Numerical simulation analysis of the magnitude and distribution of weldingresidual stress and working dynamic stress on train bearing chassis was conducted afterestablishing the train body model using Hypermesh software, thereby identifying fourbearing dangerous areas near the traction beam. Types and distribution of defects intypical welded joints were counted with slice test. Defects were normalized to a/c=0.3semi-elliptical surface cracks and a/c=1or a/c=0.1embedding cracks.Tests of fracture toughness and fatigue crack growth behaviour when the stressratio is0.1,0.4,0.7were operated on7N01S-T5,7N01P-T4, each region of7N01PT4-7N01ST5welded joints with thickness of10mm and16mm. Results offracture toughness indicate, with the same thickness, fracture toughness values of7N01S-T5are higher than7N01P-T4; KICvalues of different heat treatments arespecifically between40~44MPa m1/2and32~37MPa m1/2; fracture toughness values ofeach region of welded joints are lower than the base metal; both heat-affected zone of7N01P-T4side and7N01S-T5side are between27~30MPa m1/2; fracture toughnessvalues of perpendicular to the squeezing direction are slightly higher than the parallel ones; affects of thickness on the fracture toughness values appear no obvious rules inthis study. Results of fatigue crack growth rate indicate, each region of welded joints ishigher than the base metal.Brittle fracture of key parts in high-speed train welding structure is evaluatedaccording to SINTAP method. Results indicate that for the four dangerous positions,critical values of a/c=0.3semi-elliptical surface crack are between4~10mm; criticalvalues of embedding crack are between2.5~3.5mm. Embedding crack of a/c=0.1hassharper shape and it’s more prone to brittle fracture.Every type of initial typical crack tolerance value with the fatigue life of107werecalculated adopting the Paris and Forman formula respectively. The calculation resultsof Forman formula are larger than Paris, because the lager fracture toughness value inForman, the lower fatigue crack growth rate.