•Volcanoes indication of •Volcanoes indication of metal shearing. deformation – not expansion. •Thin sheet residual stresses are much lower in thin sheet than thicker products. 2008 (C) M. Ramulu Cold worked hole fracture Material Properties and % of elongation: L = 8% LT = 4% ST = 3% •Percent Expansion – 3% - 5% •Cold Working is not recommended in materials where cold work expansion exceed material elongation properties. 2008 (C) M. Ramulu Cold Working : Effect of Stress Rule of thumb in Metallic Structure –A 20% Reduction in Stress Doubles the Fatigue Life 2008 (C) M. Ramulu
7 HEAT TREATMENT: Different cooling rates during quenching cause thermal stresses which can lead to a residual stress distribution. 2008 (C) M. Ramulu ASSEMBLY OF COMPONENTS: Bolted joint with built-in stresses if t 1 is not equal to t 2 . 2008 (C) M. Ramulu Residual stress distribution and residual stress intensity factor, K res , for a longitudinal welded infinite plate containing a central through crack. (a) Residual stress distribution, , across the plate width. (b) Residual stress intensity factor, K res 2008 (C) M. Ramulu Influence of chromium plating and chromium plating with shot-peening on the axial loaded fatigue behavior of 4130 steel 2008 (C) M. Ramulu Effects of surface finish on the fatigue limit of steel 2008 (C) M. Ramulu MEASUREMENT OF RESIDUAL STRESSES • Analytical Methods: Local strain approach • Numerical/Computational methods: FEM • Experimental Methods: There are seven major experimental methods -The hole drilling Method: typically 1.5-3.0mm deep, strain gages -Layer removal or Sectioning methods: the curvature changes/deflections for each layer are measured. Thickness of the layer removal is 20microns -X-ray diffraction: Residual stresses cause crystal lattice distortion, and a measurement of the inter planar spacing of the crystal lattice indicates the magnitude of the residual stress. The angle of diffraction is the measured quantity. -Neutron Diffraction - Ultrasonic Method: Speed of sound travelling throgh metal is linearly proportional to the residual stresses - Magnetic Methods 2008 (C) M. Ramulu
8 Elimination of residual stress • Plate by plastic bending • Heat treating (recrystallization): Approximate temperatures at which stress relief will be significant are: • 400-450C (750-840F) for Titanium alloys • 450-500 C( 840-930F) for high temp resistant steels • 650-800 C (1200-1475F) for high temp resistant Nickel alloys • 800-900 C ( 1475-1650 F) for superalloys • Unfavourable residual tensile stresses at the surface can be reduced or reversed into favourable by peening. 2008 (C) M. Ramulu Conclusions • Cold Working can improve fatigue life of machined parts significantly. • However, Cold Working requires intensive labor that adds high cost to the operation b t that adds high cost to the operation, but engineering & manufacturing can optimize their process and concurrence engineering to achieve the lowest cost to achieve highest quality products.
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- Residual stress, M. Ramulu