Fig. 1. A fatigue crack was initiated from porosity in a sand-cast A356- T6 casting. (Image was taken using secondary electron imaging.)
Fig. 2. This fatigue crack was initiated from an (old) oxide film in a sand-cast A356-T6 casting. (Image was taken using secondary electron imaging.)
Fig. 3. Fatigue cracks were initiated from (left) fractured and (right) debonded eutectic particles in a sand-cast A356-T6 casting. (Image was taken using secondary electron imaging.)
various degrees of porosity, oxide levels and microstructural features. When pore and oxide sizes are larger than competing microstructural weak links (such as eutectic silicon particles), crack initiation life can be ignored. However, when the flaw size in the casting approaches the microstructural feature size (~25-50 µm), crack initia- tion life from microstructural con- stituents such as persistent slip bands can dominate the total fatigue life. Fatigue cracks usually initiate from flaws in aluminum castings. In coarse microstructures, the dendrite cell walls act as grain boundaries because they are surrounded by a nearly continuous array of eutectic particles. Te researchers applied MSF models to estimate the fatigue life
2
Procedure At GM, MSF models
have been developed to predict fatigue life of cast aluminum components with
of cast aluminum alloys A356 and 319. Tey used extreme values of the characteristic flaw and microstruc- ture features that were assumed to have nominal value (50% probability) and 99.94% (+3 standard deviations) probability of occurrence. A comparison of the calculated
propagation life based on both long and short crack models with empiri- cally derived constants to actual fatigue life (Nf) was made. Te calculated crack propagation life (Np) was in good agreement with the actual fatigue life. Tis indicated the total fatigue life of cast 319 alloys failing from porosity is mainly comprised of crack propaga- tion and the assumed initiation life near zero is reasonable. Compared to the short crack model, the long crack model is simpler and fits the data over the useful life range of most compo- nents. Given the inherent scatter in fatigue life data, predictions within an order of magnitude of measured
values (dashed lines) are useful. Te researchers compared the pre-
dicted fatigue life based on the long and short crack models using the extreme- value statistics (EVS) estimated maximum pore sizes to the experimen- tal data. As expected, with the upper bound EVS estimate of the maximum pore size, both long crack and short crack models gave a conservative lower bound fatigue life prediction. Tey also performed a comparison
of the calculated fatigue life of samples failed by cracked eutectic particles and debonded eutectic with actual fatigue life. Te samples tested at the stress amplitude of 85 MPa and stress ratio R = -1 had coarse unmodified microstructures and large silicon and iron-rich particles. Te fatigue cracks in all three samples were initiated by cracked silicon and iron-rich particles. Te calculated fatigue life was in good agreement with the actual fatigue life. A comparison of the calculated
May 2013 MODERN CASTING | 39
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95