Microstructure-sensitive computational modeling of fatigue crack formation

https://www.cavs.msstate.edu/publications/docs/2003/01/805Microstructure.pdf

Microstructure-based fatigue modeling of cast A356-T6 alloy

Excellent thermal conductivity and lower density make Al–Si alloys a suitable alternative for cast iron in the fabrication of engine components. The increase in the maximum operation temperature and pressure of engines necessitates improving the thermomechanical fatigue performance of Al–Si alloys. This paper has two major parts focussing on the use of Al–Si based alloys in cylinder heads and engine blocks.In the first part, the structural stress–strain and material property requirements of cylinder heads are discussed. In addition, the physical and mechanical properties of different competing materials used in the manufacture of engine components are reviewed. The physical metallurgy, solidification sequence and thermal conductivity of Al–Si based alloys are reviewed. Also discussed is the effect of microstructural features on thermomechanical fatigue lifetime. This part also includes an overview of the strengthening mechanisms of cast Al–Si alloys, by dispersed phases and heat treatment.Demands ...

Application of cast Al–Si alloys in internal combustion engine components

Failure of metals II: Fatigue

The fatigue resistance of AM-SLM AlSi10Mg samples built in the Z direction after various heat treatments was investigated. Specimens were tested in the as-built (AB) condition, after stress relief (SR) treatment and after SR and hot isostatic pressing (HIP) at either 250 °C or 500 °C. The AB machined and polished specimens displayed the highest fatigue limit ( S f = 125 MPa, N f = 10 7 cycles). SR and HIP cycles decrease the yield strength, hardness and fatigue limit. The SR and HIP treatment at 500 °C resulted in the lowest fatigue resistance due to significant microstructural changes. A relation between the yield stress and fatigue resistance was established. Linear elastic fracture mechanics were employed for evaluating fracture surface morphology. Based on the results of fracture surface characterization, values of the critical stress intensity factor ( K cr ) for AM-SLM AlSi10Mg specimens after various heat treatments were estimated.

Fatigue of AlSi10Mg specimens fabricated by additive manufacturing selective laser melting (AM-SLM)

Constant-amplitude high-cycle fatigue tests (σmax=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN 1 × 10−6 m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K max tr ≈6.0 MPa √m for the present cast A356 alloy.

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

https://www.cavs.msstate.edu/publications/docs/2000/01/825Finite%20element.pdf

Finite element analysis of the stress distributions near damaged Si particle clusters in cast Al–Si alloys

Monotonic and cyclic finite element simulations are conducted on linear-elastic inclusions and voids embedded in an elasto-plastic matrix material The elasto-plastic material is modeled with both kinematic and isotropic hardening laws cast in a hardening minus recovery format Three loading amplitudes (Δe/2=010%, 015, 020%) and three load ratios (R=−1, 0, 05) are considered From a continuum standpoint, the primary driving force for fatigue crack formation is assumed to be the local maximum plastic shear strain range, Δγmax, with respect to all possible shear strain planes For certain inhomogeneities, the Δγmax was as high as ten times the far field strains Bonded inclusions have Δγmax values two orders of magnitude smaller than voids, cracked, or debonded inclusions A cracked inclusion facilitates extremely large local stresses in the broken particle halves, which will invariably facilitate the debonding of a cracked particle Based on these two observations, debonded inclusions and voids are asserted to be the critical inhomogeneities for fatigue crack formation Furthermore, for voids and debonded inclusions, shape has a negligible effect on fatigue crack formation compared to other significant effects such as inhomogeneity size and reversed loading conditions (R ratio) Increasing the size of an inclusion by a factor of four increases Δγmax by about a factor of two At low R ratios (−1) equivalent sized voids and debonded inclusions have comparable Δγmax values At higher R ratios (0, 05) debonded inclusions have Δγmax values twice that of voids

Jinghong Fan

Papers

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Finite element analysis of the stress distributions near damaged Si particle clusters in cast Al–Si alloys

On the driving force for fatigue crack formation from inclusions and voids in a cast A356 aluminum alloy