Microstructure-based fatigue modeling of cast A356-T6 alloy

TLDR: In this article, the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor.

About: This article is published in Engineering Fracture Mechanics.The article was published on 2003-01-01 and is currently open access. It has received 322 citations till now. The article focuses on the topics: Crack closure & Stress concentration.

Figures

Fig. 10. Variation in completely reversed, uniaxial strain-life behavior as a function of maximum inclusion size for horizontally cast plate, including coalescence effects in the LCF regime. In the coalescence propagation analysis, parameters were assigned as follows based on experimental work: u ¼ 1:5 10 3, Dp ¼ 50 lm, bDpart ¼ 12 lm, and DCS ¼ 30 lm.

Fig. 4. Model predictions for uniaxial, completely reversed strain-life for incubation at various maximum pore sizes (horizontally cast plate material has a maximum pore size of approximately 50 lm), as well as for the cases of incubation at 12 lm Si particles and 500 lm oxides. Note the drop of fatigue life with maximum pore size, with a strong influence of surface proximity of large pores.

Fig. 11. Estimated variation in completely reversed, uniaxial strain-life behavior of A356-T6 as a function of maximum inclusion size for (top) control arm and (bottom) front cradle, including coalescence effects in the LCF regime. In the coalescence propagation analysis, parameters were assigned as follows based on experimental work: alloy A ( u ¼ 1:0 10 5, Dp ¼ 1 lm, bDpart ¼ 6 lm, and DCS ¼ 25 lm); alloy B ( u ¼ 1:1 10 2, Dp ¼ 128 lm, bDpart ¼ 9 lm, and DCS ¼ 45 lm).

Fig. 5. Model predictions for uniaxial, completely reversed stress-life for incubation at various maximum pore sizes.

Fig. 6. Strain-life diagram for uniaxial, completely reversed loading for incubation at various inclusion scales/types for horizontally cast plate and comparison with the upper and lower bounds of the strain-life results from the various A356-T6 castings with different as-cast microstructures.

Fig. 14. Example of parametric computational study of total fatigue life (1 mm crack) as a function of completely reversed uniaxial stress amplitude for a wide range of maximum inclusion sizes for horizontally cast A356-T6 alloy.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Microstructure-based fatigue modeling of cast a356-t6 alloy" ?

When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. 

Q2. What is the driving force for the incubation of a crack?

The incubation driving force is based on the maximum plastic shear strain amplitude at micronotches, while the MSC/PSC driving force is based on the maximum principal stress range. 

Q3. What is the impact of LCF overloads on the formation of multisite fatigue cracks?

The influence of LCF overloads on the formation of multisite fatigue cracks which may eventually coalesce under lower amplitude HCF loading is an area of uncertainty. 

Q4. How many average Si particle spacings are needed to grow a crack?

the crack may only have to grow two or three average Si particle spacings before the entire field of cracks coalesce. 

Q5. What is the effect of particle constraint on the plasticity of cast alloys?

It is commonly observed that wrought alloys obey Miner’s rule for sequences of LCF loading, and the same is predicted for the limit plasticity regime of cast alloys since the influence of particle constraint is lost. 

Q6. What is the criterion for the transition from small fatigue crack growth to long crack behavior?

A criterion is offered for the transition from small fatigue crack growth to long crack behavior; the latter is characterized by sufficient sampling of microstructure within the crack tip damage process zone and is therefore governed by LEFM, so long as the applied stress is below net section yielding. 

Q7. How can the model be used to predict the fatigue life of a casting?

If the probability distributions of these features are specified based on quantitative metallography, for example, then the probability distributions for the fatigue life can be computed directly from the model. 

Q8. What is the role of microporosity in the small crack propagation rate?

The authors assume that the role of microporosity, to be distinguished from u, is principally to affect cyclic plastic strain localization in the Al-rich matrix within the eutectic regions ahead of the crack tip and in the vicinity of debonds ahead of the crack, thereby contributing to an increase in the DCTD, which in turn governs the small crack propagation rate. 

Q9. What is the scale of the cyclic plastic zone at the tip of the crack?

For LEFM to be valid, the scale of the cyclic plastic zone at the crack tip must be small relative to crack length, as must the scale of the damage process zone. 

Q10. What is the number of cycles required for propagation of a microstructurally small crack?

NMSC is the number of cycles required for propagation of a microstructurally small crack (MSC) with length ai < a < k DCS, where k is a nondimensional factor which represents a saturation limit when the three-dimensional (3-D) crack front encounters a network of Si particles; typically k is 3–5. 

Q11. What is the simplest approach to fatigue modeling of cast alloys?

The hierarchical approach to fatigue modeling of cast alloys permits bypass of certain crack growth regimes associated with lower length scales if the cracks incubate at larger defects. 

Q12. How can the authors use the recursion relations to reduce the fatigue life of a crack?

the recursion relations can be used when a fatigue crack grows from a large pore through a field of monosize pores that are spaced according to Eq. (25) simply by considering them to play the precise role that the particles played in this formulation. 

Q13. What is the threshold for local microplasticity associated with preexisting debonds?

An increase of the applied peak stress due to an overload drops the threshold level for local microplasticity associated with preexisting debonds. 

Q14. What is the number of cycles to incubate a micronotch root scale crack?

NT ¼ Ninc þ NMSC þ NPSC þ NLC ¼ Ninc þ NMSC=PSC þ NLC ð2Þwhere Ninc is the number of cycles to incubate (nucleation plus small crack growth through the region of notch root influence) of a micronotch root scale crack with initial length, ai, on the order of 1/2 the maximum Si particle diameter, bDpart, or pore size, bDp. 

Q15. How can the authors apply the recursion relations to a crack?

the authors can apply these relations to propagation through a field of Si particles or pores of some average diameter and spacing of a crack originating at an oxide by simply replacing bDp by bDoxide. 

Q16. How many cycles does the crack grow between the large pore and the nearest Si particle?

After initial incubation at all inclusions considered, the authors must first compute the distance the crack must grow between the large pore and the nearest fractured Si particle in the same number of cycles at the point of coalescence.