Dr Bin Yang asked an interesting question: https://www.researchgate.net/post/How_to_distinguish_between_creep_voids_and_dimples
I am currently working on a phenomenological creep damage model that takes account of (i) creep cavity nucleation, (ii) diffusion controlled creep cavity growth and (iii) plastic hole growth.
The model gives a good prediction of a wide range of creep-fatigue test data (164 tests) on; Cast 1CrMoV, Cast ½CrMoV, Wrought Grade 91, Cast Type 304L, Type 347 Weld Metal, Wrought Type 321, Wrought Type 316H and Type 316H Multi-Pass HAZ; Tested at Temperatures ranging from 538 to 650°C. (see attached plots).
(Note: The black lines show 1, 2 and 0.5, which are acceptable scatter bands. The red lines show a linear fit to the data and the upper and lower 95% prediction intervals to demonstrate whether the model meets the acceptance criterion. )
However, to achieve a good prediction for all of the 164 Creep-Fatigue tests I have to make some assumptions. I am therefore looking for other evidence (such as metallography or theoretical modelling) that supports these assumptions:
The main assumption that has been made is about the conditions under which Plastic Hole Growth dominates and when Plastic Hole Growth is negligible.
Are there any metallographic observations or theoretical modelling that suggests that; Plastic Hole Growth can only dominate; (i) when the total strain is monotonically increasing and/or (ii) when the total strain exceeds a certain value; or (iii) any other relevant observation regarding Plastic Hole Growth at elevated temperature?
Maybe you can focus on the shape of the cavities.
Assume under uniaxial creep testing:
If the plastic hole growth takes control, then the cavities should be more like a standing needles which is perpendicular to the GB. Otherwise if the vacancy diffusion mechanism controls, the cavities should be like cracks that is parallel to the GB.
But to remember, when the constrained diffusional growth mechanism takes control, then the cavities growth rate actually depends on the slow creep deformation on the neighbourhoods matrix. However, the cavities look like the crack-type, since the vacancy diffusion contributes most of the growth but it is constrained by surrounding slow creep deformation rate. This is the most common type of creep cavitation.
From my understanding the Plastic hole phenomenon could be dependent on the strain rate, if plasticity is dominated by dislocation based behaviour then the strain rate could drive the development of plastic holes. There are considerable strain rate effects at high temperatures for 9CrMoV steels which could be of interest to your research.
Dear Micheal,
Underline is highlighted for your understanding. This is my view.
In my opinion plastic growth is fatigue growth and it differs from creep growth in domination. Mechanism of creep is slow. However, Creep is more dominant. delta increases and more stable.
Plastic growth can be similar in the fatigue growth of multiple cracks, of arbitrary lengths, emanating from a row of fastener holes in a bonded, riveted, lap joint in a pressurized aircraft fuselage etc. The creep crack growth behavior develops under the condition of crack tip stress concentration. If the applied loading contains a considerable cyclic component, one speaks of creep fatigue crack growth. The crack propagation mode may then change from intergranular creep cavitation to transgranular fatigue. The development of creep-fatigue damage in most power plant steels depends on temperature, strain range, strain rate, hold time, and the creep strength and ductility of the material. At high stress intensities, crack growth rates are extremely high and little fatigue life is involved. Region III is characterized by rapid, unstable crack growth. The crack growth rate accelerates as the maximum stress intensity factor approaches the fracture toughness of the material.
Example:
PVC pipe,
Energy terms (δ and Gb) of RTFP are taken the instability behavior with increase temperature degrees in the fatigue crack propagation but in the creep crack propagation they are stable behavior , δ increase and Gb decrease , with the increase temperature degrees, that refer to unagreement RTFP with experimental data of (V-ΔK) diagram for fatigue crack propagation . But, (V-△K) diagram predicts that fatigue crack growths are lesser than LRC (Creep) in the same figures for creep crack growth. (file:///C:/Users/3020/Cookies/Desktop/Downloads/aqeel2.pdf)
For Steel
The creep fatigue crack growth behaviour of P91 steel in as-received and ex-service material conditions has been examined. The crack growth data was characterized using fracture mechanics parameters ΔK and C*. The results showed that at high frequency (> 0.01 Hz), the CFCG behaviour tend to that of high cycle fatigue crack growth and is best correlated with the ΔK parameter whereas at lower frequencies, creep mechanisms have been found to dominant and best correlated with the C* parameter. ( file:///C:/Users/3020/Cookies/Desktop/Downloads/Creep-fatigue_crack_growth_behaviour_of_P91_steels.pdf)
Diffusion creep
Diffusion creep models consider the stress-assisted diffusion of ions or vacancies in the crystal as the key mechanism for creep deformation. They are the same as those proposed earlier for metallic materials. Vacancies diffuse from the grain boundaries located nearly perpendicular to the tensile axis (where the vacancy concentration is above the equilibrium value) to those located parallel to the tensile axis (where the vacancy concentration is below the equilibrium value). The role of diffusion creep as a possible deformation process in NC materials is currently under debate, with several conflicting reports arising from both the experimental and computer simulation communities. Gleiter, who first proposed this notion, rationalized that the abundance of GBs together with the absence of conventional dislocation activity would result in plasticity localized in the GB regions, namely diffusion and sliding, even at ambient temperatures.
So far, MD simulation findings on diffusion creep in NC metals were reported only at temperatures that are close to the melting point of the material (T > 0.8 Tm). Simulations at lower temperatures NOTE****report GB sliding as a dominant deformation mode(https://www.sciencedirect.com/topics/engineering/diffusion-creep).
Hope it work out for you. Above of three indicates the creep is more dominating.
Ashish
When the characteristic diffusive length is comparable to or smaller than the average cavity size. And a follow up question - are you referring this question at the microscale or at the continuum (say engineering models) scale?
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