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Thickness effect in crack-tip stress analysis

Thickness effect in crack-tip stress analysis

Thickness effect in crack-tip stress analysis

(OP)
Hi
I am analysing crack-tip to study thickness effect in Elastic Plastic fracture mechanics. Here I have taken two TPB specimen geometry with a/w 0.3 and 0.5 with 10mm and 2.5 mm thickness. I have used 'Tool-Midsurface-Assign thickness and offset' option to specify thickness rather than 'using 'Plane stress/strain thickness=' option in section. I have also given thickness 'from geometry' in 'section assignment'. 4 different input files were created. I used CPS8R plane stress element. but in the result, I get the a/w effect in crack-tip stress-strain analysis and not thickness effect (result in both thicknesses are same). I have attached a pdf with the input file of a/w 0.5 with 2.5 thickness for your reference. I suspect, I have committed some error in my abaqus analysis. Kindly help me in this regards.



RE: Thickness effect in crack-tip stress analysis

Taking the geometric thickness is only possible for shell elements. You are using 2D solid elements. Enter the plane stress/stain thickness value manually in the solid section menu.

RE: Thickness effect in crack-tip stress analysis

(OP)
Thank you Mustaine3 (Mechanical) for the reply.
I found ,no significant difference in stress when tried with CPE8R with different thicknesses. After your reply, I have changed my analysis to 3D shell planar with S4R elements with different thicknesses, but still I couldn't get the decreasing crack-tip stresses at mid-plane. The section details are given below. Where I am wrong?? whether, my element selection was wrong??? Other parameters are as same before.

*Element, type=S4R

** Section: Section-2
*Shell Section, elset=Set-9, material=Material-1, poisson=0.29
5., 11

RE: Thickness effect in crack-tip stress analysis

Shells are not applicable for your situation. As mentioned, run it 2D and define the thickness manually in the solid section.

RE: Thickness effect in crack-tip stress analysis

(OP)
Thank you Mustaine3 (Mechanical) again for the reply.
As per your advice, I ran my analysis in 2D solid with CPE8R plane strain analysis on 20,10 and 5 mm thick specimens of a/W0.5. Plane stress/strain thickness in section is also assigned. I expect decreasing S22 at the crack tip (at the same CTOD/CMOD) as thickness decreases, but i got same stress at all the thicknesses. inp files for 10 mm thick is attached herewith. need help please.

RE: Thickness effect in crack-tip stress analysis

Maybe you think again what you are doing and then you try to explain, why you expect the stresses to be different.

RE: Thickness effect in crack-tip stress analysis

(OP)
I am studying the constraint effects in fracture. I took different geometries to evaluate crack-tip constraint parameter 'Q'. The constraint variables consists of geometry of the specimens, a/W ratio, loading modes and B/W ratio. B/W ratio involves thickness effects. The critical fracture toughness increases when thickness of specimens decreases and stress-strain field in crack tip start changing to plane stress state from plane strain state. As thickness of the specimen decreases, tri-axial stresses loss their dominance due to out-of-plane effects at the crack-tip. This becomes two parameter fracture mechanics. lot of literature available on the said topic. I wanted to simulate the crack-tip stress fields (particularly Syy for quantifying 'Q')using abaqus to check all these constraint variables. Though I specify the different thicknesses in Plane Strain or Plane stress analysis, abaqus gives same results neglecting B/W effects. How to get in-plane as well as out-of-plane effects using abaqus if CPE or CPS elements doesn't work for out-of plane effects? IS 3D THE LAST OPTION??

RE: Thickness effect in crack-tip stress analysis

Isn't z the thickness direction, so S33 is the variable to look at?

RE: Thickness effect in crack-tip stress analysis

I have not paid close attention to this thread so, correct me if I am wrong in my apprehension: Are you even running a fracture mechanics simulation at all? Or, are you simply running a classic hole-in-a-plate toy example?

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