Can both neck propagation & local necking occur in the same tensile test?
Do all materials show all three phenomena (diffused/local necking and neck) propagation? If not (which I an pretty sure), then what determines how the the specimen (or a material) fails. And if plastic flow is a path dependent process then how come failure strain can be a considered constant for material?
I will also be interested to know how I can simulate a local necking phenomena in ABAQUS?
The phenomena, which you are talking about, depends on hardening function of tested material as well as the rate of tension reflecting viscous properties. For example, neck propagation observed if polymer samples are subjested to tension with small rate but local necking may take place under high-speed loading of the same material.
Serge
Sir, I totally agree with your comments. There is no denying that all three phenomena will be highly dependent on hardening function of the material.
I can understand when triaxiality dominates that would mark onset of diffused necking. But I am interested to know what cause the material to go under local necking or to neck propagation (stress state is uniaxial for neck propagation).
What one needs to do (material definition etc.) to simulate these modes of material deformation numerically (in FEA softwares). I tried Porous Metal Plasticity (Treevgaard) for modelling damage but I couldn't get desired results.
Please find attached an Abaqus input file for a simple example case in which diffuse necking is followed by localized necking. The example case is a tensile test for a stablized ferritic stainless steel sheet with a thickness of 1 mm.
This kind of behaviour is typical of ductile sheet metals with strong work hardening capacity such as ferritic and austenitic stainless steels. There is a well developed theory for predicting these phenomena, see for example
http://materialstechnology.asmedigitalcollection.asme.org/article.aspx?articleid=1422485
You do not need the porous metal plasticity model for this, a simple Von Mises yield condition with isotropic hardening will do the job provided that the work hardening capacity is good enough. In this case (Von Mises & Isotropic hardening) the limit strains for the onset of diffuse and localized necking in uniaxial tension can be easily predicted. I can send You later a reference and an excel worksheet for this.
From possible explanation for polymer, is that you have a diffuse necking because the stress level to align the macromolecule is lower than the stress to elongate them when stretched. So the process start with a local necking which will be diffused if the stress is lower than the tangent modulus. When the diffuse necking is complete then you will have again a local necking until failure. For the failure , it depends on the stress triaxiality ratio (geometry of the specimen) and strain rate. ( see paper in engineering fracture mechanics, Identification of a new failure criterion for toughened epoxy adhesive, D. Morin ⇑, G. Haugou, B. Bennani, F. Lauro)
When a material would show neck propagation where the stress state at neck will again become uniaxial?
@Timo @Franck thanks for the replies.
@ Timo As the its is an scanned pdf I couldn't highlight the text. You can read in section 2 that the stress state goes back to uniaxial in neck propagation.
If the length of neck spreading enough, the stress state in neck region become anyaxial according to geometrical conditions for this state. But in some case we must take into account the possibility of strain-induced anisotropy of material in neck.
@Abishek Thanks for sharing the article. Having seen the article, I acknowledge that the stress state will become uniaxial again if the material hardens like described in the Figure 1 in the article.
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