Discussion Forum

Posted: 1 decade ago May 17, 2013, 2:16 a.m. EDT

Hi, The quick answer: Creep is time dependent, while plasticity is not. The longer version: The terminology is not always clear, especially when we involve the term 'viscoplasticity'. Both events result in the same type of inelastic strains. In COMSOL Multiphysics we have chosen to use 'Plasticity' for material models which you can use equally well in a stationary and a time dependent analysis, i.e. no time derivatives are involved in the constitutive relation. In the creep models, the constitutive models is such that the time derivative of inelastic strain is a function of stress (and possibly temperature and time). Creep phenomena become important at higher temperatures (say > half the melting point) 'Viscoplasticity' can mean almost anything, from pure creep models to rate dependent plasticity (where the yield stress is strain rate dependent). The material model ('Anand') which in COMSOL Multiphysics is classified as viscoplastic is actually a creep model with a hardening law. But since it usually in the literature is called 'viscoplastic, we have adopted that terminology. Regards, Henrik

Posted: 1 decade ago May 18, 2013, 11:25 p.m. EDT

Thanks Henrik for the informative answer. I want to also point out another related terminology inconsistency in regards to viscoelasticity. It is frequently assumed that viscoelasticity involves no permanent deformation. That is true for many viscoelastic models such as the most commonly used generalized Maxwell model (used in COMSOL and other FEA codes). However, other viscoelastic models such as the standard Maxwell model (http://en.wikipedia.org/wiki/Maxwell_material) involve permanent deformation. Nagi Elabbasi Veryst Engineering

Posted: 1 decade ago May 27, 2013, 6:11 p.m. EDT

Hi Henrik, I'm a little bit confused, if I want to calculate the plastic deformation of my Time dependent Contact model, should I use creep rather than plasticity? Thanks a lot.

Posted: 1 decade ago May 28, 2013, 1:55 a.m. EDT

[QUOTE] Hi Henrik, I'm a little bit confused, if I want to calculate the plastic deformation of my Time dependent Contact model, should I use creep rather than plasticity? Thanks a lot. [/QUOTE] That depends on the physics of your problem. Plasticity works well in also in time domain. The difference between creep and plasticity can be seen if you increase your load fast to constant value. An elastoplastic material deforms immediately, and then the strains are constant. A material which exhibits creep will deform continuously under a constant load. By the way: If your material is elastoplastic, then maybe the whole problem is not truly time dependent (in the sense that inertial effects are important)? If so, it is better to use a parametric stationary solver instead of a time dependent solver. Regards, Henrik

Posted: 1 decade ago Jul 26, 2013, 1:36 a.m. EDT

[QUOTE] [QUOTE] Hi Henrik, I'm a little bit confused, if I want to calculate the plastic deformation of my Time dependent Contact model, should I use creep rather than plasticity? Thanks a lot. [/QUOTE] That depends on the physics of your problem. Plasticity works well in also in time domain. The difference between creep and plasticity can be seen if you increase your load fast to constant value. An elastoplastic material deforms immediately, and then the strains are constant. A material which exhibits creep will deform continuously under a constant load. By the way: If your material is elastoplastic, then maybe the whole problem is not truly time dependent (in the sense that inertial effects are important)? If so, it is better to use a parametric stationary solver instead of a time dependent solver. Regards, Henrik [/QUOTE] Hi Henrik, Thanks a lot for your reply, and I learnt a lot from your reply. Recently I am doing a project on the dynamic behavior of soil under cyclic loading, like the example,"Block Verification Model", in the model library. Let me describe my problem with this example: I am trying to replace the prescribed displacement in the z direction with a dynamic cyclic loading, such as F(t)=10kPa*sin(10*pi*t), and then add a time dependent "study" in this model, there appears an error, showing Failed to compute elastic strain variables. - Feature: Time dependent solver 1(sol2/t1) - Error: Failed to compute elastic strain variables. When I did modeling of dynamic triaxial test, the same error comes out, so how can I solve this problem? If I want to stationary instead of time dependent modeling, how can I set parametric sweep to achieve the axial cyclic loading, like wavefunction F(t)=10kPa*sin(10*pi*t). Thanks a lot! Best

Posted: 9 years ago Sep 14, 2015, 6:43 a.m. EDT

Hi, I am using a Non-linear elastic material (Sensor: made of Polyimide) and Hyperelastic material gasket (NBR-Rubber), the sensor is placed inside the gasket and stress is applied from the top plate (Prescribed displacement: -1mm) to compress the gasket to check the tensile strength of the sensor (inside) and to record the material elongation after applying stress at the rubber gasket, keeping bottom plate stationary. (Simulation is attached) My request is to know how to calculate the creep/ shrinkage behavior over time to observe the performance of gasket (O-ring) and to note the change of strain properties. My geometry parameters are as follows. Radius (Gasket/O-ring) = 2.5 mm Width (Sensor) = 0.5 mm Height (Sensor) = 0.005mm Gasket Material= NBR Rubber Sensor Material=Polyimide Two steel plates (One at top and one at bottom) In Solid Mechanics: Gasket= Hyper-Elastic material Sensor= Non-Linear Elastic Material Bottom steel plate= Fixed Top steel plate= Applying prescribed displacement 1mm downwards. Help is requested. Many Thanks, Bilal

Attachments:

• My Simulation_NEW.mph