Linear, non linear, Buckling, Thermal, Dynamic & Fatigue analysis
Friday, November 29, 2013
Wednesday, November 13, 2013
What is the difference between RBE2 and RBE 3 Elements?
RBE2 and RBE3 are multi point constraint elements, but Rigid and RBE2 elements add stiffness to original structure while RBE3 does not.
RBE2 elements also distribute the force and moment equally among all connected nodes, irrespective of position of force or moment. RBE3 element is constraint equation to distribute force and moment as per the distance (least square weighted function).
Tuesday, November 12, 2013
What are the DOFs of the following element types
Beam - 6
Solid - 3
Shell - 6
Spring - 1 per direction
Link - 3
Solid - 3
Shell - 6
Spring - 1 per direction
Link - 3
What is the basis of deciding when the structure has failed? in case of Automobile FEA
Usually for automobile metal structures, we assume a failure to be identified, when we see plastic strain crossing around 10%.
What ELFORM do you use for different elements and Why? in the context of LS-Dyna
In terms of speed and robustness I would rank shell formulations as follows:
1. type 2
2. type 2 with BWC warping stiffness and full projection (see BWC and PROJ in *CONTROL_SHELL)
3. type 10
4. type 16 (Type 16 shells require approximately 2.5 times more CPU than type 2 shells.)
5. type 7
6. type 6 [1]
The last 3 formulations listed above are fully-integrated (4 in-plane integration points) and thus do not suffer from hourglass modes [2]. Generally speaking, the underintegrated elements tend to be a little too soft. By using stiffness-based hourglass control (HG type 4) and a reduced hourglass coefficient (say, .03 to .05), the behavior is stiffened slightly and so this hourglass combination is generally recommended for most applications of the underintegrated shells. For very high velocity/rate problems, viscosity-based hourglass control is recommended.
Accuracy:
From an accuracy standpoint, shell type 16 is preferred over the underintegrated formulations provided the following are true:
- initial element shape is reasonable
- element does not distort unreasonably during the simulation
- Used together with hourglass control type 8, the type 16 shell will give the correct solution for warped geometries.
1. type 2
2. type 2 with BWC warping stiffness and full projection (see BWC and PROJ in *CONTROL_SHELL)
3. type 10
4. type 16 (Type 16 shells require approximately 2.5 times more CPU than type 2 shells.)
5. type 7
6. type 6 [1]
The last 3 formulations listed above are fully-integrated (4 in-plane integration points) and thus do not suffer from hourglass modes [2]. Generally speaking, the underintegrated elements tend to be a little too soft. By using stiffness-based hourglass control (HG type 4) and a reduced hourglass coefficient (say, .03 to .05), the behavior is stiffened slightly and so this hourglass combination is generally recommended for most applications of the underintegrated shells. For very high velocity/rate problems, viscosity-based hourglass control is recommended.
Accuracy:
From an accuracy standpoint, shell type 16 is preferred over the underintegrated formulations provided the following are true:
- initial element shape is reasonable
- element does not distort unreasonably during the simulation
- Used together with hourglass control type 8, the type 16 shell will give the correct solution for warped geometries.
What is Mass-Scaling?
Mass-scaling refers to a technique whereby nonphysical mass is added to a structure in order to achieve a larger explicit timestep.
For quasi-static solutions, optimum efficiency is obtained by arranging for all elements in the model to require the same timestep. This can be achieved by adding or subtracting mass from each element. To perform an analysis of this type, enter a positive value of DT2MS on *CONTROL_TIMESTEP. The mass of every element in the model will then be adjusted such that the timestep is SCFT*DT2MS (SCFT is normally 0.9). Note that the analyst must still select a suitably long event time for the simulation to ensure that the behaviour is sufficiently quasistatic.
Note also that if DT2MS is set to a negative value, masses are changed only for those elements whose timestep would otherwise be less than the desired timestep. With DT2MS positive, the masses of all elements are changed.
========
Mass-scaling is a term that is used for the process of scaling the element's mass in explicit simulations to adjust its timestep. The primary motivation is to change (usually increase) the global compute timestep which is limited by the Courant's stability criteria. LS-DYNA allows two different types of mass-scaling using the DT2MS parameter from *CONTROL_TIMESTEP with the default set to no mass-scaling. When DT2MS is less than zero, LS-DYNA adds mass of each element whose timestep is below abs(DT2MS) such that the element's updated DT is equal to abs(DT2MS). When DT2MS is greater than zero, LS-DYNA adds mass to elements whose DT is below abs(DT2MS) and "removes" mass from elements whose DT is greater than zero. DT2MS>0 is seldom used while DTM2<0 is frequently used for overcoming the smallest computed timestep. Care must be taken when using DT2MS<0 to ensure that the added mass does not have an adverse effect on the simulation accuracy. It is common practice to limit the percentage of added mass to less than 5% (at part level) in dynamic simulations. Optionally, users can set ENDMASS in *CONTROL_TERMINATION to terminate a simulation based on percentage of added mass based on the total mass of the model. When ENDMAS is greater than zero, LS-DYNA terminates when the percentage of added-mass reaches ENDMAS and a report of up to 20 nodes (sorted in the descending order of its added mass due to mass-scaling) is written to both standard output and D3HSP file. It must be noted that the percentage of added-mass is based on total mass of the model which included rigid body, rigid walls, etc... and could be misleading if looked at the global level. LS-DYNA outputs the percentage of added mass at component level which is a better indicator of amount of added mass due to mass-scaling. The concept of mass-scaling for both options of DT2MS is graphically illustrated below.
Link to the 2nd definition http://blog2.d3view.com/overview-of-mass-scaling/
For quasi-static solutions, optimum efficiency is obtained by arranging for all elements in the model to require the same timestep. This can be achieved by adding or subtracting mass from each element. To perform an analysis of this type, enter a positive value of DT2MS on *CONTROL_TIMESTEP. The mass of every element in the model will then be adjusted such that the timestep is SCFT*DT2MS (SCFT is normally 0.9). Note that the analyst must still select a suitably long event time for the simulation to ensure that the behaviour is sufficiently quasistatic.
Note also that if DT2MS is set to a negative value, masses are changed only for those elements whose timestep would otherwise be less than the desired timestep. With DT2MS positive, the masses of all elements are changed.
========
Mass-scaling is a term that is used for the process of scaling the element's mass in explicit simulations to adjust its timestep. The primary motivation is to change (usually increase) the global compute timestep which is limited by the Courant's stability criteria. LS-DYNA allows two different types of mass-scaling using the DT2MS parameter from *CONTROL_TIMESTEP with the default set to no mass-scaling. When DT2MS is less than zero, LS-DYNA adds mass of each element whose timestep is below abs(DT2MS) such that the element's updated DT is equal to abs(DT2MS). When DT2MS is greater than zero, LS-DYNA adds mass to elements whose DT is below abs(DT2MS) and "removes" mass from elements whose DT is greater than zero. DT2MS>0 is seldom used while DTM2<0 is frequently used for overcoming the smallest computed timestep. Care must be taken when using DT2MS<0 to ensure that the added mass does not have an adverse effect on the simulation accuracy. It is common practice to limit the percentage of added mass to less than 5% (at part level) in dynamic simulations. Optionally, users can set ENDMASS in *CONTROL_TERMINATION to terminate a simulation based on percentage of added mass based on the total mass of the model. When ENDMAS is greater than zero, LS-DYNA terminates when the percentage of added-mass reaches ENDMAS and a report of up to 20 nodes (sorted in the descending order of its added mass due to mass-scaling) is written to both standard output and D3HSP file. It must be noted that the percentage of added-mass is based on total mass of the model which included rigid body, rigid walls, etc... and could be misleading if looked at the global level. LS-DYNA outputs the percentage of added mass at component level which is a better indicator of amount of added mass due to mass-scaling. The concept of mass-scaling for both options of DT2MS is graphically illustrated below.
Link to the 2nd definition http://blog2.d3view.com/overview-of-mass-scaling/
Friday, November 8, 2013
UNIT 1 MATERIALS STRAIN ENERGY
http://www.youtube.com/v/CCENBw9yU8g?version=3&autohide=1&showinfo=1&autoplay=1&autohide=1&attribution_tag=kmbMgObs53yDg2gNguPKLg&feature=share
Thursday, November 7, 2013
What is Strain Energy? -IQ4
The external work done on an elastic member in causing it to distort from its unstressed state gets transformed into strain energy, which is a form of (PE) potential energy.
The strain energy in the form of elastic deformation is mostly recoverable in the form of mechanical work.
The strain energy in the form of elastic deformation is mostly recoverable in the form of mechanical work.
What is Hourglass energy? IQ3
1st order reduced-integration elements (which happen to be the very type of elements generally used in explicit solvers) suffer from hourglassing. These elements only have one integration point; because of this the elements can shear without introducing any energies. FEA codes which rely on 1st order reduced integration elements counter this by introducing hourglass energy. In situations where these elements would otherwise shear, this augmented energy keeps this from happening. This is reflected in your reported hourglass energy values. High hourglassing energy is often a sign that mesh issues may need to be addressed.
What are the kind of materials you have used - IQ 1 - LS-Dyna
What are the kind of materials you have used?
MATL1 -This is an isotropic elastic material, also allows modelling of fluids.
MAT 20 - Parts made from this material areconsidered to belong to a rigid body.
MATL 24 - Elasto plastic mat with arbitrary Stress Vs Strain curve and arbritary strain rate dependancy. Also failure based on plastic strain or minimum time step can be dfeined.
MATL57 - for highly compressible low density foam.
MATL 83 - Rate effects can be modelled in low and med density foams. Hysteric unloading behaviour in this model is a function of rate sensitivity with the most rate sensitivity foams providing the largest hysterisis and vice versa.
IQ Stands for interview questions
I have mentioned it in terms of Ls-Dyna, if you can explain the same in terms of Abacus/Ansys, please do post in comments
MATL1 -This is an isotropic elastic material, also allows modelling of fluids.
MAT 20 - Parts made from this material areconsidered to belong to a rigid body.
MATL 24 - Elasto plastic mat with arbitrary Stress Vs Strain curve and arbritary strain rate dependancy. Also failure based on plastic strain or minimum time step can be dfeined.
MATL57 - for highly compressible low density foam.
MATL 83 - Rate effects can be modelled in low and med density foams. Hysteric unloading behaviour in this model is a function of rate sensitivity with the most rate sensitivity foams providing the largest hysterisis and vice versa.
IQ Stands for interview questions
I have mentioned it in terms of Ls-Dyna, if you can explain the same in terms of Abacus/Ansys, please do post in comments
How to start preparing for the interview - II
Next about the performance requirements.
Well here I'm speaking more in terms of LS dyna as that is my tool of experience.
Global energy, Understand how the energy curves vary for each load case. For example how would the energy curve look for a quasi static load like SBA and how it would vary for a dynamic case like pendulum crash. Why does it differ?
Learn to draw a schematic diagram of the vehicle in terms of the load path.
For example from the bumper beam, towards the crush cans and then the rails etc, or for the same load case (full frontal) if we look at it from a slighter higher position, the load path is from the radiator, then the shotgun, hinge pillars etc.
If the frontal impact starts of with a force of 200 kN how does it get transferred towards the end of the vehicle.
Well here I'm speaking more in terms of LS dyna as that is my tool of experience.
Global energy, Understand how the energy curves vary for each load case. For example how would the energy curve look for a quasi static load like SBA and how it would vary for a dynamic case like pendulum crash. Why does it differ?
Learn to draw a schematic diagram of the vehicle in terms of the load path.
For example from the bumper beam, towards the crush cans and then the rails etc, or for the same load case (full frontal) if we look at it from a slighter higher position, the load path is from the radiator, then the shotgun, hinge pillars etc.
If the frontal impact starts of with a force of 200 kN how does it get transferred towards the end of the vehicle.
How to start preparing for the interview - I
I will answer it more in terms of an automotive engineer, you may adapt it as per your need.
Identify the load cases in which you have expertise. By load caes i mean as an example - SBS (seat belt anchorage test or Roof crush, SDI (side door intrusion or Full frontal etc.
Next identify the Regulation applicability.
Example full frontal load cases are used for M1 configuration of vehicles or maybe Roll over are used for vehicles with passenger capacity less than 21 for instance.
Maybe you could dwell on which market (European, American, Canadia, Gulf, Asisan) an SBA, SDI is important.
Next study the test setup.
By test setup, I mean the loads, the boundary conditions, be ready to draw a small picture of the test set up as this gives the interviewer more confidence in your capabilities.
Next study the performance requirements.
Identify the load cases in which you have expertise. By load caes i mean as an example - SBS (seat belt anchorage test or Roof crush, SDI (side door intrusion or Full frontal etc.
Next identify the Regulation applicability.
Example full frontal load cases are used for M1 configuration of vehicles or maybe Roll over are used for vehicles with passenger capacity less than 21 for instance.
Maybe you could dwell on which market (European, American, Canadia, Gulf, Asisan) an SBA, SDI is important.
Next study the test setup.
By test setup, I mean the loads, the boundary conditions, be ready to draw a small picture of the test set up as this gives the interviewer more confidence in your capabilities.
Next study the performance requirements.
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