  # narasimhamurthy

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## Reputation Activity

1. narasimhamurthy liked a post in a topic by Rahul R in OPTISTRUCT
Master set cannot be referred to a Node set, but elements / contact surface
2. narasimhamurthy liked a post in a topic by Koushik Chandrashekhar in Fatigue analysis
Hi Narasimha,

If the solid is coarse, it will show good difference between shell & solid.
Since you are using shell in FATDEF, fatigue will use shell stresses only. If you feel solid stresses are accurate, you can use solid elements in FATDEF.
3.
Hi,

use HvTrans tool to convert animation files A00 to h3d file.
HWD-0050_ Translating Results - HvTrans.pdf
4.
Hi,
Yes if the model barrier is rigid, it will have zero internal energy as deformation is not possible.
Contact energy is not a physical energy of the system, it is the basically the energy spent by enabling a contact force to avoid penetration.
Contact energy = contact force*distance moved to avoid penetration.
This should be minimal and can be around 0-5 % of total energy.
Any springs also modelled in simulation, if yes how the spring energy varies?
Also is the internal energy became equal to the input energy or it is less?
5.
Hi,
Internal energy is the energy absorbed by the elements during deformation. This is also referred to as strain energy.
Hourglass energy is the energy associated with the hourglass mode of element where the element stiffness become zero and tries to come out of simulation. In such scenarios, a force is applied to that particular element to displacement and the work done by that force gives the hourglass energy. This should be not more than 10-15 % of the total energy. This is purely based on shell element formulation. Like if you go to property and edit it you will see,  IShell which defines the element formulation. More details for the same is also given in help.
6.
"Learn RADIOSS HyperMesh Interface" Videos - By Prashanth AR
7.
What is the syntax for expression builder?
8. narasimhamurthy liked a post in a topic by Rahul Ponginan in Linear buckling analysis
All,
The short answer is that the contour you see is the contour of the normalized values of a particular result of the linear buckling analysis called Eigenvector δm
δm is the associated buckling displacement shape for a particular mode.
Eigenvectors δm are the primary result in a buckling analyses, they are normalized with respect to the maximum vector component, (to put it simply the maximum vector value is taken as 1 and the rest of the values are taken equivalently) using which the mode shape is plotted.
This same mode will also have a particular eigenvalue λm this is called the buckling load factor. Use the following table to interpret the BLF for engineering purposes

we know the applied load of course as referred to in the above table,
How do we obtain these two values λm and δm?
The problem of linear buckling in finite element analysis is solved by first applying a reference level of loading, pref, to the structure. A standard linear static analysis is then carried out to obtain stresses which are needed to form the geometric stiffness matrix KF. The buckling loads are then calculated by solving an eigenvalue problem:
|K + λm KF| δm = 0
Where K is the stiffness matrix of the structure and λm  is the multiplier to the reference load. The solution of the eigenvalue problem generally yields n eigenvalues λm , where n is the number of degrees of freedom (in practice, only a subset of eigenvalues is usually calculated). The vector δm is the eigenvector corresponding to the eigenvalue.
Thanks and regards
Rahul Ponginan
9. narasimhamurthy liked a post in a topic by Rahul Ponginan in Convergence NLSTAT
HI Alessio,

UPW are the convergence criteria's for a nonlinear problem.

U stands for displacement based convergence criterion, P stands for load based convergence criterion and W stands for work /strain energy based convergence criterion.

Tight convergence is recommended for accurate results and hence UPW is default..one could have loose convergence by choosing combination of criteria's but may end up compromising with result accuracy

10.
The equation of motion for a static analysis is as below:
[K] {X} = {F} ------------------------------------------ (1)
[K] --> Global Stiffness Matrix
{X} --> Unknown Displacement
{F} ---> External Force Applied.
For the body to be in static equilibrium, the net force acting at every node must be zero. Therefore, the basic statement of static equilibrium is that the internal forces, I, and the external forces, F, must balance each other:
[K] {X} is nothing but internal force 'I'
Equation (1) now becomes,
==> I = F or I - F = 0 -----------------------------------(2)
In Dynamic Analysis, the imbalance between the external and internal forces results in an acceleration:
F - I = M a.
F --> External Forces
I ---> Internal Force
M*a --> Inertial Forces (mass times acceleration)
In linear static analysis the stiffness matrix is constant and shall not change/update throughout the analysis. There are many check need to be performed once you have linear static results for well conditioned problems.
The equation (1) is decomposed one time to find the unknown displacement.
[K] {X} ={F}
After decomposition, a singularity may lead to an incorrect solution. In static analysis to obtain {X} (displacements). Using these displacements, One can calculate a “residual” loading vector as follows:
[K] {X} -{F} =δ F
This residual vector should theoretically be null (equation 2) but may not be null due to numeric roundoff.
In Nonlinear static analysis, The stiffness matrix changes in each and every iteration since the stiffness matrix is dependent on the external load. The external load in Nonlinear static analysis is applied in increments and time here has no physical meaning.
Time is just a convenient way to apply full load in nonlinear static analysis. In Optistruct the incremental load is controlled by 'NINC' field in the NLPARM card for NLSTAT load steps, this is a fixed load increment method.
If you add the PARAM,EXPERTNL,YES to the deck, the time increment method becomes automatic in which case, the increment (load) is increased or cut back based on the convergence rate.
NLGEOM loadstep has automatic time step by default. In NLGEOM loadstep the RAMP load curve can be defined using TABLED1 card and then refer this in NLOAD1 card.
In nonlinear static analysis, OptiStruct uses the Newton-Raphson method to obtain solutions for nonlinear problems to maintain the residuals close to zero (equation 2)
In a nonlinear analysis the solution usually cannot be calculated by solving a single system of equations, as would be done in a linear problem. Instead, the solution is found by applying the specified loads gradually and incrementally working toward the final solution. Therefore, OptiStruct breaks the simulation into a number of load increments (NINC) and finds the approximate equilibrium configuration at the end of each load increment.
It is important that you clearly understand the difference between an analysis step (NLSTAT / NLGEOM), a load increment (NINC of NLPARM card), and an iteration (MAXITER of NLPARM card)
The load history for a simulation consists of one or more steps. Within a step you will have many no of increments (NINC), within increment there can be many no. of iteration (MAXITER). OptiStruct checks the equilibrium equation ( equation 2) for each and every iteration. If the solution from an iteration is not converged, OptiStruct performs another iteration to try to bring the internal and external forces into balance.
An increment is part of a step. An iteration is an attempt at finding an equilibrium solution in an increment when solving with an implicit method. If the model is not in equilibrium at the end of the iteration, OptiStruct tries another iteration. With every iteration the solution OptiStruct obtains should be closer to equilibrium; sometimes OptiStruct may need many iterations to obtain an equilibrium solution. When an equilibrium solution has been obtained, the increment is complete.
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