04 CAE Crash simulation theory

Question 1

implicit vs. explicit time

Answer 1

implicit: response of future time n+1 depends on current and future status (hard)
explicit: response of future time n+1 depends only on current and previous state (easier)

Explicit time integration (explicit FEM) is good for dynamic problems with short duration, highly non-linear material behavior and complicated contacts

Question 2

stability criterion (explicit)

Answer 2

delata t <=2/w w=sqrd(k/m)

Question 3

to long time steps

Answer 3

original graph can’t be traced
following graph diverts into extremes

Question 4

finite Elements

Answer 4

• structure is divided into finite number of elements
  -> nodes, elements, stiffness as connection of nodes

Question 5

critical time step

Answer 5

delta t = Icsqrd(p(1-v^2)/E)
Ic = size of element
E = stiffness of material
p = density of material

Question 6

most important parts

Answer 6

Elements, Material modules, contacts

Question 7

different elements

Answer 7

beam o—o, shall elements, “bulky” solid elements

Question 8

Element count in regards to approximation

Answer 8

the more elements the better the approximation

Question 9

Smaller elements drawbacks

Answer 9

smaller elements result in smaller time steps

Question 10

material model

Answer 10

x: strain
Y: stress
Graph: first steep increase, that plateau and decrease
First elastic than plastic deformation

generally explicit FE codes can manage highly non-linear material models, which is very difficult in implicit FE codes

Question 11

Contacts

Answer 11

points can be connected or not
non-linear by nature
LS-Dyna uses simplification

Question 12

major explicit FE codes

Answer 12

Ls-Dyna
Radioss
Pamcrash
Abaques Explicit