Spring failure in properties TYPE1, TYPE2, TYPE4, TYPE8, TYPE13, and TYPE25 may be
considered in two ways.
Uni-directional failure, or
Multi-directional failure
This is controlled by the option Ifail. If the Ifail flag is not set/present in the property, the default uni-directional failure is
considered for the spring. For example, in TYPE4 there is no option Ifail, so uni-directional failure is used.
The failure model may consider displacement failure, force failure or internal energy
failure. This is controlled by the option
Ifail2. Similar to Ifail, if the Ifail2 option is not
set/present in the property, the displacement (or rotation) failure model is
used.
If the criteria is uni-directional, the spring will fail as soon
as the criteria is satisfied for one degree of freedom:
or with and being the failure limits in
direction =1,2,3.
or with and being the failure limits in
direction =4,5,6.
Where, is any degree of freedom. Its property
type dependent.
For property TYPE4, there is only =1, for translational X.
For
property TYPE8, there are =1,2,3,4,5,6 for translational X, Y, Z
and rotational X,Y,Z.
For property TYPE13, there are =1,2,3,4,5,6, but in this case, for
tension/compression X, shear XY, shear XZ, torsion, bending Y, bending
Z.
Examples of failure behaviors of uni-directional failure are:
If in a tension only test, then there is
spring failure, and the force goes to zero once the elongation reaches
0.04m.
The same is true for rotation, if , the spring fails and has zero force at
0.035rad.Figure 1.
If a spring is subject to two load cases, for example,
tension and torsion and Ifail = 0 (uni-directional failure) is in use, then spring failure occurs
if either one of the failure criteria is reached.Figure 2.
Here the rotation criteria is reached first (at
Time=0.58s), then the force and moment fall to zero at the same time.
Figure 3.
Multi-directional (Ifail = 1)
If the criteria is multi-directional, all degrees of freedom are
coupled and failure occurs when:
Where, and are failure criteria. Refer to Failure Criteria for more details.
For property TYPE8, and (failure criteria shown as blue curve in
Figure 4).
For properties
TYPE13 and TYPE25, arbitrary may be input with (default is ). Figure 4 shows failure criteria with
different .Figure 4.
In a test case of two load cases, tension + torsion and
with Ifail = 1. Figure 5.
Comparing the failure value against the limit set in just
one direction, it usually is smaller than the defined limit. In this
example case, the tension limit is set at 0.04m and torsion limit set at
0.035rad. The spring fails at an elongation 0.0236 < 0.04 and
rotation 0.02826 < 0.035. This is because the failure combination of
tension and torsion reaches the failure circle (Figure 6) and; therefore, the
spring failed (force and moment fall to zero).Figure 6.
Failure Model
Option Ifail2 is available in
properties TYPE8, TYPE13, and TYPE25.
This
failure criteria will allow model velocity dependent failure limits,
they are available with displacement, force and internal energy.
Therefore, translational and rotational failure are modified to take into
account velocity, as:
and
The parameter is a scale of exponent function, and
parameter effects the failure as in Figure 7.Figure 7.
The above formulas’ are valid for displacement/rotation
criteria and are also force/moment and energy criteria.
Force (or Moment) Criteria
(Ifail2 = 2) and
Internal Energy Criteria
(Ifail2 =
3)
Translational and rotational failure are:
and
The above formulas’ are valid for
displacement/rotation, force/moment and energy.
Here () are not displacement (rotational angle)
criterion, but maximum or minimum force(moment) for
Ifail2 = 2 and
internal energy for Ifail2
= 3.
The influence of velocity is also taken
into account and the relative velocity coefficient is relative to force/moment
(Ifail2 = 2) or to
internal energy (Ifail2
= 3).