Considering the individual failure modes two situations are important to discuss, namely the
situation where failures are brittle and the situation where failures are ductile. The two
situations are illustrated in Figure 9.5.
Figure 9.5: Illustration of the brittle and ductile mechanical behaviour of failure modes.
In case of brittle failure modes there is no capacity left in the considered component of the
structural system. Such failures are relevant when e.g. a steel structural system is considered
in which a welded detail may fail due to fatigue crack growth. After fatigue failure the
corresponding cross-section of the structural system has lost its load carrying capacity and
does not contribute in the redistribution of the loads in the system.
In case of ductile failure modes there is still load carrying capacity in the considered
component of the structural system and the component is still active in the redistribution of
the loads in the system after failure. The assumption of ductile failure modes is relevant in all
cases where the development of plasticity is normally permitted in the design verifications for
the ultimate strength.
The mechanical modelling of structural systems and components is also important due to the
effect that the load carrying capacity of a structural system might depend on the so-called load
path. This problem is often referred to as the load path dependency problem. As long as an
ideal elastic perfectly plastic material model can be assumed there is no load-path
dependency. However this modelling is not applicable in general and the concept of
proportional loading is often applied. However, when a certain load path has been assumed as
a basis for the probabilistic evaluations it should be kept in mind that the evaluated
probabilities are to be considered as conditional probabilities, i.e. conditional on the assumed
load path.
In block diagram representations of systems brittle and ductile failure modes are normally
identified by the symbolism illustrated in Figure 9.6.