Page 55 - Biaxial Multiaxial Fatigue and Fracture

40 G. SAVAIDIS ET AL.

material data, but this is not the subject of the current paper. On the other hand the result
reveals some aspects of the problems when dealing with life prediction in an industrial
environment: As the tapered tube is known from experience to survive quite a number of
truck lives further investigations on the component have minor priority. Nevertheless, gaining
experience with life prediction techniques is the prerequisite to apply them in design process.

Failure-critical location calculated
with hot spot approach and local
approach

Fig. 23. Experimental tested new geometry, crack initiation at tapering of tube

CONCLUSIONS
The mechanical behaviour and fatigue life of a thin-walled tube joined to a forged component
by fillet welding has been investigated. The component is loaded by nonproportional random
sequences of bending and torsion as measured during operation. The stresses in the welded
structure have been calculated using finite element analysis. The fatigue life has been
determined theoretically, by means of the hot spot and local stresses in conjunction with the
critical plane approach, and experimentally. Though the experimental data base is quite
narrow, basic trends can be derived from the present investigation concerning the possible
field of application and capability of the theoretical approaches calculating fatigue life of
vehicle components.
1. A coarse finite element model of the component has been created in accordance with the
IIW guideline for application of the hot spot stress approach. Comparison of numerically
and experimentally determined end-results (fatigue lifetimes) shows that the coarse model
is suitable to determine lifetimes with the hot spot stress approach, if a reliable hot spot
stress-life curve at constant amplitude loading is existing for the detail investigated.
Because of the coarse mesh, the geometry of the weld is not modelled in detail. For
instance, in this investigation only one failure-critical location has been detected.
2. If experimental results are missing, appropriate S-N curves from publications, e.g. the IIW
guideline or Eurocode can be used. In cases of multiaxial loading causing normal and
shear stresses, attention must be paid to the slope of the S-N curve used, since various
suggestions are reported in the literature. The major advantage of the hot spot stress
approach is a relatively low expense to model and calculate.