PSI - Issue 38

Martin Killmann et al. / Procedia Structural Integrity 38 (2022) 212–219 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

214

3

The stress state of the model process is analysed in Fig. 2 for both geometries. First, the unloaded die is pressed into the reinforcement ring, which exhibits a uniformly distributed pressure on the die. Due to the non-circular symmetrical cross-sections, the prestress is non-uniformly distributed. The areas at the minor axis of the ellipse and between the functional elements are displaced further inward than the other regions. This leads to a higher compressive prestress at the major axis and the functional elements. During the process, this stress state is reversed. The inner pressure exerted by the workpiece leads to a higher outward radial displacement. At maximum load at the process end, the die regions around the major axis and the functional elements are being pulled apart in tangential direction. This leads to high tensile tangential stresses, which are critical for fatigue failure. A local prestressing system should therefore focus on increasing the compressive stresses in these critical areas.

Without load

Prestress

During process

Process end

σ t

σ t

σ t

σ t

y

a)

x

r

r

r

r

σ t

σ t

σ t

y

b)

x

r

r

r

Stress development

Reinforcement Inner pressure Geometry scaled x50

Tangential stress σ t

Radial displacement r

-750

750 MPa

-35

µm

35

Fig. 2: Development of the tangential stress and radial displacement distribution for a) elliptical geometry and b) functional elements

3. Use of gaps for local prestressing An approach to create a local prestress is to locally adjust the interference fit between die and reinforcement. The pressure exhibited on the die from the outside should be minimized in areas with high tangential stresses. That way, a bending effect causes higher local prestresses in those areas. To minimize the pressure, the contact between reinforcement and die is prevented by inserting a gap between the parts. The gap should be applied at the die, so that the two parts can be joined without having to consider the exact placement of the reinforcement. The fundamental mechanism of this approach is illustrated in Fig. 3. By locally removing the contact pressure of the reinforcement, a bending stress is induced, which increases the prestress at the inner die surface. However, bending also creates