Einfluss des Kugelstrahlens auf die Schwingfestigkeit von Ti-6Al-4V

Translated title of the contribution: Influence of shot peening on the fatigue behavior of Ti-6Al-4V

Miroslav Bosnjak

Research output: ThesisDiploma Thesis

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Abstract

In the majority of components the stress maximum occurs at the surface layer due to the type of external load (bending, torsion, plus their combination). In the presence of notches, the notch stress is raised to a multiple of the nominal stress. In order to increase the fatigue strength, it is recommended to enhance the local strength in the region of the stress maximum. For this purpose surface hardening procedures are commonly used. One of this procedures is shot-peening, which is used in many fields of the industry, for example toolmaking, plant construction, automotive- and aircraft industry, due to its simpleness and flexibility. It is a cold work process which creates strain hardening and a layer of compressive residual stresses at the surface by bombarding the surface with a shot. These induced compressive residual stresses interact with the mechanical load and reduce the crack-initiating tensile stresses in the surface layer. In this diploma thesis the influence of shot-peening on the fatigue strength of the titanium alloy Ti-6Al-4V was investigated. The extensive experimental program comprised rotating bending-, tension/compression-, torsion- and multiaxial fatigue testing. Latter is a combination of rotating bending and alternating torsion. To investigate the influence of shot peening intensity on the fatigue behavior, the shot peening was performed with three different intensities according to the military specification (MIL). The measurement of the residual compressive stresses offered no significant differences of the residual stress distributation with varying shot peening intensity. However a roughness measurement showed a linear increase of roughness with increasing shot peening intensity. The results obtained under rotating bending showed a significant increase of the finite life fatigue strength due to shot peening. The reason for this are the induced residual compressive stresses, which inhibit crack propagation. Both crack initiation under the surface and at the surface were detected in respect of the stress amplitude. With the aid of finite-element-analyses, the residual tensile stresses, which are balanced with the compressive residual stresses, were determined. It was shown that they are the main reason for crack initiation underneath the surface. At higher stress amplitudes the equivalent stress according to v.Mises exceeds on the surface the limit of compressive flow stress of Ti-6Al-4V. This leads to cyclic plastification at these regions which leads to crack initiation at the surface. Based on the results obtained under rotating bending, a new type of S/N-curve for shot peened Ti-6Al-4V was developed. Additionally the fatigue strengths of shot peened specimens with different notches were proved. Based on the performed experiments, no influence of the relative stress gradient was determined. Utilising tension/compression tests, the mean stress effect was investigated. For fully reversed loading an increase was achieved by factor of 30 in the finite life region. For higher stress ratios the positive effect of shot peening gets completly lost. Furthermore the fatigue behavior of Ti-6Al-4V and the effects of shot peening under torsional and multiaxial loading was investigated. In both cases, an increase of fatigue strength was observed due to shot peening treatment. The existing residual stresses were identified as the main influence on the fatigue strength. Therefore they have to be considered in the design of components. Especially for Ti-6Al-4V the tensile residual stresses can have a critical impact on the fatigue behavior due to the abnormally high mean stress sensitivity. In this day and age weight reduction, optimum use of materials, and shorter and shorter product development times have enormous importance. Thereby the computer-based lifetime estimation plays an essential role. It hence necessary to implement re
Translated title of the contributionInfluence of shot peening on the fatigue behavior of Ti-6Al-4V
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Eichlseder, Wilfried, Supervisor (internal)
  • Oberwinkler, Bernd, Co-Supervisor (internal)
Award date17 Dec 2010
Publication statusPublished - 2010

Bibliographical note

embargoed until 09-11-2015

Keywords

  • fatigue behavior
  • shot peening
  • Ti-6Al-4V

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