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Research Papers

Residual Stress Distribution and the Concept of Total Fatigue Stress in Laser and Mechanically Formed Commercially Pure Grade 2 Titanium Alloy Plates

[+] Author and Article Information
Kadephi V. Mjali

Department of Mechanical Engineering,
Cape Peninsula University of Technology,
Symphony Way, Bellville,
Cape Town 7535, Western Cape, South Africa
e-mail: vuyo.mjali@gmail.com

Annelize Els-Botes

Principal Integration Specialist
Council for Scientific and Industrial Research
(CSIR),
1 Meiring Naude,
Brummeria, Pretoria 0001, South Africa

Peter M. Mashinini

Department of Mechanical and
Industrial Engineering Technology,
University of Johannesburg,
Johannesburg 2001, Gauteng, South Africa

1Corresponding author.

Manuscript received July 19, 2017; final manuscript received July 21, 2017; published online March 13, 2018. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 140(6), 061005 (Mar 13, 2018) (10 pages) Paper No: MANU-17-1456; doi: 10.1115/1.4037438 History: Received July 19, 2017; Revised July 21, 2017

This paper discusses the investigation of residual stresses developed as a result of mechanical and laser forming processes in commercially pure grade 2 titanium alloy plates as well as the concept of total fatigue stress (TFS). The intention of the study was to bend the plates using the respective processes to a final radius of 120 mm using both processes. The hole drilling method was used to measure residual strains in all the plates. High stress gradients were witnessed in the current research and possible cases analyzed and investigated. The effects of processing speeds and powers used also played a significant role in the residual stress distribution in all the formed plates. A change in laser power resulted in changes to residual stress distribution in the plates evaluated. This study also dwells into how the loads that are not normally incorporated in fatigue testing influence fatigue life of commercially pure grade 2 titanium alloy plates. Also, the parent material was used to benchmark the performance of the two forming processes in terms of stresses developed. Residual stresses developed from the two forming processes and those obtained from the parent material were used. The residual stress values were then added to the mean stress and the alternating stress from the fatigue machine to develop the concept of TFS. This exercise indicated the effect of these stresses on the fatigue life of the parent material, laser and mechanically formed plate samples. A strong link between these stresses was obtained and formulae explaining the relationship were formulated. A comparison between theory and practical application shown by test results is found to be satisfactory in explaining concerns that may arise. The laser forming process is more influential in the development of residual stress, compared to the mechanical forming process. With each parameter change in laser forming, there is a change in residual stress arrangement. Under the influence of laser forming, the stress is more tensile in nature making the laser formed plate specimens more susceptible to early fatigue failure. The laser and mechanical forming processes involve bending of the plate samples and most of these samples experienced a two-dimensional defect, which is a dislocation. The dislocation is the defect responsible for the phenomenon of slip by which most metals deform plastically. Also, the high temperatures experienced in laser forming were one of the major driving factors in bending.

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Figures

Grahic Jump Location
Fig. 4

Mechanical forming process

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Fig. 3

The scanning procedure

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Fig. 2

Laser formed commercially pure grade 2 titanium alloy with the open mold arrangement

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Fig. 1

Prime pocket monitor

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Fig. 5

Residual stress samples

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Fig. 6

Relieved strain measured on the parent material

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Fig. 8

Stress measured on the mechanically formed plate

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Fig. 9

Typical stress measured on the laser formed plate

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Fig. 10

Relieved strain measured on the mechanically formed plate

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Fig. 7

Stress measured on the parent material

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Fig. 11

Typical relieved strain measured on the laser formed plate

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