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Wear mark evolution and numerical study of impact stresses in stainless steel flapper valves
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2016 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Compressors that are used in refrigerators and air conditioners usually have flapper vales made of martensitic stainless steel to control the flow of the refrigerant in the system. During service the flapper valves are affected by both bending and impact fatigue in the very high cycle fatigue (VHCF) range with billions of cycles until failure. Due to the VHCF, it is time consuming and expensive to test the performance of the flapper valves. One approach to improve the valve testing could be to combine traditional sample testing with the finite element method (FEM).

In this paper, FEM was used to calculate the velocity and stress between a flapper valve and the seat during impact. Three different valve tongue shapes were investigated: a circular and two elliptically shaped tongues with a width to length ratio of 3:2 and 2:1. Furthermore, two different load cases were used to make the valve move: a backpressure case that was adapted from a compressor manufacturer and a springback case that was adapted from a flapper valve testing platform.

A study of the wear mark evolution was also made on the surface of flapper valves that impacts with the seat. The valves had been in use for a different amount of cycles, were supplied by a compressor manufacturer and were made of Sandvik Hiflex steel. Stereo microscopy, scanning electron microscopy (SEM) and surface measurements were used at three different areas on the valves.

It was shown with the FEM that the maximum compressive stress, at a specific point in the material, does not occur at the impact for that point. Rather, that constructive interference between stress waves in the material is the probable cause for the stress peaks that are formed. In what way the valve impacts with the seat will affect the maximum compressive stress distribution in the valve tongue. If an area close to the root of the valve impacts first with the seat, a whiplash effect will cause a higher impact velocity and impact stress in the free end of the valve.

The wear mark study showed an initially high growth rate for the wear mark. However, with an increasing amount of cycles, the wear mark growth rate will decrease. Areas at the edge of the valve tongue consistently had the lowest wear mark depth, while areas close to the root and the free end of the valve had similar wear mark depth in the longest tested valve. FEM and wear mark results indicate that the impact velocity and maximum compressive stress are important factors for wear mark growth.

Place, publisher, year, edition, pages
2016. , p. 52
Keywords [en]
Flapper valve, FEM, Impact fatigue, Wear mark
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-199301OAI: oai:DiVA.org:kth-199301DiVA, id: diva2:1075022
External cooperation
Sandvik Materials Technology
Subject / course
Physical Metallurgy
Educational program
Master of Science in Engineering - Materials Design and Engineering
Presentation
2016-12-14, Brinellvägen 23, Stockholm, 09:00 (English)
Supervisors
Examiners
Available from: 2017-04-03 Created: 2017-02-16 Last updated: 2017-04-03Bibliographically approved

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CiteExportLink to record
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