A flapper valve is made from a hardened and tempered high strength strip steel which opens and shuts as it is subjected to very high cyclic loads. Steel strip of which flapper valves are made from can   encounter a surface defect which are anticipated to influence fatigue life negatively. In this study, the influence of surface scratches on fatigue life of flapper valve strip was investigated. The analysis was carried out by using thirty samples that were blanked out of eight different steel strips in the transverse direction. Of these samples, fifteen of them had scratches on the surface and fifteen did not, all these samples were fatigue tested by constant amplitude method. An S-N curve was plotted based upon the values and results from the fatigue test, considering the curve as the nerve center in relation with fractrographic studies using the Scanning electron microscope. Therefore this master thesis work aims to explain the influence of scratches on fatigue life of flapper valve strip and suggest future improvements based on the findings.

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.

V posledních několika desetiletích se objevuje snaha o snížení firemních nákladů, stejně tak jako nákladů, které je nucen vynaložit zákazník, čímž se společnosti snaží získat výhodu vůči svým konkurentům na trhu. Spolu s tímto trendem jde i neustálá snaha snížit dopady na životní prostředí. Vývoj stávajících produktů se proto zdá být klíčovým prvkem. Tento dokument se zabývá vývojem pístového kompresoru na CO2 chladivo, který vyrábí společnost Emerson Climate Technologies. Cíl práce je zvýšit COP kompresoru při zachování stávající životnosti kompresoru. Diplomová práce je rozčleněna do několika kapitol, které se zabývají analýzou originálního designu kompresoru, návrhem a vyhodnocením designů nových. Nezbytné teoretické základy mohou být také shlédnuty v počátečních kapitolách. V poslední části dokumentu jsou sdělena možná další vylepšení a případné jiné konstrukce. Vývoj byl zaměřen na sestavu ventilové desky. Na základě několika předpokladů a výsledků analýzy původního designu kompresoru byly navrženy nové konstrukce, které byly dále testovány statickou strukturální analýzou. Pomoci modální analýzy byly také vypočteny vlastní frekvence a vlastní tvary sacího jazýčku. Mimo modální a statické strukturální analýzy byla provedena také CFD analýza. V posledním kroku byly testovány navržené prototypy a jejich výsledky byly porovnány s původním kompresorem. K správnému návrhu bylo zapotřebí programové podpory a to především v podobě MATLABu, ANSYSu WB a Microsoft Excelu. V práci jsou velmi často prezentovány obzvláště výsledky získané v programu ANSYS WB.