Fatigue Design of Autofrettaged Diesel Engine Injection Parts
Increasing economic and ecological demands on Diesel engines have raised the injection pressures up to 3000 bar. One of the favourite ways to increase the fatigue resistance of injection parts is autofrettage, where an initial overload near the plastic limit load and following unloading generates favourable compressive stress fields around the notches. In the endurance regime, autofrettage can yield a tremendous increase of fatigue limit with crack arrest in the compressive stress field. In the finite life regime the crack initiation life benefits from the compressive mean stress at the notch surface, and the crack propagation from crack growth retardation in the compressive stress field. Focus is put on endurance and on fatigue strength under variable amplitude loading. Test results on tubes and notched specimens representing typical design features in pumps, rails, injectors etc. demonstrate the advantages of autofrettage and serve as basis for the validation of design methods. For variable amplitude loading the standardized load sequence CORAL (COmmon RAil Loading) is shown, representing the life of an injection system of a Diesel passenger car as well as ’CORAL extended’ with a large amount of additional small cycles. A variety of different life prediction approaches have been investigated. The following two design methods are discussed in the paper as they are of practical use: 1. Nominal Approach using Miner’s rule 2. Local Approach for crack initiation life and explicit 3D finite element simulation for crack propagation using LEFM, with crack closure and automatic remeshing Fatigue analyses using design method 2 could predict these experimental results with a high accuracy, proving the significance of advanced design methods. For practical application, design method 2 can be recommended. Method 1 can only be applied with a high level of experience usually not yet available in design groups of the industry. A considerable advantage of autofrettage is the very small scatter in life or in fatigue strength, respectively, resulting in remarkably small safety factors for the allowable pressures at small failure probabilities. The methods developed in research for life and endurance prediction have been successfully transferred to the industry, where it has been implemented and adapted to practical needs. MAN Diesel & Turbo SE has accompanied the research work and has used the know-how from the very beginning. The know-how has also been applied in the development of the new Common Rail Injection System CR 2.2 for 4-stroke engines operating at 2200bar to fulfill future requirements for fuel consumption and emissions. An outlook is given on unsolved design problems concerning high frequency of small cycles, temperatures up to 200°C and load spectra with maximum pressures below endurance.
Rayk Thumser Wolfgang Wagner Joachim W. Bergmann
MAN Diesel and Turbo SE formerly with MFPA Weimar,Germany MAN Diesel and Turbo SE,Germany TWIN GmbH Weimar, Germany, formerly with MFPA Weimar, now Senior Scientist of MFPA, Germany
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2013-05-13（万方平台首次上网日期，不代表论文的发表时间）