Research

Vibration Resistance of Crankshafts

To ensure that vehicle crankshafts have the necessary vibration resistance, their surface layer is specially finished. By compacting the critical radii of crankshafts, resistance can be doubled. The effect of this increased fatigue strength compared with an unrolled state is based on the premise that potentially detrimental cracks in the depth range of the maximum residual compressive stress do not get larger (crack arrest). The assessment method developed in this project was based on both the FE simulation of the deep rolling process for determining residual stress as well as their fracture mechanical evaluation. The analysis focused on Q&T steel 42CrMo4 V, the precipitation-hardened ferritic-pearlitic steel 38MnVS6 BY and spheroidal graphite cast iron GJS700. The difference between the calculated and experimentally determined vibration resistance values was -10 or rather +7 per cent under bending load and -12 or +8 per cent under combined bending and torsion load.

» We could show that a computational appraisal of the fatigue strength of compacted components is possible using simulation and fracture-mechanical valuation. «
Prof. Dr.-Ing. Matthias Oechsner | Center for Structural Materials (MPA-IfW | TU Darmstadt)

Motivation

Crankshafts in combustion engines are designed in such a way that they can handle high vibration stress. Boundary layer strengthening methods are applied. Through this compacting of the critical radii of the crankshafts, an increase in strength of more than 100 percent is possible compared with non-compacted states. In order to be able to appraise the fatigue strength of typical crankshaft materials, it is necessary to have thorough knowledge of their residual stress and fracture mechanics for example.

Methodology

The application of material characterisation and modelling methods, residual stress calculations and measurements, as well as fracture detection and fracture mechanics are based on crankshaft-similar tests from precipitation-hardened, ferritic-pearlitic steel 38MnVS6 BY and spheroidal graphite cast iron GJS700. The assessment method derived from this were based on both the FE simulation of the deep-rolling process for determining residual stress as well as their fracture mechanical evaluation. The method was tested for its suitability in practice using a stock crankshaft made from Q&T steel 42CrMo4 V.

Result

The mechanism responsible for the compacting process results from the residual compressive stress in the outer layer. The FE-based methodology for evaluating the compacted residual stress using linear-elastic fracture mechanics takes into account the fact that the residual stress prevents a possible crack growth and leads to a crack arrest. The difference between the calculated and experimentally determined vibration resistance values was –10 or rather +7 per cent under flexural load and –12 or +8 per cent under combined flexural and torsion load. The new assessment model is a practical tool for designing fatigue endurable crankshafts.

Documentation

Fatigue Strength I | Fatigue strength criteria of compacted steel crankshafts | Project No. 913 | AiF Funding ID 14861 N

Fatigue Strength II | Evaluation of the vibration resistance of compacted crankshafts made from different materials | Project No. 1079 | AiF Funding ID 16675 N

Themis

Status
Finalised

Programme
Public Funding

Budget
972,350.00 EUR

Time Period
2006-08-01 to 2011-11-30 Part I
2011-08-01 to 2014-10-31 Part II

German Federation of Industrial Research Associations (AiF) e.V.

Bayenthalgürtel 23
50968 Cologne
Germany

Federal Ministry for Economic Affairs and Energy (BMWi)

Scharnhorststrasse 34-37
10115 Berlin
Germany

Industry

Dr.-Ing. Rachid Nejma
Daimler AG

RTD Performers

1 | Research Group and Institute for Materials Science (IfW) | TU Darmstadt

Head of Research:
Prof. Dr.-Ing. Matthias Oechsner

Research Associate:
Dipl.-Ing. Gilles Desmond Fomen

2 | Research Group System Reliability, Adaptive Structures and Machine Acoustics (SAM) | TU Darmstadt

Head of Research:
Prof. Dr.-Ing. Tobias Melz

Research Associate:
Dipl.-Ing. Christian Diefenbach

Technische Universität Darmstadt

System Reliability, Adaptive Structures and Machine Acoustics (SAM)

Magdalenenstrasse 4
64289 Darmstadt
Germany

Technische Universität Darmstadt

Center for Structural Materials/Institute for Materials Science (IfW) | State Materials Testing Institute Darmstadt (MPA)

Grafenstrasse 2
64283 Darmstadt
Germany

Deputy Managing Director

Martin Nitsche

FVV
+49 (0) 69 6603 1345
+49 (0) 69 6603 2345

Research Association for Combustion Engines eV

Lyoner Strasse 18
60528 Frankfurt am Main
Germany
T +49 69 6603 1345