Section 4 Full size testing
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Guidance Notes for the Calculation of Stress Concentration Factors, Fatigue Enhancement Methods and Evaluation of Fatigue Tests for Crankshafts, July 2021 - Chapter 2 Guidance for Evaluation of Fatigue Tests - Section 4 Full size testing

Section 4 Full size testing

Parent topic: Chapter 2 Guidance for Evaluation of Fatigue Tests

4.1 Hydraulic pulsation

4.1.1 A hydraulic test rig can be arranged for testing a crankshaft in 3-point or 4-point bending as well as in torsion. This allows for testing with any R-ratio.

4.1.2 Although the applied load should be verified by strain gauge measurements on plain shaft sections for the initiation of the test, it is not necessarily used during the test for controlling load. It is also pertinent to check fillet stresses with strain gauge chains.

4.1.3 Furthermore, it is important that the test rig provides boundary conditions as defined in Ch 1, 3 Load cases.

4.1.4 The (static) mechanical properties are to be determined as stipulated by the quality control procedures.

4.2 Resonance tester

4.2.1 A rig for bending fatigue normally works with an R-ratio of -1. Due to operation close to resonance, the energy consumption is moderate. Moreover, the frequency is usually relatively high, meaning that 107 cycles can be reached within a few days. Figure 2.4.1 An example of testing arrangement of the resonance tester for bending loading shows a layout of the testing arrangement.

4.2.2 The applied load should be verified by strain gauge measurements on plain shaft sections. It is also pertinent to check fillet stresses with strain gauge chains.

Figure 2.4.1 An example of testing arrangement of the resonance tester for bending loading

4.2.3 Clamping around the journals must be arranged in a way that prevents severe fretting which could lead to a failure under the edges of the clamps. If some distance between the clamps and the journal fillets is provided, the loading is consistent with 4-point bending and thus also representative for the journal fillets.

4.2.4 In an engine, the crankpin fillets normally operate with an R-ratio slightly above -1 and the journal fillets slightly below -1. If found necessary, it is possible to introduce a mean load (deviate from R = -1) by means of a spring preload.

4.2.5 A rig for torsion fatigue can also be arranged as shown in Figure 2.4.2 An example of testing arrangement of the resonance tester for torsion loading with double crank throw section. When a crank throw is subjected to torsion, the twist of the crankpin makes the journals move sideways. If one single crank throw is tested in a torsion resonance test rig, the journals with their clamped-on weights will vibrate heavily sideways.

4.2.6 This sideways movement of the clamped-on weights can be reduced by having two crank throws, especially if the cranks are almost in the same direction. However, the journal in the middle will move more.

Figure 2.4.2 An example of testing arrangement of the resonance tester for torsion loading with double crank throw section

4.2.7 Since sideway movements can cause some bending stresses, the plain portions of the crankpins should also be provided with strain gauges arranged to measure any possible bending that could have an influence on the test results.

4.2.8 Similarly to the bending case, the applied load shall be verified by strain gauge measurements on plain shaft sections. It is also pertinent to check fillet stresses with strain gauge chains as well.

4.3 Use of results and crankshaft acceptability

4.3.1 In order to combine tested bending and torsion fatigue strength results in calculation of crankshaft acceptability (see the 'Acceptability criteria' sub-Section of the applicable Rules), the Gough-Pollard approach and the maximum principal equivalent stress formulation can be applied for the following cases:
  1. Related to the crankpin diameter:
    where
    σDWCT = fatigue strength by bending testing
    τDWCT = fatigue strength by torsion testing
  2. Related to crankpin oil bore:
    where
    σDWOT = fatigue strength by means of largest principal stress from torsion testing
  3. Related to the journal diameter:
    where
    σDWJT = fatigue strength by bending testing
    τDWJT = fatigue strength by torsion testing

4.3.2 In case increase in fatigue strength due to the surface treatment is considered to be similar between the above cases, it is sufficient to test only the most critical location according to the calculation where the surface treatment had not been taken into account.

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