Технические науки/2. Механика

Maksym Gladskyi, PhD, Volodymyr Frolov, PhD, Oleksandr Matiukhin, Yevhenii Herasymenko, Dmytro Danilchenko, Danylo Shupletsov

National Technical University of Ukraine “Kyiv Polytechnic Institute”

Fatigue Life Assessment of VT1-0 and VT9 Titanium Alloys under Irregular Loading

Constructions elements are often undergo irregular multiaxial cycle loading. Though multiaxial materials fatigue has been studied for a long time and enough of experimental data has been accumulated, problem of including of loading irregularity in low-cycle fatigue area is still actual. The most wide-spread is the conception of linear damage accumulation, offered by Miner, where damages per cycle at variable loading amplitude are added linear and failure happens in the case when . This approach is easy to use but it does not give adequate estimation of life in many cases. There were many attempts to develop model based on non-linear accumulation of fatigue damages, but most of them did not consider complex influence of such factors as type of stress state, loading path, previous stress history on the process of fatigue damages accumulation. Fatemi and Yang [1] give a wide survey of the existing models and offer their classification, describe advantages and disadvantages of each one.

Experimental procedure

With the purpose of getting stress-strain state close to homogeneous were used tubular specimens with outer diameters of 11,5 mm and 11 mm, wall thickness of 0,75 mm and 0,5 mm, test portion length of 20 mm and 21 mm for VT1-0 and VT9 respectively. Specimens of VT1-0 were tested at constant deformation amplitude, and under both proportional and non-proportional regular loading. The VT1-0 alloy showed behaviour that is typical for cyclic-stabilized materials under the tested loading conditions. For the VT9 titanium allot the basic modes were: tension-compression, alternating torsion and 90° out-of-phase loading. The first stage of the programme was the block axial loading and/or torsion moment test with given strain ranges. During this test the strain path remained constant. The second stage of the programme was testing the specimens with changing of the strain path. Transfer from one strain path to another was conducted during making the value reach the 0.5 point and then the specimen was brought to failure. At the third stage the test with a multiple strain path change was carried out.

Proposed method

The assessment of VT1-0 titanium alloy fatigue life under non-proportional loading showed that the application of Pysarenko-Lebedev modified criterion resulted in good correlation of predicted and test data due to the complex consideration of the strain state type and non-proportionality of the loading [2]. That is why it is advised to apply the Pysarenko-Lebedev modified criterion as well as the chosen damage accumulation hypothesis for assessing the VT9 titanium alloy fatigue life. In the paper the two damage accumulation hypotheses were analyzed: the linear hypothesis and the Manson’s approach, according to which the damage curve is the relative fatigue life nonlinear function and looks like this:

where ; – the number of one-level loading cycles; – number of cycles before failure under the given loading level; – material constant that is calculated from the test data under sequential double-level loading.

The application of the Pysarenko-Lebedev modified criterion and the linear damage accumulation hypothesis the best correlation of the predicted and test data is obtained for alternating torsion. As a result, one can come to a conclusion about the linearity of damage accumulation process for a given loading type. The combined application of the Pysarenko-Lebedev modified criterion and of the Manson’s approach showed the high level of predicted and test data correlation for all the loading programmes except the alternating torsion. So the following modification of the Manson’s approach is proposed:

,

where ; – strain path orientation angle, which determines the dominating type of the strain state; and are fatigue strength coefficients at finite life for uniaxial and torsional loadings.

So, during the alternating torsion the damage accumulation is linear, during the tension-compression – with the application of the Manson’s approach, and during the biaxial proportional and non-proportional loading their linear interpolation.

The application of proposed approach resulted in the best correlation of the best correlation of the predicted and test data that is shown on the Figure 1.

Figure 1. Comparison of predicted fatigue lives by the proposed approach with experimental fatigue lives

References:

[1] Fatemi A., Yang L. Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials // Int. J. Fatigue. – 1998, vol.20, No.1, pp. 9-34.

[2] Shukaev, S., Gladskii, M., Zakhovaiko, A., Panasovskii, K. A method for low-cycle fatigue life assessment of metallic materials under multiaxial loading // Strength of Materials – 2008, vol.40, No.1, pp. 48-51.