Aspects of Testpiece Design Responsible for Errors in Cyclic Plasticity Experiments

Abstract

A uniaxial cyclic plasticity copper testpiece has been simulated, using the Continuum Damage Mechanics based finite element programme Damage XX, under conditions of cyclic plasticity at temperatures of 20°C and 500°C. The results obtained have been compared with the results of experiments carried out under the same conditions.

It has been shown that inhomogeneous fields of strain and stress are generated in the gauge length region of the testpiece, which result from standard cyclic plasticity testpiece features, namely, the blend radius linking testpiece shank and gauge length, and the extensometer ridges. The influence of these features on the inhomogeneity is exacerbated by the short gauge length which is necessary in cyclic plasticity testpieces to prevent buckling.

At the temperatures of 20°C and 500°C, cracks were predicted to occur, and found experimentally, at the extensometer ridges. Testpiece failure at 20°C occurred outside the gauge length; and at 500°C failure took place from cracks growing within the gauge length region. The formation and growth of the latter gauge length crack was accurately predicted.

The results obtained clearly demonstrate the dependence of cyclic plasticity data on testpiece design and suggest that the range of scatter observed experimentally in cyclic plasticity testing for nominally identical testing conditions may result from variations in testpiece design. This is of some concern, and the establishment of a methodology which overcomes these problems will be the subject of a future paper.

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References

Chaboche, J-L. , 1988, "Continuum damage mechanics. Part II: Damage growth, crack initiation and crack growth," Jnl. Apple. Mech., Vol. 55, 65-72.

Dunne, F.P.E. , Makin, J. and Hayhurst, D. R. , 1992, "Automated procedures for the determination of high temperature viscoplastic damage constitutive equations," Proc. R. Soc. Lond, A437, 527-544.

Dunne, F. P. E. and Hayhurst, D. R. , 1992a, "Continuum damage based constitutive equations for copper under high temperature creep and cyclic plasticity," Proc. R. Soc. Lond, A437, 545-566.

Dunne, F. P. E. and Hayhurst, D. R. , 1992b, "Modelling of combined high-temperature creep and cyclic plasticity in components using Continuum Damage Mechanics," Proc. R. Soc. Lond, A437, 567-589.

Dunne, F. P. E. and Hayhurst, D. R. , 1994, ";Efficient cycle jumping techniques for the modelling of materials and structures under cyclic mechanical and thermal loading," Eur. J. Mech., A/Solids, Vol. 13, No. 5, 639-660.

Greenwood, G. , 1973, "Creep life and ductility. Micro-structure and the design of alloys." Int. Conf on Metals, Vol. 2, 91-98.

Hayhurst, D. R. , 1972, "Creep rupture under multi-axial states of stress," J. Mech. Phys. Solids, Vol. 20, 381-390.

Hayhurst, D. R. and Felce, I. D. , 1986, "Creep rupture under tri-axial tension," Engineering Fracture Mechanics, Vol. 25, Nos. 5/6. 645-664.

Hayhurst, D. R. , Lin, J. and Kowalewski, Z. L. , 1995, "High-temperature creep testpiece and extensometer ridges: Experimental determination of optimum design," in press.

10.

Hayhurst, D. R. , Dimmer, P. R. and Morrison, C. J. , 1984, "Development of continuum damage in the creep rupture of notched bars," Phil. Trans. R. Soc. Lond., A311, 103-129.

11.

Kowalewski, Z. L. , Lin, J. and Hayhurst, D. R. , 1994, "Experimental and theoretical evaluation of a high-accuracy uniaxial creep testpiece with slit extensometer ridges," Int. J. Mech. Sci., Vol. 36, No. 8, 751-769.

12.

Lin, J. , Hayhurst, D. R. and Dyson, B. F. , 1993a, "The standard ridged uniaxial testpiece: computed accuracy of creep strain," J. Strain Analysis, Vol. 28, No. 2, 101-115.

13.

Lin, J. , Hayhurst, D. R. and Dyson, B. F. , 1993b, "A new design of uniaxial creep testpiece with slit extensometer ridges for improved accuracy of strain measurement," Int. J. Mech. Sci., Vol. 35, No. 1, 63-78.

14.

Lin, J. , Dunne, F. P. E. and Hayhurst, D. R. , 1995, "Physically-based temperature dependence of elastic-viscoplastic constitutive equations for copper between 20 and 500°C." Philosophical Magazine A., Vol. 74, No. 2, 359-382.

15.

Thomas, G. B. and Varma, R. K. , 1991, "Evaluation of low cycle fatigue test data in the BCRIVAMAS intercomparison programme," BCR Information, EUR 14105EN, NPL, UK.

16.

Verrilli, M. J. , Castelli, M. G. , Bressers, J. and Oehmke, R. L. T. , "Standardization activities in TMF test methodologies," Proc. of the 81st Meeting of the AGARD SMP Panel on Thermal Mechanical Fatigue of Aircraft Engine Materials, Banff, Canada, 2-4th Oct., 1995.