ε tð Þ¼εct t t1 ð Þ εct t t2 ð ÞþεR0 þεRt t t2 ð Þ, t >t2 ð45:5Þ ε tð Þ¼εR0 þεRt t t2 ð Þ, t t2 ð45:6Þ Based on (45.4), it is clearly to be seen that measuring the strain directly after drilling would produce an incorrect result. Because of the creep effect while drilling the material, the released strain directly after drilling at the time t2 in Fig. 45.9 does not only correspond to the relaxation of residual stress. It also corresponds to the viscoelastic part caused by compression during the drilling process: εct t2 t1 ð Þ in (45.4). During an experiment, strains were measured directly after drilling and an average variation of strain of about 8 μm/m was observed during a time interval of 8 s. This demonstrates the challenge connected with correct strains to be taken for the calculation of a residual stress profile. Using (45.5) one can formulate an assumption about a possible method. Under the assumption that the two components caused by the compression effect of the drilling tool ‘εct’ will compensate themselves after a sufficient period of time (45.5), the strain will only depend on the release of residual stress: εR0 þεRt t t2 ð Þ, which leads to (45.6). Assuming a creep law for the strain which depends on the relaxation of residual stresses (Fig. 45.8), we may extrapolate to get the time independent component ‘εR0’ and finally calculate residual stresses. This will be the purpose of further work. 45.5 Conclusion The results demonstrated in this publication reveal the influence of different parameters when measuring residual stresses on plastics with the hole drilling method. Neglect of temperature compensation will generally result in measuring compressive stresses near the surface of the material. Depending on the thermal expansion of the plastics, errors may be more or less important. In addition there is a stiffening effect, which reduces the strain measured by the strain gauge rosette when working on low-stiffness materials. Finally, the influence of viscoelasticity on the measured strain has to be taken into account. An assumption is proposed to determine the valid strain and therefore to compensate viscoelastic effects. The creeping behavior of the released strain may be extrapolated after each increment to get the required strain to calculate a residual stress profile. Acknowledgement The work presented is supported by the German Science Foundation DFG in the frame of SFB/TRR 30 which is gratefully acknowledged. Reference 1. ASTM E837-13, Standard test method for determining residual stresses by the hole-drilling strain-gage method, ASTM International, 2013 2. Working Group on Residual Stresses, “AIAS – TR 01:2010”, The hole-drilling strain gauge method for the measurement of uniform or non-uniform residual stresses (2010) 3. A. Nau, B. Scholtes, M. Rohleder, J.P. Nobre, Application of the hole drilling method for residual stress analyses in components made of polycarbonate. J. Plast. Technol. 7(3), 66–85 (2011) 4. A.I. Isayev, D.L. Crouthamel, Residual stress development in the injection molding of polymers. Polym.-Plast. Technol. Eng. 22(2), 177–232 (1984) 5. S. Zike, L.P. Mikkelsen, Correction of gauge factor for strain gauges used in polymer composite testing. Exp. Mech. 54(3), 393–403 (2013) 6. A. Ajovalasit, L. D’Acquisto, S. Fragapane, B. Zuccarello, Stiffness and reinforcement effect of electrical resistance strain gauges. Exp. Mech. 43(4), 299–305 (2007) 7. P.J. McGrath, D.G. Hattingh, M.N. James, I.N. Wedderburn, A novel 8-element gauge for residual stress assessment using the high-speed centre hole-drilling method. SAIMechE Res. Dev. J. 18(1), 1–6 (2002) 8. A. Nau, G. Feldmann, J. Nobre, W. Zinn, B. Scholtes, An almost user-independent evaluation formalism to determine arbitrary residual stress depth distributions with the hole-drilling method. Mater. Sci. Forum768–769, 120 (2013) 9. H.-P. Heim, A. Ries, A.K. Bledzki, Challenge of functional gradation of self-reinforced polypropylene composites, inSAMPE Europe SEICO 11–32nd (2011), pp. 148–155 10. Carrier data taken from the firm Preusser Messtechnik GmbH, https://dms-technik.de/, German 11. E. Valentini, A. Benincasa, L. Bertelli, Experimental residual stress analysis by the hole-drilling method on plastic materials, in Conference Paper, AIAS 2009-123 (2009) 12. J.P. Nobre, J.-H. Stiffel, A. Nau, J.C. Outeiro, A.C. Batista, W. Van Paepegem, B. Scholtes, Induced drilling strains in glass fibre reinforced epoxy composites. CIRP Ann. Manuf. Technol. 62(1), 87–90 (2013) 380 A. Magnier et al.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==