Residual stress distribution measurements in shot peened aluminium plates

Transcription

1 Residual stress distribution measurements in shot peened aluminium plates M. Beghini,^ L Bertini,^ F Frendo,^ V. Fontanari, P. Scardi ^Dipartimento diingegneria Meccanica, Nucleare e della Produzione, Universita di Pisa, via Diotisalvi 2, Pisa, Italy beghini(bertini, frendo)@ing. imipi. it ^Dipartimento di Ingegneria dei Materiali, Universita di Trento, Mesiano di Povo, Trento, Italy vigilio.fontanari(paolo.scardi)@ing. unitn. it Abstract Residual stress measurements in 6082 aluminium plates subjected to two different shot peening intensities are presented and discussed. Measurements were performed by both the hole drilling method using a proper developed procedure and by X-ray diffraction. The results obtained by the different methods appeared in good agreement in the surface region, were X-ray diffraction could be applied The two different shot peening intensities showed a different depth extension of the region affected by treatment, where compressive residual stress were observed. Maximum compressive stresses of 190 MPa and 220 MPa were measured for the light and intense peening respectively. 1 Introduction Shot peening is a currently employed technological process aimed at increasing the strength of metallic components particularly when subjected to cyclic loading. Typically, fatigue damage originates in the external material layers, where the highest stress levels are reached. As a consequence of shot peening, compressive residual stresses are generated just in the outer material layers, thus fatigue resistance can be increased.

2 302 Surface Treatment Depending on shot peening parameters, mainly the type (hardness, mass and dimension) and speed (magnitude and direction respect to the target surface) of the incident spheres, different residual stress distributions are generated, which may be characterised by peak values and an affected depths. The knowledge of the residual stress distribution as a function of the technological parameters, would be useful for selecting the optimal shot peening process for a given application. A dedicated software, which numerically simulate the impact of the spheres, was recently developed by Le Guernic and Eckersley*, in order to aid the selection of optimum peening parameters. Several experimental methods may also by applied to directly measure the residual stresses produced by the process. The blind hole drilling (BHD) method is a well known semidestructive technique, allowing residual stresses to be measured up to a depth of the order of a few millimetres. A rational technique for dealing with variable residual stresses was initially introduced by Schajer^' ^. Recently some of the authors proposed an improvement of this technique, based on analytical influence functions and genetic algorithms (Beghini et alii*). On the other hand, the X-ray diffraction (XRD) method can be conveniently applied to determine residual stresses nearby the surface (a few tenths of microns, depending on X-ray energy and on material). In the present work residual stress measurements were performed on a common aluminium alloy employed in automotive industry, subjected to peening with two different intensities. Residual stresses were measured by both methods (BHD and XRD), in order to investigate their effectiveness and to achieve indications on their accuracy. 2 Experimental activity and fundamentals of the methods 2.1 Material and specimens Specimens having dimension 17x97x4 mm were cut from a plate composed of 6082 aluminium alloy (Figure 1). Two different shot peening intensities were considered: intense and light peening, namely 10 A and 10 N of the Almen scale respectively. The intense treatment was obtained using Z850 zirconia spheres, while the other with B60 glass spheres. The main mechanical properties, Young's modulus, Poisson's ratio and the yield stress of the alloy were determined by standard tension tests, performed on plate specimens cut from the same metal sheet. Tensile specimens were cut parallel and orthogonal to the rolling direction (Fig. 1), in order to evidence possible material anisotropy. Data from tension tests were employed in the BHD method and will be used to discuss the reliability of results.

3 Surface Treatment 303 Specimens for res. stress measurement ALUMINIUM PLATE Figure 1. Specimen orientations. 2.2 Test methods and procedures for residual stress measurement A numerically controlled device was used for the execution of the hole (Valentini^). It uses a small milling tool (about 1.6 mm diameter) mounted on a small air turbine, which reaches a rotational speed of about rpm. The drilling device is equipped by an optical microscope for accurately centring the hole in the rosette and by an electric contact control switch, designed to detect the zero depth reference before drilling the hole. A step motor and a fine-pitch screw give a high resolution to the in-depth movement of the tool. Depth increments of 5 jiim were used in the near surface layer. Electric strain gage rosettes EA RE-120 of MM-series were employed for relaxed strains measurement. Holes were drilled for a total depth of about 1.2 mm, at which a saturated strain signal was observed for all the specimens; depth increments were varied as the hole became deeper. Residual stress determination by the BHD method is equivalent to solve the following decoupled integral equations (Schajer): p(z)=[l,(z,z)-p(z)dz (la) t(z)=l*i,(z,z)-t(z)dz (lc)

4 304 Surface Treatment where z and Z ( Z < z ) represent the hole depth and the depth co-ordinate respectively,/?, q, t are scalar functions of the strains measured by the three strain gages of the rosette, while P, Q, T are scalar functions of the residual stress tensor components. Analytical expressions for the influence functions I ^ and 1 %, which depend on the geometry and material properties, were determined by Beghini and Bertini^ on the basis of accurate finite element simulations. In the present analysis an equibiaxial stress state is expected (also confirmed by strain measures), since there is no preferred orientation determined by the peening process, therefore only eqn (la) is necessary. Measurements were interpreted on the basis of a specific software (Beghini et alii*), which, allows to select one of the following representations for the residual stress profile P(Z): a) a constant piece wise function, corresponding to the integral method (Schajer' ^); b) a linear continuous (C ) piecewise function; c) Hermitte's (i.e. C^ cubic) spline; d) Fourier series; e) exponential series. In all cases the solution can be expressed by: r=\ in which c^are coefficients and l^z are elementary functions, constituting the basis for the stress representation. By substituting eqn (2) into eqn (la), the following equation is obtained: (2) -j/,(z,z).v/,(zvz (3) During a drilling operation several (S) relaxed strain values can be obtained, one for each depth increment z^ ; therefore a set of S linear equation in the R unknowns c^ is obtained by eqn (3): where: _,'«,, (4a) r=l The solution of system (4), for which is usually S > R, is obtained by the least squares method. Moreover the software contains a genetic algorithm, which automatically determine the optimal parameters of the base representation. The target function TF to be minimised, which was defined considering experimental errors, can be expressed as follows: (J O" res exp (6)

5 Surface Treatment 305 where cr^ is the cumulative standard deviation between the experimental and the calculated strain and o^ is the intrinsic standard deviation in the measured strain. This procedure reduce the effects of random experimental errors (Beghinf). X-ray diffraction measurements were performed on the same specimens, by the sirfy method (e.g. see Noyan and Cohen^). In addition measures were conducted in two orthogonal directions, i.e. parallel and orthogonal to the rolling direction (Fig. 1). The X-ray elastic constant were firstly determined; then several measures were done with different incident angle with respect to the specimen surface, in order to achieve information in a surface layer with depth of 50 urn. 3 Results The Young's modulus and Poisson's ratio, as determined by tension tests, were 72 GPa and 0.32 respectively, while the yield stress, at the 0.2% permanent strain, was about 300 GPa. These data were used for residual stress determination by the BHD method. No difference was observed between the results obtained with specimens cut with different orientation with respect to the rolling direction. XRD response indicated the presence of a nearly constant stress state within the analysed depth (about 50 pm). Results are reported in Table 1, showing that the stress state nearby the surface is nearly equi-biaxial. Table 1. Residual stress determied by XRD (MPa). (*Nu mbers in parentheses indicate the expscted confidence intt;rval) Light peening Intense peening DHL DIR. - 1 _ 2-170(6)* -158(6) -153(5) -153(4) Two independent measures with the BHD method were performed for each peening intensity. In any case measured relaxed strains were coherent to an equibiaxial stress state, i.e. the strains acquired by the three gages were equal, within the experimental errors. In order to evaluate the stress distribution, the average of the three strain gage readings was considered for each hole depth. In Figure 2 the average strain of the three gage rosette is plotted vs. the drilled hole depth. It may be observed that both the maximum strain and the saturation depth are sensibly greater for the intense peened specimens. The maximum value observed in the light peened specimens was about 70 jam/mm, while for the heavier treatment it was about 250 jam/mm. Moreover, for the heavier treatment the saturation was found at a depth more than twice larger than the light peened specimens.

6 306 Surface Treatment Intense peening Light peening 0*0000**** oo o o o o o o Hole depth (mm) Figure 2. Relaxed average strain for the intense and light peened specimens. The residual stress distributions were evaluated on the basis of the integral method, the linear piecewise and Hermite's spline representations J_ C/ "2 'on or 100 C6 200 inn V / I/ ^^^" ' Depth (mm) Integi al method Linear piecewise Cubic spline! XRD measurements j Figure 3. Residual stress distribution in light peened specimens

7 Surface Treatment Depth (mm) Integral method Linear piecewise Cubic spline XRD measurement O.f Figure 4. Residual stress distribution in heavy peened specimens. Figures 3 and 4 show two typical residual stress profiles obtained for the heavy and light peened specimens respectively; results obtained by XRD measurements are reported in the same figures for comparison. 4 Discussion Results indicate the presence of compressive stresses in a region having extension depending on the treatment intensity. Residual stress distributions showed nearly constant value in a relatively wide depth range followed by a rapid decrease, so that the residual stress distribution could be reasonably characterised by two parameters; this trend is similar to that reported by Le Guernic and Eckersley*. Maximum compressive stresses evaluated by the integral method were about 186 GPa and 223 GPa for the light and intense peening respectively. Compressive stress near the surface were found to be slightly lower for the specimens subjected to the intense peening, probably due to a stress relaxation caused by the adjacent material layers, subjected to a higher stress intensity. Finally it should be observed that evaluated maximum residual stresses exceed 0.5 the yield stress of the material indicated by the ASTIVf as a limit for the elastic analyses of the BHD method. However, the region of high stress is relatively narrow, thus supporting that the plasticity induced by hole drilling is not so important in this case and that the results are accurate enough.

8 308 Surface Treatment This consideration is also supported by the results obtained by the XRD method. Stress values obtained by this technique appeared in good agreement with the average value obtained by the BHD method in the surface layer. 5 Conclusion The residual stress distribution in 6082 aluminium alloy subjected to two different shot peening intensities was investigated. Measurements were performed by two independent experimental techniques: the blind hole and X- ray diffraction methods. Compressive residual stresses were found, affecting a material depth, which depends on the peening intensity. Maximum compressive stresses of 190 MPa and 220 MPa were found for the light and intense treated specimens respectively. Results obtained by the blind hole method in the first material layers were in good agreement with results obtained by the X-ray diffraction method. Acknowledgements The authors wish to thank Nor Blast company (Bologna), for shot peening treatments of the specimens. References 1. Le Guernic, Y., Eckersley, J.S., Shot peening parameters selection assisted by peenstress software, Proc. 11* IFHT - 4* ASM Heat Treatment and ^w^cgemgz^g^mg, Florence October, 1998, pp Schajer, G.S., Measurement of Non-Uniform Residual Stresses Using the Hole-Drilling Method. Part I-Stress Calculation Procedures, Journal of Engineering Materials and Technology, Transactions of the ASME, 110, pp , Schajer, G.S., Measurement of Non-Uniform Residual Stresses Using the Hole-Drilling Method. Part II-Practical Application of the Integral Method, Journal of Engineering Materials and Technology, Transactions of the ASME, 110, pp , Beghini, M., Bertini, L., Rosellini, W., Genetic Algorithms Applied to the Measurements of Residual Stresses Distributions, to be published on Af/45"PP, froc. ZYK///^//^ Oo/T/grfMCf, Vicenza, 8-11 September 1999 (in Italian). 5. Valentini, E., Vangi, D., An Automatic Test Device for Residual Stress Measurement, Proc. AIASXXI conf, Genova, pp , 1992 (in Italian). 6. Beghini, M., Bertini, L., An analytical expression of the influence functions for the hole drilling residual stress measurements, Proc. XXV AIAS Conf, International Conference on Materials Engineering, 4-7 settembre, Lecce 1996, l,pp

9 Surface Treatment Beghini, M., Applications of the Least Squares Splines Interpolating Technique to Mechanics, Proc. IX ADM Cong., Caserta Aversa (1995), pp (in Italian). 8. Noyan, I.C., Cohen, J.B., Residual Stress Measurement by Diffraction and Interpretation, Material research engineering, Springer-Verlag, ASTM E Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method.