On improving the fatigue performance of a double-shear lap joint

On improving the fatigue performance of a doubleshear lap joint L. S c h w a r m a n n

Different methods of/mproving the fatigue performance o£ a double-shear lap joint which is representative for airframe structures are discussed. Considering all aspects concerning fatigue performance, fabrication problems and costs, one method is recommended for pract/cal application in aviation. This method involves the installation of cylindrical fasteners with a low degree of interference.fit into cold-worked holes. Key words: fatigue; fatigue life; air transport engineering; interference-fit; double-shear lap joints

Notation D1 D2 D~ D4 E EF

Hole diameter before cold working Hole diameter after cold working Diameter of fastener hole Diameter of fastener Young's modulus for plate material Young's modulus for fastener material

Airframe structures have to be designed to withstand operational loads. One of the main tasks for the design engineer is to guarantee a safe fatigue life in order to avoid fatal accidents. ] A thorough study of the appearance of fatigue failures shows that most failures originate at fastener holes. 2 Thus load-carrying joints have to be designed very carefully in order to ensure the safety of the structure. It is common practice in aircraft fabrication to rivet metallic components, using either solid rivets for thin-wailed components or high-strength fasteners for thick-walled components. In the latter case it is known that the fatigue performance can be unsatisfactory if the fasteners are installed with clearance-fit, ie if the diameter of the fastener hole is greater than the diameter of the fastener. In order to avoid unwanted fatigue failure it is necessary to use methods which significantly improve fatigue performance. In this paper several possible methods are investigated, none of which require a redesign of the structure or add additional weight. Fabrication and costs are compared in order to give a recommendation for practical application.

Description of t h e methods The methods investigated involve the preparation of the fastener holes and the installation of fasteners with interference-fit. The methods are known, 3 and will be described only briefly.

Method A Installing cylindrical fasteners with clearance-fit into coldworked fastener holes

SE Sy X /~ ac ~c

X

Maximum elastic stress Yield strength Degree of cold-working ( D 2 - D 1)/D1 x 100 (%) (D4-D3)/D 3 x 100 (%) Local circumferential stress Maximum tensile value of a c IJ/(IOOSy/vt3E)(O.7E/EF + I.S)

The fastener holes are expanded by pulling through an oversized mandrel. The expansion is due to a plastic flow of the plate material (cold-working). After reaming the cold-worked holes, fasteners with clearance-fit will be installed.

Method B

Installing cylindrical fasteners with interference fit into non-cold-worked fastener holes The fastenersare installedwith interference-fit,ie the diameter of the fastener is greater than that of the hole. Different degrees of interference fit will be achieved by using different drillingtools, since the diameters of the fasteners used in the aircraftindustry are given with very small tolerances,eg +-0.006 ram.3

Method C Installing cylindrical fasteners with interference-fit into cold-worked fasteners holes

Method D Installing tapered fasteners into non-cold-worked fastener holes The installation of tapered fasteners is combined with a high degree of interference-fit. 3

Method E Installing tapered fasteners into cold-worked fastener holes All these methods introduce stress distributions in the plate in the neighbourhood of the fastener holes. A knowledge of the maximum tensile values of these stresses is a precondition for determining the load-carrying capacity of the structural component. Some information

0142-1123/83/020105-07 $3.00 © 1983 Butterworth & Co (Publishers) Ltd

Int J Fatigue Vol 5 No 2 April 1983

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hole. The analysis is limited to determination of the position and of the magnitude of the maximum tensile stress, which is present circumferentially to the hole ie F and oc, respectively, in Fig. 1. %

Cold-working the fastener hole

F

L Plate

Fig. 1. Circumferential residual stress distribution in unloaded plate (oc = local circumferential stress at point P, ~c = maximum tensile value of o c at point P)

about the maximum tensile stresses can be derived from stress analysis.

Stress analysis In this section an unloaded plate containing a circular hole is analysed. For simplicity, only two cases are considered: a plate containing an open hole, which has been coldworked; and a cylindrical fastener with interference-fit which has been installed in a non-cold-worked fastener

The main purpose of cold-working is to expand the fastener hole using plastic flow of the plate material. From this process circumferential compressive and tensile stresses result, which can be calculated using a known theory. 4 The data F and 5c have been calculated for different aluminium alloys used in aircraft design; the results are given in Figs 2 and 3. From these it is evident that both data depend very strongly on the degree of cold-working, h, which is defined as (D2-D1)/D 1 X 100 (%), where D 1 is the hole diameter before cold-working, and D 2 the diameter afterwards. It is common in aviation to apply a degree of coldworking of 2.5% < h < 3.0%. For such X-values one can conclude from Fig. 2 that F --- 2 (DI/2), ie the distance between the position of maximum tensile stress and the edge of the hole is approximately one hole-radius. Furthermore, it is known that the transition from compressive to tensile stresses is nearly 70-80% of this distance 4 Therefore reaming cold-worked holes (say up to 15% of

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Int J Fatigue April 1983

Fatigue t e s t s bu ]

The specimen used for the fatigue tests is defined in Fig. 5. The specimen was designed to be representative of many joints used in airframe structures. In order to apply methods A to E, two different fasteners (both with a nominal diameter of 1/4 in (6 ram)) were installed, ie Hilok-fasteners of titanium and Taperlok-fasteners of steel. The installation of the fasteners was carried out in accordance with fabrication specifications used in the German aircraft industry. ? The values of ~ and/~ achieved using the different tools are given in Table 2.3 The/~ values for methods D and E are average values, since the shafts of the Taperlok-fastenecs are non-cylindricaL

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Installing the fastener with interference-fit After installing a cylindrical interference-fit fastener into a circular fastener hole, circumferential stresses are present in the plate. ~ and 6 c have been calculated using a known theory; 5 the results are given in Fig. 4, from which it follows that ~ = (D3/2) for × < 1. This indicates that the place of maximum tensile stress is located at the edge of the hole. For X > 1 the plate material deforms plastically, which leads to ? > (D3/2). The transition from elastic to plastic deformation of the plate material occurs at X = 1. The values of/~ for this case can be determined from Fig. 4; the results are given in Table 1. In this paper the following definitions will be used: Low-Interference Fit (LIF) for/~-values less than those given in Table 1 and High-Interference Fit (HIF) for /~-values greater than those given in Table 1. The analytical investigations show that cold-working a hole and installing an interference-fit fastener both produce residual stresses in the plate. It is known that residual stresses lead to an improvement of fatigue performance of structural components. 6 However, the degree of improvement has to be determined by performing fatigue tests.

Int J Fatigue April 1983

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The specimen was loaded in a fatigue machine by applying two different fatigue load spectra, constant amplitude and Falstaff. The former is defined by the stress amplitude, SA, and by the stress ratio, R, the latter by the maximum spectrum stress, SFThe test results are given in Figs 6 to 10. All failures have been found in the single plate of aluminium 7075T7351 of the specimen. In all figures the plotted stresses are defined to be gross-sectional. It should be noted that for every method investigated only a few test points are sufficient to establish a fatigue curve. This is due to the very small scatter found for all tests, 3 explained by the selection of specimens with very similar values of X and/~. From the test results (Figs 6 to 10) it can be seen that: the baseline configuration, ie installing Hilok-fasteners with clearance-fit into non-cold-worked holes, gives the worst values for fatigue life; there is a significant improvement in fatigue life caused by applying each of the different methods; the fatigue life at least doubles each time. However, this depends on the applied stress, ie the fatigue life increases as the applied stress decreases. From the results the following conclusions may be drawn: •

• •

•

•

The dominant parameter for the fatigue performance of the specimen tested is the degree of interferencefit. This holds true for the non-cold-worked fastener holes and for the cold-worked fastener holes. The fatigue life increases with increasing degree of interference-fit. Cold-working the fastener holes gives a significant improvement in fatigue performance for all cases considered, The joint with Taperlok-fasteners is superior to that with Hilok-fasteners. The best fatigue performance can be achieved by installingTaperlok-fasteners into cold-worked fastener holes.

Practical application Installing fasteners with clearance-fit This procedure has been used in the past because of the need to remove the fastners without damaging the plate

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Int J Fatigue April 1983

Installing tapered fasteners

120

This needs more specialized tools and more fabrication time than using cylindrical fasteners. This is because preparing a tapered hole has to be done very thoroughly in order to meet the corresponding requirements. 7 Some assessments of the costs of applying methods A to E are given in Fig. 11, from which it is evident that methods A, B, C are nearly equivalent. On the other hand, methods D and E are more expensive.

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Conclusions The installation of cylindrical fasteners with clearancefit into non-cold-worked fastener holes should be avoided. Cold-working fastener holes or installing interferencefit fasteners significantly improves fatigue performance.

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material. This procedure is well-proven and does not cause any problems.

Cold-working fastener holes It is known that any fastener hole which can be drilled can also be cold-worked without any problems of accessibility to the structural component. 8 However, cold-working requires the use of special tools and additional fabrication time compared with merely drilling a hole.

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Installing fasteners with interference-fit Fasteners with LIF can be installed by thumb or by a light hammer without major problems. On the other hand, installing fasteners with HIF cannot be guaranteed in general because of problems concerning the accessibility of the structural component.

Int J Fatigue April 1983

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The degree of interference-fit is the dominant parameter; therefore this parameter has to be taken into account when interpreting fatigue test results.

Considering all aspects (fatigue performance, fabrication and costs) then method C is recommended for practical use. This method involves expanding the fastener holes using a degree of cold-working of 2.5% < ~ < 3.0%, and installing the cylindrical fasteners with a degree of interference-fit which is slightly less than the values given in Table 1. However, if fatigue aspects dominate then the installation of tapered fasteners is the best choice. All investigations in this paper have been carried out for a double-shear lap joint specimen. Whether the recommendations given above hold true for other configurations of jointed components has yet to be investigated.

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Int J Fatigue April 1983

Acknowledgements

4.

These investigations have been sponsored by the German Ministry of Defence in a special research programme. The author wishes to thank his colleagues R. Hillbrecht and H. G. Barnett for doing the experimental work.

H=u, Y. C. and Forman, R. G. 'Elastic-plastic analysis of an infinite sheet having a circular hole under pressure' JApp Mech (June 1975) pp 3 4 7 - 3 5 2

5,

Shah, R. C. 'On through-cracks at interference-fit-fasteners' J Pressure Vessels Tech (1977) pp 7 5 - 8 2

6.

Hertel, H. "Erm~dungsfestigkeit der Konstruktion' (Springer Verlag, Berlin, 1969) (in German)

References

7.

N. N. Specifications FH2-1.5630, FH2-3.7201, AA2-3.7202 VFW, Bremen (unpublished)

1.

Campbell, G. S. 'A note on fatal aircraft accidents involving met a l fat igue' Int J Fatigue 3 N o 4 ( 1981 ) pp 181 -- 185

8.

Phillips, J. L. 'Split-sleeve-cold-working fastener holes' Technical Report 74-10 (AFML, WPAFB, Ohio, 1974)

2.

Huth, H. and Schi~tz, D. 'Sammlung und Analysen von im Betrieb von Luftfahrzeugen auftretenden Ermi~dungssch~den' Report BMVg-FBWT7910 ( LBF, Darmstadt, 1979) (in German)

3.

Sehwarmann, L., Hillbrecht, R. end Barnert, H. G. 'Optimierung von lebensdauererhi~henden Massnahmen von Schraubnietverbindungen fi'=r schwingbeanspruchte Flugzeugstrukturen' Report ZTL.FAG2-2.37 (VFW, Bremen, 1979-1980) (in German)

I n t J Fatigue A p r i l 1 9 8 3

Author Dr Schwarmann is with the Vereinigte Flugtechnische Werke GmbH, Postfach 10 78 45, 2800 Bremen 1, Federal Republic of Germany.

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