Avoiding knife-edge countersinks in GLARE through dimpling

1 Page 1 of 21 Avoiding knife -edge countersinks in GLARE through dimpling Submitted to Fatigue and Fracture of Engineering Materials and Structures on March 4, 2004 C. Rans*, Straznicky Department of Mechanical and Aerospace Engineering Carleton University 1125 Colonel By Drive Ottawa, Ontario, Canada, K1S 5B6 ABSTRACT Traditional machine countersinking practices create a knife -edge condition in one or more of the outer aluminum layers in riveted GLARE joints. Press countersinking ( dimpling ) provides an alternative method of countersinking that prevents the formation of a knife -edge; however, its application and potential benefits to fatigue performance in GLARE are not known. This paper investigates the dimple forming process and its application to GLARE , and the resulting benefits in fatigue crack initiation life in unfilled rivet holes.

2 Initial results showed that the limited formability of GLARE complicates the dimpling process, but that dimpling shows promise as a method for increasing the crack initiation life of riveted GLARE joints. Keywords: dimpling ; Riveting; GLARE ; FML; knife -edge 1. Introduction Fibre metal laminates (FMLs) are a family of hybrid laminated sheet materials consisting of alternating metallic and fibre-reinforced plastic layers. One particular FML, GLARE (GLAss REinforced aluminum), is considered a suitable candidate for application in fuselage skin panels of pressurized aircraft, with its first application in the * Corresponding author; Email: Page 2 of 21 Airbus A380-800 megaliner1.

3 The major advantage of GLARE in airframe applications is the low fatigue crack growth rate compared to monolithic aluminum alloys. The mechanisms responsible for this superior crack growth behaviour include the plane stress state in the thinner metallic layers; the branching effect of part- through cracks growing through the laminate thickness; and the crack bridging properties of the fibre layers (fibre bridging), which reduces the crack tip stress intensity factor2-5. Experiments have shown that this behaviour can result in a 10-fold improvement in overall fatigue life over monolithic aluminum alloys of similar thickness2. Such fatigue performance allows for the use of smaller sheet thicknesses in fatigue critical applications (such as fuselage skin panels), creating the opportunity for structural weight savings.

4 The crack initiation process in GLARE is analogous to that of monolithic aluminum alloys; namely, monolithic aluminum sheets and aluminum layers in GLARE with the similar stresses will have similar crack initiation lives. The lower stiffness of the fibre layers in GLARE , however, causes the aluminum layers to carry a higher proportion of load, resulting in a shorter crack initiation life compared to monolithic aluminum of a similar thickness and under the same applied load. A further reduction in crack initiation life occurs in applications where monolithic aluminum is replaced with thinner GLARE laminates; however, improvements in overall fatigue life are still obtained due to the superior crack growth properties of GLARE .

5 Mechanically fastened joints are fatigue critical locations in airframes. large stresses occur at fastener locations as a result of fastener bearing loads and elastic stress concentrations associated with the fastener hole. In jet aircraft, the external aircraft surfaces are made flush by machining countersinks for the fastener heads into the skin. Page 3 of 21 An additional stress concentration is generated by the countersink , dependent on the depth of the countersink relative to the skin thickness. In the extreme case where the countersink depth exceeds the skin thickness, a knife -edge condition is formed, and the elastic stress-concentration factor becomes 72% greater than that in a plain hole6.

6 Due to the small thickness of the individual layers in GLARE , the practice of machine countersinking creates a knife -edge in the outermost aluminum layers (lamina knife -edge). The application of thinner GLARE laminates could result in cases where the countersink depth exceeds the laminate thickness, producing a through -thickness knife -edge (Figure 1). The resulting stress concentrations likely accelerate crack nucleation; thus, if the knife - edges were avoided, the crack nucleation period could be increased thereby further enhancing the GLARE advantage. One possible approach to Avoiding a knife -edge is the use of a formed countersink by means of dimpling . Originally developed for monolithic aluminum alloys, dimpling was used to prevent knife -edge countersinks in sheet thicknesses below 7.

7 The development of reduced- countersink -depth rivets permitted machine countersinking in sheet thicknesses down to , reducing the need for dimpling in primary aircraft structures. countersink depths that avoid a lamina knife -edge in GLARE , however, are not feasible, raising the question of the potential benefits of dimpling GLARE . In addition to preventing the formation of a knife -edge, dimpling introduces the following additional factors that may influence fatigue performance: Complex load transfer mechanism resulting from the interlocking of dimpled sheets, Residual stresses resulting from dimple formation, Page 4 of 21 Potential for delamination damage, fibre fracture and other forming related failures during dimple formation in GLARE .

8 Due to the limited use of dimpling , the above factors and their effects on fatigue life are not well understood. As part of a larger study into determining the fatigue performance of dimpled GLARE lap joints8, the results from a series of dimple forming tests and crack initiation test of unfilled dimpled holes in GLARE are presented in this paper. This investigation is limited to biaxial Grade 3 GLARE (equal number of fibres in the 0 and 90 directions) with a layup consisting of two aluminum layers and one fibre layer (designated GLARE 3-2/1). Table 1 shows a comparison of the basic properties of GLARE 3-2/1 and 2024-T3 alloy. GLARE 3-2/1 was chosen for this study due to its potential application as a fuselage skin in narrow-body aircraft, its susceptibility to through -thickness and lamina knife - edges , and its higher formability compared to thicker GLARE laminates.

9 2. Dimple Forming Techniques dimpling is a forming process, where a countersunk is formed by plastically deforming the sheet material under high pressure using a set of dimpling tools (a punch and die). This process is performed after the fastener holes have been drilled and deburred, and can be performed at room temperature (cold dimpling ) or at elevated temperatures (hot dimpling ). Hot dimpling is predominantly used in thick or brittle sheet materials, where cold dimpling would produce radial cracks. The basic geometry of a set of dimpling tools is shown in Figure 2. The springback angles shown are often included in the tool geometry to minimize distortion of the sheet material surrounding the dimple due to elastic springback after dimpling .

10 Page 5 of 21 Two basic variations of the dimpling process exist: coin dimpling and radius dimpling (Figure 3). The major difference between the two variations is how they ensure proper nesting of the dimpled sheets being joined. In coin dimpling , each sheet is dimpled independently, and the geometry of the formed dimple (dimple cone) is identical in all sheets. To ensure proper nesting, a reduction in the thickness of the dimple cone is necessary. Figure 4 shows that, for a standard dimple angle of 100 , a 23% reduction in sheet thickness is required. Coin dimpling also results in a cylindrical fastener hole, as excess material resulting from the thickness reduction in the dimple wall is formed against the mandrel of the punch.