Riveting Process And Its Effects On Fatigue Life Of GLARE Laminates

Glass Laminate Aluminum Reinforced Epoxy（GLARE）has achieved successful application in aviation area due to its excellent performances,which is very likely to be applied to domestic wide-body passenger aircraft in the future.GLARE application researches have been carried out in multi fields at home and abroad.However,the mechanical joint assembly of GLARE have not been investigated systematically,which seriously hampers the engineering application of GLARE.Based on the above background,four GLARE-3/2-0.3 variants were taken as research objects.Firstly,geometric parameters of the drilling cutter were ascertained and drilling parameters were optimized to explore the reasonable process which was suitable for different GLARE laminates.Secondly,drilling quality,interference amount and riveting quality were evaluated and then the range of optimal interference amount was determined.Subsequently,S-N curves of four kinds of riveted GLARE joints were obtained and the effect of fiber laying and hygrothermal environment on the fatigue lives of riveted GLARE joints was studied in detail by means of substantial fatigue tests to reveal the degradation mechanism and the aging behavior.Finally,failure modes of riveted GLARE joints under static load and dynamic load were studied respectively,which revealed the weak positions.Consequently,feasible suggestions for optimizing the riveted joint design and riveting process were proposed.The conclusions are as follows:（1）The study of drilling process of GLARE revealed that 2-fluted TiAlN coated carbide tool with the point angle of 120°,the helix angle of 30°and the chisel edge width of 0.06d₀ was appropriate for the commonly used connecting holes with diameters of approximately 4.0 mm on aircraft.When the spindle speed was 7500 rpm and the feed rate was 0.10 mm/rev,the holes with high surface integrity were obtained efficiently.Afterwards,the applicability of the above drilling parameters was confirmed by means of three-dimensional video microscopy（3-DVM）and scanning electron microscope（SEM）.（2）The single-nail riveted GLARE 4C,GLARE 4B,GLARE 2A and GLARE 3 joints showed excellent tensile properties when the thickness of driven head was 2.5 mm,2.0 mm,3.0 mm and 2.5mm respectively,namely,the interference amount ranged from 0.98%to 1.61%.Then,SEM observations of the cross sections of single-nail riveted GLARE joints revealed that the expansions of rivets were uniform when the interference amount was in the above range.The uneven areas in the hole caused by drilling were flattened,which reduced micro crack sources and improved the stress distribution around the hole.（3）The fatigue resistance of riveted GLARE joints was closely relevant with 0°fiber volume fraction,and was partly affected by specimen thickness.The average fatigue lives of unidirectional laminates were over seven times as compared with that of orthogonal laminates in the same structure and the fatigue lives of riveted GLARE joints increased with their thicknesses under tensile-tensile fatigue load.In addition,unidirectional laminates were particularly suitable for the parts with high requirements for fatigue life and loaded unidirectionally,while orthogonal laminates were applicable for the parts bearing complex alternating load.（4）After the aging in hygrothermal environment（70°C/85%RH）for 1500 hours,resin matrix underwent a complex moisture absorption process.And the interfacial stress was produced by the mismatch of the wet expansion of fiber and resin,which led to the weakening of fiber/resin interfaces.The load transfer between epoxy resin matrix and glass fibers was hindered,and consequently tensile strengths and fatigue lives of riveted joints decreased by more than 16%.（5）Failure modes of multi-nail riveted GLARE joints under static load and fatigue load were complex,including mainly tensile failure of laminates and rivet shear.Furthermore,the outer rivet lines of three rivet rows were relatively weak according to the research results.Therefore,it was necessary to increase load capacity of outer rivet rows.Increasing the distance between outer rivet holes and edge and properly reducing the surface roughness of rivets and outer surfaces of aluminum alloy plates were also needed in the fatigue design and riveting process.