Bolted Joint designers guidelines

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Bolted Joint designers guidelines

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ChrisR	ChrisR posted 11 Years Ago HOT Topic Details
Group: Forum Members Posts: 363, Visits: 3K	Following a prompt on another thread I though I'd better put this up! - it's not a "how to" by any means but guidance based on testing a bucket load of samples! As some of you may know I did my MSc dissertation on the effect the stacking sequence has on the bearing capacity and failure modes of a single bolt connection. Background: Resin: Hunstman LY/HY 5052 Fibres: T300 carbon & Diolen Weaves: 600 Tri-ax, 200 plain, 375 Twill, 650 Twill Carbon, 265 Plain Diolen Cure: 8hrs @ 80deg Target sample thickness 6mm No of different layups tested: 17 No of samples per layup: 5 Bolt diameter 10mm End distance 3.5 x diameter The layups varied from single fabric weight quasi-isotropic through to multi material and weave, the effective elastic bearing capacity achieved was 219N/mm2 up to 466N/mm2 (to 1st ply failure / noticeable displacement) These were the conclusions to be used as design guidance. 1) To improve the transmission of load between the fibres and layers a matrix should be used that has a lower elastic modulus and a higher strain failure percentage than the fibre used. 2) The introduction of fibres with a lower elastic modulus and a higher strain failure percentage will increase the factor of safety between the elastic yield load and the peak load 3) In laminates with predominantly woven layers, having layers close to but not at the surface where the fibres are orientated at 45deg to the load path will increase the load transmission through the laminate, increasing the elastic yield load but not significantly increasing the peak load capacity 4) Grouping layers of woven fabrics of similar weight and weave will increase the elastic yield load but may reduce the peak load capacity 5) When using a 10mm dia bolt, using a fabric weight (in these tests Twill) of around 375gsm will result in higher elastic and peak loads. Heavier fabric (650gsm) with this size of bolt results in a weaker and a lower bearing stiffness in comparison to the 375gsm. 6) Mixing woven layers with large differences in weight and weave will result in a non economic design with regards to the strength and stiffness of the bearing 7) Mixing fabrics with smaller “steps” between weights i.e. 200-375-650 will result in a stronger and stiffer laminate 8) Mixing plain woven fabrics with unidirectional fabrics helps to stabilize the laminate at the bearing point resulting in a very strong and stiff bearing (the highest in these tests) but the concession being the lowest factor of safety between the elastic yield and the peak load 9) The end and edge distances suggested by CP1380 and Karakuzu (3.5d) can be applied to laminates of using woven fabrics (previously only tested with UD fabrics). All laminates tested at the suggested end and edge distance failed in bearing first and had a zone of “ductile” behaviour before progressing to other forms of failure, whether block shear, tension failure or a complex mix of the previous combined with Interlamina shear. 10) Delaying the elevated temperature cure has adverse effects on the laminate bearing capacity and behaviour, this can be seen in the variation in Layup 17 11) Due to the quasi-ductile behaviour once the elastic yield stress has been exceeded i.e. the ability to sustain load for a duration with a controlled and slow displacement post elastic yield, with care and caution the lower bound theory and safe design theorem can be applied. CAUTIONARY NOTE: This is what I achieved with MY oven, MY technique etc so any critical joints should be tested and the designer/builder satisfied that the design load can be achieved. Linky to the my dissertation: https://www.dropbox.com/s/j842j0x7up38bvw/MSc%20Research%20Project%20-%20Bolted%20Connections
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