UNIVERSITY OF CALIFORNIA Mechanical Performance of Bolted Connections

UNIVERSITY OF CALIFORNIA

Mechanical Performance of Bolted Connections of Composite Structures for Tracker Detectors G. Vallone 1, E. Anderssen 1, D. Boettcher 1, T. Claybaugh 1, C. Evans 2, J. Silber 1 1 LBNL, 2 UKRI STFC Forum on Tracking Detector Mechanics 2019 18 June 2019 06/18/19

Index • Introduction • ATLAS ITk Strip Barrel • Interlink Test • Interlink Model • Conclusion G. Vallone 3 06/18/19

Introduction • Composite structures for tracker detectors rely on high stiffness to ensure precise positioning and low motion of sensors • The structural connections are often made by gluing the components • This does not allow a lot of flexibility during assembly… • Many times we have to rely on bolted connections • These might allow, depending on the boundary conditions, relative motions between the connected components • Here we summarize: • The measured performances of bolted joints on a simplified experimental representation of a detector structure • A possible modeling approach to use the measured performances to improve the performance (stiffness) estimate from numerical models G. Vallone 4 06/18/19

ATLAS ITk Mechanical Structure G. Vallone 5 06/18/19

Strip Barrel – Design • 4 layers: shell, nine hat stiffeners, flanges at the extremities • • These components are glued together The inner layers are held by interlinks • They are connected to the flanges via bolt and pins • We want to predict, as precisely as possible, the deflection at the staves • Bolt size is not a free parameter (mass) G. Vallone 6 06/18/19

Strip Barrel - Interlink Test • Simplified representation of the flange-interlink system • We want to measure the local performances of the joints • Aluminum frame design: • Everything is as symmetric as possible • Stiffness – its own deformation is not part of the experiment… G. Vallone 7 06/18/19

Interlink Test – Experimental Set-Up • Three test configurations: • Load normal to the interlinks plane (normal) • Load in the interlinks plane (parallel) • Radial load • Interlinks in aluminum and CFRP • Two torque levels G. Vallone 8 06/18/19

Results - Static Loading • Applied torque: • 32 lb in is ~85% of the bolt yield (Ti) • Difficult to measure the bolt elongation (too short) • Normal loading: no impact of the applied torque • Small dissipation • Parallel load shows non-linearity • Stronger effect for CFRP: lower friction? G. Vallone 9 06/18/19

Results – Radial Loading • Radial loading (blue arrow) • Max deflection: 30 µm • Expected deflection ~10 µm • The load is not uniformly distributed on all the interlinks G. Vallone 10 06/18/19

Results – Dynamic Loading • Impulse excitation, response measured with a capacitive probe • Aluminum interlinks test ongoing – will be interesting to see the influence of the torque on the dissipation • Fundamental frequency (normal, parallel): 60. 2 Hz, 250 Hz G. Vallone 11 06/18/19

Interlink Test – FE Model • Interlinks modelled as shells (ACP): • (0)(90)(± 45)(90)(0) • Material properties from bending tests performed at LBNL • Remaining elements as bricks • Fixture/interlink joint stiffness can be introduced via contact stiffness or elastic joints • Elastic constants can be calibrated against the experimental measurements • Calibrated values can then be used in the strip barrel model (scripted) G. Vallone 12 06/18/19

FE Model - Methodologies • The bolted joint stiffness can be introduced as: • • Elastic contacts (e. g. normal/tangential stiffness) MPC joints + elastic elements: • Complete stiffness matrix • Can be defined via detailed FE models of the bolted connection • Difficult to extract all the parameters from an experiment G. Vallone 13 06/18/19

Strip Barrel – FE Model Calibration • MPC joints used: • Bolts: torsional + longitudinal • Pin: cylindrical joint • FE calibration against the measured data (parallel loading deflection) • We need only two constants (longitudinal and torsional stiffness) • In our case we can assume a perfect longitudinal joint G. Vallone 14 06/18/19

Interlink Test - FE Model Material Al Al CFRP Torque in lbs 16 32 16 Weight kg 2 2 2 Force N 19. 6 Max Delta Meas. FE mm mm 1. 571 1. 48 1. 558 1. 48 1. 552 1. 55 Error Material % 6% 5% 0% Normal • • • Torque Weight Force in lbs kg N Al 16 Al Max Delta FE mm mm % 11. 2 109. 9 0. 85 0. 55 55% 32 11. 2 109. 9 0. 57 0. 55 5% CFRP 16 11. 2 109. 9 1. 05 0. 47 123% CFRP 32 11. 2 109. 9 0. 77 0. 47/0. 77 65%/0% Parallel Normal load: • Reasonable correlation (5%) with the FE results for Aluminum links • Experimental stiffness is lower Parallel load: • Good correlation only for the higher torque case on Aluminum links • Experimental stiffness is lower, probably due to slipping Dynamic: • Computed: 278 Hz, 63 Hz • Measured: 250 Hz, 60 Hz G. Vallone 15 Error Meas. 06/18/19

Conclusion • The test showed the impact on structural stiffness of the bolted connections • The connection has different impact on different loading directions • Noticeable only in the rotational degree of freedom • Measured deflection was ~2 times the one from a perfect joint • The performance is a function of the applied prestress • The material/surface roughness has an impact on the results • The interlink test provided data to calibrate FE models • The experimental behavior can be reproduced with MPC + elastic elements G. Vallone 16 06/18/19