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Enhanced Composite Joint Performance through Interlacement of Metal Inserts

Time: Fri 2021-06-11 10.15

Location: Live-streaming: For physical attendance, register at least 3 days in advance by email to Stefan Hallström at, Stockholm (English)

Subject area: Aerospace Engineering

Doctoral student: Sahar Akbarpour , Lättkonstruktioner, KTH

Opponent: Prof Conor McCarthy, University of Limerick

Supervisor: Stefan Hallström, Flygteknik, Lättkonstruktioner

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The work in this thesis investigates bolted joints in fibre reinforced composites with particular focus on a novel insert concept. The concept is characterised by replacing all composite plies with stacked metal patches, locally around a bolt hole, so that they jointly form a solid metal reinforcement. An extensive experimental study is presented together with finite element analysis of the studied cases.

Reinforcing bolt holes with high-strength metals improves the bearing load capacity of the composite laminates. True enhancement of the joint performance however requires that the open-hole tensile strength is improved as well. The work started with tests of pin-loaded and open-hole tensile specimens with inserts, and significant improvement of the bearing load capacity was found. The initial tests enabled more informed design, and insert configurations having sufficient open-hole tensile strength could thereby be manufactured and tested. In parallel, composite-metal joints were numerically modelled to simulate and analyse the mechanical performance of the joints and gain a better understanding of the governing damage mechanisms.

The performance of the joints was eventually investigated by means of experiments on single-shear, single- and double-bolt specimens, with and without inserts. The allowable bolt distance and the influence from the bolt tightening torque were also examined.

The initial samples had inserts of stainless steel. Later, specimens with titanium alloy inserts were also included in the test series. Various insert configurations were designed to study the effects of different features in the composite-metal bond lines. The numerical simulations of the composite--metal interfaces were performed with two types of models, one joining the two materials directly to each other, without modelling any adhesive film in between, and the other including an elastic representation of the adhesive layer. The experimental results were then used to support verification of the results from the simulations.

The final assessment of the concept was performed on insert configurations designed either for pure tensile loading or for more general (bi-directional) loading conditions, and the bearing load capacity, open-hole tensile strength and the performance of bolted joints were compared for cases with different inserts. While higher bearing strength improvement was achieved when the holes were reinforced with inserts of stainless steel, reinforcement with inserts of titanium was even more successful since it improved virtually all studied aspects of the joints considerably.