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Enhanced Composite Joint Performance through Interlacement of Metal Inserts
KTH, School of Engineering Sciences (SCI), Engineering Mechanics, Vehicle Engineering and Solid Mechanics, Lightweight Structures. KTH.ORCID iD: 0000-0003-3482-6655
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Abstract [sv]

Arbetet som presenteras i avhandlingen undersöker bultförband i fiberarmerade kompositmaterial med särskilt fokus på ett nytt insert-koncept. Konceptet definieras av att kompositmaterialet närmast hålet ersätts med metall genom att partiellt byta ut lagren i kompositen med tunna metallskikt som tillsammans bygger upp en solid metallförstärkning. En omfattande experimentell studie är genomförd tillsammans med finita elementanalys av de studerade fallen.

 

Förstärkning med höghållfasta metaller förbättrar bärigheten hos hålen i kompositmaterialet. En komplett förstärkningseffekt kräver dock även att draghållfastheten hos den perforerade kompositen förbättras. Forskningsarbetet började med att undersöka pinn-belastade hål och draghållfastheten hos provstavar med hål och integrerade inserts, och en tydlig förbättring kunde påvisas. Tidiga tester möjliggjorde en bättre utformning av efterföljande konfigurationer så att de erhöll en tillfredsställande hållfasthet även i ren dragbelastning. Parallellt med det experimentella arbetet utvecklades numeriska modeller av de studerade konfigurationerna för att bättre kunna analysera deras mekanik och hållfasthet.

 

Kapaciteten hos förbanden undersöktes sedan genom provning av enkelskärande överlappsfogar med enkelt eller dubbla fästelement, med och utan inserts. Tillbörliga kantavstånd och effekten av olika åtdragningsmoment undersöktes också.

 

Den ursprungliga provningen utfördes med inserts gjorda av rostfritt stål. Senare testades även inserts gjorda av en titanlegering. Olika insert-konfigurationer togs fram för att studera effekterna av olika detaljer i fogarna mellan metallerna och kompositmaterialet. Datorsimuleringarna genomfördes med två olika typer av modeller; en där de olika materialen var stumt fogade till varandra, utan att modellera limfilmen emellan, och en annan som använde en elastisk beskrivning av limmet. De experimentella resultaten användes sedan för att verifiera resultaten från de numeriska simuleringarna.

 

Den slutgiltiga utvärderingen av konceptet genomfördes på konfigurationer som tagits fram antingen för renodlad dragbelastning eller för mer generell (bi-axiell) belastning. Hålkants- och draghållfastheten undersöktes tillsammans med bärfärmågan hos bultförband och jämförelser gjordes mellan fall med olika inserts. Konfigurationerna med inserts av stål uppvisade den högsta hålkantshållfastheten medan de med titan-inserts presterade bättre totalt sett. De senare uppvisade påtagliga förbättringar av samtliga undersökta egenskaper.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2021.
Series
TRITA-SCI-FOU ; 2021:22
National Category
Aerospace Engineering Composite Science and Engineering
Research subject
Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-295235ISBN: 978-91-7873-920-2 (print)OAI: oai:DiVA.org:kth-295235DiVA, id: diva2:1555543
Public defence
2021-06-11, Live-streaming: https://kth-se.zoom.us/j/62166775634 For physical attendance, register at least 3 days in advance by email to Stefan Hallström at stefanha@kth.se, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Vinnova, 2017-04877Available from: 2021-05-19 Created: 2021-05-18 Last updated: 2022-07-11Bibliographically approved
List of papers
1. Reinforcement around holes in composite materials by use of patched metal inserts
Open this publication in new window or tab >>Reinforcement around holes in composite materials by use of patched metal inserts
2019 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 225, article id 111084Article in journal (Refereed) Published
Abstract [en]

Metal inserts are sometimes used to improve the load carrying capacity of bolted joints in composite materials. In this paper a new concept is introduced where inserts are built during composite manufacturing by integrating stacked metal patches at locations where holes are to be made after consolidation. Initial tests and a parameter study enable more informed design, and specimens with improved stacked inserts are then produced and tested. The specimens with inserts show up to 60% strength improvement in pin-loaded tests. In addition to the experimental work, finite element analysis is performed to investigate the stress fields and the failure mechanisms. The model indicates that the singular stresses at the multi-material corner points are governing for the strength and give indications of the failure mechanisms. Some basic analytical estimates are also presented.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Hybrid joint, Bearing strength, Insert, Stacked, Patch
National Category
Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-257428 (URN)10.1016/j.compstruct.2019.111084 (DOI)000480330400044 ()2-s2.0-85067507808 (Scopus ID)
Note

QC 20190902

Available from: 2019-09-02 Created: 2019-09-02 Last updated: 2022-06-26Bibliographically approved
2. Strength improvement of bolted joints in composite materials by use of patched metal inserts
Open this publication in new window or tab >>Strength improvement of bolted joints in composite materials by use of patched metal inserts
2020 (English)In: Composite structures, ISSN 0263-8223, E-ISSN 1879-1085, Vol. 52, article id 112628Article in journal (Refereed) Published
Abstract [en]

Metal inserts are occasionally used to improve bearing load capacity of bolted joints in laminated composite materials. This paper investigates a new reinforcement concept where inserts are built by locally replacing composite plies with metal patches of various diameters, surrounding the holes. The inserts are built during composite manufacturing by alternately placing the metal patches through the thickness of the laminate at locations where holes are to be drilled after consolidation. An extensive experimental study including pin‐ loaded, open–hole tensile, and single‐shear testing of bolted specimens is presented. Considerable improve- ment of the bearing strength – 50‐60% – is attained for pin‐loaded specimens with inserts, demonstrating the potential of the reinforcement concept. The open–hole tensile tests show that the by‐pass strength can be maintained or even improved with up to 20% if the inserts are properly designed. Finally, the results from the single‐shear tests of bolted joints show more than 25% improvement in strength for reinforced single‐ and double‐bolt specimens. It is possible that the inserts would maintain clamping pressure over time, which could then almost double the imrovement (47%) for bolted joints.

Keywords
Fastener, Hybrid joining, Composite metal joining, Local reinforcement, Bearing strength
National Category
Aerospace Engineering Composite Science and Engineering
Research subject
Aerospace Engineering; Solid Mechanics; Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-282550 (URN)10.1016/j.compstruct.2020.112628 (DOI)000571680000009 ()2-s2.0-85088142087 (Scopus ID)
Note

QC 20201020

Available from: 2020-09-29 Created: 2020-09-29 Last updated: 2022-06-25Bibliographically approved
3. Experimental investigation and numerical analysis of multi-material interfaces related to a composite joint concept
Open this publication in new window or tab >>Experimental investigation and numerical analysis of multi-material interfaces related to a composite joint concept
2021 (English)Report (Other academic)
Abstract [en]

An insert concept for reinforcing bolt-holes with high strength metals was previously introduced by the authors, where inserts are anchored in composite laminates through interlacement of composite plies and thin metal patches. The resulting finger-joints must be strong enough to avoid composite-metal debonding happening before bearing failure at the bolt-hole. The strength of the composite-metal interfaces is thus crucial for successful implementation of the insert concept. The paper presents an experimental study investigating the strength of various interface geometries between a prepreg composite material and stainless steel or titanium alloy inserts. In addition to the experimental work, finite element simulations are performed to analyse the stresses at the interfaces. The results indicate that the stress concentrations at multi-material corner points govern the failure and that the strength can be enhanced by expedient design.

Series
TRITA-SCI-RAP ; 2021:005
National Category
Composite Science and Engineering
Research subject
Aerospace Engineering; Materials Science and Engineering; Engineering Mechanics; Fibre and Polymer Science; Solid Mechanics
Identifiers
urn:nbn:se:kth:diva-295225 (URN)
Note

QC 20210520

Available from: 2021-05-18 Created: 2021-05-18 Last updated: 2022-06-25Bibliographically approved
4. Enhancing the performance of bolted joints in composites by use of patched steel or titanium inserts
Open this publication in new window or tab >>Enhancing the performance of bolted joints in composites by use of patched steel or titanium inserts
2021 (English)Report (Other academic)
Abstract [en]

A new insert concept that interlaces metal inserts into composite laminates has earlier been shown to improve the relatively poor bearing strength of holes in fibre reinforced polymer composites, and it is here further and more thoroughly investigated. The concept was invented to increase the efficiency of joints with mechanical fasteners in composite materials and this work presents experiments on double bolt joints with inserts made of either stainless steel or a titanium (Ti) alloy. In particular the work compares different implementations of the insert concept by reinforcing one or two holes in double bolt joints, and the effect of using different metals in the inserts. Some complementary tests on pin-loaded specimens and open hole tensile specimens are also performed and compared, also with some results reported previously.                    Considerable improvements in the bearing load capacity, i.e. 50%-60% or 35%-45%, is attained. The open-hole tensile strength is also improved considerably (almost 30%)  when the holes are reinforced with Ti inserts. The fact that the inserts can improve not only the bearing strength but also the performance in open-hole tension implies that the Ti inserts bring nothing but positive effects to the strength of the joints. The test results from single-shear double-bolt specimens with inserts at one hole showed improved strengths of 30% and 20% for specimens with steel and Ti inserts, respectively. Finally, an impressive strength improvement of 40-45% is achieved for single-shear double-bolt specimens having both holes reinforced with inserts of either steel or Ti.

Series
TRITA-SCI-RAP ; 2021:006
Keywords
Bolted joint, Local reinforcement, Metal insert, Hybrid composite material, Bearing strength
National Category
Composite Science and Engineering
Research subject
Engineering Mechanics; Aerospace Engineering; Fibre and Polymer Science; Solid Mechanics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-295226 (URN)
Note

QC 20210520

Available from: 2021-05-18 Created: 2021-05-18 Last updated: 2022-06-25Bibliographically approved

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