Precision and Dimensional Stability of Bonded Joints of Carbon-Fibre-Reinforced Polymers Parts

Citation: Kupˇcák, R.; Zouhar, J.; Viliš, J.; Gregor, L.; Hrušecká, D. Precision and Dimensional Stability of Bonded Joints of Carbon-Fibre-Reinforced Polymers Parts. Appl. Sci. 2023,13, 10413. https://doi.org/10.3390/ app131810413 Academic Editors: Lubos Kascak, Emil Spišák and Jacek Mucha Received: 7 August 2023 Revised: 10 September 2023 Accepted: 16 September 2023 Published: 18 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Precision and Dimensional Stability of Bonded Joints of Carbon-Fibre-Reinforced Polymers Parts Radim Kupˇcák1, Jan Zouhar 1,* , Jindˇrich Viliš 2, Lukáš Gregor 1and Denisa Hrušecká3 1 Faculty of Mechanical Engineering, Brno University of Technology, Technická2896/2, 616 69 Brno, Czech Republic; [email protected] (R.K.) 2Faculty of Military Technology, University of Defence, Kounicova 65, 662 10 Brno, Czech Republic; [email protected] 3Faculty of Management and Economics, Tomas Bata University in Zlín, Nám. T. G. Masaryka 5555, 760 01 Zlín, Czech Republic *Correspondence: [email protected].cz Abstract: This article aims to investigate the accuracy and dimensional stability of bonded metal and CFRP (Carbon Fibre Reinforced Plastic) adherends. The motivation behind this study was to assess the suitability of CFRP for optical devices through the evaluation of precision bonding technology. A binocular was selected as a reference optical device. A technological sample was designed, with required total runout of key dimensions 0.05 mm. The sample underwent testing according to ISO 9022-1. The total runout was evaluated after production and environmental tests. Eight out of 15 samples were turned after gluing due to insufficient accuracy. None of the turned samples exceeded the total runout deviation of 0.01 mm, and the average value of the maximal deviation was 0.0041 mm. The noncalibrated samples performed significantly worse with the average value of the maximal deviation of 0.0164 mm. The measurements during the climatic tests showed that the largest deviation (on average 77.6% of the maximum achieved deviation) occurs at the first temperature loading. Subsequent temperature cycles caused lower deviations. The results highlight the significance of addressing deformations resulting from adhesive volume shrinkage-induced stress as a crucial factor in precision bonding technology. Keywords: precision bonding; epoxy; stability; carbon fibre; composite materials; sports optics 1. Introduction The popularity and use of fibre-reinforced composite materials have been growing year by year. The demand for composite materials tripled between 2010 and 2020 [ 1 ]. Aerospace, aviation, and energy applications have been the sectors with the largest share of both the volume produced and usage of composite materials. Currently, the COVID-19 pandemic has negatively affected certain sectors including aerospace [ 2 ] and the ground transportation industry. On the other hand, the energy sector has experienced significant growth. This growth is influenced by various factors, with one of the most significant factors being the widespread adoption of wind power in China, aligning with China’s planned path towards carbon neutrality by 2060 [ 2 , 3 ]. In the long term, the use of composites has grown within consumer goods and for the production of leisure equipment, with cycling, golf and fishing being the most prominent [3]. One of the reasons for using CFRP in many industries is to achieve high stiffness while maintaining low component weight [ 4 ]. Another important property of composite materials is good vibration damping, which in some applications leads to increased user comfort and reduced equipment noise. Researchers have been also dealt with hybrid composites reinforced by different types of fibres. Zouhar et al. [ 5 ] found that the application of flax fibre-reinforced hybrid composites increases damping properties compared to pure carbon-reinforced composites. Appl. Sci. 2023,13, 10413. https://doi.org/10.3390/app131810413 https://www.mdpi.com/journal/applsci Appl. Sci. 2023,13, 10413 2 of 19 Despite the aforementioned benefits, the application of CFRPs in the production of sports optics or other optical instruments remains relatively rare. One of the primary obstacles hindering the adoption of composite materials in optical devices is the limited manufacturing precision of composite parts [ 6 ]. In some cases, limited impact resistance and the absence of plastic deformation are the main limitations [ 7 ]. In other cases, it may be necessary to consider outgassing parameters, which is currently a complex topic with limited knowledge in the context of composites [ 8 ]. However, some papers suggest that CFRP could find applications in space and cleanroom environments [ 9 ]. If these issues can be resolved, composite materials could offer numerous advantages for optical devices, namely, weight savings, better vibration damping, and improved dimensional stability. In some cases, they can also save on costs or enable the manufacturing of complex parts. Finally, composites possess a certain appeal to consumers, which, especially in the case of consumer goods, plays a crucial role in determining the product’s commercial success. In particular, carbon fibre composite materials (CFRP) are considered premium and high-tech materials widely employed in aerospace and Formula 1. Under certain conditions, usage of CFRP as a construction material can result in cost savings compared to technologies like die-casting. Manufacturing precision and deformation elimination are key requirements for CFRP applications in optics. Kupˇcák et al. [ 6 ] address the evaluation of deformation during the production of a composite tubular part made by curing prepregs in an autoclave with a prototype mold. There are several strategies to eliminate deformation during composite manufacturing [ 10 ]. Changing the mold shape for compensation does not require modification of the curing process or the lay-up, but entails the production of a new mold, as it is a trial-and-error approach. Another option is to utilize a numerical model introduced by Triforia et al. [ 11 ]. Optimization of curing is another strategy to eliminate deformations. Several authors [ 12 – 14 ] have studied the relationship between spring-back during curing and dwell time at the target temperature in the autoclave, with mixed results. However, there is consensus within the scientific community that lower deformation occurs in the laminate with slower heating and cooling rates [ 14 , 15 ] as well as lower curing temperatures and longer curing times [ 12 , 16 ], albeit with increased manufacturing costs. Optimizing the contact between a part and mold also demands attention. Due to the different thermal expansions of the mold and laminate, shear stresses develop in the contact area between the part and mold, resulting in unwanted spring-back. The application of a Fluorinated Ethylene Propylene (FEP) sliding layer can reduce the shear friction and the degree of spring-back [ 17 ]. Optimizing the lay-up and prestressing the laminate during manufacturing are additional methods to enhance the discussed contact area. The dimensional stability of composites is another important requirement for CFRP applications in many fields including optics. The fibrous structure of composite materials results in their anisotropy. Through a well-designed lay-up, mechanical properties can be customized in specific directions of the laminate, such as achieving higher stiffness in the direction under greater stress. It is even possible to attain zero or negative thermal expansion in specific directions. This has been investigated by Kelly [ 18 ] and Ito et al. [ 19 ], who describe both analytical and experimental methods. Yoon and Kim [ 20 ] studied the anisotropic behavior and thermal expansion of composite materials. They observed the relationship between fibre angle and deformation under thermal loading using ‘Lshaped’ specimens and provided a comparison between experimental measurements and an analytical model. The coefficient of thermal expansion of carbon tubes is addressed by Cordero et al. [ 21 ], primarily focusing on length expansion and the development of a measurement method. Another critical factor to consider regarding the stability of composite materials is deformation associated with moisture absorption. Observations by Trigo [ 22 ] and Arao et al. [23] indicated that moisture absorption-induced deformation in the laminate significantly surpasses that caused by thermal loading. Arao et al.’s findings suggest that, for epoxy resins, the behavior aligns with Fick’s first law. Appl. Sci. 2023,13, 10413 3 of 19 Due to the difficult machinability of composites, their application often requires bonding as an alternative method to reach the required shape of the final product. Composite materials based on thermosetting resin allow the use of three subtypes of bonding technology, the so-called secondary bonding, co-bonding, and co-curing. According to the research of Hasan et al. [ 24 ] and Moretti et al. [ 25 ], co-curing achieves the lowest manufacturing deformation among these bonding technologies. In the publications by Dhilipkumar and Rajesh [26] and Kim et al. [27], the strength of various types of bonded joints is compared. Both authors concluded that co-curing produces joints with the highest strength. Based on these findings, co-curing can be considered the most suitable method for joining composite components if the device design allows for it. Leone and Genna [ 28 ] investigated the increased adhesion after laser surface treatment of co-bonded samples. The lasertreated specimens achieved twice the shear strength compared to the reference specimen. Laser surface treatment has also been investigated by Fisher et al. [ 29 ] in the context of secondary-bonded specimens. Their results indicate that within secondary bonding, the abrasive-treated surface achieves the same results as the laser-treated surface. The view on the influence of the thickness of the bonded joint on its strength has evolved and changed throughout history. Classical analytical models according to Volkersen [ 30 ] or Goland and Reissner [ 31 ] assumed that higher bonded joint thickness leads to increased strength. However, more recent research has shown that the optimum strength is achieved by a bonded joint at the thickness of 0.1 mm to 0.5 mm [ 32 ]. Da Silva et al. confirmed these findings in their experimental study of epoxy adhesives [ 33 ]. Similar results were also obtained by Zhang and Huang [ 34 ] and Shokrian et al. [ 35 ], who investigated the impact of adherend roughness in addition to epoxy adhesive layer thickness. However, some research indicates even lower values of the optimal thickness. In their study, Guo et al. [36] identified an optimal epoxy adhesive thickness of 0.03 mm and noted a significant reduction in strength when the adhesive thickness falls below 0.01 mm. A critical phenomenon associated with bonding is the volumetric shrinkage of the adhesive during curing. This is particularly significant in precise and optical applications, where shrinkage can lead to tilt or displacement of optical elements [ 37 ] or induce stresses inside the optical elements, degrading the imaging quality of the optical instrument [ 38 ]. UV-cured adhesives are often preferred due to their nearly instantaneous curing ability, which is especially valuable in assembling micro-optical systems [ 39 ]. Broquin et al. [ 40 ] presented soldering as a possible alternative to bonding lenses with the benefit of lower residual stress values. Müller et al. [ 41 ] addressed the bonding procedure for assemblies requiring accuracy below 1 µ m. Their approach involves compensating for shrinkage effects by precisely dosing and measuring the shrinkage of a specific batch of adhesive. This is because shrinkage is affected not only by the specific batch of adhesive (owing to variations in raw materials) but also by the adhesive’s age. For successful automation, both influences must be quantified. During the experimental phase, Müller observes differences in shrinkage of up to 5% for one of the adhesives. To automate this process, accurate volumetric dosing and measuring the shrinkage before a given adhesive batch is always necessary [ 41 ]. A different approach to precision bonding is presented by Niklaus et al. [ 42 ] focusing on bonding silicon wafers. They suggested employing friction elements to prevent the wafer from sliding during adhesive curing. Through this method, they consistently achieved an accuracy of 5 µ m, surpassing the 15–50 µ m accuracy typically attained using commercially available add-ons. The potential integration of carbon nanotubes (CNTs) represents a promising avenue for enhancing various aspects of composite materials. Carbon nanotubes are renowned for their exceptional mechanical properties and thermal conductivity, rendering them valuable candidates as reinforcing phases in adhesives for optical devices and precision bonding applications. Recent studies by Kiani and Pakdaman [ 43 – 45 ] as well as previous works by Kianis [ 46 – 48 ] have explored the nonlocal behavior of CNTs, demonstrating their unique mechanical characteristics. In the realm of thermal properties, CNTs exhibit remarkable Appl. Sci. 2023,13, 10413 4 of 19 thermal conductivity [ 49 ]. Additionally, CNTs have garnered attention in the field of flexible electronics and advanced packaging materials [ 50 ], showcasing their versatility in various applications. Furthermore, CNTs have been the subject of extensive research in the field of electrical conductivity within polymer nanocomposites. The work of Khromov et al. [ 51 ] underscores the potential for improved electrical properties when CNTs are integrated into composite materials. Moreover, the ultra-high optical absorption efficiency of multi-walled carbon nanotube ensembles [ 52 ] suggests their potential for use in optical instruments. In summary, the incorporation of carbon nanotubes offers a range of advantages, from enhanced mechanical properties to improved thermal and electrical conductivity, making them a promising candidate for optimizing composite materials in the domain of optical devices and precision bonding applications. These promising attributes of CNTs open up exciting opportunities for advancing the performance and functionality of such devices. The research presented in this paper is motivated by the efforts of Meopta—optika, s.r.o. to integrate CFRP into the production of optical devices, particularly those demanding a high degree of precision and accuracy. CFRP is commonly chosen for its high stiffness, lightweight properties, low coefficient of thermal expansion (CTE), and effective damping characteristics. Nevertheless, the utilization of CFRPs in the production of sports optics and other optical instruments remains relatively uncommon. One way to integrate CFRP into the manufacturing of precise devices is through precision bonding. The optical industry frequently addresses precision bonding, primarily in the context of bonding optical components to mechanical interfaces, typically employing UV-cured adhesives [ 37 , 39 , 53 ]. However, there is a gap in the research investigating the high-precision bonding of composite structures using epoxy adhesives. This paper builds on previous research on this topic [ 6 , 54 ]. The potential of CFRP in the production of optical devices was evaluated with the aim of addressing its limitations. A technological sample and testing procedure were designed in order to test the manufacturing accuracy and dimensional stability under specified environmental conditions. The aspect of dimensional stability of bonded joints in outdoor conditions is frequently underrepresented in the literature. The technology presented in this research has the potential to introduce the benefits of CFRP, including high stiffness, low weight, and a low coefficient of thermal expansion, which are well-established in other industries, to the realm of sports optics and other precision optical applications. 2. Materials and Methods The main goal of this study was to investigate the accuracy and dimensional stability of tubular-bonded joints of metal and CFRP adherends, aiming to evaluate the suitability of CFRP for the optical devices industry. ISO 9022-1:2016 defines an optical instrument as a device whose function is based on optical phenomena [ 55 ]. This definition encompasses a wide range of devices, from specialized laboratory equipment and stargazing telescopes to simpler tools like fibre optics and magnifying glasses. For our research purposes, a reference instrument in the form of a sports optics device, primarily a binocular or spotting scope, was chosen based on several considerations. In comparison to more sophisticated optical devices, the accuracy requirements are significantly lower (although still quite high in the context of conventional engineering), increasing the likelihood of successful production. The testing conditions were established by ISO 14490-6:2014 [ 56 ] and ISO14490-7:2016 [ 57 ] for testing of optical parameters and by ISO 9022-1:2016 [ 55 ], ISO 9022-2:2015 [ 58 ], ISO 9022-3:2015 [ 59 ] and ISO 9022-8:2015 [ 60 ] for testing of mechanical properties and climatic resistance. 2.1. Material Preparation Considering that optical instruments almost always feature rotationally symmetric lenses, the mechanical components are typically also rotationally symmetric. Therefore, winding or braiding technology may seem like a suitable choice. Parts that deviate from a cylindrical shape present challenges for winding, primarily due to fibre slippage on Appl. Sci. 2023,13, 10413 5 of 19 contoured surfaces [ 61 ] or the bridging of concave surfaces [ 62 ]. Another viable option is one of the manual laying technologies (wet laying, prepreg, infusion). These technologies offer greater flexibility in terms of manufactured shapes, but they are more labor-intensive compared to winding [63]. Despite the aforementioned limitations, winding technology was selected for the technological samples, primarily due to the availability of a wide range of commercially wound cylindrical tubes. The technological sample (Figure 1) consists of a wound quasiisotropic CFRP tube made of [90 ◦ /0 ◦ / ± 30 ◦ ] layers of T700 12k fibres with a length of 90 mm, an inner diameter of 50 mm, and a wall thickness of 1.5 mm. The tube was wound using a multi-axis winding machine and cured at room temperature with subsequent post-curing in an oven. The tube was supplied by a third party along with a technical datasheet, which indicated the following mechanical properties: Axial Young’s Modulus 66.6 MPa, Bending Stiffness 4.4 × 10 9 N · mm 2 , and coefficient of thermal expansion in axial direction 1.2 × 10 −6 K −1 No further specifications are available since the material was acquired from a third party. This tube was chosen based on matching dimensions with the requirements of the reference device and, as there were no increased stiffness requirements in this particular case, the quasi-isotropic pan-fibre composition is sufficient. The CFRP tube is bonded to anodized EN_AW-6061-T651 aluminium alloy sleeves, which represent the interface for mounting optical assemblies, such as objective and eyepiece of the Keplerian telescope. A 1 mm wide gluing gap allows for compensation of manufacturing inaccuracies of the carbon components. The precise alignment of the two aluminium alloy sleeves, in accordance with the specified tolerances defined by a total runout of the inner diameters of sleeves of 0.05 mm, is ensured by precise bonding or turning after bonding. Appl.Sci.2023,13,xFORPEERREVIEW5of20  2.1.MaterialPreparation Consideringthatopticalinstrumentsalmostalwaysfeaturerotationallysymmetric lenses,themechanicalcomponentsaretypicallyalsorotationallysymmetric.Therefore, windingorbraidingtechnologymayseemlikeasuitablechoice.Partsthatdeviatefrom acylindricalshapepresentchallengesforwinding,primarilyduetofibreslippageoncon‐ touredsurfaces[61]orthebridgingofconcavesurfaces[62].Anotherviableoptionisone ofthemanuallayingtechnologies(wetlaying,prepreg,infusion).Thesetechnologiesoffer greaterflexibilityintermsofmanufacturedshapes,buttheyaremorelabor‐intensive comparedtowinding[63]. Despitetheaforementionedlimitations,windingtechnologywasselectedforthe technologicalsamples,primarilyduetotheavailabilityofawiderangeofcommercially woundcylindricaltubes.Thetechnologicalsample(Figure1)consistsofawoundquasi‐ isotropicCFRPtubemadeof[90°/0°/±30°]layersofT70012kfibreswithalengthof90 mm,aninnerdiameterof50mm,andawallthicknessof1.5mm.Thetubewaswound usingamulti‐axiswindingmachineandcuredatroomtemperaturewithsubsequent post‐curinginanoven.Thetubewassuppliedbyathirdpartyalongwithatechnical datasheet,whichindicatedthefollowingmechanicalproperties:AxialYoung’sModulus 66.6MPa,BendingStiffness4.4∙10 9 N∙mm 2 ,andcoefficientofthermalexpansioninaxial direction1.2∙10 −6 K −1 Nofurtherspecificationsareavailablesincethematerialwasac‐ quiredfromathirdparty.Thistubewaschosenbasedonmatchingdimensionswiththe requirementsofthereferencedeviceand,astherewerenoincreasedstiffnessrequire‐ mentsinthisparticularcase,thequasi‐isotropicpan‐fibrecompositionissufficient.The CFRPtubeisbondedtoanodizedEN_AW‐6061‐T651aluminiumalloysleeves,whichrep‐ resenttheinterfaceformountingopticalassemblies,suchasobjectiveandeyepieceofthe Kepleriantelescope.A1mmwidegluinggapallowsforcompensationofmanufacturing inaccuraciesofthecarboncomponents.Theprecisealignmentofthetwoaluminiumalloy sleeves,inaccordancewiththespecifiedtolerancesdefinedbyatotalrunoutoftheinner diametersofsleevesof0.05mm,isensuredbyprecisebondingorturningafterbonding.  Figure1.Geometryofatechnologicalsample. Anassemblyfixturewasdesignedforbondingthetechnologicalsamplewiththere‐ quiredprecision.(Figure2). Figure 1. Geometry of a technological sample. An assembly fixture was designed for bonding the technological sample with the required precision (Figure 2). Appl.Sci.2023,13,xFORPEERREVIEW6of20   Figure2.Anassemblyfixture. FortheproposedtechnologytobearelevantalternativetoCNCmachining,itmust achievecomparableaccuracies.Thetotalrunoutoftheinnerdiametersofbothsleeves wasidentifiedasacriticalparameter,withamaximumallowablevaluesetat0.05mm (Figure1).Theserequirementsmustbemetbothpost‐productionandfollowingaseries ofmechanicalandclimatictests,asdetailedinthesubsequentchapter. 2.2.DesignofExperiments Toensurealogicalstructureforthetestsandexperiments,itwasessentialtoestablish amethodologyandscheduletobeadheredtoduringtheexperiments.Theprocesseswere dividedintothreestages: 2.2.1.Stage1=DesignandPreparationofTechnologicalSamples Thefirststageconcentratedonmaterialselectionandthedesignofthesampleand assemblyfixture.Subsequently,thesamplesweremanufacturedthroughtheturningof metalparts,shorteningofcarbontubes,andtheassemblyofthesamplesusingtheassem‐ blyfixture. 2.2.2.Stage2=VerificationofDimensionsandGeometricTolerances TheinspectionofthepieceswascarriedoutusingCMMZeissConturaG2,Formline Roundscan555,andasetofOK/NOKgauges. 2.2.3.Stage3=VerificationoftheDimensionalStabilityofaBondedJointunderOperat‐ ingConditions Duringthefinalstage,sampleswithunsatisfactorytoleranceswererectifiedbyturn‐ ingandtestedagainaccordingtothemethodologyofstage2.Subsequently,climaticre‐ sistancetestsbasedonISO9022‐2:2015wereconductedusingtheclimaticchamberVötsch VT37030S2. Initially,thetechnologicalsamplewasplacedintoaclimaticchambertemperedto +70±2°Candheldatthistemperaturefor16h.Then,thesameprocesswasrepeatedat temperaturesof−40±3°C.Thefinalclimaticresistancetestwasfocusedonthermalshock testing,duringwhichthetechnologicalsamplewassubjectedtothermalshocksattem‐ peraturesof—25°C/+40°Cin6cyclesandchangetimenotlongerthan20s. Followingallclimaticresistancetests,thedimensionsandgeometrictolerancesofthe sampleswereverifiedagainusingthesameequipmentandmeasurementmethodsasin stage2.Thefinalsetofmechanicalresistancetests,asperISO9022‐3:2015,includeda Figure 2. An assembly fixture. Appl. Sci. 2023,13, 10413 6 of 19 For the proposed technology to be a relevant alternative to CNC machining, it must achieve comparable accuracies. The total runout of the inner diameters of both sleeves was identified as a critical parameter, with a maximum allowable value set at 0.05 mm (Figure 1). These requirements must be met both post-production and following a series of mechanical and climatic tests, as detailed in the subsequent chapter. 2.2. Design of Experiments To ensure a logical structure for the tests and experiments, it was essential to establish a methodology and schedule to be adhered to during the experiments. The processes were divided into three stages: 2.2.1. Stage 1 = Design and Preparation of Technological Samples The first stage concentrated on material selection and the design of the sample and assembly fixture. Subsequently, the samples were manufactured through the turning of metal parts, shortening of carbon tubes, and the assembly of the samples using the assembly fixture. 2.2.2. Stage 2 = Verification of Dimensions and Geometric Tolerances The inspection of the pieces was carried out using CMM Zeiss Contura G2, Formline Roundscan 555, and a set of OK/NOK gauges. 2.2.3. Stage 3 = Verification of the Dimensional Stability of a Bonded Joint under Operating Conditions During the final stage, samples with unsatisfactory tolerances were rectified by turning and tested again according to the methodology of stage 2. Subsequently, climatic resistance tests based on ISO 9022-2:2015 were conducted using the climatic chamber Vötsch VT3 7030 S2. Initially, the technological sample was placed into a climatic chamber tempered to +70 ± 2 ◦ C and held at this temperature for 16 h. Then, the same process was repeated at temperatures of − 40 ± 3 ◦ C. The final climatic resistance test was focused on thermal shock testing, during which the technological sample was subjected to thermal shocks at temperatures of −25 ◦C/+40 ◦C in 6 cycles and change time not longer than 20 s. Following all climatic resistance tests, the dimensions and geometric tolerances of the samples were verified again using the same equipment and measurement methods as in stage 2. The final set of mechanical resistance tests, as per ISO 9022-3:2015, included a shock test, a bump test utilizing TIRAshock 4110, and a sinusoidal vibration test employing a Derritron TW 6000 device. Subsequent to the completion of all mechanical resistance tests, the dimensions and geometric tolerances of the samples were verified according to the methodology of stage 2. 2.3. Selection of Adhesives Epoxy adhesives are widely used in composite materials [ 7 ]. Specific adhesives were selected according to the following criteria: •the selected adhesive is suitable for bonding metals and thermoplastics; •it has high environmental resistance (−40 ◦C to +70 ◦C); •and low shrinkage (although most manufacturers do not specify this parameter). Finally, 5 adhesives that fulfilled all the above-mentioned criteria were selected and used in our experiments, more specifically, 3 types of 3M Scotch-Weld TM Epoxy Adhesive (DP110 Gray, DP125 Gray, and DP190 Gray), Gurit Spabond 340 LV HT (further just Spabond 340 LV), and Stachema Eprosin Flex (further just Eprosin Flex). Their technical properties are presented in Table 1. Appl. Sci. 2023,13, 10413 7 of 19 Table 1. Selected adhesives and their properties [64–68]. Adhesive Type Working Time Clamping Time Cure Time Viscosity at Room Temperature Density Shear Strength Volumetric Shrinkage [min] [min] [Days] [Pa.s] [g/cm3][MPa] [%] DP110 9–15 20 2 65.0 1.12 17 not specified DP125 25 150 1 52.5 1.13 13.4 not specified DP190 90 480 7 86.3 1.31 11.7 not specified Spabond 340 LV 45 1060 28 264 1.12 29 1.91–1.94 Eprosin Flex 70 1440 7 15 1.55 15.9 not specified 3. Results According to the prescribed methodology and schedule, 15 samples were produced (Figure 3). Their purpose was to evaluate the manufacturing precision and dimensional stability of the bonded joint under environmental conditions specified in Section 2.2. Appl.Sci.2023,13,xFORPEERREVIEW7of20  shocktest,abumptestutilizingTIRAshock4110,andasinusoidalvibrationtestemploy‐ ingaDerritronTW6000device.Subsequenttothecompletionofallmechanicalresistance tests,thedimensionsandgeometrictolerancesofthesampleswereverifiedaccordingto themethodologyofstage2. 2.3.SelectionofAdhesives Epoxyadhesivesarewidelyusedincompositematerials[7].Specificadhesiveswere selectedaccordingtothefollowingcriteria:  theselectedadhesiveissuitableforbondingmetalsandthermoplastics;  ithashighenvironmentalresistance(−40°Cto+70°C);  andlowshrinkage(althoughmostmanufacturersdonotspecifythisparameter). Finally,5adhesivesthatfulfilledalltheabove‐mentionedcriteriawereselectedand usedinourexperiments,morespecifically,3typesof3MScotch‐WeldTMEpoxyAdhesive (DP110Gray,DP125Gray,andDP190Gray),GuritSpabond340LVHT(furtherjustSpa‐ bond340LV),andStachemaEprosinFlex(furtherjustEprosinFlex).Theirtechnicalprop‐ ertiesarepresentedinTable1. Table1.Selectedadhesivesandtheirproperties[64–68]. AdhesiveTypeWorking Time Clamping Time Cure Time ViscosityatRoom TemperatureDensityShear Strength Volumetric Shrinkage [min][min][Days][Pa.s][g/cm3][MPa][%] DP1109–1520265.01.1217notspecified DP12525150152.51.1313.4notspecified DP19090480786.31.3111.7notspecified Spabond340LV451060282641.12291.91–1.94 EprosinFlex7014407151.5515.9notspecified 3.Results Accordingtotheprescribedmethodologyandschedule,15sampleswereproduced (Figure3).Theirpurposewastoevaluatethemanufacturingprecisionanddimensional stabilityofthebondedjointunderenvironmentalconditionsspecifiedinSection2.2.  Figure3.Geometryofatechnologicalsample. 3.1.VerificationofManufacturingAccuracyafterPrecisionBonding Theprimaryevaluationparameterformanufacturingprecisionwasthetotalrunout ofØ40relativetobaseB(Figure1).SampleinspectionwasconductedusingaZeiss ConturaG23DCMMandaFormlineRoundscan555(Figure4).Themeasuredvaluesare Figure 3. Geometry of a technological sample. 3.1. Verification of Manufacturing Accuracy after Precision Bonding The primary evaluation parameter for manufacturing precision was the total runout of Ø40 relative to base B (Figure 1). Sample inspection was conducted using a Zeiss Contura G2 3D CMM and a Formline Roundscan 555 (Figure 4). The measured values are presented in the graph (Figure 5). Out of the 15 samples, seven (46.6%) achieved the required accuracy of 0.05 mm. Appl.Sci.2023,13,xFORPEERREVIEW8of20  presentedinthegraph(Figure5).Outofthe15samples,seven(46.6%)achievedthere‐ quiredaccuracyof0.05mm.  Figure4.SampleinspectionusingFormlineRoundscan555.  Figure5.Valuesoftotalrunout. Table2illustratesthatnoneoftheadhesivesresultedina100%successrateforthe samples.Notably,adhesiveDP125didnotproduceasinglesamplewiththerequired precision,whichcouldinitiallysuggestunsatisfactoryperformancewhencomparedto theotheradhesivetypes.However,itisimportanttoconsiderthesmallstatisticalsample size,whichmayintroducedeviationsduetofactorssuchasincorrectassembly(particu‐ larlyevidentinthehighdeviationforDP125samples)orsuboptimaladhesiveapplica‐ tionsresultinginthepresenceofbubbles/voidsinthebondedjoint.Whenexaminingthe averagedeviations,DP125andDP190adhesivesexhibitlowerperformance,whileDP 110,Eprosin,andSpabondachievecomparableresults. Table2.Post‐bondingaccuracyrate. AdhesiveTypeAccuracyRateTheAverageValueofTotal Runout[mm] DP1102/30.041 DP1250/30.159 DP1901/30.074 Spabond2/30.051 Eprosin2/30.041 Figure 4. Sample inspection using Formline Roundscan 555. Appl. Sci. 2023,13, 10413 8 of 19 Appl.Sci.2023,13,xFORPEERREVIEW8of20  presentedinthegraph(Figure5).Outofthe15samples,seven(46.6%)achievedthere‐ quiredaccuracyof0.05mm.  Figure4.SampleinspectionusingFormlineRoundscan555.  Figure5.Valuesoftotalrunout. Table2illustratesthatnoneoftheadhesivesresultedina100%successrateforthe samples.Notably,adhesiveDP125didnotproduceasinglesamplewiththerequired precision,whichcouldinitiallysuggestunsatisfactoryperformancewhencomparedto theotheradhesivetypes.However,itisimportanttoconsiderthesmallstatisticalsample size,whichmayintroducedeviationsduetofactorssuchasincorrectassembly(particu‐ larlyevidentinthehighdeviationforDP125samples)orsuboptimaladhesiveapplica‐ tionsresultinginthepresenceofbubbles/voidsinthebondedjoint.Whenexaminingthe averagedeviations,DP125andDP190adhesivesexhibitlowerperformance,whileDP 110,Eprosin,andSpabondachievecomparableresults. Table2.Post‐bondingaccuracyrate. AdhesiveTypeAccuracyRateTheAverageValueofTotal Runout[mm] DP1102/30.041 DP1250/30.159 DP1901/30.074 Spabond2/30.051 Eprosin2/30.041 Figure 5. Values of total runout. Table 2illustrates that none of the adhesives resulted in a 100% success rate for the samples. Notably, adhesive DP 125 did not produce a single sample with the required precision, which could initially suggest unsatisfactory performance when compared to the other adhesive types. However, it is important to consider the small statistical sample size, which may introduce deviations due to factors such as incorrect assembly (particularly evident in the high deviation for DP 125 samples) or suboptimal adhesive applications resulting in the presence of bubbles/voids in the bonded joint. When examining the average deviations, DP 125 and DP 190 adhesives exhibit lower performance, while DP 110, Eprosin, and Spabond achieve comparable results. Table 2. Post-bonding accuracy rate. Adhesive Type Accuracy Rate The Average Value of Total Runout [mm] DP 110 2/3 0.041 DP 125 0/3 0.159 DP 190 1/3 0.074 Spabond 2/3 0.051 Eprosin 2/3 0.041 The technological suitability of the adhesives is an important factor as well, determined by parameters such as processing time, viscosity, mixability, and the tendency to leak out of the bonded joint. These parameters are detailed in Table 3. Table 3. Technological suitability of adhesives. Adhesive Type Working Time [min] Viscosity at Room Temperature [Pa.s] Ease of Mixing Tendency to Leak out DP110 9–15 65.0 Good Medium DP125 25 52.5 Good Medium DP190 90 86.3 Good Medium Spabond 340 LV 45 264 Harder to mix because of higher viscosity Very low Eprosin Flex 70 15 Very good, for best results the adhesive needs to be partially cured when applying Very high Appl. Sci. 2023,13, 10413 9 of 19 Based on the conducted experiments and measurements, the following conclusions can be drawn: • A total of 46.6% of the samples exhibited a total runout deviation below 0.05 mm, meeting the specified requirements and thus were considered satisfactory. • The noncompliant samples likely experienced stress generation during the curing process, leading to a radial shift after removal from the assembly fixture. Potential stress sources include excessive adhesive shrinkage and uneven filling of the adhesive joint, which may be further verified through a CT scan. • Among the tested adhesives, DP 110, Eprosin, and Spabond yielded the most favorable results and appear to be suitable choices for these bonding applications. • Enhancing the accuracy of the assembly fixture and the precision of metal counterparts could lead to increased sample accuracy, consequently improving the guaranteed geometric precision. 3.2. Verification of Dimensional Stability of Bonded Joint When assessing the feasibility of using composite materials in optical instruments, it is crucial to consider not only manufacturing precision but also the dimensional stability during the instrument’s operation. The aim of stage three of the study is to experimentally verify the dimensional stability of bonded joints during environmental testing based on the standards ISO 9022-1:2016, [ 55 ] ISO 9022-2:2015, [ 58 ] ISO 9022-3:2015 [ 59 ] and ISO 9022-8:2015 [60]. The noncompliant samples from the previous stage required calibration by turning to achieve the necessary accuracy before undergoing environmental testing. As shown in Figure 6, the total runout of all the samples met the required standards after calibration, as indicated by the highlighted portion of the columns in comparison to the previous values in lighter color. Appl.Sci.2023,13,xFORPEERREVIEW10of20   Figure6.Valuesoftotalrunoutaftercalibration. Followingthecalibrationofthesamples,twocyclesofthermalresistancetestsand onecycleofmechanicalresistancetestswerecarriedout.Themeasuredtotalrunoutval‐ uesaftereachtestaredetailedinTable4.Throughoutandafterthetests,therewereno observedissuesrelatedtoadhesion,cohesion,orvisibledamage. Table4.Valuesoftotalrunoutaftertestsofthermalandmechanicalresistance. SampleInfo Before Environmental Tests After1stSetof ClimaticTests After2ndSetof ClimaticTests AfteraSetof MechanicalTests Number Adhesive Calibrated TotalRunout [mm] TotalRunout [mm] Deviation [mm] TotalRunout [mm] Deviation [mm] TotalRunout [mm] Deviation [mm] 7DP110no0.0120.026−0.0130.020−0.0080.0060.006 8DP110yes0.0140.0060.0080.0140.0000.024−0.010 9DP110no0.0280.0230.0050.0280.0000.0270.000 2DP125yes0.0140.0130.0010.016−0.0020.018−0.004 4DP125yes0.0050.009−0.0040.007−0.0020.007−0.002 6DP125yes0.0120.015−0.0040.016−0.0050.016−0.004 1DP190yes0.0200.0200.0000.0180.0020.0180.002 3DP190no0.0230.0110.0120.0180.0040.0170.005 5DP190yes0.0330.0310.0030.0320.0010.0330.000 10Spabondyes0.0170.021−0.0040.018−0.0020.020−0.003 11Spabondno0.0370.0160.0200.0100.0270.0190.018 12Spabondno0.0220.052−0.0300.052−0.0300.057−0.036 13Eprosinno0.0220.0100.0120.027−0.0050.0190.004 14Eprosinyes0.0170.0150.0020.0150.0020.0170.000 15Eprosinno0.0350.0340.0010.0250.0110.0340.002 Figure 6. Values of total runout after calibration. Following the calibration of the samples, two cycles of thermal resistance tests and one cycle of mechanical resistance tests were carried out. The measured total runout values after each test are detailed in Table 4. Throughout and after the tests, there were no observed issues related to adhesion, cohesion, or visible damage. Appl. Sci. 2023,13, 10413 16 of 19 the requirements placed on the binoculars, a methodology for the environmental testing of the technological sample was developed. The necessary level of accuracy was determined by considering the tolerances typically attainable in the production of optical instruments through traditional manufacturing methods. The technological samples were composed of a CFRP tube bonded to aluminum alloy sleeves using an assembly fixture. A total of 46.6% of the samples achieved the required precision after bonding. The failed samples were calibrated by turning. Throughout the stability tests of the bonded joint, which encompassed two climate resistance tests and one mechanical resistance test, the following observations were made: • Among the calibrated samples, 87.5% (seven out of eight) did not exceed a deviation of 0.005 mm. • In the case of noncalibrated samples, 85% (six out of seven) exceeded a deviation of 0.01 mm at least once during the environmental tests. This difference is likely attributed to the shrinkage stress in the bonded joint, which was partially released during the turning process in the calibrated samples. • The most significant deformation occurred during the initial temperature load. Noncalibrated samples reached an average of 77.6% of the maximum deviation after the first temperature load. This suggests that after the first temperature cycle, the bonded joint stabilizes considerably, possibly due to the stress relief mentioned earlier. Based on these findings, it is clear that further research should focus on understanding and mitigating deformations in the bonded joint. Shrinkage stress appears to be a significant factor contributing to these deformations, and the results suggest that it can be effectively addressed through post-bonding machining or heat treatment. These technological steps may need to be integrated into the production process. If future research can develop a manufacturing technology that consistently and reliably achieves the required precision in CFRP bonding, it is highly likely that this technology will find practical applications in the production of precision CFRP devices. Author Contributions: Conceptualization, R.K. and J.Z.; methodology, R.K. and J.Z.; software, J.V. and L.G.; validation, R.K., J.Z. and D.H.; formal analysis, D.H.; investigation, R.K. and D.H.; resources, R.K. and D.H.; data curation, J.V. and L.G.; writing—original draft preparation, R.K. and D.H.; writing—review and editing, R.K. and D.H.; visualization, R.K. and L.G.; supervision, J.Z.; project administration, J.Z.; funding acquisition, J.Z. and L.G. All authors have read and agreed to the published version of the manuscript. Funding: This work has been supported by the project No. FV 22–10 funded by The Ministry of Education, Youth and Sports (MEYS, MŠMT in Czech) support. The part of the presented work was supported by the Project for the Development of the Organ-ization “VAROPS (DZRO VAROPS) Military autonomous and robotic assets” by the Ministry of Defence of Czech Republic. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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