Fabrication Processes of Thermosetting Matrix Composite Materials PDF

Fabrication processes of thermosetting matrix composite materials materials Properties Physical, Mechanical and Environmental Fibre Matrix PROCESS...

Fabrication processes of thermosetting matrix composite materials materials Properties Physical, Mechanical and Environmental Fibre Matrix PROCESS (Prepregs) Composite part (Fluid dynamic Cure conditions Heat transfer Impregnation Chemical reaction) Lay-up Geometry: Tolerances and surface properties Know-how  Fabrication processes of composite materials (Additive manufacturing processes) High performances Autoclave lamination Filament winding Resin Transfer Moulding, Pultrusion VARI, Resin infusion Hand lay-up and SMC spray-up and BMC (discontinuous fibers) Low Performances  Fabrication processes of composite materials (Additive manufacturing processes) High costs Autoclave lamination Filament winding Resin Transfer Moulding, Pultrusion VARI, Resin infusion Hand lay-up and SMC spray-up and BMC (discontinuous fibers) Low costs Manufacturing of fiber reinforced plastic Pultrusion Comp. molding RTM Thermoplastic compression molding (potential) Filament Winding Hand lay up Autoclave Fiber content Vf=0.65 Chemical and physical phenomena occurring in all technologies Fibre Impregnation Fluid dynamics Heating and curing Heat transfer and cure kinetics Mechanics of materials and Residual stresses viscoelasticity  Autoclave lamination Lamination= manual or automated stacking layer by layer of UD (or fabric) laminae, using uncured resin. The so-called “prepregs” are used Autoclave = an oven in which it is possible to control temperature and pressure (8 bar) The curing cycle consists of a program of temperature and pressure devoted to polymerization of resin and reduction of defects (voids, tolerances, etc.) below a threshold depending on the component It is mainly used to produce advanced composite with carbon and Kevlar fibers and with epoxy resins  Prepregs Laminae made of fibres impregnated with resin (thickness= a few tens of mm, width from 1.25 cm to 150 cm) The reinforcement can be unidirectional (tape) or a fabric (woven) The resin is characterized at room temperature by a very high viscosity  Prepregs fabrication by solvent impregnation – The fibers go through a bath where the resin is in solution in a solvent and subsequently between rollers that regulate the amount applied. The solvent is then removed in a dryer  Prepreg fabrication by melt impregnation Step 1: a resin film of constant thickness is applied on a release paper. Step 2: is transferred on fibres (fabric or tapes) going through heated rolls where also pressure is applied Step 1 Step 2  Ply thickness as a function of areal weight (surface density) Epoxy carbon HS Epoxy/glass  Why prepregs are used? Optimal control on the ratio between resin and hardener Uniform resin distribution on the surface and through the thickness Lay up with different fiber orientation and complex shapes can be obtained Better control on: the final composite properties, its homogeneity ripetibility of mechanical properties  Lay-up in clean room 1. The prepregs are stored at about -20 °C. After extraction they must be kept at room temperature in their packaging films for at least 24 h, so avoiding moisture condensation 2. Prepreg cutting Manual Automatic Fiber placement machine (automatic cutting and lay up) 3. Lay up on tools 4. vacuum bagging → consolidation and void removal are promoted Prepreg cutting  Laser projectors for proper lamina placement on tools Accuracy between 0.3 and 1 mm Data transferred from CAD The ply table provides the stacking sequence Ply Table ply table: bottom laminate ply table: stringer  Hand lay up and automated fiber placement Hand lay up: fiber placement machine (simultaneous cutting and Limited access to large tools laying up of prepregs) Lay up rate: about 1-3 kg/h Lay up rate: 25 kg/h today, up to 40 kg/h perspective GLAss-REinforced" Fibre Metal Laminate https://www.youtube.com/watch?v=xK4gMDduHgA https://www.youtube.com/watch?v=ulv50nbap5k  automated fiber placement VOC removal fiber feeding duct robot arm compaction roll IR lamp A shape with cutting signs prepreg tape bobins Deposition head around curve boundaries  Automated tape laying (ATL) Ingersoll: Leonardo helicopters Airborne tape laying head Electroimpact: B777X wing skins On line inspection under implementation  AFP: On-line defect detection Images of typical defects, taken with an on-line camera Twist Missing tow Gap JEC composites July 2018  Prepreg characterization Beside resin and fiber type, a prepreg is characterized by Gel time Tack level Drape Resin viscosity and “flow” Resin content Resin to hardener ratio (FTIR, HPLC) Resin reactivity and glass transition temperature (DSC) Key properties are also the shelf life and the mechanical life  Prepreg characterization Gel time: – At a given temperature (usually curing Temp), it represents the time to reach gelation Tack level and drape: – Qualitative parameters that describe the ability of a prepreg plies to bond each other during stacking and to adapt to complex mold shapes, respectively. They can change during storage Qualitative Definition/Description tack level 5 Sticks to hand, but no residual left 4 Sticks to untreated tool indefinitely in vertical configuration 3 Sticks to untreated tool for about 30 s in vertical configuration 2 Falls off untreated tool immediately in vertical configuration In1 No tack – In AFP the placement parameters (Infrared heating, roller pressure, lay-up speed) for the first ply may be expected to differ from the rest of lay-up. – To adhere the first ply laid by AFP to the tool a” tackifier” or a first ply are hand laid up  Prepreg characterization Resin viscosity and flow – The resin viscosity is measured either as a control on the prepreg batch either as a control on the aging resulting form prolonged storage. – Sometimes the technological parameter named “flow” is performed: a prepreg disc is pressed at the curing temperature and amount of squeezed resin is measured Resin content – It is usually a little bit higher than the amount that should remain in the composite after curing. It comprises the resin that is lost during curing in order to remove volatiles – Typical values: – 35%-45% by volume expected in the cured in the composite + an excess of a few % Volatiles are ususally: Solvent residues, absorbed water, Reactive monomers or any low moecular weight compound that results from former synthesis of monomers  Prepreg characterization DSC – It is used to measure the Tg of the reactive mixture (Tgo) to check if it was properly stored. It is used to measure the Tg of cured resin – It is used to measure the reactivity through the exothermal effects associated to the reactions (exothermal reaction peak shape and position). FTIR – It is used as acceptance control to check if the ratio between epoxy and amine groups is that expected. The ratio between the height of epoxy and amine absorption peaks is measured. Shelf life: the time the resin (and the prepreg) can be stored at low temperature, usually -20 °C (usually about 1 year) Mechanical life: the time the resin (and the prepreg) can remain at room temperature before curing in autoclave (usually a few hundreds of hours)  Prepreg characteriztion AGATE (Advanced General Aviation Transport Experiments FAA qual. Procedure da MIL-HDBK-17  Properties of resins for prepregs High viscosity at stratification temperature (room temperature or a little higher): usually > 1000 Pa s Low reactivity at room temperature in order to maximize the mechanical life. Low reactivity at storage temperatures (- 20 °C) in order to maximize shelf life. High reactivity at curing temperature to minimize curing time. Solubility can play a key role. For instance, dicyandiamide is a solid powder poorly soluble at mixing and room temperatures. This strongly limits reactivity. Then only at curing temperature (about 125 °C) the dissolution of the amine in the resin can occur, so activating the reaction.  Static Strength Reduction of Composites Comes From Many Sources Pristine materials Reduction of Processing anomalies the allowable Surface irregularities stress Splicing Waviness Inclusions Stress Voids Damage Visible damage Nonvisible damage Allowable design region Repair (holes, etc.) Design Environment Strain Allowable strain reduction  vacuum bagging Vacuum bag: a film (usually polyamide) characterized by a high stretching properties also at high temperatures (cure temperature). Sealant tape: a bi-adhesive thick tape use to seal the vacuum bag Release film or fabric: a release layer in some cases perforrated or porous to allow resin flow and volatiles extraction.  Vacuum bagging Breather: always present, is a porous mat deputed to allow air extraction on the entire surface of the tool. It allows the extraction of volatiles. In some cases a single layer acts as breather/bleeder. Bleeder: a porous mat deputed to the absorption of the resin in excess lost the laminate surface Peel ply: a thin fabric added as last layer on the prepreg stack in order to protect the laminate surface or to produce surface roughness for further adhesive bonding. In some cases used as breather. Sandwich panel: minimum required auxiliary materials for vacuum bagging  Vacuum bagging https://www.youtube.com/watch?v=t22vJLHWxYA  Corner Bridging SOLUTION Pressure intensifiers: rubber inserts Dog ears on the bag Defects-Foreign Objects  Ply drop-off a way to change the thickness of a laminate If drop-off areas are further covered by continuous plies (internal ply drop-off), void can occur Debulking Honeycomb: core crush  Ribs and fillers Ribs can be co cured on a laminate using proper tools Fillers are usually needed Fillers can be pre-shaped assuming proper triangular sections with the aid of additional tools. Fillers can be also used simply rolling UD or fabric prepregs In some cases fillers are pre-cured and used as solid composite parts Vacuum bagging: section 46 B787 Vacuum bagging: section 44 B787 Vacuum bagging: sections 44 and 46 B787  Boeing 787: fuselage structure A A-A co-cured stringers A Frames Shear ties: a link between frames and skin by titanium fasteners Boeing 787: fuselage structure  Autoclave lamination Autoclave= an oven where temperature and pressure (max 8 bar) are controlled The cure cycle is a defined program of temperature and pressure Prepregs are always used Sketch of an autoclave Sketch of an autoclave Some typical tools Thermocouples (TC1, TC2) are put in the laminate, in the trimming area The carts that bring the tools into the autoclave  Tool properties Thermal inertia – It has effect on the rate of heating and cooling in autoclave curing Themal expansion – Coefficients of thermal expansion (°F-1 10-6) of composite and tool are often very different: Alluminium 13 Composite 10-1 The different shrinkage of the tool and of the composite in the cooling phase can induce residual stresses and deformations. This may be partly overcome with: Low Cooling rates Tools made in composite (Carbon reinforced) High stiffness Free of any porosity  Tool properties Low weight – Low density materials Tolerances – The tolerances of the tool will be reflected on those of the composite laminated. Usually larger tolerances are obtained for the composite in comparison with those of the tool Repairability Capable to be used for several hundreds autoclave cycles  Coefficient of Coefficient of Specific Thermal Tooling Gravity Specific Heat Mass Thermal Thermal Material (Btu/lb/°F) Conductivity Expansion (CTE) (g/cm3) (Btu/ft3/°F) (Btu/ft2/hr/°F) (in/in/°F) METALS Cu-Be (C1751 O) 8.80 0.10 0.88 1680 10.0 Cast Aluminum 2.70 0.23 0.62 1395 12.9 Steel 7.86 0.11 0.86 360 6.7 304 Stainless Steel 8.02 0.12 0.96 113 9.6 Nickel 8.90 0.10 0.89 500 7.4 Zinc 7.14 0.09 0.64 746 19.0 Invar 36 8.11 0.12 0.97 73 0.8 Invar 42 8.13 0.12 0.98 106 2.9 CERAMICS MgO 2.90-3.58 1.13-1.50 3.28-5.37 79 6.1 Al203 2-90-3.98 0.86-1.03 2.49-4.10 22 3.3 State Change (2-phase 0.45 na na 0.83 1.1-3.3 technology inc.) PLASTER Gvpsum Based 1.4-1.6 0.84-1.00 1.18-1.60 10 8.3  Coefficient of Coefficient of Specific Thermal Thermal Tooling Specific Heat Thermal Gravity Mass Conductivity Material (Btu/lb/°F) Expansion (CTE) (g/cm3) (Btu/ft3/°F) (Btu/ft2/hr/°F/i (in/in/°F) n) COMPOSITES (small numbers per year) Glass/Epoxy 1.8-2.0 0.3 0.54-0.60 21.8-30.0 8.0-9.0 Carbon/Epoxy 1.5-1.6 0.3 0.45-0.48 24.0-42.0 0.1-3.0 GRAPHITE Monolithic graphite 1.74-2.00 0.27-0.30 0.47-0.60 160-220 0.1-1.0 foams FOAM PU Foam Board (Models and 0.24-0.80 na na na 27 prototypes) Carbon Fiber Foam 0.1-1.6 na na 1.7-173 2.7-3.2 WATER SOLUBLE MATERIALS: CORES IN PRESENCE OF UNDERCUTS Water Soluble (3D na na na na na printed) The thermal expansion of the composite depends on the stacking sequence. In a UD lamina it is dominated by that of the fibers in direction 1 (that of fibers) and is dominated by the matrix in the directions 2 and 3 Tooling materials and shapes  Tooling materials and processes:3D printing Polycarbonate, amorphous PET-G or ABS are also used, reinforced with short glass or carbon fibres The picture: ultem 1010 polyetherimide (PEI), with a Tg of 216°C Parts are vacuum-bagged and oven-cured at 121°C Surface smoothing needed Soluble resins available when undercuts are present Main advantage: – Dramatically reduced the time for tool fabrication 3D printing is candicate to substitute PU and epoxy foam boards  Tooling materials and processes:3D printing Anisotropic strength and stiffness is obtained due to the layer-by-layer fabrication (see the rough surfaces not machined) Growth direction Material: PC+CF Equipment: machine for additive and subtractive manufacturing Growth direction (lower mechanical properties) https://www.youtube.co m/watch?v=g3mNEeUU FZQ&ab_channel=CEA DGroup https://www.youtube.com/watch?v=ii_tTYgKQWo&ab_channel=BelottiSpa  Tooling materials and processes:3D printing «Massivit» process (https://youtu.be/KCgnueUyPhU): Anisotropic strength and stiffness resulting from the layer-by-layer fabrication can be by-passed making first an external «mould» by 3D printing and casting into it an epoxy resin more stiff and strong. Growth direction 1- An external sacrificial shell is 3D-printed by UV polymerization of an acrylic thermosetting resin. This resin after casting is dissolved 1- Cast epoxy resin still reproducing the rugosity resulting from 3D printing layer-by- layer deposition. 1- Epoxy resin after polishing  Effects of shrinkage The assembly of anisotropic laminae determines a state of residual stress that can never be eliminated. Relaxation mechanisms allow to reduce the values ​calculated for perfect elastic laminae considering an initial state of zero stress at the temperature of cure in the case of elastic behavior Direction of highest shrinkage= 2 1 2 1 A laminate made of two superimposed plies with fibers oriented at 0 ° and 90 ° (as in the figure) will deform to a different extent in 1 and 2 directions being characterized by different stiffnesses in these directions→ After cooling from cure temperature a bend laminate will be obtained  Void content Air, water and VOC (volatile organic compunds) Microvoids (porosity) Mechanical properties reduction (in particular the interlaminar shear strength, ILSS) Up to 4% void content, ILSS is reduced of 7% for every 1% of void. In aeronautic primary structures the void content should be below 1% Void content and interlaminar shear stress In this test the failure occurs under shear loads Void content and fatigue  The most used cure cycle 200 10 and ----- pressure 3 180 10 160 8 2 140 10 Temperature (°C) Pressure (bar) 120 6 10 viscosity (Pa s) 100 80 4 60 1 40 2 temperature -1 20 10 _____ viscosity 0 0 0 50 100 150 200 250 time (min)  Viscosity changes during cure: a real case At gelatiom the resin becomes rubbery and viscosity goes to infinite Reaction is not stopped 10000 1000 Viscosity (Pa*s) 100 10 1 0.5 °C/min 0.75 °C/min 1 °C/min 0.1 40 60 80 100 120 140 160 180 200 Temperature (°C) A real cure cycle: temperature measured on composite parts 140 Tcura,min=122°C Tcuremin 120 100 50°C __TTgas ambiente in the autoclave autocl. 80 T [°C] 102 min 60 40 20 0 0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 time [min] tempo  Cure cycle with a dwell (used for thick parts in order to keep more uniform the temperature across the thickness)  Monitored (imposed) parameters in a typical curing cycle to each composite part Cure temperature + a tolerance (e.g. 180+5 °C) measured by the thermocouples placed on the parts Minimum time at the curing temperature as measured by the thermocouples placed on the parts Heating and cooling rates + a tolerance (typical heating rate 2 +1 °C/min, typical cooling 4+1.5 °C/min), as measured by the thermocouples placed on the parts Max temperature for autoclave door opening Gas pressure and pressure profile (with a tolerance) Vacuum in the bag: pressure below a given limit Prepregs for aeronautic applications To be Noted: Recommended cure cycle Tg dry and wet Toughened epoxy VBO (Vacuum bag only) BMI (Bis-maleimide) and Cyanate ester resins (high Temp.) Type of reinforcement Prepregs for Honeycomb sandwich structures  Resin flow Resin flow depends on the applied pressure (3 to 8 bar). It can occur: – At laminate edges (undesirable) This must be avoided to limit non uniform resin distribution and thickness gradients in the plane – At laminate surfaces (desirable) Promote void removal Uniformity of resin distribution  Void formation and growth Void growth can occur if the vapour pressure (Tv) of any potential volatile compound exceeds the actual pressure (Pr) in the resin (i.e. the hydrostatic pressure in the resin) If Tv>Pr voids can be generated and grow Water vapour saturation pressure at 180 oC is 9.9 bar at 130 oC 2.7 bar At high viscosity and above gel point voids become entrapped in the matrix Temperature °C Vapour pressure, atm Interply and intraply voids 100 1.001 110 1.42 120 1.96 130 2.67 140 3.57 150 4.70 160 6.10 180 9.90  Voids formation and growth Most of the void is due to water absorbed by the prepreg. Unreacted epoxy resins are strongly hydophillic. Other volatiles can originate from solvents used in the impregnation process. The hydrostatic resin pressure, capable to prevent the formation of voids, is only a fraction of the autoclave pressure: also fiber beds have a load carrying capability In the mechanical analogy, the spring represents the fiber bed and the liquid in the dashpot the liquid resin. Resin leakage in the bleeder (or by borders) is represented by the top valve. (also air entrapped during lay up may be the origin of voids)  Resin pressure:Vacuum bagging At room temperature: there is no flow The top ply in the stack is kept under vacuum (between -700 and -980 mbar depending on the process) Autoclave pressure reduces the breather thickness. A single layer of breather under 8 bar pressure with a thickness of 0.5 mm was adopted The resin flow in autoclave process can be sketched vacuum as a dashpot and a spring in parallel (a Maxwell-Voigt viscoelastic element). The spring represents the stiffness of the reinforcement The dashpot the viscous drag due to resin flow breather laminate tool  Resin Pressure: heating and resin flow During heating: viscosity decreases A pressure gradient is developed across the lay up The resin flows through composite thickness filling the breather (in plane flow is neglected) under autoclave pressure The upper resin layer still under vacuum vacuum Resin filled breather Unfilled breather Resin front position xf  Resin Pressure: end of flow, pressure build up When the resin touch the vacuum bag: The flow ends The pressure is distributed between that supported by the reinforcement, under compression strain, and the hydrostatic pressure in the resin. The resin pressure is always lower than autoclave pressure autoclave pressure vacuum bag/resin contact  A three stage process and the viscoelastic model autoclave pressure autoclave pressure autoclave pressure vacuum vacuum vacuum bleeder laminate tool Room temperature Heating: reinforcement vacuum bag in contact No flow. compaction and flow. to resin Eventual lost of volatiles Still possible volatile Flow end by diffusion extraction Pressure is distributed between the spring (elastic reaction of fiber stack) and the dashpot (hydrostatic pressure in the resin)  Other possible causes for flow end vacuum Resin filled breather Unfilled breather Resin front position xf The resin flow ends for three reasons: 1. the bleeder is filled of resin(as formerly described): bag-resin contact occurs. Pressure distributed between resin and fibers Resin pressure=0 2.the spring takes all the load 3. resin viscosity is very high and the dashpot becomes not deformable Resin pressure=0 and takes the load at high viscosity. Low viscous resin is never in (at low viscosity) contact with bag  Consolidation and voids formation Step 1: room temperature very high viscosity Step 2: Temp. increases, viscosity decreases and the liquid escapes from the damper. Fiber bed is compacted Step 3: the fiber bed start carrying load and its permeability reduces: the resin flow decreases Step 4: if gelation is not occurred all the load is carried by spring (compacted fiber bed)→ the hydrostatic pressure in the liquid drops and void formation is likely to occur  Hydrostatic resin pressure measurements The stiff screen prevents deflection into the hole in contatct with the pressure transducer Test with high and low flow resins in UD laminates 10 to 40 plies Campbell, J. Adv. Mat. 1995  Hydrostatic resin pressure measurements: results Gross porosity No porosity Case 2 of former slide Case 1 of former slide Higher resin flow allowed with thick bleeder (overbleeding) Resin flow is associated to resin pressure decrease Resin flow is promoted by viscosity decrease and is associated to a decrease of the resin pressure  Hydrostatic resin pressure measurements: results In thicker laminates a higher resin pressure is obtained compared to thinner ones: the same amount of resin absorbed in the bleeder results in a lower fraction of resin loss  Cure process objectives Resin polymerization up to a degree of reaction of 0.9-0.95 in order to reach a Tg a bit lower than Tgmax Resin excess removal and consolidation VOC, water and air removal contain resin in excess entrapped, air, water and VOCs prepregs Removal of : resin in excess entrapped, air, water and VOCs Polymerization Cure cycle Mechanical properties development (shear, etc.) Cured Laminate The resin and fiber content must be constant and always the same Void content