Lecture 21 - Composite Materials (FRPs) PDF

Civil and Environmental Engineering Hamzeh Hajiloo, PhD, PEng CIVE 2700: Civil Engineering Materials Lecture 21: Composite Materials Chapter 11 Chapter 16  Outline Introduction to campsites Constituents 1. Fibers 2. Matrix Fabrications...

Civil and Environmental Engineering Hamzeh Hajiloo, PhD, PEng CIVE 2700: Civil Engineering Materials Lecture 21: Composite Materials Chapter 11 Chapter 16  Outline Introduction to campsites Constituents 1. Fibers 2. Matrix Fabrications 1. Pultrusion 2. Filament winding Strengthening with FRP Properties of composites CIVE 2700 Lecture 21 /2  A Composite Material Definition: A combination of two or more distinct materials which have improved properties over the individual materials. Composites combine the best properties of their constituents (e.g. strength, stiffness, weight, corrosion resistance, wear resistance, fatigue life, impact damage, etc…) Used in marine, aviation, civil engineering structures, automotive and sport industries, etc… More familiar composite materials: Concrete → stone, sand, and cement paste. Reinforced concrete → concrete and steel. Wood → cellulose and lignin. CIVE 2700 Lecture 21 /3  fiber-reinforced polymers (FRP) Recently: fiber-reinforced polymers (FRP) Generally, the constituent materials have significantly different properties Properties of composite material are significantly different than constituents Auto and aero industries use high strength composite materials to build lightweight vehicles Wood is a natural composite of cellulose fibers (cell walls) and lignin (glue) An electron micrograph of a glass FRP Material components that are combined composite cross-section. to create an FRP composite. Constituents: (1) Fibers and (2) Matrix CIVE 2700 Lecture 21 /5  (1) Fibers classification scheme When discussing composite materials, the terms "microscopic composites" and "macroscopic composites" refer to the scale of the constituents within the material. Microscopic composites include fibers or particles in sizes up to a few hundred microns. This category includes materials where small fibers or particles are dispersed within a matrix. These components are usually too small to be individually distinguished without the aid of a microscope. Macroscopic composites have constituents of much larger size; Concrete is a good example of a macroscopic composite. It consists of larger aggregate particles and may also include reinforcing elements like steel rebars. These components are easily visible to the naked eye. Macroplastics CIVE 2700 Lecture 21 /6  Microscopic Composites Consist of: Hair 1. continuous phase or matrix – usually polymer (plastic) 2. dispersed or reinforcing phase E-Glass The matrix phase Aramid surrounds, suspends, and binds fibers or particles Carbon transfers load to them protects them against environmental attack and damage due to The average diameter of handling fibers used is usually less The dispersed phase than 0.01 mm generally harder and stiffer than the matrix phase Schematic representations of the various geometrical and spatial characteristics of particles of the dispersed phase that may influence the properties of composites: (a) concentration, (b) size, (c) shape, (d) distribution, and (e) orientation CIVE 2700 Lecture 21 /7  Structural classification of composites (1) Laminates Indeed, the process of joining sheets containing aligned fibers to create a multi-layered composite is known as laminating or laminated composite manufacturing. In this process, individual layers or plies with aligned fibers are stacked and bonded together to form a composite material with enhanced mechanical properties. The orientation of the fibers in each layer can be adjusted to achieve specific strength and stiffness characteristics. Lamina: A single layer of a composite material, consisting of fibers and a matrix. Laminate: A composite material made by stacking and bonding together multiple laminas or plies, often with different orientations, to create a material with enhanced mechanical properties. UD-Laminas Laminate A Laminate Remember: tensile strength of fibers is the strongest along the fiber direction. CIVE 2700 Lecture 21 /8  Structural classification of composites (2) Sandwich panels Sandwich panels are composite materials that consist of a lightweight core material, often a honeycomb structure, sandwiched between two facing sheets or skins. The combination of the core material and facing sheets creates a structure that offers a balance of strength, stiffness, and low weight. The core of sandwich panels is typically made of a honeycomb structure. This core material is usually composed of lightweight materials, such as aluminum arranged in a hexagonal pattern. This structure provides excellent strength and stiffness relative to its weight. Sandwich panels are used in a wide variety of applications including roofs, floors, and walls of buildings and in aerospace and aircraft (i.e., for wings, fuselage). Benefits of Sandwich Panels in Building Construction: Lightweight: The use of lightweight materials in sandwich panels contributes to the overall reduction in the weight of the building structure. Insulation: The core material provides thermal insulation, contributing to energy efficiency and occupant comfort. Strength: The combination of the facing sheets and the core material creates a strong and rigid structure, enhancing the overall strength of the building. Speed of Construction: Prefabricated sandwich panels can speed up the construction process, reducing on-site labor requirements and potential delays. face sheet adhesive layer honeycomb CIVE 2700 Lecture 21 /9  (1) Fibers Common fibers Common fibers used in fiber-reinforced composites: (a) steel, (b) glass, and (c) polypropylene Scanning electron micrograph of concrete mortar mixed with carbon fibers CIVE 2700 Lecture 21 /10  (1) Fibers Properties Woven patterns: a) plain weave, b) basket weave, others available. a) b) Glass fiber In Civil Engineering, the common fibers used are: Glass (E = 80-90 GPa): Most common, produced in the largest quantities and the least expensive, high strength, low stiffness. It is commonly used for reinforcement in composite materials, such as fiberglass-reinforced plastics (FRP) in boats, car parts, and building components. Carbon (E = 250-1000 GPa): More expensive than glass fibers, higher strength and stiffness. They are used in various industries, including aerospace, automotive, Carbon fiber and sports equipment. Aramid (E = 60-120 GPa)-“Kevlar” is the Dupont trade name; High strength, modulus and impact resistant, but absorbs moisture, and is expensive. is used in applications such as body armor, tires, and aerospace components. Aramid, Kevlar The average diameter of fibers used is usually less than 0.01 mm CIVE 2700 Lecture 21 /11  (1) Fibers Comparison of Stiffness and Strength of Materials The choice of fiber depends on the specific requirements of the application. Carbon fibers are often chosen for applications demanding high stiffness and strength, while aramid fibers are known for their high strength-to-weight ratio. Glass fibers strike a balance between stiffness and strength. CIVE 2700 Lecture 21 /12  (1) Fibers Orientation Continuous fibers are fibers that extend uninterrupted through the entire length of a composite material in a specific direction. When continuous fibers are used in a composite, the resulting material is anisotropic, meaning it exhibits different mechanical properties in different directions. The strength and stiffness are typically highest along the direction of the fibers. Discontinuous fibers, sometimes referred to as short fibers or chopped fibers, are fibers that are not continuous throughout the entire length of the composite. They are randomly oriented in different directions. The use of discontinuous, randomly oriented fibers tends to produce a more isotropic composite. In an isotropic material, the mechanical properties are the same in all directions. Unidirectional- all fibers run in Example: Effect of fiber orientation on the tensile strength of E- one direction (e.g. prepreg). glass fiber-reinforced epoxy composites. CIVE 2700 Lecture 21 /13  (1) Fibers Forms Filament is a single fiber, and Strand is twisted filaments gathered into bundles. Common glass fibers include: Rovings are bundles of untwisted continuous filaments or strands. Mat, particularly Chopped Strand Mat (CSM), is a form of reinforcement in which chopped rovings are randomly deposited and held together with a binder. it provides reinforcement in multiple directions due to the random orientation of the chopped strands. This random orientation contributes to isotropic properties. Roving, Rovings spool Mat Individual fibers have fewer internal defects, and are much stronger than the bulk material (e.g., glass fibers are about 300x stronger than glass plates) CIVE 2700 Lecture 21 /14  (1) Fibers Forms Chopped Rovings are bundles of continuous fibers (rovings) that have been cut or chopped into shorter lengths. Rovings are typically long strands of continuous fibers. Powder generally refers to finely ground particles of a material, often in solid form. In composite materials, powdered fillers or additives may be incorporated into resins or matrix materials to achieve specific properties, such as improved thermal conductivity, flame resistance, or enhanced mechanical properties CIVE 2700 Lecture 21 /15  Glass fibre manufacturing The continuous glass fibers are wound onto spools for further processing. Multiple strands of glass fibers may be gathered together to form rovings, which are bundles of continuous fibers. Formation of Fibers: The molten glass is passed through fine openings, forming continuous filaments. The filaments are rapidly cooled to solidify them into glass fibers. CIVE 2700 Lecture 21 /16  Matrix Matrix component: Matrix (polymer) has three important functions: Holds the fibre in place. It ensures proper alignment and distribution of the fibers within the composite structure, contributing to the overall integrity of the material. Transfers loads to the high-stiffness fibres. While the fibers provide strength and stiffness, the matrix helps distribute and transfer external loads to the fibers, ensuring a more uniform distribution of stress throughout the material. Protects fiber from environmental and abrasion damage. More flexible and ductile than fibers Epoxy resin and curing agent (‘hardener’) CIVE 2700 Lecture 21 /17  Matrix Types Thermosets: Irreversible cross-linking, no melting, high performance. Thermoplastics: Reversible melting, processing flexibility, recyclability. CIVE 2700 Lecture 21 /18 Fabrications: (1) Pultrusion and (2) Filament winding CIVE 2700 Lecture 21 /19  Fabrication (1) Pultrusion Pultrusion process: Continuous fiber rovings are drawn from spools and pulled through a resin bath for impregnation. Fibers gather together to produce a particular shape (preforming die). Pass through a heated die for curing. Cured profile is cut into lengths. Products: structural shapes with a constant cross-section such as; round, rectangular, pipe, plate or sheets. Placing a thermocouple in a GFRP rebar (for research) CIVE 2700 Lecture 21 /20  (1) Pultrusion Civil Engineering Applications Structural shapes: Rods, round and square tubes, I-beams, C-channels. General applications: Reinforcing rods for concrete (rebar). Beams, girders and cellular panels to support large loads (vehicular and pedestrian bridges). Columns, posts and pilings to carry vertical loads (bridge columns, marine pilings, and utility poles). Trusses in a wide variety of structures (bridges, transmission towers, and Cross-sections of FRP industrial plants). shapes (e.g. by pultrusion) Tanks and pressure vessels. CIVE 2700 Lecture 21 /21  (1) Pultrusion Pultruded FRP Frame Structure FRP is about four times lighter than steel of an equal strength. FRP is inherently resistant to corrosion, making it ideal for applications in marine environments or where exposure to harsh chemicals is a concern. FRP beam, column and plates to produce an all fiber-glass building. Pultruded FRP Frame Structure GFRP/polyester structural sections developed by Strongwell Co. USA CIVE 2700 Lecture 21 /22  (1) Pultrusion Pultruded Bridge Structures Long-span pedestrian bridge Homestead Bridge (1997, Los Alamos, NM). Placed by helicopter in a remote location. A short-span all FRP road bridge (2001, UK) CIVE 2700 Lecture 21 /23  (1) Pultrusion Pultruded FRP Rebar Deformed of sand coated surface to enhance an FRP bar’s mechanical bond with concrete. Incorporate sand on the surface… …or a fibre braid Some companies involved in producing FRP bar: V.ROD® SchockCom Bar® TUF-Bar® MST Bar® CIVE 2700 Lecture 21 /24  Fabrication (2) Filament winding Products: hollow, circular components, such as pipes and tanks. Process: Continuous fiber strands are drawn from spools and pulled through a resin bath for impregnation. Fibers are wound onto a mandrel in a variety of orientations. After the appropriate number of layers are applied, the part is cured and the mandrel is removed. CIVE 2700 Lecture 21 /25  (2) Filament winding Tanks and Pipes Underground storage tanks or pipes: Less chance of corrosion compared to metal Has a 2-3 longer life than steel and concrete.While filament-wound tanks and pipes may have higher upfront costs than some traditional materials, the long-term benefits, such as reduced maintenance and longer lifespan, can make them cost-effective over the life of the structure. FRP pipes FRP tanks-storing corrosive liquids CIVE 2700 Lecture 21 /26 Strengthening with FRP CIVE 2700 Lecture 21 /27  Why Post Strengthening? Deterioration of bridge decks, beams, girders and columns, buildings, parking structures and others due to ageing Crashing of vehicles to bridge components environmentally induced degradation such as corrosion of steel reinforcement poor initial design and/or construction, Lack of maintenance, Accidental events such as earthquakes. CIVE 2700 Lecture 21 /28  Why Post Strengthening? Need for upgrading: Increasing service loads (such as traffic load on bridges), Changing of the structural system (such as removing of walls, columns) Large crack width Large deformation Seismic retrofit in areas of high seismic risk. CIVE 2700 Lecture 21 /29  Why FRP for Post Strengthening? Immunity to corrosion, Low weight (1/4 of steel), therefore easier application No need for scaffold High tensile strength Unlimited availability in FRP sizes Disadvantages of FRP for Post Strengthening: Linear elastic to failure, Material cost, Incompatible thermal expansion coeff. with concrete, Softening at high temperature (such as 45° to 70°C for some epoxy resins. CIVE 2700 Lecture 21 /30  Strengthening materials UD-Strips: (thickness appr. 1 mm) made by pultrusion Prepreg: Flexible sheets or fabrics (in one or two directions) and sometimes pre- impregnated with resin. CIVE 2700 Lecture 21 /31  Applications: (1) Prepreg process: A continuous fiber reinforcement impregnated with a polymer resin. Applied to surface requiring strengthening. A number of sheets are laid up to provide a desired thickness. At room temperature, the matrix undergoes a curing reaction (the prepreg sheets must be stored at a lower temperature, e.g. in a freezer). Flexible carbon FRP sheet can be used for repair of concrete structures. CIVE 2700 Lecture 21 /32  Applications: (1) Wet lay-up Wet lay-up, hand lay-up or contact molding structural rehabilitation applications FRP sheets or fabrics are bonded Wet lay-up application of GFRP sheets on a rectangular column CIVE 2700 Lecture 21 /33  FRP stay-in-place formwork The use of hybrid FRP (Fiber-Reinforced Polymer) and concrete members, particularly concrete-filled FRP tubes, is an innovative solution in civil engineering, offering advantages such as corrosion resistance and enhanced structural performance. Concrete-filled FRP tubes are used as both foundation piles and bridge piers. CIVE 2700 Lecture 21 /34  Laminates and Wraps Severely corroded steel in these bridge columns Carbon FRP column wrapping. can be repaired with FRPs. Concrete bridge girder strengthened in shear with carbon Strengthening a fatigue-damaged aluminum overhead sign FRP. The FRP is then painted with a UV-resistant paint. structure with externally-bonded glass FRP wraps. CIVE 2700 Lecture 21 /35 Civil Engineering Applications Properties of Composites CIVE 2700 Lecture 21 /37  Some Definitions: Homogeneous: Properties are not function of the position of the material points Isotropy: Properties are not function of the orientation. Anisotropy: Properties are function of the orientation with no planes of symmetry. CIVE 2700 Lecture 21 /38  Properties of Composites Ductility Ductility and Strength of FRP fibers increase ductility and strength of the matrix depending on: fiber properties volume fraction orientation Adding steel fibers to plain concrete has the following benefits: High-energy absorption (toughness) High flexural fatigue resistance Cracking resistance/redistribution of cracking Redistribution of moments Increased impact resistance Fiber bridging cracks in concrete CIVE 2700 Lecture 21 /39  Properties of Composites Improved toughness of Fiber reinforced concrete (FRC) Stress–strain relations of randomly oriented fiber-reinforced concrete with different fiber contents CIVE 2700 Lecture 21 /40  Properties of Composites (Unidirectional Fibers) The properties of a composite are affected by; mechanical properties, volume fraction of each component and the orientation of the fibers. Subscript f, m, c and ν refer to fiber, matrix, composite ply and Elastic properties are a function of: volume fraction. Fiber modulus, Ef Fiber volume fraction, ν f ν m + ν f=1.0 Matrix modulus, Em Matrix volume fraction, ν m 3 2 The density of the composite can be E22 written as: ρc = νf ρf + ν m ρm or ρc = ν f ρf + (1- ν f)ρm 1 E11 CIVE 2700 Lecture 21 /41  Loading parallel to fibres 𝜀𝑐 = 𝜀𝑚 = 𝜀𝑓 = 𝜀 𝜀𝑐 = 𝐶𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 𝜀𝑚 = Matrix strain 𝜀𝑓 = fibre strain 𝐹𝑐 = 𝐹𝑚 + 𝐹𝑓 𝜎𝑐 × 𝐴𝑐 = 𝜎𝑚 × 𝐴𝑚 +𝜎𝑓 × 𝐴𝑓 𝜎𝑖 = 𝑠𝑡𝑟𝑒𝑠𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑖 𝐴𝑖 = 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑖 𝐸𝑐 × 𝜀 × 𝐴𝑐 = 𝐸𝑚 × 𝜀 × 𝐴𝑚 +𝐸𝑓 × 𝜀 × 𝐴𝑓 𝐴𝑚 𝐴𝑓 𝐸𝑐 = 𝐸11 = 𝐸𝑚 𝜈𝑚 +𝐸𝑓 𝜈𝑓 𝐸𝑐 = 𝐸𝑚 × +𝐸𝑓 × 𝐴𝑐 𝐴𝑐 𝐹𝑓 𝜎𝑓 × 𝐴𝑓 𝐸𝑓 × 𝜀 × 𝐴𝑓 𝐸𝑓 = = = × 𝜈𝑓 𝐹𝑐 𝜎𝑐 × 𝐴𝑐 𝐸𝑐 × 𝜀 × 𝐴𝑐 𝐸𝑐 CIVE 2700 Lecture 21 /42  Properties of Composites Stiffness (E) and strength of the composite are proportional to the amount of fibres (volume fraction of fibers) in the matrix. When the load is applied to continuous, aligned fibre composite, parallel to the fibres, the elastic modulus is: 𝐸11 = 𝐸𝑐 = 𝐸𝑚 𝜈𝑚 +𝐸𝑓 𝜈𝑓 When the load is applied to an aligned fibre composite, perpendicular to the fibres, the elastic modulus is: 𝐸𝑚 𝐸𝑓 𝐸22 = 𝐸𝑐 = 𝐸𝑓 𝜈𝑚 + 𝐸𝑚 𝜈𝑓 CIVE 2700 Lecture 21 /43  Example 1 Calculate the modulus of elasticity of fiberglass composite if the fiberglass volume is 70% and epoxy is 30%. Also, calculate the percentage of load carried by the glass fibres. Ef is 70.5 Gpa, and Em is 6.9 Gpa. E11 = E c = EmVm + E f V f Ec = (6.9 GPa)(0.3) + (70.5 GPa)(0.7) = 51.42 GPa CIVE 2700 Lecture 21 /44  Example 2 Calculate the composite modulus in the longitudinal direction for polyester reinforced with 60 vol% E-glass. → E-polyester = 6.9 x 103 MPa → E-glass = 72.4 x 103 MPa E11 = E c = EmVm + E f V f ………………………………………………………………………………………………………………………………………………… Solution: 3 3 Ec = (6.9x10 Mpa)(0.4) + (72.4 x10 MPa)(0.6) ……………………………………………………………………………………..……….……………………………………………… = 2760 MPa + 43,440 MPa …………………………………………………………………………………………………………..……..…………………………… = 46,200 MPa …………………………………………………………………….………………………………………………………………………… ………………………………………….…………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………… ………………………………………………………………..……………………………………..……………………………………… ……………………………………………………………………………………….………………………………………….………….. CIVE 2700 Lecture 21 /45  Tensile Stress-Strain Behavior of Composites Broken fibers in a matrix The star (*) indicates the fracture strength and strain of the fiber and matrix. 1) Not all fibers fracture at the same time (the fibers are not loaded all the same amount) 2) The matrix keeps plastically deforming and still attached to some fibers which are still able to carry some load. CIVE 2700 Lecture 21 /46