Introduction PartI

Part I

The intent of this thesis is to investigate, through an experimental and theoretical study, the mechanical behavior of Glass Fiber Reinforced Polymers (GFRP) pultruded materials in the long term.

GFRP composites are widely used in aerospace and automotive industry. More recently, GFRP composites have found several applications in civil engineering as both primary and secondary structural elements. Although GFRP composites have considerable advantages, some weak points limit their applicability. One of these is creep. Creep is mainly attributed to the viscosity of the polymer matrix and to the gradual damage of the reinforcing fibers over time. Significant experimental research has been focused on the characterization of the creep behavior of composite materials. Nevertheless, viscoelastic behavior of GFRP pultruded elements in the long term is still under investigation as well as their creep failure over time. Actually, creep models that are mostly found in the literature are based on the extrapolation of experimental data acquired from short-term experiments.

For these reasons, the purpose of this work is to investigate the possibilities of predicting viscoelastic behavior of composite elements in the long term through several approaches.

Part I addresses general information about Fiber Reinforced Polymer (FRP) composites. In particular, the composition and technological production are described.

Then, Part II recalls the mechanics of FRP composites by a literature review. Linear elastic and viscoelastic types of behavior are widely investigated. Moreover, the main mate-rial failure modes such as static, creep, and fatigue ruptures are analyzed and the influence of environmental conditions on the mechanical response of such materials is discussed.

Part III focuses on the state of the art of the standards and codes currently available in order to design structures made of FRP composite materials. In particular, with concern to the viscous behavior, several standards are discussed and compared.

Later, Part IV concerns the experimental activities developed during a six-month stage at Laboratoire Navier of Ecole des P onts − P arisT ech. In particular, the stage involved an experimental campaign focusing on the validation of the Time-Temperature-Stress Su-perposition Principle (TTSSP) through 4-points bending tests on several GFRP pultruded composite bars having an epoxy matrix. Actually, according to the TTSSP, creep response of composite elements in the long-term at a reference conditions (stress and temperature) can be predicted through several short-term creep tests at various levels of stress and tem-perature. Indeed, since composite materials generally have lower stiffness combined with high strength, Serviceability Limit State (SLS) is often more important for the design of the overall structure than Ultimate Limit State (ULS); therefore, it is really necessary to investigate on the creep behavior of such materials because creep strain could induce higher deflections (added to elastic deflections) over time which could not satisfy limit values of SLS.

Finally, in Part V, a micromechanical model is developed in order to study the possibility of tertiary creep and hence the applicability of the TTSSP is explored.

Chapter 1

Fiber Reinforced Polymer

(FRP) composites

Fiber Reinforced Polymer (FRP) composites are made of a polymer matrix reinforced with fibers, such as glass or carbon fibers, as schematically shown in Figure 1.1. In particular, the fibers support almost all the load while the matrix, surrounding the fibers, protects the fibers assuring the loading distribution to the adjacent or broken fibers.

The advantage of composite materials is that, if well designed, they usually exhibit the best qualities of their components or constituents and often some qualities that neither constituent possesses. Moreover, composite materials can have higher performance, such as high strength, than other typical materials with lower weight, cost, volume, or thickness.

Figure 1.1: FRP composite

Conversely, some weaknesses limit the application of composite materials. For instance, they show a remarkable creep behavior, mainly due to the viscosity of the polymer matrix. The long-term creep behavior has not yet been fully investigated and the recent normative codes are based on the extrapolation of short-term experimental data. Furthermore, the mechanical behavior of such materials is also strongly conditioned by the environmental conditions, due to the temperature and humidity sensitivity of the polymeric matrix.

FRP composites have been used on a limited basis in structural engineering for many years except for repair and rehabilitation of existing structures, Figure 1.2 (a). Even though these materials have not yet been recognized by official building codes, recently such

materi-als have found several applications in new civil structures, e.g. for footbridges, small houses, and gridshells, Figure 1.2 (b).

Figure 1.2: Examples of applications of FRP composites in civil engineering structures

1.1

Fibers

Thanks to their high mechanical qualities, fibers (Figure 1.3) are used in composite materials to furnish the requested levels of stiffness and strength. Actually, long fibers in various forms are inherently much stiffer and stronger than the same material in bulk form. For example, ordinary plate glass has a ultimate stress of only 20 MPa, yet commercial glass fibers have strengths of 2800÷4800 MPa. It should be pointed out that this large difference of strength is due to the almost perfect structure of a fiber; in fact, a glass fiber has less defects than a glass plate because the fiber is very small in size, as well as really selected and controlled. This aspect is actually important because, from fracture mechanics theory, we know that several defects can produce a quicker fracture with a lower stress. Obviously, the geometry and physical makeup of a fiber are crucial to the evaluation of its strength.

Figure 1.3: Glass fibers

As previously mentioned, the major purpose of the fibers, in composite materials, is to carry the larger part of the applied load. Thus, the strength of a composite material is mainly

a function of fiber strength, although the matrix also contributes. A fiber is characterized geometrically not only by a very high length-to-diameter ratio but also by a near-crystal-sized diameter. Table 1.1 shows the strength and stiffness properties of the most common fibers.

Unity weight Tensile strength S/ρ Tensile stiffness E/ρ Fibers and Wires ρ(kN/m3_{) S (MP a) (km) E (MP a) (Mm)}

Aluminum 26.3 620 24 73000 2.8 Titanium 46.1 1900 41 115000 2.5 Steel 76.6 4100 54 207000 2.7 E-glass 25.0 3400 136 72000 2.9 S-glass 24.4 4800 197 86000 3.5 Carbon 13.8 1700 123 190000 14 Graphite 13.8 1700 123 250000 18

Table 1.1: Fiber properties [31]

Carbon fibers present great stiffness but their commercial cost is still high. In recent years, since in engineering applications the problem of environmental sustainability has became really important, also natural fibers have found several applications as reinforcement in FRP composite materials.

However, in structural applications, glass fibers are still mostly used combined with polymeric matrix. Glass fibers exhibit high strength and low cost. On the contrary, they exhibit small stiffness and fatigue lifetime.

Commercially there exist four types of glass fibers:

E-glass A low alkali borosilicate glass with good electrical and mechanical properties and

good chemical resistance. The designation E is for electrical.

C-glass A type of glass made and applied specifically for high chemical resistance. Its

mechanical resistance is lower than E-glass. The composition is 64.6% SiO2, 4.1% Al2O3•Fe2O3, 13.4% CaO, 3.3% MgO, 9.6% Na2O•K2O, 4.7% B2O3 and 0.9% BaO. The symbol C was originally chosen for chemical resistance.

S-glass A magnesia-alumina-silicate glass for aerospace applications with high strength. Its

mechanical resistance is higher than E-glass. Originally S stood for high strength.

ECR-glass A type of glass with high chemical resistance and properties similar to E-glass.

1.2

Matrix

All the fibers inside the composite material are surrounded by a matrix volume. The purpose of the matrix is to protect and bond the fibers and then to distribute the loading as uniformly as possible to the adjacent fibers; the matrix must also transfer the shear stresses between broken fibers.

Generally, matrix materials can be polymers PMC (Polymer-Matrix Composite), metals MMC (Metallic-Matrix Composite) or ceramics CMC (Ceramic-Matrix Composite). The cost of each matrix material escalates in that order as does the temperature resistance. However, the objective of this work is to address the polymer matrices.

As shown in Figure 1.4, polymers exist in at least three major forms:

Linear A linear polymer is merely a chains of mers

Branched A branched polymer consists of a primary chain of mers with other chains that

are attached in three dimensions just like tree branches

Cross-linked A cross-linked polymer has a large number of three-dimensional highly

in-terconnected chains

Figure 1.4: Polymer structures: (a) linear polymer; (b) cross-linked polymer Linear polymers have the lowest strength and stiffness, whereas cross-linked polymers have the highest because of their inherently stiffer and stronger internal structure.

There are three main classes of structural polymers:

Rubbers: This polymers are cross-linked polymers that have a semi-crystalline state well

below room temperature, but act as the rubber we all know above room temperature; rubbers exhibit a large range of elastic behavior due to their strong chemical bonds. They present generally an amorphous structure.

Thermoplastics: The thermoplastics structure is a branch structure, but generally do

not cross-link very much, if at all. Thus, as shown in Figure 1.5, they usually can be repeatedly softened by heating and hardened by cooling; moreover thermoplas-tics exhibit elastic behavior for high stress levels and plastic behavior for high stress. Thermoplastics include nylon, polyethylene and polysulfone.

Thermosets: Thermosets are polymers that are chemically reacted until almost all the

molecules are irreversibly cross-linked in a three-dimensional network. Thus, once an epoxy has “set”, it cannot be changed in form. Thermosettings are generally amorphous and then they have usually brittle behavior; glass transition temperature (Tg) is a important parameter of thermosetting behavior because (Tg) is the critical

temperature at which the material changes its behavior from being “glassy” to being “rubbery”. in this context “glassy” means hard and brittle (and therefore relatively easy to break), while “rubbery” means elastic and flexible. Thermosets include epoxies, phenolics and polyimides.

Generally, FRP composite materials are made of thermoset polymers such as epoxy resins, vinylester and polyurethane or thermoplastics polymers, e.g. polyetheretherketone and polyester. Nevertheless, commercial FRP composites are mostly made of epoxy resin. Ac-tually, the type of resin most commonly used is ipophthalic polyester though other resins

exhibit better behavior in specific applications. For example, vinylester matrix has high cor-rosion resistance, urethane methacrylate has very good fire, smoke and toxicity performance and epoxy is used in the higher performance applications and almost always with carbon fibers. εy E0 T σy σb εb

Figure 1.5: Mechanical behavior of thermoplastics

1.3

Pultrusion

Pultrusion is a continuous process for the production of constant section profiles in composite materials as well as steel beams. The first pultrusion machine, called the Glastruder, was developed by Brandt Goldsworthy in the early 1950s (Goldsworthy, 1954). In 1959, a U.S. patent for a pultrusion machine and the method for producing pultruded parts were awarded to Goldsworthy and Landgraf.

The pultrusion process includes several steps which are briefly presented. Firstly, the resin is applied to the fibers either by the use of a dip tank or by injection. In the first approach, fibers are impregnated through a tank containing the unpolymerized resin matrix and then pulled through the pultrusion die (heated steel die), whereas in the injection method the fibers are firstly pulled through a cavity fixed to the front of the die and then the resin is injected under pressure into the cavity impregnating the reinforcement. The pultrusion die preshapes the material thanks to various guides and rollers as schematically shown in Figure 1.6. The temperature of the die is such that the resin system is caused to react and cure within the die. Finally, the profile is pulled by either reciprocating pullers or a caterpillar haul-off and it is then automatically cut to length with a saw.

The pultrusion process is apparently simple, nevertheless high quality technical resources and experiences are required in order to produce a reliable composite profile. For example, the position of the fibers and the quantities of reinforcement and resin mix are crucial issues to create an excellent production. Moreover, the machine speed, die temperature and resin reactivity are parameters which interact and their balance requires high technologies. Lastly, the internal stress due to the cooling of the profile must be extensively investigated using particular shapes and appropriate cooling time.

The reinforcement used is generally a combination of unidirectional roving and random mat. This combination provides a reasonably optimized set of properties in the longitudinal and transverse directions respectively; however, other reinforcement types may be used.

Figure 1.7: Pultrusion process of Kenaf Fiber

The shapes produced can be simple such as a rod or more complex; Figure 1.8 shows typical pultruded profiles. Although there is great flexibility in the shape, thickness vari-ation, and size, the cross section must remain constant along its length. In addition, the part must be straight and cannot be cured into a curved shape. Pultruded profiles can be as small as 3 mm diameter or up to 1 m wide and 250 mm deep and are produced at a rate of between 180 m and 12 m per hour, depending on size and complexity.

1.4

Durability

FRP composites as well as all engineering materials, are subject to mechanical and physical deterioration with time, load, and exposure to environmental conditions. During the service life, structures made of FRP composites are exposed to a variety of potentially harmful physical and chemical environments. Although FRP materials are not susceptible to elec-trochemical corrosion, several harmful environmental conditions can damage such material if used improperly.

Durability of composite materials is not well investigated and some uncertain are still presents. Numerous factors that might influence FRP’s durability have been studied and discussed in the research literature. Actually, durability of FRP composites is certainly influenced by the matrix and fiber types used, the relative proportions of the constituents, the manufacturing process, the short and long-term loading, and exposure conditions.

Figure 1.9: Schematic showing typical deterioration in mechanical and bond properties for unidirectional glass FRP bars (reproduced after Bisby, 2003)

Generally, harmful effects which produce degradation of the FRP composites can be listed in two different types: environmental and physical effects. The physical effects are sustained loads which produce creep phenomenon and also cyclic loadings which cause fatigue failure. The environmental effects are:

• Moisture and marine environments: Although FRP composites are not suscep-tible to electrochemical corrosion, they are not immune to the potentially harmful effects of moist or marine environments. Research works have observed FRP compos-ites degradations under prolonged exposure to moist environments linked to the rate of absorption of fluid into the polymer matrix. Actually, all polymers absorb moisture, which, depending on the chemistry of the specific polymer involved, can cause a host of reversible or irreversible physical, thermal, mechanical and/or chemical changes. Indeed moisture absorption typically results in plasticization of the matrix caused by interruption of weak bonding between polymer chains producing reductions in the polymer’s strength, modulus, strain at failure, and toughness. Moreover, moisture-induced swelling of the polymer matrix can cause irreversible damage through matrix cracking and fiber-matrix debonding.

• Heat and fire: FRP materials are susceptible to elevated temperatures during fire or high temperature service environments. In particular, most FRPs currently used in infrastructure applications suffer significant degradation of mechanical and bond properties at temperatures exceeding the glass transition temperature of their polymer matrix, Tg, due to matrix softening and plasticization, as shown in Figure 1.9.

• Cold and freeze-thaw cycling: Cold and freeze-thaw cycling may affect the dura-bility performance of FRP components through changes that occur in the behavior of the component materials at low temperatures, or due to differential thermal expansion between the polymer matrix and fiber components which may produce residual stresses in the material. These stresses contribute to matrix micro-cracking and fiber-matrix debonding.

• Alkalinity and corrosion: Laboratory tests have shown that glass fibers suffer degradation of mechanical properties under exposure to high pH solutions (alkaline environments). Nevertheless, in FRP composites, damage to glass fibers depends also on the protection provided by the polymer matrix, the level of applied stress, and the temperature. Finally, direct contact between carbon FRP materials and metals could potentially cause galvanic corrosion of the metal component.

• Ultraviolet radiation: Direct exposure to UV light causes degradation of polymer constituents through a mechanism known as photodegradation in which UV radiation within a certain range of specific wavelengths breaks chemical bonds between polymer chains. Although UV degradation is typically confined to a relatively thin layer near the surface, UV-induced surface flaws can cause stress concentrations that may result in failure of FRP components at lower loads than unexposed specimens. FRP components can be protected from UV radiation through the use of UV resistant paints, coatings, sacrificial surfaces, or various UV-resistant polymer resins.