Sistemas MIMO-UWB

The majority of civil engineers have little experience in selecting appro-priate adhesives. This section describes properties of adhesives—in their unmixed, mixed, curing, and hardened states—that should be considered when specifying an adhesive system.

1. Unmixed State (Shelf Life): As with any chemical materials, there is a period during which the unmixed adhesive components may be stored without undergoing signifi cant deterioration. This period is often called “shelf life.” Most commercially produced structural adhesives have an average shelf life ranging from 6 months to several years. The specifi ed shelf life may be prolonged by storing the adhesives at low temperature (e.g., in refrigerators). Depending on the application schedule and the expected construction delays, the engineer and/or the contractor should estimate the ordering time of adhesives and ensure that no material is used at or after the specifi ed expiration date provided by the manufacturer or supplier.

2. Freshly Mixed State: The engineer should evaluate the following properties of mixed adhesives:

a. Viscosity: For ease of spreading, a viscosity range of between 20 and 150 Pa at a shear rate of 10⁻¹ is recommended. When

adhesives are applied on a vertical surface, a yield stress of at least 20 Pa is required. It should be noted that the viscosity of the adhesives is altered by changing the ambient temperature and by the addition of fi llers. Figure 4-1 illustrates the infl uence of temperature and fi ller content on adhesive fl ow curves (Mays and Hutchinson 1992).

b. Usable Life (Pot Life): The cross-linking process starts as soon as the resin and the hardener of a room-temperature-cure adhe-sive are mixed. The rate of cross-linking, and consequently hardening, depends on both the reactivity of the adhesive for-mulation and the mobility of the molecules. The pot life of an adhesive defi nes the workability limit, while the “gel time”

defi nes the point at which solidifi cation commences. In many cases, the pot life and the gel time are very similar. However, the engineer is more interested in the pot life.

c. Wetting Ability: The ability of an adhesive to wet a substrate surface is essential to develop the adhesion process. Adhesives applied toward the end of their pot life tend to lack wetting ability.

d. Joint Open Time: The joint open time starts as soon as the adhe-sive is applied to the composite parts. It represents the time limit during which the joint should be closed; otherwise, an appreciable reduction in the adhesive bond strength may result.

Joint open time may be reduced at higher temperature and higher relative humidity.

3. Curing State: The impact of surrounding temperature on the curing process varies from one adhesive to another. In general, the curing time is inversely proportionate to the surrounding temperature. The curing time is reduced at higher temperatures, while a colder envi-ronment will result in a prolonged curing time. In fact, many epoxies stop curing below 41 °F (5 °C) unless a special formulation designed for the colder environment is used. Figure 4-2 illustrates the effect of formulation and cure temperature on the fl exural strength devel-opment of two-part epoxy adhesives. Figure 4-3 shows a typical cure temperature–time relationship for one-part toughened epoxy. The civil engineer should provide adequate environmental information to the manufacturer in order to specify the appropriate adhesive system. It should be noted that manufacturers can custom-design adhesives for different environments (Fig. 4-2).

4. Hardened State: The mechanical properties of a structural adhesive in the hardened state are related to its internal structure, including the molecular interactions of the adhesive system. It should be noted that, if the internal structure of the adhesive is modifi ed to change certain mechanical properties (e.g., strength, stiffness, and

Figure 4-1. Infl uence of temperature and fi ller content on adhesives fl ow curves.

A, Temperature effect on one-part hot-cure toughened epoxy; B, fi ller content effect on three-part fl exibilized epoxy polyamine.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

toughness), this will be at the expense of reducing other properties.

For example, if the desired property is higher toughness, the trade-off will be lower stiffness. For this reason, the engineer should set the required criteria for the application before specifying the adhesive system. In other words, be aware of the impact of requiring a higher value for one property on the other structural properties of the adhesive system under consideration. The mechanical properties of adhesives at the hardened state are usually provided by the Figure 4-2. Effect of formulation and cure temperature on fl exural strength development of two-part epoxy adhesives. A, Normal type; B, rapid-cure type.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

Figure 4-3. Cure temperature vs. time relationship for one-part epoxy.

manufacturer. However, the engineer may require verifi cation tests for some critical properties for both short- and/or long-term behav-ior of the specifi ed adhesive system.

Figure 4-4 shows a typical test specimen used to evaluate the tensile, shear, and fl exural strength of hardened adhesives (Mays and Hutchinson

Figure 4-4. Typical adhesives tests. A, Tensile specimen; B, shear box test on adhesive prism; and C, fl exural test on adhesive prism.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

1992). A typical tensile stress-strain curve for a range of epoxy adhesives is shown in Fig. 4-5. Chapter 7 of the MIL-17 Handbook (ASTM 2002) con-tains detailed information on mechanical testing of adhesively bonded joints.

Fracture toughness is another property that the engineer should evalu-ate when using adhesives, especially in seismic zones and for structures subjected to vibration and dynamic loading. For the hardened state, the engineer should also determine the state of the surrounding service envi-ronment and the expected temperature variation. One of the physical properties that should be evaluated is the glass-transition temperature (Tg), which, for any polymer, is defi ned as the specifi c temperature at which the polymers change from a relatively rigid, “glass-like” substance to a relatively viscous, “rubbery” material. This transition temperature varies from one adhesive to another and depends on several factors, including the polymer molecular weight, adhesive curing temperature, and rate of loading if the measurement process involves mechanical deformation.

One of the most convenient methods of measuring Tg is using a dif-ferential scanning calorimeter (DSC), as shown in Fig. 4-6. For engineers, quasi-static mechanical methods using a fl exural test on a hardened

Figure 4-5. Typical tensile stress–strain curves for a range of epoxy adhesives.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

Figure 4-6. Temperature dependence of bulk adhesive fl exural modulus.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

adhesive prism are more convenient for determining the heat distortion temperature (HDT). It should be noted that the results of such quasi-static methods are good only for comparative purposes since the results of these tests depend on the specimen confi guration and the selected rate of loading. Table 4-2 presents typical values for HDT for cold-cure epoxies.

As shown in Table 4-2, the HDT values range from 93.2 °F to 118.4 °F (34

°C to 48 °C), which, in many cases, will be exceeded, especially when the composite joints or composite/metallic bonded joints are directly exposed to high temperatures. In these cases, the engineer may evaluate the use of a one-part hot cure epoxy product with a Tg value of 212 °F (100 °C).

Figures 4-7 and 4-8 illustrate the temperature effect on the bulk adhesive fl exural modulus and bulk adhesive shear strength, respectively.

Table 4-2. Heat Distortion Temperatures of Cold-Cure Epoxies

Adhesive Type HDT (°C/°F)

Two-part cold-cure polyamide 40/104

Two-part cold-cure polyamine (aliphatic) 41/105.8 Two-part cold-cure polyamine (aliphatic adduct) 43/109.4 Two-part cold-cure polyamine (aromatic) 48/118.4

Two-part cold-cure polysulphide 34/93.2

HDT, heat distortion temperature.

Figure 4-7. Temperature dependence of bulk adhesive shear strength.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

Figure 4-8. Water uptake plots for a range of epoxy adhesives.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

Another important factor that needs to be considered by the structural engineer when specifying a structural adhesive is the possibility of water or water vapor attacks. All adhesives are sensitive to exposure to water or water vapors (except those specifi cally formulated for underwater applications). The water uptake is accommodated to a large extent by swelling.

To measure the water uptake properties of a structural adhesive, a thin-fi lm adhesive specimen may be immersed in water at a known tem-perature or stored in an environmental humidity chamber where the humidity and temperature are controlled for a specifi ed time. The water uptake is then measured after a specifi ed exposure time and the fractional uptake is plotted against the square root of time per unit thickness, as shown in Fig. 4-9. The strength loss of composite bonded joints due to water exposure may be dictated by adhesive plasticization or by displace-ment of the adhesive from the substrate due to water “wicking” along the interface, or by both indications (Mays and Hutchinson 1992).

Like other polymers, adhesives are viscoelastic materials and are sus-ceptible to “creep.” The engineer should be aware of this property when estimating the long-term stiffness of composite bonded joints. Creep is a phenomenon of movement, strain, or deformation in excess of the normal movement that results from the elastic qualities of the bonded joint. There are three creep stages, as shown in Fig. 4-10: (1) primary creep (1st stage);

(2) secondary creep (2nd stage); and (3) tertiary (3rd stage) or creep rupture. It should be noted that creep curves obtained from bulk hard-ened adhesive specimens do not necessarily compare with those obtained from bonded composite joints under similar stress and environmental conditions due to the nature of adherend restraint.

Figure 4-9. Typical creep and creep rupture time curves.

Strain (e)

High Stress Level

Low Stress Level

Figure 4-10. Schematic topography of solid surfaces.

Source: Mays and Hutchinson (1992), reprinted with the permission of Cambridge University Press.

Creep rates of adhesives follow exponential laws similar to those of stress-rupture analysis (Lubin 1982). Generally, the creep rate, v., of an adhesive can be expressed by the following equation:

ν =Ae⁻

Q

RT (4-1)

where

A, Q = constants dependent on stress level and material properties R = gas constant

T = temperature in the joint.

Brittle (e.g., high shear modulus) adhesives are less sensitive to creep as compared to ductile adhesives. In general, the more highly cross-linked the hardened adhesive structure and the higher the curing temperature, and hence the higher the Tg, the better the creep resistance (Mays and Hutchinson 1992).

In addition to moisture, three major factors that affect the creep of adhesives are stress, time, and temperature. Generally, high levels of stress are accompanied by high rates of creep; thus, the higher the level of stress, the greater the rate of creep. As the temperature increases (above ambient temperature), the creep rate increases with a greater rate when Tg is reached. As mentioned earlier, the Tg is sensitive to the rate of loading. For this reason, a “time/temperature superposition technique”

was developed to characterize the long-term response of polymers. This technique, initially articulated by Leaderman (1943) and further devel-oped by Findley and Onaran (1976), allows for predicting the long-term creep behavior of a specifi c adhesive system at different temperatures and loading conditions from relatively short-term test data.

Another important consideration in specifying the structural adhesive system is the fatigue characteristic of the adhesives. This is especially critical when the composite structure is subjected to cyclic or dynamic loading, such as bridges, machinery-supporting fl oors, and structures subjected to high wind loads.

The fatigue performance of an adhesive is affected by the viscoelastic nature of the material and its resistance to crack propagation, or fracture toughness. When adhesives are subjected to dynamic loads with low fre-quencies in a high-temperature environment, their viscoelastic effects will predominate their performance in a manner similar to that experienced with creep. Adhesives fracture when subjected to high-frequency dynamic loads coupled with low-temperature environment, due to crack propaga-tion either within the adhesive layer or at the adhesive/substrate inter-face. In this case, the crack propagation tends to control the number of load cycles that can be resisted by the adhesive prior to failure. In general, the fatigue performance of a structural adhesive in a composite bonded joint is linked to the joint confi guration and the stress distribution within the joint. For this reason, this issue is discussed further in Chapter 5.

4.5 GENERAL PROPERTIES OF ADHESIVES AND ADHERENDS Generally, adhesives used with polymer composites display character-istics similar to the composites they join. Stress–strain charactercharacter-istics of adhesives are nonlinear, and their mechanical properties are signifi cantly affected by elevated service temperatures and humidity as well as sus-tained loadings.

Surface preparation plays a dominant role in the reliability of an adhe-sive bond. Surface properties, such as a composite’s wettability and roughness, affect the ability of structural composites to be reliably bonded.

Wetting is a measure of an adhesive’s ability to spread across a solid surface. Liquids tend to bead on surfaces that have poor wetting charac-teristics. Materials such as polyethylenes and polypropylenes display poor wettability characteristics and are very diffi cult to bond. In contrast, plastics such as epoxies and polyesters are readily wetted and bonded.

Roughened adherend surfaces usually enhance the performance of an adhesive bond by developing greater mechanical interlock.

Most adhesives display higher compressive and shear capacities than tensile and peel strengths. In addition, most display signifi cantly greater shear than tensile deformations prior to fracture.

As a rule, thinner bond lines are preferred for maximum bond strengths and rigidities. Thinner bond lines are more resistant to cracking when fl exed, and they display less creep. They also display lower residual thermal stresses and have a lower probability of adverse inclusions.

Although adhesive joints provide a more uniform stress distribution within the joint as compared to mechanically fastened connections, stress concentrations are still present. The stress concentrations experienced in a simple lap joint are a signifi cant concern in polymer composites. These stresses can induce peeling along the ends of the joints, causing an

interlaminar failure in a laminate. For this reason, it is preferred that adhesively bonded composite parts be designed such that they are stressed parallel to their reinforcement. Maintaining small angles between layers of laminates can also minimize the potential for delaminations.

Adverse peeling loads can be avoided by either providing tapered laps or eliminating eccentricities within the connection by employing a dou-ble-lap, scarf, or similar confi guration. They can also be avoided by employing mechanical fasteners together with an adhesive.

Another factor in the selection of a joint confi guration is the need to choose a joint design in which the component parts are pressed down upon the adhesive rather than slid into position during assembly. With this method there is less likelihood that the adhesive will be pushed out of place.