136 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair Longitudinal Pavement 日ase Subgrade Pavement Subgrade Figure 5.5.A pavement with base(gravel layer)undemeath and subgrade(soil layer)undemeath the base.The base layer allows water to flow through it in order to avoid the local collection of water.The longitudinal direction refers to the direction of traffic precipitate-known as ductile iron-is used instead.Ductile iron is akin to but different from cast iron,which has flake graphite precipitate.Due to the spheroidal graphite precipitate,ductile iron is stronger and more ductile than cast iron.This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection.Corrosion is particularly serious when the water is acidic.It causes tuberculation,which refers to the formation of small mounds of corrosion products on the inside surface of a pipe.Due to the centrifugal casting used to manufacture iron pipes,there is variation in the wall thickness along the length of a pipe.As a result,the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement,but may have asbestos re- inforcement.Asbestos fibers are an effective reinforcement,but their carcinogenic character results in health concerns.Due to the water in the pipe and in the soil surrounding the pipe,corrosion is an issue.As a result,the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication,since the creation of a composite involves the joining of various components,such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite
136 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Pavement Base Subgrade Pavement Subgrade Base Longitudinal Figure 5.5. A pavement with base (gravel layer) underneath and subgrade (soil layer) underneath the base. The base layer allows water to flow through it in order to avoid the local collection of water. The longitudinal direction refers to the direction of traffic precipitate – known as ductile iron – is used instead. Ductile iron is akin to but different from cast iron, which has flake graphite precipitate. Due to the spheroidal graphite precipitate, ductile iron is stronger and more ductile than cast iron. This provides an example of a layered composite involving the use of metal as one layer and a cement-based material as the other layer. A ductile iron pipe has a cement mortar lining inside the pipe for the purpose of corrosion protection. Corrosion is particularly serious when the water is acidic. It causes tuberculation, which refers to the formation of small mounds of corrosion products on the inside surface of a pipe. Due to the centrifugal casting used to manufacture iron pipes, there is variation in the wall thickness along the length of a pipe. As a result, the durability of a pipe varies along its length. Both unreinforced concrete and steel-reinforced concrete are used for pipes. The unreinforced concrete has no steel reinforcement, but may have asbestos reinforcement. Asbestos fibers are an effective reinforcement, but their carcinogenic character results in health concerns. Due to the water in the pipe and in the soil surrounding the pipe, corrosion is an issue. As a result, the corrosion of the steel rebars and the consequent degradation of the steel-concrete bond are an important consideration for steel-reinforced concrete pipes. 5.3 Materials for Joining Joining is at the heart of composite fabrication, since the creation of a composite involves the joining of various components, such as the joining of fiber and matrix. The bonding between the reinforcement and the matrix is critical to the mechanical integrity of a composite
5.3 Materials for Joining 137 Due to the limited size of a structural component (for example,the size of a panel is limited by the equipment used to fabricate it),most structures involve the joining of components.The structural integrity of the resulting joints is critical to the mechanical integrity of the overall structure. Joining may be achieved by sintering,welding,brazing,soldering,adhesion,or fastening.This section covers various types of joints. 5.3.1 Sintering or Autohesion Diffusive adhesion refers to bonding that results from the diffusion of certain species from one party to another.Sintering is one of the forms of diffusive adhe- sion.It involves solid-state diffusion.Sintering commonly refers to powder met- allurgy-the bonding of solid particles due to the solid-state diffusion of atoms between adjacent particles,as illustrated in Fig.1.7 and shown in Fig.5.6 for the sintering of silver particles.In the field of thermoplastic polymers,sintering is known as autohesion,which refers to diffusive adhesion at temperatures above the glass transition temperature Tg and below the melting temperature Tm. In liquid-state sintering,at least one but not all of the elements exist in the liquid state.This means that liquid-state sintering (a rather confusing term)involves solid-state diffusion that is supplemented by the presence of a liquid,the flow of which aids the sintering.For example,the fabrication of a silicon carbide whisker copper-matrix composite may be achieved by sintering a mixture of silicon car- bide whiskers and copper particles at a temperature slightly below the melting temperature of copper,so that only solid-state diffusion occurs(i.e.,the admix- ture method).The diffusion involves copper and does not involve silicon carbide, which has a much higher melting temperature than copper.The addition of a mi- nor proportion of zinc particles to the mixture causes the presence of zinc liquid during the sintering,since the melting temperature of zinc is much less than that of 1Hm30000X Figure 5.6.Scanning electron microscope photograph of the surface morphology of a silver particle organic-based thick-film paste on an alumina substrate after heating(known as firing)in air at 300C for 30 min.The heating causes the buring out of the organic vehide,in addition to the sintering of the silver particles.The arrows show the necks between adjacent silver partices.The silver particles are irregularly shaped,with sizes ranging from 1.5 to 2.5 um.(From [1])
5.3 Materials for Joining 137 Due to the limited size of a structural component (for example, the size of a panel is limited by the equipment used to fabricate it), most structures involve the joining of components. The structural integrity of the resulting joints is critical to the mechanical integrity of the overall structure. Joining may be achieved by sintering, welding, brazing, soldering, adhesion, or fastening. This section covers various types of joints. 5.3.1 Sintering or Autohesion Diffusive adhesion refers to bonding that results from the diffusion of certain species from one party to another. Sintering is one of the forms of diffusive adhesion. It involves solid-state diffusion. Sintering commonly refers to powder metallurgy – the bonding of solid particles due to the solid-state diffusion of atoms between adjacent particles, as illustrated in Fig. 1.7 and shown in Fig. 5.6 for the sintering of silver particles. In the field of thermoplastic polymers, sintering is known as autohesion, which refers to diffusive adhesion at temperatures above the glass transition temperature Tg and below the melting temperature Tm. In liquid-state sintering, at least one but not all of the elements exist in the liquid state. This means that liquid-state sintering (a rather confusing term) involves solid-state diffusion that is supplemented by the presence of a liquid, the flow of which aids the sintering. For example, the fabrication of a silicon carbide whisker copper-matrix composite may be achieved by sintering a mixture of silicon carbide whiskers and copper particles at a temperature slightly below the melting temperature of copper, so that only solid-state diffusion occurs (i.e., the admixture method). The diffusion involves copper and does not involve silicon carbide, which has a much higher melting temperature than copper. The addition of a minor proportion of zinc particles to the mixture causes the presence of zinc liquid during the sintering, since the melting temperature of zinc is much less than that of Figure 5.6. Scanning electron microscope photograph of the surface morphology of a silver particle organic-based thick-film paste on an alumina substrate after heating (known as firing) in air at 300°C for 30min. The heating causes the burning out of the organic vehicle, in addition to the sintering of the silver particles. The arrows show the necks between adjacent silver particles. The silver particles are irregularly shaped, with sizes ranging from 1.5 to 2.5μm. (From [1])
138 5 Materials for Lightweight Structures,Civil Infrastructure,Joining and Repair 25 ●Cu/SiCw Coated Cu/SiCw Admixture 20 ▲Brass/SiCw Coated △Brass/SiCw Admixture IOA) 15 10上 0 102030405060 Vol.SiCw Figure 5.7.Variation of the porosity with the silicon carbide whisker volume fraction in copper-matrix and brass-matrix composites made by the coated filler method and the admixture method.(From[2]) copper.Thus,the presence of zinc results in liquid-state sintering;in other words, the solid-state diffusion of copper that is supplemented by the dissolution of cop- per in the zinc liquid,which flows,thereby enhancing the mass transport during sintering.Figure 5.7 shows that,for the same filler volume fraction,the presence of zinc results in a composite of lower porosity,whether or not the conventional admixture method of powder metallurgy is used for the composite fabrication (Sect.1.3.2).If the coated filler method of powder metallurgy is used,the presence of zinc does not make much difference to the porosity of the composite since this method is highly effective at producing composites of low porosity.The presence of zinc in the copper matrix makes the matrix brass. 5.3.2 Welding A welded joint refers to a joint made by melting parts of the two members involved in the joint at their interface and their subsequent solidification upon cooling.The joining commonly arises from the presence of van der Waals forces-electrostatic interactions between the electric dipoles of one member and those of the other member.These dipoles may be permanent or temporary.Joining based on van der Waals forces is known as dispersive adhesion or adsorption.However,excess material (called the filler metal)that is ideally of the same composition as the members is placed at or around the joint to provide additional mechanical support. The filler metal undergoes melting during welding.Figure 5.8 shows an I-beam (i.e.,a beam in the shape of the letter I)obtained by welding a vertical member and two horizontal members together.The filler metal takes the form of a fillet at the
138 5 Materials for Lightweight Structures, Civil Infrastructure, Joining and Repair Figure 5.7. Variation of the porosity with the silicon carbide whisker volume fraction in copper-matrix and brass-matrix composites made by the coated filler method and the admixture method. (From [2]) copper. Thus, the presence of zinc results in liquid-state sintering; in other words, the solid-state diffusion of copper that is supplemented by the dissolution of copper in the zinc liquid, which flows, thereby enhancing the mass transport during sintering. Figure 5.7 shows that, for the same filler volume fraction, the presence of zinc results in a composite of lower porosity, whether or not the conventional admixture method of powder metallurgy is used for the composite fabrication (Sect. 1.3.2). If the coated filler method of powder metallurgy is used, the presence of zinc does not make much difference to the porosity of the composite since this method is highly effective at producing composites of low porosity. The presence of zinc in the copper matrix makes the matrix brass. 5.3.2 Welding A welded joint refers to a joint made by melting parts of the two members involved in the joint at their interface and their subsequent solidification upon cooling. The joining commonly arises from the presence of van der Waals forces – electrostatic interactions between the electric dipoles of one member and those of the other member. These dipoles may be permanent or temporary. Joining based on van der Waals forces is known as dispersive adhesion or adsorption. However, excess material (called the filler metal) that is ideally of the same composition as the members is placed at or around the joint to provide additional mechanical support. The filler metal undergoes melting during welding. Figure 5.8 shows an I-beam (i.e., a beam in the shape of the letter I) obtained by welding a vertical member and two horizontal members together. The filler metal takes the form of a fillet at the
5.3 Materials for Joining 139 Figure 5.8.A steel I-beam,where the components have been joined by welding junction between the vertical member and the horizontal member being joined. The molten filler metal penetrates the space between the two members,although this penetration is not shown in Fig.5.8.This type of weld is called a fillet weld, which is widely used to make lap joints,corner joints,and T-joints. Because the heat associated with welding affects the microstructure of the mem- bers near the joint,the members next to the joint region tend to have a different microstructure than the parts of the members away from the joint region.The zone where the heat has affected the microstructure is known as the heat-affected zone, which is immediately next to the fusion zone(the zone that underwent melting during welding).The change in microstructure is usually undesirable in terms of mechanical properties,as it can take the form of precipitate coalescence(thereby forming larger precipitates),grain growth,and recrystallization.In addition,ther- mal expansion of the members in the heat-affected zone during welding can cause thermal stress.Therefore,subsequent use of the welded structure under dynamic loading tends to cause fatigue cracks near the toe(tip)of a fillet weld,as illustrated in Fig.5.9. A special type of welding involves the joining of members in the form of a ther- moplastic polymer,which melts upon heating and subsequently solidifies upon cooling.The joining results from the presence of van der Waals forces,as in the Fatigue crack Weld Figure 5.9.A fatigue crack near the toe of the fillet of a weldjoining two members that are perpendicular to one another
5.3 Materials for Joining 139 Figure 5.8. A steel I-beam, where the components have been joined by welding junction between the vertical member and the horizontal member being joined. The molten filler metal penetrates the space between the two members, although this penetration is not shown in Fig. 5.8. This type of weld is called a fillet weld, which is widely used to make lap joints, corner joints, and T-joints. Because the heat associated with welding affects the microstructure of the members near the joint, the members next to the joint region tend to have a different microstructure than the parts of the members away from the joint region. The zone where the heat has affected the microstructure is known as the heat-affected zone, which is immediately next to the fusion zone (the zone that underwent melting during welding). The change in microstructure is usually undesirable in terms of mechanical properties, as it can take the form of precipitate coalescence (thereby forming larger precipitates), grain growth, and recrystallization. In addition, thermal expansion of the members in the heat-affected zone during welding can cause thermal stress. Therefore, subsequent use of the welded structure under dynamic loading tends to cause fatigue cracks near the toe (tip) of a fillet weld, as illustrated in Fig. 5.9. A special type of welding involves the joining of members in the form of a thermoplastic polymer, which melts upon heating and subsequently solidifies upon cooling. The joining results from the presence of van der Waals forces, as in the Figure 5.9. A fatigue crack near the toe of the fillet of a weld joining two members that are perpendicular to one another