153 Joining of metal foams

Metal foams can be soldered and welded. Foams have a cellular structure resembling, in some ways, that of wood. Because of this they can be joined in ways developed for wood, using wood screws, glue joints or embedded fasteners. Figure 15.1 summarizes joining methods.

Solder

è

i

r

i

Wood screws Soldering/brazing Glue line „—n

Wood screws Soldering/brazing Glue line „—n

Gluing

Welding

Embedded fastening

Embedded fastening

Figure 15.1 Ways of fastening and joining metal foams

Welding, brazing and soldering

Welding and brazing are best used for foams with integral skins. Studies of laser welding (Burzer et al., 1998) show promise, but the technique requires careful control. Brazing of Al-based foams with aluminum--eutectic alloys is practical. The soldering of aluminum foams requires a flux to remove the oxide film. If the flux penetrates the foam it causes corrosion, so soldering is only practical for sandwiches or skinned structures, restricting the solder to the outer surface of the skin. Soldered joints weaken the foam, which fails at a stress less that the tensile strength of the foam itself.

Adhesives

Foams can be glued with the same adhesives used to bond the base metal (Olurin, et al., 1999). The glue joints are usually stronger than the foam itself. There are some drawbacks: low thermal stability, mismatch of expansion coefficient and the possible creation of a thermal and electrical isolation barrier. Provided these are not critical to the design, adhesives (particularly epoxies)

allow simple, effective attachment. Typical of their use is the attachment of face-sheets to metal foam cores in sandwich-panel construction.

Fasteners

Embedded fasteners (Figure 15.2), when strongly bonded by threads or adhe-sives to the foam itself, pull out at an axial load

Figure 15.2 Embedded fasteners: wood screws and inserts

where 2R is the diameter, I is the embedded length of the fastener, x ^ R is a small end correction allowing for the tapered tip of the fastener if there is one, and as is the shear-yield strength of the foam. This last can be estimated as

y,s giving

\PsJ

where ay,s is the yield strength of the solid of which the foam is made. Figure 15.3 shows the dependence of pull-out load on foam density, on logarithmic scales, for a range of fasteners. All scale with density in the way described by Equation (15.3). Experiments confirm the dependence on length and diameter.

Bolted fasteners (Figure 15.4) fail when the head of the fastener pulls through the foam. The pull-through load is

The first term is simply the crushing strength, ac, of the foam times the contact area of the washer, of radius Rw. The second accounts for the tearing of the

o

Insert (dry)

Nail (dry)

0

Wood Screw (dry)

A

Studs (dry)

Insert (with epoxy)

Nail (with epoxy)

Wood Screw (with epoxy)

Stud (with epoxy)

Slope 1.5

0.06

Relative Density p/pc

Figure 15.3 The dependence of pull-out load on relative density for a range of embedded fastners (wood screw diameter 2 a = 4.8 mm; nail diameter 2 a = 4.5 mm; stud diameter 2 a = 6 mm; threaded insert diameter 2 a = 20 mm; all have embedded length I = 20 mm)

Figure 15.4 A bolted fastener

foam around the periphery of the washer. The tear-energy per unit area, y, has been measured. It is adequately described by

where y0 is a characteristic of the material of which the foam is made; for Alporas foam, its value is y0 = 260kJ/m2.

Through-fasteners may be required to carry bearing loads (Figure 15.5). Initial yield occurs when the mean bearing load exceeds the crushing strength of the foam, that is, when

i ,

2R ^

F

F

Figure 15.5 A through-fastener carrying bearing loads a c where oc is the crushing strength of the foam, approximated by

(see Chapter 4, Equation (4.2)). Once yielding has occurred the fastener is no longer secure and fretting or local plasticity caused by tilting make its behavior unpredictable. Equation (15.6), with an appropriate safety factor, becomes the safe design criterion.

Fmax/Fp

0.55

i

y

0.50

y

0.40

10 102 103 104 105 106 Number of cycles

Figure 15.6 The accumulated displacement under tension-tension cyclic loading of embedded fastners at various levels of peak cyclic stress

Cyclic loading of fasteners

Cyclic loading leads to a response typified by Figure 15.6. At peak cyclic loads, Fmax, below the monotonic pull-out load, Ff, the response (both for pull-out, Equation (15.3), and for bearing loads, Equation (15.6)) is essentially elastic until, at a critical number of cycles which depends on Fmax/Ff, the displacement per cycle increases dramatically. This response parallels that for fatigue of plain specimens (Chapter 8). Figure 15.7 shows a plot of load, normalized by the monotonic pull-out load, versus number of cycles to failure, defined as the number of cycles at the knee of the curves of Figure 15.6, for an embedded fastner subjected to cyclic pull-out loads. It resembles the S-N curves for metal foams, but the slope is slightly steeper, suggesting that damage accumulates slightly faster than in the cyclic loading of plain specimens.

1 101 102 103 104 105 106 107 Number of cycles to failure, nt fmax/fp = a nfa

1 101 102 103 104 105 106 107 Number of cycles to failure, nt

Figure 15.7 Cyclic peak-load plotted against number of cycles to failure for embedded fasteners

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