Information about laminated materials?

[Adam]

Can someone give me some information about laminated materials? Is a bar of steel made of many layers, for example, stronger than one solid piece?

 

[rdt2]

For many applications, it's not just strength you need but toughness (i.e lots of energy need to cause fracture). Laminating steel (as in samurai swords) improves the toughness rather than the strength (all other things being equal).

However, laminated materials are usually used for thin components in which the loads are in the plane of the laminate (e.g GRP car bodies). The loads can then be carried by strong-but-light fibres.

Laminates are not so good when the stresses are fully three-dimensional (e.g with engine blocks) - they tend to split across the plies (like wet plywood does).

Cheers,

Ron.

 

[red_fox77]

For improved strength/toughness you need a material that is not homogeneous, properties equal in all directions. Laminate material are usually very strong in 1D, so they alternate the direction of the material in layerd plies, which gives you good 2D strength/toughness.

Metals don't work this way, but are sometimes added to laminates to get other properties, like electrical performance or impact toughness.

You can, however, put metal fibers into a metal biner system and improve the properties of the material along the direction of the fibers. These are called metal-metal composites and are common.

 

[Tyro]

Hmm...laminates. You could take a PhD in that topic and still not know everything, the field is that vast.

When people talk about strength, there are usually 3 primary types of 'strengths' they could be referring to - yield strength, Elastic modulus and toughness.

One definition of the YS is, for example "Often defined as the stress needed to produce a specified amount of plastic deformation (usually, a 0.2 percent change in length)." For the layperson, "the stress at which a bar of material X when pulled, fails" usually suffices.

The Elastic modulus of a material is defined as its stress/strain ratio. It is analogous to the (normalised version of the) spring constant,k, in Hooke's Law, F=kx.

Toughness is a bit trickier to define, because of the multitude of tests one can do to abuse a material, but basically, the tougher the material, the more abuse it can take before failing.

The yield strength of a laminate material under uniaxial load depends on the laminate's construction and layout. For example, if the layers in your laminate were transverse to the axis at which you were applying the load (e.g. you pulling a sandwich apart) then it is weak. If we return to the sandwich analogy the only resistance to your force is the mayo and butter. If you pull the laminate in the plane of its laminae, then it is much stronger.

Same issue with the Elastic modulus.

As for toughness, in most cases laminates increase the ability of an otherwise homogeneous material to withstand abuse. There are exceptions to these, of course. CFRP panels on aircraft, for example, have terrible toughness (they weren't designed for that!) and the oft-mentioned example of dropping a spanner on them will severely damage the composite; while the famed Chobham armour on Challenger and Abrams tanks is a composite itself (they were designed for that!).

 

[Njorl]

One of the principle uses of laminates is to combine strengths of different materials. The simplest version of this is plywood. One layer resists splitting in one direction, the next resists splitting in the orthoganal direction, provided you lay the grains at right angles. Metals too, have directionally dependant structures. If you can disperse the directionality, you can improve the strength for some purposes (though weaken it for others).

Then there are countertops - the hard plastic surface does not gouge easily, but would shatter if left unsupported. The wood beneath would be easily gouged, but prevents the top from shattering.

That latter is how ceramic tank armor works. A high-velocity tungsten rod would melt its way through 18" of steel, or shatter ceramic like it was glass, but layer the ceramic on top of steel, and the rod can be deflected.

Njorl