What is the simple reason for some materials being stronger under compress?

Ceramic Welding Backing | Ceramic Backing Welding| Ceramic Backing Taps

Most, if not all, materials that are restrained or self-restrained by 
their bulk are stronger in compression than in tension.  This is because 
compression forces molecules together and puts less stress on molecular, 
ionic, or metallic bonds or on intermolecular forces. A material under 
compression usually becomes harder to compress its modulus increases.  The 
modulus of a material under tension is effectively  constant until it 
breaks [fracture].  Materials that have large elastic and plastic 
deformation ranges are strong in tension.

Reasons for being weak in tension 
1. Weak intermolecular forces- liquids and gases that are under 
confinement such as in pistons.  These have almost no tensile strength. 
2. High elasticity and/or hardness combined with no ductility or plastic 
deformation; most ceramics and even some hardened metals fall in this 
category. These materials are strong but in tension they fail at crystal 
imperfections, inclusions and crystal vacancies because the stress is 
concentrated at these points and the material lacks ductility to relieve 
the stress.  Materials with these characteristics tend to fail 
catastrophically.  The properties are a strong function of impurities and 
preparation methods.  Progress has been made in more ductile ceramics that 
have improved tensile properties.
3. Materials that just are weak and usually ductile.  Lead and gold have 
weak tensile properties with good ductility and malleability.  Copper has 
moderate tensile strength with high ductility.
4. Agglomerates that are a mixture of high modulus materials held together 
by weak glues[concrete, cement, dental amalgam] or even by no glue at all 
such as a stack of bricks or blocks or powders such as sand or flour.  The 
latter, if constrained, can be compressionally strong but are not 
predictable.  I think the Leaning Tower of Pisa has a problem with this.

Reasons for being weak in compression
1. No flexural strength for example a piece of nylon monofilament , a thin 
wire
2. Poor orthogonal constraint, a chain, a rope, a woven wire.
3. Plastic materials that flow. 
4. Molecular and crystal alignment, which can affect tension also.  
Layered crystals such as mica or graphite have different strengths in 
different planes.  Some rubbery and plastic materials flow readily because 
of internal molecular arrangement.

There are many materials that behave very differently with changing 
temperature, frequency, or built in stress, or with crystal structure.  

These are some of the general reasons; I have probably omitted many more.

A good reference for this is any good materials science or metallurgy 
text.  Also discussion with Engineers about reasons for materials choices 
they make can be very enlightening.  Much progress is being made in 
composite and layered materials that have improved properties.  The 
important thing is a sound evaluation of the requirements for a particular 
application and a thorough knowledge of the properties of materials chosen 
for the application and correct mating of the two.

A good reference: Lawrence Van Vlack,  Elements of Materials Science and 
Engineering,  Addison Wesley

Date: 2023-04-10     hits: 450    Return