Strength of Materials and Properties of Matter

Collapsible Cow

What it shows:

A small crude spindly model of a cow is able to support five times its own weight. Another model, scaled up exactly six times in all dimensions, collapses under its own weight! Assuming that strength is proportional to cross-sectional area (∝ dimension 2) and weight is proportional to volume (∝ dimension 3), simply scaling the model up geometrically leads to the situation where the weight is too great for its strength.

How it works:

This demonstration was inspired by R.H. Stinson's apparatus note in the AJP (see References...

Read more about Collapsible Cow
Creep of Lead

What it shows:

A metal under stress will not fracture straight away, but will deform plastically due to the dislocation of crystal boundaries; this is called creep.

How it works:

Here we use lead as the test sample because there is significant creep compared to other metals. The lead is loaded (see fig.1) to a value that is just below the breaking stress of the sample. When creep occurs, the lead is drawn thinner at its weakest point (called 'necking', see fig.2) until its reduced cross-sectional area causes the sample to exceed its breaking...

Read more about Creep of Lead
Fracture Strength of Chalk

What it shows:

This demonstration allows you to compare chalk’s compressive strength with its tensile strength.

How it works:

We use railroad chalk, which although being softer and harder to work, is nice and big and easy to see. A sample is placed in each of the two types of testing assembly (details in Setting it Up), and loads carefully applied. Railroad chalk has a tensile strength of 195kNm-2 ± 30kNm-2 (a load of 2.5 to 3.5kg) and a compressive strength of 500kNm-2 ± 65kNm-2 ( a load of 7 to 9kg).

...

Read more about Fracture Strength of Chalk
Egg Shell Strength

What it shows:

Eggs have a reputation for being quite strong under compressional loads. You won't believe your eyes when you see how strong they actually are — an egg can support a person!

How it works:

An egg shell is a composite material, but primarily calcium carbonate, "nature's ceramic."1 We assume the ultimate compressional strength of the material to be about the same as bone (which is mostly calcium phosphate, but never mind): 170x106 N/m2. The diameter of a "large" chicken egg is about 1.75" and the typical shell...

Read more about Egg Shell Strength
Spiral Fracture

What it shows:

A spiral fracture is incurred when a torque is directed along the axis of a limb or shaft. Planes perpendicular to the axis are unaffected, but those parallel are twisted, which causes pure tensile forces in one part of the limb, pure compressive forces in another. Fracture occurs when either the compressive or tensile limit of the material is exceeded. This demo shows a spiral fracture in a simulated skiing accident.

How it works:

An old ski boot has a wooden plug placed snugly inside it acting as a foot. A 3 × 4cm square hole accommodates a 0.5m...

Read more about Spiral Fracture