JULY/AUGUST 2019. DISCOVER 57
Seeing
Stress in
Action
Stretch a plastic bag between two
polarizers — screens that filter light waves
in one direction. You’ll see trippy bands of
color following stresses across the baggie.
This so-called photoelastic effect, exhibited by
transparent polymers and glasses, was once
used to test the integrity of structures. Before
computer simulations did the trick, engineers
built miniature models with photoelastic
materials, subjected them to forces
and observed the stress
colorscape.
Temperature
Matters
Hot water makes a cold glass crack or
even shatter due to thermally induced stress,
forces exerted by changing temperature. The
cup’s interior warms up and expands faster than the
exterior. With no room to grow, the glass cracks alongpre-existing defects.
Temperature also affects strength
— the ability to withstand stress — a point made
tragically clear by the World Trade Center collapse. The
110-story towers should have withstood an airplane
collision. But according to a 2002 paper in the Journal
of Engineering Mechanics, the explosion and
subsequent fire heated the support columns
to more than 1,400 degrees Fahrenheit,
causing them to lose the strength to
support upper floors.
Slow
and Steady
Stress
Fatigue refers to weakening due
to moderate stresses applied again
and again. A fatigued material will
develop microscopic cracks and
eventually break completely.
Bone stress fractures,
common in athletes,
form in this way.
Different Types
of Stress
Physicists and engineers distinguish between
several varieties of mechanical stress.
COMPRESSION
TENSION
SHEAR
BENDING
TORSION