What is a critical crack length
most associated with: a) creep permanent strain, b) plastic yielding, c)
nonprotective oxide films, d) fast fracture, e) high ductility?
This question then is all about
what a critical crack length is and what it’s most commonly associated with. To see what the best connection
might be between critical crack length and one of our answer choices, let’s consider
some of those choices and what they mean. Starting with choice a, creep
permanent strain, we see from the word creep that whatever strain is developing is
long lasting and it happens very slowly. A creep permanent strain would
involve a very slow deformation that takes place.
Moving on to choice b, plastic
yielding, this involves the change in shape or the deformation of an object, which
can then be returned to its original shape. That’s why the yielding is called
plastic. Next we have nonprotective oxide
films, where we can tell what these films are constructed of but we’re not sure
their purpose as they’re called nonprotective. If we jump answer choice d and move
down to answer choice e, high ductility, this is a term that may be more
familiar. Ductility is a measure of a
material’s ability to be drawn out into a thin wire. Imagine a wire made of some type of
metal such as aluminum or copper. The ductility of that material is a
measure of how easy it is to do that with a given element.
And lastly we look at answer choice
d, fast fracture. Fast fracture is often associated
with the failure of a structural element. What’s a structural element? Say we had a large column of some
material and that this column was subject to a large stress along its length. Imagine that over time that stress
grows greater and greater and greater with the column under more and more force. Eventually we know from experience
that column will begin to crack. Very small at first and invisible
to the eye, but these cracks will grow as the force increases. Now let’s say that one of these
cracks gets to be a certain size; that is, it gets to be what we would call the
critical crack length. Once the length of a crack in our
column has reached that size, then any increase in the load on the column will tend
to have a very specific and dramatic effect.
That increase in load will
typically cause a very rapid expansion of the crack, with the tip of the crack as it
expands through the material, moving as fast as the speed of sound. This splitting or cracking or
fracturing of this column is known as fast fracture. It occurs when the slow degradation
of a supportive element, in this case evidenced by the small cracks which grew over
time, lead suddenly to a rapid loss of ability to support weight, hence the name
fast fracture. In other words, a crack of critical
length is right on the borderline of expanding via fast fracture to be a significant
structural defect in a support element.
We can say then that these two
terms are closely connected, critical crack length and fast fracture. Answer choice d then stands out as
our best option: critical crack length is most associated with fast fracture.