Video: Identifying the Phenomenon Most Associated with the Critical Length of a Crack

What is a critical crack length most associated with? [A] creep permanent strain [B] plastic yielding [C] non-protective oxide films [D] fast fracture [E] high ductility


Video Transcript

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.

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