Question

a.Why is the radius of crack opening critical?

b.Explain the crack growth from a surface crack and how it grows. Why does it not need to be over half the cross sectional surface area to fracture?

c.Use the Hunt Library or your preferred source and research how alloying and heat treatment can reduce or worsen the expected life.

Answer 1

1.

1. The smaller the radius of curvature, the greatest the stress concentration.

Near the crack tip, the stresses look like =

Sigma= K1gQ/(2pir)^.5

The total stress concentration depends on the radius of curvature of crack. The smaller the radius curvature greater will be the concentration.

Greater the radius of curvature smaller will be the stress concentration.

This is why radius of crack opening is critical.

How a crack grows is called "crack propagation" and this takes into account when a crack can start to grow, how far it grows, and what direction it goes in. So when can it grow? Well there are two ways to look at it. First, from the point of view of the stress intensity factor, KI. The material resists cracking with everything it has, which is called its fracture toughness. The crack can grow when its stress intensity factor reaches the fracture toughness of the material: KIc=KI The other way to look at when a crack can grow is the energy way, When a crack is formed, new surfaces are also formed, along the edges where the material has split apart. The material has to have enough energy to create these new surfaces or it will not crack. If G is the energy necessary for the crack to grow and R is the material's resistance to crack growth, the condition for a crack to grow is: G = R But once the crack has grown, will it keep growing? Things change once a crack has grown. The resistance of the material may have gone up or down; the energy necessary to grow the crack may also have gone up or down. In order for the crack to continue growing each time, the change in energy must equal the change in resistance. If the change in energy is less than the change in resistance, then the crack will not grow any more unless more force is applied. If the change in energy is greater than the change in resistance, there will be unstable crack growth. In this case the crack may grow until the structure fails.

Heat Treatment often requires some form of heat treatment to achieve an increase in hardness and obtain maximum strength and durability. Through the many different processes of heat treatment, the properties of steel are changed via physical and mechanical channels. As an added benefit, heat treatment can also aid in the manufacturing process.   The primary focus of the heat treatment process is to increase the strength and durability. In this process Fatigue Life of a metal decreases. The heat treatment can be done at higher temperature after that Quenching process is there then tempering. While going through all of these processes there is probability of formation of small cracks due to quenching. These cracks reduce the life span of a Metal. Since alloys are created to improve upon properties (usually mechanical strength) of the constituent pure metals, I think if there are any properties of the original base metals that are better than in the alloyed state. Yes, definitely. Two such properties are electrical conductivity and corrosion resistance. When a pure metal is alloyed, the secondary element acts as scattering centers that reduce the mean free path length for electrons, which increases electrical resistivity and thus lowers electrical conductivity. Therefore, a desirable property of metals, electrical conductivity, is degraded by alloying. The corrosion resistance of pure aluminum is excellent. Alloying aluminum with copper, iron and silicon is commonly done to improve mechanical and physical properties; however, these secondary elements also form phases that are catholic to the base aluminum matrix. The result is that although alloying does improve the strength of pure aluminum, it also reduces its corrosion resistance. This is a general trend for metals: the corrosion resistance of high purity metals usually decreases with alloying. Overall, alloying certainly produces desirable properties in metals but is not without its drawbacks either. But in most of those situations the benefits of alloying outweigh any potential disadvantages. That’s why the commercial version of a metal is so often used instead of its high purity counterpart, which is the whole point of alloying.