Liberty Ship Failures
Failure Analysis Overview
In engineering, failure analysis is the process of determining how and why a component failed. Basically it is the process of learning from your mistake. To fix the problem it is necessary to first determine what went wrong. There are two basic steps to this process:
- Determine where the failure originated
- Determine what mechanism caused the crack to propagate to failure
Numerous mechanisms can cause failure, and these will be explained in the next section.
The first thing to do in failure analysis is to look at the failure surfaces of the component. The photo below shows a failure surface of a turbine blade. The area in the lower right is an open chamber which cooling air passes through to keep the blade temperatures down to an acceptable level. Above this chamber is an area shaped like about a fifth of a pie. The vertex of this angle is where the crack initiated. Once the crack origin is determined, the next step is to determine what cause the crack to grow. Possible propagation mechanisms include:
Fatigue failures are the result of repeated loadings which are applied and removed. Although none of the loads could cause failure on its own, this repeated cycling can initiate and propagate a crack to failure. Experts estimate that 50-90% of mechanical failures are due to fatigue. This is because potential fatigue failures are difficult to detect. Testing for fatigue takes a lot of time and money, since many loading cycles must be applied to cause failure. Engineers attempt to design components so that they have adequate fatigue life, but sometimes they are unsuccessful. This is usually because local stress concentrators (holes, notches, etc.) are greater than expected. These concentrators can cause a crack to initiate which will grow to failure.
Corrosion and Erosion
Corrosion is a chemical process. The most common example of corrosion is rust, which is a chemical interaction between iron and oxygen. Rust is greatly accelerated by the presence of water and salt. Rust is particularly damaging because it flakes off exposing more base metal. Other metals, such as aluminum, also react with oxygen. In the case of aluminum, an oxide is created that tighly adheres to the surface, and becomes a protective layer that prevents oxygen from reacting with the base metal. The chemical processes of corrosion tend to attack certain portions of the microstructure and leave other portions untouched. In the photo below, corrosion caused the pitting on the left. Notice the white ridges in the cavity. This is microstrutural material that was left behind after corrosion.
Erosion is a mechanical process where an abrasive material is blasted against a component. A good example of this would be the damage done to helicopter blades when operating in a desert environment. When sand gets kicked up into the air, the helicopter blades smash into the sand particles at high speed, causing damage to the blades. As shown on the right in the photo below, erosion pitting tends to be smoother, since it does not selectively attack the microstructure.
Corrosion & Erosion
Overload failures are relatively rare. This is the case when the applied load is more than the component can handle, and it fails as soon as the load is applied. This occurs when the component is very poorly designed, or it is exposed to unexpected loads which it was not designed to accommodate. An example of this is an airplane that is loaded way beyond its design specifications, resulting in failure of the landing gear when it touches down.
Tensile overloads of ductile materials show what are called "cup & cone" failures. This is shown in the photo below.