Despite the best attempts of engineers, products still fail. When they do, the first step in fixing the design is performing a failure analysis to determine where the failure originated what caused it to propagate.
The initial task in failure analysis is to determine the crack origin. This is often where the component is week, such as a defect, like a void (area where there is no material) or an inclusion (foreign material). It can also be where the component is sound, but the local stress is high due to a stress concentrator. Stress concentrators can greatly increase the local stress. Consider a panel which is held at one end and has a load applied at the other end. If a small hole is drilled in the panel perpendicular to the direction of loading, there will be two spots along the hole where the local stress will be three times the nominal stress.
The second step is to determine what mechanism caused the crack to grow once it initiated. There are numerous possibilities, including:
Overload
Fatigue
Creep
Overload is the simplest type of failure. The applied load exceeds the material capability and the component breaks. Overload failures can be identified by looking at the fracture surface. They have a distinctive "Cup and Cone" appearance. Overload failure often occur after another component fails. Large wind turbines are designed so that the blade pitch will be altered in high winds to reduce the aerodynamic force on them, preventing the blades from rotating too fast. If this system does not work properly, the blades can fail catastrophically.
Fatigue is the most common type of failure. It occurs when a component is cyclically loaded. One cycle is applied each time the load is applied and removed. The simplest example of fatigue is to take a paper clip and straighten it out. Then bend it backward and forward. After doing this a few times, it will most likely break. When cracks propagate due to fatigue, they leave what are called "beach marks". These are so named because they look like the patterns left by waves on a beach. These marks show the crack front at different points in time as it propagates. One of the most famous examples of fatigue is the De Havilland Comet. It became the first production commercial jet when it debuted in 1952. It soon encountered problems, and several aircraft were lost. After extensive testing, it was discovered that fatigue cracks initiated at the corners of the windows. Since the aircraft is pressurized, the fuselage underwent a loading cycle every time it took off and climbed to cruising altitude. After the location of the crack origination site, the windows were redesigned to make them more round, which reduced the stress concentrator. Unfortunately the aircraft's reputation was ruined, and sales of the aircraft never rebounded.
Creep cracking occurs when load is applied to a component at a high temperature and held for an extended time. This type of cracking tends to follow grain boundaries, which are less resistant to creep cracking. Because of this, high performance gas turbine blades are often cast so that as a single crystal, so that they have no grain boundaries. Single crystal blades can run approximately 50F hotter than conventionally cast blades.
The initial task in failure analysis is to determine the crack origin. This is often where the component is week, such as a defect, like a void (area where there is no material) or an inclusion (foreign material). It can also be where the component is sound, but the local stress is high due to a stress concentrator. Stress concentrators can greatly increase the local stress. Consider a panel which is held at one end and has a load applied at the other end. If a small hole is drilled in the panel perpendicular to the direction of loading, there will be two spots along the hole where the local stress will be three times the nominal stress.
The second step is to determine what mechanism caused the crack to grow once it initiated. There are numerous possibilities, including:
Overload
Fatigue
Creep
Overload is the simplest type of failure. The applied load exceeds the material capability and the component breaks. Overload failures can be identified by looking at the fracture surface. They have a distinctive "Cup and Cone" appearance. Overload failure often occur after another component fails. Large wind turbines are designed so that the blade pitch will be altered in high winds to reduce the aerodynamic force on them, preventing the blades from rotating too fast. If this system does not work properly, the blades can fail catastrophically.
Fatigue is the most common type of failure. It occurs when a component is cyclically loaded. One cycle is applied each time the load is applied and removed. The simplest example of fatigue is to take a paper clip and straighten it out. Then bend it backward and forward. After doing this a few times, it will most likely break. When cracks propagate due to fatigue, they leave what are called "beach marks". These are so named because they look like the patterns left by waves on a beach. These marks show the crack front at different points in time as it propagates. One of the most famous examples of fatigue is the De Havilland Comet. It became the first production commercial jet when it debuted in 1952. It soon encountered problems, and several aircraft were lost. After extensive testing, it was discovered that fatigue cracks initiated at the corners of the windows. Since the aircraft is pressurized, the fuselage underwent a loading cycle every time it took off and climbed to cruising altitude. After the location of the crack origination site, the windows were redesigned to make them more round, which reduced the stress concentrator. Unfortunately the aircraft's reputation was ruined, and sales of the aircraft never rebounded.
Creep cracking occurs when load is applied to a component at a high temperature and held for an extended time. This type of cracking tends to follow grain boundaries, which are less resistant to creep cracking. Because of this, high performance gas turbine blades are often cast so that as a single crystal, so that they have no grain boundaries. Single crystal blades can run approximately 50F hotter than conventionally cast blades.
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