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Theory (by Ellis Prach)
A Crack is the formation of two new surfaces from a single surface. One way in which cracks form is by repetitive pulling on the metal. Thermal expansion and contraction caused by alternate heating and cooling creates repetitive pulling and is a common cause for crack initiation. The greater the differentiation in temperatures the fewer the cycles are required to start a crack.
Cracks generally form at sharp corners and welds, also on the backside of the heat exchanger, due to restrictive air flow. The stresses and forces in these locations make the metal more susceptible to crack initiation. The initial crack commonly is very small. It increases in length with each expansion and contraction. At first, the crack growth is barely discernable. With each thermal cycle, the amount of growth is greater than it was in the preceding cycle. However, the amount of crack growth is not a constantly increasing increment, but is a function of the differences in temperature, time and crack length.
Therefore, the rate of crack growth is not readily predictable under normal circumstances or in service, as opposed to a controlled laboratory environment. It is not possible to predict weather a crack will become critical in a matter of months, seasons or years. Any discernible crack has the potential for becoming a significant crack within an unknown time.
Some cracks may be through thickness cracks, which means the crack has actually penetrated the metal all the way through. Some cracks are part-through thickness cracks, that have not yet penetrated the opposite face of the metal. These conditions describe crack depth. Frequently, crack depth can be determined by the length of the crack. As a general rule of thumb, it may be said that a crack one to two times longer than the thickness of the metal will be a through thickness crack. Short cracks tend to be tight cracks. The opening between the surfaces, or the crack width tends to be minimal. As the crack grows in length, the crack width tends to enlarge.
Any crack has length, depth and width and the potential to grow in all directions. Thermal variables influence the rate at which the crack will grow and the shape it will take. Gases will escape through the smallest possible thickness crack. As the crack grows longer and, consequently wider, the quantity of escaping gases increase greatly.