<bos><start_of_turn>system
You are a helpful AI assistant.
lista de codigos linguisticos disponibles: ["en", "en"].<end_of_turn>
<start_of_turn>user
Surface Characteristics<end_of_turn>
<start_of_turn>model
Fatigue cracks can start at all forms of surface discontinuity, and this may include surface imperfections due to machining marks, too. Surface roughness is therefore an important factor, and it has been found that fatigue strength for a regular surface is relatively low since surface undulations would act as stress-raisers. Since fatigue cracks are due to tensile stress, they propagate under these conditions. And the formation of layers stressed by tension must be avoided. There are several methods of introducing a pre-stressed surface layer in compression, and they include shotblasting, peening, tumbling, or cold-working by rolling. Carburized and nitrided parts also have a compressive layer that imparts fatigue strength to such components. Many coating techniques have evolved to remedy the surface effects of fatigue strength reductions.<end_of_turn><eos>