El ángulo de rozamiento interno o ángulo de fricción es una propiedad de los materiales granulares el cual tiene una interpretación física sencilla, al estar relacionado con el ángulo de reposo o máximo ángulo posible para la pendiente de un conjunto de dicho material granular. En un material granuloso cualquiera, el ángulo de reposo está determinado por la fricción, la cohesión y la forma de las partículas pero en un material sin cohesión y donde las partículas son muy pequeñas en relación al tamaño del conjunto el ángulo de reposo coincide con el ángulo de rozamiento interno. Es especialmente importante en mecánica de suelos para determinar tanto la capacidad portante como la resistencia al deslizamiento de un terreno arenoso.

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The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope face is on the verge of sliding. The angle of repose can range from 0° to 90°. The morphology of the material affects the angle of repose; smooth, rounded sand grains cannot be piled as steeply as can rough, interlocking sands. The angle of repose can also be affected by additions of solvents; if a small amount of water is able to bridge the gaps between particles, electrostatic attraction of the water to mineral surfaces will increase the angle of repose, and related quantities such as the soil strength. When bulk granular materials are poured onto a horizontal surface, a conical pile will form. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area and shapes of the particles, and the coefficient of friction of the material. However, a 2011 study shows that the angle of repose is also gravity-dependent, while a 2013 study of martian dunes suggests that it is not gravity dependent. Material with a low angle of repose forms flatter piles than material with a high angle of repose. The term has a related usage in mechanics, where it refers to the maximum angle at which an object can rest on an inclined plane without sliding down. This angle is equal to the arctangent of the coefficient of static friction μs between the surfaces.

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