spalling: one construct, many systems

In geomorphology, spalling (exfoliation) is the detachment of curved shells or flakes from the surface of a rock mass when near-surface stresses (from unloading and pressure release, thermal cycling, or moisture) exceed the rock's tensile strength parallel to the free surface. Blackwelder's 1925 Exfoliation as a phase of rock weathering described how granite domes shed concentric sheets in just this way. The referent is a fragment separating from the free surface of a brittle solid under surface-parallel stress, the same mechanical idea that recurs in decaying building stone and in fatigued steel.[1]

Unit

The thickness and rate of rock shells detaching from an exposed surface

Recovery means

Not applicable

Measured by

Assessed by the thickness and spacing of exfoliation sheets, monitored surface detachment, and modelled near-surface stress

In the study of masonry and building-stone durability, spalling is the loss of flakes or outer layers from the dressed face of stone or concrete when pressures build up just below the surface (ice growing in pores during freeze–thaw, or salts crystallising) and break the outer skin away. Hall's 2006 treatment of freeze–thaw in Fracture and failure of natural building stones ties this surface loss to the water held in the stone's pore network and the stress it generates on freezing. It is the same surface-parallel fracture as natural rock exfoliation, here decaying a wall rather than a cliff, and it is measured by the depth and area of material shed.[2]

Unit

The depth and area of stone or concrete lost from a face

Recovery means

Not applicable

Measured by

Quantified by surface recession and lost area, laboratory freeze–thaw and salt-crystallisation cycling, and the stone's porosity and saturation

In tribology, spalling is the dominant failure mode of rolling bearings and gears: repeated rolling contact drives subsurface shear stresses that nucleate cracks below the raceway, and those cracks grow back to the surface and release a flake of metal, leaving a pit. Lundberg and Palmgren's 1949 Dynamic capacity of rolling bearings founded the statistical life theory that ties a bearing's endurance to the subsurface stresses which seed these spalls. It is the identical idea of a surface fragment detaching under near-surface stress seen in weathering rock and stone, here driven by cyclic contact fatigue in metal and clocked as the life to the first spall.[3]

Unit

The number of load cycles until the first surface spall forms

Recovery means

Not applicable

Measured by

Expressed as rated fatigue life (e.g. the L10 life), cycles or hours to the first spall, and the growth of spalled area with continued running