What term describes a timber defect that involves a fracture or separation across the grain?

The term that describes a timber defect involving a fracture or separation across the grain is referred to as "checking." This is a natural occurrence in wood, particularly when it dries or undergoes changes in moisture content. Checking typically appears as cracks or splits that run parallel to the grain, often beginning at the ends of the piece of timber and extending inward. Such structural fissures can affect the integrity and performance of the timber, which is especially important in the context of bridge safety and maintenance.

In contrast, other timber defects mentioned in the options describe different issues. For example, "surface checking" generally pertains to smaller, superficial cracks that occur on the surface layer of the wood; "blistering" may indicate a separation of layers often seen in laminates; and "shake" refers to separations that are often larger and run along the growth rings of the wood rather than transverse to them. Understanding these differences is crucial for assessing wood quality and ensuring the structural soundness of timber components in bridge construction and maintenance.