field head
The head formed by upsetting the bucktail of a rivet after it is inserted through the materials to be connected, as opposed to the factory head made when the rivet is manufactured; the field head of a solid rivet is typically pancake-shaped, depending on the shape of the bucking bar or other tool used to upset it. The field head of a blind rivet is typically doughnut-shaped.
field head: the second head you hammer onto a rivet
A field head is the head formed on a rivet after it has been driven through overlapping materials and upset (deformed) by impact or pressure. The factory head, made during rivet manufacture, remains untouched; the field head is created on site to complete the joint. In solid rivets, this is done by striking the protruding tail (bucktail) against a bucking bar held on the opposite side. In blind rivets, a mandrel is pulled through the rivet body to create the field head without access to the back side.
The shape and quality of the field head depend entirely on the tooling and technique used. Solid rivets driven with hand or pneumatic hammers against a flat bucking bar produce a roughly pancake-shaped field head; different bar contours (dished, conical) yield varied results. Blind rivets typically form a characteristic doughnut-shaped field head as the mandrel splays the rivet tail radially outward. The head must be large enough and well-formed to develop full clamping force across the joint and resist pull-through under load.
Quality and failure modes
A poorly formed field head is a common cause of rivet joint failure. If the tail is not fully upset, the head may be undersized and offer insufficient bearing area; the rivet can then pull through the material under shear or tensile load. Conversely, if upset is excessive, material flows away from the rivet axis, thinning the shank and reducing its effective diameter. Cold rivets and dull bucking bars increase hardness and reduce deformability, making it difficult to achieve a sound head. Corrosion pitting on the rivet or materials also impairs upset flow.
The term "field" distinguishes this second head from the factory head because it is formed in the field, on the assembly line or at the structure, not in the rivet mill. This vocabulary became standard in aircraft and structural steel work, where riveted joints were dominant and the quality of field heads directly affected airworthiness and safety. Modern riveted construction in aerospace, shipbuilding, and bridge work still relies on field head integrity, though many applications have shifted to bolts or welds.