Final answer:
Head markings on rivets indicate their grade and composition, with 'A' grade having no marking, 'B' grade having a raised cross, 'D' grade featuring a raised dimple, 'AD' grade having a raised double dash, and 'DD' grade rivets showcasing a raised double dash and a dot.
Step-by-step explanation:
The question refers to the identification of the different types of rivets based on the markings on their heads. Each type of rivet has a unique head marking that helps in identifying its grade and material composition. Rivets are identified primarily by letters (A, B, D, AD, DD), and the presence of raised or recessed dots helps to further distinguish the grades.
Rivets with an 'A' grade are typically made of 1100 aluminum and do not have a marking on the head. 'B' grade rivets are made of 5056 aluminum and are marked with a raised cross on the head. The 'D' grade rivets, made of 2017-T aluminum, are identifiable by a raised dimple. An 'AD' rivet is an A-grade material that is heat-treated to the D condition, and it is marked with a raised double dash. Lastly, the 'DD' grade rivets are made of 2024-T aluminum and carry a raised double dash and a dot. It's important to note these rivet head markings during aircraft maintenance and repair works as they are often used in the aerospace industry.