Flexible joints are commonly referred to as expansion joints

Flexible joints are commonly referred to as expansion joints, and they can be made from a variety of materials, not just rubber. While some flexible joints may indeed be made primarily of rubber or elastomeric material, they can also incorporate metals and other materials to accommodate movement and absorb vibrations in piping systems.
Flexible joints, or expansion joints, are designed to compensate for thermal expansion, contraction of piping systems, absorb equipment vibration, and reduce noise. They can be constructed from a range of materials including rubber, plastic, metal, and fabric. These materials offer various properties necessary for specific applications, including resistance to temperature, chemicals, pressure, and movements in different directions.
Furthermore, expansion joints can come in various forms such as rubber expansion joints, metallic expansion joints, fabric expansion joints, and more. Each type of expansion joint has its own unique set of characteristics, strengths, and weaknesses. For instance, rubber expansion joints are known for their flexibility and ability to absorb sound and vibration, while metallic expansion joints offer high-temperature resistance and durability.
In conclusion, while flexible joints can be made from rubber or elastomeric materials, they are not exclusively limited to these materials. Instead, they encompass a broad range of materials and designs tailored to meet specific performance requirements, making them a versatile solution for accommodating movement in piping systems.