monocoque is seen in very low volumes because there are few applications where it is structurally and economically viable. Generally, these three designs are associated with the following materials:
Unibody—steel-based structure (mostly steel stampings) usually with steel skin panels but sometimes plastic or aluminum skin panels. This design has high investment (engineering and tooling) costs and is designed for high volume.
Space frame—usually an aluminum-based structure (aluminum castings, extrusions, and sheet). This design is less complex than the unibody and has lower investment costs, which are typically offset by higher material costs. Because of the high material costs (that are variable with volume), this is typically a low-volume design.
Monocoque—reinforced resin/composite body structure using the skin to bear loads. Today, this architecture is uncommon for passenger automobiles and more common for aircraft or ships.
The space frame and monocoque structures are associated today with niche vehicle markets, whereas the unibody with its steel-based structure is common (perhaps found in more than 99 percent of today’s automobiles). These design approaches differ from the body-on-frame design that is well suited for heavier “working” vehicles like trucks and SUVs. Body-on-frame readily achieves all the desired design criteria, except that it is heavy because of the large frame components.
There are several methods to make steel structures lighter, regardless of their design construction:
Substitute higher-strength steel for lower-strength steel. Higher-strength steel can be down-gauged (made thinner). There are, however, forming and joining issues with higher-strength steel that limit where it can be applied, and down-gauging can reduce the ability to meet stiffness criteria.
Substitute sandwich metal material for conventional steel. Sandwich material has layers of steel or aluminum (usually three), often with the internal layer in the form of honeycomb or foam. Other layered materials can include bonded steel with plastic/polymers. This cladding material can achieve high stiffness and strength levels with low mass. Sandwich material is light, is very stiff, and can be formed for many parts. On the downside, joining it to other parts can be difficult, its availability is limited today, and it is expensive to produce.
Introduce new steel designs that are available, such as with laser welded blanks and hydro-formed tubes or hydro-formed sheet metal. The use of tubes and laser blanks can make more optimal use of metal (steel or aluminum) and result in less mass in the structure without compromising design criteria. These methods may increase or decrease costs depending on the application.
Most steel and mixed-material vehicles (e.g., steel and aluminum) today are unibody, and aluminum-intensive vehicles tend to be space frame designs, but these are low volume due to cost. The unibody design was developed primarily for steel, and the conventional vehicle today is composed of about 65 percent steel (both mild and high strength). Various components of a unibody can have alternative lightweight materials, including high-strength steel, polymers/composites, and aluminum directly substituted on a part-by-part basis to help reduce mass on a limited basis. Sheet molding compound (SMC plastic) body panels are sometimes used for fenders or exterior closure panels to save weight, and in the case of low-volume vehicles, to save costs. The ability to substitute alternative materials, however, can be limited because of forming (part shape), joining, and interface issues between mixed materials. Steel unibody designs can accommodate polymer/composite or aluminum closure panels because these parts can be easily isolated from the remainder of the structure since they are fastened onto the structure. Many unibody steel-based vehicles made in North America have aluminum hoods and deck lids, but steel doors. Hoods and deck lids are simpler designs than doors (they are flatter and have fewer parts, and therefore are less expensive and less complex to switch over to aluminum). Steel doors could also be converted to aluminum in many cases, as is often done in Europe, but in North America their size and geometry would make this conversion relatively expensive.
The mass savings by introducing high-strength steel results from the ability to down-gauge the thickness over mild steel while maintaining the same strength as the thicker mild steel part. Down-gauging reduces stiffness, and so this is not a solution in some cases where stiffness is important. Also, as the strength of steel increases, its ability to be formed into different shapes is reduced (its allowable percent elongation is reduced). This reduced formability also limits where high-strength steel can be applied. The outside panels (skin panels) on a unibody are predominantly non-structural and subject to dents, thus also limiting the ability to down-gauge these panels. The tools that form high-strength steel parts cost more, require greater maintenance because they are subject to wear, and require greater forming pressures in production. In most cases, high-strength steel parts cost more than comparable mild steel parts. New, advanced high-strength steels are being developed to give high-strength steel greater formability and weldability. These advanced high-strength steels, expected to be available within a few years, can reduce mass on some compatible parts by around 35 percent. This is achieved by using high-strength steel to