Comparative analysis of stiffness and strength of carbon fiber and common metal materials

Carbon fiber is now widely used in fields that require high strength and stiffness relative to weight, including aerospace, automated machines, racing, professional bicycles, rehabilitation equipment, and more. Due to its unique design, carbon fiber is also used in the manufacture of luxury goods including watches, wallets and more.

Comparing the performance of carbon fiber to metallic materials such as steel or aluminum is not easy. Unlike carbon fiber, metals are generally homogeneous (isotropic), which ensures the same properties in all directions. The strength and stiffness of carbon fiber components is created by positioning the fibers or fabric in a specific way.

In this series of articles, a series of important properties such as specific strength, specific stiffness, thermal expansion, thermal conductivity, long-term use, etc. will be compared with metals such as carbon fiber and aluminum. In this article, strength and stiffness in relation to material weight will be introduced first.

Material strength and stiffness in relation to weight

To explain stiffness in relation to weight, let's take a sheet 5 cm wide, 50 cm long and 2 mm thick as an example. When a 5 kg weight is suspended from the end of the sheet, the load will cause the sheet to bend to an extent that corresponds to stiffness.

For different materials, sheets of the same thickness will exhibit different bending properties. The stiffer the material, the less it bends. After the load is removed, the sheet will return to its original shape.

Now imagine that if a strip of material were exposed to a very high load, this would cause the material to yield, and after the load was released, the material would return to its original shape. The higher the strength of the material, the greater the load it can withstand before yielding permanently.

In addition to the aforementioned strength and stiffness, another key property that design engineers focus on is the weight of the material, which is determined by density.

Material stiffness is expressed as Young's modulus. However, this parameter alone is not sufficient to account for material stiffness without considering the weight of a given material. For example, where a bicycle frame (size, geometry, wall thickness) is made of two different metals (steel and aluminum), the steel frame will be 3 times as stiff as the aluminum frame. However, if the weight of the material is taken into account, a steel frame, while 3 times stiffer than an aluminum frame, is also 3 times heavier.

Of course, these numbers above are only approximations, as in practice engineers will design the geometry of the material chosen, for example in the case of an aluminum bicycle frame, it is common to increase the frame diameter as well as the wall thickness. For bicycle frames, stiffness and strength are directly related to geometry and wall thickness (a 2x increase in wall thickness results in about an 8x increase in stiffness). Therefore, the relationship of stiffness to weight (i.e. specific modulus) is actually the most efficient way to determine the stiffness of a material, and for most engineers both stiffness and weight are the most critical parameters.

Carbon fiber is a material that provides high stiffness and high strength at low densities because it is much lighter than aluminum and steel. By weight, carbon fiber is 2 to 5 times more rigid (depending on the fiber used) than aluminum and steel. For a specific part that is compressed along only one plane, made of unidirectional carbon fiber, it will be 5-10 times stiffer than steel or aluminum (same weight).

The table below compares the stiffness and damage resistance of different materials of the same weight. For analysis purposes, bidirectional carbon fiber, which is the most commonly used composite material, was used, and occasional unidirectional carbon fiber, mainly used in products that generate stress along only one plane.

(Comparison of specific modulus and specific strength of metal and bidirectional carbon fiber)

(Comparison of specific modulus and specific strength of metal and unidirectional carbon fiber)

The carbon fiber sheet data above refers to samples made using epoxy infusion techniques (70/30% carbon fiber to resin). Its data shows the advantages of carbon fiber, improved and highest modulus special materials (very expensive) with 2 or 3 times the stiffness properties of standard carbon fiber, they are mainly used in military applications and the aerospace industry.

To interpret the results shown in the table, assume that the engineer wants to fabricate a high-strength, light-weight carbon fiber panel with a maximum weight of 10 kg and an area of 1 square meter, and consider aluminum, steel, and carbon fiber.

Within the 10kg weight limit, engineers can choose from:

The thickness of the steel plate is about 1.5 mm

Aluminum plate about 4mm thick

Carbon fiber plate about 7mm thick

Carbon fiber provides greater stiffness at lower densities, so a product of the same weight can be thicker, in short, 2 times the thickness of the material provides 23 times the stiffness. Thanks to the use of carbon fiber, there are many opportunities for weight reduction.