 One of the main selling points of FRP composites is that they combine low weight with high strength and relatively high stiffness. It is time to give some numbers to back up this statement. In this video we are going to compare the mechanical performance of FRPs to that of traditional building materials. So how is the performance of FRPs in terms of stiffness and strength? And how light is lightweight, actually? We will compare with concrete and steel, but also include the lightweight metal alloys of aluminum and titanium. Let's first consider stiffness. We will plot the stiffness versus density to classify the different materials. The plot is in log-log space so that we can cover different orders of magnitude in the stiffness. Our ideal material is going to be located in the top left corner. Low weight and high stiffness. We start by placing steel and concrete. Steel has a density of almost 8000 kg per cubic meter and a stiffness of 210 GPa. The stiffness of concrete lies between 30 and 40 GPa at a density of 2400 kg per cubic meter. Aluminum alloys are approximately two times more stiff than concrete at almost the same density while titanium lies somewhere in between aluminum and steel. Now let's put some ingredients of FRPs in this chart. Glass fibers have a density that is comparable to concrete and aluminum and a stiffness that is somewhat higher. The Young's modulus of glass fibers lies around 70 to 80 GPa. Carbon fibers are lighter than glass fibers with a density around 1800 kg per cubic meter and carbon fibers have a stiffness that is higher than steel at 230 to 350 GPa. To be fair, I should say that the values shown here are for the stiffness in axial direction. Aluminum fibers have different stiffness in different directions. The axial direction is the stiff direction but also the most relevant one. Carbon is close to our ideal material, stiffer than steel, lighter than aluminum. The properties of glass are also good. However, these values are for the fibers only. Fibers have to be combined with a matrix material to form a material that can be used for structural applications. In FRPs, the fibers are combined with a polymer matrix. The polymers that are used, epoxy, polyester, maybe a thermoplastic like peak, are all very light with a density between 111280 kg per cubic meter. But their stiffness is also much lower than any of the other materials in our chart. So FRPs, for FRPs to have a good stiffness, we must have a high fiber content. The composite stiffness is obviously going to be lower than the stiffness of the fibers. The good news is that the weight will also be lower, especially for GFRP with the heavier glass fibers. This is not the end of the story. These properties are for unidirectional FRP composites that have low stiffness and strength in the direction perpendicular to the fibers. To mitigate this weakness, laminates are made, layered structures with different fiber direction in the different layers. Depending on the application, the fiber distribution can be UD dominated or it can be such that the laminate has equal properties in all directions. This last extreme is obtained with a quasiisotropic layup. If we also include the quasiisotropic layup in the chart, we see that the performance is less impressive. For a real FRP design, the stiffness in primary load carrying direction can be in between the UD and the quasiisotropic values. A stiffness in one direction that is higher than the quasiisotropic one is going to have negative consequences for the stiffness in other directions, which will be even lower than the quasiisotropic value. One more point of attention is that the out of plane shear stiffness of laminates remains low irrespective of the fiber orientation in the layers. In any case, there are no fibers in the direction of this shear stress. If we remake this chart for strength, the relative location of the metals is similar. With a strength of 370 to 770 MPa for steel and 200 to 414 for aluminum. The strength of concrete, even the compressive strength which may go up to 90 MPa, is significantly lower than that of the metals. The polymer matrices that we use have a strength in the range of 40 to 80 MPa. Accounting for the low weight, that is already good. The fibers are performing really well when it comes to strength. Both glass and carbon have a much higher strength than the classical engineering materials, at 2700 to 5000 MPa. When combining fibers and polymer, the strength that can be reached in UD FRP is still higher than that of steel at 1 to 3000 MPa. So for strength, the comparison between FRP composites and traditional materials is more favorable than for stiffness. However, here too the material behavior is directional. The UD material is weak in transverse direction. In multidirectional laminates, the strength is lower than in the UD material. In all cases, the interlaminar strength remains low. Moreover, we have been looking at tensile strength. The compressive strength of FRPs is typically lower than this tensile strength. So we end up with a range of strength values. Moreover, strength is not the complete story when it comes to failure. The utility of composites is often poor, which makes it difficult to exploit this high strength fully. In conclusion, FRP composites are indeed light. Much lighter than steel. The stiffness of GFRP is lower than steel, similar to concrete, but the strength can be higher. It is important to realize that both strength and stiffness can be optimized by playing with the fiber direction, although this always comes at the expense of the performance in other directions. And no matter how the laminate is designed, the out-of-plane performance remains polymer dominated.