Variations of materials, layup techniques and process variations affect carbon fiber composite strength, so samples are tested to determine bulk properties. This video conveys testing in action. Figure 1 shows several strips after pulling to failure, and Figure 2 shows a strip in the tensile test instrument.
Figure 1: The strips after they are pulled apart
Figure 2: Metal tabs are placed on either end of the strips and they are pulled apart until they break (the force at which breakage occurs is measured).
Figure 3: Squares of carbon fiber are cut before laying up
Figure 4: Squares of carbon fiber
Figure 5: A resin is applied to the carbon fiber so that when it cures, it will harden
Figure 6: The carbon fiber squares (with resin applied) are put in a bin and wrapped in plastic. A breathable fabric is placd between the carbon fiber and plastic and the air is sucked out with a vacuum. The fabric prevents resin from getting stuck in the hose. This is all put in an oven and baked to cure the resin.
Figure 7: The composite comes out of the oven cured, which means it is hard. It is cut into thin strips with a saw.