How can you test the breaking point of a substance or a material? Engineers have developed several tests to measure the breaking point of various elements by assessing what amount of pressure can be applied to a substance to cause it to break or become brittle. Flexural tests are one kind of analysis used to measure the flexibility of the material. A flexural test measures the flexural strength of material or fabrics, assessing the flexing modulus, of materials by applying as much pressure at the ends of the element with another opposing force being applied at the center of the material being tested (Hirohide 23). Flexural strength is thus defined as the maximum amount of pressure of a fiber or fabric used to the outermost parts of the structure in comparison to the amount of force it can hold.
There are two main types of flexural tests, which are the two-point flexural tests and the four-point flexural test. In the latter, a narrow straight beam of the material is placed on the electromechanical load frame with pressure being applied extensively on the sides of the load against a strong force applied at the center, until the load failure level is achieved (Hirohide 46). Imposing a load at the center also allows for the measurement of the flexural strength if the outer sides of the fabric remain constant. The same approach is used for the four-point flexural strength, used to assess the residual strength after fixture failure. The four-point flexural test is more reliable than the two-point flexural strength since it is more objective and realistic since most materials have larger surface areas. A flexural test produces a V-shaped specimen after failure, while a bend test mostly produces a U-shaped specimen after failure, but uses the same testing approach as a flexural test.
Maximum stress and strengths of the tests are calculated by the incremental amount of load applied to the material in question. The progressiveness of the flexural strength is recorded then presented graphically. For aircraft, the flexural test is done with the goal of testing the resistance of the carbon-fiber exterior to lighting and therefore employs a different approach. Instead of using a heavy load to exert pressure at the center, a lighting strike generator is used in the place, which stimulates waveforms of lighting as per the SAE documentation. Automobili Lamborghini developed the lighting strike generator in Hirohide’s (2011) experiment (Hirohide 51).
Although flexural tests assess the visible damage by assessing the breaking points of materials, some damage is usually not visible to the naked eye, yet the external pressure adversely affects the fiber or fabric in question. Composite materials, which include layers of different materials, are harder to test for flexural strength since some of the material used in the fabric might be malleable causing them to bend while others might be brittle and break. This kind of force causes damage that is invisible to the eye, yet one that poses a great danger to the structure in case of external power (Hirohide 315). Composite repairs are strategies employed to manage such damages and repair such fabrics and ensure that they are fully functional and in excellent condition.
When high and low-velocity impact encounters a surface, it might cause damage to the metallic material used while causing the plastic to bend slightly. As such, controlling such cost can be a little complicated in a variety of ways. One of the best strategies used to repair composite materials is superficial non-structural fillers, which acts as a temporary repair, or a kind of repair where strength is not as important as functionality. This type of repair strategy is known as a Cosmetic repair. Another approach widely used is the Resin Injection, which employs the use of a syringe to inject repairing fluids to damaged parts of the composite material, in a way that slows down the spreading of delamination (Hirohide 316). This strategy is also a temporary way to stop damages.
A more permanent approach to composite repairs is the Semi-structural Plug/Patch, whereby damages parts of the composite material are replaced with mechanically fastened plugs. This approach is more applicable for materials with thick solid laminates and helps the material to gain some strength. Another permanent approach is the Structural Mechanically–fastened Doubler, which employs the use of bolts and solid laminates to cover the damaged areas in a way that allows the entire structure to transfer loads across the entire structure. These structural repairs are aerodynamically smooth and preferable for strengthening damaged composite materials. Structural Flush Repair is the most effective approach, which entails the replacement of entire layers within the damaged areas of the composite material with similar fabrics. The size of the patch used must fit the damaged area exactly.
In conclusion, flexural tests are necessary for testing the rigidity and flexibility of the material used in building structures, be it airplanes or even concrete structures. By asserting as much force as possible and measuring the flexural modulus, engineers can determine the best material for constructing the best structures. In the event of damages, especially to composite materials, engineers developed several repair strategies to help these materials function appropriately. Some damages are temporary and do not help in the restoration of fabric strength. However, others are more permanent and completely restore the fabric strength.