Carbon fiber reinforced resin matrix composites are widely used in various aerospace structures due to their high specific strength and high modulus, corrosion resistance and designability. It is easy to produce impact damage, mainly manifested as internal matrix cracking, delamination and fiber breakage. These damages are difficult to see on the surface of the material, but the compressive performance of the component is significantly reduced, which poses a huge threat to the safety of composite components. In the structural design and strength analysis of the aircraft, the impact resistance of the composite material is a very important design basis, and a reasonable design value is an important guarantee for the safety of the composite material structure of the aircraft.
As the representative of the new generation of high-strength medium-modulus carbon fiber, T800 grade carbon fiber has been increasingly used in the aerospace field. The application of domestic T800 grade carbon fiber composite material system in aviation structure is in the stage of demonstration and design. Therefore, the allowable value and structural design of domestic T800 grade composite material The study of allowable value is necessary.
The material used is domestic T800 grade carbon fiber epoxy resin composite material. The specimen includes three typical layups [45/0/-45/90]3s,
[45/0/-45/90] 4s, [45/0/-45/90] 5s; serial numbers are A, B.C; thicknesses are 3.42mm, 4.46mm, 5.55mm, respectively. Carry out with reference to ASTMD7136-2007. The automatic drop weight impact testing machine is adopted. The total mass of the drop weight is (5.5 ± 0.25) kg, and the diameter of the impact head is (12.7 ± 0.1) mm.
Damage resistance performance refers to the ability to resist impact, which can be characterized by the impact energy-dimple depth curve of the composite material after impact compression test. The pit depth and impact energy of the three groups of specimens A, B, and C after impact are plotted. There is an inflection point in the impact energy-pit depth relationship curves of the three groups of A, B, and C specimens. Before the inflection point, the impact energy increases. , the crater depth increases slowly, and after the inflection point, with the increase of the impact energy, the crater depth increases rapidly, indicating that the damage resistance performance of the composite material has changed abruptly near the inflection point. The reason for this phenomenon is: before the inflection point, the impact damage only causes a small amount of delamination of the laminate, and after the inflection point, the fiber breaks in the composite material, resulting in the composite laminate basically losing the ability to continue to resist the impact. As a result, the impact energy increases and the depth of the composite crater increases rapidly.
The inflection points of the impact energy-pit depth relationship curves of the three groups of specimens are: 0.70mm in group A, 0.76mm in group B, and 0.45mm in group C, all of which are less than the pit depth corresponding to BVID. Under the same impact energy, the depth of the generated pits is group C The CAl in the material allowable value is the post-impact compressive strength obtained after impacting with an energy of 6.7J/mm, which can be expressed as CAL6.7J/mm. In the structural design allowable value, CAI is the post-impact compressive strength when barely visible damage (BVID) occurs, that is, the post-impact compressive strength when a 1.3mm pit depth occurs after impact, which can be expressed by CAIBVID. For the CAIBVID value is lower than CA16.7J/mm. With the improvement of material properties, the impact energy of 6.7J/mm in domestic T8OO grade composite materials generally cannot appear BVID. Therefore, it is necessary to consider the impact energy and residual compressive strength when BVID is generated in the research stage of material allowable value.
The impact energy required for the three groups of specimens to generate BVID (pit depth 1.3mm) was plotted and curve fitting was performed.
The material thickness has a linear relationship with the impact energy required for barely visible impact damage (1.3mm). At present, when carrying out the experiment of the allowable value of the newly developed composite material after impact compression design, the usual practice is to first obtain the energy value required for the generation of BVID through a large number of test pieces. After the impact energy relationship is required, when performing the post-impact compression design allowable value experiment, the initial calculation can be performed according to the formula to obtain the required impact energy, and then the experimental verification is carried out with a small number of test pieces, so as to avoid wasting a large number of tests. parts, saving costs and improving efficiency.
The damage resistance performance (impact energy-pit depth curve) and its damage tolerance performance (pit depth residual compressive strength curve, pit depth-compressive failure strain curve) of domestic T800 composite laminates all show inflection point phenomenon, and the inflection point The corresponding pit depths are all less than the pit depth specified by the barely visible damage (BVD). In the case of the same layup, with the increase of the thickness of the composite laminate, the impact resistance of the composite increases. For domestic T800 grade composite materials, the CAL6.7J/mm value is high, while the CAIBVID value is low, so the residual compressive strength when BVID is generated should be considered when selecting materials.
