Unlike existing metals, composite materials have high specific strengths and specific stiffnesses. Structures made of these materials are lighter, leading to performance improvements and energy savings. Owing to these advantages, composite materials are being widely applied to various areas such as the aerospace, shipping, and automobile industries. Recently, adhesive-bonding methods without fasteners have been widely used in manufacturing composite structures of aircrafts. Particularly, the cocuring method connects composite parts in just one step because they are bonded together by the resin pre-impregnated in the prepreg. By using the cocuring method, potential defect and energy consumption can be reduced during the assembly process. However, even though the cocuring method is used, the possibility of delamination by through-thickness stress still exists. Especially, joints without fastener are vulnerable to fatigue failure and humid conditions. Through-thickness direction reinforcements which can induce a minimal increase of weight should be considered in order to use composite joints in primary structures reliably.
We confirmed the reinforcing effects of jagged z-pins on the static and fatigue strengths of composite single-lap joints and conducted tests on composite joints reinforced with jagged stainless-steel pins under the three environmental test conditions: room temperature and dry (RTD), elevated temperature and dry (ETD), and elevated temperature and wet (ETW). A total of 153 specimens were fabricated in an autoclave by the cocuring method. Specimens were exposed to 71°C temperature and 85% relative humidity environment until moisture saturation was achieved. The composite adherends were made by using T700-12K-31E#2510 carbon-epoxy unidirectional prepreg from Toray Composites Inc. Stainless steel pins of 0.5 mm-diameter were inserted into the prepreg through the thickness to achieve a 2.0% pin volume density. To increase the frictional force between the reinforcing pin and the base material, the surface of the z-pin was physically and chemically treated. The physical treatment involved making a jagged shape on the surface of the pin with a forging process. The jagged shape had a diameter of 0.3 mm and a depth of 0.02 mm at 0.6 mm intervals. For the chemical treatment, the pins were soaked in an aqua regia solution with a 3:1 ratio of hydrochloric and nitric acid for 5 min. Fatigue test conducted that tension-tension cyclic loads were applied to the specimens with the stress ratio of 0.5 and the maximum stress level ranging from 60% to 90% of the static strengths of the joints.
Test results showed that reinforcement with z-pins improved joint strength under all test conditions. In particular, when jagged z-pins were used under the ETW condition, static and fatigue strengths at a million cycles increased by 32.2% and 65.8%, respectively, compared with the corresponding values for an unpinned joint. When standard z-pins?without the jagged pattern?were used, the static and fatigue strengths increased by 13.4% and 11.8%, respectively. In the future, if the jagged shape and pin density are optimized, we expect that the effect of z-pinning can be further improved.