Bearing damage in composite bolted connections (such as those commonly used in aerospace applications) is investigated through the application of ultrasonic guided waves. Specifically, identical macro fiber composite sensor arrays were bonded to each of two identical composite plates. One plate was then inspected ultrasonically in the unloaded condition with bearing damage being introduced through successive uniaxial tensile tests, while the other plate was assessed while under load in the tensile testing machine. A scattering matrix approach is employed to characterize the interaction of the guided waves with the target bolt hole. The effects of applied load and the bolt fixture on the ultrasonic results are explored. A parametric study is carried out to determine the optimal actuation frequency and interrogation angle for this application. The results demonstrate that the system is capable of detecting bolt bearing damage as well as monitoring of the applied load in the elastic region.
This study investigates the numerical simulation and theoretical modeling of longitudinal compressive failure in fiber reinforced composite materials. Firstly the numerical simulation of longitudinal compressive failure is conducted. The simulated results show that at one moment of the loading, the localized deformation catastrophically appears in the material, and in this initiation of the localized deformation, the reduction of tangent shear stiffness plays an important role. Secondly the theoretical modeling of longitudinal compressive failure is implemented. A set of mathematical equations is obtained for the deformation of composite materials, and the mathematical solution of the equations is considered. There exists a state where arbitrariness appears in the solution of equations expressing deformation of composite materials, and it is indicated that the onset of arbitrariness in solution of equations expressing deformation of composite materials is closely related with the initiation of longitudinal compressive failure, and also related with the initiation of narrow localized band in the materials. Finally the numerical simulation is conducted for compressive failure in quasi-isotropic laminate. The localized deformation also appears in the laminate, and the simulated deformation of the material agrees with the microscope picture of the experimental result.
Structural health monitoring refers to the process of making an assessment, based on nondestructive, in-situ, autonomous measurements, about the ability of a structure to perform its intended function. This paper presents work done on a bolted connection in carbon-fiber reinforced polymer composite materials. A composite specimen is bolted in a double lap joint configuration to a test apparatus that applies an increasing tensile load. Ultimately, the load results in bearing failure of the material around the bolt hole. To monitor the progression of damage, macro fiber composite sensors are bonded in a circular array around the bolt hole. These sensors are then used to generate ultrasonic guided waves, a popular technique in nondestructive evaluation because of the favorable combination of propagation distance and sensitivity to damage. As the specimen is subjected to increasing load levels, measurements are taken repeatedly and compared with one another. Because damage will change the local mechanical properties of the material, the ultrasonic waves passing through the damaged region will be scattered differently in each direction, resulting in a different waveform arriving at the other surrounding sensors. By applying appropriate signal processing techniques, these changes may be interpreted as indicating the extent of damage that has occurred in the specimen. Preliminary analysis is presented demonstrating the correlation between changes in received strain signals and increasing damage levels.
The authors proposed fiber-optic-based damage monitoring of carbon fiber reinforced plastic (CFRP) bolted joints. Optical fibers were embedded along bolt holes and strain change along the optical fiber induced by internal damage was measured by a Brillouin Optical Correlation Domain Analysis (BOCDA), which is a high spatial resolution distributed strain sensing system. This study began by investigating damage modes of CFRP bolted joints after bearing failure. Effective embedding positions of optical fibers were then proposed and their feasibility was evaluated by finite element analysis simulating the damage propagation in the bolted joint and consequent strain change. Finally, verification tests were conducted using specimens with embedded optical fibers at various positions. It was clearly shown that damage could be detected using residual strain due to fiber-microbuckling (kinking) damage or permanent deformation of neighboring plies. Furthermore, damage size and direction could be estimated from the change in the strain distribution. The system developed is quite useful for a first inspection of large-scale composite structures in aerospace applications.
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