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Dimpling in the composite face sheets of honeycomb sandwich structures due to mismatch in the thermal expansion coefficients of the constituent materials was studied with emphasis on its monitoring and prediction. Strain distributions along optical fibers embedded in the face sheet were monitored during manufacturing. Dimple formation and in-plane strain distributions in the face sheets were studied using finite element analysis, and an analytical model based on the beam theory was constructed to predict the dimple depths from the strain data. A system using twin optical fiber sensors was proposed to accurately measure the dimpling-induced strains. The usability and performance of the system was evaluated using small scale specimens and finally on a more realistic large-scale specimen. The system could measure the strain changes due to dimpling of the face sheets and provided decent prediction of the dimple depth distribution along the sandwich panels. [ABSTRACT FROM AUTHOR]

L-shaped composites are fundamental parts of complex-shaped structures, and it is well-known that shape distortion (i.e., spring-in deformation) arises after curing due to the orthotropic nature of composites. This residual deformation increases the manufacturing costs, attracting significant attention to the mechanisms of spring-in deformation. Meanwhile recent aerospace-grade composites include inter-laminar toughened layers composed of thermosetting resin and thermoplastic particles to enhance the inter-laminar fracture toughness and to improve the impact resistance. Even though the inter-laminar resin layers can affect the process-induced strain and deformation, their effects have not yet been studied in detail. Therefore, this current study performed fiber-optic-based internal strain measurement and evaluated the residual deformation of L-shaped parts with inter-laminar resin layers. Through-thickness cure shrinkage strain at the corner part was relaxed and the spring-in angle decreased by holding the parts at the cure temperature, indicating that viscoelasticity of the thermoplastic particles is important. Viscoelastic finite element analysis supported this finding and indicated that the effect of inter-laminar toughened layers on the residual deformation should be considered to optimize curing processes. [ABSTRACT FROM AUTHOR]

CFRP pipes are used in spacecraft to support heavy optical instruments. CFRPs' low coefficients of thermal expansion and high stiffness improve the instrument performance. However, significant through-thickness strain arises in thick CFRP pipes in curing and operation at low temperature, resulting in delamination failure. In this study, we developed a fiber-optic-based life-cycle monitoring system to measure through-thickness strain development. First, we addressed the mechanism of strain development using a theoretical approach and finite element analysis (FEA). We confirmed that geometrical constraint arising from the cylindrical shapes causes significant out-of-plane stress in the through-thickness center, and the stress increases with increased thickness and stiffness. A fiber Bragg grating (FBG) sensor was then embedded at the through-thickness center of a pipe during lay-up. Non-axisymmetric strain change in the FBG sensor was continuously measured using the birefringence effect throughout the life cycle, including curing and simulated operation in a low temperature environment. Through comparison between pipe and plate specimens, it was clearly demonstrated that the system could sensitively capture through-thickness strain development and detect delamination failure in the pipe. In addition, FEA was conducted and the result agreed well with the experiment data, confirming the validity of the experiment results. [ABSTRACT FROM AUTHOR]

The present study investigated, both experimentally and numerically, the improvement of low-velocity impact damage resistance of carbon fiber reinforced plastic (CFRP) laminates due to through-the-thickness stitching. First, we conducted drop-weight impact tests for stitched and unstitched laminates. The results of damage inspection confirmed that stitching did improve the impact damage resistance, and revealed that the improvement effect became greater as the impact energy increased. Moreover, the stitching affected the through-the-thickness damage distribution. Next, we performed FEM analysis and calculated the energy release rate of the delamination crack using the virtual crack closure technique (VCCT). The numerical results revealed that the stitching affected the through-the-thickness damage distribution because the stitch threads had a marked effect on decreasing both the modes I and II energy release rate around the bottom of the laminate. Comparison of the results for models that contained delaminations of various sizes revealed that the energy release rate became lower as delamination size increased; therefore the stitching improved the impact resistance more effectively when the impact energy was higher. [ABSTRACT FROM AUTHOR]

The authors focus on the construction of a structural health monitoring (SHM) system with an advanced grid structure (AGS) made of carbon-fiber reinforced plastic (CFRP). AGS is often applied to aerospace structures because the ribs carry only axial forces in the carbon fiber direction, making AGS structurally effective and lightweight, and because the repetition of many ribs in the AGS composition results in damage tolerance. The failure of a single rib hardly affects the fracture of the whole structure, making AGS a fail-safe structure. In this research, the authors have embedded multiplexed-fiber Bragg grating (FBG) sensors into an AGS rib in the longitudinal direction to measure mechanical strains of all ribs in order to detect the existence and regions of AGS rib fractures. Monitoring the change in riblongitudinal strains is the most effective SHM system for AGS. To confirm the proposal, the authors explore the following. First, various damage characteristics under low-velocity impact loading are investigated and it is verified that partial rib cracks are the most typical damage in AGS. An AGS is then fabricated with embedded FBG sensors and verified that the SHM system is able to measure all rib strains. Subsequently, it is analytically determined that the change in longitudinal-rib strains is the most appropriate mechanical feature for damage detection. Moreover, a statistical outlier analysis is introduced into the SHM system for automatic damage detection. Finally. AGS is established with the SHM system and verified experimentally. Results confirm that the existence of damage and its regions in AGS can be detected with the proposed SHM system. [ABSTRACT FROM AUTHOR]

The effect of transverse cracks on the ultimate tensile strength (UTS) was experimentally and analytically studied for carbon fiber reinforced plastic (CFRP) cross-ply laminates. For coupon specimens of various cross-ply stacking sequences, quasi-static tensile tests were carried out and transverse cracks were observed. The experimental results showed that the measured fiber bundle strength of 0° ply was almost constant, which indicated that transverse cracks had little influence on the notch sensibility of the bundle strength. Then, we proposed a new numerical model based on the finite element method to investigate the damage extension to the fracture. This model considered the elastoplastic behavior of epoxy matrix and the fiber breakages. Using this model, we applied a Monte-Carlo method to the damage extension simulation. It was found that the plastic region at the tip of the transverse crack reduced the stress concentration due to the crack, and that the bundle strength was barely affected by transverse cracks. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a numerical simulation of interlaminar damage propagation in FRP laminates under transverse loading, using the finite element method. First, we conducted drop-weight impact tests on CFRP cross-ply laminates. A ply crack was generated at the center of the lowermost ply, and then a butterfly-shaped interlaminar delamination was propagated at the 90/0 ply interface. Based on these experimental observations, we present a numerical simulation of interlaminar damage propagation, using a cohesive zone model to address the energy-based criterion for damage propagation. This simulation can address the interlaminar delamination with high accuracy by locating a fine mesh near the damage process zone, while maintaining computational efficiency with the use of automatic mesh generation. The simulated results of interlaminar delamination agreed well with the experiment results. Moreover, we demonstrated that the proposed method reduces the computational cost of the simulation. [Copyright &y& Elsevier]

Abstract: This paper examines the stress distribution around a fiber break in alumina fiber-reinforced aluminum matrix (Al2O3/Al) composites using finite element analysis and predicts their tensile strengths using tensile failure simulations. In particular, we discuss the effect of matrix hardening on the tensile failure of the Al2O3/Al composites. First, we clarify the differences in the stress distribution around a fiber break between an elastic–perfect plastic matrix and an elastic–plastic hardening matrix using finite element analysis. Second, the procedures for simulating fiber damage evolution in the Al2O3/Al composites are presented. The simulation incorporates the analytical solution to the stress distribution of a broken fiber in the spring element model for the stress analysis of the whole composite. Finally, we conduct Monte Carlo simulations of fiber damage evolution to predict the tensile strength of the Al2O3/Al composites. Coupled with a size-scaling analysis, the simulated results express the size effect on the strengths of the composites seen in experimental results. [Copyright &y& Elsevier]

One of the difficulties in composite structural application is joining components. A T-joint is a critical component in aircraft structures that transfers loads between vertical and horizontal panels. However, in T-joints, a critical concern is that cracks occur in the deltoid and immediately propagate along the flange-skin interface. Therefore, introducing crack arrest features at the flange-skin interface is effective in improving the damage tolerance of T-joints. This study begins by investigating the crack propagation mode at the flange-skin interface using finite element analysis and then proposes an appropriate crack arresting mechanism using interlocking fiber features that we have been studying. The effectiveness of the crack arrester in T-joints is evaluated by pull-up tests. The specimens with the arrester showed totally different failure progress and much higher energy absorption capability as compared to the specimens without the arrester. Some important findings on the efficient arrester arrangement and configuration were also obtained. [ABSTRACT FROM AUTHOR]