Applied Composite Materials (v.21, #5)

Process Factors and Edgewise Compressive Properties of Scarf-repaired Honeycomb Sandwich Structures by Sui Liu; Zhidong Guan; Xia Guo; Kai Sun; Jiaoyue Kong; Dongxiu Yan (689-705).
Bonded repairs were conducted on flat and edge-closed composite sandwich panels that had undergone different levels of initial damage, and edgewise compression behaviors of repaired panel were tested. Experimental results indicate that these repair techniques can restore the compression performance of damaged panels effectively. The repaired specimens recovered an average of over 83 % of their strength. A k-sample Anderson-Darling test was used to analyze the influence of various parameters, including curing temperature, curing pressure, and repair configurations. After a thorough comparison, it was concluded that a high-temperature, high-pressure treatment can improve the mechanical performance of repaired panels, but the improvement is closely related to the structural complexity of the repaired region. A double-side repair scheme could be used to prevent the degradation of mechanical performance caused by the additional bending moment. The conclusions drawn in the present study provide further insight into the mechanical performance of repaired sandwich panels under edgewise compressive loads. These data facilitate the improved design methodology on bonded repair of composite sandwich structures.
Keywords: Composite sandwich panel; Bonded repair; Edgewise compression; k-sample Anderson-Darling test

In this paper, the classic embedding technique, with bared sensors, and a recent proposal, the monitoring patch, are compared with the aim to improve the composites in-core instrumentation. The monitoring patch emerges with the need to industrialize sensors integration inside composite structures; thus, a complete evaluation of its mechanical performance has to be done. Numerical and experimental campaigns are carried out on elementary carbon-epoxy coupons to evaluate the benefits and disadvantages of this procedure compared with the typical interlayer sensor embedding. The results show that the use of monitoring patch does not affect significantly the mechanical performance of instrumented coupons. An instrumentation transfer function (ITF) is proposed to link the information that electronic devices can detect, the mechanical phenomena around these electronic devices and the measurements data acquired by global or local techniques (DIC, FEM, gauges). A good correlation between the strain data acquired and the strain values calculated by FEM confirms the approach of the ITF to evaluate the influence of the monitoring patch on the measured signal.
Keywords: Composite; Embedding; Bared sensor; Monitoring patch; In-core instrumentation

Experimental Investigation About Stamping Behaviour of 3D Warp Interlock Composite Preforms by Clément Dufour; Peng Wang; François Boussu; Damien Soulat (725-738).
Forming of continuous fibre reinforcements and thermoplastic resin commingled prepregs can be performed at room temperature due to its similar textile structure. The “cool” forming stage is better controlled and more economical. The increase of temperature and the resin consolidation phases after the forming can be carried out under the isothermal condition thanks to a closed system. It can avoid the manufacturing defects easily experienced in the non-isothermal thermoforming, in particular the wrinkling [1]. Glass/Polypropylene commingled yarns have been woven inside different three-dimensional (3D) warp interlock fabrics and then formed using a double-curved shape stamping tool. The present study investigates the in-plane and through-thickness behaviour of the 3D warp interlock fibrous reinforcements during forming with a hemispherical punch. Experimental data allow analysing the forming behaviour in the warp and weft directions and on the influence of warp interlock architectures. The results point out that the layer to layer warp interlock preform has a better stamping behaviour, in particular no forming defects and good homogeneity in thickness.
Keywords: Textile composites; Fabric-reinforced thermoplastic; 3D warp interlock structure; Forming; Double-curved shape

Six typical composite grid cylindrical shells are constructed by superimposing three basic types of ribs. Then buckling behavior and structural efficiency of these shells are analyzed under axial compression, pure bending, torsion and transverse bending by finite element (FE) models. The FE models are created by a parametrical FE modeling approach that defines FE models with original natural twisted geometry and orients cross-sections of beam elements exactly. And the approach is parameterized and coded by Patran Command Language (PCL). The demonstrations of FE modeling indicate the program enables efficient generation of FE models and facilitates parametric studies and design of grid shells. Using the program, the effects of helical angles on the buckling behavior of six typical grid cylindrical shells are determined. The results of these studies indicate that the triangle grid and rotated triangle grid cylindrical shell are more efficient than others under axial compression and pure bending, whereas under torsion and transverse bending, the hexagon grid cylindrical shell is most efficient. Additionally, buckling mode shapes are compared and provide an understanding of composite grid cylindrical shells that is useful in preliminary design of such structures.
Keywords: Composite grid structure; Parameterized finite element modeling; Buckling analysis; Cylindrical shell

Crack deflection along the fiber/matrix interface for fiber-reinforced composites is an important condition upon which the toughening mechanisms depend. Sound control for the interface debonding of composites contributes to improving the fracture toughness of composites. Combined with the virtual crack closure technique, a finite element model of composites is proposed to predict the competition between the matrix crack deflection along the interface and the matrix crack penetration into the fibers under the thermomechanical coupling fields. For C/C composites, the effects of the geometry size, fiber volume fraction, fiber coating materials and thermal mismatch on the energy release rate and the crack deflection mechanisms are studied. Results show the fiber coating increases the ability to deflect at large thermal mismatch and small crack sizes, and the TaC coating shows larger effect than the SiC coating. The research provides fundamental method for promoting the toughening design of C/C composites.
Keywords: Fiber-reinforced composites; Toughening; Virtual crack closure technique

The Experiment and Numerical Simulation of Composite Countersunk-head Fasteners Pull-through Mechanical Behavior by Junwu Mu; Zhidong Guan; Tianya Bian; Zengshan Li; Kailun Wang; Sui Liu (773-787).
Fasteners made of the anisotropic carbon/carbon (C/C) composite material have been developed for joining C/C composite material components in the high-temperature environment. The fastener specimens are fabricated from the C/C composites which are made from laminated carbon cloths with Z-direction carbon fibers being punctured as perform. Densification process cycles such as the thermal gradient chemical vapor infiltration (CVI) technology were repeated to obtain high density C/C composites fastener. The fasteners were machined parallel to the carbon cloths (X-Y direction). A method was proposed to test pull-through mechanical behavior of the countersunk-head C/C composite material fasteners. The damage morphologies of the fasteners were observed through the charge coupled device (CCD) and the scanning electron microscope (SEM). The internal micro-structure were observed through the high-resolution Mirco-CT systems. Finally, an excellent simulation of the C/C composite countersunk-head fasteners were performed with the finite element method (FEM), in which the damage evolution model of the fastener was established based on continuum damage mechanics. The simulation is correspond well with the test result . The damage evolution process and the relation between the countersunk depth and the ultimate load was investigated.
Keywords: Composite fasteners; Pull-through test; FEM model

Optimization and Reliability Analysis of 2.5D C/SiC Composites Turbine Stator Vane by Zhigang Sun; Chunyuan Kong; Xuming Niu; Yingdong Song; Xianqiao Wang (789-803).
This paper presents a feasible and efficient methodology to design 2.5D C/SiC composites vane system. To better represent the architecture of 2.5D C/SiC composites, here we define five geometric parameters to describe its microstructure based on the optical photomicrographs. The double scale model for mechanical properties of 2.5D C/SiC composites has been presented to provide a reliable validation with the experimental results. Meanwhile, Monte Carlo (MC) simulation method has been employed to investigate the stochastic behavior of 2.5D C/SiC composites mechanical properties. MC simulation results show that mechanical properties of 2.5D C/SiC composites heavily depends on the stochastic behavior of components and the microstructure of 2.5D composites. To fully explore the potential of 2.5D C/SiC composite, finally we present a vane optimization model and investigate its reliability by integrating the analytical model for mechanical properties with the finite element model analysis. These findings provide an effective method to assess the risk of vane design.
Keywords: 2.5D C/SiC composites; Optimization; Reliability analysis; Mechanical properties; Safety factor