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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.18, #5)

Experimental Investigation of the Compaction and Tensile Strength of Co-cured Skin-to-Stiffener Structures by Xueming Wang; Fuyuan Xie; Min Li; Zuoguang Zhang (pp. 371-383).

The integral technology of co-curing has been attracted increasing attention due to its resulting in fewer parts and reduction the assembly times and cost of aircraft structures. In this study, an experimental co-curing technique was applied to fabricate T-stiffened panels in the autoclave, with an emphasis to investigate experimentally on the influence of tool assembly schemes on the manufacture and performance evaluation of co-cured panels. The pressure transferring characteristic of the tools was measured. The thickness analysis and the microscopy observation were examined to discuss the influence of various tool assembly schemes on the compaction of T-stiffened panels. Moreover, a special pull-off test was performed to evaluate the effect of the manufacturing quality on the mechanical performance. The experimental results indicated that during co-curing period, an interesting observation was found that the manufacturing quality of T-stiffened panels was significantly influenced by a position limit for the tools. In case of absence of a position limit for the tools, a micro-cut showed that the fiber waviness obviously appeared in skins and stiffener corners of T-stiffened panels, which was extremely attributed to the pressure distribution in stiffener corners. Applying a position limit for the tools can effectively eliminate the fiber waviness in stiffener corners and a well consolidated T-stiffened panel with uniform thickness was obtained, almost free of fiber waviness and resin-rich defect. Under tension condition, the fiber waviness in stiffener corners remarkably caused a decrease of the initial damage load, but showed no significant influence on the peak load.

Keywords: Composites; Co-curing; T-stiffened panels; Compaction; Tensile strength

Effects of Steam Environment on Fatigue Behavior of Two SiC/[SiC+Si₃N₄] Ceramic Composites at 1300°C by Marina B. Ruggles-Wrenn; Vipul Sharma (pp. 385-396).

The fatigue behaviors of two SiC/[SiC+Si₃N₄] ceramic matrix composites (CMC) were investigated at 1,300°C in laboratory air and in steam. Composites consisted of a crystalline [SiC+Si₃N₄] matrix reinforced with either Sylramic™ or Sylramic-iBN fibers (treated Sylramic™ fibers that possess an in situ BN coating) woven in a five-harness satin weave fabric and coated with a proprietary boron-containing dual-layer interphase. The tensile stress–strain behaviors were investigated and the tensile properties measured at 1,300°C. Tension–tension fatigue behaviors of both CMCs were studied for fatigue stresses ranging from 100 to 180 MPa. The fatigue limit (based on a run-out condition of 2 × 10⁵ cycles) in both air and steam was 100 MPa for the CMC containing Sylramic™ fibers and 140 MPa for the CMC reinforced with Sylramic-iBN fibers. At higher fatigue stresses, the presence of steam caused noticeable degradation in fatigue performance of both composites. The retained strength and modulus of all run-out specimens were characterized. The materials tested in air retained 100% of their tensile strength, while the materials tested in steam retained only about 90% of their tensile strength.

Keywords: Ceramic-matrix composites (CMCs); Fatigue; High-temperature properties; Fractography

Temperature Dependence of Flexural Strength of a CF-SMC Composite by Keiji Ogi; Masahiro Yamanouchi (pp. 397-408).

This paper presents the temperature dependence of predicting the flexural strength of a carbon fiber-reinforced sheet molding compound (CF-SMC). First, three-point flexural tests were performed to measure strength and elastic modulus at a variety of temperatures for CF-SMC specimens. Next, simple equations for predicting flexural strength were derived based on the fracture mechanics approach. The predicted flexural strength was in reasonably good agreement with the experiment results. The scatter of flexural strength was ascribed to the variation of location and size of the initial damage. In addition, the effect of temperature on flexural strength and delamination behavior was explained in association with the temperature dependence of the elastic modulus and fracture toughness.

Keywords: SMC; Flexural strength; Fracture mechanics; Fracture toughness

Fatigue Damage Characterization by NDT in Polypropylene/Glass Fibre Composites by Paulo N. B. Reis; José A. M. Ferreira; Mel O. W. Richardson (pp. 409-419).

This paper presents the results of a study on glass-fibre-reinforced polypropylene composite in which the fatigue damage was investigated in terms of residual stiffness and temperature rise. Thermographic and acoustic emission techniques were used to aid the interpretation the fatigue damage mechanisms. Different laminates were tested. For one series, all the layers have one of the two fibre directions oriented with the axis of the plate. For the other two series layer distribution was obtained with the following laminate orientation in respect to the axis of the sheet: +45°/0°/−45°/0°/+45°/0°/−45° and +30°/−30°/+30°/0°/+30°/−30°/+30°. It was possible to conclude that the residual stiffness and temperature rise can be used to predict final failure of a structure and/or component. With thermographic technique it is possible to obtain temperature maps and the precise site where the failure will occur.

Keywords: Fatigue; Non Destructive Testing (NDT); Composite materials

PLLA/Flax Mat/Balsa Bio-Sandwich Manufacture and Mechanical Properties by Antoine Le Duigou; Jean-Marc Deux; Peter Davies; Christophe Baley (pp. 421-438).

This paper describes the manufacture and mechanical characterization of a sandwich material which is 100% bio-sourced. The flax mat/PLLA facings and balsa core can also be composted at end of service life. Manufacture is by vacuum bag moulding. The optimum moulding time and temperature are a compromise between ensuring good impregnation and avoiding degradation, and holding for 60 min at 180°C was found to be satisfactory. The mechanical properties of the bio-sandwich obtained are compared to those of a traditional glass reinforced polyester balsa sandwich. The flexural strength is 30% lower, as predicted based on the facing properties. Skin/core adhesion is also measured using debonding tests. Crack propagation occurs at the skin/core interface in the traditional sandwich but within the facing in the bio-sandwich. The impregnation of the core in the two materials is examined using X-ray micro-tomography.

Keywords: Sandwich; Biofibre; Biopolymer; Vacuum forming

Dynamic Experimental Study of Deployable Composite Structure by Chao Xiong; Yi-ming Lei; Xue-feng Yao (pp. 439-448).

In this paper, dynamic behaviors of deployable composite structure are experimentally studied. Firstly, both the composite tape springs and the deployable composite structure are fabricated using the carbon/epoxy and glass/epoxy composite thin cylindrical shells, respectively. Secondly, the deployable behaviors of thin cylindrical shell specimens with section angle θ = 70° and 90° are investigated based on the strain history in three directions (0º, 45ºand 90º). Finally, the deployment behaviors of the deployable carbon/epoxy composite structure with two hinges and three hinges are also studied including the deployment process and dynamic strain history.

Keywords: Deployable composite structure; Deployable process; Dynamic characterization; Strain history

Effect of Interface Properties on Mechanical Behavior of 3D Four-Directional Braided Composites with Large Braid Angle Subjected to Uniaxial Tension by Guodong Fang; Jun Liang; Baolai Wang; Yu Wang (pp. 449-465).

A Representative Volume Cell (RVC) chosen to epitomize the entire three dimensional four-directional braided composites is investigated to evaluate the mechanical behavior of the material by computational micromechanics. In addition to including several damage modes of braid yarn and matrix within the braided composites, the numerical model also takes into account interface damage mode by using a Cohesive Zone Model (CZM). A parametrical study is conducted to evaluate the influence of interface properties on the macro stress-strain curve and the interaction of different failure modes of the braided composites under uniaxial tensile loading. The interface damage evolution of the braided composites with large braid angle is also provided further. Preliminary results indicate that the interface damage, which is one of the key factors to cause the nonlinearity of the stress-strain relationship, can decrease the elastic modulus but not obviously control the ultimate strength of the braided composites with large braid angle.

Keywords: 3D braided composites; Damage; Interfaces; Cohesive zone model; Finite element modeling

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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.18, #5)

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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.18, #5)