J.J. Xiong

Tear Resistance of Orthogonal Kevlar-PWF-reinforced TPU Film

Abstract This work seeks to investigate the notch sensitivity and fracture behaviour of orthogonal Kevlar-plain woven fabric (PWF)- reinforced thermoplastic polyurethanes (TPU) film applied to high altitude balloon. Four types of specimens are implemented to measure notched strength and fracture toughness by conducting static tension and tear tests on an MTS system respectively. The damage and failure mechanisms are discussed and the results for notched strength and tear resistance are evaluated and compared with each other. From the experiments, it is found that the notch sensitivity of the film increases with the increase in the size of the hole, but the notch sensitivity and the stress concentration of the notch are insignificant and there is a decrease of only about 4%-10% in tensile strength for the notched specimens with different hole sizes in diameter compared with the unnotched specimen. In contrast, the tear resistance containing a central slit with only 1 mm length is about half of tensile strength of the unnotched film, which implies that the tear resistance exists an significant notch sensitivity. The results of this study provide an insight into notch sensitivity and fracture behaviour of the Kevlar-PWF-reinforced TPU film and constitute a fundamental basis for the design of high altitude balloon.

Mechanical Properties of RTM-made Composite Cross-joints

Abstract Tension and shear tests are carried out on composite cross-joints, produced by resin transfer moulding (RTM), stitch-RTM and cobonding techniques separately, to investigate the influences of different production methods on their mechanical properties and their failure mechanism. It is concluded from test results that, in terms of mechanical properties, the RTM-made cross-joint holds superiority over other two, and both stitch-RTM and cobonding methods have significant adverse effects on mechanical properties. Experimental observation and finite element (FE) numerical simulation show that the delamination first takes place at the upper triangle-like resin-rich zone of joints, then propagates along the interface between the zone and the curved webs/flanges to the bottom plate, and grows along the horizontal bottom plate till failure of the joint accurs at last. Results demonstrate that there is a good agreement between the FE model and the tests.

Static Pull and Push Bending Properties of RTM-made TWF Composite Tee-joints

This paper deals with static pull and push bending tests on two-dimensional (2D) orthogonal EW220/5284 twill weave fabric (TWF) composite tee-joints processed with the resin transfer moulding (RTM) technique. Static pull and push bending properties are determined and failure initiation mechanism is deduced from experimental observations. The experiments show that the failure initiation load, on average, is greater for push bending than for pull bending, whereas the scatter is smaller for push bending than for pull bending. The failure mode of RTM-made tee-joints in pull bending tests can be reckoned to be characteristic of debonding of resin matrix at the interface between the triangular resin-rich zone and the curved web of tee-joint until complete separation of the curved web from the bottom plate. In contrast, as distinct from the products subject to pull bending loading, the RTM tee-joints in push bending tests experience matrix cracking and fibre fracture from outer layers to inner layers of the bottom plate until catastrophic collapse resulting from the bending. Three-dimensional finite element (FE) models are presented to simulate the load transfer path and failure initiation mechanism of RTM-made TWF composite tee-joint based on the maximum stress criterion. Good correlation between experimental and numerical results is achieved.

Small sample theory for reliability design

This paper outlines a new technique to address the paucity of data in assessing fatigue strength reliability. New formulations are presented for scatter factors, dealing with conditions where the population standard deviation is unavailable and where fatigue test results are incomplete. Reduction factor formulae for these two cases are also given. Finally, the concepts are applied to two sets of experimental data, demonstrating the practical and economic use of the proposed technique. It is shown from these examples that, when the standard deviation of the population is unknown, then safe fatigue life and fatigue strength can be obtained realistically according to the small sample test data using the new formulae for the fatigue life scatter factors and for the fatigue strength reduction factor.

On static and fatigue strength determination of carbon fibre/epoxy composites Part 1: Experiments:

This work seeks to investigate the static and fatigue properties and the failure mechanisms of T300/QY8911 carbon-fibre-reinforced composite laminates with different layups to optimize the stacking sequence effect. Three different layups have been studied, namely, [45/02/-45/902/-45/0/45/90]s, [45/-45/0/-45/0/45/90/45/0/-45]s and [45/0/-45/0/-45/0/45/0/90/0]s. The results for static strength and fatigue residual strength under different fatigue stress amplitudes are evaluated and compared with each other. The damage and failure mechanisms of the composite laminates are discussed. It is observed that all tension-tension fatigue damage patterns of the notched laminates are similar while the tension-tension fatigue properties vary with the laminate layup. The reduction of stress concentration caused by the tension-tension fatigue damage leads to an improvement of the residual strength of the notched specimens in contrast to their static strength properties. The damage mechanics of the notched laminates under compression-compression loading are more complex than those of the tension-tension fatigue specimens; their damage patterns are influenced by the test clamping fixture, the layup, the size of specimens and the diameter of the hole. The residual strengths are lower than those of the specimens without fatigue damage. It is verified that in a high-temperature and moisture environment, there is a decrease in static compression strength. The results of this study provide an insight into fatigue damage development in composites and constitute a fundamental basis for the development of a strain-based residual strength model.

On static and fatigue strength determination of carbon fibre/epoxy composites Part 2: theoretical formulation

This paper outlines a practical model to evaluate fatigue residual strength for reliability-based design. A new formulation is presented for a fatigue-driven residual strength surface based on controlling fatigue strain. The parameter determination formulae of the residual strength surface are established to deal with the test data effectively and easily. Finally, the model is applied to experimental data, demonstrating the practical and effective use of the proposed model. It is shown that fatigue-driven residual strength can be obtained realistically according to the small sample test data using the new formulae.

Analytical Solution for Predicting In-plane Elastic Shear Properties of 2D Orthogonal PWF Composites

Abstract This paper proposes a new analytical solution to predict the shear modulus of a two-dimensional (2D) plain weave fabric (PWF) composite accounting for the interaction of orthogonal interlacing strands with coupled shear deformation modes including not only relative bending but also torsion, etc. The two orthogonal yarns in a micromechanical unit cell are idealized as curved beams with a path depicted by using sinusoidal shape functions. The internal forces and macroscopic deformations carried by the yarn families, together with macroscopic shear modulus of PWFs are derived by means of a strain energy approach founded on micromechanics. Three sets of experimental data pertinent to three kinds of 2D orthogonal PWF composites have been implemented to validate the new model. The calculations from the new model are also compared with those by using two models in the earlier literature. It is shown that the experimental results correlate well with predictions from the new model.

Reliability-based minimal sample factor formulation for a corrosion damage assessment in aluminium alloy plates

Accelerated corrosion tests and atmospheric exposure tests are conducted to investigate the corrosion performance of LY12CZ aluminium alloy. Based on the experimental data, the corrosion depth at different corrosion times is determined using a fuzzy discrimination method which follows a log-Gauss distribution. From the experimental observations, the minimal sample factor formulation and a power law function model with three parameters are presented to assess safe corrosion damage at different corrosion times and to describe the corrosion depth law with corrosion time. From the application examples of this model, it is shown that the data for corrosion depth are better defined by a three-parameter power law function shown in the figure. Statistical analysis shows a strong correlation between pit depth and observed corrosion time. Finally, the equivalent relationship between accelerated corrosion and atmospheric exposure corrosion is established.

Analytical Solutions for Predicting Tensile and Shear Moduli of Triaxial Weave Fabric Composites

Novel micromechanical curved beam models were presented for predicting the tensile and shear moduli of triaxial weave fabric (TWF) composites by considering the interactions between the triaxial yarns of 0° and ±60°. The triaxial yarns in micromechanical representative unit cell (RUC) were idealized as curved beams with a path depicted using the sinusoidal shape functions, and the tensile and shear moduli of TWF composites were derived by means of the strain energy approach founded on micromechanics. In order to validate the new models, the predictions were compared with the experimental data from literature. It was shown that the predictions from the new model agree well with the experimental results. Using these models, the tensile and shear properties of TWF composites could be predicted based only on the properties of basic woven fabric.

Effects of stress ratio on the temperature-dependent high-cycle fatigue properties of alloy steels

This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue (HCF) properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they function as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.