Xiaolu Gong

Determination of Residual Stress in Composite Laminates Using the Incremental Hole-drilling Method:

The cooling conditions used in the forming process of composite materials play an important role in the creation ofresidual stress. In this study, a new method for measuring residual stress in composite laminates is presented. Three cooling conditions were used to produce different residual stress levels. Residual stresses in [02/902]s and [08] laminate have been measured by the incremental holedrilling method combined with 3-D finite element modelling. A software which quickly calculates all the coefficients for each increment was developed. The automatic procedure can be used to calculate the calibration coefficient for any type oflaminate (ply number, mechanic characteristics,...) and whatever the number of increments and their depths. The different results show that this method provides access to the in-depth distribution and through thickness ofresidual stress in the laminate with a good accuracy and practicality.

Influence of experimental parameters on determination of residual stress using the incremental hole-drilling method

Abstract The incremental hole-drilling method is an effective semi-destructive technique for determining the profile and magnitude of residual stresses in composite laminates. This method provides access to the residual stress profile of the intra and inter plies in the through thickness of the laminate. Correct application of the method requires optimisation of the relevant experimental parameters. Tests were performed to investigate the influence of two experimental parameters—the depth of each drilled increment and the influence of the relative position of the strain gages compared with the radius of the hole drilled. The development of a rapid automatic method for numerical calculation of the calibration coefficients enhanced the interest of the method while extending the experimental data range being investigated. The results clearly show that these parameters have an appreciable effect on determination of the residual stress gradient, and especially the magnitude and stability of the residual stress.

Acoustic properties of air-saturated porous materials containing dead-end porosity

This study examines the acoustic properties of materials with complex micro-geometry containing partially open or dead-end (DE) porosity. One of these kinds of materials can be obtained from dissolving salt grains embedded in a solid metal matrix with the help of water. The solid matrix is obtained after the metal, in liquid form, has invaded the granular material formed by the salt particles at negative pressure and high temperature, and after cooling and solidification of the metal. Comparisons between theoretical and experimental results show that the classical Johnson-Champoux-Allard model does not quite accurately predict the acoustic behavior. These results suggest that the assumptions of the Biot theory may not all be fulfilled and that cavity resonators and dead ends can be present in the material. The first part of the study proposes a simple model to account for this geometry. Based upon this model, two acoustic transfer matrices are developed: one for non-symmetric and one for symmetric dead-en...

Synthesis of ZnFe2O4/SiO2 composites derived from a diatomite template

A novel porous ZnFe2O4/SiO2 composite product has been generated with a template-directed assembly method from porous diatomite under different synthesis conditions, such as precursor concentrations (metallic nitrates), calcination temperature and diatomite type. The phase composition and morphology of all the materials were examined by x-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The results indicated that an inherited hierarchical porous structure from the diatomite template can be obtained, and the synthesis conditions were found to have clear effects on the formation of the ZnFe2O4/SiO2 composite. The ideal composite of ZnFe2O4/SiO2 can be obtained through optimization of diatomite template type, precursor solution and calcination temperature. Furthermore, the adsorption abilities of two types of diatomites were analyzed in detail using FTIR spectra and nitrogen adsorption measurements etc, which proved that A-diatomite (Shengzhou-diatomite) is better than B-diatomite (Changbai-diatomite) on the aspect of adsorbing Zn and Fe ions, and of forming the ZnFe2O4.

Influence of residual stresses on the mechanical behavior of composite laminate materials

Residual stresses occur during most manufacturing processes (from shrinkage of the resin, difference of thermal expansion coefficients, etc.). Residual stresses may cause local yielding and damage initiation and propagation, and can severely affect performances of a composite structure component. The aim of this work is to investigate the influence of residual stresses on the mechanical behavior of carbon/epoxy cross-ply laminates under tensile and torsion loading. The residual stresses field was determined using the incremental blind-hole drilling technique combined with the finite element analysis. The effect of various cure cycles on the residual stress level is analysed. The quantitative effect of the residual stresses on the mechanical behaviour and the damage initiation and growth is studied by using acoustic emission technique.

Determination of the Residual Stresses in Composite Laminate Using the Compliance Method

Residual stresses play an important role on the mechanical behavior of composite laminate. The development of new methods to determine the residual stresses gradient within the laminates is necessary. This article presents the adaptation of the compliance method in the case of composite laminates carbon/epoxy [02/902]s. The incremental drilling of a constant width groove allows for each increment to measure the strains (using strain gages) and displacements (using an optical device) of particularly points of the structure surface. These experimental data are compared with results given by a finite elements simulation. This comparison allows to raise the residual stresses in the composite laminate.

Experimental determination of residual stresses in composite laminates [02/θ2]s

The present work aims to determine the residual stresses in carbon fiber/epoxy composite laminates, by means of the incremental hole-drilling method. Based on mechanical theories of composite laminates and an elastic plate with a circular hole, the relationship between the relaxed strains on the surface of laminates and the residual stresses in laminates was established. This newly deduced theoretical formula was adapted into the incremental hole-drilling method and allowed us to further study the residual stresses in the through-thickness direction for various composite laminates. Related numerical modeling of composite laminate with a hole was built to calibrate the coefficients within the formula. Experiments were conducted and the residual stresses in composite laminates [02/θ2]s are presented. The proposed approach was validated with the consistence between our results for cross-ply laminates and those in literature.

Thermo-Mechanical Behavior Analysis of Pressure Infiltration Casting Process for Manufacturing Open-Cell Metal Foam

Pressure infiltration casting technique is widely used to manufacture open-cell metal foam and it has been well performed in manufacture practice in our research group. In the present study, an attempt is made to highlight the influence of the infiltration casting process on the physical structure of metal foam. According to the production practice, several parameters: the pressure difference, the temperature condition, the particle size and the mould dimension, need to be well considered. A 2D model coupling porous zone and multiphase is created with a fixed grid. The cooling and solidifying process of molten metal are as well carried out with enthalpy method. Reasonable convergence rules are used to proceed high quality calculations. This work provides excellent experimental and numerical comprehension of pressure infiltration casing process to manufacture open-cell metal foam. These understandings can well guide and optimize the product manufacture, the physical structure and the mechanical behaviors of open-cell metal foams within their industrial application.

Mechanical behavior in bending deformation of thermoplastic composite laminates with different stacking sequences

This work aims to numerically analyze the mechanical properties of thermoplastic composite laminates with the [0°/90°]4s, [±25°]s4 and [0°/±45°/90°]2s stacking sequences under three-point bending loading. Three-dimensional multilayer structural models of composite laminates are established and then orthotropic elastics, Hill yield criterion and isotropic hardening rule are applied on these structural models of composite laminates to conduct the bending mechanical deformation. Fine enough meshes and reasonable loads and boundary conditions can efficiently reduce the computing cost. Afterwards, three-dimensional fiber–matrix structural models with disordered fiber arrangements of 0°/90°, 25°/−25° and 0°/45° interlaminates at the free edges of composite laminates are also created, in which the boundary conditions are transferred step by step using a three-step structural submodeling. Once more, orthotropic elastics and isotropic hardening are applied on these fiber–matrix structural models. This work provides a good way to understand the bending mechanical behavior of thermoplastic composite laminates with different stacking sequences.

Effect of Ultraviolet and Moisture Aging on the Mechanical Properties of Hemp Fibers Composites

Polypropylene reinforced by hemp fibers composites were prepared by injection method. An aging chamber was used to simulate and accelerate the natural ageing process. The influence of moisture and UV aging on the mechanical properties of the composites is studied in this paper. A modified law of mixtures applying to polypropylene/hemp fibers composites is proposed. The microstructures of the composites during aging treatment were analyzed by means of digital microscope. We can see that the moisture and UV aging reduced the mechanical properties of the composites. The UV aging is more penalizing on the mechanical properties of composites, compared to the moisture aging after 4 weeks of exposure.