Papers by Keyword: Micro-Mechanics

Abstract: In this study, joining of AlN and Al with compositional graded layer is made by centrifugal mixed-powder method (CMPM). The mixed-powder of AlN particles and Al particles is inserted into a spinning mold with bulk-shaped AlN, and then molten Al is poured into the spinning mold with the mixed-powder and bulk-shaped AlN. As a result, the molten Al penetrates into the space between the mixed-powder by the centrifugal force, and at the same time, the Al particles can be melted by heat from the molten Al. Then AlN and Al can be joined with compositional graded layer after solidification. Micromechanics-based analysis is also employed to understand the thermal stress relaxation by the compositional graded layer.

Abstract: Because reinforced-fiber has characteristic of single step forming in three-dimensional braided composites, analysis of mechanical performance is very different from laminates. This paper proceeds from the micro-structure of 3-D braided composites, some review and research are presented detailedly, and the several model’s merit and demerit are analyzed, which can conduct dynamicists to choose reasonable numerical model. The mechanical performance can be predicted accurately, and the next research can be guided using above analysis.

Abstract: Based on 2-D PFC, non-circular particle with direction was constructed and specimens with different distribution of long axis orientation were established. Loads were applied in vertical direction. Changes of fabric such as the long axis orientation, particle contact direction and particle contact force in the process of loading were analyzed. It was shown that the larger angle of the loading direction and the long axis orientation, the greater strength of specimen and more anisotropic. It was proved by the numerical analysis that tendency of particle contact stress direction is the micro – mechanics reasons of soil’s anisotropy under loading.

Abstract: In order to gain a deep understanding of energy absorption property and failure mechanism of closed-cell foam metals, the dynamic energy absorption property of closed-cell foam metals with matrix Al, matrix Al-Si6, matrix Al-Si12 CCAF, Mg matrix, aluminum-fiber and RE-Al alloys are discussed based on the compression and energy absorption test. The influence of different basis materials on absorption property of closed-cell foam metals is discussed, too. It also shows the micro topography of CCAF during the compression process. In addition, micro-mechanics failure mechanism of CCAF is discussed. Results obtained in this research include: (1) the compression deformation curves of different foam metals indicate that matrix Al CCAF is better for energy absorption materials, because of the best energy absorption property and the high strength. (2) the strength of metal foams with matrix Al-Si6, matrix Al-Si12 and matrix Mg are higher than that of matrix Al CCAF, and also have better energy absorption property. However, the failure characteristics of brittle fracture indicate obvious in matrix Al-Si6, matrix Al-Si12 and matrix Mg. (3) the micro-topography of CCAF with matrix Al is simple and uniform. Also the fracture has obvious tear trace. Aluminum-fiber and RE-Al alloys have better compression property, but lower strength and no strength increasing stage lead to destroy suddenly. (4) the curves of energy absorption for matrix Al and matrix Al-Si6CCAF are on the rise. And the maximum of energy absorption is about 2.1MJ/m³. (5) micropore and crack of closed-cell foam metals provide a good space for the compression and energy absorption. Meanwhile, it is the main failure parts of damage. Keywords: closed-cell foam metal; dynamic energy absorption; micro-topography; micro-mechanics

Abstract: This paper investigates nonlinear responses of honeycomb sandwich composite under externally applied loads theoretically and experimentally. In the experimental work, honeycomb sandwich composites made of an aluminum honeycomb core and glass fiber reinforced polymer (FRP) laminate surfaces were loaded under in-plane tension and out-of plane bending up to failure. Stress-strain curves or load deflection plot together with elastic moduli and ultimate strengths were obtained. An energy approach was used to establish a nonlinear constitutive relationship for the honeycomb sandwich composites. Making use of the superimposition ability of the strain energies of all of the walls of the RVE, a nonlinear constitutive relationship for the honeycomb core was obtained. The thus obtained relationship was incorporated with a laminate theory and the bridging model to analyze nonlinear responses of the honeycomb core and FRP surface sandwich composites up to failure. The composite failure was detected micromechanically, i.e., based on the failure status of its constituent aluminum core, reinforcing fiber, polymer matrix, and adhesion layer. Stiffness discount was applied respectively to the failed composing element. The predicted nonlinear stress-strain curves under tension and load-deflection relationship under three-point bending were compared with the experimental measurements. Favorable correlations have been obtained.

Abstract: For the flexible three-dimensional microstructure, the GMC single-cell method was applied to establish the mechanical model of the bending interface, and then the interface micro-element method based on composite materials was performed to work on the interfacial properties of weak interfaces and micro-mechanical behavior. As a result, a flexible interface model for the interface stress was constructed and it would be helpful to improve the bending properties of the flexible three-dimensional microstructure.

Abstract: This paper deals with an analysis of a crack-tip field of particulate-reinforced composites which can describe the evolution of debonding damage, matrix plasticity and particle size effect on deformation and damage. Numerical analyses were carried out on a crack-tip field in elastic-plastic matrix composites reinforced with elastic particles by using a finite element method developed based on an incremental damage theory. The particle size effect on damage is described by a critical energy criterion for particle-matrix interfacial debonding. The effect of debonding damage on a crack-tip field is discussed based on numerical results. The debonding damage initiates and progresses ahead of a crack-tip. The stress distribution shifts downward in the debonding damage area. It is concluded that a crack-tip field is strongly affected by debonding damage.

Abstract: For micro-fiber reinforced strain-hardening cementitious materials, in addition to the basic characteristics and mechanical properties of fiber and interfacial properties between fiber and matrix, mechanical properties of matrix such as strength and crack resistance are essential parameters for material design, too. Therefore, the fracture properties of cement paste and mortar which are two most basal cementitious materials were studied, using three-point bending beams of which strength and depth are varied. Complete load versus crack mouth opening displacement (P-CMOD) curve directly obtained, and double-K fracture parameters ini Ic K and un Ic K were subsequently determined. The initial cracking load Pini was determined using resistant strain gauges. The results show that an apparent stable crack propagation before unstable failure was observed both in cement paste and in mortar. For cement paste, due to the influence of shrinkage crack, the divergence of the unstable fracture toughness un Ic K is more evident than initial fracture toughness ini Ic K .

Abstract: The main purpose of this paper is to develop a set of improved micro-metrology system, which consists of a digital image-measuring device and a microscope with a long focal length, for the study of displacement fields, strain fields in the area across the interphases, and also the effect of temperature on the micro-mechanics properties of interphases in thermoplastic composites. Meanwhile, the precision of the micro-metrology system is assessed by using an extra solution within of the framework of infinitesimal strain in particular, the choice of the size of subset in the correlation technique is discussed when the displacements are not infinitesimal. Experimental results show that, under practical service conditions of thermo-mechanical coupled loads, a significant residual thermal stress mismatch due to the difference in coefficients of thermal expansion between the fiber and the matrix is balanced out.