Bending strength of delaminated aerospace composites

Buckling-driven delamination is considered among the most critical failure modes in composite laminates. This paper examines the propagation of delaminations in a beam under pure bending. A pre-developed analytical model to predict the critical buckling moment of a thin sub-laminate is extended to account for propagation prediction, using mixed-mode fracture analysis. Fractography analysis is performed to distinguish between mode I and mode II contributions to the final failure of specimens. Comparison between experimental results and analysis shows agreement to within 5 per cent in static propagation moment for two different materials. It is concluded that static fracture is almost entirely driven by mode II effects. This result was unexpected because it arises from a buckling mode that opens the delamination. For this reason, and because of the excellent repeatability of the experiments, the method of testing may be a promising means of establishing the critical value of mode II fracture toughness, G(IIC), of the material. Fatigue testing on similar samples showed that buckled delamination resulted in a fatigue threshold that was over 80 per cent lower than the static propagation moment. Such an outcome highlights the significance of predicting snap-buckling moment and subsequent propagation for design purposes.     

Compressive strength of unidirectional fiber composites with matrix non-linearity

A study has been made of the effect of fiber misalignment and non-linear behavior of the matrix on fiber microbuckling and the compressive strength of a unidirectional fiber composite. The initial fiber misalignment constituted the combined axial and shear stress state in the matrix, and the state of stress just prior to the buckling was considered to be the initial state of stress in bifurcation analysis. The expression for the critical microbuckling stress was found to be the same as that for the elastic shear-mode microbuckling stress except that the matrix elastic shear modulus was replaced by the matrix elastic-plastic shear modulus. Incremental theory of plasticity and deformation theory of plasticity were used to model the matrix non-linearity. The analysis results showed reasonable correlation with available experimental data for AS4/3501-6 and AS4/PEEK graphite composites with 2° to 4° range of initial fiber misalignment.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

CNT composites for aerospace applications

Carbon nanotubes were synthesized by thermal arc plasma process after optimization of the synthesis parameters. These samples were then analysed by scanning and transmission electron microscopes (SEM and TEM), in order to establish the morphology of the nanostructures. Atomic force microscopy (AFM) and electron diffraction studies were also carried out before using the sample for the composite material preparation. Composites of epoxy resin with curing agent as well as a mixture of graphite and carbon nanotubes were prepared with varying proportions of the mixture. The electrical resistivity of the material was studied under varying pressure and voltage conditions. Preliminary results of these studies present interesting features which are reported here.     

Compressive strength of cement-based composites: Roles of aggregate diameter and water saturation degree

Concrete structures experience microcracking leading to mechanical damage when submitted to desiccation. This change in mechanical properties can be dependant on the level of drying and also on constituents of the material. This paper focuses on one particular aspect, which is to determine the role of the size of rigid inclusions over cementitious materials under drying. However, concrete is a complex heterogeneous material, where the shape of natural aggregates is variable, and the number of factors to analyze is large. Thus the study was performed on a geometrically simplified material, initially proposed by Bisschop and van Mier (2002) [1] compound of spherical glass inclusions of several diameters in a cementitious matrix to explore the influence of aggregate size. A set of uniaxial and triaxial compressive tests is performed for materials with various water saturation levels. A great dependence is observed between mechanical properties, notably peak strength and damage, linked to aggregate size and water saturation degree. Complementary microtomographic acquisitions are also done, and tend to validate the cracking pattern dependency to the aggregate diameter, and therefore explain changes in mechanical behaviour.     

Compressive strength of aluminium foams

Compressive strength was measured for aluminium foam specimens having different density and size. Larger specimens exhibited lower mean strength and narrower scattering of the strength values versus material density than the smaller ones. This behaviour is explained in terms of a greater probability of the existence of lower density regions in the former specimens. Both small and large low-density samples show more reproducible properties than the higher density ones.     

碳纤维截面特性对碳纤维/环氧树脂复合材料压缩强度的影响

基于压拉平衡为特征的新一代先进复合材料的需求,开展了碳纤维截面形状和尺寸对碳纤维/环氧树脂复合材料压缩强度的影响研究。有限元模拟和试验结果均表明,增大碳纤维直径可以提高复合材料压缩强度。另外碳纤维截面形状也对复合材料压缩强度有影响,圆形截面优于椭圆形截面。      

Asymptotic analysis considerations on the initial postbuckling behavior of delaminated composites

Summary An asymptotic analysis for the initial postbuckling behavior of delaminated beam/plates is performed. Under the assumptions of inextensional deformation, the exact expressions that govern the plane elastic deformation of the different parts of the system are expanded in a Taylor series in terms of the distortion variable. Order of magnitude arguments are used to relate the distortion variables and these are subsequently used in an asymptotic solution for the deflections and the load. Finally, a comparison with experiments is performed.     

Investigation of failure mechanisms in aged aerospace composites

The effect of isothermal ageing on two high temperature, bismaleimide composite materials, a novel CSIRO CBR 320/328 composite and a commercial CIBA GEIGY Matrimid^® 5292 composite, was examined at 204 and 250 °C. Delamination is a major cause of failure in composite materials, therefore, the Mode I interlaminar fracture toughness (G_IC) of both materials was measured using the double cantilever beam (DCB) test. Chemical degradation of the matrix was monitored concurrently using Fourier transform infrared (FTIR) and Raman spectroscopy. Chemical changes at the core of both of these materials were found to occur concomitantly with the observed changes in interlaminar fracture toughness. FTIR analysis of both matrix materials revealed the predominant degradation mechanism to be the oxidation of the methylene group bridging two aromatic rings common to the structure of both resins, and was substantiated by the ingrowth of a broad peak centred at 1600 cm⁻¹ . In addition to this, the pyromellitic anhydride unit present only in the CBR 320/328 composites was found to be highly resistant to the effects of ageing, whereas the saturated imide, common to the cured structures of both materials, was observed to degrade. Raman spectroscopy indicated that the predominant degradation mechanism of the composites differed at the two ageing temperatures.     

Compressive failure of hybrid multidirectional fibre-reinforced composites

In this paper, the hybridisation of multidirectional carbon fibre-reinforced composites as a means of improving the compressive performance is studied. The aim is to thoroughly investigate how hybridisation influences the laminate behaviour under different compression conditions and thus provide an explanation of the “hybrid effect”. The chosen approach was to compare the compressive performance of two monolithic carbon fibre/epoxy systems, CYTEC HTS/MTM44-1 and IMS/MTM44-1, with that of their respective hybrids. This was done by keeping the same layup throughout ((0/90/45/−45)_2S) while replacing the angle plies in one case or the orthogonal plies in the other case with the second material, thus producing two hybrid systems. To investigate the compressive performance of these configurations, compact and plain compression test methods were employed which also allowed studying the sensitivity of compressive failure to specimen geometry and loading conditions. The experimental results and the subsequent fractographic analysis revealed that the hybridisation of selective ply interfaces influenced the location and severity of the failure mechanisms. Finally, in light of this knowledge, an update of the generic sequence of events, previously suggested by the authors, which lead to global fracture in multidirectional fibre-reinforced composites under compression is presented.     

Compressive strength of coated rigid-rod polymer fibres

One limitation to the use of high-strength/high-modulus rigid-rod polymer fibres like poly-(p-phenylene benzobisthiazole) (PBZT) and poly-(p-phenylene benzobisoxazole) (PBZO) in composite structures is their low compressive strength. Various theories have been developed to predict compressive strength of rigid-rod fibres. In this study the critical buckling stress for rigid-rod fibres with stiff external coatings has been theoretically modelled assuming that the failure mode in compression is the microbuckling of the fibrils in shear. Our model predicts that significant improvement in fibre compressive strength will occur only when relatively thick coatings, with thickness to diameter (t/D) ratios in excess of > 0.05, are used. Experimentally measured compressive strength of aluminium coated PBZT fibres shows values in good agreement to the theory at t/D ratios of 0.006 and below. Factors related to the selection of suitable coating materials and problems associated with establishing coating performance are identified.Nomenclature P axial compressive load - P _f axial compressive load on the fibre - P _c axial compressive load on the coating - P _cr ⁱ critical buckling load in the ith case - _cr critical buckling stress - _co compressive strength of the uncoated fibre - _c compressive strength of the coated fibre - v(x) lateral deflection of a buckled fibril or coating - V _m amplitude of the lateral deflection in the mth mode - m number of half-sine waves in the deflection mode - x coordinate distance along axial direction - y coordinate distance along radial direction - coordinate distance along circumferential direction - l length of the buckling unit - N number of fibrils in the fibre - D fibre diameter - d fibril diameter - t coating thickness - I _f moment of inertia of the fibril - A _f cross-sectional area of the fibril - E _f tensile modulus of the fibre - E _c tensile modulus of the coating material - E tensile modulus of the coated fibre - G torsional shear modulus of the fibre - v_c Poisson's ratio of the coating material - _f density of the fibre - _c density of the coating material - density of the coated fibre - U _f strain-energy change in the fibre - U _c strain-energy change in the coating - T _f external work done on the fibre - T _c external work done on the coating - d/D - t/D     

Beryllium metal matrix composites for aerospace and commercial applications

Abstract

There is a growing need in both aerospace and commercial markets for lighter weight, higher stiffness, higher thermal stability materials to solve the design engineers’ problems of reduced mass, higher access speeds, improved mechanical and thermal stability for today's advanced technology. To address those needs, Brush Wellman Inc. has developed, characterised, and put into high volume production a family of beryllium metal matrix composites. There are two classes of materials that have been developed to provide these engineering benefits to the designer in both the commercial and aerospace markets. The first family of materials is aluminium beryllium (AlBeMet, which is a registered tradename of Brush Wellman). This material is a metal matrix composite consisting of pure beryllium and aluminium, with 20–62 vol.-%Be and the remainder aluminium. The material is produced by both powder metallurgy and net shape technologies such as investment casting and semi-solid forming. The materials properties that make it attractive for the design engineer are a density that is 25% less than that of aluminium, a specific stiffness four times those of aluminium, titanium, steel, and magnesium, a higher dampening capacity than aluminium, and a coefficient of thermal expansion almost 50%lower than aluminium. The second family of materials is a beryllium–beryllium oxide metal matrix composite, which are called E materials. This material was developed to address the thermal management needs of the electronic packaging design engineer. The material properties that make this material attractive to the electronic packaging engineer are: a density 20–25% that of Kovar, Invar, and CuMoCu, and 30% less than Al–SiC; a thermal conductivity ranging from 210 to 240 W m^-1 K^-1and a tailorable coefficient of thermal expansion, ranging from 6×10^-6 to 8.7×10^-6 K^-1.