Compressive strength of delaminated aerospace composites

An efficient analytical model is described which predicts the value of compressive strain below which buckle-driven propagation of delaminations in aerospace composites will not occur. An extension of this efficient strip model which accounts for propagation transverse to the direction of applied compression is derived. In order to provide validation for the strip model a number of laminates were artificially delaminated producing a range of thin anisotropic sub-laminates made up of 0°, ±45° and 90° plies that displayed varied buckling and delamination propagation phenomena. These laminates were subsequently subject to experimental compression testing and nonlinear finite element analysis (FEA) using cohesive elements. Comparison of strip model results with those from experiments indicates that the model can conservatively predict the strain at which propagation occurs to within 10 per cent of experimental values provided (i) the thin-film assumption made in the modelling methodology holds and (ii) full elastic coupling effects do not play a significant role in the post-buckling of the sub-laminate. With such provision, the model was more accurate and produced fewer non-conservative results than FEA. The accuracy and efficiency of the model make it well suited to application in optimum ply-stacking algorithms to maximize laminate strength.     

Bending behavior of delaminated composite plates with contact effects

Laminated plates with strip-type delamination under pure bending were investigated analytically and experimentally. In the analysis, a two-dimensional nonlinear finite element code based on updated Lagrangian formulation was developed to analyze the bending behavior of the laminated plates and the local buckling phenomenon of the sublaminates in the delaminated region. The formulation includes large displacements and large rotations needed to describe the local buckling phenomenon of the delaminated region. The transformation matrix method, which satisfies the compatibility of displacements between the upper and lower delaminated surfaces, can reduce the number of system unknowns and was used to cope with this contact problem. The modified Newton-Raphson method was used to solve the resulting nonlinear system equation and a load-controlled scheme used in the incremental solution procedure. In each increment, the iteration process was performed until the contact condition was satisfied. In the experiment, a tensile-test machine equipped with a set of four-point bending device was used to record the load-displacement response of the delaminated plates; the bending moment-curvature relation was calculated therefrom. The analytical results are improved by contact analysis. The results shows that the size of delaminated region had no significant influence on the bending strength of laminated plates, no matter whether the local buckling occurred or not. The normal contact force on the surface of the delaminated region kept constant along the delaminated length.     

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.     

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.     

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.     

Bending strength of silica glass

Tensile and bending tests are useful to characterize the mechanical behavior of ceramics. Theoretical comparisons between results of both tests are usually done based on Weibull statistics. In previous experiments on borosilicate glass, no agreement was found between experimental and theoretical values of the ratio of the maximum bending and tensile stresses at 50 percent probability of fracture. In this investigation, additional experiments in bending have been performed, to measure the distribution of fracture initiation points. Good agreement with theory is found. The previous disagreement could be attributed to fatigue effects.     

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.     

Bending strength of oil-lubricated cylindrical plastic gears

Forschung im Ingenieurwesen - Oil-lubricated plastic gears enable significantly higher power transmission compared to dry-running gears. One of the most relevant damage mechanisms herein is tooth...     

Tensile strength of hybrid composites

This paper presents an approximate, statistical analysis for the tensile strength of unidirectional hybrid composite materials consisting of two-dimensional arrays of alternating low elongation and high elongation fibres in a common matrix. Expressions for ineffective length and fibre strain concentration factor in hybrid composites are developed. The analysis identifies a number of important material properties that affect the failure process in hybrids: statistical fibre tensile failure strain characteristics, and fibre extensional moduli and cross-sectional areas. The influence of these parameters on the failure process is examined and differences between failure mechanisms in hybrids and composites containing only one type of fibre are considered. The analysis predicts that, in general, the failure strain of a hybrid should be different from those of composites reinforced with either of the parent fibres alone. It is found that the theory can explain the hybrid effect that has been observed by several authors: hybrids made by combining high elongation and low elongation fibres, such as graphite and glass, often display tensile failure strains greater than those of composites made from the low elongation fibres alone. Predicted failure strains are compared with available experimental data. Suggestions for future work in the area are presented.     

Bending strength and effective modulus of atmospheric ice

Bending strength and the effective modulus of atmospheric ice accumulated in a closed loop wind tunnel at temperatures − 6 °C, − 10 °C and − 20 °C with a liquid water content of 2.5 g/m³ have been studied at different strain rates. More than 120 tests have been conducted. Ice samples, accumulated at each temperature, have been tested at the accumulation temperature. In addition, tests have been performed at temperatures of − 3 °C and − 20 °C, for the ice accumulated at − 10 °C. These tests showed a clear dependency of bending strength of atmospheric ice on test temperature at low strain rates. Strain rate effects are implied because the spread in bending strength for the different temperatures diminishes as strain rate increases. The results also reveal that, in most cases, the effective modulus of atmospheric ice increases with increasing strain rate. The bending strength of atmospheric ice accumulated at − 10 °C has been found to be greater than that of ice accumulated at − 6 °C and − 20 °C. The results show that the effective modulus of ice accumulated at − 20 °C at higher strain rates is less than that of the two other types.     

纤维增强形状记忆聚合物复合材料及其航天应用

形状记忆聚合物(SMP)是一种能够保持临时形状,并在外界刺激下自发回复到其初始形状的智能材料,具有高形状固定率、高形状回复率、转变温度可调、变形能力强、质量轻等优点,但其应用受到响应方式单一和承载能力差的限制,通过向聚合物中添加功能颗粒或增强纤维制成形状记忆聚合物复合材料(SMPC),可有效解决这一问题.首先介绍了SM...     

Post-impact behaviour of aerospace composites for high-temperature applications: experiments and simulations

The post-impact performance of different carbon-fabric-reinforced composite materials were studied experimentally and analytically. Three types of thermosetting matrix were considered: conventional epoxy, high-temperature curing epoxy and epoxy-isocyanate. Experimental testing consisted of impacting rectangular specimens at different energy levels by using a spring-driven impact apparatus that was able to impart velocities of up to 5 m s⁻¹ to masses of 0.5, 1.0, 2.5 and 5.0 kg travelling horizontally. After impact, coupons were non-destructively inspected by means of opaque-enhanced dye-penetrant X-radiography and tested in static compression to correlate impact energy, damage extent and residual strength. Epoxy composites contain damage within a narrow region, while epoxy-isocyanate materials propagate the damage far away from impact point. Epoxy composites show an asymptotically decreasing failure strength with impact energy up to a lower threshold (0.3–0.4 times that of the undamaged material), while epoxy-isocyanate material shows a trend of ever decreasing residual strength. An analytical study was performed by means of the finite element code PAM-FISS, used to simulate the compression-after-impact (CAI) tests. Type, size and location of damage, as well as the mechanisms leading to final failure, were reproduced quite well by the finite element analysis (FEA), while some discrepancies between FEA and experimental CAI residual strength tests were found (7% for undamaged specimens and 10% for blister-delaminated specimens); higher errors were found in the case of completely delaminated specimens, mainly owing to the inability of the present software and hardware to conveniently model the complete state of damage.     

Creep strength of discontinuous fibre composites

A unidirectional, discontinuous fibre composite is considered under conditions of steady state creep in the direction of reinforcement. The composite consists of noncreeping, discontinuous, perfectly aligned, uniformly distributed fibres which are perfectly bonded to a matrix obeying a power relation between stress and strain rate. Expressions for the interface stress, the creep velocity profile adjacent to the fibres and the creep strength of the composite are derived. Previous results for the creep strength,σ _c obtained for composites of the same type are briefly reviewed and compared with the present result. It is shown that all results reduce to the same general expression $$\sigma _c = \alpha V_{f^{\sigma _0 } } \left( {\frac{{\dot \in }}{{\dot \in _{0 } }}} \right)1/n_{\rho ^{1 + 1/n} }$$ in whichρ is the fibre aspect ratio, \(\dot \in\) is the composite creep rate,V _f is the fibre volume fraction,σ ₀,ε ₀ andn are the constants in the matrix creep law. The creep strength coefficient α is found to be very weakly dependent onV _f and practically independent ofn whenn is greater than about 6.