Thermal de-consolidation of thermoplastic matrix composites—I. Growth of voids

Thermoplastic matrix composites have one unique advantage, being reprocessable or recycleable through post-thermoforming and fusion bonding subject to a re-heating/cooling cycle. However, this advantage is not unconditionally available, and thermoplastic matrix composites may suffer greatly from unexpected deterioration in meso-structures during the thermal processing, leading to de-consolidation. Based on experimental observations, the mechanisms of thermal de-consolidation are discussed in detail, and a void growth model is established to evaluate the degree of de-consolidation in a post-thermal operation. Using this model the dependence of void growth on material properties and processing parameters, such as the compressibility of fibre reinforcement, the effects of elasticity/viscosity of matrix, the applied external pressure and processing temperature is evaluated. It is shown that the results obtained are in a good agreement with the experimental data.     

Postbuckling analysis and imperfection sensitivity of metal matrix laminated cylindrical panels

The postbuckling behaviour of metal matrix composite (MMC) laminated cylindrical panels under quasistatic in-plane loading is investigated. A micro-to-macro analysis is used to obtain the response of the composite structure. The micromechanical analysis allows us to establish the overall instantaneous elastic-viscoplastic behaviour of the MMC composite at each load increment. The macromechanical analysis provides the response of the geometrically imperfect cylindrical panel to the applied external loading.

Results are presented for unidirectional and antisymmetric angle-ply SiC/Ti composite panels, subjected to uniaxial compressive loadings. The effects of the panel curvature, initial imperfections and rate of loading on the postbuckling response are illustrated. Comparisons with the response of the corresponding perfectly elastic panels are shown.     

Asymmetric behaviour of fibrous metal matrix composites

Abstract

Experimental investigations have illustrated that unidirectional metal matrix composites (MMCs) show asymmetric behaviour under uniaxial tensile and compressive loading. This asymmetry occurs when the material is loaded along the fibre direction and also when loaded in the transverse direction. In this paper, results from finite element micromechanical models are presented. The models were used to study the asymmetric behaviour of unidirectional fibre reinforced MMCs subjected to longitudinal and transverse loading. The effects of the thermal residual stresses arising from the manufacturing process were included in the study. Also, the influence of the degree of bonding of fibre to matrix was examined, from perfectly bonded to completely debonded. Results reveal that thermal residual stresses are responsible for the asymmetric behaviour of the MMCs in the longitudinal direction. In transverse loading, both the degree of interface bonding and residual stresses account for the asymmetric behaviour. The predicted stress–strain response of the MMC shows good agreement with the available experimental data for both tènsile and compressive loading. Results also suggest that in order to predict accurately the yielding behaviour of MMCs, the current symmetric yield criteria require modification.     

Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites—part I: uni-weave composites

Moisture diffusion and hygrothermal aging in uni-weave bismaleimide composites were systematically studied in this research. The moisture weight gain curves of the composites were compared with those of the neat resin in order to determine the interface effect on moisture absorption. Both the neat resin and composites display similar two-stage diffusion behavior, with the first and second stages being diffusion- and relaxation-controlled, respectively. When the diffusivities of the composites were compared to theoretical predications of an appropriate mathematical model, it is found that the fiber-matrix interface has little effect on the short term moisture diffusion. However, the long term absorption is suppressed in the composites. The rigid fiber–matrix interface seems to constrain long-range segmental motions of the matrix and in so doing reduce water absorption in the relaxation-controlled second stage. Although the fiber–matrix bonding is initially very strong, prolonged water absorption at elevated temperatures eventually damage the interface and causes interfacial cracking.     

Woven fabric composites—developments in engineering bounds,homogenization and applications

Various approaches for approximating upper and lower bounds for the elastic stiffness tensor for general woven fabric composites are first described. Well accepted minimum energy principles are briefly presented to establish the foundation for practical finite element procedures for determining these bounds. Secondly, comparisons of four common homogenization procedures are shown: the strain energy balance method, the plate approximation method, a direct approach via area averaging, and asymptotic expansion homogenization. As a limiting case, all of the methods obtain the well‐known Rule of Mixtures for a unidirectional uniaxial specimen. In attempting to consolidate much of the existing knowledge of structural constitutive models for woven fabric composites, this research seeks to summarize and compare various homogenization methods via finite element analyses. Finally, some illustrative applications are presented. Copyright © 1999 John Wiley & Sons, Ltd.     

Non destructive ultrasonic evaluation of fibrous metal matrix composites

An iterative numerical elastic constants optimization method associated to a classical ultrasonic immersion device is used for the entire characterization of two kinds of metal matrix composites. The usual methods of optimization are generally based on Newton's algorithm. Sometimes this algorithm converges towards relative minima that gives elastic constants which do not correspond to the physical reality of the material. As a remedy, we have developed a numerical analysis technique based on a different algorithm allowing a better convergence to the unique global solution. Using the Levenberg-Marquard algorithm, the methodology to recover the elastic constant, consists in minimizing the square deviation between calculated velocities and the experimental ones measured under variable incidence from a computer controlled ultrasonic immersion device.     

The effect of interaction of loadings and geometry imperfection on the buckling state of frames

In this paper the effect of the interaction of two loadings, and of the deviation from right angle on the buckling mechanism of a rectangular two-bar frame is discussed. Using a nonlinear stability analysis it is found that there is a critical value of the ratio of the two loadings, as well as of the deviation from right angle, for which the physical equilibrium path and the physically unacceptable complementary path meet each other at an asymmetric bifurcation point. At this point, the response of the frame changes from an elastic limit point instability to a stable equilibrium path associated with inelastic buckling. The analysis is supplemented by a variety of numerical results obtained by an efficient and reliable simplified nonlinear stability analysis applied for the first time to a non-rectangular frame.     

Fracture characteristics of a particulate-reinforced metal matrix composite

The effect of particulates on the failure mechanism of an Al-Mg-Si alloy 6061 with 20% angular alumina particles was studied. Fracture toughness tests were conducted on compact tension peak-aged specimens. The interaction of the reinforcement phase with the crack was investigated by optical microscopy and scanning electron microscopy, both on the surface and in the mid-thickness of the fractured specimen. It is shown that the fractured particles ahead of the crack tip, in particular the larger particles, play an important role in the void-initiation phase of the fracture process. Particle size and aspect ratio determine the likelihood of fracture. Some differences in the failure mechanisms have been observed between the mid-thickness and the surface of the specimen because of the difference between plane strain and plane stress fractures.     

The damage tolerance of carbon fiber reinforced composites—A workshop summary

The principal objective of this workshop was to identify the critical problem areas associated with the damage tolerance of carbon-fiber reinforced composite materials. The discussion was divided into six areas: (1) damage tolerant materials; (2) testing methods; (3) structural life prediction; (4) damage tolerant design concepts; (5) repair methods; and (6) nondestructive testing. Approximately 1 h was devoted to the discussion of each of these topics. Discussion was stimulated by having one, two, or three introductory presentations by the discussion leaders of each topic followed by open discussion. In this report, my impressions of the discussion on each topic are combined with those of the discussion leaders and presented in the following format: Summary of Presentations; Summary of Discussion; and Critical Issues. It is possible to obtain the final conclusions of the workshop by examining the critical issues listed at the end of each section.

The discussion was lively, controversial, and open. The main conclusions to be drawn from the workshop are: (1) damage from impact is the worst type of damage for these materials—significant reductions in the compressive strength will occur following impact; (2) fatigue damage at the present time does not limit the use of these materials but as new materials are developed fatigue failure may become an issue; (3) very little is understood about the micromechanics of damage, hence it is impossible to predict the effect of changing the properties of the individual components (fibers, matrix, and fiber-matrix interface) on the bulk material properties; (4) better analytical models to describe the formation and growth of impact-damage are badly needed; and (5) rapid NDT methods to inspect large components are required.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—II. impact behaviour

Quasi-static, low-hanging and high-velocity impact tests have been conducted in order to study the effect of fibre/matrix adhesion on the impact properties of fibre-reinforced metal laminates. Differences in fibre/matrix adhesion were achieved by using treated or untreated carbon fibres in an epoxy resin system. Chemical removal of the aluminium layers and a sectioning technique were applied to examine and characterize the impact damage in the laminates. The results show that the laminates with the weaker fibre/matrix adhesion exhibit larger damage zones, although the back face crack length and permanent indentation after impact are smaller for a given impact energy. Residual tensile strength after impact is also higher for the untreated fibre laminates due to increased fibre/matrix splitting in the composite layer.     

Dual scale non-linear stress analysis of a fibrous metal matrix composite

Precise estimation of local stress profiles in individual phases of a fiber reinforced metal matrix composite is a crucial concern for design of composites. Stress profiles are significantly affected by plastic relaxation of soft matrix. In this work, an analytical model was developed to compute local stress profiles in individual phases of fibrous metal matrix composites. To this end, embedded cell cylindrical composite model was applied in which a layered concentric cylinder consisting of a fiber-, matrix- and homogenized composite layers was used. Mean field micromechanics was integrated into the conventional elasticity solution process so that micro-macro dual scale analysis could be performed. The algorithm was formulated in an iterative incremental structure which was able to perform plastic analysis. This also allows temperature dependence of flow stress to be considered. Taking copper-SiC system as a reference composite, stress profiles were obtained for mechanical and thermal loading cases. For comparison, independent finite element analyses were carried out for two different unit cell models. Excellent agreement between analytical and numerical solutions was found for the mechanical loading case even for plastic range. In the case of thermal loading, however, plastic solutions revealed notable difference in quantity, especially for the axial stress component.     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—I.: Residual strength

The role of interfacial adhesion between fibre and matrix on the residual strength behaviour of carbon-fibre-reinforced metal laminates (FRMLs) has been investigated. Differences in fibre/matrix adhesion were achieved by using treated and untreated carbon fibres in an epoxy resin system. Mechanical characterisation tests were conducted on bulk composite specimens to determine various properties such as interlaminar shear strength (ILSS) and transverse tension strength which clearly illustrate the difference in fibre/matrix interfacial adhesion. Scanning electron microscopy confirmed the difference in fracture surfaces, the untreated fibre composites showing interfacial failure while the treated fibre composites showed matrix failure. No clear differences were found for the mechanical properties such as tensile strength and Young's modulus of the FRMLs despite the differences in the bulk composite properties. A reduction of 7·5% in the apparent value of the ILSS was identified for the untreated fibre laminates by both three-point and five-point bend tests. Residual strength and blunt notch tests showed remarkable increases in strength for the untreated fibre specimens over the treated ones. Increases of up to 20% and 14% were found for specimens with a circular hole and saw cut, respectively. The increase in strength is attributed to the promotion of fibre/matrix splitting and large delamination zones in the untreated fibre specimens owing to the weak fibre/matrix interface.     

Continuous manufacturing of fiber-reinforced metal matrix composite wires — technology and product characteristics

A new continuous processing method is presented for the production of continuous-fiber-reinforced metal matrix composite (MMC) wires. The process currently yields MMC wires with the diameter of 0.5–1.6 mm at a maximum speed of 0.3 m/s for (fiber)–(metal) combinations of (Al₂O₃, Si–Ti Carbide, SiC, Carbon)–(Aluminum, 2024A1, 6061A1) with a fiber volume fraction of 0.50–0.65. The process operation, mechanical/materials characteristics of the MMC wires as well as their relationships are described as an overview of the new materials processing technology.     

Woven fabric composites—part I: Predictions of homogenized elastic properties and micromechanical damage analysis

In Part 1, the finite element developments are described to first predict the homogenized elastic properties of woven fabric composites, followed by a progressive damage analysis for the subsequent investigation of micromechanical damage initiations and propagations in the representative unit cell of woven fabric composites. For progressive damage analysis, in‐plane uniaxial tensile and shear loading are applied, respectively, to the unit cell. In order to make the boundaries of the unit cell remain straight, displacement control loading is applied instead of force control loading. The tensile and shear stress–strain curves are obtained and shown to be in good agreement with available experimental results. Utilizing the computational efforts in Part 1, the characterization of macro‐crack initiation loads is carried out in Part 2 for the global damage analysis without the need to resorting to experimental data for the prediction of damaged properties. Copyright © 2001 John Wiley & Sons, Ltd.     

Woven fabric composites—part II: Characterization of macro‐crack initiation loads for global damage analysis

In Part 2, finite element techniques are developed to additionally investigate the global damage of woven fabric composites, focusing on plain weaves. The homogenized elastic properties of the original undamaged and the damaged woven fabric composites from the micromechanical homogenization and the micromechanical progressive damage analysis investigated earlier in Part 1 are subsequently employed in the present global damage analysis. The theory of continuum damage mechanics is utilized in the global damage analysis. The damage variables, the most important material properties of the continuum damage mechanics, are measures of average material degradation at a macro‐mechanics scale. In the present study, these damage variables are calculated numerically using the results from the micromechanical damage analysis in Part 1, instead of being obtained experimentally. Subsequently, a finite element formulation for the global damage analysis of woven fabric composites is developed to predict the initiation loads of macro‐cracks. Copyright © 2001 John Wiley & Sons, Ltd.     

FMEA—A diagraph and matrix approach

This paper presents a method for failure mode and effects analysis (FMEA) of mechanical and hydraulic systems based on a diagraph and matrix approach. The method takes into account structural as well as functional interaction of the system. This is desirable as failures in these systems are not independent. A failure mode and effects diagraph, derived from the structure of the system, models the effects of failure modes of the system and consists of nodes, subnodes and edges. For efficient computer processing, matrices are defined to represent the diagraph. A function (VCM-F_me or VPF-F_me) characteristic of the system failure mode and effects is obtained from the matrix and this aids in the detailed analysis leading to the identification of various structural components of failure mode and effects. In addition, the number of tests for failure mode and effects are derived. An index I_fme, a measure of failure mode and effects of the system, is obtained using VPF-F_me. The methodology is applicable not only at the design stage during the operation stage also.     

The effect of heat treatment on mechanical properties of carbon nanofiber reinforced copper matrix composites

The effect of heat treatment of carbon nanofibers (CNFs) on the mechanical properties of CNF (Ni/Y)–Cu composites was investigated. CNF (Ni/Y)–Cu composite powder mixtures were prepared by a combination of in situ chemical vapor deposition (CVD) and co-deposition processes. The in situ CNF (Ni/Y)–Cu powder synthesized by CVD was subject to heat treatment at temperatures ranging from 700 to 1,000 °C. The morphology and quality of CNFs were characterized by transmission electron microscope, scanning electron microscope, and Raman spectroscopy. Heat treatment can improve the CNFs by eliminating the amorphous carbon and disordered graphite. Bulk composites containing various fractions of CNFs were fabricated from the powder by cold pressing and sintering followed by repressing. With the same fraction of CNFs (2.5 wt%), the strengthening efficiency of the CNFs heat treated at 800 °C is 88% higher than that of as-synthesized CNFs. The strengthening mechanism of CNFs in the composites is discussed in detail.     

The effect of matrix properties on reinforcement in short alumina fibre-aluminium metal matrix composites

The effect of the alloy matrix on room-temperature strengthening in -alumina-reinforced aluminium alloys has been investigated. Alloy matrices fell into two families exhibiting significantly different fibre-strengthening response. The first gave rise to little or no improvement in the room-temperature strength, while the second gave significant improvements by up to 300%. It is shown that a simple Rule of Mixtures (ROM) strength analysis, modified to account for the discontinuous and random orientation of the reinforcement, can adequately explain these responses. Little or no reinforcement occurs when the matrix properties result in a high value for the critical volume fraction VCRIT which must be exceeded to produce any increase in strength. However, by careful selection of the matrix alloyV _CRIT can be reduced, thus giving significant reinforcement of the room-temperature strength. This analysis shows that for optimum room-temperature reinforcement the matrix alloys should exhibit a low rate of work-hardening. In certain alloys reinforcement levels were in excess of those predicted by the ROM analysis. It is proposed that this occurs in relatively low-strength matrices as a result of dispersion strengthening of the matrix due to the presence of the fibre array.     

Improvement of plant based natural fibers for toughening green composites—Effect of load application during mercerization of ramie fibers

Effect of mercerization to tensile properties of a ramie fiber was explored. Load application technique during mercerization has been employed in order to improve mechanical properties of the fiber. A chemical treatment apparatus with tensile loading portion for applying monofilaments was newly developed. The ramie fiber was alkali-treated by 15% NaOH solution with applied loads of 0.049 and 0.098 N. The results showed that tensile strength of the treated ramie fiber was improved, 4–18% higher than that of the untreated ramie fiber, while Young’s modulus of the treated fibers decreased. It should be noted that fracture strains of the treated ramie fiber drastically increased to 0.045–0.072, that is, twice to three times higher than those of the untreated ramie fiber. It was considered that such property improvements upon mercerization were correlated with change of morphological and chemical structures in microfibrils of the fiber. Finally, the plastic deformation behavior and fracture mechanism of the mercerized fibers under tensile loading process was explained using a schematic model.