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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.4, #5)

Effect of BN Coating on the Strength of a Mullite Type Fiber by K. K. Chawla; Z. R. Xu; J.-S. Ha; M. Schmücker; H. Schneider (pp. 263-272).

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al₂O₃₊28 wt.-% SiO₂+ 2 wt.-% B₂O₃). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the thin coatingup to 0.3 µm) while in the case of thick BN coating (1 µm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.

Keywords: fiber; mullite; Boron Nitride; Nextel 480; Weibull

Effect of BN coating on the strength of a mullite type fiber by K. K. Chawla; Z. R. Xu; J. -S. Ha; M. Schmücker; H. Schneider (pp. 263-272).

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al₂O₃+28 wt.-% SiO₂+2 wt.-% B₂O₃). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.

Keywords: fiber; mullite; Boron Nitride; Nextel 480; Weibull

Folding of continuous fibre thermoplastic composites by R. Marissen; L. Th. Van Der Drift; J. C. Sterk (pp. 273-282).

Thermoplastic composites with continuous fibres are attractive construction materials because of their good specific mechanical properties and their possibility to be processed very rapidly. An extremely fast processing method is folding of sheet material. Folding can be performed by local heating of a sheet along a line. Heating is done to a temperature, well above the softening or melting point of the thermoplastic polymer. Subsequent folding along the heated line requires very low forces. The folded geometry becomes permanent after cooling below the softening point of the polymer. Unfortunately, folding causes microbuckling of the reinforcing fibres at the compression side of the fold causing a severe reduction in the local strength. The present report describes a method for the folding of thermoplastic composites, which controls the direction of fibre microbuckling. The method results in a smaller strength reduction.The folding equipment is designed in such a way that microbuckling of the fibres occurs in a direction in the local plane of the sheet rather than being perpendicular to that plane. The result is a folded sheet with a reduction of about half of the original strength, as compared to a strength of only 15% of the original strength, which is typical for folds produced with more conventional means. In other words, the remaining strength is more then doubled using the new folding technology.

Keywords: fibre buckling; buckling control; bending strength; tilted moulds; processing; heating; melting; cooling; glass fibres; woven fabric; polyethyleneterephtalate

Folding of Continuous Fibre Thermoplastic Composites by R. Marissen; L. Th. van der Drift; J. C. Sterk (pp. 273-282).

Thermoplastic composites with continuous fibres are attractive construction materials because of their good specific mechanical properties and their possibility to be processed very rapidly. An extremely fast processing method is folding of sheet material. Folding can be performed by local heating of a sheet along a line. Heating is done to a temperature, well above the softening or melting point of the thermoplastic polymer. Subsequent folding along the heated line requires very low forces. The folded geometry becomes permanent after cooling below the softening point of the polymer. Unfortunately, folding causes microbuckling of the reinforcing fibres at the compression side of the fold causing a severe reduction in the local strength. The present report describes a method for the folding of thermoplastic composites, which controls the direction of fibre micro-buckling. The method results in a smaller strength reduction.The folding equipment is designed in such a way that microbuckling of the fibres occurs in a direction in the local plane of the sheet rather than being perpendicular to that plane. The result is a folded sheet with a reduction of about half of the original strength, as compared to a strength of only 15 the original strength, which is typical for folds produced with more conventional means. In other words, the remaining strength is more then doubled using the new folding technology.

Keywords: fibre buckling; buckling control; bending strength; tilted moulds; processing; heating; melting; cooling; glass fibres; woven fabric; polyethyleneterephtalate

Thermoelastic Stress Analysis of a GRP Tee Joint by J. M. Dulieu-Smith; S. Quinn; R. A. Shenoi; P. J. C. L. Read; S. S. J. Moy (pp. 283-303).

A detailed study of the stresses that are developed in a glass reinforced plastic (GRP) tee joint under service loads is described. The joints are fabricated by laminating a boundary angle over a radiused fillet on either side of the ‘tee’. Full-field stress characterisation data is provided by a thermoelastic analysis of the tee joint. Calibration procedures that allow the thermoelastic data to be compared with the results of a finite element analysis are detailed. The results of the thermoelastic analysis are compared with values obtained from the finite element analysis. The applicability of thermoelastic analysis as a validation tool for finite element models of composite materials is assessed.

Keywords: thermoelastic stress analysis; calibration; tee joints; marine structures; glass fibre reinforced plastics; SPATE; non-destructive testing; finite element modelling

Thermoelastic stress analysis of a GRP tee joint by J. M. Dulieu-Smith; S. Quinn; R. A. Shenoi; P. J. C. L. Read; S. S. J. Moy (pp. 283-303).

Keywords: thermoelastic stress analysis; calibration; tee joints; marine structures; glass fibre reinforced plastics; SPATE; non-destructive testing; finite element modelling

Corrosion Behavior of High Energy–High Rate Consolidated Graphite/Copper Metal Matrix Composites in Chloride Media by J. E. Orth; H. G. Wheat (pp. 305-320).

The corrosion behavior of particulate reinforced graphite/copper (Gr_p/Cu) metal matrix composites (MMCs) was studied in 3.5 wt.% solution using electrochemical techniques, ionic solution analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques. The materials under investigation were high energy–high rate (HEHR) consolidated Gr_p/Cu metal matrix composites. HEHR processing employs a 10 MJ homopolar generator that supplies a 100 kA pulse to rapidly heat and solidify the composite powder compact. This short time at high temperature and the preferential heating and melting at the graphite-copper interface serve to encapsulate the graphite reinforcement, thus providing a highly densified composite product.Initially the open circuit potential corrosion behavior of 1.2, 5, 15, 25, and 40 volume percent Gr_{_p}Cu composites was studied in aerated and deaerated 3.5 wt.% NaCl solution using SEM and EDX. Subsequently, the environmental stability of these composites was studied using electrochemical techniques such as polarization resistance and potentiodynamic polarization. The severity of corrosive attack increased with increasing graphite content and in aerated solutions. In addition, solutions from these tests were analyzed to determine the relative amounts of copper and carbon present in the electrolyte after polarization tests. Microscopic analysis techniques were used to characterize the corrosion morphologies and the extensive localized corrosion occurring at the graphite-copper interface. The effectiveness of benzotriazole as a corrosion inhibitor for the copper MMCs was also studied.

Keywords: metal matrix composites; copper; uniform corrosion; galvanic corrosion

Corrosion behavior of high energy-high rate consolidated graphite/copper metal matrix composites in chloride media by J. E. Orth; H. G. Wheat (pp. 305-320).

The corrosion behavior of particulate reinforced graphite/copper (Gr_p/Cu) metal matrix composites (MMCs) was studied in 3.5 wt.% sodium chloride solution using electrochemical techniques, ionic solution analysis, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques. The materials under investigation were high energy-high rate (HEHR) consolidated Gr_p/Cu metal matrix composites. HEHR processing employs a 10 MJ homopolar generator that supplies a 100 kA pulse to rapidly heat and solidify the composite powder compact. This short time at high temperature and the preferential heating and melting at the graphite-copper interface serve to encapsulate the graphite reinforcement, thus providing a highly densified composite product.Initially the open circuit potential corrosion behavior of 1.2, 5, 15, 25, and 40 volume percent Gr_pCu composites was studied in aerated and deaerated 3.5 wt.% NaCl solution using SEM and EDX. Subsequently, the environmental stability of these composites was studied using electrochemical techniques such as polarization resistance and potentiodynamic polarization. The severity of corrosive attack increased with increasing graphite content and in aerated solutions. In addition, solutions from these tests were analyzed to determine the relative amounts of copper and carbon present in the electrolyte after polarization tests. Microscopic analysis techniques were used to characterize the corrosion morphologies and the extensive localized corrosion occurring at the graphite-copper interface. The effectiveness of benzotriazole as a corrosion inhibitor for the copper MMCs was also studied.

Keywords: metal matrix composites; copper; uniform corrosion; galvanic corrosion

Physically based failure criterion for dimensioning of thick-walled laminates by Lothar Kroll; Werner Hufenbach (pp. 321-332).

The extreme lightweight potential of modern composites for the application in highly strained and thick walled components can only be sucessfully utilized with the help of adapted design procedures. Therefore, the stress and strain analysis of fibre reinforced components has experienced a tremendous improvement in recent years. The derived mechanical methods and the existing computing facilities are now capable of calculating complex and three-dimensional states of stress for single layers within laminated structures. The adequate development of appropriate failure criteria for the evaluation of such stress states has unfortunately not been promoted in the desired manner. In 1980, Hashin proposed a new generation of physically based failure analysis which could only be realized by a considerably increased numerical effort. Recently, Puck made a new attempt based on Hashin's concept using fundamental elements of the failure criterion by Mohr and Coulomb. Applying this model, the three-dimensional state of stress is evaluated in a realistic manner. It is assumed that besides the occurence of fibre failure only tensile stresses and shear stresses in loading planes tangential to the fibre direction induce the inter-fibre failure of the unidirectionally reinforced composite, whereas compressive stresses in these planes obstruct failure.

Keywords: failure analysis; 3D-failure criterion; laminate design; inter-fibre fracture; strength analysis; fracture plane; thick laminates; 3D-stress analysis

Physically Based Failure Criterion for Dimensioning of Thick-Walled Laminates by Lothar Kroll; Werner Hufenbach (pp. 321-332).

The extreme lightweight potential of modern composites for the application in highly strained and thick walled components can only be sucessfully utilized with the help of adapted design procedures. Therefore, the stress and strain analysis of fibre reinforced components has experienced a tremendous improvement in recent years. The derived mechanical methods and the existing computing facilities are now capable of calculating complex and three-dimensional states of stress for single layers within laminated structures. The adequate development of appropriate failure criteria for the evaluation of such stress states has unfortunately not been promoted in the desired manner. In 1980, Hashin proposed a new generation of physically based failure analysis which could only be realized by a considerably increased numerical effort. Recently, Puck made a new attempt based on Hashin's concept using fundamental elements of the failure criterion by Mohr and Coulomb. Applying this model, th e three-dimensional state of stress is evaluated in a realistic manner. It is assumed that besides the occurence of fibre failure only tensile stresses and shear stresses in loading planes tangential to the fibre direction induce the inter-fibre failure of the unidirectionally reinforced composite, whereas compressive stresses in these planes obstruct failure.

Keywords: failure analysis; 3D-failure criterion; laminate design; inter-fibre fracture; strength analysis; fracture plane; thick laminates; 3D-stress analysis

Deformation Behaviour and Microstructure of a 20% Al₂O₃ Reinforced 6061 Al Composite by Xiaoxin Xia; H. J. McQueen (pp. 333-347).

Deformation and microstructural behaviours of a 20% (volumepercent) particle reinforced 6061 Al matrix composite have been studied bytorsion from 25 to 540°C with strain rates of 0.1, 1 and5 s^-1. The logarithmic stress versus reciprocal temperaturerelationship exhibits two slopes indicating different deformationmechanisms. The 20% Al₂O₃/6061 Alcomposite shows a greater hardening behaviour than those of the 10% Al₂O₃/6061 Al composite and of the monolithic alloy. Above 250°C, TEM investigations reveal muchsmaller subgrain size and higher volume of non-cellular substructures, aswell as dynamic recrystallization nuclei in the 20% Al₂O₃/6061 Al composite in comparison to those of the10% Al₂O₃/6061 Al composite and matrixalloy the same test condition. The torsion fracture surface was studied andcompared to the three point bending failure specimens.

Keywords: softening; dynamic recovery; dynamic recrystallization; microstructure; peak strain; subgrain; Al₂O₃ reinforcement; Al matrix; torsion

Off-axis Fatigue Crack Growth and the Associated Energy Release Rate in Composite Laminates by J. Tong; F. J. Guild; S. L. Ogin; P. A. Smith (pp. 349-359).

Stable matrix crack growth behaviour under mechanical fatigue loading hasbeen studied in a quasi-isotropic (0/90/-45/+45)_s GFRPlaminate. Detailed experimental observations were made on the accumulationof cracks and on the growth of individual cracks in +45° as well as 90° plies. A generalised plain strain finiteelement model of the damaged laminate has been constructed. This model hasbeen used to relate the energy release rate of growing cracks to the crackgrowth rate via a Paris relation.

Keywords: quasi-isotropic laminates; off-axis fatigue crack growth; energy release rate; finite element analysis

Deformation and Fracture Properties of Damage Tolerant In-situ Titanium Matrix Composites by S. Dubey; W. O. Soboyejo; T. S. Srivatsan (pp. 361-374).

This paper discusses the tensile response and fracture toughness ofin-situ titanium alloy metal matrices discontinuously-reinforced withwhiskers of titanium boride which were successfully produced by ingotmetallurgy techniques. Additions of elemental boron resulted in a nearuniform dispersion of the rod-like titanium boride (TiB) reinforcements inthe alloy matrix. Such composites have engendered considerable scientificand technological interest due to their attractive combinations of improvedmechanical properties and low manufacturing cost. The improved elasticmoduli of the composites are explained using shear lag and rule-of-mixturestheories. The increased strengths of the in-situ composites are rationalizedby considering the combined effects of deformation restraints imposed by thestiff whiskers and strengthening contributions arising from the substructurethat evolves from the presence of additional dislocations.

In Situ Monitoring of Thermally Cycled Metal Matrix Composites by Neutron Diffraction and Laser Extensometry by Mark R. Daymond; Philip J. Withers (pp. 375-393).

A novel stroboscopic neutron diffraction data collection system has beendeveloped. In combination with scanning laser extensometry this has beenused to investigate the thermal cycling behaviour of SiC short fibrereinforced Al matrix composites. Three-dimensional unit cell finite elementmodels have been produced, incorporating matrix deformation both by creepand plasticity. Comparison of the experimental results with modelpredictions has allowed conclusions to be drawn about the deformationprocesses which dominate at different parts of the thermal cycle.

Keywords: neutron diffraction; thermal cycling; internal stress; thermal stress; relaxation; extensometry; stroboscopic; metal matrix composite; residual stress

Author Index (pp. 399-399).

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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.4, #5)

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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.4, #5)