Applied Composite Materials (v.21, #1)

A Celebration of the Work of Professor Tony Kelly ScD FRS FREng PhD CBE DL by P. W. R Beaumont; Costas Soutis; Alma Hodzic (1-3).

Predicting precisely where a crack will develop in a material under stress and exactly when in time catastrophic fracture of the component will occur is one the oldest unsolved mysteries in the design and building of large engineering structures. Where human life depends upon engineering ingenuity, the burden of testing to prove a “fracture safe design” is immense. For example, when human life depends upon structural integrity as an essential design requirement, it takes ten thousand material test coupons per composite laminate configuration to evaluate an airframe plus loading to ultimate failure tails, wing boxes, and fuselages to achieve a commercial aircraft airworthiness certification. Fitness considerations for long-life implementation of aerospace composites include understanding phenomena such as impact, fatigue, creep, and stress corrosion cracking that affect reliability, life expectancy, and durability of structure. Structural integrity analysis treats the design, the materials used, and figures out how best components and parts can be joined. Furthermore, SI takes into account service duty. However, there are conflicting aims in the complete design process of designing simultaneously for high efficiency and safety assurance throughout an economically viable lifetime with an acceptable level of risk.
Keywords: Failure mechanisms; Damage mechanics; Multi-scale modelling; Physical modelling; Numerical modelling; Computer simulation; Structural integrity; Life prediction

Atomic Models of Strong Solids Interfaces Viewed as Composite Structures by I. Staffell; J. L. Shang; K. Kendall (45-55).
This paper looks back through the 1960s to the invention of carbon fibres and the theories of Strong Solids. In particular it focuses on the fracture mechanics paradox of strong composites containing weak interfaces. From Griffith theory, it is clear that three parameters must be considered in producing a high strength composite:- minimising defects; maximising the elastic modulus; and raising the fracture energy along the crack path. The interface then introduces two further factors:- elastic modulus mismatch causing crack stopping; and debonding along a brittle interface due to low interface fracture energy. Consequently, an understanding of the fracture energy of a composite interface is needed. Using an interface model based on atomic interaction forces, it is shown that a single layer of contaminant atoms between the matrix and the reinforcement can reduce the interface fracture energy by an order of magnitude, giving a large delamination effect. The paper also looks to a future in which cars will be made largely from composite materials. Radical improvements in automobile design are necessary because the number of cars worldwide is predicted to double. This paper predicts gains in fuel economy by suggesting a new theory of automobile fuel consumption using an adaptation of Coulomb’s friction law. It is demonstrated both by experiment and by theoretical argument that the energy dissipated in standard vehicle tests depends only on weight. Consequently, moving from metal to fibre construction can give a factor 2 improved fuel economy performance, roughly the same as moving from a petrol combustion drive to hydrogen fuel cell propulsion. Using both options together can give a factor 4 improvement, as demonstrated by testing a composite car using the ECE15 protocol.
Keywords: Interfaces; Fracture energy; Computer model; Coulomb’s Law; Dissipative functions; Composite vehicles

In this paper, the transverse matrix (resin) cracking developed in multidirectional composite laminates loaded in tension was numerically investigated by a finite element (FE) model implemented in the commercially available software Abaqus/Explicit 6.10. A theoretical solution using the equivalent constraint model (ECM) of the damaged laminate developed by Soutis et al. was employed to describe matrix cracking evolution and compared to the proposed numerical approach. In the numerical model, interface cohesive elements were inserted between neighbouring finite elements that run parallel to fibre orientation in each lamina to simulate matrix cracking with the assumption of equally spaced cracks (based on experimental measurements and observations). The stress based traction-separation law was introduced to simulate initiation of matrix cracking and propagation under mixed-mode loading. The numerically predicted crack density was found to depend on the mesh size of the model and the material fracture parameters defined for the cohesive elements. Numerical predictions of matrix crack density as a function of applied stress are in a good agreement to experimentally measured and theoretically (ECM) obtained values, but some further refinement will be required in near future work.
Keywords: Composite laminates; Finite element analysis; Cohesive elements; Crack density; Equivalent constraint model; Damage; Matrix cracking

Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre de-bonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance in terms of the global properties and design methods for such materials. This paper presents a numerical modelling based analysis of the effect of material randomness on delamination damage in CFRP materials by the implementation of a stochastic cohesive-zone model (CZM) within the framework of the finite-element (FE) method. The initiation and propagation of delamination in a unidirectional CFRP double-cantilever beam (DCB) specimen loaded under mode-I was analyzed, accounting for the inherent microstructural stochasticity exhibited by such laminates via the stochastic CZM. Various statistical realizations for a half-scatter of 50 % of fracture energy were performed, with a probability distribution based on Weibull’s two-parameter probability density function. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. This indicates that deterministic analysis of composites using average properties may be non-conservative and a method based on probability may be more appropriate.
Keywords: CFRP laminates; Delamination; Stochasticity; Cohesive zone elements; Microstructural randomness; Weibull’s distribution

Damage Assessment of Composite Structures Using Digital Image Correlation by M. A. Caminero; M. Lopez-Pedrosa; C. Pinna; C. Soutis (91-106).
The steady increase of Carbon-Fiber Reinforced Polymer (CFRP) Structures in modern aircraft will reach a new dimension with the entry into service of the Boeing 787 and Airbus 350. Replacement of damaged parts will not be a preferable solution due to the high level of integration and the large size of the components involved. Consequently the need to develop repair techniques and processes for composite components is readily apparent. Bonded patch repair technologies provide an alternative to mechanically fastened repairs with significantly higher performance, especially for relatively thin skins. Carefully designed adhesively bonded patches can lead to cost effective and highly efficient repairs in comparison with conventional riveted patch repairs that cut fibers and introduce highly strained regions. In this work, the assessment of the damage process taking place in notched (open-hole) specimens under uniaxial tensile loading was studied. Two-dimensional (2D) and three-dimensional (3D) Digital Image Correlation (DIC) techniques were employed to obtain full-field surface strain measurements in carbon-fiber/epoxy T700/M21 composite plates with different stacking sequences in the presence of an open circular hole. Penetrant enhanced X-ray radiographs were taken to identify damage location and extent after loading around the hole. DIC strain fields were compared to finite element predictions. In addition, DIC techniques were used to characterise damage and performance of adhesively bonded patch repairs in composite panels under tensile loading. This part of work relates to strength/stiffness restoration of damaged composite aircraft that becomes more important as composites are used more extensively in the construction of modern jet airliners. The behaviour of bonded patches under loading was monitored using DIC full-field strain measurements. Location and extent of damage identified by X-ray radiography correlates well with DIC strain results giving confidence to the technique for structural health monitoring of bonded patches.
Keywords: Composites; Stress concentration; Open hole tension; Bonded patch repair; Damage detection; Digital image correlation

The determination of intrinsic safety factors for glass and carbon fibre unidirectional composites and filament wound internally pressurised structures, is described. In such structures the fibres are placed on geodesic paths and the pressure induces tensile forces in them. The fibres ensure the strength of the composite and must break for it to fail. Failure is seen in such structures, to depend mainly on the accumulation of fibre breaks. These are initially randomly distributed but become critical when clusters of breaks develop. Long term behaviour of carbon fibre composites is controlled by the viscoelastic relaxation of the matrix around breaks, which can lead to further delayed fibre breaks. Failure in glass fibre structures can additionally be induced by stress corrosion of the glass fibres. This process does not seem to occur with carbon fibres and as the latter are increasingly used in critical structures emphasis is given to them. Until the development of clusters of fibre breaks, in a filament wound structure, no macroscopic changes in the composite behaviour are evident so that failure occurs in a sudden death manner. Multi-scale simulation, taking into account the characteristics of the composite components and scaling up their behaviour under load, accurately describes the overall behaviour of the composite structure. This approach not only allows the behaviour to be described, as a function of time, but also calculates the scatter which will occur in the behaviour of the structure. This allows the intrinsic safety factors of the composite structure to be quantified.
Keywords: Reliability; Modelling; Fibre failure; Intrinsic limiting properties

Damage Tolerance of Pre-Stressed Composite Panels Under Impact Loads by Alastair F. Johnson; Nathalie Toso-Pentecôte; Dominik Schueler (123-147).
An experimental test campaign studied the structural integrity of carbon fibre/epoxy panels preloaded in tension or compression then subjected to gas gun impact tests causing significant damage. The test programme used representative composite aircraft fuselage panels composed of aerospace carbon fibre toughened epoxy prepreg laminates. Preload levels in tension were representative of design limit loads for fuselage panels of this size, and maximum compression preloads were in the post-buckle region. Two main impact scenarios were considered: notch damage from a 12 mm steel cube projectile, at velocities in the range 93–136 m/s; blunt impact damage from 25 mm diameter glass balls, at velocities 64–86 m/s. The combined influence of preload and impact damage on panel residual strengths was measured and results analysed in the context of damage tolerance requirements for composite aircraft panels. The tests showed structural integrity well above design limit loads for composite panels preloaded in tension and compression with visible notch impact damage from hard body impact tests. However, blunt impact tests on buckled compression loaded panels caused large delamination damage regions which lowered plate bending stiffness and reduced significantly compression strengths in buckling.
Keywords: Carbon fibre/epoxy laminates; High velocity impact tests; Impact damage in pre-stressed plates; Damage tolerance of composites

Modeling Lightning Impact Thermo-Mechanical Damage on Composite Materials by Raúl Muñoz; Sofía Delgado; Carlos González; Bernardo López-Romano; De-Yi Wang; Javier LLorca (149-164).
Carbon fiber-reinforced polymers, used in primary structures for aircraft due to an excellent strength-to-weight ratio when compared with conventional aluminium alloy counterparts, may nowadays be considered as mature structural materials. Their use has been extended in recent decades, with several aircraft manufacturers delivering fuselages entirely manufactured with carbon composites and using advanced processing technologies. However, one of the main drawbacks of using such composites entails their poor electrical conductivity when compared with aluminium alloy competitors that leads to lightning strikes being considered a significant threat during the service life of the aircraft. Traditionally, this problem was overcome with the use of a protective copper/bronze mesh that added additional weight and reduced the effectiveness of use of the material. Moreover, this traditional sizing method is based on vast experimental campaigns carried out by subjecting composite panels to simulated lightning strike events. While this method has proven its validity, and is necessary for certification of the structure, it may be optimized with the aid provided by physically based numerical models. This paper presents a model based on the finite element method that includes the sources of damage observed in a lightning strike, such as thermal damage caused by Joule overheating and electromagnetic/acoustic pressures induced by the arc around the attachment points. The results of the model are compared with lightning strike experiments carried out in a carbon woven composite.
Keywords: Carbon fiber laminates; Lightning impact; Damage modeling; Finite element method

Accidental impact loading of Composite laminates during manufacture and in-service can occur in different locations including near the edge or on the edge of a composite structure. This paper describes investigation of the effect of impact to composite laminates and compares the damage arising from central, near edge and on edge impact events. The damage tolerance of impact damaged laminates using both compression and tension tests has been measured. These results reveal the different damage mechanisms arising from different locations of impact. These different damage mechanisms have been investigated using X-Ray computed tomography. Impact on the edge of composite laminates is found to lead to smaller damage area, but more fibre failure; the severity of this damage is not revealed in standard compression after impact tests.
Keywords: Composite laminates; Edge impact; Damage tolerance; X-Ray computed tomography

In this present work, we investigate damping behavior of filled and layered composite material that has its inclusions coated by viscoelastic coating material. To analyze its behavior, we use generalized self-consistent Eshelby method with correspondence principle approach. The viscous coating layer is assumed to possess properties at its glass transition temperature. This analytical study reveals that at ultra thin coating layer, the composite exhibits very high loss characteristics where its effective loss moduli significantly exceed the loss moduli of both coating and matrix materials. High shearing dissipation mechanism in ultra thin layer of viscoelastic coating material is found to be responsible for this peculiar behavior. This remarkable loss amplification effect is technologically appealing as such composites with high damping and high stiffness properties might be attainable.
Keywords: Filled and layered composites; Inclusions; Viscoelastic coating; Self-consistent Eshelby model; Effective loss modulus; Dissipation mechanism; Optimal design

Micro-Mechanical Parameters in Short Fibre Composite by Ioannou Ioannis; Hodzic Alma; Gitman Inna; Soutis Costas; M. A. Almaadeed (197-211).
The aim of this paper is to analyse the contribution of micro-mechanical parameters, on the macroscopic behaviour of a short fibre reinforced thermoplastic composites (SFRTC). By developing an algorithm to provide a representative random micro-structure, a comparative analysis of different micro-mechanical parameters, such as aspect ratio (AR) and fibre orientation (FO), was conducted and compared with the existing analytical models. A study of different aspect ratios and different fibre orientations has been carried out in order to examine their effect on the linear elastic properties of SFRTC. Aspect ratios from one to ten have been analysed for the cases of fully oriented 0° fibres, miss-oriented fibres and randomly oriented fibres. A representative volume element (RVE) was used to investigate the effect of the representative size. Results were analysed statistically through X 2 test, and the subsequent representative realisations were compared with the theoretical predictions.
Keywords: Representative volume element; Homogenisation; Randomness

Creep behavior in interlaminar shear of a non-oxide ceramic composite with a multilayered matrix was investigated at 1,200 °C in laboratory air and in steam environment. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated Hi-Nicalon™ fibers woven in a five-harness-satin weave. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. The interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 16–22 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air and in steam. In air and in steam, creep run-out defined as 100 h at creep stress was achieved at 16 MPa. Larger creep strains were accumulated in steam. However, creep strain rates and creep lifetimes were only moderately affected by the presence of steam. The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.
Keywords: Ceramic-matrix composites (CMCs); Creep; High-temperature properties; Mechanical properties; Fractography

Composite ship hull structures are in contact with water on the external surfaces and may support vibrating equipment on the internal surfaces. This study examined how the Fluid-structure Interaction (FSI) coupled with the harmonic excitation could affect the composite structural response. A multiphysics computational technique based on the Finite Element Method (FEM) and Cellular Automata (CA) was developed and applied for this research. A comparison was made on the structural responses with and without FSI for composite beams and plates. Furthermore, metallic structures made of either aluminum or steel were also compared to composite structures to investigate the effect of the coupled FSI and a harmonically vibrating machine. The study showed that FSI could magnify the displacement and stress level in composite structures supporting harmonically vibrating equipment. Analysis of composite structures supporting such equipment without considering the FSI effect would result in non-conservative design leading to pre-mature failure. A parametric study was conducted to determine what parameter could signify the FSI effect on composite structures.
Keywords: Fluid-structure interaction; Vibrating equipment; Multiphysics analysis

Analysis of Bolted Flanged Panel Joint for GRP Sectional Tanks by S. M. Radhakrishnan; B. Dyer; M. Kashtalyan; A. R. Akisanya; I. Guz; C. Wilkinson (247-261).
The performance of flanged panel bolted joints used in Glass Reinforced Plastic (GRP) sectional tanks is investigated using a combination of experimental and computational methods. A four-panel bolted assembly is subjected to varying pressure in a rupture test rig to study damage development at the intersection of the four panels. It is found that cracking initiates at a panel corner at the four panel intersection at a pressure of 35 kPa and propagates to other panel corners with increasing pressure. This is attributed to the excessive deformation at the four panel intersection. The effect of bolt spacing, varying end distances and bolt pre-tension in decreasing the localized deformation and maximum induced stresses are investigated using finite element analysis. It is found that varying the amount of bolt spacing and end distances had a considerable influence on the joint performance whereas varying bolt pretension had very negligible effect. Consequently, this study establishes the maximum pressure which the GRP panel joint can withstand without failure and the corresponding optimum joint parameters.
Keywords: Bolted flanged panel joints; Sheet moulding compounds; Joint parameters; Experimental testing; Finite element modelling

Recycling of Reinforced Plastics by R. D. Adams; Andrew Collins; Duncan Cooper; Mark Wingfield-Digby; Archibald Watts-Farmer; Anna Laurence; Kayur Patel; Mark Stevens; Rhodri Watkins (263-284).
This work has shown is that it is possible to recycle continuous and short fibre reinforced thermosetting resins while keeping almost the whole of the original material, both fibres and matrix, within the recyclate. By splitting, crushing hot or cold, and hot forming, it is possible to create a recyclable material, which we designate a Remat, which can then be used to remanufacture other shapes, examples of plates and tubes being demonstrated. Not only can remanufacturing be done, but it has been shown that over 50 % of the original mechanical properties, such as the E modulus, tensile strength, and interlaminar shear strength, can be retained. Four different forms of composite were investigated, a random mat Glass Fibre Reinforced Plastic (GFRP) bathroom component and boat hull, woven glass and carbon fibre cloth impregnated with an epoxy resin, and unidirectional carbon fibre pre-preg. One of the main factors found to affect composite recyclability was the type of resin matrix used in the composite. Thermoset resins tested were shown to have a temperature range around the Glass Transition Temperature (Tg) where they exhibit ductile behaviour, hence aiding reforming of the material. The high-grade carbon fibre prepreg was found to be less easy to recycle than the woven of random fibre laminates. One method of remanufacturing was by heating the Remat to above its glass transition temperature, bending it to shape, and then cooling it. However, unless precautions are taken, the geometric form may revert. This does not happen with the crushed material.
Keywords: Recycling; Remanufacturing; Composite materials; GFRP; CFRP