Interfacial debonding in laminated titanium matrix composites

The results of an experimental program in which multiaxial loads were applied to [0₄] and [±45]_s silicon carbide/titanium (SiC/Ti) tubes are reviewed showing that stress coupling, matrix viscoplasticity (including room temperature creep) and fiber/matrix interfacial damage all contribute to nonlinear response and permanent strains in titanium matrix composites (TMC). A micromechanical model that explicitly considers the aforementioned phenomena is presented herein. The model assumes a periodic microstructure and uses finite elements to analyze a representative volume element. The composite is assumed to be in a state of generalized plane strain making it possible to discretize only a generic transverse plane while still being able to apply three-dimensional loading through appropriate boundary conditions. The response of laminated composites is predicted by incorporating the micromechanical results into nonlinear lamination theory. Predictions are presented to show the influence of the model parameters on the effective composite response of unidirectional [0₄] and angle-ply [±45]_s TMC laminates.     

Impact properties of laminated angle ply composites

The impact properties of laminated composites have been studied as a function of fiber orientation angle, lamination configuration and specimen geometry. The energy absorbing mechanisms havebeen identified. The impact properties of laminated composites are influenced significantly by the fiber orientation angle, lamination configuration and specimen geometry. All off-axis composites (0° < θ < 90°) fail by brittle inter-fiber cleavage mode with little or no interlayer delamination. The longitudinal composites (θ = 0°), both unidirectional and crossplied, fail by a combination of failure modes which take place in a sequential manner—fiber failure and interfacial splitting followed by a layer-to-layer delamination. The presence of 0° layer(s) in transverse composite (θ ±90°) improves their impact performance.     

Dynamic pulse-buckling behavior of ‘quasi-ductile'' carbon/epoxy and E-glass/epoxy laminated composite beams

Dynamic pulse-buckling response of carbon/epoxy and E-glass/epoxy laminated composite beams with [(±67.5)_n]_s ply sequence, subject to axial impact was investigated experimentally and numerically. The laminated beams deformed like ductile metals, retaining a residual deformed shape after being axially impacted, exhibiting no obvious delamination. The ‘crest' deflection of the beams was found to be linearly proportional to the impact energy. The numerical investigation showed that the beams' top and bottom surfaces experienced stresses (transverse stress component) in excess of the tensile strength limits of the matrices.     

Magneto-elastic interchange in ferromagnetic alloys

The internal friction δ, exchange integral A, magnetocrystalline anisotropic constant K_I and saturation magnetization M_s of Fe–Cr–Al and Fe–Cr–Al–Si alloys annealed at 1373 and 1473 K are measured. The energy density and volume fraction of domain walls (DWs) of these alloys are calculated based on the theories of ferromagnetism and the magnetic parameters measured. The physical process of irreversible movement of 90° DWs is suggested. The results indicate the dissipated elastic energy per unit volume due to the irreversible movements of 90° DWs is equal in value to the energy density of DWs, that is γ_w/δ_w=λ_sE/2. It is an effect of magneto-elastic interchange in ferromagnetic alloys.     

A study on material damping of 0° laminated composite sandwich cantilever beams with a viscoelastic layer

The damping behavior of a 0° laminated sandwich composite beam inserted with a viscoelastic layer is investigated. A quantitative analysis of damping in the sandwich laminated composite beam has been conducted through the theoretical method. Results showed that the viscoelastic core thickness in the sandwich beam and the length of the beam have a great effect on the damping loss factor. They also demonstrate the great capability of laminated sandwich composites with embedded viscoelastic layer to considerably enhance structural damping. It is shown that the extended Ni–Adams’ theory can be efficiently used to identify the damping characteristics of the laminated sandwich composite beam.     

Role of matrix resin in delamination onset and growth in composite laminates

The effect of the matrix resin on the onset and growth of delamination in composite laminates has been investigated in this work. Two kinds of graphite/epoxy composite materials (T300/648-BF₃/MEA and T300/634-DDS) with quite different matrix properties have been used. The study was done on two different layups, [(±30)₃/90₂]_s and [(±45)₂/O₂/90₂]_s. Out-of-plane moiré interferometry and diiodomethane-enhanced X-radiography were used to detect delamination. A strength criterion for the onset of delaminatoin is proposed and an assessment made of the effect of matrix properties on delamination onset. A modified energy release rate model is presented for characterization of delamination growth emphasis being placed on assessing the behavior of delamination resistance curves and delamination growth rate. The results indicate that enhancement of matrix strength and ductility increases the critical loads for delamination onset and delamination resistance in the composite laminates under static loading, and significantly reduces the delamination growth rate under cyclic loading.     

Physical aging behavior of high-performance composites

The effect of physical aging on the viscoelastic creep properties of a thermoplastic-toughened cyanate ester resin (Fiberite 954-2) and its IM8/954-2 composites, and a semi-crystalline thermoplastic (Fiberite ITX) and its IM8/ITX composites was investigated. The study was carried out by using dynamic mechanical analysis (DMA) and tensile creep tests. Tests were performed on plain resin, [90 °]_4s, and [± 45 °]_2s composite specimens. Creep tests were conducted up to an aging time of 54 h with the logarithmic aging shift rate, μ, and its dependence on sub-glass transition aging temperature, being determined. The results showed significant physical aging in both material systems. To study the effect of long-term aging on creep behavior, momentary creep tests were conducted on the [± 45 °]_2s composites of both material systems at temperatures between 140 and 200 °C. Master curve plots were drawn from these momentary creep tests using the time/temperature superposition principle (TTSP). Effective time theory (ETT) was then used to modify TTSP and incorporate physical aging effects.     

The active buckling control of some composite column strips using piezoceramic actuators

The overall flexural buckling control of composite column strips using piezoceramic actuators is examined in this paper. The buckling control is investigated through the use of induced strain actuation associated with the piezoelectric effect and in conjunction with a closed-loop control system. Three column strip specimens have been fabricated from commercially available carbon-epoxy pre-impregnated sheets. The layup configurations are [90₂/0]_s, [± 45/0]_s, and [90₂/± 45]_s. The length and width of each test specimen is 280 mm and 35 mm respectively. After manufacture of the composite column strips, piezoceramic actuators were surface bonded at their mid-heights on both sides of the column. Due to imperfections in the material, and of a geometrical nature, the composite column strips, with inactivated piezoceramic actuators, will deflect from the onset of loading and reach an ultimate load capability at high deflection levels. As a result of the presence of imperfections, this ultimate load will be less than the critical buckling load of the ideal structure. By applying a controlled voltage to the actuators a reactive moment will be induced at the column centre thereby removing the lateral deflections and enforcing the column to behave in a perfectly straight manner. An exact theoretical buckling analyses is outlined. This is used to evaluate the critical buckling loads of the individual composite test specimens. The test procedure is outlined and load-deflection plots, obtained with and without active control, are presented. The composite column strips with active control are shown to clearly demonstrate an increase in axial compressive load capacity compared to those without control. For the layup configurations considered, increases in load carrying capability are of the order of 19.8%–37.1%.     

Damage detection of surface cracks in composite laminates using modal analysis and strain energy method

This paper presents an approach to detect surface cracks in various composite laminates. Carbon/epoxy composite AS4/PEEK was used to fabricate laminated plates, [0]₁₆, [90]₁₆, [(0/90)₄]_S and [±45/0/90]_2S. Surface crack damage was created on one side of the plate using a laser cutting machine. Modal analysis was performed to obtain the mode shapes from both experimental and finite element analysis results. The mode shapes were then used to calculate strain energy using the differential quadrature method (DQM). Consequently, the strain energies of laminated plates before and after damaged were used to define a damage index which successfully identified the surface crack location.     

An elastic–plastic thermal stress analysis of aluminum metal–matrix composite beams

In this study, an elastic–plastic thermal stress analysis is carried out on steel fiber-reinforced aluminum metal–matrix composite beams. Temperature is chosen to vary linearly. It is zero and T₀ at the upper and lower surfaces, respectively. The beam is fixed by two rigid planes at the ends. The solution is performed at 0°, 30°, 45°, 60° and 90° orientation angles. The plastic region is expanded at the lower side of the beam. It is found that the intensity of the residual stress component of σ_x and the equivalent plastic strain are maximum at lower surface of the beam. The residual stress is found to be greatest for the 0° orientation angle. In addition, the intensity of the equivalent plastic strain is the greatest for the same angle.     

Beam analysis of angle-ply laminate end-notched flexure specimens

Analysis and experiments on quasi-unidirectional and angle-ply laminate end-notched flexure specimens are presented. The analysis is based on laminated beam theory incorporating first-order shear deformation theory. Compliance and strain-energy release rate determined for relatively thin unidirectional and angle-ply laminate ENF specimens were in good agreement with a previous classical plate theory formulation. For thicker laminates, however, effects of shear deformation on the compliance of the ENF specimen become significant. An experimental study on glass/polyester quasi-unidirectional and angle-ply laminate ENF specimens was conducted. Specifically, [0]₆, [±30]₅ and [±45]₅ laminates with mid-plane delaminations were considered. Experimental compliance data agreed well with analytical predictions. The fracture toughness increased with increased angle θ at the ±θ interface. This is attributed to the fracture work associated with the debonding of transversely oriented fiber bundles in the quasi-unidirectional plies. The angle-ply laminates displayed more yarn debonding than the quasi-unidirectional laminate. For all laminates it was observed that the crack propagated in a non-uniform manner which is correlated with elastic coupling effects with cracked regions of the laminate beams.     

Advanced calculation of the room-temperature shapes of thin unsymmetric composite laminates

It is well known that the room-temperature shapes of unsymmetric laminates do not always conform to the predictions of classical lamination theory. Instead of being saddle shaped, as classical lamination theory predicts, the room-temperature shapes of unsymmetrically laminated composites are often cylindrical in nature. In addition, a second cylindrical shape can sometimes be obtained from the first by a simple snap-through action. Hyer developed for the class of all square unsymmetric cross-ply laminates which can be fabricated from four layers i.e., [0₃/90], [0₂/90/0], [0/90/0/90], [0₂/90₂], an extended classical lamination theory to predict whether these laminates have a saddle shape or one or two cylindrical shapes. The Finite Element Analysis (FEA) has just recently been used for the calculation of the room-temperature shapes of unsymmetric laminates, because more sophisticated finite element codes are now available and the calculations can be made in an acceptable time. The hope is to get more accurate results for the shape and the stresses and forces that occur during the snap through action. These results are needed for the development of active deformable composite structures based on unsymmetric laminates and incorporated shape memory alloy wires [Schlecht M. & Schulte K., Development of active deformable structures due to thermal residual stresses and incorporating shape memory alloys. In Proc. ECCM Smart Composites Workshop, ECCM6, Bordeaux, 1993, pp. 20–115.] Results for different lay-ups are presented and compared.     

An investigation of graphite PEEK composite under compression with a centrally located circular discontinuity

The failure characteristic of graphite polyetheretherketone (Gr/PEEK) under compression with a centrally located circular discontinuity was investigated through experimentation and a nonlinear ply-by-ply finite element technique. The stacking sequence of the laminates investigated were: [0 °₁₆], [90 °₁₆], [±45 °]_4S [0 °/90 °]_4S, and [0 °/ ± 45 0°/90 °]_2S. In the experimentation, [90 °]₁₆, [0 °/90 °]_4S, and [0 °/ ±45 °/90 °]_2S laminates, as well as three of the [0 °]₁₆, failed due to a crack that was normal to the loading direction and initiated from the edge of the hole progressing to the outer edges of the specimen. The [±45 °]_4S specimens failed to support the load due to an internal crack that originated from the hole's edge and then traveled at an angle of about 42% to the direction of loading. The finite element method used to analytically model the failure of Gr/PEEK accurately modeled the response of the specimens tested experimentally.     

The influence of the stress state on the plastic zone size

“Linear elastic” fracture mechanics is based on the “plane strain” condition which means dε_zz = 0. Really stress states can occur in structures or components where dε_zz > 0. Two examples are discussed. Using the v. Mises criterion and the Sneddon equations the influence of an external stress σ^e_zz acting parallel to the crack front on the plastic zone size is calculated and demonstrated by examples.     

Calculation of the ferrite volume in some dual phase steels

The relation between the γ/γ + boundary temperature, T, and the equivalent values of [Cr] and [Ni], as well as the variation of the ferrite volume, V_f, with the temperature in + γ dual-phase steels have been studied. With the aid of a computer, the regressive expressions derived from the experimental results are: T (°C) = T₃ + 21.2 [Cr] − 15.8 [Ni] + 223; V_f (%) = 0.715 exp [0.015(T − T_δ)] − exp[0.015(T_c − T_δ)] + 1.85 exp [0.0083(T − T_c]).     

Determination of fracture energy and tensile cohesive strength in Mode I delamination of angle-ply laminated composites

The energy release rate in delamination of angle-ply laminated double cantilever composite beam specimens was calculated using the compliance equation, and interlaminar cohesive strengths were obtained. Instead of the traditional approach of a beam on an elastic foundation, a second-order shear-thickness deformation beam theory (SSTDBT) was considered. The equilibrium equations were obtained using the principle of minimum total potential energy and the system of ordinary differential equations were solved analytically. The problem was solved for [0°]₆ , [±30°]₅, and [±45°]₅ laminates with mid-plane delaminations and the results were verified using experimental evidence available in the literature.     

Thermal and mechanical fatigue analysis of CFRP laminates

[0°/90°]_s and [±45°]_s CFRP laminated plates were analysed using a finite element formulation for their fatigue behaviour. A fatigue criterion which is based on the laminate interlaminar stresses and the basic lamina fatigue parameters was used. Thermal effects were included in the formulation. In particular, initial thermal stresses resulting from the curing of the laminate were also included in the analysis. The results showed that both laminates had predicted S-N behaviour similar to that from experiments of past investigators. Also, the fatigue behaviour for the [±45°]_s laminate between room temperature and the curing temperature were found to be the same. However, in the case of the [0°/90°]_s laminate the fatigue strength at high temperatures was found to be lower than that at low temperatures.     

Characterization of the vibration damping loss factor of glass and graphite fiber composites

Material damping of laminated composites is experimentally determined by the half-power bandwidth method for cantilever beam specimens excited with an impulse excitation. Data acquisition and manipulation are carried out using both an IBM PC-AT and a GenRad 2500 Series FFT Analyzer. Unidirectional continuous fiber 0° and 90° laminates were fabricated from glass/epoxy (Hercules S2-Glass/3501-6), graphite/epoxy (Hercules AS4/3501-6) and graphite/poly (ether ether ketone) (ICI AS4/PEEK[APC-2]) to investigate the effect of fiber and matrix properties as a function of frequency, up to 1000 Hz, on the damping of composites. The S2-glass/3501-6 composite had a higher loss factor than the AS4/3501-6 in the 0° orientation with the loss factor for the AS4/3501-6 exhibiting a linear increase with increasing frequency and the loss factor for the S2-glass varying nonlinearly with frequency. The 90° material exhibited a higher damping loss factor than the 0°, varying nonlinearly with increasing frequency. In the 90° orientation, the glass fiber composite had loss factors that were approximately fourfold greater than the 0° orientation at frequencies greater than 200 Hz. The 0° AS4/PEEK had a loss factor that was approximately equal to that of the 0° AS4/3501-6. The 90° AS4/PEEK had a loss factor that was approximately 50% less than the AS4/3501-6 and 25% greater than the S2-glass/3501-6 composite.     

Bearing strength of carbon epoxy laminates under static and dynamic loading

An experimental study has been carried out to investigate both the static and dynamic bearing strengths of a pinned-joint carbon epoxy composite plate with [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking configurations. The static and dynamic experiments have been carried out according to the ASTM D953 standards and ASTM STP 749, respectively [ASTM D 953-D, Standard Test method for Bearing Strength of Plastics, ASTM Designation. 342; Joining of Composite Materials, ASTM STP 749, American Society for Testing and Materials (1981) 131]. The ratio of the edge distance to the pin diameter (E/D), and that of the width to the pin diameter (W/D) of the specimens were varied to obtain the static bearing strength and the S–N fatigue curve. The experiments show that the static bearing strengths reach their upper limit when E/D and W/D ratios are equal to or greater than 4 for both [0°/45°/−45°/90°]_S and [90°/45°/−45°/0°]_S stacking sequences. The fatigue strength, on the other hand, reduces by up to 65% as E/D and W/D ratios increase for both stacking configurations.     

High strain rate compressional behavior of stitched and unstitched composite laminates with radial constraint

This paper is concerned with the high strain rate compressional behaviour of glass/epoxy (Hy-E 9134B, Fiberite, USA) composite laminates with or without stitching reinforcement by untwisted Kevlar-49 threads (1140 denier). The split Hopkinson pressure bar (SHPB) apparatus is used in performing the high strain rate tests. Test data are analyzed in a manner similar to that reported by Hauser Exp. Mech., 6 (1966) 395. Specimens are tested at strain rates up to 10⁴ s⁻¹. Unidirectional laminated parallelepiped samples are impacted along their fiber direction. Their high velocity compressive ductility is observed. Both [0°]₂₄ and [(0°/90°)₆]_S glass/epoxy circular specimens with disc diameters of 10 and 50·8 mm are transversely impacted by an input bar in order to study their high strain rate behavior. Moreover, two sets of stitched circular specimens with disc diameters of 10 and 50·8 mm are also examined. The effect of strain rate and radial constraint on the dynamic properties of stitched and unstitched GFRP laminated specimens and their associated damage patterns are described.