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.     

复合材料开孔层板压缩渐进损伤试验

为研究碳纤维增强树脂基复合材料开孔层板在压缩加载过程中的损伤起始、演化方式和损伤特点,采用微距拍摄、逐级加载超声C扫描、X光扫描和扫描电子显微镜观测4种观测手段对国产CCF300/5228A[45/0/-45/90]4s、[452/02/-452/902]2s、[454/04/-454/904]s3种铺层方式的开孔层板进行了压缩试验研究。对压缩载荷作用下开孔层板的损伤起始和损伤演化进行了观察和对比。对试验中观测到的纤维微屈曲、纤维挤出、孔边开裂和分层扩展等现象之间的关系进行了分析和说明。试验结果表明:压缩载荷下45°和90°铺层相邻位置为层板易分层位置,含45°和90°铺层相邻位置的开孔层板渐进损伤过程较为明显:开孔层板在压缩载荷下较早出现损伤,损伤的起始和演化缓解了孔边应力集中,促使压缩应变能在孔边逐步释放,推迟开孔层板压缩破坏的发生,提高层板压缩承载能力。研究结果可为材料结构损伤容限设计提供依据。     

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 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.     

聚酰亚胺树脂基MT300/KH420复合材料高温力学性能(Ⅱ)——弯曲性能

从宏、微观的角度研究了碳纤维增强聚酰亚胺树脂基MT300/KH420复合材料的高温力学性能,重点揭示了MT300/KH420复合材料[0°]14和[±45°/0°/90°/+45°/0°2]s层合板在常温~500℃的弯曲性能变化规律。研究表明:MT300/KH420复合材料高温力学性能优异,[0°]14层合板在420℃的弯曲强度保持在51%以上,弯曲模量在500℃以内变化很小。[0°]14层合板在常温下断口粗糙,且贯穿厚度,表现为脆性破坏;随温度升高,树脂流动性增强,呈现出黏弹效应,破坏逐渐集中在加载点处,在500℃,部分树脂热解,纤维束脱离基体并氧化。[±45°/0°/90°/+45°/0°2]s层合板高温弯曲性能较为稳定,主要破坏为上、下表面沿45°方向开裂,并伴有层间分离,在500℃出现严重分层破坏;相比于受基体控制的层合板弯曲性能,温度对受纤维控制的层合板弯曲性能影响较小。     

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%.     

Fatigue damage mechanics of composite materials Part III: Prediction of post-fatigue strength

A damage-based model for post-fatigue notch strength is presented. The model is an extension of a method developed previously to predict the notch strength of laminated composites. A simple finite element representation of the notch tip damage zone is used to obtain the localized damage-modified stress distribution. A uniaxial tensile stress failure criterion is applied to the 0° plies from which the laminate strength is evaluated. In conjunction with the fatigue damage growth law described in Part II, residual strength is calculated as a function of the applied loading conditions, specimen geometry and lay-up for (90/0)_s, (90/0)_2s and (90₂/0₂)_s T300/914C carbon-fibre/epoxy laminates subjected to tension-tension fatigue cycling.     

Measurement of the static compressive strength of carbon-fibre/epoxy laminates

This paper describes an experimental study of the compressive failure of T800/924C carbon-fibre/efoxy composite laminates. Undirectional laminates loaded parallel to the fibres have compressive strengths that are 70% of the tensile strength and fail by fibre-microbuckling. During microbuckling the fibre debonds from the matrix, and the fibres break in bending. Multidirectional [(±45/0₂)₃]_sm laminates were also tested in compression, and the critical failure mechanism observed was microbuckling of the 0° plies. The failure strain was almost the same as for the undirectional laminate, The failure strain was almost the same as for the unidirectional laminate, which indicated that the ±45° plies have no significant influence on the failure strength of the 0° plies.     

A damping analysis of composite laminates using the closed form expression for the basic damping of Poisson's ratio

Various methods have been presented to obtain the effective damping of a symmetric laminated composite. In this paper, modified classical lamination theory based upon the elastic–viscoelastic correspondence principle was utilized by developing the basic damping of Poisson's ratio for accurately predicting the damping of laminated composite beams. The analysis involves an extension of the elastic–viscoelastic approach. Futhermore, Ni and Adams' theory was used for verifying the modified classical lamination theory. Six typical laminated composites with [±θ]_s,[0/θ]_s,[0/±θ/90]_s,[90/±θ/0]_s,[0/90/±θ]_s and [90/0/±θ]_s, stacking sequences were employed for this study. Numerical results have shown that damping values were in some difference among prediction methods over the particular range of fiber orientation.     

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.     

Effects of temperature on impact damages in CFRP composite laminates

In this paper, the effect of temperature variations (low and high temperatures) was studied experimentally on impact damage to CFRP laminates. The composite laminates used in this experiment were CF/EPOXY orthotropic laminated plates with lay-up [0₆/90₆]_s and [0₄/90₄]_s, and CF/PEEK orthotropic laminated plates with a lay-up of [0₆/90₆]_s. A steel ball launched by the air gun was used to generate the CFRP laminate impact damage. For impact-damaged specimens, nondestructive evaluation (NDE), such as a scanning acoustic microscopy (SAM) was performed on the delamination-damaged samples to characterize damage growth at different temperatures.

Therefore, this study was undertaken to experimentally determine the interrelations between impact energy and impact damage (i.e. the delamination area and matrix) of CFRP laminates (CF/EPOXY and CF/PEEK) subjected to foreign object damages (FOD) at low and high temperatures.     

Fracture behavior of cross-ply graphite/ epoxy laminates

Fracture behavior of cross-ply (0/90)_4s, (0/90)_10s, (0₂/90₂)_2s and (0₄/90₈/0₄)_T laminates of T300/934 graphite/epoxy material was studied using compact tension specimens of several widths and thicknesses, center notched tension and three point bend specimens. The process of damage initiation and propagation was studied and is discussed in detail. The critical stress intensity factor was evaluated and its variation with specimen size and type is shown. On the basis of these investigations, a suitable specimen for the evaluation of meaningful fracture toughness is suggested.     

Matrix cracking behavior of K3B/IM7 composite laminates subject to static and fatigue loading

The matrix cracking behavior of a new high-performance thermoplastic composite material, K3B/IM7, was systematically investigated. Laminates in various grouped thickness and ply stacking sequences, [0₂/90₂/0₂], [0₂/90₄/0₂], and a quasi-isotropic laminate [+45/0/−45/90]_s were tested under static and tension–tension fatigue loading. Depending on the stacking sequence of the laminates and the type of loading, various matrix cracking behavior were found. Under static loading, the matrix cracks were mainly close to the specimen edges. A few cracks were found to penetrate the specimen width, even when the load was large enough to break the specimen. However, under fatigue cyclic load, the edge initiated cracks propagated fully across the specimen width. Combined with the fatigue Paris Rule and considering the ply thickness and stacking sequence, the energy release rate method was applied to predict the relations between the loading strain amplitude and fatigue cycles for matrix cracking failure.     

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.     

Analysis of stiffness reduction of cracked cross-ply laminates

Stiffness reduction of cracked [0°_m/90°_n]_s laminates is analyzed by variational methods on the basis of the principle of minimum complementary energy. For this purpose admissible stress systems are constructed which satisfy equilibrium and all boundary and interface conditions. The optimal stress field is then determined by minimization of complementary energy. The analysis allows for crack interaction and random crack distribution. Results are given for Young's modulus, shear modulus and Poisson's ratio. Young's modulus results are in excellent agreement with experimental data for [[0°/90°₃]]_s glass/epoxy laminate.     

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.     

Microdebonding investigation of the effects of thermal residual stress on the bond strength of a graphite/polymide composite

The microdebonding test was used to investigate the effects of thermal residual stresses resulting from different lay-ups in fabrication on the fiber/matrix bond strength of a graphite-fiber-reinforced polyimide composite. This was accomplished by comparing the results of a cross-plied laminate with those of a unidirectional laminate. The results indicated that the measured interfacial bond strength of the unidirectional composites was greater than that of the cross-plied laminate. The thermal radial stress distribution around the fiber for the unidirectional and the [0₂, 90₂]_s laminates were estimated, to explain this reduction of the interfacial bond strength.     

Interaction between transverse cracks and delamination during damage progress in CFRP cross-ply laminates

In previous papers the microscopic failure process of (0/90_n/0) (n = 4,8,12) cross-ply laminates was investigated. Progressive damage parameters, such as the transverse crack density and the delamination ratio, were measured. A simple modified shear-lag analysis including the thermal residual strains was conducted to predict the transverse crack density and the delamination length. The analysis did not consider the interaction between the transverse cracks and the delamination. In the present paper, a prediction is presented for the transverse crack density including the effect of delamination growth. The prediction shows better agreement with the experimental results, especially for laminates with thicker 90 ° plies in which extensive delamination occurs.

Loading/unloading tests have also been performed to obtain the Young's modulus reduction and the permanent strain as functions of the damage state. The shear-lag predictions of the Young's modulus reduction and the permanent strain are compared with the experimental data. Better agreement is obtained when the interaction between transverse cracks and delamination is considered.     

Impact fatigue properties of epoxy resin filled with SiO2 particles

Impact fatigue tests were carried out on epoxy resin filled with SiO₂ particles. The effects of the percentage of SiO₂ particles and the impact cyclic loading frequency on the impact fatigue strength was investigated. The micromechanism of impact fatigue failure was examined and correlated with the morphology of the fracture surface. The impact stress amplitude, σ_t, can be estimated by the formula, σ₂(N_f · Te)^mt = D_t where (N_f· Te) is the cumulative duration time, and mt and D_t, are parameters describing impact fatigue characteristics. The impact fatigue strength and the static strength are governed by the percent of SiO₂ particles. Crack initiation under monotonie cyclic impact loading was attributed to decision of the epoxy-SiO₂ interface. Unstable crack propagation occurs when the crack passes through the SiO₂ particles.     

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.