Durability of filament-wound composite flywheel rotors

This paper predicts the durability of two types of flywheels, one assumes to fail in the radial direction and the other assumes to fail in the circumferential direction. The flywheel failing in the radial direction is a conventional filament-wound composite flywheel and the one failing in the circumferential direction is a tailor-made type. The durability of the former is predicted by Micromechanics of Failure (MMF) (Ha et al. in J. Compos. Mater. 42:1873–1875, 2008), employing time-dependent matrix strength, and that of the latter is predicted by Simultaneous Fiber Failure (SFF) (Koyanagi et al. in J. Compos. Mater. 43:1901–1914, 2009), employing identical time-dependent matrix strength. The predicted durability of the latter is much greater than that of the former, depending on the interface strength. This study suggests that a relatively weak interface is necessary for high-durability composite flywheel fabrication.     

Progressive damage modeling in fiber-reinforced materials

This paper presents an anisotropic damage model suitable for predicting failure and post-failure behavior in fiber-reinforced materials. In the model the plane stress formulation is used and the response of the undamaged material is assumed to be linearly elastic. The model is intended to predict behavior of elastic-brittle materials that show no significant plastic deformation before failure. Four different failure modes – fiber tension, fiber compression, matrix tension, and matrix compression – are considered and modeled separately. The onset of damage is predicted using Hashin’s initiation criteria [Hashin Z, Rotem A. A fatigue failure criterion for fiber-reinforced materials. J Compos Mater 1973;7:448; Hashin Z. Failure criteria for unidirectional fiber composites. J Appl Mech 1980;47:329–34] and the progression of damage is controlled by a new damage evolution law, which is easy to implement in a finite element code. The evolution law is based on fracture energy dissipation during the damage process and the increase in damage is controlled by equivalent displacements. The issues related to numerical implementation, such as mesh sensitivity and convergence in the softening regime, are also addressed.     

Relationship between lifetime under creep and constant stress rate for polymer-matrix composites

The present article reviews two existing theoretical approaches for creep failure criteria of viscoelastic materials. One criterion is based on the continuum damage mechanics (CDM) and the other is based on the fracture mechanics extended to viscoelastic materials. Although both theoretical frameworks are based on different physical concepts, the deduced lifetime expressions turn out to be equivalent even though its parameters have different physical interpretation. It is proved that both theoretical frameworks, when extended to variable stress loading cases, imply the linear cumulative damage (LCD) law. Additionally the relationship obtained between the creep–rupture and constant stress rate until failure is very simple. Moreover this simple relationship is obtained independently by two different cumulative damage laws, which do not obey the LCD law, and by experimental evidence using published data for two different polymer-matrix composites (PMC). Finally a micromechanical model, used for creep–rupture of unidirectional composites, is extended for constant stress rate until failure to corroborate the simple relationship obtained between the creep–rupture and constant stress rate until failure.     

Deterministic and probabilistic lifetimes from kinetic crack growth—generalized forms

The lifetime prediction methodology developed here is an addendum to and a generalization of that given earlier by Christensen and Miyano [Christensen RM, Miyano Y (2006) Int J Frac 137:77–87]. The previous results were not sufficiently general to model some of the results in the intermediate time ranges. The present results still retain the kinetic crack formalism but include more general forms that are in accordance with data. This new method admits both deterministic and probabilistic forms. Specific applications are given for creep rupture and constant strain rate programs. Possible applications are for any materials types whose very long term creep rupture behavior takes a power law form.     

Numerical characterization of compressive response and damage evolution in laminated plates containing a cutout

A micromechanical constitutive model [Liang Z, Lee HK, Suaris W. Micromechanics-based constitutive modeling for unidirectional laminated composites. Int J Solid Struct 2006;43:5674–89], based on the concept of the ensemble-volume average for laminated composites, is implemented into a finite element program to numerically characterize the compressive response and damage evolution in laminated plates containing a cutout. Prior to the implementation of the model into the finite element program, the predicted moduli of laminated composites are compared with analytical bounds and experimental data for the validation and verification of the constitutive model. A series of numerical simulations for a uniaxial test of laminated plate specimens containing a cutout are conducted using the implemented constitutive model. The predictions are compared with experimental data [Lessard LB, Chang FK. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: Part II-Experiment. J Compos Mater 1991;25:44–64; Chang FK, Lessard LB. Damage tolerance of laminated composites containing an open hole and subjected to compressive loadings: Part I-Analysis. J Compos Mater 1991;25:2–43] to verify the accuracy of the implemented constitutive model. A parametric study is also carried out to illustrate the influence of the geometry of the specimens on the behavior of laminated plates. It is shown that the implemented constitutive model is suitable for the analysis of the constitutive behavior of laminated plates having a dilute or moderate fiber volume fraction.     

Elastic analysis of hybrid bonded joints and bonded composite repairs

The authors extend the closed-form bonded joint linear elastic analysis method of Delale et al. [Delale F, Erdogan F, Aydinoglu MN. Stresses in adhesively bonded joints: a closed-form solution. J Compos Mater 1981;15:249–71] and Bigwood and Crocrombie [Bigwood DA, Crocombe AD. Elastic analysis and engineering design formulae for bonded joints. Int J Adhes Adhes 1989;9(4):229–42] to include the composite deformation mechanisms and the thermal residual strains that arise in hybrid metal-composite joints such as those presented by bonded composite repairs applied to metallic aircraft structures. The analytical predictions for the adhesive stresses and the compliance are compared to the results of a linear elastic finite element model that has itself been validated by comparison with experimental results. The results are applied to the problem of coupled linear extension and bending of a bonded composite repair applied to a cracked aluminum substrate. The resulting stress intensity factor and crack-opening displacement in the repaired plate are compared to the results of a three-dimensional finite element analysis, and also exhibit excellent results. Throughout the text, observations are made regarding the practical application of the results to failure prediction in hybrid joints, whereby the authors demonstrate the need for consistency in the analytical methods used to determine the fatigue and failure of composites from the coupon level to the analysis of the final structural details.     

Damage accumulation model for aluminium‐closed cell foams subjected to fully reversed cyclic loading

Although metal foams are a relatively new material, substantial knowledge has been accumulated about their mechanical properties and behaviour under monotonic loads and tension–tension and compression–compression cyclic loads. However, there are very few reports of the behaviour of metal foams under tension–compression‐reversed loading. In this paper, we examine some of the rare published data regarding the tension–compression cyclic response of metal foams, develop a statistical model of the fatigue lifetime and propose two damage accumulation models for aluminium‐closed cell foams subjected to a fully reversed cyclic loading. In developing these models a fatigue analysis and a failure criterion for the material are needed; the fatigue models considered are the Coffin–Manson and the statistical Weibull model, and the failure criterion used is the one described by Ingraham et al. (Ingraham, M.D., DeMaria, C.J., Issen, K.A. and Morrison, D.J.L. (2009). Mater. Sci. Eng. A. 504:150–156). The models developed are compared with the experimental published data by Ingraham et al. (Ingraham, M.D., DeMaria, C.J., Issen, K.A. and Morrison, D.J.L. (2009). Mater. Sci. Eng. A. 504:150–156) and a final analysis was performed to determine whether it is preferable to use the total or plastic strain amplitude for the fatigue analysis.     

Off-axis fatigue behaviour and its damage mechanics modelling for unidirectional fibre–metal hybrid composite: GLARE 2

The fatigue behaviour of a unidirectional fibre–metal laminate GLARE 2 has been studied under various off-axis loading conditions. Tension–tension fatigue tests were first performed at room temperature on nine kinds of plain coupon specimen with a different off-axis angle. A non-dimensional effective stress defined on the basis of the classical static failure theory was applied as an off-axis fatigue strength parameter. A macroscopic fatigue damage mechanics model was then developed using the non-dimensional effective stress, and it was compared with the classical fatigue failure models for composites. The absolute off-axis fatigue strength decreases as the off-axis angle increases. The longitudinal fatigue strength of GLARE 2 is about two times as high as that of the high-strength aluminium alloy, while the transverse fatigue strength is almost one-half. The S–N relationships are almost linear for all off-axis angles in the intermediate range of fatigue life 10³<N_f<10⁵, and they are followed by fatigue limits. The off-axis fatigue data plotted using the strength ratio (i.e. the maximum fatigue stress normalized by the static strength) are approximately represented by a single master S–N curve. The non-dimensional effective stress succeeds in describing this characteristic of the off-axis fatigue behaviour. The damage mechanics model developed using the non-dimensional effective stress can favourably reproduce the directional nature of the constant amplitude off-axis fatigue behaviour of GLARE 2. This model has an advantage over the classical fatigue failure models for composites with respect to the numerical procedure for fatigue life analysis.     

Use of the polar method to predict the hygroscopic transient stresses for balanced [θ/−θ] nS laminates under asymmetrical environmental loadings

This paper presents an analytical method to assess the transient hygroscopic stresses in laminated composite plates due to asymmetrical constant environmental conditions. The developed method permits us to determine directly the values of such stresses without the calculation of the moisture concentration through laminated plates. The present method is the extension of the method developed by Tounsi and Adda-Bedia [Tounsi, A., Adda-Bedia, E.A.: Appl Compos Mater 10, 1–18 (2003)] which is limited only to the problems with symmetrical environmental conditions. Thus by the present method, we can study the effect of symmetrical and asymmetrical environmental conditions. A validation was made with the results of the same authors [Tounsi, A., Adda-Bedia, E.A.: Appl Compos Mater 10, 1–18 (2003); Sereir, Z., et al.: J. Thermal Stres. 28(9), 889–910 (2005)] for symmetrical environmental conditions and the results of benkeddad [Benkeddad, A., et al.: Compos. Struct. 30(2), 201–215 (1995)] for asymmetrical environmental conditions. In order to evaluate the performances of each balanced [θ/?θ] _S laminate proposed for this applications, we use the polar method introduced by Verchery [Vannucci, P., Verchery, G.: Int. J. Solids Struct. 38, 9281–9294 (2001), Vincenti, A., et al.: Composites: Part A, 1525–1532 (2001), Valot, E., et al.: Compos. Struct. 60, 413–429 (2003), Vannucci, P., Verchery, G.: Compos. Sci. Technol. 61, 1465–1473 (2001)]. This method enables us to locate the favourite directions of the mechanical characteristics and to estimate the degree of anisotropy of all stacking sequences. Therefore, we can reduce the transient hygroscopic stresses, particularly at the edges of laminated plates. Through this theoretical study presented here, we hope to contribute to the understanding of hygrothermal behaviour of composite laminated plates.     

C/SiC复合材料应力氧化失效机理

研究了干氧和湿氧两种气氛、疲劳和蠕变两种应力下C/SiC复合材料在1300℃的应力氧化行为. 试验结果和断口形貌SEM分析表明: C/SiC复合材料在疲劳应力下比在蠕变应力下具有更强的抗氧化能力和更长的持续时间; 干氧环境中的蠕变试样以C纤维氧化失效为主; 水蒸气的存在加剧了SiC基体的氧化, 并且使受蠕变应力的C/SiC复合材料以SiC基体氧化失效为主.     

Deformation behaviour,short crack growth and fatigue livesunder multiaxial nonproportional loading

Experimental results of a research project on short crack growth under multiaxial nonproportional loading are presented. Fatigue lives, crack growth curves and the deformation behaviour of hollow tube specimens and notched specimens were investigated under combined tension and torsion loading. The results served as basis for the development of a cyclic plasticity model [Döring R, Hoffmeyer J, Vormwald M, Seeger T. A plasticity model for calculating stress–strain sequences under multiaxial nonproportional cyclic loading. In: Comput Mater Sci. 28(3–4);2003:587–96; Döring R, Hoffmeyer J, Seeger T, Vormwald M. Constitutive modelling of nonproportional hardening, cyclic hardening and ratchetting. In: Proceedings of the seventh international conference on biaxial/multiaxial fatigue and fracture, DVM, Berlin; 2004. p. 291–6; Hoffmeyer J. Anrisslebensdauervorhersage bei mehrachsiger Beanspruchung auf Basis des Kurzrisskonzepts. PhD-Thesis, TU Darmstadt; 2004.] and a short crack model [Hoffmeyer J. Anrisslebensdauervorhersage bei mehrachsiger Beanspruchung auf Basis des Kurzrisskonzepts. PhD-Thesis, TU Darmstadt; 2004; Döring R, Hoffmeyer J, Seeger T, Vormwald M. Fatigue lifetime prediction based on a short crack growth model for multiaxial nonproportional loading. In: Proceedings of the seventh international conference on biaxial and multiaxial fatigue and fracture, DVM, Berlin; 2004. p. 253–8].Stress–strain paths including nonproportional hardening and experimental fatigue lives of the unnotched specimens under different loading cases are discussed and compared with calculations. Load-time-sequences were in-phase, 45° and 90° out-of-phase loading with constant and variable amplitudes, torsion without and with superimposed static normal stress, and strain paths like box, butterfly, diamond and cross path. For the notched specimens fatigue lives under 0° and 90° out-of-phase loading are compared with calculations based on finite element results and the short crack model. During some tests the initiation, growth and orientation of short cracks was studied using the plastic replica technique.     

Applicability of Fatigue Life Prediction Method to Polymer Composites

A prediction method of fatigue strength under an arbitraryfrequency, temperature, and stress ratio is proposed for polymercomposites and its validity is confirmed for the flexural fatiguestrength of satin-woven CFRP laminates. This method is based upon fourhypotheses: (a) same failure process under constant strain-rate (CSR),creep, and fatigue loadings, (b) same time-temperature superpositionprinciple for all failure strengths, (c) linear cumulative damage lawfor nondecreasing stress process, and (d) linear dependence of fatiguestrength upon stress ratio. This method was applied to the flexuralfatigue strength of various unidirectional CFRPs, and the verificationand limitations of this method were discussed.     

Experimental study and modeling of fatigue life prediction of plain weave carbon/polymer composite under constant amplitude loading

For fiber-reinforced composites, the anisotropy and the tension-compression asymmetry have very important influence on fatigue performance. The quasi-static and fatigue mechanical behavior of plain weave laminates of carbon/polymer composites were experimentally investigated in this paper. The quasi-static and stress-controlled fatigue tests were carried out on the servo-hydraulic material testing machine. Fracture failure surfaces were observed in micro-mechanism with scanning electron microscopy. A phenomenological and nonlinear constant life diagram (CLD) model was developed based on quasi-static strength and S–N curve data-sets of different stress ratios. The relationship between six parameters of the proposed model and the failure cycles was studied. The fatigue experimental results showed that the fatigue failure type changed from tensile mode to compressive mode at nearby stress ratio R = ?0.2. A satisfactory agreement between the predicted value of cycle life and experimental data was observed. The results indicated that the fatigue performance was adequately described by the Basquin S–N formulation and proposed CLD.     

Intrinsic ductility for structural materials as a function of stress and temperature

Abstract

In recent years, a method has been developed to measure current creep strength at nearly constant structure in a high precision stress relaxation test (SRT), covering at least five decades in creep rate in a one day test. Results have been reported on a wide range of metallic alloys, polymers, composites and ceramics. In the present paper it is shown that these same data can be used to determine a measure of intrinsic ductility over the same range of stress, using results on a low alloy Cr–Mo–V steel. This is based on an experimental and theoretical correlation between elongation at failure and strain rate sensitivity, m. This refined SRT test can now be used to evaluate both the intrinsic creep strength and the intrinsic ductility as a function of stress in a single short-time test. The test can detect embrittling phenomena at very low creep rates as a function of temperature. This measure of ductility may be used directly in engineering design and remaining life assessment.     

Effective moduli and failure considerations for composites with periodic fiber waviness

A finite element micromechanical model for fibrous materials introduced in a previous work [J. Compos. Mater. 38 (4) (2004) 273] is used to further study the effects of periodic and localized fiber waviness. A periodic unit cell based on hexagonal fiber packing and sinusoidal fiber waviness was assumed as a representative volume element. Equivalent to this wavy-shaped unit cell, a straight unit cell but with wavy material-orientation is introduced. This type of homogenized continuum modeling simplifies the analysis since the wavy geometry with details of constituent materials is avoided. Thus, stiffness parameters associated with individual lamina with waviness are estimated when subject to the constraining effects of neighboring isotropic or straight fiber material layers. It is shown that the shear constraint of the added layers increases the effective moduli of the wavy layer by inhibiting the fiber straightening deformation mechanism. The local stress distribution is also examined and the potential for material failure is investigated. The methodology provides a platform to study the behavior of wavy fiber composites in a systematic manner.     

Simulation of creep crack growth in ceramic composites

The elevated temperature response resulting from tensile creep of fiber reinforced ceramic composites was modeled using Monte Carlo simulation. The model consisted of a uniaxially loaded fiber tow aligned with the direction of applied load, and modeled the growth of matrix cracks resulting from creep failure of bridging fibers. A creep strain rate consisting of primary and steady state components was assumed, and each component was modeled by a power law relationship. Power law creep exponents in the range of 2.0–2.5 for a selected SiC/SiC system at stress levels ranging from 60 MPa to 200 MPa were evaluated. Fatigue-like behavior was predicted as a result of tensile creep, and a fatigue exponent of 3.03 ± 0.07 was predicted for nominal stress levels less than 200 GPa. The influence of initial crack length on failure lifetime was also studied, but was found to have little influence on the predicted lifetime. The predicted failure response suggested a stress dependent creep process could be used to model experimental data and evaluate the failure mechanism of reinforced composites.     

Tensile strength at elevated temperature and its applicability as an accelerated testing methodology for unidirectional composites

The applicability of a macroscopic time-temperature superposition principle (TTSP) to unidirectional composite strength is discussed based on the microscopic Simultaneous Fiber-Failure (SFF) model that has been presented by Koyanagi et al. (J. Compos. Mater. 43:1901–1914, 2009a). The SFF model estimates composite strengths as functions of fiber, matrix, and interface strengths. This paper first investigates the applicability of SFF to the complicated temperature dependence of composite strengths, i.e., one composite exhibits significant temperature dependence and another does not, considering the temperature dependence of the components, which results in successful estimations for the two composite systems used in the present study. The long-term durability predicted by the SFF and that predicted by the TTSP are then compared. They typically correspond to each other in various cases; accelerated testing methodology (ATM) employing TTSP is thus proved to be valid from the micromechanical viewpoint, assuming the SFF applicability.     

Fatigue strength prediction of ultra high strength steel butt‐welded joints

First, fatigue tests were performed on butt‐welded joints made of novel direct quenched ultra high strength steel with high quality welds. Two different welding processes were used: MAG and Pulsed MAG. The weld profiles, misalignments and residual stresses were measured, and the material properties of the heat‐affected zone were determined. Fatigue tests were carried out with constant amplitude tensile loading both for joints in as‐welded condition and for joints after ultrasonic peening treatment. Finally, in fatigue strength predictions, the crack initiation phase was estimated using the procedures described by Lawrence et al. [Lawrence F V, Ho N J and Mazumdar P K (1981) Predicting the fatigue resistance of welds. Annu. Rev. Mater. Sci, 11, 401–425]. The propagation phase was simply estimated using S–N curves for normal quality butt welds, which may contain pre‐existing cracks or crack‐like defects eliminating the crack initiation stage.     

Effects of moisture and UV exposure on liquid molded carbon fabric reinforced nylon 6 composite laminates

Liquid molding of thermoplastics has been limited by high resin viscosity, high temperature processing requirements, and a short processing window [Sibal PW, Camargo RE, Macosko CW. Designing nylon 6 polymerization for RIM. In: Proceedings of the second international conference on reactive processing of polymers, Pittsburgh, PA; 1982, p. 97–125.]. The processing parameters for vacuum assisted resin transfer molding (VARTM) developed by the authors and previously reported [Pillay S, Vaidya UK, Janowski GM. Liquid molding of carbon fabric-reinforced nylon matrix composite laminates. J Thermoplast Compos Mater 2005;18:509–27] have been adapted to process carbon/nylon 6 composite panels. The present work addresses the effects of moisture and ultraviolet (UV) exposure on the static and dynamic mechanical properties of carbon fabric reinforced, thermoplastic polyamide 6 matrix panels processed using VARTM. The Bao and Yee dual diffusivity model [Bao LR, Yee AF. Moisture diffusion and hygrothermal aging in bismaleimide matrix carbon fiber composites: Part II – Woven and hybrid composites. Compos Sci Technol 2002;62:2111–9] was applied to evaluate the moisture uptake for the C/PA6, fully immersed in distilled water at 100 °C. SEM results show that moisture exposure result in surface micro-cracks compromise of the fiber–matrix interface. The flexural strength is lowered by 45%, after exposure to moisture at 100 °C. UV exposure up to 600 h causes yellowing of the samples and an increase in crystallinity from 40% to 44%.