A MECHANISTIC-BASED THERMOMECHANICAL FATIGUE LIFE PREDICTION MODEL FOR METAL MATRIX COMPOSITES

The framework for developing a mechanistic-based life prediction model for metal matrix composites is described. For a composite consisting of unidirectional silicon carbide fibers in a titanium aluminide matrix, SCS-6/Ti-24A1-1INb (at%) [0]₈, three dominant damage mechanisms were identified: (1) matrix fatigue damage, (2) surface-initiated environmental damage, and (3) fiber-dominated damage. Damage expressions were developed for each mechanism along with a method for determining the constants. The damage is summed to obtain the total life. The model is capable of making predictions for a wide range of histories, including isothermal fatigue at different frequencies and stress-ratios, thermomechanical fatigue (TMF) under in-phase and out-of-phase cycling conditions, thermal cycling at constant stress, and stress holds at either maximum or minimum stress. Considering the wide range of cyclic conditions, the predictions compare favorably with experiments. In addition, the controlling damage mechanism for each history is predicted.     

Interfaces and fatigue damage in a metastable beta titanium matrix composite

The results of a systematic study of the effects of interfacial microstructure on fatigue damage in a metastable β Ti---15V---3Cr---3Al---3Sn/SiC (SCS-9) composite are presented. Interfacial microstructure is controlled by heat treatment in the β phase field of the matrix, which promotes coarsening of the fiber-matrix interface without significant changes in the metastable β matrix microstructure (grain size). The effects of interfacial microstructure on debone and friction strengths are also discussed using results from fiber push-out tests. Fatigue damage initiation and propagation mechanisms are elucidated via optical/scanning electron microscopy and acoustic emission analysis of specimens that were deformed to failure in incremental cyclic loading steps. The effects of cyclic deformation on matrix hardness and composite modulus are also examined prior to the presentation of a fracture mechanics (micromechanics) approach for the prediction of fatigue life, and the effects of crack-tip shielding via bridging mechanisms. The paper highlights the potential for the development of accurate life prediction methodologies that are based on experimental observations of damage in titanium matrix composites. It also illustrates the need for multidisciplinary mechanics and materials approaches in the study of composite behavior.     

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.     

A cost-effective P/M titanium matrix composite for automobile use

The aim of the present study is to obtain a new high-performance titanium matrix composite appropriate for automobile parts using a new low-cost powder metallurgy process. The results can be summarized as follows:

1. A production process was developed for a sintered titanium alloy from cheap, low-purity titanium powder (sponge fines) which in its as-sintered form (without expensive hot isostatic pressing or heat treatment) achieves superior fatigue properties to hot-isostatic-pressed titanium alloy made from expensive high purity hydride-dehydride titanium powder.
2. TiB was found to be a superior reinforcing compound for blended elemental titanium matrix composites than SiC, B₄C, TiAl, TiB₂, TiN and TiC tested previously and it was used in the above low-cost production process to make the new disperse-particle titanium matrix composites.
3. The developed titanium matrix composite allows considerably cheaper production of parts from titanium alloy than by conventional ingot forging methods and was confirmed to be far superior to conventional titatium alloys in tensile strength, fatigue properties, rigidity, heat resistance, and wear resistance.

     

Interaction between cyclic loading and residual stresses in titanium matrix composites

Metal matrix composites are gaining popularity for applications where high performance materials are needed. Titanium matrix composites (TMCs) continuously reinforced by silicon carbide fibres are under development for applications in aeroengines. Their use in blades, rings and shafts promises a significant weight reduction and performance improvement due to their high specific strength and stiffness. To obtain the whole capabilities of the material not only advanced processing techniques but also post-processing treatments are necessary. A detailed analysis of the residual stress development during cyclic loading leads to the necessity of residual stress modifications to optimise the fatigue behaviour of TMCs. Since the aerospace industry requires high reliability of the materials used, models for predicting failure and life time are of special interest. Predictive models based on the properties of the single constituents of the composite are most suitable to reduce the number of experiments and to develop methodologies to improve specific mechanical properties. Nevertheless, both experiments on the single constituents as well as on the composite are necessary to validate the model. A previously developed rheological model is used to assess different post-processing procedures to improve the fatigue behaviour of a titanium matrix composite. The usage of the model and experiments on the system SCS-6/Ti-6Al-2Sn-4Zr-2Mo are presented.     

Micromechanics modeling of fatigue failure mechanisms in a hybrid polymer matrix composite

Initiation of fatigue damage for a hybrid polymer matrix composite material was studied via 3-Dimensional viscoelastic representative volume element modeling in order to gain further understanding. It was found that carbon fiber reinforced composites perform better in fatigue loading, in comparison to glass fiber reinforced composites, due to the fact that the state of stress within the matrix material was considerably lower for carbon fiber reinforced composites eliminating (or at least prolonging) fatigue damage initiation. The effect of polymer aging was also evaluated through thermal aging of neat resin specimens. Short-term viscoelastic material properties of unaged and aged neat resin specimens were measured using Dynamic Mechanical Analysis. With increasing aging time a corresponding increase in storage modulus was found. Increases in the storage modulus of the epoxy matrix subsequently resulted in a higher state of predicted stress within the matrix material from representative volume element analyses. Various parameters common to unidirectional composites were numerically investigated and found to have varying levels of impact on the prediction of the initiation of fatigue damage.     

Isothermal fatigue behavior of a titanium matrix composite under a hybrid strain-controlled loading condition

The fatigue response of an eight-ply, unidirectional, titanium-based metal-matrix composite (MMC) (SCS-6/Ti-15-3) was investigated at elevated temperature (427 °C) using a hybrid strain-controlled loading mode. This hybrid control mode did not allow the thin MMC specimen to experience any compressive stress and, thus, prevented buckling. All fatigue testing was conducted at a constant strain rate of 0.2% s⁻¹. Damage mechanisms were systematically identified for the cases when loading was parallel or perpendicular to the fiber direction. When the fibers were parallel to the loading direction, the dominant damage mechanism was either fiber fracture or matrix cracking. Matrix creep occurred at all levels of strain, and matrix plasticity was observed when the strain level was greater than 0.55%. When loading was perpendicular to the fiber direction, the fiber-matrix interfacial damage was the dominant damage mechanism. The severity of this damage varied depending upon the maximum strain level. Matrix cracks also had a critical effect on the fatigue response when the maximum strain level was greater than 0.35%. Plastic deformation in the matrix material occurred for strain levels greater than 0.23%, and matrix creep was a key factor at all strain levels. Fatigue-life diagrams along with dominant deformation and damage mechanisms were established for both cases and are compared with previous studies.     

(Al2O3)f/Al复合材料在强界面结合下的疲劳损伤模式

在拉-拉载荷下测定了(Al₂O₃)_f/Al复合材料的疲劳寿命(S-N)曲线。通过夭折试验以及SEM疲劳断口和纵截面组织结构分析,研究了复合材料的疲劳损伤模式。研究结果表明,(Al₂O₃)_f/Al复合材料的疲劳极限为750MPa,远高于SCS-6碳化硅纤维增强钛基复合材料。该复合材料兼有钛基和树脂基纤维复合材料疲劳损伤的特点,高应力下由单个裂纹的起源和生长导致复合材料的失效;低应力下,疲劳损伤模式包括纤维劈裂、众多基体裂纹和单个基体裂纹的横向扩展。其中纤维劈裂是主控机制。其更高的疲劳极限可归因于低应力下纤维的纵向劈裂。     

Fatigue of beta titanium alloy at 20, 482 and 648 °C

Fatigue life of fibrous metal matrix composites is limited by the distribution of fibre strengths, the fibre‐matrix interfacial strength, and the fatigue resistance of the matrix. The aim of this work is to provide fatigue results for a beta titanium alloy over a range of temperatures and stresses that can be used as input for predicting fatigue life of a titanium matrix composite. Stress controlled tests having fatigue ratios between ?1 and ?0.2 were conducted on a limited number of samples machined from unreinforced laminated Ti‐15Mo‐3Al‐2.7Nb‐0.2Si (TIMETAL^®21S) sheets to represent as closely as possible the in situ matrix material. Stress control was used to enable quantification of strain ratcheting for tensile mean stresses and a fast loading rate was used to minimize time‐dependent (creep) deformation. Stress amplitude‐life data at 20, 482 and 648 °C for fully reversed loading are well fit by a power law. Normalizing the stress amplitude with respect to the power law coefficient appears to account for the temperature dependence of the S–N curves. As the tests had large strains and lives were in the low‐cycle fatigue range, strain range at the half‐life was also correlated to life. For tensile mean stress cycling at 482 and 648 °C, the rate of strain ratcheting per cycle increased to failure; shakedown was not observed.     

Explosive shock-compression processing of titanium aluminide/titanium diboride composites

Explosive shock-compression processing is used to fabricate Ti₃Al and TiAl composites reinforced with TiB₂. The reinforcement ceramic phase is either added as TiB₂ particulates or as an elemental mixture of Ti + B or both TiB₂ + Ti + B. In the case of fine TiB₂ particulates added to TiAl and Ti₃Al powders, the shock energy is localized at the fine particles, which undergo extensive plastic deformation thereby assisting in bonding the coarse aluminide powders. With the addition of elemental titanium and boron powder mixtures, the passage of the shock wave triggers an exothermic combustion reaction between titanium and boron. The resulting ceramic-based reaction product provides a chemically compatible binder phase, and the heat generated assists in the consolidation process. In these composites the reinforcement phase has a microhardness value significantly greater than that of the intermetallic matrix. Furthermore, no obvious interface reaction is observed between the intermetallic matrix and the ceramic reinforcement.     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Synthesis,microstructure and mechanical properties of Al2O3 reinforced Ni3Al matrix composite

A new method to synthesize alumina reinforced Ni₃Al intermetallic matrix composites has been described. The powder mixture of nickel and aluminium was mechanically alloyed. The powder mixture was excessively heated during mechanical alloying and then exposed to atmosphere for oxidation. The oxidized powder mixture was transformed into alumina reinforced nickel aluminide matrix composite on subsequent pulse current processing. Alumina reinforcements were generated in the nickel aluminide matrix by in situ precipitation. The microstructure of the composite showed that the alumina reinforcements were 50–150 nm in size. The fine alumina reinforcements were homogeneously distributed in the matrix phase. The mechanical properties of the alumina reinforced nickel aluminide matrix composite fairly exceeded the nickel aluminide alloys. This novel synthesis approach allowed the rapid and facile production of high strength alumina reinforced Ni₃Al matrix composites.     

A micromechanistic-based approach to fatigue life modeling of titanium-matrix composites

Fatigue life modeling of titanium-based metal-matrix composites (MMCs) was accomplished by combining a unified viscoplastic theory, a non-linear micromechanics analysis and a damage accumulation model. The micromechanics analysis employed the Bodner-Partom unified viscoplastic theory with directional hardening. This analysis was then combined with a life-fraction fatigue model to account for the time-dependent component of fatigue damage. The life-fraction fatigue model involved the linear summation of damage from the fiber and matrix constituents of the composite. A single set of empirical constants for the life-fraction fatigue model were established for each of two titanium MMCs reinforced with silicon carbide fibers: SCS-6/Ti-15-3 and SCS-6/ TIMETAL®21s. The predicted fatigue lives were within one order of magnitude of the experimental data for different loading conditions: isothermal fatigue, and both in-phase and out-of-phase thermomechanical fatigue. MMCs modeled included cross-ply, quasi-isotropic and unidirectional SCS-6/TIMETAL®21s, and cross-ply and quasi-isotropic SCS-6/Ti-15-3 laminates.     

碳纤维复合材料假脚工艺参数对其冲击后疲劳性能的影响

基于三维逐渐损伤理论和有限元法,对碳纤维复合材料假脚的冲击及冲击后疲劳破坏过程进行分析,研究了不同的复合材料体系、几何尺寸、纤维铺设方式等工艺参数对碳纤维假脚的冲击损伤及疲劳性能的影响规律。结果表明,在冲击载荷作用下,碳纤维复合材料假脚的损伤模式主要为基体开裂、纤维压缩和分层;复合材料体系的横向和法向拉伸强度以及剪切强度等参数越小,假脚的冲击损伤面积越大,所能承受的疲劳循环次数越低;随着后龙骨厚度的增加,基体开裂损伤面积越来越大,分层损伤面积略有减小,而纤维压缩损伤几乎没有变化。尽管随着后龙骨厚度的增加,假脚的疲劳循环次数逐渐增大,但是相对于厚度的增加量,疲劳循环次数的增加量相对较小;不同铺层参数对碳纤维复合材料假脚的冲击损伤模式几乎没有影响。适度增加0°铺层的含量,可有效提高碳纤维复合材料假脚的疲劳性能。     

Tensile fatigue behaviour of FRP and hybrid FRP sheets

This paper presents the fatigue behaviour of various fibre reinforced polymer (FRP) composites, namely, carbon, glass, polyparaphenylenl benzobisoxazole (PBO), and basalt fibres, including the effect of hybrid applications such as carbon/glass and carbon/basalt composites. A coupon test was conducted to examine the mechanical characteristics of the FRP composites subjected to monotonic and cyclic loads. Test parameters included the applied load range and different types of hybridization. Study results show that (1) the mechanical properties of the emerging PBO and basalt fibres are comparable to those of the conventional carbon and glass fibres; (2) the tensile modulus of the fibres influences the failure mode of the composite coupons; (3) the progressive damage propagation causes fatigue failure of the composites; (4) the hybrid composites of carbon/basalt significantly improves the fatigue resistance in comparison to the homogeneous basalt composite, whereas the resistance of the carbon/glass hybrid composites does not provide such effects.     

Effect of a high-temperature cycle on the mechanical properties of silicon carbide/titanium metal matrix composites

The effects of fibre/matrix interface strength and thermal residual stresses on the mechanical properties of a silicon carbide/titanium composite were investigated. A 3-ply [0/90/0] composite was subjected to a simulated superplastic forming/diffusion bonding (SPF/DB) temperature cycle which changed fibre/matrix interfacial strength and thermal residual stresses in the composite. The [0/90/0] composite subjected to the SPF/DB process showed a 25% decrease in ultimate tensile strength (UTS) and a 30% decrease in failure strain compared to the as-fabricated (ASF) material. The fatigue life for the SPF/DB specimens was approximately 50% lower than the ASF specimens. The fracture surface of the ASF specimens was very irregular accompanied by substantial fibre pull-out as compared to the planar fracture surface of the SPF/DB cycled specimens that showed negligible fibre pull-out. The large changes in the tensile strength and fatigue life due to the SPF/DB cycle are explained by a difference in the failure mechanisms occurring as a result of the SPF/DB-induced changes in the strength of the fibre/matrix interface and higher thermal residual stresses. Unreinforced titanium was also tested to study the effect of the SPF/DB cycle on the matrix static properties. Fibres were etched from the composite and then individually tested for modulus and strength. Finally, a microscopic examination of the fibre/matrix interface was performed to study the effects of the SPF/DB cycle on the interface.     

Development of SiC/TiAl composites: processing and interfacial phenomena

The needs of the aerospace industry for materials with low density and suitable mechanical properties at elevated temperature have oriented research to TiAl/SiC intermetallic matrix composites. A fabrication method specific for such composites was developed in this study. Two types of long fibers that have been used are the BP SM 1240 and Textron SCS-6 silicon carbide monofilaments. To fabricate these composites, firstly, the arc spraying conditions have been optimized by preparing sprayed Ti–48Al–2V material on a steel substrate and both types of Ti–48Al–2V/SCS-6 and Ti–48Al–2V/SM 1240 monotapes. Secondly, we have investigated the conditions under which a Ti–48Al–2V alloy powder could be consolidated and the kinetics of fiber damage by chemical reaction with the TiAl base matrix observed during consolidation. Then composite samples have been fabricated by uniaxial hot pressing of monotapes. In both composites, an interaction occurs between the fiber and the matrix and the reaction zone thickness follows approximately a parabolic growth law. It was determined that the plasma spray process is well adapted to produce titanium aluminide matrix composites. However, efforts are in progress to solve some of the problems inherent in the choice of the fiber, matrix and interfacial characteristics.     

连续碳化硅纤维增强钛基(SiCf/Ti)复合材料的制备技术及界面特性研究综述

连续碳化硅纤维(SiCf)由于具有比强度、比模量高,耐磨性、热稳定性好等性能优点,常作为增强体制备SiC纤维增强钛基复合材料。与钛合金基体相比,其具有密度更低、强度更高、疲劳蠕变性能大幅提升等优点,但横向性能却明显下降。因此,该类材料常被设计制作成单向增强性部件,广泛应用在航空航天等领域,如发动机的传动轴、整体叶环、盘类及风扇叶片等多种复合材料的结构件。碳化硅纤维增强钛基复合材料的性能主要由碳化硅纤维的性能、基体性能及纤维与基体之间的结合界面性能决定。目前批量生产的SiC纤维性能较差,界面结合状态与复合材料性能之间关系的研究开展较少,还不能为钛基复合材料构件设计提供足够的数据支持。因此,近年来研究者们主要从SiCf/Ti基复合材料力学行为的研究角度出发,探究不同基体及纤维类型、复合材料制备工艺方法、界面特性及产物对SiCf/Ti基复合材料界面结合力及破坏机制的影响,获得了大量有价值的数据,以期开发出成本低、产物稳定性好、可批量生产SiCf/Ti基复合材料的制造工艺方法。目前较为成熟的碳化硅纤维有英国DERA-Sigma公司提供的Sigma系列SiCf及美国Textron公司提供的SCS系列SiCf,后者强度最高达到6 200 MPa。SiCf/Ti基复合材料的制备工艺包括金属箔-纤维-金属箔工艺(FFF)、单层带工艺(MT)、基体-涂层纤维工艺(MCT)等,制备复合材料的工艺根据零部件的用途来定,FFF适用于制备板材等大尺寸构件,MCT适用于制备叶环、轴、管、叶片等复杂结构件。界面是增强体与基体之间的纽带和桥梁,界面结构设计、界面反应控制及反应产物均影响着界面的力学特性。在SiCf/Ti基复合材料的纤维和基体之间添加过渡层能够减缓它们之间的相互扩散及化学反应,过渡层选用反应层和惰性涂层组成的双层涂层较好。界面反应产物受涂层成分、基体组织、复合和热处理工艺、环境因素等的影响,增强纤维及基体性能、优选制备工艺、控制界面反应及产物有利于提高复合材料的力学性能。本文总结了连续SiC纤维(SiCf)增强钛基复合材料的应用研究现状,详述了SiCf/Ti基复合材料的钛合金基体材料、SiCf的种类及性能,SiCf与SiCf/Ti基复合材料的制备方法,分析了SiCf/Ti基复合材料界面结构设计及反应产物,阐明了界面力学特性与复合材料性能的关系,指出国内SiCf/Ti基复合材料发展的重点应放在高性能SiC纤维的研究与开发、界面层设计及界面与性能的关系以及复合材料分析检测手段三个方面,为SiCf/Ti基复合材料的制备及其今后的实际应用提供了参考。     

Finite-element analysis of selectively SiC fibre-reinforced titanium composites

Finite element analysis has been applied to selectively SiC fibre reinforced titanium matrix composites. The influence of thermal residual stress on the behaviour of clad titanium fibre composites with initial unbridged sharp cracks under external loading has been investigated. Through-thickness stress distributions have been obtained for the composites as a function of loading configuration, fibre volume fraction, cladding thickness and composite thickness. It has enabled a quantitative understanding of the premature delamination of clad titanium fibre composites ahead of mode I crack that has been observed experimentally. Potential solutions to the problem have been suggested based on finite element predictions of the variation of tensile through-thickness stress close to the transition boundary between cladding material and composite as a function of fibre volume fraction and, especially, with thickness of the cladding layers.     

High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion

Pure titanium matrix composite reinforced with carbon nanotubes (CNTs) was prepared by spark plasma sintering and hot extrusion via powder metallurgy process. Titanium (Ti) powders were coated with CNTs via a wet process using a zwitterionic surfactant solution containing 1.0, 2.0 and 3.0 wt.% of CNTs. In situ TiC formation via reaction of CNTs with titanium occurred during sintering, and TiC particles were uniformly dispersed in the matrix. As-extruded Ti/TiCs composite rods were annealed at 473 K for 3.6 ks to reduce the residual stress during processing. After annealing process, the tensile properties of the composites were evaluated at room temperature, 473, 573 and 673 K, respectively. Hardness test was also performed at room temperature up to 573 K with a step of 50 K. The mechanical properties of extruded Ti/CNTs composites at elevated temperature were remarkably improved by adding a small amount of CNTs, compared to extruded Ti matrix. These were due to the TiC dispersoids originated from CNTs effectively stabilized the microstructure of extruded Ti composites by their pinning effect. Moreover, the coarsening and growth of Ti grain never occurred even though they were annealed at 573, 673 K for 36 ks and 673 K for 360 ks, respectively.