Improved Fracture Toughness of a Titanium Alloy Laminate by Controlled Diffusion Bonding

The use of laminate composites containing a weak interface to increase the fracture toughness of high strength titanium alloys has been studied. Billets were fabricated from Ti-6A1-4V sheet material using a diffusion bonding process. Six billets were fabricated, each billet having an interface with different properties. Results indicate that toughness, as measured by the precracked Charpy test, may be increased when delamination or splitting of the bond occurs.

A simple model to predict the conditions necessary for delamination has been formulated. Correlations between the model and experimental results are made. The model can account for the effect of different base metal and interface material properties and thicknesses. It is seen that a thin, low yield strength interface material with a full strength diffusion bond to a high yield strength, fairly tough base metal leads to optimum composite toughness.     

A simple way to make tough diamond/metal laminates

Diamond foils are exceptionally strong yet brittle. One approach to make ceramic foils less susceptible to brittle fracture is to introduce interfaces into the material that provide pathways for crack deflection. In this study, we were able to produce strong yet tough diamond/metal laminates (DMLs) from freestanding diamond foils using a brazing process. The mechanical behavior was characterized via three-point bending (3PB) where the laminates exhibit step-like fracture. Crack deflection at interfaces induces toughening within the laminates. At approx. 3.0 MJ/m³ diamond/metal laminates exhibit more than twice the fracture energy of monolithic diamond foils while maintaining 90 % stiffness and about 70 % nominal strength. Classical laminate theory (CLT) supports the assessment of the deformation behavior and step-like fracture of diamond/metal laminates. We find that the diamond-to-metal interface plays a critical role: it must be strong enough to enable the transfer of shear stress, while being weak enough to deflect a crack.     

Modeling complex crack paths in ceramic laminates: A novel variational framework combining the phase field method of fracture and the cohesive zone model

The competition between crack penetration in the layers and cohesive delamination along interfaces is herein investigated in reference to laminate ceramics, with special attention to the occurrence of crack deflection and crack branching. These phenomena are simulated according to a recent variational approach coupling the phase field model for brittle fracture in the laminae and the cohesive zone model for quasi-brittle interfaces. It is shown that the proposed variational approach is particularly suitable for the prediction of complex crack paths involving crack branching, crack deflection and cohesive delamination. The effect of different interface properties on the predicted crack path tortuosity is investigated and the ability of the method to simulate fracture in layered ceramics is proven in relation to experimental data taken from the literature.     

Transverse compression failure of unidirectional composites

The mechanical response of unidirectional composites subject to uniaxial transverse compressive loads was measured and analyzed by finite element simulation. Consistency in failure plane orientation was observed when comparing simulated matrix shear band angle to measured crack angle. A model based on hexagonal packing of fibers was proposed and the shear band angle was shown to depend on the fiber volume fraction. The effects of strong and weak fiber–matrix interfaces were considered using models with randomly distributed fibers for a valid statistical analysis. The results of these models showed that the composite compressive strength increased with the fiber loading for the strong interface case, while the strength was independent of the fiber loading for the weak interface case because of interface debonding. POLYM. COMPOS., 36:756–766, 2015. © 2014 Society of Plastics Engineers     

Crack growth and fracture behavior of fabric reinforced polymer composites

The objective of this study is to understand and characterize crack deflection and sub-crack growth in fabric laminate composites. The theory of crack digression based on the Cook-Gordon mechanism of crack blunting and the criterion developed by Kendall were used to study the crack propagation phenomenon. A simple approach has been developed to evaluate the cohesive and adhesive fracture energies, which play a vital role in the study of strength and toughness of the fabric laminate composites. The effects of strain rate and quasi-static crack velocity on these energy values were identified. This study explored the possibility of selftoughening in an otherwise brittle composite system. Two competing mechanisms have been identified that control crack propagation in fabric laminate composites.     

Alumina/NiCu Laminate under Thermal Shock up to 1000°C: II, Prediction of the Laminate Behavior

In this investigation, a multilayered, multimaterial system with strong interface subjected to thermal shock loading was analyzed. The analysis was based on a one-dimensional spatio-temporal finite difference scheme of the temperature field, and the thermal residual stresses and zero misfit stress temperature were considered. Using a failure criterion based on crack initiation, the number of broken layers due to thermal shock and residual mechanical strength at room temperature could be predicted. Furthermore, the room temperature residual strength of the laminate as a function of thermal shock temperature was constructed, demonstrating steplike behavior. Using this model, the mechanical behavior of the alumina/NiCu laminate system subjected to thermal shock loading of up to 1000°C was predicted. The model revealed the superiority of this material system over monolithic ceramics under thermal shock conditions.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

Stress and failure analyses of scarf repaired CFRP laminates using a cohesive damage model

This study describes stress and failure analyses of tensile loaded repaired Carbon Fibre Reinforced Composite (CFRP) laminates, using scarf configuration. A numerical model including interface finite elements was used to obtain peel and shear-stress distributions in the directions tangent and normal to the scarf. These stresses were evaluated at several locations in the repair, namely in the middle of the adhesive, at interfaces between adhesive and patch, and between adhesive and parent material. Several scarf angle values were considered in the analysis. A cohesive mixed-mode damage model was also used to carry out the failure analysis, in order to assess the efficiency of the repairs, for different stacking sequences. A study was performed to evaluate the influence of the mechanical properties of the adhesive and parent laminate/adhesive and adhesive/patch interfaces on the strength and failure modes of the joint. It was concluded that the strengths of the adhesive and interfaces are more important than the fracture properties in the failure process of the repair. It was also verified that the strength of the repair increased exponentially with the scarf angle reduction.     

Damage tolerant oxide/oxide fiber laminate composites

Oxide-fiber/oxide-matrix composites were developed using non-infiltrated woven fiber layers between matrix-infiltrated fiber layers in order to achieve damage tolerant behavior. A fiber interface coating was not used. This technique enables damage tolerance in materials with strong fiber-matrix bonding and under oxidizing conditions. Fabrication of composites was carried out through a slurry infiltration technique. Slurries for fiber (Nextel™ 720, 3M) infiltration were prepared using a submicron α-Al₂O₃ powder coated with an amorphous SiO₂-layer through a sol–gel process. Hot-pressing was used to densify and bond the laminate layers together, followed by pressureless heat-treatment to allow mullite to form. Room temperature three-point bending tests were performed on as-received samples and on samples which underwent long-term annealing at high temperatures (1200–1300°C) in air. Subsequent examination revealed that due to the lack of a fiber interface coating, matrix-infiltrated fiber layers behaved in a quasi-monolithic manner with little or no crack deflection. Layers of non-infiltrated fibers, however, provided damage tolerance by deflecting cracks in the plane of the laminate and by serving as a mechanical bond between matrix-infiltrated layers. The laminate composites demonstrate reasonable room-temperature fracture strength both in the as-received state (88 MPa) and after exposure to 1300°C air for 200 h (72 MPa) along with extensive fracture deflection through the layers of non-infiltrated fiber. Composite properties, specifically fracture strength and damage tolerance, can be tailored by varying lay-up and processing parameters such as fiber-matrix ratio and type of fiber weave.     

Fracture of model concrete: 2. Fracture energy and characteristic length

The specific fracture energy G_F was measured in six types of simple concrete: all from the same matrix. The aggregates were spheres of the same diameter (strong aggregates, that debonded during concrete fracture, and weak aggregates, able to break); three kinds of matrix-aggregate interface (weak, intermediate and strong) were used. All in all, 55 test results are reported. These results are intended to be used as an experimental benchmark for checking numerical models of concrete fracture.A meso-level analysis of these results showed a correlation between the measured G_F values and the properties of the matrix, aggregates and interfaces, particularly with the actual area of the fracture surface. The strength of the matrix-aggregate interface correlates quite well with G_F, and concrete ductility, measured by means of the characteristic length, correlates also with the strength of the matrix-aggregate interface.     

Al2O3/TiC/(MoSi2+Mo2B5) multilayer composites prepared by tape casting

A new multilayer composite (MLC) with a super-plastic layer, a hard layer and a weak interface was proposed in this paper. The hard layer can provide the MLC high temperature strength, the super-plastic layer can deform plastically at high temperatures and disperse the applied stress and stop the advance of the crack, and the weak interface can deflect the propagating crack at room temperature. Such MLC was prepared by tape casting in the Al₂O₃/TiC/MoSi₂+Mo₂B₅ system in the present work. In this system Al₂O₃ was as the hard layer, MoSi₂+Mo₂B₅ was as the super-plastic layer and TiC was as the weak interface. The microstructures and the stress-displacement behaviors of the MLCs were investigated. It was found that such design is effective on the increase of fracture energy both at room temperature and at high temperatures, and the strength at high temperatures could be remained in a relatively high level. ©     

Impact of Interface Heterogeneity on Joint Fracture

The effects of heterogeneities (weak zones in particular) in adhesive joints and their importance on overall fracture properties are relatively unknown, but doubtlessly they may be crucial in many applications. Using a model heterogeneous adhesive bond, represented by a given mixture of regions of strong and weak adhesion, we have studied the influence of interface variability on overall fracture energy (global energy release rate). Adopting the original Griffith-Irwin arguments, we have employed a simple, fracture scaling law. By varying relative proportions of the weak and strong interfaces, a nonlinear evolution of fracture energy was observed. This was contrary to expectations, cf. rules of mixtures. Inspired by rheological models (Maxwell, Voigt/Kelvin, etc.), an appropriate model was found empirically.     

Cracks at adhesive interfaces

The application of Griffith's energy balance argument to cracks at adhesive interfaces is studied. Adhesive interfaces are generally brittle, representing the simplest form of fracture mechanics geometry because cracks are constrained to travel along the interface, giving a defined crack path which eases analysis. Experimentally, such cracks may be propagated along the interface between optically smooth rubber pieces, and measured through the transparent material. The development of adhesive fracture test-pieces since Griffith's time reveals difficulties in his reasoning, and allows improved understanding of the energy balance method. The most important conclusion is that stress does not normally enter the cracking criterion. It is demonstrated experimentally that stress may remain constant while the crack criterion changes. The strength of an adhesive interface is shown to be a meaningless parameter; instead, the work of adhesion, or adhesive energy, which is the work of adhesion together with energy losses, should be used to define the behaviour of cracks at interfaces.     

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800?℃ and C₂H₄/H₂ ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection     

The effect of glass-resin interface strength on the impact strength of fiber reinforced plastics

The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.     

界面渗透率对厚复合材料层板流动-压实过程影响的数值分析

基于有效应力原理与达西渗流定律,建立了厚复合材料层板流动-压实过程的多场耦合有限元数值模型,通过与厚单向板试验结果的对比,验证了模型的正确性。建立了含界面层的厚正交层合板流动-压实计算模型,分析了垂直于层间界面方向的界面渗透率对正交层合板流动-压实过程的影响。通过与同等厚度单向板的分析结果对比表明,当不同方向铺层层间界面渗透率高时,厚正交层合板的流动-压实过程几乎与相同厚度单向板的流动-压实过程相同。但当层间界面的渗透率低时,会阻碍内部树脂的流动,导致正交层合板内部纤维体积含量提升慢,且越靠近内部,界面渗透率的影响越明显,最终在界面处纤维含量出现明显的跳跃分布。     

界面类型对三维Nextel 720纤维增韧莫来石陶瓷基复合材料力学性能的影响

采用化学气相渗透工艺在Nextel 720纤维表面制备PyC和PyC／SiC两种涂层，然后以正硅酸乙酯和异丙醇铝作为先驱体，以先驱体浸渗热解法制备三维Nextd 720纤维增韧莫来石陶瓷基复合材料，比较分析了两种涂层复合材料的力学性能和断裂模式。结果表明：具预先涂覆PyC的复合材料中纤维与基体直接接触，发生烧结形成强结合界面，复合材料脆性断裂，三点抗弯强度仅56MPa。PyC／SiC涂层则演化为间隙／SiC复合界面层，SiC成为阻滞纤维与基体接触的阻挡层，间隙保证了纤维拔出，复合材料韧性断裂且三点抗弯强度高达267．2MPa。     

Multilayered ceramic composites – a review

With the aim of improving the toughness of ceramic materials, laminated composites have been successfully developed since Clegg et al. (1990) inserted weak interfaces using very thin graphite layers between silicon carbide sheets and obtained a composite that exhibited non-catastrophic fracture characteristics. The weak interface must allow the crack to deviate either by deflection or delamination; in other words, the interface must exhibit a fracture resistance that is lower than that of the matrix layer. In parallel, ceramic laminated composites with strong interfaces were developed in which the residual tensile and compressive stresses appeared in alternate layers during cooling after sintering. These composites are prepared by stacking ceramic sheets produced by lamination or tape casting or by the sequential formation of layers by slip casting, centrifugation or electrophoretic deposition. The techniques may be combined to obtain a composite with the most adequate configuration. This work presents a review about the obtainment of multilayered ceramic composites as a toughening mechanism of ceramic plates.     

Homogenized Finite Element Analysis on Effective Elastoplastic Mechanical Behaviors of Composite with Imperfect Interfaces

A three-dimensional (3D) representative volume element (RVE) model was developed for analyzing effective mechanical behavior of fiber-reinforced ceramic matrix composites with imperfect interfaces. In the model, the fiber is assumed to be perfectly elastic until its tensile strength, and the ceramic material is modeled by an elasto-plastic Drucker-Prager constitutive law. The RVE model is then used to study the elastic properties and the tensile strength of composites with imperfect interfaces and validated through experiments. The imperfect interfaces between the fiber and the matrix are taken into account by introducing some cohesive contact surfaces. The influences of the interface on the elastic constants and the tensile strengths are examined through these interface models.