An Orowan-based deformation model for layered metallic materials is presented and used to calculate the stress-strain behavior for two deformation modes. This model assumes that layer thicknesses are sufficiently small so that single rather than multiple dislocation pileups form. Deformation then proceeds by increasing the density of single dislocation pileups. Furthermore, it is assumed that the controlling stress for plastic deformation is that to propagate a tunneling dislocation loop inside an embedded elastic-plastic layer. Initially, the resolved stress required to propagate an isolated tunneling loop does not depend on whether the loop shears the layer perpendicular to an interface or stretches it parallel to an interface. At larger strains, the tunneling arrays become sufficiently dense such that local dislocation interaction changes the line energy of a tunneling dislocation. As a result, the elastic-plastic layers may exhibit modest softening when sheared or substantial hardening when stretched. When the elastic-plastic layers are embedded into a multilayered specimen with alternating elastic-only layers, no macroscopic strain softening is observed. However, the predicted macroscopic stress-strain curves for stretching and shearing are significantly different in their dependence on layer thickness.

In this study the failure of the ductile layers in laminated composite systems was studied numerically. The results indicate that similar maximum stress values develop in the ductile layers as in the fracture test of the same ductile material if the crack tip in the brittle layer is already at the interface. For nondebonding interfaces brittle behavior of the ductile layers is dependent upon the extent of the cracks and the fracture characteristic of the brittle layers.

Neutron diffraction was used to investigate the residual stress distribution in an axisymmetric A12O3-Ni joint bonded with a 40 vol%A12O3-60 vol%Ni composite layer. A series of measurements was taken along the axis of symmetry through the A12O3 and composite layers. It is shown that after taking into account the finite neutron diffraction sampling volume, both the trends and peak values of the experimentally determined strain distribution were in excellent agreement with calculations of a simple finite element model, where the rule-of-mixtures approach was used to describe the constitutive behavior of the composite interlayer. In particular, the predicted steep strain gradient near the interface was confirmed by the experimental data.

The mechanical properties of intermetallic / metallic microlaminates were studied by determining the fundamental composite properties that control the fracture behavior: namely, the stress-displacement functions of the metal layers. Finite element methods were used to model the stress-displacement function of a constrained metal layer and to examine the effect of constituentproperties, residual stress, offset cracks in adjacent intermetallic layers, and debonding inclusions in the metal layer. Finally, FEM models representative of four specific microlaminates weredeveloped and the results were compared to experimentally determined σ(u)'s for those composites. Determining these fundamental composite properties and showing how they control the mechanical behavior gives insight into the optimum design of this composite system.

Carbon nitride/titanium nitride composite coatings have been deposited on Si (100) and 7059 coming glass by in-situ pulsed laser deposition technique. A pulsed laser (λ = 248 nm) has been used to ablate the both pyrolytic graphite and TiN targets. It has been shown that TiN provides a lattice-matched structural template to seed the growth of carbon nitride crystallites (W. D. Sproul et. al., Appl. Phys. Lett. Vol. 67, 203–205; 1995). This paper describes the same approach to grow carbon nitride composite coatings with varying thicknesses of buffer layer and carbon nitride films at different temperatures and pressures. Our preliminary results show the superior mechanical properties (hardness and modulli). The films have been characterized by Xray diffractometer, SEM, FTIR and Raman spectroscopic techniques.

The design of multilayered ceramic composites is reviewed, with the aim of relating the properties that can be achieved to the microstructure of the composite. Limitations on some properties, such as damage tolerance and strength, are discussed. Failure mechanism maps that define some of these limits are given.

The fracture toughness of laminated metal composites consisting of alternating layers of a metal matrix composite (Al6090/SiC/25p) and a monolithic aluminum alloy (Al5182) has been studied as a function of the volume fraction of the component materials. Finite element simulations of the fracture toughness tests have been used to study the mechanisms of crack growth and extrinsic toughening. The mechanisms responsible for toughening in laminated metal composites are described.

Discontinuously reinforced aluminum (DRA) composites have been processed to contain discrete regions of unreinforced aluminum with the objective of enhancing the damage tolerance. The effects of changes in the ductile phase size, shape and strength as well as the SiCp reinforcement distribution on the toughness were studied. The incorporation of the ductile phase can increase the crack growth resistance of the DRA composite. In such cases, stable crack propagation (i.e. R curve behavior) is observed in contrast to the behavior of the conventional DRA composite which fails catastrophically under the conditions tested. The level of toughening is affected by the size and mechanical properties of the ductile phase as well as the orientation and shape of the ductile regions with respect to the test geometry (i.e. crack arrestor vs crack divider).

Discontinuously reinforced aluminum (DRA) composites with enhanced fracture toughness have recently been developed at ALCOA. The approach consists of producing a composite microstructure in which discrete ductile phases have been incorporated into the DRA through traditional powder processing routes. In the present paper, the high strain rate behavior of these toughened composites is investigated by obtaining (i) the dynamic flow characteristics at various levels of elevated strain rates using a split Hopkinson compression bar, and (ii) energy absorption during dynamic crack initiation and crack propagation using three-point bend specimens loaded on a modified Hopkinson bar configuration.

The mechanisms contributing to the fracture resistance of refractory metal intermetallic composites containing a BCC metallic phase (niobium) were investigated using model Nb-Si laminates and in situ composites. The controlling influence of ductile phase yield strength and fracture behavior were investigated by varying laminate processing parameters, and/or altering temperatures and applied strain rates during fracture experiments on all materials. The fracture behavior of “ductile” constituents were found to be influenced by phase grain size, solid solution content, constraint (as influenced by interfacial bond strengths), and the testing condition (high strain rates and low temperatures). The measured fracture resistance, when compared to theoretical models, was shown to be controlled by the “toughness” of the “ductile” phase and independent of the fracture behavior promoted (cleavage and ductile). The loss in ductility due to cleavage by high constraint, high strain rates and/or low temperatures was compensated by high yield and cleavage fracture stresses in order to provide a level of toughening similar to that contributed by ligaments which failed with lower yield stresses and greater strains.

A brittle intermetallic, Nb3Al, reinforced with a ductile metal, Nb, has been used to investigate the resistance curve and cyclic fatigue behavior of a relatively coarse laminated composite. With this system, the toughness of Nb3Al was found to increase from ∼1 MPa√m to well over 20 MPa√m after several millimeters of stable crack growth; this was attributed to extensive crack bridging and plastic deformation within the Nb layers in the crack wake. Cyclic fatigue-crack growth resistance was also improved in the laminate microstructures compared to pure Nb3Al and Nb-particulate reinforced Nb3Al composites with crack arrester orientations in the laminate providing better fatigue resistance than either the matrix or pure Nb.

JIc measurements were performed on a SiCp/Al-7093 MMC with controlled heat treatments, and the damage mechanisms were evaluated to understand the influence of microstructural parameters on the fracture toughness and crack resistance behavior. The deformed materials showed widely different damage and flow localization for different matrix microstructures. In an effort to improve fracture toughness, large Al particles were incorporated into the powder-metallurgy based MMC, and extruded to obtain pancake shaped Al phases. In the extruded condition, the effect of Al particles on the crack initiation toughness was negligible. However, significant improvement in the toughness was observed when the extruded material was further rolled. These issues are discussed in the context of observed deformation and damage mechanisms.

The fabrication of aluminum-aluminide layered composites by reactive bonding of elemental Al and Ni foils was investigated. It was observed that after hot-pressing, thin Ni foils were converted to NiAl. The as-processed Al-NiAl layered structure could be heat-treated to produce an equilibrium Al-Al3Ni layered composite. Tensile tests revealed that composites could be produced that failed in a “tough” manner and were stronger and stiffer than aluminum.

In this study the behavior of stationary interface cracks in layered materials at creep regime in plane-strain condition and pure opening dominated mode-I load state was studied numerically. The results indicate that the introduction of a transitional layer to the interface of the elastic-creeping bimaterials significantly elevates the stress values ahead of the interface cracks under identically applied load levels at creep regime.

A constitutive model for deformation of a novel laminated metal composite (LMC) which is comprised of 21 alternating layers of Al 5182 alloy and Al 6090/SiC/25p metal matrix composite (MMC) has been proposed. The LMC as well as the constituent or neat structures have been deformed in uniaxial tension within a broad range of strain-rates (i.e. 10−5 to 100 s−1) and homologous temperatures (i.e. 0.8 ≥ 0.95 Tm). The results of these experiments have led to a thorough characterization of the mechanical behavior and a subsequent semiempirical constitutive rate equation for both the Al 5182 and Al 6090/SiC/25p when tested monolithically. These predictive relations have been coupled with a proposed model which takes into account the dynamic load sharing between the elastically stiffer and softer layers when loaded axially during isostrain deformation of the LMC. This model has led to the development of a constitutive relationship between flow stress and applied strain-rate for the laminated structure.

The mechanical behavior and damage mechanisms of the Ni/TiC microlaminate composites under static and cyclic loading were investigated. The relationship between the ultimate tensile strength and the layer thickness at both room temperature and 600°C was studied. The fatigue life and the evolution of the stiffness reduction under various maximum applied stress levels were determined. The results revealed that the ultimate tensile strength linearly increased as the laminate layer thickness decreased. Also, the microlaminate exhibited a non-progressive fatigue behavior.

Fatigue crack growth has been measured in a laminated metal composite (LMC) consisting of alternating layers of AA6090/SiC/25p metal matrix composite (MMC) and AA5182 alloy. This material was tested in both as-pressed (F temper) and aged (T6 temper) conditions. Corresponding crack growth measurements were made in self-laminates of both the MMC and AA5182 materials to examine the role of the interfaces.

The LMC-T6 material has a significantly higher fracture toughness than its MMC constituent but exhibits only modest improvements in nominal fatigue crack growth resistance and a lower nominal threshold. The LMC-T6 shows high levels of crack closure which reveal poor intrinsic crack growth resistance. Self-laminated AA6090/SiC/25p MMC in the F temper has lower microhardness, lower strength and superior intrinsic crack growth resistance compared to this material in the T6 condition. The fatigue fracture surface of T6 temper MMC remains macroscopically smooth despite the development of a large amount of crack closure. The selflaminated AA5182 exhibits a significant improvement in fatigue crack growth resistance after heat treatment and a corresponding change in crack morphology from smooth to very rough. This change leads to a large increase in roughness-induced closure. The effects of heat treatment are attributed to differences in interfacial strength.

The fatigue behavior of the SCS-6 silicon carbide fiber-reinforced Ti-6Al-4V/Ti-25Al- 10Nb hybrid laminated composite was investigated at room temperature. The accumulation of fatigue damage in the form of matrix cracking was measured as a function of loading cycles and applied stress levels. The residual stiffness and residual tensile strength of the post-fatigued specimens were determined. The comparison of the crack growth behavior of the hybrid composite with both the SCS-6/Ti-6-4 and SCS-6/Ti-25-10 composites will also be discussed.

It is generally recognized that fiber-reinforced laminated composites are susceptible to damage resulting from low-velocity impacts. Over recent years, many strategies have been devised to increase the fracture toughness of resin matrix materials with the aim of improving the composites overall resistance to impact damage. One popular strategy for enhancing the fracture toughness of thermosets involves increasing its molecular weight between crosslinks which, in turn, enhances the resins ductility. In this paper, we investigate the efficiency of this toughening approach with regard to resisting damage in composite laminates subjected to lowvelocity impacts. Generic damage characteristics and mechanisms are reviewed and it is shown that two different events occur during the impact process. First, the laminate experiences a local failure which resembles a Hertzian fracture process followed by subsequent delamination between the plies. Results are presented illustrating the effects that systematically increasing the molecular weight between crosslinks of the resin has on each of these mechanisms. Also, the residual compressive strength (Compression After Impact) of the laminates made with these resins is presented.

Real-time monitoring of fatigue damage and dynamic strain in inherently smart andcontinuous unidirectional (0° and 90°) and bidirectional (0°/90°) carbon fiber epoxy-matrix composites by electrical resistance (R) measurement was achieved. Upon cyclic tension (0°) of 0° or 0°/90° composites, R (0°) decreased reversibly, while R perpendicular to the fiber layers increased reversibly, though R in both directions changed irreversibly by a small amount after the first cycle. Upon fatigue testing of the 0° composite at a maximum stress of 57–58% of the fracture stress, the peak R (0°) in a cycle irreversibly increased both in spurts and continuously, due to fiber breakage, which started at 30% of the fatigue life. For the 90° composite, R(0°) increased reversibly upon tension (0°) and decreased reversibly upon compression (0°).