James M. Larsen

An evaluation of fiber-reinforced titanium matrix composites for advanced high-temperature aerospace applications

The current capabilities of continuous silicon-carbide fiber-reinforced titanium matrix composites (TMCs) are reviewed with respect to application needs and compared to the capabilities of conventional high-temperature monolithic alloys and aluminides. In particular, the properties of a firstgeneration titanium aluminide composite, SCS-6/Ti-24Al-11Nb, and a second-generation metastable beta alloy composite, SCS-6/TIMETAL 21S, are compared with the nickel-base superalloy IN100, the high-temperature titanium alloy Ti-1100, and a relatively new titanium aluminide alloy. Emphasis is given to life-limiting cyclic and monotonie properties and to the influence of time-dependent deformation and environmental effects on these properties. The composite materials offer a wide range of performance capabilities, depending on laminate architecture. In many instances, unidirectional composites exhibit outstanding properties, although the same materials loaded transverse to the fiber direction typically exhibit very poor properties, primarily due to the weak fiber/matrix interface. Depending on the specific mechanical property under consideration, composite cross-ply laminates often show no improvement over the capability of conventional monolithic materials. Thus, it is essential that these composite materials be tailored to achieve a balance of properties suitable to the specific application needs if these materials are to be attractive candidates to replace more conventional materials.

Cumulative-damage modeling of fatigue crack growth in turbine engine materials

Abstract Life predictions of turbine engine structural components utilize fracture mechanics principles to determine fatigue crack growth rates. Fatigue cracks grow under conditions of variable temperature, frequency, hold time, stress ratio and stress level. At elevated temperatures, time-dependent material behavior can play a significant role in the material behavior. Cumulative-damage models must account for all these variables as well as interaction effects. The earliest modeling involved interaction schemes and, primarily, time-independent material behavior. More recent work has focused on time-dependence and creep-fatigue interaction effects. A review of current modeling concepts is presented.

Thermal expansion anisotropy in a Ti-6Al-4V/SiC composite

Abstract We studied the thermal expansion behavior of a Ti–6Al–4V/35 vol.% continuous SiC fiber composite using in situ high temperature neutron diffraction (ND). The lattice expansion of constituent phases within the composite was monitored from axial (parallel to the unidirectionally aligned fibers) and transverse (perpendicular to the fibers) directions during heating from room temperature (RT) to 1170 K. The phase-specific thermal expansion of the Ti–6Al–4V matrix and SiC fibers in the composite is discussed in the context of thermal load partitioning between the matrix and fibers. In the axial direction, the matrix and the fiber share the thermal load and co-expand up to about 800–900 K, above which the thermal load transfer becomes ineffective. In the transverse direction, the matrix and fibers expand independently over the whole temperature range. Using the Schapery model (J. Comp. Mater. 2 (1968) 380) and the rule-of-mixtures (ROM), the macroscopic thermal expansion of the composite is predicted and compared with the experimental results.

Microstructure and crack-shape effects on the growth behavior of small fatigue cracks in Ti24Al11Nb

Abstract The effects of four different microstructures in the titanium aluminide alloy Ti24Al11Nb (at.%) on the fatigue crack growth behavior of small surface cracks and large cracks have been investigated. The four microstructures were a Widmanstatten basketweave, a Widmanstatten aligned colony, an equiaxed primary α2 in a Widmanstatten matrix and a completely equiaxed α2 structure. Small cracks were found to develop arbitrary shapes owing to the effects of microstructure. The crack shapes (aspect ratios) were measured using a laser interferometric and photomicroscopic system, and these measurements allowed accurate calculation of crack growth rates at the surface position as well as at the depth position for the surface cracks. After accounting for the continuous variation in crack shape in crack growth rate calculations, the trends in small-crack growth rates agreed reasonably well with the corresponding large-crack growth rates. While the crack growth rates at depth positions for small cracks correlated well with large-crack data in the basketweave microstructure, crack growth rates at surface positions correlated well with the corresponding large-crack data in the other microstructures. The microstructural factors that may be responsible for this behavior are discussed.

Fatigue of unidirectional SCS-6/Ti-24Al-11Nb composite containing a circular hole

Fatigue tests of notched unidirectional titanium aluminide composite, SCS-6/Ti-24Al-11Nb (at. pct), were performed to characterize the failure modes and mechanisms that control the life of this material. Using middle-hole specimens tested over a range of stress levels, fatigue damage was documented by optical photography, elastic compliance measurements, and electrical potential drop measurements. It was found that a few dominant cracks form near the notch root and propagate into the composite matrix. Chemical removal of the matrix material and metallographic sectioning of fatigue specimens examined prior to failure revealed that extensive fiber bridging of the matrix cracks dominated the fatigue life. A detailed description of the failure modes and mechanisms of damage initiating in the vicinity of a circular hole is presented, and the relative roles of crack initiation and crack growth are discussed.

Influence of mode of initiation on the growth of small surface cracks in titanium aluminides

Abstract Fatigue crack growth behaviour of small surface cracks in two α2 (Ti3Al) based titanium aluminide alloys, Ti-24Al-11Nb (at%) and Ti-25Al-17Nb-1Mo, was investigated. The primary objective was to monitor the variations in surface crack aspect ratio (a/c; a = crack depth and c = half surface length) as influenced by the microstructure in which the cracks were initiated and propagated. The continuous changes in a/c have been estimated using compliance data acquired using a laser interferometric system and surface crack length (2c) data collected using a photomicroscopic system. The changes in a/c have been used in the calculations of stress intensity factor range (ΔK) and crack growth rate. The aspect ratios of small cracks in Ti-24Al-11Nb, primarily initiated at grain boundaries, varied significantly as the cracks grew, owing to the influence of local microstructure. In the Ti-25Al-17Nb-1Mo alloy, cracks initiated from electro-discharge machined (EDM) notches grew with a nearly semicircular shape. The differences in crack aspect ratio variations between the two materials appear to be due to the differences in the microstructural environment through which the cracks grew. The growth rates of large cracks were found to be similar in both alloys. The differences in crack growth behaviour of small versus large cracks appear to be due to the reduced closure levels of small cracks relative to large cracks.

Probabilistic Modeling of Residual Stress Data in IN100

Compressive residual stresses (RS) are known to be beneficial in extending the fatigue life of metal components. The RS are imparted into the material through a plastic deformation process such as shot peening, low plasticity burnishing, laser shot peening, ultrasonic peening, among others with shot peening being the most prevalent due to simplicity and cost. Evaluation of the effect of residual stresses on fatigue life is challenging due to the uncertainty in the magnitude of the residual stress resulting from the deformation process and possible relaxation due to mechanical and thermal cycling. As such, a probabilistic approach is undertaken to characterize the variation in RS in a powder nickel material, IN100, subjected to shot peening. RS data from X-ray diffraction of six replicate specimens were analyzed and a three parameter probabilistic model determined through nonlinear regression. The parameters of the regression model can be used as random variables and form the basis for a probabilistic model of residual stress.

Understanding Materials Uncertainty for Prognosis of Advanced Turbine Engine Materials

Abstract : Materials damage prognosis offers the opportunity to revolutionize life management of advanced materials and structures through a combination of improved state awareness, physically based predictive models of damage and failure, and autonomic reasoning. Historically, lifetime and reliability limits for advanced fracture-critical turbine engine materials have been based on expected worst-case total life under fatigue. Recent findings in a variety of advanced propulsion alloys indicate that the life-limiting mechanisms are typically dominated by the growth of damage that begins at the scale of key microstructural features. Such behavior provides new avenues for management and reduction of uncertainty in prognosis capability under conditions that depend on damage tolerance. To examine a range of sources of uncertainty in behavior and models of such behavior, this paper explores the following topics: (1) Duality in Fatigue, (2) Relaxation of Surface Residual Stresses in Laboratory Specimens, (3) Relaxation of Bulk Residual Stresses in Components, (4) Nonlinear Acoustic Parameter for the Detection of Precursor Fatigue Damage, (5) Elevated Temperature Fretting Fatigue, (6) Crack Growth under Spin Pit Environments, and (7) Crack Growth Under Variable Amplitude High Cycle Fatigue (HCF) Loading. Based on the findings, we outline avenues for further technology development, maturation, validation, and transition of mechanistically based models that have the potential to reduce predictive uncertainty for current and future materials.

Prediction of transverse fatigue behavior of unidirectionally reinforced metal matrix composites

Unidirectionally reinforced metal matrix composites (MMC) are targeted for use in many aerospace applications which require high specific strength and stiffness at elevated temperatures. Such applications include blings and disks. The primary weakness of a component made of unidirectionally reinforced MMC is its susceptibility to transverse loads. The strength of the component in the transverse direction is significantly lower than that in the longitudinal direction under monotonic, sustained and fatigue loading conditions. Hence, replacement of monolithic components with MMC components requires that the transverse strength of the MMC should be predicted accurately. This paper discusses the applicability of a net-section based model to predict the fatigue behavior of [909] MMC under transverse loading.