Reji John

Stress intensity factor and compliance solutions for a single edge notched specimen with clamped ends

Abstract A single edge notched geometry [SE(T)] with clamped ends is well suited for fracture toughness and fatigue crack growth testing of composites. Closed form expressions for the stress intensity factor and the compliance for an SE(T) with clamped ends were developed using finite element analysis. Using these expressions, automated crack growth tests were conducted on a monolithic Ti-1100 and a [90]8 SCS-6/Ti-24Al-l 1Nb composite. The crack growth behavior, characterized in terms of the crack growth rate versus the applied stress intensity factor range, correlated well with available data obtained using the compact tension geometry. The results of this investigation indicate that the SE(T) geometry with clamped ends can be successfully used for fracture and fatigue crack growth testing of monolithic and composite specimens.

Weight function for a single edge cracked geometry with clamped ends

A single edge cracked geometry with clamped ends is well suited for fracture toughness and fatigue crack growth testing of composites and thin materials. Analysis of fiber bridging phenomenon in the composites and determination of stress intensity factors due to non-uniform stress distributions such as residual and thermal stresses generally require the use of a weight function. This paper describes the development and verification of a weight function for the single edge cracked geometry with clamped ends. Finite element analyses were conducted to determine the stress intensity factors (K) and crack opening displacements (COD) due to different types of stress distributions. The weight function was developed using the K and COD solution for a constant stress distribution. K and COD predicted using this weight function correlated well with the finite element results for non-uniform crack surface stress distributions.

Determination of Crack Bridging Stresses from Crack Opening Displacement Profiles

This paper discusses the development of an optimization procedure to deduce the bridging stress from the crack opening displacements (COD) measured during fatigue crack growth. Finite element analysis was conducted using the center-cracked geometry to verify the optimization procedure. The proposed procedure successfully predicted the bridging stress distributions with zero stresses at the crack tip and the bridging stress distributions with non-zero stresses at the crack tip. The results also showed that COD measurements spaced at ≈ 0.8-1.0 mm are sufficient for reliable prediction of bridging stresses. Accurate prediction of bridging stresses near the crack tip required COD data within ≈ 0.1 mm from the crack tip. The application of the proposed procedure to a metal matrix composite (SCS-6/TIMETAL®21S) is also discussed.

Fatigue crack propagation parallel to fibers in unidirectionally reinforced SCS-6/Timetal®21S

The primary weakness of a component made of unidirectionally reinforced SCS-6/Timetal{reg_sign}21S 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. The previous investigations did not investigate the effect of applied stress ratio on the crack growth behavior of the composite. This paper describes extensive characterization of the fatigue crack growth parallel to fibers in unidirectional SCS-6/Timetal{reg_sign}21S using two geometries, three applied stress ratios and closure measurements.

Crack-Bridging Effects in Notch Fatigue of SCS-6/TIMETAL 21S Composite Laminates

Fatigue tests of middle-hole tension specimens of SCS-6/TIMETAL 21S composite (silicon-carbide fibers reinforcing a matrix of Ti-15Mo-2.6Nb-3Al-0.2Si alloy) were performed on three laminate architectures: unidirectional, cross ply, and quasi-isotropic. Specimens were tested over a range of stress levels, and fatigue damage was documented in situ by macrophotography and direct-current electric potential drop measurements. Typically, failure evolved by the formation of a few dominant cracks at the notch that propagated into the composite matrix and, in many instances, were substantially affected by unbroken fibers bridging the cracks. Fractographic and failure mode characterization revealed key differences in the effectiveness of crack bridging in the three laminates. A shear-lag crack-bridging model was shown to correlate crack growth data in the laminates based on an empirical value of fiber/matrix interfacial shear stress. Crack-bridging stress distributions were predicted using the shear lag model and verified by comparing the predicted crack opening displacement profiles with measurements made using a laser interferometric displacement gage system. Implications of the results are discussed with respect to the potential use of these materials in practical structural applications.

Prediction of creep-rupture life of unidirectional titanium matrix composites subjected to transverse loading

Titanium matrix composites (TMCs) incorporating unidirectional fiber reinforcement are considered as enabling materials technology for advanced engines which require high specific strength and elevated temperature capability. The resistance of unidirectional TMCs to deformation under longitudinally applied sustained loading at elevated temperatures has been well documented. Many investigators have shown that the primary weakness of the unidirectional TMC is its susceptibility to failure under very low transverse loads, especially under sustained loading. Hence, a reliable model is required to predict the creep-rupture life of TMCs subjected to different transverse stress levels over a wide range of temperatures. In this article, we propose a model to predict the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture behavior of the corresponding fiberless matrix. The model assumes that during transverse loading, the effective load-carrying matrix ligament along a row of fibers controls the creep-rupture strength and the fibers do not contribute to the creep resistance of the composite. The proposed model was verified using data obtained from different TMC fabricated using three matrix compositions, which exhibited distinctly different types of creep behavior. The results show that the creep-rupture life of the transverse TMC decreases linearly with increasing ratio of the fiber diameter to the ply thickness. The creeprupture life is also predicted to be independent of fiber spacing along the length of the specimen.

Bridging fiber stress distribution during fatigue crack growth in [0]4 SCS-6/timetal®21S

Abstract A procedure was developed to determine the bridging fiber stress distribution from crack opening displacements measured during crack growth. The bridging fiber stress range is near-linear along the crack and non-zero stresses at the crack tip. The magnitude of the bridging fiber stress range is proportional to the applied stress range.

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.

Rupture life of unidirectionally reinforced titanium matrix composites subjected to sustained transverse loading

Abstract A new model was developed to predict the creep-rupture life of continuously reinforced titanium matrix composite subjected to sustained transverse loading. The proposed model assumes that the maximum net-section stress in the matrix ligament dictates the rupture life of the transverse composite. A good correlation was observed between the predictions of the net-section model and the measured creep-rupture life. The failure depends on the ratio of fiber diameter to ply thickness and the failure is independent of fiber spacing parallel to the loading direction. The proposed model predicted a reasonable lower bound solution and should be an useful conservative design tool The Wright-Crossman (W-C) model was found to be non-conservative. The verification of the net-section model using data from other TMC systems is discussed elsewhere [9].

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.