Jack Telesman

Modeling of crack bridging in a unidirectional metal matrix composite

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, ΔKeff, was calculated using both models to correlate the measured crack growth rate in the composite. The calculated ΔKeff from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

Abstract An investigation of the fatigue crack growth (FCG) behavior of PWA 1480 single crystal nickel base superalloy was conducted. Typical Paris region behavior was observed above a δK of 8 MPa √m . However, below that stress intensity range, the alloy exhibited highly unusual behavior. This behavior consisted of a region where the crack growth rate became essentially independent of the applied stress intensity. The transition in the FCG behavior was related to a change in the observed crack growth mechanisms. In the Paris region, fatigue failure occurred along {111} facets, however at the lower stress intensities, (001) fatigue failure was observed. A mechanism was proposed, based on barriers to dislocation motion, to explain the changes in the observed FCG behavior. The FCG data were also evaluated in terms of a recently proposed stress intensity parameter, K rss . This parameter, based on the resolved shear stresses on the slip planes, quantified the crack driving force as well as the mode I ΔK , and at the same time was also able to predict the microscopic crack path under different stress states.

Fatigue crack growth study of SCS6/Ti-15-3 composite

A study was performed to determine the fatigue crack growth (FCG) behavior and the associated fatigue damage processes in a (0)(8) and (90)(8) oriented SCS(6)/Ti-15-3 composite. Companion testing (CT) was also done on identically processed Ti-15-3 unreinforced material. The active fatigue crack growth failure processes were very similar for both composite orientations tested. For both orientations, fatigue crack growth was along the fiber direction. It was found that the composite constituent most susceptible to fatigue damage was the interface region and in particular the carbon coating surrounding the fiber. The failure of the interface region lead to crack initiation and also strongly influenced the FCG behavior in this composite. The failure of the interface region was apparently driven by normal stresses perpendicular to the fiber direction. The FCG rates were considerably higher for the (90)(8) oriented CT specimens in comparison to the unreinforced material. This is consistent with the scenario in which the interface has lower fatigue resistance than the matrix, causing lower composite fatigue resistance. The FCG rates of the (0)(8) composite could not be directly compared to the (90)(8) composite but were shown to increase with an increase in the crack length.

A study of spectrum fatigue crack propagation in two aluminum alloys—II. Influence of microstructures

Abstract An investigation to determine the important metallurgical factors that influence both constant amplitude and spectrum crack growth behavior in aluminum alloys was performed. The effect of microstructural features such as grain size, inclusions, and dispersoids was evaluated. It was shown that at lower stress intensities, the I/M 7050 alloy showed better FCP resistance than P/M 7091 alloy for both constant amplitude and spectrum testing. It was suggested that the most important microstructural variable accounting for superior FCP resistance of 7050 alloy is its large grain size. It was further postulated that the inhomogeneous planar slip and large grain size of 7050 limit dislocation interactions and thus increase slip reversibility which improves FCP performance. The hypothesis was supported by establishing that the cyclic strain hardening exponent for the 7091 alloy is higher than that of 7050.

Fatigue resistance of a hot corrosion exposed disk superalloy at varied test temperatures

The fatigue resistance of the hot corrosion pitted ME3 disk superalloy was investigated. Low cycle fatigue specimens were subjected to hot corrosion exposures that produced pits on the gage sections. These specimens were tested at varied temperatures and strain ranges. Corrosion pitting influenced fatigue life and failure mode by varying degrees, depending on temperature and strain range. As observed through interrupted tests, fatigue cracks initiated at a smaller fraction of life for high-temperature tests, in comparison to that at low temperatures. Correspondingly, the crack initiation failure mode changed significantly with test temperature. While cracks initiated from the hot corrosion pits for all test conditions, at 704 °C the intergranular initiation failure mode was dominant, whereas at the lower temperatures cracks initiated within the pits from crystallographic facets. Finite element analyses were performed to quantify the effect of varying pit dimensions and spacing on elastic stress concentration. The highest stress concentration was calculated to occur at the narrow ligaments between overlapping hot corrosion pits. Increasing the number of overlapping pits did not further add to the stress concentration. There was good qualitative agreement between the calculated stress concentrations and the location of crack initiations for tests conducted at 704 °C but not for tests conducted at 204 °C.

Effects of Long Term Exposures on Fatigue of PM Disk Superalloys

Turbine disks in some advanced engine applications may be exposed to temperatures above 700°C for extended periods of time, approaching 1000 h. These exposures could affect near-surface composition and microstructure through formation of damaged and often embrittled layers. The creation of such damaged layers could significantly affect local mechanical properties. Powder metal disk superalloys LSHR and ME3 were exposed at temperatures of 704, 760, and 815°C for times up to 2020 h, and the types and depths of environmental attacked were measured. Fatigue tests were performed for selected cases at 704 and 760°C, to determine the impacts of these exposures on properties. Fatigue resistance was reduced up to 98 % in both superalloys for some exposure conditions. The changes in surface composition and phases, depths of these changed layers, failure responses, and failure initiation modes were compared.

A study of spectrum fatigue crack propagation in two aluminum alloys—I. Spectrum simplification

An investigation of the fatigue crack propagation FCP behavior of two aluminum alloys is performed to simulate spectrum loading conditions found at critical locations in high performance fighter aircraft. Negative loads are shown to be eliminated for the tension-compression spectrum for low to intermediate maximum stress intensities, and load interactions are found to be more significant at higher stress intensities and with more plasticity at the crack tip. In the second part, the influence of microstructural features including grain size, inclusions, and dispersoids on constant amplitude and spectrum crack growth behavior in aluminum alloys is studied. At low stress intensities the I/M alloy demonstrated better FCP resistance than the P/M 7091 alloy for both constant amplitude and spectrum testing, and the inhomogeneous planar slip and large grain size of 7050 limit dislocation interactions, thereby improving FCP performance.

Fatigue Crack Growth and Crack Bridging in SCS-6/Ti-24-11

Interfacial damage induced by relative fiber/matrix sliding was found to occur in the bridged zone of unidirectional SCS-6/Ti-24Al-11Nb intermetallic matrix composite specimens subjected to fatigue crack growth conditions. The degree of interfacial damage was not uniform along the bridged crack wake. Higher damage zones were observed near the machined notch in comparison to the crack tip. The interfacial friction shear strength tau(sub f) measured in the crack wake using pushout testing revealed lower values than the as-received interface. Interfacial wear also reduced the strength of the bridging fibers. The reduction in fiber strength is thought to be a function of the magnitude of relative fiber/matrix displacements ind the degree of interfacial damage. Furthermore, two different fiber bridging models were used to predict the influence of bridging on the fatigue crack driving force. The shear lag model required a variable tau(sub f) in the crack wake (reflecting the degradation of the interface) before its predictions agreed with trends exhibited by the experimental data. The fiber pressure model did an excellent job in predicting both the FCG data and the DeltaCOD in the bridged zone even though it does not require a knowledge of tau(sub f).

An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys

Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased ∆Kth values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions.

Accelerated crack growth rate at low ?K in a single crystal superalloy

The low Delta K crack growth behavior of a single crystal of the PWA 1480 nickel-based superalloy was investigated. The crystal was tested in the near (100) orientation with the side faces being in the near (001) orientation. Although in the higher Delta K region the fatigue crack growth (FCG) behavior is rather normal, at Delta K of about 8 MPa sq rt m, a transition occurs where the FCG rate appears to be independent of Delta K. This region is found to continue until Delta K of about 2.5 MPa sq rt m, where the FCG rate again decreases with decreasing Delta K.