Jeffrey Calcaterra

Development of an Improved High Cycle Fatigue Criterion

An integrated computational-experimental approach for prediction of total fatigue life applied to a uniaxial stress state is developed. The approach consists of the following elements: (1) development of a vibration based fatigue testing procedure to achieve low cost bending fatigue experiments and (2) development of a life prediction and estimation implementation scheme for calculating effective fatigue cycles. A series of fully reversed bending fatigue tests were carried out using a vibration-based testing procedure to investigate the effects of bending stress on fatigue limit. The results indicate that the fatigue limit for 6061-T6 aluminum is approximately 20% higher than the respective limit in fully reversed tension-compression (axial). To validate the experimental observations and further evaluate the possibility of prediction of fatigue life, an improved high cycle fatigue criterion has been developed, which allows one to systematically determine the fatigue life based on the amount of energy loss per fatigue cycle. A comparison between the prediction and the experimental results was conducted and shows that the criterion is capable of providing accurate fatigue life prediction.

Investigation of variable amplitude loading on fretting fatigue behavior of Ti–6Al–4V☆

Abstract The fretting fatigue behavior of Ti–6Al–4V under both constant amplitude and variable amplitude loadings was investigated. Constant amplitude fretting fatigue tests were conducted at a frequency of 200 Hz as well as at 1 Hz, while the variable amplitude loading combined the interaction of these two frequencies. The axial load ratios ranged from nominal values of 0.1 to nominal values of 0.8. Fretting fatigue lives at the higher frequency were found to be less than those from tests conducted at the lower frequency with a larger difference being noted at higher axial load ratios. In general, fretting fatigue resistance for the variable amplitude loading conditions fell within the scatter band of the 200 Hz constant amplitude fretting fatigue tests. Miners linear damage summation model provided a reasonable, yet slightly unconservative, prediction of the variable amplitude loading tests.

Goodman Diagram Via Vibration-Based Fatigue Testing

A new vibration-based fatigue testing methodology for assessing high-cycle turbine engine material fatigue strength at various stress ratios is presented. The idea is to accumulate fatigue energy on a base-excited plate specimen at high frequency resonant modes and to complete a fatigue test in a much more efficient way at very low cost. The methodology consists of (1) a geometrical design procedure, incorporating a finite-element model to characterize the shape of the specimen for ensuring the required stress state/ pattern; (2) a vibration feedback empirical procedure for achieving the high-cycle fatigue experiments with variable-amplitude loading; and finally (3) a pre-strain procedure for achieving various uniaxial stress ratios. The performance of the methodology is demonstrated with experimental results for mild steel, 6061-T6 aluminum, and Ti-6Al-4V plate specimens subjected to a fully reversed bending, uniaxial stress state.

A Promising New Energy-Based Fatigue Life Prediction Framework

An energy-based fatigue life prediction framework has been developed for prediction of axial and bending fatigue life at various stress ratios. The framework for the prediction of fatigue life via energy analysis was developed in accordance with the approach in our previous study which states: the total strain energy dissipated during a monotonic fracture process is a material property that can be determined by measuring the area underneath the monotonic true stress-strain curve. The framework consists of the following two elements: (1) Development of a bending fatigue criterion by observing the total strain energy of the effective volume, which is achieved by computing the total plastic strain energy with consideration of the stress gradient influence through the thickness of a specimen, in the fatigue area, during cyclic loading. A comparison between the prediction and the experimental results from 6061-T6 aluminum specimens was conducted and shows that the new energy-based fatigue criterion is capable of predicting accurate fully reversed bending fatigue life. (2) Development of a new life prediction criterion for axial fatigue at various stress ratios. The criterion was constructed by accounting for both the residual energy dissipated, monotonically, due to the mean stress, and the incorporation of the mean stress effect into the total strain energy density dissipated per cycle. The performance of the criterion was demonstrated by experimental results from 6061-T6 aluminum dog-bone specimens subjected to axial stress at various stress ratios. The comparison shows very good agreement, thus validating the capability of producing accurate fatigue life predictions.Copyright

A New Energy-Based Uniaxial Fatigue Life Prediction Method for a Gas Turbine Engine Material

A new energy-based fatigue life prediction framework for calculation of axial and bending fatigue life at various stress ratios has been developed. The purpose of the life prediction framework is to account for materials used in gas turbine engines, such as Titanium 6Al-4V, which experience an endurance stress limit as the number of cycles increase towards infinity. The work conducted to develop this energy-based framework consist of the following entities: (1) A new life prediction criterion for axial and bending fatigue at various stress ratios for Aluminum 6061-T6, (2) use of the previously developed improved uniaxial energy-based method to acquire fatigue life prior to endurance limit behavior [1], (3) and the incorporation of a statistical energy-based fatigue life calculation scheme to the uniaxial life criterion (the first entity of the framework), which is capable of constructing prediction intervals based on a specified percent confidence level. The exactitude of this work was verified by comparison between theoretical approximations and experimental results from recently acquired Al 606-T6 and Ti 6Al-4V data. The comparison shows very good agreement, thus validating the capability of the framework to produce accurate fatigue life predictions.Copyright

Residual Strength Degradation of a Titanium Matrix Composite Subjected to Elevated Temperature Fatigue

Most of the experimental fatigue studies performed on Titanium Matrix Composite (TMC) systems have subjected the test specimens to complete failure. Unfortunately, this type of investigation provides little information about the progression of damage mechanisms or the residual strength degradation of the material. To address this concern, an experimental test program was conducted on SCS-6/Ti-15-3 composite with laminate orientations of [0]8, [0/90]2s, and [0/±45/90]s. The main goal of this research was to determine the effect of elevated-temperature (427°C) fatigue on the degradation of residual strength in TMCs. Specimens were cycled until they had reached a pre-determined percentage of their fatigue life. After cycling, specimens were loaded monotonically in tension until failure. Fractography of the specimens provided information pertaining to the progression of damage mechanisms during fatigue. The laminate orientation was the primary factor for controlling the damage progression morphology as well as the residual strength degradation.

Residual Strength Degradation of Cross-Ply Titanium Matrix Composite Due to Elevated Temperature

This study investigates the residual strength degradation of cross-ply, SCS-6/Ti-15-3 titanium matrix composite (TMC) due to elevated temperature fatigue. To accomplish this, several specimens were cycled at 427°C to certain fractions of their expected fatigue lives, then monotonically loaded to failure. Fatigue tests were conducted in both load-control mode with a load ratio (Rσ) of 0.05 and strain-control mode with a strain ratio (Re) of − 1. Maximum stresses in the load-controlled tests were 300 and 450 MPa with frequencies of 5 and 10 Hz, respectively. The strain amplitudes ranged from 0.25 to 0.4% in the strain-controlled tests. Rather than being conducted at a constant frequency, these tests were performed at a strain rate of 0.1% per s. Various mechanical responses during cycling are discussed and compared for the load-controlled and strain-controlled tests. After failure, specimens were sectioned and studied using both optical and scanning electron microscopy. Residual strength data were then correlated to the amount and type of each damage mechanism. It was found that the residual strength degradation in cross-ply laminate, when exposed to different fatigue conditions, correlates together with the fraction of the cyclic life left in the composite.

Investigation of the Fatigue Behavior of SCS-Ultra/Ti-6-4

The focus of this study is to evaluate the fatigue behavior of SCS-Ultra/Ti6-4 at 4270C. The improvement in ultimate strength over SCS-6/Ti-6-4 was less than expected and the fatigue lives of the new material were much shorter at all stress levels. Posttest evaluation with a scanning electron microscope revealed several processing flaws in the new composites. Because of these flaws, the SCS-Ultra fibers failed at a lower composite stress than predicted and there were numerous crack initiation sites to greatly accelerate the progression of damage in fatigue tests. Despite the poor fatigue performance of the SCS-Ultra/Ti-6-4 specimens, this material shows promise for greatly increased ultimate strength. Both the stress and strain to failure are greater than SCS-6/Ti-6-4, even with the previously mentioned processing defects.