Energy Dependence of Fatigue-Enhanced Photoemission

This is a progress report on the subject of fatigue-enhanced photoemission. In our studies on fatigue-enhanced photoemission, the ultimate goal is to see whether or not the photoyield can be used as a tool to uniquely define the early and later stages of fatigue on structural materials. This yield results in an electron current off the specimen

In Vitro Two-Dimensional Echocardiographic Imaging of a Stented Porcine Bioprosthetic Valve: The Bent Strut Artifact

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72898/1/j.1540-8175.2008.00753.x.pd

Measurement of Flow Stress Related Phenomena by Nonlinear Acoustics

Before going into acoustic harmonic generation, I should bring to your attention the definition of internal stresses which was made about 40 years ago. Basically, we have to distinguish between two kinds of internal stresses. Internal stresses of the first kind are those which spread out over macroscopic distances of the order of millimeters. Applying x-rays, one obtains a line shift in the Bragg reflection due to a lattice parameter change. A simple example of internal stresses of the first kind is shown in Fig. 1, top: in bending a piece of material elastically, a line shift in the Bragg reflection will be found on the upper and lower surface. Internal stresses of the second kind are restricted to much, much smaller dimensions, say of the order of 1 um or below. Bragg reflection does not show a line shift, but merely a line broadeninq. A typical example is shown in Fig. 1, bottom. Assume dislocations are distributed in a material. The· variation of the elastic stress field surrounding the dislocations is assumed to be sinusoidal and of the periodicity of the dislocation arrangement. If another dislocation is pushed against this chain of dislocations, it will see the stress field of the dislocation arrangement. In work hardening theories, this model is used to calculate the work hardening coefficient. The present paper will be concerned mainly with the internal stresses of the second kind

Fatigue Lifetime Prediction with the Aid of SAW NDE

The scattered radiation of elastic waves from fatigue cracks in metals is interpreted for their detection and characterizations. The studies concentrated on predicting the remaining fatigue life for single fatigue cracks above the threshold value for macrocrack propagation. Acoustic surface waves were used to interrogate the crack during cyclic fatigue. The inversion of the scattering data provided crack depth and crack length as a function of fatigue. Auxiliary experiments were conducted to study effects of crack closure. Since the closure load depends strongly upon crack depth, especially in the near-threshold regime, improved life prediction is the expected result

Acoustic Interactions with Internal Stresses in Metals

It became evident at the residual stress meeting in San Antonio last yearl that it is necessar2 to restate the definitions for internal stresses. In general, we are talking about three kinds of residual stresses as shown in Table I. The one that\u27s called the first kind of internal stress ranges over millimeters or centimeters {or long range internal stress) and can be identified by x-rays through a line shift. The second kind ranges over dimensions of microns and with the x-ray method gives rise to a shift as well as a line broadening. This type is usually due to particles within the material or particles of a different phase or something similar. The third kind, which ranges over 100 to 1,000 Angstroms, is a microscopic internal stress in a true sense and is indicated by x-ray line broadening only. In the following, we will be talking about the internal stresses of the third kind, due to dislocations, and later about the first kind of internal stresses

Crack Depth Measurements with the Aid of SAW NDE

This report presents results of measurements of crack depth with the aid of acoustic bulk and surface waves. Both simulated and real fatigue cracks were examined. Two techniques were employed, one took advantage of the very efficient mode conversion between acoustic surface waves and shear waves at the crack tip; the other technique used the diffraction of shear waves at the crack tip. Both techniques were used on a number of simulated {spark eroded) and real cracks in Al 2024. In one fatigue specimen which contained an elliptical crack 4.5 mm in length and 1.25 mm in depth, crack closure studies were carried out. The precision of crack depth determination was judged to be better than 10%

Remaining Fatigue Lifetime Prediction for Retirement-for-Cause in Metals

A methodology is evaluated to predict the probability of specimen failure with subsequent fatigue, after a short surface crack has been detected in Al 2219-T851 alloy. Cracks are detected and tracked to failure using optical microscopy. Predictions of remaining lifetime distributions are made with a Monte Carlo procedure in conjunction with growth laws which model the effect of grains of differing size, shape and crystallographic orientation in the crack path on propagation rate. Because the surface of the alloy cyclically hardens, the average rate of crack growth is less for cracks formed later during fatigue. The predictive methodology successfully describes this phenomenon, as well as predicts the probability of early failure arising from the statistical nature of the growth process, for failure probabilities substantially smaller than conveniently measureable in the laboratory