In direct laser metal deposition technologies, such as the laser engineered net shaping (LENS) process, it is important to understand and control the thermal behavior during fabrication. With this control, components can be reliably fabricated with desired material properties. This paper will describe the use of contact and imaging techniques to monitor the thermal signature during LENS processing. Development of an understanding of solidification behavior, residual stress, and microstructural evolution with respect to thermal behavior will be discussed.

A method to determine process-induced residual stress in fiber-reinforced composite materials using strain measurements from embedded fiber optic sensors is presented. This method allows non-destructive, real-time determination of residual macrostress in these materials and may be useful for both process monitoring and control. Extrinsic Fabry-Perot interferometer strain sensors were embedded in Hercules AS4/3501-6 graphite/epoxy composite specimens prior to cure. The specimens were cured in a press, and the internal strains and temperatures developed during processing were monitored and recorded. Residual macrostresses were computed from these measurements using a viscoelastic model of the material. The results compare favorably with analytical predictions, previous experimental measurements from a destructive technique, and with measurements of warpage of a non-symmetric laminate.

This paper presents the results of experiments to measure the internal strains and temperatures that are generated in carbon fiber/epoxy composite specimens during processing using embedded fiber optic strain sensors and thermocouples. Measurements of strain and temperature, combined with a computational model, offer the potential for non-destructive, real-time determination of residual stress in composites, and may be useful for process monitoring and control. Extrinsic Fabry-Perot interferometers, Bragg grating strain sensors, and thermocouples were embedded in graphite/epoxy composite laminates prior to cure. The specimens were cured in a press, and the internal strains and temperatures developed during processing were monitored and recorded. The results are compared with expected values, and limitations of the experimental technique are discussed.

Current approaches widely used in fatigue and fracture evaluations of mechanical components are summarized. Metallic alloys, and fatigue and fracture behavior in the absence of environmental effects such as corrosion or elevated temperatures, are considered. Certain shortcomings in the approaches are then discussed, followed by samples of recent work to improve the capabilities to make fatigue and fracture assessments. Finally, future developments that could further enhance those capabilities are suggested.

An ultrasonic surface acoustic wave technique for studying the growth behaviour of small fatigue cracks is described. The technique allows crack depth and opening stress to be monitored continuously during the course of a fatigue test. Results are given for a 1740 MPa yield strength, silicon-modified, AISI 4340 steel tested under zero-to-tension cyclic loading. Good agreement is shown between acoustically determined crack depth and that measured by post-fracture optical microscopy. The monitoring of changing crack depth-to-surface length ratios during tests is also demonstrated. Acoustically determined crack opening stresses were found to be about 10% higher than values determined by measurements of crack tip opening displacements by scanning electron microscopy. Effects on crack growth of two different specimen surface preparations, electropolishing and diamond paste polishing, are also reported. Growth rates in electropolished specimens were as much as an order of magnitude higher than in diamond paste polished specimens which had a shallow but significant layer of compressive residual stress.

The strain gage blind hole-drilling technique may be used to determine residual stresses at and below the surface of components. In this paper, the hole-drilling analysis methodology for thick plates is reviewed, and experimental data are used to evaluate the methodology and to assess its applicability to thin plates. Data on the effects of gage pattern, surface preparation, hole spacing, hole eccentricity, and stress level are also presented.

Load models for fatigue analysis and testing are tailored to the level of complexity required for the application. Random variable models are developed and applied to analyses in which load sequence effects can be neglected. Conventional narrow-band load peak and range distributions are applied to crack initiation and growth. It is shown that narrow-band load models provide useful, conservative life estimates for general Gaussian loadings. Distributions of significant peaks and ranges for wide-band loadings are developed empirically through simulations with racetrack filtering. An efficient “sequential” simulation technique is introduced for continuous generation of both narrow- and wide-band random loads. Based on a simplified crack closure model, simulations of crack growth suggest that sequence effects are most influential when any or a combination of the following are present: larger ratios of crack opening stress to maximum applied stress, lower values of applied tensile mean stress, smaller values of yield stress and crack growth coefficient. When sequence effects are present, the regularity of the spacing between tensile overloads can be important. In particular, assumption of regularly spaced overloads can be nonconservative.

A ligament model, which performs a simplified, one-dimensional elastic-plastic analysis to predict fatigue crack closure behavior, is described. The model is a modification and extension of a similar one developed by Newman and has the ability to predict the influence of pre-existing residual stress fields on crack closure. Different aspects of the model are examined and, where possible, compared with experimental data and with results from theoretical studies in the literature. For specimens without pre-existing residual stresses, the aspects examined include prediction of plasticity and closure induced residual stresses near the crack tip and prediction of the influence of applied mean stress on crack opening stress. For specimens with pre-existing residual stresses, aspects examined include prediction of residual stress redistribution caused by crack growth and prediction of the influence of such stresses on crack growth behavior, using the closure concept of an effective stress intensity range. Predictions of crack growth are also compared to those based on the superposition approach, for both compressive and tensile residual stresses.

A promising new type of tilting pad journal bearing—the Fluid Pivot journal bearing—is described. The long development history of this bearing is summarized, showing how the design evolved through research and testing. Static and dynamic performance features of the bearing are presented, with sample test results compared with predictions based on an advanced computer analysis. Comparisons of the Fluid Pivot journal bearing with conventional mechanically pivoted journal bearings are given.