Drew V. Nelson

UNDERSTANDING THERMAL BEHAVIOR IN THE LENS PROCESS

Abstract 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.

Evaluation of an energy-based approach and a critical plane approach for predicting constant amplitude multiaxial fatigue life

A new energy-based approach for predicting constant amplitude multiaxial fatigue life is described. The approach is based on cyclic plastic and elastic strain energy densities and takes into account effects of stress-state and mean stresses. A wide range of test data is used to evaluate the approach as well as a critical plane approach based on maximum amplitude of shear strain modified by the maximum normal stress on the plane of maximum shear strain amplitude. Predictive capabilities of both approaches are found to be comparable, with some differences for certain types of multiaxial loading.

A fiber optic sensor for transverse strain measurement

A fiber optic sensor capable of measuring two independent components of transverse strain is described. The sensor consists of a single Bragg grating written into high-birefringent, polarization-maintaining optical fiber. When light from a broadband source is used to illuminate the sensor, the spectra of light reflected from the Bragg grating contain two peaks corresponding to the two orthogonal polarization modes of the fiber. Two independent components of transverse strain in the core of the fiber can be computed from the changes in wavelength of the two peaks if axial strain and temperature changes in the fiber are zero or known. Experiments were performed to determine the response of the sensor by loading an uncoated sensor in diametral compression over a range of fiber orientations relative to the loading. The results of these experiments were used with a finite element model to determine a calibration matrix relating the transverse strain in the sensor to the wavelength shifts of the Bragg peaks. The performance of the sensor was then verified by measuring the transverse strains produced by loading the fiber in a V-groove fixture.

Residual-stress determination through combined use of holographic interferometry and blind-hole drilling

Holographic interferometry is used to determine in-plane radial displacements due to release of residual stresses by hole drilling. A method is derived for relating radial displacements measured in three directions of illumination to the state of residual stress, analogous to relations used in the conventional strain-rosette technique. Residual stress is produced by an interference fit of two circular tubes. Agreement between stress determined holographically with a computed value and with that determined by the conventional technique is good. Advantages of the holographic technique in overcoming various shortcomings of the conventional technique are discussed. A modification of the holographic technique involving data collection in only two directions of illumination is described.

Residual-stress determination by single-axis holographic interferometry and hole drilling—Part I: Theory

A method is described for the rapid, accurate determination of residual stresses from a holographic interference fringe pattern. The pattern is generated by the displacement field caused by localized relief of residual stresses via the introduction of a small, shallow hole into the surface of a component or test specimen. The theoretical development of the holographic method is summarized. An example is given showing how the method can be applied to a typical experimentally observed fringe pattern to determine principal residual stresses and directions.

A multi-parameter Bragg grating fiber optic sensor and triaxial strain measurement

The principle of operation of a multi-parameter dual Bragg grating fiber optic sensor formed in a polarization maintaining fiber is described. The response of the sensor to separately applied longitudinal loading, transverse loading and temperature change is presented in terms of wavelength shifts of the Bragg peaks. Results of those tests are used as sensor calibration data. A model to predict the longitudinal and two orthogonal transverse strain components from measured wavelength shifts of the Bragg peaks observed in combined loading tests is described. The model is based on an assumed linear behavior between wavelength shifts and sensor loadings and uses the calibration data as input. Predictions of transverse strain are compared with applied strains in the combined loading tests and found to be inadequate for transverse strain components. A model for predicting strains is then developed to account for non-linearities between transverse loading and wavelength shifts observed in sensor calibration data. Predictions of longitudinal strain and both transverse strain components from measured wavelength shifts in combined loading are seen to be in excellent agreement with applied test strains.

Measurement of transverse strains with fiber Bragg gratings

In this paper, we present a method to measure two components of transverse strain in an optical fiber using a single Bragg grating written into high-birefringent, polarization- maintaining (PM) fiber. The reflected spectrum from this grating contains two peaks corresponding to the two orthogonal polarization modes of the fiber. If the axial strain and temperature in the fiber is known, then two components of transverse strain can be computed from the changes in wavelength of the two peaks. A Bragg grating written near 1300 nm in PM fiber was loaded in diametrical compression, and the changes in wavelength of the Bragg peaks were monitored using an optical spectrum analyzer. Transverse strains were computed from the changes in wavelength using available strain-optic coefficients for low-birefringent optical fiber. These strains are compared to finite element analysis predictions, and it is shown that the observed sensor response is greater than the response predicted by the low-birefringent analysis. A calibration factor is developed for the sensor to allow the determination of transverse strains from the measured wavelength shifts.

Multidimensional strain field measurements using fiber optic grating sensors

Fiber optic grating sensors written into polarization preserving optical fiber may be used to monitor multidimensional strain fields in composite materials. This paper provides an overview of the characterization and test of multiaxis fiber grating sensors formed by writing 1300 and 1550 nm fiber gratings into polarization preserving optical fiber. A discussion of the usage of these multiaxis fiber grating sensors to measure two and three dimensional strain fields will be made. A brief review of practical applications of the technology to measure shear strain, transverse strain gradients as well as axial and traverse strain will be made with emphasis on aerospace and civil structure applications.

Residual-stress determination by single-axis holographic interferometry and hole drilling—Part II: Experiments

Experiments to assess the ability of the holographic/hole-drilling technique to accurately determine uniaxial stresses are described. The experimental data are in the form of optical interference fringe patterns. Different patterns obtained by varying the direction of laser light illuminating a test specimen with respect to the direction of stress are shown. Stresses estimated by the technique are compared with known values in specimens of aluminum alloy 7075-T651 and hardened Type 304L stainless steel.

Death by small forces: a fracture and fatigue analysis of wave-swept macroalgae

SUMMARY Wave-swept macroalgae are subjected to large hydrodynamic forces as each wave breaks on shore, loads that are repeated thousands of times per day. Previous studies have shown that macroalgae can easily withstand isolated impositions of maximal field forces. Nonetheless, macroalgae break frequently. Here we investigate the possibility that repeated loading by sub-lethal forces can eventually cause fracture by fatigue. We determine fracture toughness, in the form of critical strain energy release rate, for several flat-bladed macroalgae, thereby assessing their resistance to complete fracture in the presence of cracks. Critical energy release rates are evaluated through single-edge-notch, pull-to-break tests and single-edge-notch, repeated-loading tests. Crack growth at sub-critical energy release rates is measured in repeated-loading tests, providing a first assessment of algal breakage under conditions of repeated loading. We then estimate the number of imposed waves required for un-notched algal blades to reach the point of complete fracture. We find that, if not checked by repair, fatigue crack growth from repeated sub-lethal stresses may completely fracture individuals within days. Our results suggest that fatigue may play an important role in macroalgal breakage.