Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location.

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.

Monitoring stress changes in carbon fiber reinforced polymer composites with GHz radiation.

We performed proof of concept experiments to demonstrate that the reflected power of GHz illumination from the surface of carbon fiber reinforced polymer (CFRP) composites is linearly related to the stress in the material. We introduce a stress coefficient to describe the change in normalized power with applied stress, analogous to the stress-optic coefficient, because the effect is attributed to changes in the refractive index of the effective medium comprising the polymer matrix and carbon fibers. Stress coefficients of -0.549±0.134/GPa and -0.154±0.024/GPa were measured for two different composite materials, both linear in the measurement range of 40 MPa and 100 Mpa, respectively. This technique opens up the possibility of non-destructive evaluation of stresses in CFRP components for quality assurance in manufacturing and in structural health monitoring of in-service aerospace and automotive parts.

Measurement of direct strain optic coefficient of YSZ thermal barrier coatings at GHz frequencies.

We report the first observation of stress induced birefringence in air plasma sprayed (APS) thermal barrier coatings (TBCs) using a reflection based polariscope and GHz illumination. Strain optic coefficients of (-0.0133 ± 0.0102) × 10-9 and (-0.0190 ± 0.0043) × 10-9 were measured for yttria-stabilized zirconia (YSZ) APS coatings of the same thickness deposited on substrates of 3 mm and 1 mm mild steel. The reflection measurement approach was validated by additional measurements of the stress optic coefficient of bulk yttria-partially stabilized zirconia (YTZP) ceramic that were in agreement with previously reported transmission measurements. The ultimate application of this technique is the prediction of remaining life in TBCs.

Direct stress optic coefficients for YTZP ceramic and PTFE at GHz frequencies.

We report the first measurement of the direct stress optic coefficient for yttria-partially stabilized zirconia (YTZP) ceramic, using illumination between 260 and 380 GHz with applied stresses up to 27 MPa. YTZP exhibited a linear change in refractive index as a function of stress across the entire applied stress domain. A direct stress optic coefficient was also measured for polytetrafluoroethylene (PTFE). PTFE showed viscoelastic behavior at stress values above 4.5 MPa. These results open the way for quantitative sub-surface stress measurements in structural ceramics and ceramic coating systems at GHz and THz frequencies.

Elastic proteins in the flight muscle of Manduca sexta.

The flight muscles (DLM1) of the Hawkmoth, Manduca sexta are synchronous, requiring a neural spike for each contraction. Stress/strain curves of skinned DLM1 showed hysteresis indicating the presence of titin-like elastic proteins. Projectin and kettin are titin-like proteins previously identified in Lethocerus and Drosophila flight muscles. Analysis of Manduca muscles with 1% SDS-agarose gels and western blots showed two bands near 1 MDa that cross-reacted with antibodies to Drosophila projectin. Antibodies to Drosophila kettin cross-reacted to bands at ∼500 and ∼700 kDa, but also to bands at ∼1.6 and ∼2.1 MDa, that had not been previously observed in insect flight muscles. Mass spectrometry identified the 2.1 MDa protein as a product of the Sallimus (sls) gene. Analysis of the gene sequence showed that all 4 putative Sallimus and kettin isoforms could be explained as products of alternative splicing of the single sls gene. Both projectin and sallimus isoforms were expressed to higher levels in ventrally located DLM1 subunits, primarily responsible for active work production, as compared to dorsally located subunits, which may act as damped springs. The different expression levels of the 2 projectin isoforms and 4 sallimus/kettin isoforms may be adaptations to the specific requirements of individual muscle subunits.

Modular spiral phase plate design for orbital angular momentum generation at millimetre wavelengths.

Proof of concept measurements of a modular spiral phase plate design able to generate millimetre wavelength beams with an azimuthal mode number of l = ±10 are presented. The plate is comprised of ten single modules that interlock to create the full plate assembly, allowing improved machining accuracy compared to standard techniques. Therefore, this design could be used in millimetre wavelength systems that require the manipulation of large OAM modes. The plate was manufactured from polypropylene (index of refraction n ≈ 1.5), and was measured at 100GHz. A three dimensional field scanner was used to measure three near field surfaces behind the plate. Intensity measurements showed the expected OAM intensity ring, and phase measurements showed ten phase dislocations, implying proper functionality.

Three-dimensional measurements of a millimeter wave orbital angular momentum vortex.

Initial three-dimensional phase and intensity measurements of a 100 GHz l=±1 orbital angular momentum (OAM) vortex are presented. The vortex was generated by illuminating a polypropylene spiral phase plate. Measurements were taken with a three-dimensional field scanner operating in the W-band (75-100 GHz). Early analysis shows splitting of the OAM phase dislocation at the vortex center, resulting in a complex inner vortex intensity pattern.

Calcium sensitivity and myofilament lattice structure in titin N2B KO mice.

The cellular basis of the Frank-Starling "Law of the Heart" is the length-dependence of activation, but the mechanisms by which the sarcomere detects length changes and converts this information to altered calcium sensitivity has remained elusive. Here the effect of titin-based passive tension on the length-dependence of activation (LDA) was studied by measuring the tension-pCa relation in skinned mouse LV muscle at two sarcomere lengths (SLs). N2B KO myocardium, where the N2B spring element in titin is deleted and passive tension is elevated, was compared to WT myocardium. Myofilament lattice structure was studied with low-angle X-ray diffraction; the myofilament lattice spacing (d1,0) was measured as well as the ratio of the intensities of the 1,1 and 1,0 diffraction peaks (I1,1/I1,0) as an estimate of the degree of association of myosin heads with the thin filaments. Experiments were carried out in skinned muscle in which the lattice spacing was reduced with Dextran-T500. Experiments with and without lattice compression were also carried out following PKA phosphorylation of the skinned muscle. Under all conditions that were tested, LDA was significantly larger in N2B KO myocardium compared to WT myocardium, with the largest differences following PKA phosphorylation. A positive correlation between passive tension and LDA was found that persisted when the myofilament lattice was compressed with Dextran and that was enhanced following PKA phosphorylation. Low-angle X-ray diffraction revealed a shift in mass from thin filaments to thick filaments as sarcomere length was increased. Furthermore, a positive correlation was obtained between myofilament lattice spacing and passive tension and the change in I1,1/I1,0 and passive tension and these provide possible explanations for how titin-based passive tension might regulate calcium sensitivity.