Elizabeth S. Drexler

A Microstructural Hyperelastic Model of Pulmonary Arteries Under Normo- and Hypertensive Conditions

This work represents the first application of a statistical mechanics based microstructural orthotropic hyperelastic model to pulmonary artery mechanics under normotensive and hypertensive conditions. The model provides an analogy between the entangled network of long molecular chains and the structural protein framework seen in the medial layer, and relates the mechanical response at macro-level to the deformation (entropy change) of individual molecular chains at the micro-level. A finite element approach was adopted to implement the model. Material parameters were determined via comparing model output to measured pressure–stretch results from normotensive and hypertensive trunks and branches obtained from a rat model of pulmonary arterial hypertension. Results from this initial study show that this model appears reasonable for the study of hyperelastic and anisotropic pulmonary artery mechanics. Typical tangent modulus values ranged from 200 to 800 kPa for normotensive arteries—this increased to beyond 1 MPa for hypertensive vessels. Our study also provokes the hypothesis that increase of cross-linking density may be one mechanism by which the pulmonary artery stiffens in hypertension.

Stiffening of the Extrapulmonary Arteries From Rats in Chronic Hypoxic Pulmonary Hypertension.

Changes in the compliance properties of large blood vessels are critical determinants of ventricular afterload and ultimately dysfunction. Little is known of the mechanical properties of large vessels exhibiting pulmonary hypertension, particularly the trunk and right main artery. We initiated a study to investigate the influence of chronic hypoxic pulmonary hypertension on the mechanical properties of the extrapulmonary arteries of rats. One group of animals was housed at the equivalent of 5000 m elevation for three weeks and the other held at ambient conditions of ~1600 m. The two groups were matched in age and gender. The animals exposed to hypobaric hypoxia exhibited signs of pulmonary hypertension, as evidenced by an increase in the RV/(LV+S) heart weight ratio. The extrapulmonary arteries of the hypoxic animals were also thicker than those of the control population. Histological examination revealed increased thickness of the media and additional deposits of collagen in the adventitia. The mechanical properties of the trunk, and the right and left main pulmonary arteries were assessed; at a representative pressure (7 kPa), the two populations exhibited different quantities of stretch for each section. At higher pressures we noted less deformation among the arteries from hypoxic animals as compared with controls. A four-parameter constitutive model was employed to fit and analyze the data. We conclude that chronic hypoxic pulmonary hypertension is associated with a stiffening of all the extrapulmonary arteries.

An Experimental Method for Measuring Mechanical Properties of Rat Pulmonary Arteries Verified With Latex

This paper describes a test method for measuring the mechanical properties of small, nonlinear membrane samples from a rat model for pulmonary hypertension. The size and nonlinearity of the pulmonary artery samples poses a challenge for developing a test method that will generate quality, reproducible data in the pressure range experienced by the hypertensive pulmonary artery. The experimental method described here has sufficient precision to yield a combined relative standard uncertainty of 4 %. The method is calibrated against 75 µm thick latex and the data agree well with the neo-Hookian model.

Error analysis and thermal expansion measurement with electron-beam moiré

In this paper, the authors study erros incurred when using the experimental technique of electron-beam moiré. There are two sources of error: error manifested as an apparent magnificant drift and error due to fringe tracing. The error due to fringe tracing is nearly negligible in comparison to the error due to magnification drift. By investigating the thermal expansion of commercially pure copper, the authors demonstrate the usefulness of the error estimate. The average result for the coefficient of thermal expansion is within 1.8 percent of handbook values for this materials, with a possible error due to apparent magnification drift of 9 percent.

Quantifying nonlinear anisotropic elastic material properties of biological tissue by use of membrane inflation

Determination of material parameters for soft tissue frequently involves regression of material parameters for nonlinear, anisotropic constitutive models against experimental data from heterogeneous tests. Here, parameter estimation based on membrane inflation is considered. A four parameter nonlinear, anisotropic hyperelastic strain energy function was used to model the material, in which the parameters are cast in terms of key response features. The experiment was simulated using finite element (FE) analysis in order to predict the experimental measurements of pressure versus profile strain. Material parameter regression was automated using inverse FE analysis; parameter values were updated by use of both local and global techniques, and the ability of these techniques to efficiently converge to a best case was examined. This approach provides a framework in which additional experimental data, including surface strain measurements or local structural information, may be incorporated in order to quantify heterogeneous nonlinear material properties.

Strain Measurements in a Thermally-Cycled Flip-Chip PBGA Solderball

The mismatch between the coefficients of thermal expansion of silicon chips and their organic substrates has been mitigated through the practice of using underfill in flip-chip packages. Yet solder fatigue and package failures still occur. This is particularly true for flip-chips on organic substrates that are thermally cycled between low (−55 °C) and high temperatures (125 °C). In this study, I used electron-beam moir6 to measure displacements and calculate strains in a solderball contained in a flip-chip plastic-ball grid array package. A crossed-line grating with a pitch of 450 nm was used to allow detailed measurements of the local displacements from a cross section of a flip-chip package. Elastic displacements were observed and measured, and the v-field displacements, out of the plane of the chip, were more significant than the shear or u-field, in the plane of the chip, displacements. Larger v-field displacements were measured near the center of the silicon chip than at the edge of the chip.

SE(T) Testing of Pipeline Welds

Single edge-notch tension (referred to as SE(T) or SENT) tests are increasingly being used in the pipeline community, as they are a laboratory-scale fracture toughness test, capable of being performed on linepipe steels and welds. The constraint and loading conditions of the SE(T) specimens more closely correspond with actual field flaws than those of the conventional three-point-bend CTOD (crack tip opening displacement) specimens. The test matrix covered in this paper consists of two nominally X65 pipes and one X80 pipe. Two welding procedures were applied to one of the X65 pipes, resulting in two different welds. Consequently four girth welds were in the test matrix. Notches were cut with electrical discharge machining (EDM) from the outer-diameter (OD) surface of the pipe with the target locations in the base metal, weld centerline, and heat-affected zone (HAZ). The EDM notches were grown by fatigue precracking in a three-point bend fixture to generate sharp flaws. The specimens were loaded in tension and periodically unloaded to generate J-integral resistance curves. The specimens with the weld and HAZ flaws were tested at room temperature and three to four lower temperatures. This paper covers the specimen preparation and the comparison of test results among specimens with different flaw locations at a wide range of temperatures. The specimen preparation and fatigue crack front straightness presented significant challenges. In general, at a given temperature, cracks propagated at lower energies in the weld material than in the HAZ or base material. Comparison of the J-integral curves for the even-matched and over-matched welds showed greater toughness in the over-matched weld at lower temperatures (but still on the upper-shelf of the curves of the ductile-to-brittle transition temperature (DBTT)). Testing at low temperatures appears to affect the HAZ differently than the weld material, as significant increases in toughness were observed between room temperature and −80°C in the HAZ.Copyright

Strength and toughness at 4 K of forged, heavy-section 316LN

The through-thickness mechanical properties of heavy-section 316LN forged stainless-steel plate were measured at 4 K. The properties studied were tensile and fracture toughness. Grain size, hardness, and chemistry variations were also obtained. The forged material came from cores of a 700-mm-thick plate for the MIT Alcator C-MOD. The average tensile yield strength was 889 MPa and the maximum variations from the average surface value were ± 5%. The average fracture toughness (estimates of K Ic from J Ic measurements) of the forging was unusually high, 471 MPa/m, with consistent values having been obtained from the interior of the forging, but variations of 25% from the average interior value were found.

THE EFFECT OF MICROSTRUCTURE ON THE HYDROGEN-ASSISTED FATIGUE OF PIPELINE STEELS

Tests on the fatigue crack growth rate were conducted on four pipeline steels, two of grade API 5L-X52 and two API 5L-X70. One X52 material was manufactured in the mid-1960s and the other was manufactured in 2011. The two X70 materials had a similar vintage and chemistry, but the microstructure differs. The fatigue tests were performed in 5.5 and 34 MPa pressurized hydrogen gas, at 1 Hz and (load ratio) R = 0.5. At high pressures of hydrogen and high values of the stress intensity factor range (ΔK) there is no difference in the fatigue crack growth rates (da/dN), regardless of strength or microstructure. At low values of ΔK, however, significant differences in the da/dN are observed. The older X52 material has a ferrite-pearlite microstructure; whereas, the modern X52 has a mixture of polygonal and acicular ferrites. The X70 materials are both predominantly polygonal ferrite, but one has small amounts (∼5%) of upper bainite, and the other has small amounts of pearlite (<2%) and acicular ferrite (∼5%). We discuss the fatigue test results with respect to the different microstructures, with particular emphasis on the low ΔK regime.Copyright

Chamber for mechanical testing in H2 with observation by neutron scattering

A gas-pressure chamber has been designed, constructed, and tested at a moderate pressure (3.4 MPa, 500 psi) and has the capability of mechanical loading of steel specimens for neutron scattering measurements. The chamber will allow a variety of in situ neutron scattering measurements: in particular, diffraction, quasielastic scattering, inelastic scattering, and imaging. The chamber is compatible with load frames available at the user facilities at the NIST Center for Neutron Research and Oak Ridge National Laboratory Spallation Neutron Source. A demonstration of neutron Bragg edge imaging using the chamber is presented.