Christopher N. McCowan

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.

Tensile strength and ductility of indium

Abstract The tensile properties of indium were measured at temperatures ranging from 4 to 295 K. Indium recrystallizes during deformation at room temperature and twins during deformation at low temperatures. Strain-hardening characteristics were assessed, using true stress-strain curves, and were found to be similar to soft f.c.c. metals. The logarithm of the flow strength was dependent on temperature. Photomicrographs depicting deformation twinning and slip of indium at 76 and 295 K are discussed.

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.

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.

Effects of Nb3Sn Heat Treatment on the Strength and Toughness of 316LN Alloys with Different Carbon Contents

A series of six 316LN-type alloys with different carbon contents were evaluated for suitability as conduit-sheath materials for the Nb3Sn superconductors that will be used in high-field magnets. Impact energies (at 76 to 295 K) and tensile strengths (at 4, 76, and 295 K) of annealed and “sensitized” alloys were compared. Embrittlement by aging at 700°C for 100 h (the Nb3Sn reaction conditions) was determined to be a function of carbon content; alloys with very low carbon content retained adequate toughness.

Percent Shear Area Determination in Charpy Impact Testing

The Charpy test is used throughout the world in a wide range of industries because of its low cost and the fact that notching and dynamic loading produces a crack tip stress field that is conservative for many applications. As a result of its widespread use, there is a compelling motivation to extract as much data as possible from the Charpy test. In a Charpy impact test, three key measurements are typically made: total absorbed energy, lateral expansion, and percent shear fracture area. At present, the measurements of absorbed energy and lateral expansion are quantitative and well defined, but the methods used by most laboratories in the measurement of percent shear are qualitative at best. This is ironic for a 100–year-old test because, as discussed in this paper, it can be reasonably argued that percent shear is the most fundamental and physically meaningful of the three Charpy parameters for brittle fracture characterization. Digital image analysis for shear fracture area is shown to have low uncertainty and be both repeatable and easy to use on a routine basis. Recommendations for changes to the ASTM E23 standard are provided.

Welding Consumable Development for a Cryogenic (4 K) Application

This paper summarizes the development and qualification of an appropriate welding consumable for a demanding cryogenic magnet application. This research shows that higher oxygen content in the weld manifests itself as inclusions, which have a severe detrimental effect upon the fracture toughness at 4 K. Also, welds enriched with manganese and nickel have demonstrated improved fracture toughness. These discoveries were combined in the development of a nitrogen- and manganese-modified, high-nickel stainless-steel alloy. It produced gas metal arc welds with superior cryogenic mechanical properties when welding procedures were modified to reduce the oxygen content.

Cryogenic material properties of stainless steel tube-to-flange welds

Abstract The mechanical properties of stainless steel tube-to-flange welds for a cryogenic piping application were measured. A planar specimen was developed to duplicate the constraint, loading and heat-sink properties of the circular joint, while reducing preparation time and cost. Specimens were evaluated containing welds between the tube material (21 Cr-6Ni-9Mn) and the three stainless steels being considered for the flange materials: type 304L, type 316L and 21 Cr-6Ni-9Mn. The mechanical property tests consisted of three phases: simple tensile testing to failure, tensile testing of notched specimens (where the notch simulated fabrication flaws) and fatigue testing of notched specimens for the 4 × 104 cycle design life of the structure. The type 316L stainless steel flange produced welds with the best combination of strength and ductility at 295 and 4 K in all three phases of testing.

Measurements of Fatigue Crack Growth Rates of the Heat-Affected Zones of Welds of Pipeline Steels

Pipelines are widely accepted to be the most economical method for transporting large volumes of hydrogen, needed to fuel hydrogen-powered vehicles. Some work has been previously conducted on the fatigue crack growth rates of base metals of pipeline materials currently in use for hydrogen transport and on pipeline materials that may be used in the future. However, welds and their heat-affected zones are oftentimes the source and pathway for crack initiation and growth. The heat-affected zones of welds can exhibit low resistance to crack propagation relative to the base metal or the weld itself. Microstructural irregularities such as chemical segregation or grain-size coarsening can lead to this low resistance. Therefore, in order to have adequate information for pipeline design, the microstructures of the heat-affected zones must be characterized, and their mechanical properties must be measured in a hydrogen environment. With that in mind, data on the fatigue crack growth rate is a critical need. We present data on the fatigue crack growth rate of the heat-affected zones for two girth welds and one seam weld from two API 5L X52 pipes. The materials were tested in hydrogen gas pressurized to 5.5 MPa and 34 MPa at a cyclic loading rate of 1 Hz, and an R ratio of 0.5.Copyright