Manuel L. Lovato

Mechanical response of zirconium—II. Experimental and finite element analysis of bent beams

Abstract In a companion paper [Acta mater. 2001, 49(15), 3085–3096] we develop a polycrystal constitutive law that incorporates the deformation mechanisms operating in high purity zirconium (Zr) at liquid nitrogen (LN) and room temperature (RT). In this paper we present results of 4-point bending tests performed on beams of highly textured zirconium. These tests have been performed at LN and RT, in two orthogonal bending planes, and up to a strain of approximately 20% in the outermost fibers of the beams. A novel experimental technique, dot-matrix deposition and mapping (DMDM), has been developed and employed to analyze the distribution of local plastic strain and macroscopic deformation in the deformed beams. Automated electron backscatter diffraction (EBSD) pattern analysis has been used to evaluate the textures just below the outermost tensile and compressive surfaces and at the neutral plane. Experimental results compare very well with the predictions of finite element (FE) simulations obtained using the constitutive law developed in Part I. Specifically, we compare local deformation, macroscopic deformation and local texture in the beam. We show that the contribution of twinning to deformation results in different qualitative responses in the compressive and tensile fibers of the bent beam. Our results indicate the necessity of using a constitutive description that accounts for the anisotropy of the aggregate and for its evolution with deformation.

Known Residual Stress Specimens Using Opposed Indentation

In order to test new theories for residual stress measurement or to test the effects of residual stress on fatigue, fracture, and stress corrosion cracking, a known stress test specimen was designed and then fabricated, modeled, and experimentally validated. To provide a unique biaxial stress state, a 60 mm diameter 10 mm thick disk of 316L stainless steel was plastically compressed through the thickness with an opposing 15 mm diameter hard steel indenters in the center of the disk. For validation, the stresses in the specimen were first mapped using time-of-flight neutron diffraction and Rietveld full pattern analysis. Next, the hoop stresses were mapped on a cross section of two disks using the contour method. The contour results were very repeatable and agreed well with the neutron results. The indentation process was modeled using the finite element method. Because of a significant Bauschinger effect, accurate modeling required testing the cyclic behavior of the steel and then modeling it using a Chaboche-type combined hardening law. The model results agreed very well with the measurements. The duplicate contour measurements demonstrated stress repeatability better than 0.01% of the elastic modulus and allowed discussion of implications of measurements of parts with complicated geometries.

In-Situ DIC and Strain Gauges to Isolate the Deficiencies in a Model for Indentation Including Anisotropic Plasticity

A 60-mm diameter disk of 2024 aluminum was indented by opposing steel indenters over a central 10 mm region. Residual stress measurements made using neutron diffraction and the contour method matched each other, but not a finite element (FE) model with a calibrated model for plastic anisotropy of the aluminum. Since residual stresses are only the endpoint of the process, in situ data was needed to determine which portion of the load/unload process was causing model deficiencies. The indentation process was repeated on a new specimen with three-dimensional Digital Image Correlation (3D-DIC) to map full-field strain information and with resistance strain gauges to obtain high fidelity strain information at discrete locations. The DIC data was too noisy to extract strains, so displacements were analyzed after rigid body motion was removed. The deformation field revealed geometric imperfections of the indenters that were within tolerance, but had significant effect on the stress state. An updated FE model including geometric imperfections in the indenters gave better agreement with the DIC data. It did not however allow the material model to become the dominant effect and thus model calibration was unsuccessful.

Damage & Fracture of High-Explosive Mock Subject to Cyclic Loading

We use four-point bend specimen with a single shallow edge notch to study the fracture process in Mock 900-21, a PBX 9501 high explosive simulant mock. Subject to monotonic loading we determine quantitatively the threshold load for macroscopic crack initiation from the notch tip. The four-point bend specimen is then subject to cyclic loading in such a way that during the first cycle, the applied force approaches but does not exceed the threshold load determined from the monotonic loading test and in the subsequent cycles, the overall maximum deformation is maintained to be equal to that of the first cycle. It is expected and is also confirmed that no macroscopic damage and cracking occur during the first cycle. However, we observe that sizable macroscopic crack is generated and enlarged during the subsequent cycles, even though the applied force never exceeds the threshold load. Details of the process of damage formation, accumulation, and crack extension are presented and the mechanical mechanism responsible for such failure process is postulated and discussed.

THE YOUNG’S MODULUS OF 1018 STEEL AND 6061‐T6 ALUMINIUM MEASURED FROM QUASI‐STATIC TO ELASTIC PRECURSOR STRAIN‐RATES

The assumption that Youngs modulus is strain-rate invariant is tested for 6061-T6 aluminium alloy and 1018 steel over 10 decades of strain-rate. For the same billets of material, 3 quasi-static strain-rates are investigated with foil strain gauges at room temperature. The ultrasonic sound speeds are measured and used to calculate the moduli at approximately 10{sup 4} s{sup -1}. Finally, ID plate impact is used to generate an elastic pre-cursor in the alloys at a strain-rate of approximately 10{sup 6} s{sup -1} from which the longitudinal sound speed may be obtained. It is found that indeed the Youngs modulus is strain-rate independent within the experimental accuracy.

Mechanical behavior and strength in spalled copper

The bulk compressive and tensile response of samples machined from the different regions of dynamically damaged OFHC copper is examined in combination with post-mortem analysis of the recovered sample. This was done using optical microscopy and electron back-scatter diffraction to understand the strength and mechanical behavior of a material deformed under dynamic loading conditions. Stored plastic work in the shocked samples produced significant hardening in all the samples. It was also observed that the samples containing a dynamically induced damage field were slightly harder and showed enhanced yield strength over material simply subjected to shock and release that showed no dynamic damage. This was attributed to the excess work hardening around voids induced during incipient spall. By comparing the responses of two specimens with differing amounts of dynamic damage, it was concluded that there appears to be a limit to the amount of dynamic damage, after which the enhanced yield strength in the materi...

Plasma Sprayed Zirconium Diffusion Barrier Development for Monolithic U-Mo Metallic Fuel

This presentation discusses the DOE/NNSA Convert program and how plasma spray is implemented in nuclear fuel plate production.

Characterization of Carbon Epoxy-Filled Composite

Please find attached a summary of the characterization work performed at Los Alamos between 2014 and 2015 on epoxy-filled carbon composite material.

Bulge Testing and Interface Fracture Characterization of Plasma-Sprayed and HIP Bonded Zr Coatings on U-Mo

Bulge testing using a pressurized fluid to fracture the interface between bonded material layers along with three-dimensional digital image correlation to measure the sample distortion caused by pressurized fluid was applied to plasma-sprayed coatings. The initiation fracture toughness associated with the bonded materials was measured during the testing. The bulge testing of the uranium-molybdenum alloy plasma sprayed with zirconium and clad in aluminum is presented. The initiation fracture toughness was observed to increase with the increasing cathodic arc-cleaning current and the use of alternating polarity transferred arc current. This dependence was linked to the interface composition of oxide and mixed metal phases along with the interface temperature during spray deposition.