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At least 1,027 records · Page 57

All sources NASA NTRS DOE OSTI

DOE OSTI2015

Measurement of fracture toughness by nanoindentation methods: Recent advances and future challenges

In this study, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials. For pillar splitting, cohesive zone finite element simulations are used to validate a simple relationship between the critical load at failure, the pillar radius, and the fracture toughness for a range of material properties and coating/substrate combinations. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties. Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ~25% is found between the pillar splitting technique and double-cantilever testing. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred.

36 MATERIALS SCIENCE↗

DOE OSTI2016

The second Sandia Fracture Challenge: predictions of ductile failure under quasi-static and moderate-rate dynamic loading

Not Available

36 MATERIALS SCIENCE↗

DOE OSTI2022

Scalable preconditioning for the stabilized contact mechanics problem

We present a family of preconditioning strategies for the contact problem in fractured and faulted porous media. We combine low-order continuous finite elements to simulate the bulk deformation with piecewise constant Lagrange multipliers to impose the frictional contact constraints. This formulation is not uniformly inf-sup stable and requires stabilization. We improve previous work by Franceschini et al. (2020) by introducing a novel jump stabilization technique that requires only local geometrical and mechanical properties. We then design scalable preconditioning strategies that take advantage of the block structure of the Jacobian matrix using a physics-based partitioning of the unknowns by field type, namely displacement and Lagrange multipliers. The key to the success of the proposed preconditioners is a pseudo-Schur complement obtained by eliminating the Lagrange multiplier degrees of freedom, which can then be efficiently solved using an optimal multigrid method. Numerical results, including complex real-world problems, are presented to illustrate theoretical properties, scalability and robustness of the preconditioner. A comparison with other approaches available in the literature is also provided.

58 GEOSCIENCES↗

DOE OSTI2020

DECOVALEX-2019 (Task G Final Report)

The DECOVALEX Project is an on-going international research collaboration, established in 1992, to advance the understanding and modeling of coupled Thermal (T), Hydrological (H), Mechanical (M) and Chemical (C) processes in geological in geological systems. This document is the final report of Task G which was proposed by the Swedish Radiation Safety Authority (SSM) and coordinated by geomecon GmbH, presenting the technical definitions of the problems studied, approaches applied, achievements made and outstanding issues for future research. Task G in DECOVALEX-2019 is primarily concerned with the evolution of transmissivity throughout the lifetime of a repository for radioactive waste, and in particular of spent nuclear fuel, in sparsely fractured and competent crystalline rock masses, and the representation of these processes via numerical simulation. The numerical simulations were to be evaluated for their suitability, and to be validated. Over the course of the work presented here, not only were transmissivity changes and the best approaches to simulate those change examined, but also strategies were to be developed on how to monitor transmissivity change. Guidelines on what a control program and a monitoring system for a repository should include and be able to measure, and how this can be done in practice, was a key objective of the research project.

58 GEOSCIENCES↗

NASA NTRS1978

Analysis and Test of Deep Flaws in Thin Sheets of Aluminum and Titanium. Volume 2: Crack Opening Displacement and Stress-Strain Data

Static fracture tests were performed on surface flawed specimens of aluminum and titanium alloys. A simulated proof overload cycle was applied prior to all of the cyclic tests. Variables included in each test series were flaw shapes and thickness. Additionally, test temperature was a variable for the aluminum test series. The crack opening displacement and stress-strain data obtained are presented.

Finger, R. W.↗

NASA NTRS1991

Energy coupling in catastrophic collisions

The prediction of events leading to the catastrophic collisions and disruption of solar system bodies is fraught with the same difficulties as are other theories of impact events; since one simply cannot perform experiments in the regime of interest. In the catastrophic collisions of asteroids that regime involves bodies of a few tons to hundred of kilometers in diameter, and velocities of several kilometers pre second. For hundred kilometer bodies, gravitational stresses dominate material fracture strengths, but those gravitational stresses are essentially absent for laboratory experiments. Only numerical simulations using hydrocodes can in principle analyze the true problems, but they have their own major uncertainties about the correctness of the physical models and properties. The question of the measure of the impactor and its energy coupling is investigated using numerical code calculations. The material model was that of a generic silicate rock, including high pressure melt and vapor phases, and includes material nonlinearity and dissipation via a Mie-Gruniesen model. A series of calculations with various size ratios and impact velocities are reported.

Holsapple, K. A.↗

NASA NTRS1991

Microfracture in high temperature metal matrix laminates

Computational simulation procedures are described to evaluate the composite microfracture behavior, establish the hierarchy/sequence of fracture modes, and the influence of compliant layers and partial debonding on composite properties and microfracture initiation. These procedures are based upon three-dimensional finite element analysis and composite micromechanics equations. Typical results for the effects of compliant layers and partial debonding, microfracture initiation, and propagation and the thermomechanical cyclic loading on a SiC/Ti15 composite system are presented and discussed. The results show that interfacial debonding follows fiber or matrix fracture, and the thermomechanical cyclic loading severely degrades the composite integrity.

Mital, Subodh K.↗

NASA NTRS2013

A Comparison of Materials Issues for Cermet and Graphite-Based NTP Fuels

This paper compares material issues for cermet and graphite fuel elements. In particular, two issues in NTP fuel element performance are considered here: ductile to brittle transition in relation to crack propagation, and orificing individual coolant channels in fuel elements. Their relevance to fuel element performance is supported by considering material properties, experimental data, and results from multidisciplinary fluid/thermal/structural simulations. Ductile to brittle transition results in a fuel element region prone to brittle fracture under stress, while outside this region, stresses lead to deformation and resilience under stress. Poor coolant distribution between fuel element channels can increase stresses in certain channels. NERVA fuel element experimental results are consistent with this interpretation. An understanding of these mechanisms will help interpret fuel element testing results.

Nuclear Propulsion↗

NASA NTRS2017

Simulating Matrix Crack and Delamination Interaction in a Clamped Tapered Beam

Blind predictions were conducted to validate a discrete crack methodology based on the Floating Node Method to simulate matrix-crack/delamination interaction. The main novel aspects of the approach are: (1) the implementation of the floating node method via an 'extended interface element' to represent delaminations, matrix-cracks and their interaction, (2) application of directional cohesive elements to infer overall delamination direction, and (3) use of delamination direction and stress state at the delamination front to determine migration onset. Overall, good agreement was obtained between simulations and experiments. However, the validation exercise revealed the strong dependence of the simulation of matrix-crack/delamination interaction on the strength data (in this case transverse interlaminar strength, YT) used within the cohesive zone approach applied in this work. This strength value, YT, is itself dependent on the test geometry from which the strength measurement is taken. Thus, choosing an appropriate strength value becomes an ad-hoc step. As a consequence, further work is needed to adequately characterize and assess the accuracy and adequacy of cohesive zone approaches to model small crack growth and crack onset. Additionally, often when simulating damage progression with cohesive zone elements, the strength is lowered while keeping the fracture toughness constant to enable the use of coarser meshes. Results from the present study suggest that this approach is not recommended for any problem involving crack initiation, small crack growth or multiple crack interaction.

De Carvalho, N. V.↗

DOE OSTI2017

DUCTILE-PHASE TOUGHENED TUNGSTEN FOR PLASMA-FACING MATERIALS IN FUSION REACTORS

Tungsten (W) and W-alloys are the leading candidates for plasma-facing components in nuclear fusion reactor designs because of their high melting point, strength retention at high temperatures, high thermal conductivity, and low sputtering yield. However, tungsten is brittle and does not exhibit the required fracture toughness for licensing in nuclear applications. A promising approach to increasing fracture toughness of W-alloys is by ductile-phase toughening (DPT). In this method, a ductile phase is included in a brittle matrix to prevent on inhibit crack propagation by crack blunting, crack bridging, crack deflection, and crack branching. Model examples of DPT tungsten are explored in this study, including W-Cu and W-Ni-Fe powder product composites. Three-point and four-point notched and/or pre-cracked bend samples were tested at several strain rates and temperatures to help understand deformation, cracking, and toughening in these materials. Data from these tests are used for developing and calibrating crack-bridging models. Finite element damage mechanics models are introduced as a modeling method that appears to capture the complexity of crack growth in these materials.

Ductile Phase Toughening↗

DOE OSTI2023

Structure and Process of Managing the UO2 Dry In-Pile Fracture Test Irradiation Experiment

The management and structure of a novel research and development (R&D) project can be hard to navigate. This is especially true when it involves nuclear fuel. Familiarity with institutional safety and procedural requirements, a well defined scope, and accurate cost estimates can all help ensure successful project management from start to finish. The Dry In-Pile Fracture Test (DRIFT) experiments performed at the Transient Test Reactor Facility are an excellent example of managing an R&D project within its scope and budget constraints. The objective of the DRIFT test was to develop fracture propagation data in a manner consistent with light-water reactors to validate and improve the fracture propagation models of uranium oxide fuels in the MOOSE (Multiphysics Object-Oriented Simulation Environment)-BISON-Marmot code framework [1].

99 GENERAL AND MISCELLANEOUS↗

DOE OSTI2015

Crack Front Segmentation and Facet Coarsening in Mixed-Mode Fracture

A planar crack generically segments into an array of “daughter cracks” shaped as tilted facets when loaded with both a tensile stress normal to the crack plane (mode I) and a shear stress parallel to the crack front (mode III). We investigate facet propagation and coarsening using in situ microscopy observations of fracture surfaces at different stages of quasistatic mixed-mode crack propagation and phase-field simulations. The results demonstrate that the bifurcation from propagating a planar to segmented crack front is strongly subcritical, reconciling previous theoretical predictions of linear stability analysis with experimental observations. As a result, they further show that facet coarsening is a self-similar process driven by a spatial period-doubling instability of facet arrays.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Immobile Pore-Water Storage Enhancement and Retardation of Gas Transport in Fractured Rock

The effect of immobile pore-water on gas transport in fractured rock has implications for numerical modeling of soil vapor extraction, methane leakage, gaseous CO 2 leakage from sequestration operations, radionuclide transport from underground nuclear explosions, and nuclear waste disposal. While the ability for immobile pore-water storage to effect gas transport has been recognized in the past, the details and specific scenarios leading to enhanced, retarded, or unaffected gas transport have not been explored. In this paper, we performed numerical investigations into the enhancement and retardation of gas transport due to immobile pore-water storage in order to identify implications for gas transport applications. To do this, we developed a numerical approach to model gas transport with a single-phase flow solution coupled to the advection–dispersion equation modified to account for immobile pore-water storage. Other than the immobility of pore water, the formulation contains all other physics included in two-phase formulations (advective and diffusive gas transport in fractures and rock matrix and dissolution in immobile pore water). The assumption of immobile pore water is valid here since for many applications involving transport of soluble gases in fractured rock, the rate of aqueous transport is insignificant compared to gas transport. We verify our modeling approach with analytical solutions of: (1) 1D gas diffusion, (2) 1D gas advection, (3) barometric pumping of a fracture, and (4) gas transport with uniform fracture flow and diffusion into the matrix. We account for pore-water storage in our model by implementing a kinetic formulation of gas dissolution wherein the dissolved (aqueous) phase is considered an immobile constituent. Using this formulation, we model the effect of dissolution rate and saturation on the retardation of gas transport during pure diffusion and pure advection. We also demonstrate that although it is commonly believed that pore-water storage will always enhance gas transport in fractures during oscillatory flow (e.g., during reversing pressure gradients such as barometric pumping cycles), our simulations indicate that this may not always be the case. Our numerical investigations indicate that scenarios with lower effective diffusion coefficients ( ⪅ 10 − 5  m 2 / s ) and lower dissolution coefficients (i.e., the dissolution diffusion coefficient) ( ⪅ 10 − 11  m 2 / s ) will result in enhanced gas transport. Other combinations of gaseous diffusion and dissolution coefficients result in delayed gas transport or insignificant effect on gas transport. Based on these results, and given the fact that the free-air diffusion coefficients for many gases are slightly above 10 − 5  m 2 / s (near the boundary of enhanced/non-enhanced gas transport), tortuosity would have to be significant for gas transport enhancement to occur. Similarly, if we consider the free-water diffusion coefficient of gases (around 10 − 9  m 2 / s for many gases of interest) to be a maximum bound for the effective dissolution coefficients (i.e., the rate of diffusion into the water away from the air/water interface will limit the dissolution rate), the effective dissolution coefficient would have to be at least 2 orders of magnitude less for enhanced gas transport to occur. Otherwise, pore-water storage will delay or have negligible effect on the gas transport. The results also indicate that the assumption of instantaneous equilibrium, often invoked in numerical codes for dissolution processes (i.e., dissolution coefficient is effectively infinite), will fail to accurately capture important details of soluble gas transport. Finally, the implication of our results is that gas breakthrough times in fractured rock during oscillatory flow may be highly dependent on the effective gaseous diffusion and dissolution coefficients.

58 GEOSCIENCES↗

DOE OSTI2013

Crack Growth Rate and Fracture Toughness Tests on Irradiated Cast Stainless Steels

Cast austenitic stainless steels are used in the cooling system of light water reactors for components with complex shapes, such as pump casings, valve bodies, and coolant piping. In the present study, crack growth rate and fracture toughness JR curve tests were performed on irradiated cast stainless steels in low-corrosion-potential environments (low-dissolved-oxygen high-purity water or simulated pressurized water reactor environment) at 320°C. Both as received and thermally aged materials were included to investigate the combined effect of thermal aging and irradiation embrittlement on the fracture behavior of cast stainless steels. The samples were irradiated to approximately 0.08 dpa at the Halden reactor. Good resistance to corrosion fatigue and stress corrosion cracking was observed for all samples. Thermal aging had little effect on the crack growth behavior at this dose level. Cleavage-like fracture was the dominant cracking morphology during the crack growth rate tests, and the ferrite phase was deformed to a lesser extent compared with the surrounding austenite phase. The fracture toughness results showed a dominant effect of neutron irradiation, and the fracture resistances were decreased considerably for all cast specimens regardless of their thermal aging conditions. The reduction in fracture toughness was more significant in the unaged than thermally aged materials. Nonetheless, the fracture toughness values of thermally aged specimens were about 30% lower than their unaged counterparts, suggesting a combined effect of thermal aging and neutron irradiation in cast stainless steel.

Chen, Y.↗

NASA NTRS1999

Crack Growth Simulation and Residual Strength Prediction in Airplane Fuselages

This is the final report for the NASA funded project entitled "Crack Growth Prediction Methodology for Multi-Site Damage." The primary objective of the project was to create a capability to simulate curvilinear fatigue crack growth and ductile tearing in aircraft fuselages subjected to widespread fatigue damage. The second objective was to validate the capability by way of comparisons to experimental results. Both objectives have been achieved and the results are detailed herein. In the first part of the report, the crack tip opening angle (CTOA) fracture criterion, obtained and correlated from coupon tests to predict fracture behavior and residual strength of built-up aircraft fuselages, is discussed. Geometrically nonlinear, elastic-plastic, thin shell finite element analyses are used to simulate stable crack growth and to predict residual strength. Both measured and predicted results of laboratory flat panel tests and full-scale fuselage panel tests show substantial reduction of residual strength due to the occurrence of multi-site damage (MSD). Detailed comparisons of n stable crack growth history, and residual strength between the predicted and experimental results are used to assess the validity of the analysis methodology. In the second part of the report, issues related to crack trajectory prediction in thin shells; an evolving methodology uses the crack turning phenomenon to improve the structural integrity of aircraft structures are discussed, A directional criterion is developed based on the maximum tangential stress theory, but taking into account the effect of T-stress and fracture toughness orthotropy. Possible extensions of the current crack growth directional criterion to handle geometrically and materially nonlinear problems are discussed. The path independent contour integral method for T-stress evaluation is derived and its accuracy is assessed using a p- and hp-version adaptive finite element method. Curvilinear crack growth is simulated in coupon tests and in full-scale fuselage panel tests. Both T-stress and fracture toughness orthotropy are found to be essential to predict the observed crack paths. The analysis methodology and software program (FRANC3D/STAGS) developed herein allows engineers to maintain aging aircraft economically while insuring continuous airworthiness. Consequently, it will improve the technology to support the safe operation of the current aircraft fleet as well as the design of more damage-tolerant aircraft for the next generation fleet.

Chen, Chuin-Shan↗

NASA NTRS1982

Molecular dynamics simulation of propagating cracks

Steady state crack propagation is investigated numerically using a model consisting of 236 free atoms in two (010) planes of bcc alpha iron. The continuum region is modeled using the finite element method with 175 nodes and 288 elements. The model shows clear (010) plane fracture to the edge of the discrete region at moderate loads. Analysis of the results obtained indicates that models of this type can provide realistic simulation of steady state crack propagation.

Mullins, M.↗

NASA NTRS1996

Probabilistic Fatigue Life Analysis of High Density Electronics Packaging

The fatigue of thin film metal interconnections in high density electronics packaging subjected to thermal cycling has been evaluated using a probabilistic fracture mechanics methodology. This probabilistic methodology includes characterization of thin film stress using an experimentally calibrated finite element model and simulation of flaw growth in the thin films using a stochastic crack growth model.

thermal cycling fatigue of thin film metal interco↗

DOE OSTI2017

Molecular dynamics modeling of PPTA crystallite mechanical properties in the presence of defects

Here, the mechanical properties of PPTA crystallites, the fundamental building blocks of aramid polymer fibers such as Kevlar® and Twaron®, are studied here using molecular dynamics simulations. The ReaxFF interatomic potential is employed to study crystallite failure via covalent and hydrogen bond rupture in constant strain-rate tensile loading simulations. Emphasis is placed on analyzing how chain-end defects in the crystallite influence its mechanical response and fracture strength. Chain-end defects are found to affect the behavior of nearby chains in a region of the PPTA crystallite that is small relative to the typical crystallite size in manufactured aramid fibers. The central Csingle bondN bond along the backbone chain is identified as the weakest in the PPTA polymer chain backbone in dynamic strain-to-failure simulations of the crystallite. It is found that clustering of chain-ends leads to reduced crystallite strength and crystallite failure via hydrogen bond rupture and chain sliding, whereas randomly scattered defects impact the strength less and failure is by covalent bond rupture and chain scission. The axial crystallite modulus increases with increasing chain length and is independent of chain-end defect locations. On the basis of these findings, a theoretical model is proposed to predict the axial modulus as a function of chain length.

36 MATERIALS SCIENCE↗