Andrew B. Geltmacher

A modeling study of the effect of stress state on void linking during ductile fracture

Abstract The deformation and fracture behavior of sheet specimens containing either pairs of “pseudo”-random arrays of equi-sized holes has been examined in both uniaxial and equal-biaxial tension utilizing experiment as well as computer simulation. Our results show for this plane-stress situation that hole linking is always caused by flow localization within the ligaments between neighboring holes. The imposed strains to initiate flow localization and subsequent ligament failure are sensitive to stress state (uniaxial versus biaxial) and the location of the neighboring hole(s). A significant observation is the influence of stress state on the multidirectionality of hole linking paths: specifically, increasing the biaxial component of the stress state increases the number of holes that can participate in a hole-linking process. A related implication is that the strain range over which void linking occurs decreases with increasing triaxiality of the stress state; in effect, after the initiation of void linking, its propagation is accelerated under biaxial or triaxial tension.

Quality Improvement of Non-manifold Hexahedral Meshes for Critical Feature Determination of Microstructure Materials

This paper describes a novel approach to improve the quality of non-manifold hexahedral meshes with feature preservation for microstructure materials. In earlier works, we developed an octree-based isocontouring method to construct unstructured hexahedral meshes for domains with multiple materials by introducing the notion of material change edge to identify the interface between two or more materials. However, quality improvement of non-manifold hexahedral meshes is still a challenge. In the present algorithm, all the vertices are categorized into seven groups, and then a comprehensive method based on pillowing, geometric flow and optimization techniques is developed for mesh quality improvement. The shrink set in the modified pillowing technique is defined automatically as the boundary of each material region with the exception of local non-manifolds. In the relaxation-based smoothing process, non-manifold points are identified and fixed. Planar boundary curves and interior spatial curves are distinguished, and then regularized using B-spline interpolation and resampling. Grain boundary surface patches and interior vertices are improved as well. Finally, the local optimization method eliminates negative Jacobians of all the vertices. We have applied our algorithms to two beta titanium datasets, and the constructed meshes are validated via a statistics study. Finite element analysis of the 92-grain titanium is carried out based on the improved mesh, and compared with the direct voxel-to-element technique.

A cellular automaton-based technique for modeling mesoscale damage evolution

A 2D stochastic cellular automaton model was developed to create a framework to examine the evolution of damage on the mesoscale. The model is based on an expanding representative volume element (RVE) in a cellular domain. The state of a cell, which can be either solid material or void, is determined by mapping existing voids through plastic convection and creating new damage through the cellular automaton. The cellular automaton formulation addresses the effect of damage initiation, propagation and coalescence through the iterative evaluation of individual cells based on their current state and the current states of their eight neighboring cells. The amount of damage in each time step is controlled by conservation of mass in the expanding RVE. By modifying the ratio of damage to plastic convection and the probabilities in the cellular automaton rule table, the model can generate microcrack or microvoid damage morphologies at different spatial scales. Very rapid simulations are achieved using this formulation. Thus, a wide range of material damage morphologies can be rapidly modeled in a single simulation architecture based solely on local cell geometry. However, one will be able to characterize cellular automaton rules and parameters to simulate specific constitutive models of material damage and fracture.

The Effect of Material Cleanliness on Dynamic Damage Evolution in 10100 Cu

Two different plates of 10100 Cu (> 99.99 % Cu) were incipiently failed using flyer plate impact in a gas gun equipped with soft recovery. One plate has a grain size of 53 μm and the other, 63 μm. The cleanliness of the plates was assessed using the cold crucible melt technique. The smaller grain sized material has a number density of second phase particles about four times higher than the other material, but with a smaller mean size. The size distribution of second phase particles is well represented by a log normal distribution. The Stepanov corrected spall strength, as measured by a VISAR, of larger grain sized material is about 50 % higher than that of the other material under incipient loading conditions. The difference in spall strength between the materials decreases with increasing shock pressure. Void nucleation plays a dominant role in describing the resulting porosity distribution in the incipiently failed samples. The cause of the difference in spall strength between the plates is attributed t...

Soldering a 3D Wire Lattice Structure

Soldering the junctions in a novel woven structure constructed from metallic wires is expected to significantly enhance the mechanical properties of that structure. The ideal bond geometry has solder localized at wire junctions and a uniform amount of solder at each junction throughout the structure. By examining the effects of solvent variations, liquid flux concentration, and solder powder concentration on the soldered joint geometry and distribution, we have developed a new procedure to bond these wire junctions using a very fine solder powder suspended in a flux solution.

Variations in mode shape for sensor placement in health monitoring systems

A generalization for a two-tier approach to damage identification based on structural performance and levels or magnitude of damage is presented. The two tiers are defined as health or damage monitoring and situation assessment. Damage monitoring involves the inspection of a structure for continual degradation caused by accumulated damage. Situation assessment results from a known incident with a high probably of damage. Initial work on damage monitoring of structural components examines the response of a flat plate as the first step in a series of analyses that will address more complex structures. Damage is included in the computational study in the form of damage to joints such as weld lines. Trends in local and global responses have been evaluated in order to develop an understanding of the implications of varying amounts of damage in the joints on structural response. Numerical based visualization techniques are used to isolate regions of mode shape variation with increasing damage. Implications and use of the developed techniques for monitoring and sensor placement requirements are noted.

Laser-micromachined defect arrays for DC potential drop fatigue studies

The experimental characterization of fatigue crack initiation and growth of structural materials can be very expensive and time consuming. Fatigue specimens are typically controlled by a single dominant defect and several specimens are needed to examine the fatigue response for each loading condition of interest. Time and expense add up as millions of load cycles are sometimes required to initiate a crack, and replicate tests are necessary to characterize the inherent statistical nature of fatigue. In order to improve the efficiency of experimentation, we are developing laser-based techniques to produce fatigue test samples with arrays of defects. Controlled arrays of oval shaped micro-defects are laser-micromachined in titanium alloy (Ti-6Al-4V). Crack initiation from the individual defects in the arrays is monitored using a DC potential drop technique. Results indicate the utility of this approach in multiplying the amount of fatigue data generated per specimen-test. The new fatigue test approach is applicable to a wide range of material systems and initial defect structures.

Integrated experimental–computational characterization of TIMETAL 21S ☆

Abstract Accurate material constitutive parameters are important for the finite element simulation of processing, and component and structural loading. Simulations have shown that standard mechanical testing and data reduction practices can produce material property uncertainties of the order of the desired manufacturing tolerances for future components. The present study refines an integrated experimental–computational constitutive response characterization methodology that yields more accurate finite element simulations. Contrary to the previous data reduction techniques, where the goal was to produce a single uniform stress state, the new methodology seeks to generate a wide range of multiaxial stress states and then deconvolve the material response from the specimen behavior through simulation. This methodology extracts the constitutive response from experimental load-displacement test data and specimen shape evolution by constructing the material input curve from finite element simulations of the test specimen. This research includes the development of a novel test specimen to supplement the ASTM E8 specimen design, which exhibits variations in the position of strain localization along the gage length and generates a limited range of multiaxial stress states induced in the material undergoing testing. The novel specimen design examined in the present study uses a single through-thickness hole to generate the multiaxial stress and strain states. This has the added benefit of more nearly matching the stress states in actual structural components.

Computational aspects of steel fracturing pertinent to naval requirements.

Modern high strength and ductile steels are a key element of US Navy ship structural technology. The development of these alloys spurred the development of modern structural integrity analysis methods over the past 70 years. Strength and ductility provided the designers and builders of navy surface ships and submarines with the opportunity to reduce ship structural weight, increase hull stiffness, increase damage resistance, improve construction practices and reduce maintenance costs. This paper reviews how analytical and computational tools, driving simulation methods and experimental techniques, were developed to provide ongoing insights into the material, damage and fracture characteristics of these alloys. The need to understand alloy fracture mechanics provided unique motivations to measure and model performance from structural to microstructural scales. This was done while accounting for the highly nonlinear behaviours of both materials and underlying fracture processes. Theoretical methods, data acquisition strategies, computational simulation and scientific imaging were applied to increasingly smaller scales and complex materials phenomena under deformation. Knowledge gained about fracture resistance was used to meet minimum fracture initiation, crack growth and crack arrest characteristics as part of overall structural integrity considerations.

Virtual Processing of Hybrid Shape Memory Alloy Composites

The capability of using recoverable martensitic transformation to modify the residual stress-state of hybrid Shape Memory Alloy (SMA) composites is explored. It is shown that through careful selection of a thermomechanical loading path the composite can be “processed” such that the constituent phases have a beneficial residual stress-state. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension) resulting in improved material properties, such processing places the considered phase into a preferred stress state. This processing is explored here by considering composites with an SMA phase whose constititutive behavior is described by a recent phenomenological model and an elasto-plastic second phase. To consider realistic microstructural effects, a 3D numerical representation of the composite is generated using microtomography. It is shown that through an actuation (isobaric) loading path, the martensitic transformation of the SMA phase generates irrecoverable strains in the elasto-plastic phase which, upon unloading, results in a favorable residual stress-state. To consider the applicability of this methodology for a variety of composites, the effect of thermal residual stresses due to thermal expansion mismatch is identified and matrix phases with different elastic moduli and plastic hardenings are considered. Specifically, it is shown that martensitic transformation is the driving force behind the generation of the new composite residual stress-state. Through computational simulation, it is shown that increased elastic moduli or plastic hardening coefficients of the elasto-plastic phase yield small increases in residual stresses.© 2011 ASME