Tonya W. Stone

Nanomechanics of phospholipid bilayer failure under strip biaxial stretching using molecular dynamics

The current study presents a nanoscale in silico investigation of strain rate dependency of membrane (phospholipid bilayer) failure when placed under strip biaxial tension with two planar areas. The nanoscale simulations were conducted in the context of a multiscale modelling framework in which the macroscale damage (pore volume fraction) progression is delineated into pore nucleation (number density of pores), pore growth (size of pores), and pore coalescence (inverse of nearest neighbor distance) mechanisms. As such, the number density, area fraction, and nearest neighbor distances were quantified in association with the stress–strain behavior. Deformations of a 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) bilayer were performed using molecular dynamics to simulate mechanoporation of a neuronal cell membrane due to injury, which in turn can result in long-term detrimental effects that could ultimately lead to cell death. Structures with 72 and 144 phospholipids were subjected to strip biaxial tensile deformations at multiple strain rates. Formation of a water bridge through the phospholipid bilayer was the metric to indicate structural failure. Both the larger and smaller bilayers had similar behavior regarding pore nucleation and the strain rate effect on pore growth post water penetration. The applied strain rates, planar area, and cross-sectional area had no effect on the von Mises strains at which pores greater than 0.1 nm2 were detected (0.509 ± 7.8%) or the von Mises strain at failure (e failure = 0.68 ± 4.8%). Additionally, changes in bilayer planar and cross-sectional areas did not affect the stress response. However, as the strain rate increased from 2.0 × 108 s−1 to 1.0 × 109 s−1, the yield stress increased from 26.5 MPa to 66.7 MPa and the yield strain increased from 0.056 to 0.226.

Length scale effects of friction in particle compaction using atomistic simulations and a friction scaling model

The objective of this study is to illustrate and quantify the length scale effects related to interparticle friction under compaction. Previous studies have shown as the length scale of a specimen decreases, the strength of a single crystal metal or ceramic increases. The question underlying this research effort continues the thought—If there is a length scale parameter related to the strength of a material, is there a length scale parameter related to friction? To explore the length scale effects of friction, molecular dynamics (MD) simulations using an embedded atom method potential were performed to analyze the compression of two spherical FCC nickel nanoparticles at different contact angles. In the MD model study, we applied a macroscopic plastic contact formulation to determine the normal plastic contact force at the particle interfaces and used the average shear stress from the MD simulations to determine the tangential contact forces. Combining this information with the Coulomb friction law, we quantified the MD interparticle coefficient of friction and showed good agreement with experimental studies and a Discrete Element Method prediction as a function of contact angle. Lastly, we compared our MD simulation friction values to the tribological predictions of Bhushan and Nosonovsky (BN), who developed a friction scaling model based on strain gradient plasticity and dislocation-assisted sliding that included a length scale parameter. The comparison revealed that the BN elastic friction scaling model did a much better job than the BN plastic scaling model of predicting the coefficient of friction values obtained from the MD simulations.

Characterization and Failure Analysis of an Automotive Ball Joint

This case study describes the failure analysis of an automotive lower ball joint that fractured under normal driving conditions. Through spectroscopy, the material was determined to be SAE-AISI 5135H steel. The metallographic examination revealed a tempered martensitic structure, and hardness measurements radially across the surface of the cut ball stud suggested that the stud was through hardened. Scanning electron microscopy of the fracture surfaces indicated fatigue as the main failure mechanism. Finite element analysis was used to analyze the performance of the part under a normal loading condition. A detailed fatigue analysis to determine the effect of various loads on the life of the ball joint was completed using three methods: S-N curve approximation using hardness values, S-N approach using Basquin’s equation, and a linear elastic crack-growth model. The cause of failure was determined to be from surface cracks forming in the high stress concentration neck region where the ball and stud are joined. The presence of a small surface flaw in this region was shown to significantly reduce the fatigue life of the ball joint.

Failure Analysis and Simulation Evaluation of an Al 6061 Alloy Wheel Hub

This paper details the failure analysis of a wheel hub from a student designed Formula SAE® race car that fractured at the roots of the rim finger attachment region. The wheel hub was identified to be manufactured from a rolled Al 6061 alloy. The experimental characterization included fracture surface analysis and microstructural analysis using scanning electron microscopy, as well as compressive stress–strain testing and micro-hardness testing to determine its mechanical properties. Analysis of the fractured surfaces of the hub revealed beach marks and striations, suggesting a fatigue failure. A kinematic model was developed to determine wheel hub loadings as defined by the car driving history. Detailed loads calculated from a kinematic equilibrium model and material properties obtained from the experiment results were used in a finite element model to simulate the stress distribution and fatigue life of the wheel hub. The wheel simulation results were consistent with the failure mode determined from the fractography study.

Smooth Yield Surface Constitutive Modeling for Granular Materials

In this paper, the authors present an internal state variable (ISV) cap plasticity model to provide a physical representation of inelastic mechanical behaviors of granular materials under pressure and shear conditions. The formulation is dependent on several factors: nonlinear elasticity, yield limit, stress invariants, plastic flow, and ISV hardening laws to represent various mechanical states. Constitutive equations are established based on a modified Drucker–Prager cap plasticity model to describe the mechanical densification process. To avoid potential numerical difficulties, a transition yield surface function is introduced to smooth the intersection between the failure and cap surfaces for different shapes and octahedral profiles of the shear failure yield surface. The ISV model for the test case of a linear-shaped shear failure surface with Mises octahedral profile is implemented into a finite element code. Numerical simulations using a steel metal powder are presented to demonstrate the capabilities of the ISV cap plasticity model to represent densification of a steel powder during compaction. The formulation is general enough to also apply to other powder metals and geomaterials. [DOI: 10.1115/1.4034987]

Failure Analysis and Mechanical Performance Evaluation of a Cast Aluminum Hybrid-Iron Golf Club Hosel

This article details the failure analysis of a commercial golf club hybrid-iron that fractured through the hosel during normal use. The golf club hosel was manufactured from a cast aluminum alloy, and the optical analysis revealed casting pores up to 20% through the hosel thickness. Mechanical properties of the aluminum alloy were determined for material characterization and used to construct a finite element model to analyze the performance of the material under failure conditions. In addition, a full structural scale experiment was conducted to determine the failure strength.

Sensitivity and uncertainty analysis of microstructure–property relationships for compacted powder metals

Abstract In an earlier study, the authors presented a characterisation of the FC-0205 Ancorsteel powders containing 0·6 and 1·0% Acrawax to define the evolution of the failure line and cap surface of the modified Drucker/Prager cap model during compaction. Using the results of that study (i.e. FC-0205 material parameters), this paper presents sensitivity and uncertainty analysis of the microstructure–property relationships for powder metallurgy compaction. It is found for all of the responses of interest (the compressibility curve, the interparticle friction, the material cohesion, the cap eccentricity and the elastic modulus) that the most dominant parameter is the initial (or tap) density. It is also observed that the uncertainty in output parameters for the case of 1% wax is much larger than those for the case of 0·6% wax, due to the large uncertainty in the failure stress (in particular, the compressive failure stress).