Gbadebo Owolabi

A Comparison Between Two Models That Predict the Elastic-Plastic Behavior of Particulate Metal Matrix Composites Under Multiaxial Fatigue Type Loading

This paper is an effort to first modify two cyclic plasticity models developed for homogeneous metals to address the heterogeneous nature of particulate metal matrix composites (PMMCs), and subsequently to evaluate the resulting relations both theoretically and experimentally. Specifically, using the original Mroz model and the endochronic theory of plasticity as their bases, two sets of elastic-plastic constitutive relations are identified. These sets of relations account for the interaction in stress fields between adjacent particles in PMMCs. The behavior predicted by each model is compared with experimental results obtained from a series of uniaxial and biaxial (tension-torsion) tests performed on circular specimens made of the 6061/Al2 O3 /20p-T6 PMMCs with 20% volume fraction of particles. The materials are tested for a variety of applied monotonic and cyclic loading paths.Copyright

Shear band formation in AISI 4340 steel under dynamic impact loads: Modeling and experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Microstructure-Sensitive Fatigue Design for Notched Components

The current drive for the discovery and use of advanced materials in safety-critical components necessitates the development of reliable fatigue design tools. However, most of the current tools require a number of approximations based on heuristics and phenomenological data rather than solid theoretical underpinning and still yield unsatisfactory and inconsistent results when applied to complex components under in service loads. Microstructural inhomogeneities in the materials induce notch size effects, but are not explicitly accounted for in phenomenological methods. Accordingly, notch sensitivity remains a highly empirical subject in spite of significant advances in microstructure-sensitive modeling. More robust fatigue design tools should capture the cause and effect relation of microstructure to distribution of slip and driving forces for formation and early growth of small cracks in the notch root field. Recent developments in computational crystal plasticity and microstructure-scale modeling have provided deeper understanding of the complex correlations between properties and structures and further indicate the limitations of conventional fatigue life prediction approaches. These modeling approaches have the potential to substantially reduce the need for costly large scale experimental programs to determine scatter in fatigue, for example. At present, however, there is a lack of simulation-based methodologies for considering interactive effects of stress and strain field gradients at the notch-root and microstructure-scale behavior in predicting notch-root fatigue crack initiation. In this paper results from simulations within a well-defined notch root damage process zone are used along with a probabilistic mesomechanics approach to develop a framework for new microstructure-sensitive fatigue notch factor. In addition, probability distributions of fatigue strength and fatigue life to form small cracks are estimated from simulations, extending notch sensitivity to explicitly incorporate microstructure sensitivity and attendant size effects via probabilistic arguments.© 2009 ASME

Mechanical Properties of Ultrafine Grain 2519 Aluminum Alloy

The effect of percentage thickness reduction and annealing time on the mechanical properties of cryo-rolled AA 2519 aluminum (Al) alloy were examined. Tensile tests was performed on samples in the longitudinal, transverse and at 45° to the rolling direction. The mechanical properties such as the Yield Strength (YS) and the Ultimate Tensile Strength (UTS) were observed to improve when compared to as-received sample of the 2519 alloy. This is in agreement with the Hall-Petch relationship. The highest variations in these properties were observed in the longitudinal direction, followed by the 45° and the lowest values were obtained in the transverse direction. However, the difference between the mechanical properties in the various directions decreased with an increase in annealing time showing homogeneous distribution of the fine particles.

Effects of regenerative mechanical vibration on the mechanical integrity of ceramic diesel particulate filters

In this study, the effects of mechanical vibration on the mechanical properties of ceramic diesel particulate filters (DPFs) were investigated. The goal is to determine how the mechanical vibration used in the regenerative ash cleaning process for these filters affects their mechanical integrity during subsequent reuse. Both virgin and vibrated DPF samples were subjected to com-pressive and 3-point flexural loading at three different loading rates along axial and tangential directions. Statistical analysis was conducted to determine the significance of variation in the compressive and flexural strengths of the DPFs as a result of exposure to mechanical vibration. The results show that there is no statistically significant difference in both compressive and flexural strengths of the virgin DPFs and the DPFs subjected to the same level of mechanical vibration typically used in ash cleaning of DPFs. When the intensity of vibration was doubled, the drop in compressive strength became statistically significant, but less than 10% under axial loading. However, no drop in flexural strength was observed for DPFs subjected to this high intensity of mechanical vibration. The safe threshold for mechanical vibration of ceramic filters is considered to be much higher than that currently used in vibration-based ash cleaning process.

Laser surface modification of Ti6Al4V-Cu for improved microhardness and wear resistance properties

To modify the properties of Ti6Al4V alloy, Cu has been added to host an antimicrobial effect in the revised alloy for marine application. The Laser Metal Deposition (LMD) process on the Ti6Al4V alloy and Cu was been investigated for surface modification in order to combat the problem of biofouling in the marine industry. The investigations focused on the microstructural observations, micro-hardness measurements and dry sliding wear in the presence of 3 and 5 weight percents of Cu. The microstructure results showed that Widmanstatten microstructures were formed in all the samples and lose their robustness towards the fusion zone as a result of the transition of heat sink towards the substrate. The microhardness values of Ti6Al4V-3Cu and Ti6Al4V-5Cu alloys were greatly improved to 547±16 VHN

Dynamic Behavior of Acrylonitrile Butadiene Styrene Under Impact Loads

This paper is aimed at providing a better understanding of the potential energy absorption benefits of components fabricated using fused deposition modeling (FDM) additive manufacturing. Using FDM, it is possible to print three-dimensional (3-D) objects created through the use of computer-aided design and computer-aided manufacturing software coupled with computer codes that enable the layer-by-layer deposition of material to form the 3-D component. Also known as direct digital manufacturing or 3-D printing, AM offers the benefit of being able to rotate printing orientation during processing to manipulate the design build and ultimately control mechanical and structural properties when subjected to dynamic loads. In this work, tensile test specimens were first fabricated to characterize the general mechanical behavior of the of 3D-printed Acrylonitrile Butadiene Styrene (ABS) material to assess its potential strain rate dependency. The mechanical evaluation under the quasi-static load was also necessary to determine the properties necessary to characterize the dynamic evolution of ABS in compression at various strain rates. ABS specimens were subsequently subjected to high strain rate deformation through the use of the Split Hopkinson Pressure Bar. During compression a new phenomenon described as a multistage collapse in which the samples undergo multiple stages of contraction and expansion was observed as the impact load was applied.Copyright

Dynamic Failure of Aluminum Alloy 2219-T8 Under High Strain Rate

The suitability of aluminum alloys in a vast majority of engineering applications forms the basis for the need to understand the mechanisms responsible for their deformation and failure under various loading conditions. The material investigated in this study is AA 2219-T8 aluminum alloy. Supplied by the NASA Research Center, with high strength to weight ratio and corrosive resistance. Containing a unique mixture of aluminum, copper, and other trace elements, this alloy has potential applications in multiple fields including aerospace, defense, and commercial industries. In this paper, the dynamic high strain rate impact deformation of the AA2219-T8 aluminum alloy was performed using the split Hopkinson pressure bars. The evolution of localized strain in the aluminum samples during the deformation process obtained using high speed digital cameras is reported. Microstructural analysis of deformed aluminum samples was also performed using optical microscopes in order to determine the influence of impact strain rate on localized strain along narrow adiabatic shear bands in the AA2219-T8 aluminum alloys. Results obtained indicate that peak flow stress in the deformed aluminum sample depends on the strain rate at which the deformation test was performed. The non-uniformity of the strain obtained using the digital image correlation as deformation time progresses shows two distinct areas of non-uniform strains that may be indicating potential sites for the formation of adiabatic shear bands in the tested samples.Copyright

Dynamic Deformation Behavior of AA2099-T8 Under Compression and Torsion Loads

The suitability of aluminum alloys in a vast majority of engineering applications forms the basis for the need to understand the mechanisms responsible for their deformation and failure under various loading conditions. Aluminum AA2099 alloy finds application in fuselage structures that are statically and dynamically loaded, stiffness dominated designs, and in lower wing structures. The fuselage structures and wings of aircraft experience huge damage due to foreign object impacts. AA2099 aluminum alloy has an advantage of high specific strength compared with other alloys in the AA2000, 6000, and 7000 series; this characteristic makes it the material of choice in high performance aerospace structures. In this paper, the dynamic high strain rate impact deformation of AA2099 aluminum alloy under compression and torsion loading conditions using the split Hopkinson pressure and Kolsky torsion bars was performed. Digital image photogrammetric evolution of localized strain in aluminum samples during deformation process using high speed digital camera is reported. Microstructural analysis of deformed aluminum samples was performed using high resolution electron microscopes in order to determine the influence of impact strain rate on localized strain along narrow adiabatic shear bands in the AA2099 aluminum alloys. Results obtained indicate that peak flow stress in the deformed aluminum sample depends on the strain rate at which the deformation test was performed. An increase in impact strain rate results into an increase in the peak flow stress observed in the impacted aluminum sample. The type of adiabatic shear band localized in the aluminum sample also depends on the strain rate at which material was impacted.

On Fatigue Strength Reduction Factor: State of the Art

Numerous theoretical models have been developed to predict the fatigue strength reduction factor (also known as fatigue notch factor), an important parameter in fatigue life prediction of notched components. These models include: the classical average stress method, the fracture mechanics method, the stress field intensity method, the strain energy method, and the weakest link method. However, most of these methods do not incorporate explicit sensitivity to materials microstructure. Accordingly, notch sensitivity remains a highly empirical subject in spite of significant advances in microstructure-sensitive modeling. This paper gives a detailed literature review of these methods and addresses their limitations. It also discusses a recently developed probabilistic method for microstructure-sensitive fatigue notch factor. The probabilistic method provides a very strong physical basis for fatigue strength reduction and associated notch sensitivity; thus it can be used to determine the effect of notches on reduction of fatigue resistance in a way that directly incorporates microstructure. The results obtained using the new probabilistic framework and other conventional methods are compared with experimental data for notched components. The probabilistic framework gives better correlation with experimental results for the notch sensitivity and notch size effect than the conventional approaches including the Neuber’s, the Peterson, and the fracture mechanics methods.Copyright