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Lee Jaehong, Kim Kwangwon, Ju Jaehyung, et al. Compliant Cellular Materials With Elliptical Holes for Extremely High Positive and Negative Poisson's Ratios J. Eng. Mater. Technol. 137, 011001 (2014) (17 pages);   Paper No: MATS-14-1072;   doi:10.1115/1.4028317

Cellular materials' two important properties—structure and mechanism—can be selectively used for materials design; in particular, they are used to determine the modulus and yield strain. The objective of this study is to gain a better understanding of these two properties and to explore the synthesis of compliant cellular materials (CCMs) with compliant porous structures (CPSs) generated from modified hexagonal honeycombs. An in-plane constitutive CCM model with CPSs of elliptical holes is constructed using the strain energy method, which uses the deformation of hinges around holes and the rotation of links. A finite element (FE) based simulation is conducted to validate the analytical model. The moduli and yield strains of the CCMs with an aluminum alloy are about 4.42 GPa and 0.57% in one direction and about 2.14 MPa and 20.9% in the other direction. CCMs have extremely high positive and negative Poisson's ratios (NPRs) (νxy* ∼ ±40) due to the large rotation of the link member in the transverse direction caused by an input displacement in the longitudinal direction. A parametric study of CCMs with varying flexure hinge geometries using different porous shapes shows that the hinge shape can control the yield strength and strain but does not affect Poisson's ratio which is mainly influenced by rotation of the link members. The synthesized CPSs can also be used to design a new CCM with a Poisson's ratio of zero using a puzzle-piece CPS assembly. This paper demonstrates that compliant mesostructures can be used for next generation materials design in tailoring mechanical properties such as moduli, strength, strain, and Poisson's ratios.

Maniatty Antoinette, Karvani Payman. Constitutive Relations for Modeling Single Crystal GaN at Elevated Temperatures J. Eng. Mater. Technol. 137, 011002 (2014) (7 pages);   Paper No: MATS-14-1094;   doi:10.1115/1.4028441

Thermal–mechanical constitutive relations for bulk, single-crystal, wurtzite gallium nitride (GaN) at elevated temperatures, suitable for modeling crystal growth processes, are presented. A crystal plasticity model that considers slip and the evolution of mobile and immobile dislocation densities on the prismatic and basal slip systems is developed. The experimental stress–strain data from Yonenaga and Motoki (2001, “Yield Strength and Dislocation Mobility in Plastically Deformed Bulk Single-Crystal GaN,” J. Appl. Phys., 90(12), pp. 6539–6541) for GaN is analyzed in detail and used to define model parameters for prismatic slip. The sensitivity to the model parameters is discussed and ranges for parameters are given. Estimates for basal slip are also provided.

Park Dong-Yeob, Gravel Jean-Philippe, Arafin Muhammad, et al. Evaluation of Two Low-Constraint Toughness Test Methods in a Single Specimen J. Eng. Mater. Technol. 137, 011003 (2014) (9 pages);   Paper No: MATS-14-1100;   doi:10.1115/1.4028728

In previous studies, the single- and double-clip gauge methods were successfully consolidated in a single-edge notched tension (SE(T)) single specimen so that crack tip opening displacement (CTOD) values obtained from both SE(T) methods could be compared under identical test conditions. The current study investigated the effect of unloading compliance crack size equations on resistance curves obtained from both gauging methods combined in a single specimen. It was found that the unloading compliance crack size equations of Cravero and Ruggieri and the single clip gauge method predict crack sizes well within approximately 2% error in average. Two CTOD-resistance curves obtained from both gauging methods produce approximately the same results until peak loads, and thereafter the curves deviate. The results obtained from the double clip gauge method are consistently higher than those from the single clip gauge, although the difference between two resistance curves is reduced when the same unloading compliance crack size prediction procedure is used. This observation is very important within the framework of engineering critical assessment (ECA) and defect assessment procedures. An “apparent” higher resistance curve will generate larger tolerable defects thereby reducing the conservatism of an ECA analysis.

Masih Saeed, Mashayekhi Mohammad, Torabian Noushin. Identification and Validation of a Low Cycle Fatigue Damage Model for Al 7075-T6 Alloy J. Eng. Mater. Technol. 137, 011004 (2015) (11 pages);   Paper No: MATS-13-1089;   doi:10.1115/1.4028840

In this paper, the behavior of 7075-T6 aluminum alloy under low cycle fatigue (LCF) loading is experimentally and numerically investigated using continuum damage mechanics (CDM). An experimental procedure is established to identify the damage parameters for Al 7075-T6. A damage-coupled explicit finite element code is developed using the experimentally extracted damage parameters to study the material behavior under LCF loading. Moreover, fractographic examinations are conducted to identify the fatigue crack initiation locations and propagation mechanisms. The model is employed for life-time assessment of stringer-skin connection of a fuselage and the results are compared with the data available in the literature.

Joshi Shashikant, Tewari Asim, Joshi Suhas S. Microstructural Characterization of Chip Segmentation Under Different Machining Environments in Orthogonal Machining of Ti6Al4V J. Eng. Mater. Technol. 137, 011005 (2015) (16 pages);   Paper No: MATS-14-1009;   doi:10.1115/1.4028841

Segmented chips are known to form in machining of titanium alloys due to localization of heat in the shear zone, which is a function of machining environment. To investigate the correlation between machining environments and microstructural aspects of chip segmentation, orthogonal turning experiments were performed under three machining environments, viz., room, LN2, and 260 °C. Scanning electron and optical microscopy of chip roots show that the mechanism of chip segment formation changes from plastic strain and mode II fracture at room temperature, to predominant mode I fracture at LN2 and plastic strain leading to shear band formation at 260 °C. The chip segment pitch and shear plane length predicted using Deform™ matched well with the experimental values at room temperature. The microstructural analysis of chips show that higher shear localization occurs at room temperature than the other two temperatures. The depth of machining affected zone (MAZ) on work surfaces was lower at the two temperatures than that of at the room temperature at a higher cutting speed of 91.8 m/min.

Masurtschak S., Friel R. J., Gillner A., et al. Laser-Machined Microchannel Effect on Microstructure and Oxide Formation of an Ultrasonically Processed Aluminum Alloy J. Eng. Mater. Technol. 137, 011006 (2015) (11 pages);   Paper No: MATS-13-1149;   doi:10.1115/1.4028926

Ultrasonic consolidation (UC) has been proven to be a suitable method for fiber embedment into metal matrices. To aid successful embedment of high fiber volumes and to ensure their accurate positioning, research on producing microchannels in combination with adjacent shoulders formed by distribution of the melt onto unique UC sample surfaces with a fiber laser was carried out. This paper investigated the effect of the laser on the microstructure surrounding the channel within an Al 3003-H18 sample. The heat input and the extent of the heat-affected zone (HAZ) from one and multiple passes was examined. The paper explored the influence of air, as an assist gas, on the shoulders and possible oxide formation with regards to future bonding requirements during UC. The authors found that one laser pass resulted in a keyhole-shaped channel filled with a mixture of aluminum and oxides and a symmetrical HAZ surrounding the channel. Multiple passes resulted in the desired channel shape and a wide HAZ which appeared to be an eutectic microstructure. The distribution of molten material showed oxide formation all along the channel outline and especially within the shoulder.

Topics: Lasers , Aluminum , Heat , Fibers
Violaine Tinard, Quang Tam Nguyen, Christophe Fond. Experimental Study on High Damping Rubber Under Combined Action of Compression and Shear J. Eng. Mater. Technol. 137, 011007 (2015) (7 pages);   Paper No: MATS-14-1095;   doi:10.1115/1.4028891

High damping rubber (HDR) is used in HDR bearings (HDRBs) which are dissipating devices in structural systems. These devices actually have to support permanent static load in compression and potential cyclic shear when earthquakes occur. Mastering the behavior of bearings implies an accurate understanding of HDR response in such configuration. The behavior of HDR is, however, complex due to the nonlinearity and time dependance of stress–strain response and especially Mullins effect. To the authors' knowledge, tests on HDR under combined quasi-static compression and cyclic shear (QC-CS) have not been performed with regard to Mullins effect yet. The purpose of this study is thus to assess experimentally Mullins effect in HDR, especially under QC-CS. In order to achieve this aim, cyclic tensile and compression tests were first carried out to confirm the occurrence of Mullins effect in the considered HDR. Then, an original biaxial setup allowing testing HDR specimen under QC-CS was developed. This setup enables us to identify Mullins effect of the considered HDR under this kind of loading. Tests carried out with this setup were thus widened to the study of the influence of compression stress on shear response under this loading, especially in terms of shear modulus and density of energy dissipation.

Susmel Luca, Taylor David. Estimating Lifetime of Notched Components Subjected to Variable Amplitude Fatigue Loading According to the Elastoplastic Theory of Critical Distances J. Eng. Mater. Technol. 137, 011008 (2015) (15 pages);   Paper No: MATS-13-1176;   doi:10.1115/1.4028927

The present paper is concerned with the use of the elastoplastic theory of critical distances (TCD) to perform the fatigue assessment of notched components subjected to in-service variable amplitude (VA) fatigue loading. The elastoplastic TCD takes as its starting point the assumption that the detrimental effect of stress/strain concentrators of any kind can efficiently be taken into account by directly postprocessing the entire elastoplastic stress/strain field in the vicinity of the notch apex. Thanks to its specific features, the TCD can be formalized in different ways by simply changing size and geometrical features of the domain used to calculate the required effective stress. The so-called point method (PM) is the simplest form in which this theory can be applied. This formalization of the TCD postulates that the elastoplastic stress/strain state to be used to estimate fatigue damage has to be determined at a given distance from the tip of the notch being assessed. According to the TCD's philosophy, such a distance is treated as a fatigue property. Therefore, given the material, this critical length does not change as either the features of the assessed stress/strain concentrator or the profile of the investigated loading path vary. In the present study, the above design strategy is attempted to be used to estimate lifetime of notched component subjected to VA loading, the required critical distance being determined under constant amplitude (CA) loading. The accuracy and reliability of the devised approach were checked by using a number of experimental results generated by testing, under both concave upward and concave downward spectra, notched samples containing geometrical features having a different sharpness. Such a validation exercise allowed us to prove that the elastoplastic TCD, used in the form of the PM, is highly accurate in estimating fatigue damage also in notched components subjected to in-service VA loading.

Topics: Fatigue , Stress

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