Background: Using a thin-walled tube torsion test to characterize a material’s shear response is a well-known technique; however, the thin walled specimen tends to buckle before reaching large shear deformation and failure. An alternative technique is the surface stress method (Nadai 1950; Wu et al. J Test Eval 20:396–402, 1992), which derives a shear stress-strain curve from the torque-angular displacement relationship of a solid cylindrical bar. The solid bar torsion test uniquely stabilizes the deformation which allows us to control and explore very large shear deformation up to failure. However, this method has rarely been considered in the literature, possibly due to the complexity of the analysis and experimental issues such as twist measurement and specimen uniformity. Objective: In this investigation, we develop a method to measure the large angular displacement in the solid bar torsion experiments to study the large shear deformation of two common engineering materials, Al6061-T6 and SS304L, which have distinctive hardening behaviors. Methods: Modern stereo-DIC methods were applied to make deformation measurements. The large angular displacement of the specimen posed challenges for the DIC analysis. An analysis method using multiple reference configurations and transformation of deformation gradient is developed to make the large shear deformation measurement successful. Results: We successfully applied the solid bar torsion experiment and the new analysis method to measure the large shear deformation of Al6061-T6 and SS304L till specimen failure. The engineering shear strains at failure are on the order of 2–3 for Al6061-T6 and 3–4 for SS304L. Shear stress-strain curves of Al6061-T6 and SS304L are also obtained. Conclusions: Solid bar torsion experiments coupled with 3D-DIC technique and the new analysis method of deformation gradient transformation enable measurement of very large shear deformation up to specimen failure.
The method of thin-wall tube torsion to characterize metal’s shear response is well-known. Unfortunately, the thin wall tube specimen tends to buckle before reaching large shear deformation and failure. An alternative technique, which has rarely been considered, is Nadai’s surface stress method (Nadai, Theory of Flow and Fracture of Solids. McGraw-Hill, New York, 1950). It derives shear stress-strain curve from the torque-twist relationship of a solid bar. Although the analysis is more complex due to nonlinear shear stress distribution along the radius, the deformation is stable through large shear deformation to failure. Solid bar torsion experiments were conducted to study large shear deformation of Al6061-T6. Two experiments were described in this study. Since few tests were available in the literature, these experiments were to explore the large deformation behaviors of an engineering alloy and the application of modern measurement techniques, such as 3D DIC method, under torsion. Results show during twisting, the surface shear strain distribution was uniform initially and then localized on a narrow band; eventually, the specimen was cracked and failed within the band. Depending on the specimen size, the twist could be greater than 360°. Details are discussed.
Foam materials are extensively utilized in aerospace, military, and transportation applications to mitigate blast or shock impact. When foam materials are subjected to an external high-speed impact, shock, or blast loading, an elastic wave or shock wave will form and propagate through the thickness of the foam materials. In this study, silicone foam pads, which were confined laterally and pre-strained to different levels, were experimentally characterized and theoretically analyzed to understand their effects on wave propagation characteristics under impact loading. Depending on impact velocity, either an elastic strain wave or a shock wave would be generated in the silicone foam pad with different pre-strains. Above a certain impact velocity, a shock wave will be generated whereas, below this threshold impact velocity, an elastic strain wave will be generated. This threshold impact velocity depends on the pre-strain applied to the silicone foam pad. Equations are provided to estimate the wave propagation speed for either an elastic or a shock wave from the amount of pre-strain in the silicone foam pads and the impact velocity. These equations are expected to help improve silicone foam design and assembly processes for shock or blast mitigation applications.
Polymeric foams have been extensively used in shock isolation applications because of their superior shock or impact energy absorption capability. However, as a type of soft condensed matter, the highly nonlinear, heterogeneous, and dissipative behavior of polymeric foams may result in an ineffective mitigation or isolation to shock/blast loading. To meet certain desired shock mitigation or isolation requirements, the polymeric foams need to be experimentally characterized to obtain their intrinsic material response. However, radial inertia during dynamic compression has become a severe issue and needs to be fully understood. In this study, we developed an analytical method to calculate the additional stress induced by radial inertia in a polymeric foam specimen. The radial inertia is generally caused by Poisson’s effect and associated with three different mechanisms – axial strain acceleration, large deformation, and Poisson’s ratio change. The effect of Poisson’s ratio change during deformation on radial inertia was specifically investigated for hyperelastic foam materials, and verified with experimental results obtained from Kolsky compression bar tests on a silicone foam.
Polymeric porous materials have a wide range of applications. An important one in structural engineering is to use foams for cushioning or absorbing the kinetic energy from impact. Conventional foaming processes produce polymeric foams with disordered three-dimensional networks, which are dispersion in cell shape, size, etc. Since mechanical properties depend on the shape and structure of the cell, these foams are difficult to characterize and predict due to complexity and variation of cells. The new 3D printing fabrication method can now prepare components of foams with perfect regular array of cells. The printed foams potentially could be tuned or designed for application. In this study, foam pads of various porosities were printed using the same polymer. They all have a Body Centered Cubic (BCC) cell structured but with different span sizes. Experiments were conducted to characterize these foam pads in compression and shear, including off-axis loadings. The property of printing polymer was also characterized for analyzing the behaviors of these foam pads. Results are compared.
This report describes the mechanical characterization of six types of woven composites that Sandia National Laboratories are interested in. These six composites have various combinations of two types of fibers (Carbon-IM7 and Glass-S2) and three types of resins (UF- 3362, TC275-1, TC350-1). In this work, two sets of experiments were conducted: quasi-static loading with displacement rate of 2 mm/min (1.3x10^( -3 ) in/s) and high rate loading with displacement of 5.08 m/s (200 in/s). Quasi-static experiments were performed at three loading orientations of 0deg, 45deg, 90deg for all the six composites to fully characterize their mechanical properties. The elastic properties Young's modulus and Poisson's ratio, as well as ultimate stress and strain were obtained from the quasi-static experiments. The high strain rate experiments were performed only on glass fiber composites along 0deg angle of loading. The high rate experiments were mainly to study how the strain rate affects the ultimate stress of the glass-fiber composites with different resins.
Experiments were performed to characterize the mechanical response of a 15 pcf flexible polyurethane foam to large deformation at different strain rates and temperatures. Results from these experiments indicated that at room temperature, flexible polyurethane foams exhibit significant nonlinear elastic deformation and nearly return to their original undeformed shape when unloaded. However, when these foams are cooled to temperatures below their glass transition temperature of approximately -35 o C, they behave like rigid polyurethane foams and exhibit significant permanent deformation when compressed. Thus, a new model which captures this dramatic change in behavior with temperature was developed and implemented into SIERRA with the name Flex_Foam to describe the mechanical response of both flexible and rigid foams to large deformation at a variety of temperatures and strain rates. This report includes a description of recent experiments. Next, development of the Flex Foam model for flexible polyurethane and other flexible foams is described. Selection of material parameters are discussed and finite element simulations with the new Flex Foam model are compared with experimental results to show behavior that can be captured with this new model.
Laser Engineered Net Shaping (LENS) and Powder Bed Fusion (PBF) are 3-D additive manufacturing (AM) processes. They are capable of printing metal parts with complex geometries and dimensions effectively. Studies have shown that AM processes create metals with distinctive microstructure features and material properties, which are highly dependent on the processing parameters. The mechanical properties of an AM material may appear to be similar to the corresponding wrought material in some way. This investigation focuses on the relationships among AM process, microstructure features, and material properties. The study involves several AM SS316L components made from 3D LENS and PBF printing. Specimens were taken from different locations and orientations of AM components to obtain the associated tensile properties, including yield, strength, and ductility, and to conduct microstructure analyses.
We investigate the process-structure-property relationships for 316L stainless steel prototyping utilizing 3-D laser engineered net shaping (LENS), a commercial direct energy deposition additive manufacturing process. The study concluded that the resultant physical metallurgy of 3-D LENS 316L prototypes is dictated by the interactive metallurgical reactions, during instantaneous powder feeding/melting, molten metal flow and liquid metal solidification. The study also showed 3-D LENS manufacturing is capable of building high strength and ductile 316L prototypes due to its fine cellular spacing from fast solidification cooling, and the well-fused epitaxial interfaces at metal flow trails and interpass boundaries. However, without further LENS process control and optimization, the deposits are vulnerable to localized hardness variation attributed to heterogeneous microstructure, i.e., the interpass heat-affected zone (HAZ) from repetitive thermal heating during successive layer depositions. Most significantly, the current deposits exhibit anisotropic tensile behavior, i.e., lower strain and/or premature interpass delamination parallel to build direction (axial). This anisotropic behavior is attributed to the presence of interpass HAZ, which coexists with flying feedstock inclusions and porosity from incomplete molten metal fusion. The current observations and findings contribute to the scientific basis for future process control and optimization necessary for material property control and defect mitigation.