Characterization of the Formability of High-Purity Polycrystalline Niobium Sheets for SRF Applications

The forming limit diagram of high-purity niobium sheets used for the manufacturing of superconducting radiofrequency (SRF) cavities is presented. The Marciniak (in-plane) test was used with niobium blanks with a thickness of 1 mm and blank carriers of annealed oxygen-free electronic copper. A high formability was measured, with an approximate true major strain at necking for plane-strain of 0.441. The high formability of high-purity niobium is likely caused by its high strain rate sensitivity of 0.112. Plastic strain anisotropies (r-values) of 1.66, 1.00, and 2.30 were measured in the 0°, 45°, and 90° directions. However, stress–strain curves at a nominal strain rate of ~10−3 s−1 showed similar mechanical properties in the three directions. Theoretical calculations of the forming limit curves (FLCs) were conducted using an analytical two-zone model. The obtained results indicate that the anisotropy and strain rate sensitivity of niobium affect its formability. The model was used to investigate the influence of strain rate on strains at necking. The obtained results suggest that the use of high-speed sheet forming should further increase the formability of niobium.

Plastic Behavior of Laser-Deposited Inconel 718 Superalloy at High Strain Rate and Temperature

Nickel-based superalloys have several applications for components exposed to high temperatures and high strain rate loading conditions during services. The objective of this study was to investigate the tensile properties of Inconel 718 produced using the laser metal deposition technique. Specimens with different heat treatments were investigated. Experimental tests were performed at the DYNLab at Politecnico di Torino (Italy). The temperature sensitivity was investigated between 20 °C and 1000 °C on a Hopkinson bar setup at a nominal strain rate of 1500 s−1. The specimens heating was obtained by means of an induction heating system, and the temperature control was performed by thermocouples, an infrared pyrometer, and a high-speed infrared camera. The thermal images were analyzed to check the uniformity of the heating and to investigate the presence of adiabatic self-heating. The results showed that the materials strength exhibited a significant drop starting from 800 °C. The strain rate influence was investigated at room temperature, and limited sensitivity was found covering six orders of magnitude in the strain rate. A preliminary analysis of the fracture mode was performed. Finally, different solutions for the strength material modeling were proposed and discussed with the aim of identifying models to be used in finite element simulations.

Thermomechanical tensile properties of deposited Inconel 718 superalloy over a wide range of strain rate and temperature

Nickel-based superalloys show high strength retained also at high temperature and they are widespread used for structural components exposed during services to high temperature combined with high strain rate or impact loading conditions. The objective of this study was the investigation of the plastic flow behaviour of Laser Metal Deposited Nickel-based superalloy Inconel718. The material was manufactured at Northwestern Polytechnical University in China. Specimens with three different heat treatment conditions were investigated: as-deposited, directly aged and aged after homogenization and solution. High strain rate tensile tests were performed on the direct Hopkinson bar setup developed at DYNLab laboratory at Politecnico di Torino. At a nominal strain rate of 1500 s-1 the temperature sensitivity was investigated between 20 and 1000°C. An induction heating system was adopted, and the temperature was monitored by thermocouples and infrared pyrometer and high-speed camera. The results showed the materials strength decreases as a function of temperature with a significant drop starting from 800 °C. An asymmetric tension-compression behaviour was found by comparing the results with data in compression. The strain rate influence was investigated at room temperature and very limited or negligible sensitivity was found covering six orders of magnitude in strain rate.

Strain rate and temperature dependent strain localization of dynamically stretched lightweight bars: from metal to polymer

This work studies the dynamic strain localization and constitutive relationship of a Ti3Al2.5V alloy in jet engine containment system and a transparent polycarbonate conceived for aircraft canopy application by Digital Image Correlation (DIC) technique from quasi-static condition to high strain rates at different temperatures. The responses of two materials show significant strain rate and temperature sensitivities. Observations of Ti3Al2.5V alloy show that the dynamic local strain rate can reach values up to 1000 % of the nominal strain rate in the necking zone. However, dynamic local strain rate of polycarbonate soars up during strain softening then decreases rapidly with necking propagation, and eventually becomes 20 % of the nominal strain rate until fracture. Appropriate viscoplastic constitutive models are determined for both materials, which are incorporated in finite element simulations to reveal the trend of dynamic local strain rate evolution in dynamic tensile tests. The present work shows two different kinds of strain localization in typical lightweight materials, which should be addressed carefully from Split Hopkinson Tension Bar (SHTB) tests.

Variation of Mechanical Properties across the Dissimilar Joints: Ferritic steel and Stainless Steel

Grade 91 ferritic steel (also known as P91) is widely used for constructing steam generators in prototype fast breeder reactors (PFBR) because of their stress corrosion cracking resistance, high thermal conductivity and low thermal expansion co-efficient for intermediate heat exchangers and austenitic stainless steels (SS304L, SS316L and SS316LN.) Due to their high temperature creep strength and good corrosion resistance, the dissimilar metal weld (DMW) joints between these materials are unavoidable. Inconel 82/182 filler metal is recommended to join these materials since its thermal expansion co-efficient lies between that of ferritic steel and stainless steel. In the present investigation, the tensile properties of each region of DMW joint have been evaluated. DMW joint between P91 and SS 316LN were fabricated using manual metal arc welding (MMAW) process with inconel 82/182 filler metals. The tensile properties of various regions of DMW were examined at a nominal strain rate of 1×10-3 s-1. Microstructural features of various regions of DMW were examined through optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Out of different regions, P91 parent metal exhibited higher tensile strength while the transverse tensile specimens failed at the outer edge of the HAZ of P91 steel. This heterogeneity in mechanical properties may be due to the evolution of complex microstructures developed during welding.

Empirical Equations for a Study on Strain Rate Sensitivity of Polypropylene Syntactic Foam

The strain rate sensitivity of polypropylene syntactic foams with polymer microballoons in the relative density from 0.5 to 0.8 is studied at the nominal strain rate ranged from 10-1 to 102 s -1. Two equations of matrix materials are introduced to represent the viscoelastic properties, and another two equations are proposed with respect to the parameters of matrix materials to estimate the elastic moduli and yield stresses of polymeric syntactic foams.

Strain-Rate Sensitivity of Aluminum Alloys AA1200 and AA3103

Tensile tests are carried out for the aluminum alloys AA1200 and AA3103 at various strain-rates in the range from 10−4 s−1 to 1 s−1. Tests with constant nominal strain-rate and strain-rate jump tests are conducted, and the instantaneous rate sensitivity and the rate sensitivity of strain hardening are investigated. For both materials, the instantaneous rate sensitivity is found to be rather independent of strain, while the rate sensitivity of the strain hardening is important and the saturation stress increases with increasing strain-rate. A phenomenological constitutive model is described that comprises a kinetic equation governing the instantaneous rate sensitivity of the flow stress and a structural parameter that determines the mechanical state of the material. The evolution of the structure parameter is assumed to depend on strain-rate. The model parameters are determined for the two materials using the available experimental information. It is found that the constitutive model provides a good representation of the experimental results.

The Wavelike Plastic Deformation of Single Crystal Copper

The purpose of this work is to explore nonuniform plastic flow at small length- and time-scales. Pure single crystal copper tensile specimens were pulled along the [6¯ 5 6] crystal axis at three nominal strain-rates: 0.01%/s, 0.04%/s, and 0.10%/s. Simultaneously, the surface deformation was monitored with in situ digital image correlation over a length-scale of ∼100 μm and a time-scale of 0.07–0.2 s. Sequential digital image correlation strain-rate fields show compelling evidence of a wavelike plastic deformation that is proportional to the nominal strain-rate and decelerates with increasing strain hardening. While a mechanism responsible for the waves is not identified, a methodology correlating observations of multiple researchers is forwarded.

Plane Strain Indentation of Snow at the Microscale

Indentation tests are commonly used to characterize terrain properties in tire-terrain interaction. By way of motivation, in order to understand and quantify the sources of uncertainty in tire-snow interaction, we present a new direction of research studying the indentation behavior of snow at the microscale. The mechanical behavior of snow is known to be influenced significantly by its microstructure which can evolve due to environmental conditions. The traction and compaction of snow due to interfacial friction and contact exerted by various parts of the tire thus should also be dependent on snow’s microstructure. Due to the geometric scales of parts of the tire, from sipes to tread blocks, tire-snow interaction is inherently a multi-scale problem ranging from microscale to macroscale. This paper addresses the microscale behavior of medium-density snow (density 387 kg / m3) whose microstructure is obtained via 3-D X-Ray Microtomography (XMT). A physically based, history and rate dependent viscoplastic model for polycrystalline ice was incorporated into a meshfree Material Point Method simulation code using an implicit algorithm on a parallel computer for the indentation of a punch on snow under the plane strain condition at a nominal strain rate of 0.0014 Sec−1. Load/pressure vs. sinkage/strain relationship was obtained and compared with macroscopic pressure-sinkage relationship. Micromechanical behavior of the indentation test was also presented and discussed.

High Temperature Mechanical Behavior of a Mo-Si-B Solid Solution Alloy

Multi phase alloys at the Mo-rich end of the Mo-Si-B system have drawn recent attention because of their high temperature performance capabilities. Previous studies on two- and three-phase alloys have confirmed the central role of the Mo-rich solid solution phase in affecting creep resistance and low-temperature toughness in these multiphase alloys. Thus, it is important to understand the intrinsic mechanical response of the matrix solid solution. In this study, compression and tensile tests were conducted over a nominal strain rate regime spanning 10-4 s-1 to 10-7 s-1 and temperature ranging from 1000°C to 1300°C in vacuum on a Mo-Si-B solid solution alloy (Mo-3Si-1.3B in at.%) that contained a low fraction (~5 %) of the T2 phase. The microstructure of the deformed specimens was examined to elucidate the underlying deformation mechanisms.