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Standards Development
This proposal specifies terms, definitions, symbols, principles, apparatus, specimen, test procedure, data processing, result evaluation, report and other content for the high strain rates tensile test at elevated temperature for metallic materials by using split Hopkinson tensile bar. This proposal is applicable to the determination of mechanical properties of metallic materials at elevated temperature within the strain rate of 102s-1 ~ 103s-1, which are applied for the study of impact-related problems, such as analysis of vehicle collision safety based on numerical simulation in automotive industry, numerical predictive assessment of aircraft impact in aviation industry, etc.
In many engineering application scenarios, Metallic materials are subjected to both high temperature and high strain rate. For example, the aircraft engine, especially the blades. Nuclear plant safety, with the high pressure steam pipeline, airplane and Spacecraft crash or collision, etc. In structural applications, various components must be designed to function over a broad range of strain rates and temperatures. During impact events, for example, stress or strain waves propagate inside the impacted bodies, inelastic deformations may develop and the impacted bodies may be excited leading to destructive damage. As well known that mechanical behaviour at elevated temperature and high strain rates differs considerably from that observed at room temperature and quasi-static strain rates. Experimental results, for example, indicate that the yield stress of many metals decreases with the temperature increases with the rate of loading. Therefore, to determine the high strain rate tensile properties of metallic materials at high temperature by the Hopkinson bar test method is of great importance for the engineering design, structural optimization, processing and evaluation of metallic structures. Conventional servo-hydraulic machines are generally used for testing at quasi-static strain rates of 1 s- 1 or less With special design, it is possible to attain greater strain rates, up to about 100 s-1, with conventional load frames. For higher strain rates, other test methods are required. Measurements at high strain rates cannot be performed by conventional testing machines. For this reason, special methods and experimental arrangements are used. Like Hopkinson techniques. The key principles of the split-Hopkinson bar in tension are (a) the methods of generating a tensile loading pulse and (b) the method of attaching the specimen to the incident and transmission bars. The method started from John Hopkinson and his son Bertram Hopkinson in 1914 who proposed a technique to measure the shape of an impact stress pulse in a long elastic bar. Kolsky in 1949 extended the Hopkinson bar method to measure stress–strain response of materials under impact loading. The pressure bar technique of Kolsky was similar to that developed by Davis in 1948, except that Kolsky used two bars and the specimen was sandwiched in between. This allows deformation of a specimen of ductile material at high strain rates, while maintaining a uniform uniaxial state of stress within the specimen. The maximum strain rate in a Kolsky bar varies inversely with the length of the specimen. It is limited by the elastic limit of the two bars that are used to transmit the stress pulse to the specimen. With the Kolsky bar it is possible to develop the uniaxial stress–strain behavior of materials at a variety of strain rates. This allows the development of constitutive relations that express the uniaxial stress as a function of strain, strain rate and temperature. The first edition of ISO 26203-1 standards was released in 2010, and revised in 2018.While, since 1980s, it has become more common to use SHTB to carry out high temperature and high strain rates tensile tests. High temperature SHTB has become an important branch of SHTB testing methods. Unfortunately, temperature-related contents are not included in the ISO 26203-1:2018. Up to present, many standards have been published for the mechanical properties of metallic materials at high strain rates. For instance, ISO 26023-1: Metallic materials - Tensile testing at high strain rates - Part 1: Elastic-bar-type system; ISO 6892-2: Metallic materials — Tensile testing — Part 2: Method of test at elevated temperature, etc. However, both of them have no comments or not sufficient on thehigh temperature high stain rate test method for metallic materials. Even though the high temperature split Hopkinson (or Kolsky) bar has proved to be a robust piece of equipment, having been widely used to determine the dynamic mechanical properties for variety of materials including composites, aluminum, stainless steel, and the super alloy Inconel at elevated temperature etc. However, high temperature dynamic test method has not been embodied in the international standard. The main reasons are as following. Firstly, there are many heating methods for samples, but how to ensure the uniformity of specimen still have great controversies. Secondly, Temperature gradient has great influence on the propagation of stress wave in waveguide bar. The correction method of the influence of temperature on the waveguide needs to be unified. Thirdly, the fixing method of high temperature sample and the non-contact temperature measurement method need to be standardized. Finally, eliminate the flexibility and give the recommended temperature calibration methods. This proposal is designed on the basis of the nearly 2-Year applicable Chinese standard GB/T 37783- 2019 Metallic Materials – High strain rate tensile test method at elevated temperature. Seeking the unification of high strain rate tensile test method at high temperature. The proposed new standard (ISO:26203-1) is applicable to determine the dynamic tensile properties of metallic materials under various strain rates at elevated temperature by classical experimental technique named Split Hopkinson Tension Bar (SHTB). This proposal aims to standard high strain rate tensile test method at elevated temperature for determination of the dynamic mechanical properties of metallic materials in the strain rate range of 102s-1-103s-1 at high temperature. The obtained properties refer to tensile stress-strain curves, yield strength in tension, failure strain at variable temperature. This proposal includes draft, which describes the experimental method for high temperature high-strain-rate measurements of dynamic mechanical parameters applied for the study of impact-related problems, such as analysis of aircraft engine reliability based on numerical simulation in aviation industry, numerical predictive assessment of pipeline safety in chemical industry, etc.
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