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Test Methods for Measuring High Temperature Tensile Properties of Subscale Specimen Geometries for Additively Manufactured Metallic Materials

An overview of the state-of-the-art for subscale high temperature tensile testing of metallic materials.

Figure 1. A generalized specimen layout. (Image: Air Force Research Laboratory)

With the rise of additive manufacturing (AM) for metallic materials, concerted efforts are underway to integrate the technology into present and future aerospace systems to enhance performance capability, reduce cost, and minimize production lead-times. One advantage of AM components is the generation of complex and thin-walled geometries for location-specific performance otherwise unachievable through conventional manufacturing means. Yet, as a new manufacturing process, AM leads to unique microstructures that must be properly assessed for material properties and performance.

At times, regions of interest at the component level will require subscale specimen excision and evaluation for proper characterization as witness coupons may not fully capture location-specific performance. Other industries, e.g., nuclear power, have adopted non-standardized testing with the goal of test specimen miniaturization so to characterize material response using minimal material volume without the sacrifice of accurately capturing bulk material properties. Material scarcity, costs, and handling hazards all motivate the need for developing such a testing capability.

Two of the principal standards recognized for elevated temperature tensile testing of metallic materials are ASTM E21 and ISO 6892-2. Notably, these standards heavily pull from their room temperature equivalents – ASTM E8 and ISO 6892-1 – to specify acceptable specimen geometries and general test setup. However, none of the standards directly addresses how to approach the intricacies specific to subscale specimen testing.

There are current efforts within ASTM Subcommittee E28.04.01 (Task Group on Small Specimens in E8/E8M) to publish an annex providing guidelines to uniaxial tensile testing of subscale geometries. Yet, the annex has limited scope to room temperature evaluation only. Additional efforts are underway through ASTM Subcommittee F42.01 (New Test Method for Additive Manufacturing – Test Artifacts – Miniature Tension Testing of Metallic Materials), which seeks to develop a miniature rectangular cross-section tension specimen with a gauge length of 10-15 mm. However, the title and scope of the effort presently is in draft form.

ASTM E8 and ISO 6892-1 provide guidance on acceptable standard tensile specimen geometries. A generalized specimen layout is presented in Figure 1. The ratios have been established to bound the design parameters to ensure proportional response, i.e., interlaboratory comparability. Often, standard geometries serve as the basis for subscale specimen design. ASTM E8 specifically outlines a subsize geometry for rectangular tension test specimens. The subsize specimen has a gauge length of 25.0 mm, width of 6.0 mm, and variable thickness not to exceed the width dimension. Yet, despite explicitly defining a subsize geometry, the scale of its standardized form remains large in the context of subscale testing. Hence, efforts have been undertaken to retain high fidelity assessment while minimizing material volumes.

See the author information for this Tech Brief in the continuation of this report featured in the next section.

For more information, download the Technical Support Package (free white paper) below. AFRL-08234

Topics:
3D Printing & Additive Manufacturing Additive manufacturing Advanced manufacturing Aerospace Manufacturing & Prototyping Manufacturing processes Materials properties Metals Nanotechnology Production Tensile strength Test & Measurement Test procedures
This Brief includes a Technical Support Package (TSP).
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Test Methods for Measuring High Temperature Tensile Properties of Subscale Specimen Geometries for Additively Manufactured Metallic Materials

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    Aerospace & Defense Technology Magazine

    This article first appeared in the August, 2023 issue of Aerospace & Defense Technology Magazine (Vol. 8 No. 5).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Assessment of Test Methods for Measuring High Temperature Tensile Properties of Subscale Specimen Geometries for Additively Manufactured Metallic Materials" is an interim report published by the Air Force Research Laboratory on May 15, 2023. It covers the evaluation of methodologies and equipment used to assess the tensile properties of subscale specimens, particularly focusing on materials produced through additive manufacturing (AM).

    The report begins with an executive summary that outlines the objectives and significance of the research. It emphasizes the need for reliable testing methods to understand the mechanical properties of AM metallic materials, especially at high temperatures, which are critical for various aerospace and defense applications.

    The introduction sets the context for the study, highlighting the growing use of AM technologies and the challenges associated with characterizing the mechanical properties of subscale specimens. The document details current methodologies, equipment, and instrumentation used in tensile testing, including standard and non-standard specimen geometries, uniaxial test frame setups, and various heating sources such as radiant, resistance, and induction heating.

    A significant portion of the report discusses identified gaps in the current testing methodologies, particularly in high-temperature tensile testing. It addresses the effect of specimen size on tensile properties and the implications for additive manufacturing processes. The report also explores considerations specific to AM, such as the impact of as-built surface finishes on performance assessment and the intrinsic versus extrinsic performance of AM metallic materials.

    The conclusions drawn from the research highlight the necessity for standardized testing protocols that can accurately reflect the mechanical properties of AM materials. Recommendations for future research and testing methodologies are provided to bridge the identified gaps and improve the reliability of tensile property assessments.

    Overall, this document serves as a comprehensive resource for researchers and engineers involved in the development and testing of additively manufactured metallic materials, offering insights into current practices and future directions for enhancing testing methodologies in high-temperature environments. The report is publicly available, facilitating knowledge exchange within the scientific and technical communities.

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