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.

August 1, 2023

Air Force Research Laboratory, Wright-Patterson Air Force Base, OH

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.

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