Methodologies and Equipment for Measuring High Temperature Tensile Properties of Subscale Specimen Geometries for Additively Manufactured Metallic Materials (Continued)

Editorial Note: With the growth in adoption of addititively manufactured materials across aerospace and defense manufacturing, we decided to include two parts of this Air Force Research Lab report.

August 1, 2023

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

An overview of a universal tensile tester. (Image: Air Force Research Laboratory Materials and Manufacturing Directorate)

Universal test frames are generally either screw-driven or servohydraulic, which are both perfectly suited for uniaxial tensile testing experiments. A uniaxial test frame is comprised of several basic fixture components: loading device, a load cell, and a specimen gripping apparatus.

An example of a commercially available tensile testing frame is shown below. Load cells are available in a wide range of load limits to accommodate the sensitivity needs for a wide range of materials and specimen geometries. ASTM E74 outlines the calibration procedure for load cells and must be followed to ensure proper measurement during testing, regardless of specimen size. Further details about alignment and gripping will be provided given their overall importance to testing accuracy. Standardized testing procedures have been established to assist in test frame and specimen alignment through the quantification of bending strains and their acceptable limits during testing.

Test frames may be configured with some type of universal joint that allows for concentricity and angularity adjustments for alignment. The largest contributor to bending strains originates from the test specimen-grip interface. ASTM E1012, established to address adequate alignment under tensile and compressive loading, outlines the usage of strain-gaged specimens to verify test equipment alignment through a series of loading-unloading and re-gripping cycles.

Specific to high temperature tension testing of metallic materials, standards mandate maximum bending strains do not exceed 10% of the axial strain.

Despite the existence of these standards, the issue persists where, in general, equipment is unavailable to measure maximum bending strains at elevated test temperatures. As a result, often, the only viable option is to qualify the alignment at room temperature using the intended machine setup for elevating temperature testing.

Notably, alignment requirements are designed for standard specimen sizes. Alignment procedures for test setup of subscale specimens are rarely reported in detail and seem to be generally considered a “best effort” based upon experimental expertise of the user.

This is somewhat troublesome because as specimen size and gauge lengths decrease, concentric and angular misalignments become more pronounced, i.e., larger bending strains. The subscale specimens may also require higher machining tolerance for more precise alignment in the specimen-grip interface. Furthermore, subscale specimen sizes frequently make it impractical to manufacture a strain-gaged specimen for proper test frame alignment due to physical constraints.

As a result, custom grips and alignment fixtures often are engineered to accommodate specifically designed subscale test specimen geometries and to ensure uniaxiality throughout testing. Self-aligning grips have also been used at room temperature and elevated temperatures.

This work was performed by Thomas Gallmeyer for the Air Force Research Laboratory Materials and Manufacturing Directorate. For more information, download the Technical Support Package (free white paper) below. AFRL-08232