ASTM B598 - Offset Yield Strength in Tension for Copper Alloys

UNDERSTANDING the Determination of Offset Yield Strength in Tension for Copper Alloys

Introduction

In the realm of material science, particularly concerning copper alloys, understanding the offset yield strength is crucial. It not only helps in the characterization of the material but is integral in predicting its behavior under various operational environments. The offset yield strength is the stress at which a specified amount of plastic deformation occurs. This article elaborates on standard practice ASTM B 598, adopted for determining offset yield strength in tension for copper alloys, a methodology approved for use by agencies of the Department of Defense.

Scope:

Standard practice ASTM B 598 specifically focuses on the determination of offset yield strengths (0.01%, 0.02%, and 0.05% offset) at room temperature. Although primarily applied to flat strip materials, the practice is versatile enough to be employed for other forms including wire, rod, and bar. The test is applicable to materials thicker than 0.010 in. (0.25 mm), particularly copper alloys commonly used in spring applications.

Test Procedure:

Apparatus:

  • A standard testing machine, conforming to Practices E 4 and E 74, ensuring precision and accuracy.

  • Class B-1 or more accurate Extensometers adhering to Practice E 83.

  • Extensometer Calibrator or a device of similar accuracy for precise measurements.

Test Specimen:

Tension test specimens should adhere to Test Methods E 8, ensuring relevance to the product form and consistency in testing.

Procedure:

  1. Preparation: Follow Test Methods E 8 for preparing and conducting a tension test. Pay special attention to the preparation of thin-gauge samples.

  2. Load Range: Ensure the yield strength falls within the upper half of the chart load range used. Choose a strain magnification for precise determination of strain increments.

  3. Preload Application: Apply a nominal 10% of the known or anticipated offset yield strength to remove slack and ensure alignment.

  4. Load Application: Attach the extensometer and apply the load uniformly. Ensure the rate of stress application does not exceed 100,000 psi/min.

  5. Extensometer Removal: Once the linear behavior is adequately departed, remove the extensometer without halting the test.

  6. Yield Strength Calculation: Refer to the stress-strain diagram and determine the offset yield strength according to Test Methods E 8.

Calculation and Report:

Calculate the yield strength by identifying where the offset line intersects the stress-strain curve. This is achieved by dividing the load in pounds at the intersection by the original cross-section area.

Precision and Bias:

The precision of this practice is influenced by various factors including specimen characteristics, testing conditions, and data interpretation methods. A comprehensive assessment requires addressing these variables to ensure accurate and reliable results. Currently, standard reference values or round-robin test data to determine bias are not available, indicating a need for further research and data accumulation.

Conclusion:

Standard Practice ASTM B598 stands as a fundamental approach in determining the offset yield strength of copper alloys. While it offers a comprehensive methodology, considerations regarding precision and bias underline the continuous need for refinement and advancement. By adhering to this standard practice, professionals in the field can ensure consistency, reliability, and accuracy in determining the offset yield strength, contributing significantly to the broader landscape of material science and engineering.

Reference documents associated with the standard

ASTM E4 - Practices for Force Verification of Testing Machines

This standard outlines the procedures for verifying that the force exerted by testing machines is accurate and reliable. In the context of determining the offset yield strength of copper alloys, ensuring that the testing machines are exerting the correct force is foundational. It establishes that the readings garnered during the tests are trustworthy, laying the groundwork for further analysis.

ASTM E6 - Terminology Relating to Methods of Mechanical Testing

Communication and understanding are bolstered by a common language. ASTM E6 provides definitions and terminologies used in mechanical testing, ensuring that professionals, whether they are testing, analyzing, or utilizing the data, have a unified understanding. This standard is essential in eliminating ambiguities and ensuring that the outcomes of tests are interpreted and communicated effectively.

ASTM E8 - Test Methods for Tension Testing of Metallic Materials

This standard is pivotal, outlining the methods for tension testing of metallic materials. In application to copper alloys, it ensures that the material is tested under standardized conditions, ensuring the reliability and comparability of results. Every step from the preparation of test samples to the process of testing is detailed, ensuring that the yield strength determined is accurate and reliable.

ASTM E74 - Practice for Calibration of Force Measuring Instruments for Verifying the Load Indication of Testing Machines

Precision in measurement is a cornerstone in material testing. ASTM E74 ensures that the force measuring instruments are calibrated and provide accurate readings. In the determination of offset yield strength, this practice ensures that the force readings during the tension tests are not just accurate but are verifiable, instilling confidence in the results obtained.

ASTM E83 - Practice for Verification and Classification of Extensometers

The measurement of deformation and strain is central in determining the yield strength. ASTM E83 outlines the practices for verifying that the extensometers, instruments measuring the elongation and deformation of materials under stress, are accurate. It ensures that the measurements of strain, pivotal in determining the offset yield strength, are precise and reliable.

Key Words:

  • Offset yield

  • Tensile test

  • Yield strength

  • Copper alloys

  • Material testing

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ASTM B754 - Measuring Flatness in Copper and Copper Alloy Strip

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ASTM B577 - Detection of Cuprous Oxide (Hydrogen Embrittlement Susceptibility) in Copper