What is ASTM D638?
ASTM D638 is a widely recognized standard test method developed by the American Society for Testing and Materials (ASTM) for determining the tensile properties of plastics. It provides a standardized procedure for measuring the strength, stiffness, and elongation characteristics of plastic materials when subjected to a tensile force.
The tensile properties of plastics are essential to understand their mechanical behavior and suitability for various applications. ASTM D638 helps ensure consistent and comparable data across different materials and manufacturers, facilitating material selection, quality control, and product development processes.
The test is typically conducted on a universal testing machine, also known as a tensile testing machine, which applies a controlled force to a test specimen in a linear direction until it breaks. During the test, various mechanical properties are measured, including tensile strength, yield strength, modulus of elasticity, elongation at break, and Poisson's ratio.
The ASTM D638 standard provides guidelines on specimen preparation, conditioning, and testing parameters, ensuring that results are accurate and reproducible. It outlines specific requirements for specimen dimensions, including gauge length and cross-sectional area, as well as recommendations for testing speed and environmental conditions.
By following ASTM D638, manufacturers, researchers, and engineers can obtain critical data on the performance and behavior of plastic materials, aiding in the selection of appropriate materials for specific applications. The standard also enables the comparison of different materials, quality control assessments, and the development of material specifications and regulations.
It's important to note that while this introduction provides a general overview of ASTM D638, it is always advisable to refer to the specific standard document for complete and up-to-date information on the test method and any associated requirements or modifications.
Scope of ASTM D638
The scope of ASTM D638, titled "Standard Test Method for Tensile Properties of Plastics," encompasses the evaluation and measurement of the tensile properties of plastic materials. It provides a standardized methodology for determining the mechanical behavior of plastic materials under tension. Here are the key aspects covered by the scope of ASTM D638
1. Material Types: The standard is applicable to a wide range of plastic materials, including thermoplastics, thermosetting plastics, and reinforced plastics. It covers both rigid and flexible materials, allowing for the testing of various types of plastics commonly used in industrial, commercial, and consumer applications.
2. Tensile Properties: ASTM D638 focuses on the determination of key tensile properties of plastics, including tensile strength, yield strength, elongation at break, modulus of elasticity, and Poisson's ratio. These properties provide insights into the material's ability to withstand tensile forces and its elongation behavior before failure.
3. Test Specimens: The standard specifies the dimensions and shape of the test specimens, which are typically dog bone-shaped samples with a reduced cross-sectional area in the center. It provides guidelines for specimen preparation, including specimen-cutting methods and dimensions based on the thickness of the plastic material.
4. Test Conditions: ASTM D638 outlines the environmental conditions for specimen conditioning and testing. This includes recommendations for temperature and humidity during conditioning to achieve equilibrium moisture content and consistent test conditions. The standard also suggests specific testing speeds for uniform strain rates.
5. Testing Equipment: The standard does not provide specific requirements for testing machines or equipment. However, it emphasizes the need for properly calibrated and verified equipment to ensure accurate measurements of the applied load and elongation. The selection and calibration of the testing equipment are critical for obtaining reliable and consistent results.
6. Data Analysis: ASTM D638 provides guidelines for calculating and reporting the various tensile properties based on the recorded load and elongation data. These calculations allow for meaningful comparison and interpretation of the mechanical behavior of different plastic materials.
The scope of ASTM D638 is extensive, covering a broad range of plastic materials and providing standardized procedures for testing their tensile properties. By adhering to the standard, manufacturers, engineers, researchers, and regulatory bodies can obtain consistent and reliable data for material selection, quality control, research and development, and compliance with industry standards and regulations.
Why ASTM D638 is important?
ASTM D638 is important for several reasons:
1. Standardization: ASTM D638 provides a standardized test method for evaluating the tensile properties of plastics. Standardization ensures consistency and comparability of test results across different materials, manufacturers, and laboratories. This allows for meaningful comparisons and facilitates material selection, quality control, and research and development efforts.
2. Material Selection: The tensile properties of plastics, such as tensile strength, yield strength, and elongation, are crucial for determining their suitability for specific applications. By conducting ASTM D638 tests, manufacturers and engineers can assess the mechanical performance of different plastic materials and make informed decisions regarding material selection. This helps ensure that the chosen materials meet the required performance criteria and can withstand the anticipated stresses and loads.
3. Quality Control: ASTM D638 enables manufacturers to establish quality control procedures for plastic materials. By regularly testing samples using this standard, manufacturers can monitor the mechanical properties of their materials and ensure that they consistently meet specified requirements. This helps maintain product quality, reliability, and safety.
4. Research and Development: Researchers and scientists utilize ASTM D638 to evaluate new plastic formulations, additives, and processing techniques. By performing systematic tensile testing, they can assess how these factors affect the mechanical behavior of plastics. This information aids in optimizing material formulations, improving product performance, and developing new materials with enhanced mechanical properties.
5. Regulatory Compliance: Many industries have specific regulatory requirements and standards that plastics must meet. ASTM D638 is often referenced in industry-specific standards and regulations, providing a recognized methodology for testing the tensile properties of plastic materials. Compliance with these standards ensures that materials meet the necessary performance and safety criteria.
Overall, ASTM D638 plays a crucial role in ensuring consistent and reliable assessment of the tensile properties of plastics. It assists in material selection, quality control, research, and development efforts, and compliance with industry standards and regulations. By using this standard, stakeholders can make informed decisions, enhance product performance, and maintain the integrity and safety of plastic materials.
Apparatus of the ASTM D638
Here is a detailed description of each apparatus used in the ASTM D638 test:
1. Tensile Testing Machine: A tensile testing machine, also known as a universal testing machine, is a mechanical device used to apply a controlled tensile force to the plastic specimen until it breaks. It consists of a load frame, load cell, grips, and a crosshead. The load frame provides a stable structure for the test, while the load cell measures the force applied to the specimen. The grips securely hold the specimen and prevent slippage during the test. The crosshead is attached to the grips and moves at a specified rate to elongate the specimen until failure.
2. Grips: Grips are essential for securely holding the plastic specimen during the test. They come in different types depending on the specimen's shape, dimensions, and material properties. Commonly used grips include wedge grips, which use mechanical wedges to hold the specimen, and screw grips, which utilize screws to grip the specimen tightly. Pneumatic grips are also available, using air pressure to secure the specimen.
3. Extensometer: An extensometer is an optional device used to measure the elongation or strain of the specimen during the test accurately. It consists of two arms or points that are attached to the specimen and measures the displacement as the specimen is elongated. The extensometer provides more precise measurements of elongation at specific points along the specimen gauge length, allowing for accurate calculation of strain and modulus of elasticity.
4. Specimen Cutting Die: A specimen cutting die is used to prepare standardized specimens with specific dimensions as required by the test method. The die is usually made of hardened steel and has a shape that matches the desired specimen dimensions. It ensures consistency in specimen dimensions and shape, allowing for accurate and reproducible test results.
5. Vernier Calipers or Micrometer: Vernier calipers or micrometers are precision measuring instruments used to measure the dimensions of the test specimen. They are used to measure the width, thickness, and gauge length of the specimen. Vernier calipers use a sliding scale and a Vernier scale to provide precise measurements, while a micrometer uses a screw mechanism for accurate measurements. Accurate specimen measurements are crucial for calculating stress and strain values during the test.
6. Conditioning Chamber: A conditioning chamber, also known as an environmental chamber, is used to maintain a specific temperature and humidity during specimen preparation and testing. The test specimens are typically conditioned to standard temperature and humidity conditions before testing to ensure consistent results. The conditioning chamber helps control the environmental factors that may affect the mechanical properties of the plastic material.
7. Test Specimens: The test specimens are prepared from the plastic material being evaluated. They are typically in the form of dogbone-shaped samples, with a reduced cross-sectional area in the center to promote consistent failure in that region during the test. The dimensions and shape of the test specimens are defined by the standard, and they are prepared using a specimen cutting die or other suitable cutting methods.
These are the key apparatus used in the ASTM D638 test. Each plays a crucial role in conducting the test accurately and ensuring consistent and reliable measurements of the tensile properties of plastics. It is important to follow the specific requirements and guidelines outlined in the ASTM D638 standard for precise and standardized testing procedures.
Test specimen and condition of ASTM D638
The ASTM D638 standard specifies the requirements for test specimens and the conditioning procedures for conducting a tensile test. Here are the key details.
Shape: The standard specifies that the test specimens should have a dogbone shape. This shape is characterized by a reduced cross-sectional area in the center and wider ends, allowing for consistent failure within the gauge length.
Dimensions: The dimensions of the test specimens are defined based on the thickness of the material. The standard provides specific requirements for gauge length, width, and thickness based on the thickness range of the plastic material being tested. For example, for specimens less than 1 mm in thickness, the standard recommends a gauge length of 4 times the thickness, a width of 10 times the thickness, and a thickness between 0.4 mm and 0.6 mm.
1. Test Specimens:
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| Tensile test specimen |
Specimen Preparation: The test specimens can be prepared using a specimen cutting die or other appropriate methods to ensure the desired dimensions and shape. Care should be taken to avoid any damage or irregularities during specimen preparation.
2. Conditioning:
Temperature and Humidity: The standard recommends conditioning the test specimens to a specific temperature and humidity before testing. This conditioning is typically done in a controlled environment, such as a conditioning chamber, to ensure consistent testing conditions. The standard specifies the temperature and humidity conditions based on the desired test environment or as specified by the relevant material or product standards.
Conditioning Time: The conditioning time can vary depending on the material and test requirements. The standard typically recommends a minimum conditioning period of 40 hours. However, the specific conditioning time may be influenced by factors such as material type, thickness, and the desired equilibrium moisture content.
It is important to note that the specific requirements for test specimens and conditioning procedures may vary depending on the material being tested, the intended application, and any relevant industry standards or specifications. It is crucial to refer to the latest version of the ASTM D638 standard or any specific material or product standards for detailed and up-to-date information on specimen dimensions and conditioning requirements.
Test procedure of ASTM D638
The test procedure for ASTM D638 involves several steps to measure the tensile properties of plastics. Here is a general overview of the test procedure:
1. Specimen Preparation:
- Prepare the test specimens according to the dimensions specified in the standard. Use specimen-cutting dies or appropriate cutting methods to ensure accurate dimensions and a dogbone shape.
- Record the specimen dimensions, including gauge length, width, and thickness, using vernier calipers or micrometers.
2. Conditioning:
- Place the test specimens in a conditioning chamber or controlled environment at the specified temperature and humidity conditions for a sufficient period to reach equilibrium moisture content.
- Follow the recommended conditioning time specified in the standard or any relevant material or product standards.
3. Test Setup:
- Set up the tensile testing machine according to the manufacturer's instructions.
- Install appropriate grips (e.g., wedge grips, screw grips) on the machine.
- Calibrate the machine and verify its accuracy using known reference standards.
4. Mount the Specimen:
- Securely mount one end of the test specimen in the lower grip and the other end in the upper grip of the testing machine, aligning the specimen axially.
5. Preload:
- Apply a small preload to the specimen to ensure proper alignment and eliminate any initial slack or misalignment.
6. Test Configuration:
- Set the test parameters such as the test speed, load cell range, and data acquisition settings based on the requirements of the standard or any applicable specifications
- The test speed is typically specified in the standard, but a common rate is 2 mm/min.
7. Tensile Testing:
- Start the test machine to apply a continuous and controlled tensile force to the specimen at the specified test speed.
- Monitor the load (force) and elongation (displacement) values throughout the test.
- Continue the test until the specimen breaks, capturing the maximum load applied (tensile strength) and the corresponding elongation values.
8. Data Analysis:
- Calculate various tensile properties, including tensile strength, yield strength, modulus of elasticity, elongation at break, and Poisson's ratio, using the recorded load and elongation values.
- Use the specimen dimensions to determine cross-sectional area and calculate stress and strain values.
It's important to note that the specific details of the test procedure, including test speed, data acquisition, and calculations, may vary based on the material being tested and any applicable material or product standards. Always refer to the latest version of the ASTM D638 standard or any specific material standards for comprehensive and up-to-date instructions on conducting the test.
The test result of ASTM D638
The ASTM D test results are the outcomes obtained by conducting a specific test method outlined in an ASTM standard. The exact test results will vary depending on the specific test being performed and the material or product being evaluated.
For example, in the case of ASTM D638, which is the test method for tensile properties of plastics, the typical test results include:
1. Tensile Strength: It is the maximum stress or load at which the specimen fractures during the tensile test. Tensile strength is expressed in units of force per unit cross-sectional area, such as megapascals (MPa) or pounds per square inch (psi).
2. Yield Strength: It is the stress at which the material exhibits a specific amount of permanent deformation or offset from the linear elastic region. Yield strength indicates the material's ability to withstand deformation before undergoing significant plastic deformation.
3. Modulus of Elasticity: Also known as Young's modulus, it represents the stiffness or rigidity of the material. It is a measure of the material's resistance to elastic deformation and is expressed in units of stress over strain, such as MPa or psi.
4. Elongation at Break: It is the percentage increase in gauge length of the specimen at the point of fracture compared to its original length. Elongation at break indicates the material's ability to stretch or deform before failure.
5. Poisson's Ratio: It is a dimensionless value that describes the lateral contraction of the material when subjected to axial tension. Poisson's ratio is the ratio of the transverse strain to the axial strain and provides insights into the material's response to applied tensile forces.
These are some of the common test results obtained from ASTM D tests. The actual test results will depend on the specific material, test conditions, and any additional parameters defined in the respective ASTM standard. It is important to consult the specific ASTM standard being used for the test to understand the precise test results and their interpretation.
Factor affecting the test results in ASTM D638
Several factors can influence the test results in ASTM D638 for the tensile properties of plastics. It is important to consider these factors to ensure accurate and reliable test results. Here are some key factors that can affect the test results:
1. Specimen Preparation: The dimensions and shape of the test specimens should comply with the requirements specified in the standard. Any deviations or irregularities in specimen dimensions can affect the stress distribution and lead to inaccurate results. Care should be taken to ensure proper specimen cutting and avoid any damage or defects during preparation.
2. Conditioning: The conditioning of the test specimens is crucial to reach an equilibrium moisture content and establish consistent test conditions. Failure to properly condition the specimens can introduce variations in the material properties, affecting the test results. Follow the recommended conditioning time and environmental conditions specified in the standard or relevant material standards.
3. Grip Alignment and Slippage: Improper alignment of the test specimen in the grips or slippage during testing can result in inaccurate measurements. It is important to ensure that the specimen is securely and evenly gripped to prevent any slippage or misalignment. Proper alignment and gripping techniques help maintain consistent loading conditions throughout the test.
4. Test Speed: The test speed or crosshead speed determines the rate at which the force is applied to the specimen. The standard typically specifies a specific test speed. Deviations from the recommended test speed can affect the strain rate and may alter the material response. Ensure that the test speed adheres to the requirements of the standard being followed.
5. Environmental Conditions: Environmental factors such as temperature and humidity can influence the mechanical properties of plastics. Changes in these conditions during specimen preparation and testing can impact the material behavior. It is important to control and monitor the environmental conditions within the specified range throughout the test to minimize their effects on the results.
6. Instrument Calibration: Regular calibration and verification of the testing machine, load cell, and extensometer (if used) are essential to ensure accurate measurements. Calibration should be performed following the manufacturer's recommendations and traceable to national or international standards.
7. Material Variations: Variations in the composition, manufacturing processes, and thermal history of the plastic material can affect its mechanical properties. These variations may lead to differences in the test results even for specimens from the same material. Understanding the material's characteristics and considering its variations can help interpret and compare the test results effectively.
It is crucial to follow the guidelines and requirements specified in the ASTM D638 standard and any relevant material or product standards to minimize the influence of these factors on the test results. Additionally, conducting proper quality control measures and performing multiple tests can help improve the accuracy and reliability of the test results.
What are the Drawbacks and issues of ASTM D638?
While ASTM D638 is a widely recognized and utilized standard for testing the tensile properties of plastics, there are certain drawbacks and issues associated with it. Here are some of the notable ones:
1. Simplified Testing Conditions: ASTM D638 provides general guidelines for testing conditions, including temperature, humidity, and test speed. However, these conditions may not accurately reflect the real-world environmental conditions to which plastic materials are exposed. In certain applications, the performance of plastics can be significantly influenced by factors such as temperature, moisture, and chemical exposure, which are not fully addressed by the standard.
2. Single Test Configuration: ASTM D638 specifies a single test configuration, primarily focusing on uniaxial tension. This limitation may not fully capture the complex stress states experienced by plastics in practical applications. Plastic materials can experience different loading conditions, such as multiaxial loading, cyclic loading, and impact loading, which are not directly addressed by the standard.
3. Material Variability: Plastic materials exhibit inherent variability in their mechanical properties due to factors like molecular structure, processing conditions, and additives. ASTM D638 does not provide explicit guidelines for addressing material variability, and the standard does not differentiate between different types of plastics. This can limit the ability to assess the specific performance of different plastic materials accurately.
4. Sample Size and Representative Behavior: The standard specifies specific dimensions for test specimens. However, the small sample size may not always accurately represent the behavior of the entire plastic material or component. Large-scale plastic components may exhibit different mechanical behavior due to variations in processing, orientation, and stress distribution.
5. Time-Dependent Behavior: ASTM D638 primarily focuses on measuring the static tensile properties of plastics. It does not address time-dependent behavior, such as creep, stress relaxation, and fatigue, which are important considerations in applications where plastics are subjected to sustained or cyclic loading over extended periods.
6. Influence of Testing Equipment: While ASTM D638 provides general guidelines for testing equipment, it does not offer specific requirements or tolerances for the testing apparatus. The accuracy and precision of the testing machine, grips, and load cells can influence the test results, and variations in equipment and calibration practices across different laboratories can introduce discrepancies in the reported data.
Despite these drawbacks and issues, ASTM D638 remains a widely accepted and valuable standard for characterizing the tensile properties of plastics. It provides a consistent framework for comparing materials and assessing their performance under uniaxial tension. However, it is important to consider these limitations and supplement the standard with additional tests, evaluations, and relevant material-specific standards when necessary to obtain a comprehensive understanding of the mechanical behavior of plastics.
