Standard Test Method for Tensile Properties of Plastics - PDF

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ASTM D638: Comprehensive Guide to Tensile Testing of Plastics​

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ASTM D638 is the definitive standard for determining the tensile properties of unreinforced and reinforced plastics. It provides essential data for material selection, quality control, and engineering design. This guide covers specimen preparation, testing equipment, procedures, and best practices to ensure accurate and repeatable results.​

🔍 What Does ASTM D638 Measure?

ASTM D638 evaluates the following mechanical properties of plastics:

• Ultimate Tensile Strength (UTS): The maximum stress the material can withstand while being stretched before breaking.​

• Yield Strength: The stress at which a material begins to deform plastically.​

• Elongation at Break: The strain on the material at the point of fracture.​

• Tensile Modulus (Young’s Modulus): The ratio of stress to strain in the elastic deformation phase.​

• Stress-Strain Behavior: The relationship between the applied stress and the resulting strain.​

These properties are critical for predicting how plastics will perform under mechanical loads in various applications.​

🧪 Specimen Types and Preparation

ASTM D638 specifies five standard specimen types (Types I–V), each designed for different material thicknesses and properties:

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• Type I: Preferred for rigid and semi-rigid plastics, thickness up to 7 mm.​

• Type II: Used for materials with thicknesses between 7 mm and 14 mm.​

• Type III: Alternative for rigid plastics when Type I is not suitable.​

• Type IV: Designed for thin sheets and films, thickness up to 1 mm.​

• Type V: Smaller specimens for limited material quantities.​

Specimens are typically prepared by:​

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• Injection Molding: For producing uniform specimens with consistent properties.​

• Machining: Cutting specimens from larger sheets or molded parts.​

• Die Cutting: Using precision dies to punch specimens from sheets.​

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Proper specimen preparation is crucial to ensure accurate and consistent test results.​

⚙️ Cutting Dies and Presses

Precision cutting dies are essential for producing specimens that meet ASTM D638 dimensional requirements. These dies are used in conjunction with hydraulic or manual presses to cut specimens from plastic sheets or plates.​

Key considerations when selecting cutting dies and presses include:​

• Die Material: High-quality steel for durability and precision.​

• Die Maintenance: Regular sharpening and inspection to maintain cutting accuracy.​

• Press Compatibility: Ensuring the press provides sufficient force and alignment for clean cuts.​

Using appropriate cutting equipment helps prevent specimen deformation and ensures compliance with ASTM standards.​

🧰 Universal Testing Machines (UTMs)

ASTM D638 tests are conducted using Universal Testing Machines capable of applying tensile forces to specimens. Key features of suitable UTMs include:​

• Load Capacity: Adequate force range to accommodate various plastic materials.​

• Crosshead Speed Control: Precise control over the rate of extension, typically ranging from 1 to 500 mm/min.​

• Data Acquisition Systems: High-resolution sensors and software for capturing force and elongation data.​

Ensuring the UTM meets ASTM D638 specifications is vital for obtaining reliable test results.​

🧲 Grips and Fixtures

Proper gripping of specimens is essential to prevent slippage and ensure accurate measurements. Common grip types used in ASTM D638 testing include:​

• Manual Wedge Grips: Suitable for a wide range of materials and specimen sizes.​

• Pneumatic Grips: Provide consistent clamping force, ideal for high-throughput testing.​

• Hydraulic Grips: Offer high clamping forces for testing high-strength materials.​

Selecting the appropriate grips and fixtures based on specimen type and material properties is crucial for test accuracy.​

📏 Extensometers

Extensometers measure the elongation of specimens during testing, providing data on strain and modulus. Types of extensometers used in ASTM D638 testing include:​

• Contact Extensometers: Physically attach to the specimen, offering high accuracy for rigid materials.​

• Non-Contact Extensometers: Use optical methods (e.g., video or laser) to measure elongation, suitable for delicate or highly extensible materials.​

All extensometers should be calibrated according to ASTM E83 to ensure measurement accuracy.​

🧪 Test Procedure Overview

1. Specimen Conditioning: Condition specimens at 23 ± 2°C and 50 ± 5% relative humidity for at least 40 hours prior to testing.​

2. Measurement: Measure specimen dimensions (width, thickness, gauge length) accurately.​

3. Mounting: Secure the specimen in the grips of the UTM, ensuring alignment to prevent bending.​

4. Extensometer Attachment: Attach the extensometer according to the manufacturer's instructions.​

5. Testing: Apply tensile load at the specified crosshead speed until specimen failure.

6. Data Collection: During the test, the system records force and elongation continuously. This data is used to create a stress-strain curve, from which tensile strength, yield strength, elongation, and tensile modulus are calculated.

7. Post-Test Inspection:
   After specimen breakage, examine the failure mode (e.g., necking, brittle fracture, slippage). This information is helpful for material behavior analysis and test validation.

📉 Stress-Strain Curve Interpretation

The stress-strain curve generated from ASTM D638 provides valuable insights into material behavior:

• Elastic Region: A straight-line portion of the graph representing elastic deformation, where strain is proportional to stress.

• Yield Point: The transition from elastic to plastic deformation.

• Plastic Region: The curved section where permanent deformation occurs.

• Ultimate Tensile Strength (UTS): The peak of the curve, representing the maximum stress the material can withstand.

• Fracture Point: The final point where the material fails.

Understanding this curve allows engineers to choose materials that match design requirements such as flexibility, rigidity, and toughness.

🔧 Calibration and Test Accuracy

To ensure compliance with ASTM D638 and the repeatability of results:

• Calibrate Load Cells regularly per ASTM E4.

• Verify Extensometer Accuracy using a standard gauge block or as per ASTM E83.

• Perform Routine Grip Inspections to check for slippage, wear, or alignment issues.

• Maintain the Cutting Die to prevent dimensional errors during sample preparation.

🧱 Common Materials Tested

ASTM D638 is commonly used to test the tensile properties of:

• Thermoplastics: e.g., Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polycarbonate (PC)

• Thermosets: e.g., Epoxies, Phenolics

• Composite Materials: Filled or reinforced plastic blends

• 3D Printed Polymers: For prototyping and additive manufacturing R&D

Each material may require a specific specimen type and testing setup to ensure accuracy.

🏗️ Applications and Industry Relevance

ASTM D638 is widely used in:

• Automotive & Aerospace: To verify strength of interior/exterior plastic components

• Consumer Products: For durability validation of plastic housings, toys, and containers

• Construction: For tensile properties of piping and plastic sheet goods

• Medical Devices: For compliance and material qualification

• Quality Control: For batch testing in manufacturing environments

Accurate tensile testing is critical for meeting safety, regulatory, and functional requirements across these industries.

🛠️ Equipment Summary

Here’s a quick summary of the equipment needed to run ASTM D638 tests:

Component Description:

1. Cutting Die & Press: To create specimens per Type I–V dimensions

2. Universal Testing Machine:  Capable of applying tensile forces with controlled crosshead speed

3. Grips: Manual, pneumatic, or hydraulic grips suitable for plastic samples

4. Extensometer: Contact or non-contact system for strain measurement

5. Data Acquisition Software: To capture and analyze test data

✅ Best Practices for ASTM D638

• Use a properly aligned test setup to avoid bending stresses.

• Condition specimens consistently to reduce variability.

• Match grip type to specimen thickness and brittleness.

• Use non-contact extensometers for soft or stretchable plastics.

• Document all parameters for traceability and standard compliance.

📚 Related Standards

• ASTM D412 – Tensile testing for rubber and elastomers

• ASTM D882 – Tensile testing for thin plastic films

• ASTM E8 – Tensile testing of metals (non-plastic counterpart)

• ISO 527 – International equivalent of ASTM D638

📎 Final Thoughts

ASTM D638 remains the cornerstone test for evaluating tensile properties of plastic materials. By understanding the full testing ecosystem—specimen preparation, machine configuration, grips, extensometers, and data interpretation—labs and manufacturers can ensure high-quality, consistent, and standards-compliant material testing.

Whether you're validating a new compound, performing quality checks, or conducting R&D, mastering ASTM D638 gives you the confidence that your plastic materials will meet the mechanical demands of the real world.