Standard Test Method for Shear Strength of Plastics by Punch Tool - PDF

 

 

Introduction: What Is ASTM D732?

ASTM D732 is a standardized test method used to determine the shear strength of plastic materials using a punch-type shear fixture. This test measures the force required to shear a hole through a plastic specimen and is commonly applied in the plastics, automotive, aerospace, and consumer goods industries to assess the mechanical properties of thermoplastics and thermosets.

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It is particularly valuable for materials used in structural components where shear loading is a factor.

Common Applications and Materials

• Plastic sheets and films used in packaging or construction

• Thermoset laminates for aerospace interiors

• Engineering polymers used in automotive parts

• Composite panels and injection-molded parts

This test is frequently used for quality control, material comparison, and design validation.

Sample Preparation for ASTM D732

Proper sample preparation is critical for repeatable results:

• Shape: Square or rectangular sheets

• Dimensions: 2.0 inches x 2.0 inches x 0.050 – 0.500 inches thick

• Number of samples: At least five specimens are recommended

• Conditioning: As per ASTM D618 – typically 23°C and 50% relative humidity for 40+ hours

Samples must be flat and free of visible defects, and edge quality can affect results due to stress concentrations during shear.

Required Equipment for ASTM D732

1. Universal Testing Machine (UTM)

A UTM capable of reaching up to 50 kN (11,000 lbf) or more is typically required, depending on the material strength. Machines must support:

• Constant crosshead speed

• Load cell with suitable resolution (e.g., 0.1% of full scale)

2. Punch Shear Test Fixture

The fixture consists of:

• A cylindrical punch (typically 1.0-inch diameter)

• A die plate with a centered hole

• A specimen clamp to hold the sample securely

This setup shears the specimen as the punch is driven through the sample into the die hole.

3. Data Acquisition System

• Force vs. displacement

• Peak load capture

• Optional: software for calculating stress

Test Setup and Procedure

Speed:

• Crosshead speed: 0.05 in/min to 0.20 in/min (depending on material type)

Test Procedure:

1. Insert the sample between the punch and die.

2. Align the punch over the center of the specimen.

3. Start the test at the specified crosshead speed.

4. Record the maximum force required to shear the punch through the sample.

Data Captured:

• Peak force

• Displacement

• Shear stress

Relevant Equation

Shear strength is calculated using this formula:

Shear Strength (psi or MPa) = Maximum Load (lbf or N) / Punch Area

Where:

• Punch Area = π × (d²) / 4

• d is the punch diameter (in inches or millimeters)

For a 1.0-inch punch:

• Area = π × (1.0²) / 4 = 0.785 in²

Real-World Applications

• Automotive: Fastener plate validation in dashboards and trim

• Aerospace: Plastic laminate bonding quality for interiors

• Consumer Products: Durability of phone cases or plastic housings

• Packaging: Strength validation for plastic crates and bins

Common Pitfalls and Testing Tips

Common Issues:

• Improper sample thickness

• Misaligned punch and die

• Slipping due to inadequate clamping

• Overly high test speeds

Tips for Success:

• Use hardened steel punches and dies for repeat testing

• Calibrate load cells regularly

• Use lubrication if allowed to reduce frictional heat

Comparison to Related Standards

Standard - Description - Key Difference

ASTM D256 - Impact resistance (Izod method) - Focuses on impact, not shear

ASTM D638 - Tensile strength of plastics - Measures tensile, not shear forces

ISO 2039-1 - Plastic hardness and compression strength - Different measurement principle

ASTM D695 - Compressive strength of plastics - Does not involve shearing action

Conclusion: Key Takeaways

• ASTM D732 is essential for understanding the shear performance of plastic materials.

• Requires specific test fixtures and consistent sample prep.

• Universal Grip Company provides the punch dies, shear fixtures, and testing machines for full compliance with the method.

• Avoid test errors by ensuring alignment, correct clamping, and standard test speed.

By following the guidance in this article, engineers and lab techs can produce accurate and reproducible shear strength results for a wide range of plastics.