ASTM D638: Tensile Testing For Plastics

Introduction

Mechanical properties of a material are needed to know in plastics engineering and polymer science. Among all the material properties, the most significant one is tensile strength, i.e., the amount of stress that can be withstood by a material without failure when subjected to tension. ASTM D638, an internationally recognised standard by the American Society for Testing and Materials (ASTM), provides a method for testing the tensile properties of plastics in laboratory conditions.

What is ASTM D638?

ASTM D638, or "Standard Test Method for Tensile Properties of Plastics," describes the method, specimen type, and test conditions for testing the tensile strength, elongation at break, and elastic modulus of thermoplastic and thermosetting polymers. It is applicable to semi-rigid and rigid plastics and serves to provide a means for comparison of products from various manufacturers or various methods of processing.

The material is standardised to ensure tensile testing to be repeatable, minimising test speed, grip type, and specimen form variation. ASTM D638 finds extensive usage and forms the foundation of engineering design, material choice, and regulatory assistance.

Specimen Types and Preparation

ASTM D638 specification categorises five types of test specimens (Type I to Type V) based on size. Types I and IV are used most often:

Type I: 165 mm total length, 13 mm thin section width, 3.2 mm thickness. Employed most often for general tensile testing of common plastics.

Type IV: Total of 115 mm, 6 mm thin section width, 3 mm thickness. Where material availability is constrained or as a comparison base study.

Specimens can be produced through injection moulding, compression moulding, extrusion, or by cutting from plates. Also, attention must be given that specimens are free from flaws, i.e., voids, scratches, or irregularities on their surfaces, because they can also affect tensile results quite significantly.

Testing Procedure

The tensile test is where the specimen is placed inside a universal testing machine (UTM) and the specimen loaded to a controlled, uniaxial tensile load up to failure. Some of the important factors involved in the process are:

1. The grip selection: The specimens are held by pneumatic or mechanical grips. The grip must not cause slippage and the grip must not be a source of stress concentrations.

2. Test Speed: The crosshead speed varies with material type and test specimen thickness. For nominal Type I specimens with 3.2 mm thickness, the speed is often 50 mm/min. The speed can influence elongation as measured and tensile strength in case of viscoelastic plastics.

3. Conditioning Environment: Specimens are conditioned at 50% relative humidity and 23°C for a minimum of 40 hours according to ASTM D618 so they are as stable as possible with regard to moisture content and temperature.

Key Properties Measured

ASTM D638 testing involves some of the most critical material properties:

Tensile strength at yield: Maximum stress material will tolerate before undergoing permanent deformation. HDPE, for example, will usually yield at 20–30 MPa, whereas polycarbonate (PC) may yield at 60–70 MPa.

Tensile strength at break: Stress at which specimen eventually fractures. Brittle plastics like polystyrene break at or near the yield point, whereas ductile plastics like polyethylene stretch significantly before they break.

Elongation at Break: Elongation in length at break in percentage terms. HDPE may have 400–600% elongation, whereas that of polystyrene is less than 5%.

Modulus of Elasticity (Young's Modulus): A measure of stiffness from the slope of the first linear part of the stress-strain curve. For instance, polypropylene has around 1.5–2 GPa in modulus and PVC up to 3–3.5 GPa.

These values are applied by engineers to determine whether a material can sustain operation loads or not without breaking or permanent deformation.

Applications of ASTM D638 Testing

1. Engineering Material Selection

Designers contrast plastics for structural components from ASTM D638 data. For example, when choosing material for vehicle components such as interior panels, tensile strength and elongation determine whether a plastic is impact-resistant and flexible without cracking.

2. Manufacturing Quality Control

Plastic manufacturers may on occasion test batches to ensure mechanical properties are consistent. Varying extrusion temperature, injection pressure, or cooling rates will be transferred to tensile properties. ASTM D638 is an accepted method to ensure materials meet specifications before shipping.

3. Research and Development

In R&D, tensile testing determines the effect of additives, fillers, or reinforcement in raw materials. For example, the addition of glass fibres to polypropylene doubles tensile strength from 30 MPa to over 80 MPa and reduces elongation at break by half. ASTM D638 ensures effective comparison between formulations.

4. Compliance with Regulations

Medical devices, food packs, and electronic components often require mechanical testing according to ASTM D638 to meet ISO, FDA, or ASTM regulation standards. Polyethylene tubing that is FDA-approved is one such instance where tensile properties uniformity must be assured in order to ensure safe use in medical devices.

Conclusion

ASTM D638 is a standard underlying the tensile testing of plastics for their properties. Providing credible procedures for testing, specimen preparation, and analysis, it enables engineers and manufacturers to make well-informed decisions on material choice, product design, and quality assurance. Ranging from automotive panels to medical devices, the standard ensures plastics behave consistently and safely when under mechanical stress. For more information, please check Stanford Advanced Materials (SAM).