Tensile Strength of Plastics per ASTM D638: Procedure, Specimens & Results
What Is ASTM D638?
ASTM D638 is the standard test method for tensile properties of plastics. It is one of the most frequently specified and universally applied test methods in the plastics industry, covering the determination of tensile strength, elongation at break, yield strength, and Young’s modulus for rigid and semi-rigid unreinforced and reinforced thermoplastics and thermosets.
Understanding tensile properties is fundamental to material selection, product design, quality control, and regulatory compliance across the automotive, packaging, consumer goods, electronics, and medical device industries.
Why Tensile Testing of Plastics Is Essential
Plastics are subjected to tensile loads in nearly every structural and semi-structural application. Whether a plastic clip must snap without breaking, a polymer hose must not burst under internal pressure, or a medical device housing must resist deformation during use, the material’s tensile properties govern these outcomes. ASTM D638 data forms the basis of finite element analysis (FEA) models, material datasheets, and procurement specifications.
ASTM D638 Specimen Geometry
ASTM D638 defines five specimen types (I, II, III, IV, V) based on material thickness and application:
Specimen Type | Gage Length | Width at Gage | Thickness | Typical Use |
Type I | 50 mm | 13 mm | ≤7 mm | Most rigid plastics |
Type II | 50 mm | 6 mm | ≤4 mm | Rigid and semi-rigid |
Type III | 50 mm | 19 mm | 7–14 mm | Thick specimens |
Type IV | 25 mm | 6 mm | ≤4 mm | Flexible and semi-rigid |
Type V | 7.62 mm | 3.18 mm | ≤4 mm | Limited material, micro |
Type I is the most commonly used geometry for injection-molded and extruded plastic specimens.
Key Measurements from ASTM D638
Tensile Strength at Break (σB)
The maximum stress sustained by the specimen during the test, or the stress at fracture if fracture occurs below the maximum load.
Tensile Strength at Yield (σY)
The first stress at which strain increases without a corresponding increase in stress is the yield point. Not all plastics exhibit a distinct yield point.
Elongation at Break (εB)
The percentage increase in gage length at the point of specimen fracture. Indicates ductility. Highly ductile plastics (PE, nylon) may elongate 200–800% before breaking; rigid, brittle plastics (PS, thermosets) may break below 5%.
Young’s Modulus (Tensile Modulus, E)
The slope of the linear portion of the stress-strain curve. Reflects stiffness. Determined per the chord modulus method between 0.05% and 0.25% strain per ASTM D638.
Test Speed Selection
ASTM D638 specifies five test speeds (1, 5, 50, 500, 5000 mm/min). The appropriate speed depends on the material type:
- Rigid plastics (PS, ABS, PC): 5 mm/min
- Semi-rigid plastics (PP, HDPE): 50 mm/min
- Flexible plastics (LDPE, elastomers): 500 mm/min
Conditioning Requirements
Specimens must be conditioned at 23°C ± 2°C and 50% ± 5% RH for at least 40 hours before testing per ASTM D618. For hygroscopic materials (PA, PC), conditioning significantly affects results.
Conclusion
ASTM D638 is a fundamental test method for evaluating the tensile behaviour of plastics, providing critical data on strength, stiffness, and ductility. These properties are essential for material selection, design validation, and quality control across a wide range of applications. By ensuring standardised and reproducible results, ASTM D638 enables engineers to predict performance, optimise designs, and ensure the reliability and safety of plastic components in real-world use.
Why Choose Infinita Lab for ASTM D638 Tensile Testing?
Infinita Lab offers comprehensive ASTM D638 tensile testing services with state-of-the-art servo-hydraulic and electromechanical test frames, calibrated extensometers, and ISO/IEC 17025-accredited test methods. Our SPOC model ensures seamless project management from specimen preparation through the final report.
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Frequently Asked Questions (FAQs)
What is the difference between tensile strength and yield strength in plastics? Tensile strength is the peak stress the specimen can withstand. Yield strength is the stress at which permanent plastic deformation begins. For ductile plastics, yield occurs before the ultimate tensile strength. For brittle plastics, fracture occurs before or at the maximum stress with no distinct yield point.
Why must plastic tensile specimens be conditioned before testing? Moisture absorbed by hygroscopic polymers (PA, PC, ABS) plasticizes the polymer chains, reducing stiffness and increasing elongation. Conditioning to a standard moisture state ensures results are reproducible and comparable between laboratories.
Can ASTM D638 be used for rubber and very flexible materials? ASTM D638 covers rigid to semi-rigid plastics. For rubbers and very flexible elastomers, ASTM D412 (dumbbell and cut ring specimens) is the appropriate tensile test method, as it accommodates the large strains involved.
How many specimens are required for a valid ASTM D638 test? ASTM D638 recommends a minimum of five specimens per test condition. Additional specimens may be required if results are highly variable or if statistical confidence intervals are needed.
What is the difference between ASTM D638 and ISO 527 for tensile testing of plastics? Both standards cover tensile properties of plastics using similar dog-bone specimens, but they differ in specimen geometry (ISO 527 uses Type 1A/1B specimens), test speed specifications, and modulus measurement methods. Results from the two standards are generally comparable but not always directly interchangeable without cross-reference testing.
