ASTM D638 Tensile Properties Plastics Checklist: A Practical Guide

ASTM D638 is the standard test method for tensile properties of plastics. It answers a direct engineering question: how does a plastic respond when pulled? The data supports material selection, lot release, design validation, and supplier qualification. A disciplined ASTM D638 tensile properties plastics checklist keeps the test repeatable and defensible.

Why ASTM D638 Matters for Plastic Part Decisions

Tensile data from ASTM D638 is often the first mechanical property reviewed for a new plastic grade. It helps compare formulations, verify incoming material, and check whether a molded part meets a drawing or specification. ASTM D638 is a test method, not a pass/fail limit. The material specification, purchase order, or drawing sets the acceptance criteria. The method defines how to prepare specimens, condition them, pull them, and calculate results.

ASTM D638 is frequently compared with ISO 527. The two standards share the same general goal but differ in specimen geometry, test speed, and reporting details. Do not treat the results as interchangeable without a correlation study. If a supplier reports ISO 527 data and your specification cites ASTM D638, ask for the test method, specimen type, and speed before accepting the comparison.

Scope and Limits of ASTM D638

ASTM D638 covers rigid and semi-rigid plastics, including injection-molded, compression-molded, and sheet materials. It provides tensile strength, elongation at yield, elongation at break, tensile modulus, and related stress-strain data. It does not cover every polymer family. Elastomers often use ASTM D412, composites may use ASTM D3039, and foams may need ASTM D3574. Check the material specification before assuming ASTM D638 is the right method.

Results are sensitive to specimen preparation, conditioning, alignment, and crosshead speed. A small change in any of these can shift strength or elongation. That is why a written ASTM D638 tensile properties plastics checklist is more useful than a single remembered procedure. The method is used for quality control, research and development, and specification compliance. It is not a substitute for long-term creep, fatigue, or impact testing.

The ASTM D638 tensile properties plastics checklist: Before the Test

Specimen Selection and Preparation

Select the specimen type required by the material specification or the standard. ASTM D638 includes Type I, II, III, IV, and V specimens. Type I and Type II are common for rigid plastics. Type IV is often used for thin sheet or film-like materials. Type V is a smaller specimen for limited material. Machine or mold specimens to the specified dimensions. Inspect for nicks, voids, flash, bubbles, and incomplete fills. Measure width and thickness at several points along the gauge section. Record the average dimensions and the specimen orientation. Test at least five specimens per sample unless the governing specification says otherwise.

Conditioning and Test Environment

Condition specimens in accordance with ASTM D618 unless the material specification sets a different schedule. A common laboratory atmosphere is 23 ± 2°C and 50 ± 10% relative humidity. Many plastics require at least 40 hours of conditioning, but the exact time depends on the polymer and thickness. Test in the same atmosphere. For hygroscopic materials such as polyamides, moisture history can change tensile results. Document the conditioning time, temperature, and humidity.

Running the Test: Speed, Grips, and Data Capture

Set the crosshead speed according to ASTM D638 and the specimen type. Common speeds are 5 mm/min, 50 mm/min, and 500 mm/min. Rigid plastics often use the slower speeds, while flexible materials may use faster speeds. Use grips that prevent slippage without crushing the specimen. Align the specimen with the load axis. Misalignment causes bending and lowers measured strength.

Use an extensometer when modulus or yield elongation is required. The extensometer should be calibrated and positioned on the gauge section. Record force and displacement continuously. Convert the data to stress and strain using the original cross-sectional area and gauge length. Note the failure mode: gauge break, grip break, jaw slip, or necking. A specimen that breaks outside the gauge section may not be valid for strength or elongation, but the result should still be recorded and explained. For modulus, the strain range and rate matter. ASTM D638 provides guidance on modulus calculation, but the material specification may define a specific secant modulus, such as 1% secant.

Calculations, Reporting, and Compliance Records

Calculate tensile strength by dividing the maximum force by the original cross-sectional area. Calculate elongation by dividing the change in gauge length by the original gauge length and multiplying by 100. Tensile modulus comes from the initial linear portion of the stress-strain curve, often as a secant or slope value. Report the mean, standard deviation, and number of specimens. Include specimen type, dimensions, conditioning, test speed, and failure mode. Report units clearly. Stress is usually in MPa, strain in percent, and modulus in MPa or GPa. Keep the original units and conversion factors in the record.

Treat the ASTM D638 tensile properties plastics checklist as a controlled record. Link it to the material test certificate and the lot number. If your quality system follows ISO 9001, the checklist can serve as a documented test procedure and evidence of traceability. For certificate requirements, see ASTM material test certificate requirements.

Common Pitfalls and Quality Checks

Watch for inconsistent specimen preparation, wrong crosshead speed, poor grip alignment, and incomplete conditioning. Do not mix data from different specimen types or speeds without a note. Do not compare ASTM D638 results directly with ISO 527 results unless a correlation has been established. Keep the raw force-displacement data, not only the summary table. Check the calibration status of the load cell, extensometer, and crosshead speed. Verify the grips are appropriate for the material stiffness. Review the data for outliers and investigate before excluding.

The ASTM D638 tensile properties plastics checklist should be reviewed before each test campaign. Confirm the current edition of ASTM D638, the material specification, and the conditioning requirements. If the plastic is used in packaging or shipping, distribution testing may also apply; see ASTM D4169 distribution testing schedule guide or the ASTM D4169 distribution testing schedule. For a contrast with metals testing, see ASTM A36 tensile strength requirements.