Why North American projects specify D638

Owners, engineering firms and third-party laboratories in North America most often cite ASTM D638 when describing the tensile properties of plastics and reinforced plastics. It was not written specifically for grating, but tensile data for grating resins, molded products and some profiles is usually issued to it. Understanding D638's specimen and speed rules is how you judge whether a report is usable on a North American project.

What the standard is

• Current edition: ASTM D638-22, approved and published in July 2022, superseding D638-14;
• Scope: unreinforced and reinforced plastics up to about 14 mm thick;
• Poisson's ratio is optional, and the method is not identical to ISO 527-1;
• Maintained by ASTM Committee D20 on plastics.

Choosing among five specimen types

Specimen	Typical use
Type I	Preferred for rigid and semi-rigid plastics; about 50 mm gauge length, 165 mm overall, 3.2 mm thick
Type II	Alternative when a rigid material fails outside the Type I gauge section
Type III	Thicker material, about 7–14 mm
Type IV	Flexible or non-rigid plastics, up to about 4 mm thick
Type V	Miniature specimen, about 25 mm gauge, for limited material or environmental chambers

Cast resin samples for grating usually use Type I; thin sections cut from molded grating may use Type IV; thick profiles or special sampling use Type III. The wrong specimen can change the strength value substantially, so the report must state the type.

How test speed is chosen

D638 selects the lowest speed in its table that ruptures the specimen in 0.5–5 minutes, unless a material specification says otherwise. For Types I–IV, rigid materials commonly run at 5 mm/min, ductile materials at 50 mm/min, and non-rigid materials up to 500 mm/min; Type V uses 1, 10 or 100 mm/min. Strength and elongation change with speed, so reports must be aligned on speed before being compared.

A key difference in modulus measurement

D638 measures tensile modulus at the same speed as the other properties; ISO 527 normally uses a separate 1 mm/min modulus test. Both require an extensometer, and D638 specifies its accuracy class. This is one reason the same material can show slightly different modulus in D638 and ISO 527 reports.

What it means for FRP grating

D638 is a suitable method for molded grating, grating resins and short-fibre reinforced parts. For axial tensile testing of continuous-fibre pultruded profiles, the industry more often uses a method such as ASTM D3039, which is written for continuous-fibre composites; if a customer specifies D638, the report should state the sampling direction and its applicability. In either case, transverse tensile strength is usually much lower than longitudinal and must be tested separately.

Common pitfalls

• Strength without specimen type. Type I and Type IV have different geometries and their values are not directly comparable.
• Missing speed. Without it, strength and elongation cannot be verified.
• Mixing D638 with GB/T 1447. Specimens, speeds and modulus ranges differ; there is no one-to-one conversion.
• Using D638 data as the axial design value for continuous-fibre profiles. If the sample direction does not represent the profile's fibre direction, the data is misleading.

Edition history

D638-22 was published in July 2022 and supersedes D638-14. It keeps the Type I–V system and the practice of measuring modulus at the same speed as the other properties. Purchasing documents should cite “D638-22” specifically; writing only “D638” lets the laboratory choose the edition.

Where to find it

The official entry is in the Niteck FRP grating standards library. For the load data North American projects usually ask for, see load capacity; for specifications and tolerances, see specifications, for checking against project conditions.

This article explains the method and does not replace the standard text; the current edition governs all test conditions and criteria.