ASTM D638 vs ASTM D790: When to Use Tensile vs Flexural Testing

Key Takeaways

• Use ASTM D638 when the plastic must resist pulling, stretching, or tensile loading.
• Use ASTM D790 when the plastic must resist bending, flexing, or stiffness loss.
• Use both tests when the part will face tensile and bending forces in real use.
• Choose these testing methods based on how the plastic will actually be loaded. 

When evaluating the mechanical properties of plastics, engineers and product teams frequently rely on two foundational standards: ASTM D638 and ASTM D790. Both methods are often used together in polymer mechanical testing, but they measure fundamentally different kinds of material behavior. 

For engineers and product teams, the ASTM D638 vs ASTM D790 (or tensile vs flexural testing) decision comes down to how the material will actually experience force in use. Let’s begin by comparing these testing methods to understand the difference between tensile and flexural testing.

Criteria	ASTM D638	ASTM D790
Type of load applied	Tensile pulling force	Bending force
Properties measured	Tensile strength, elongation, tensile modulus, and stress-strain curve data for plastics	Flexural strength, flexural modulus, and bending response
Typical specimen types	Dog-bone-shaped plastic specimens	Rectangular bars cut or molded from plastic materials.
Common applications	Structural components under tension, fasteners, plastic ties, and other parts requiring tensile strength data	Support beams, structural panels, casings, shelving, and other parts requiring flexural strength data
Typical industries	Aerospace, automotive, consumer goods, medical, plastics	Construction, automotive, electronics, aerospace products

 

ASTM D638 vs ASTM D790: Definition and Use Cases

ASTM D638 is a tensile test that evaluates how a material behaves when pulled. ASTM D790 is a flexural test that evaluates how a material behaves under bending loads.

When to Use ASTM D638

Use ASTM D638 testing when you need to understand how a plastic behaves under pulling, stretching, or elongation. With this test, you can compare resin formulations, validate molded parts, review material data sheets, or check how much tensile load a material can tolerate before failure. 

When to Use ASTM D790

In these ASTM D638 vs ASTM D790 decisions, use ASTM D790 when the material must resist bending or maintain stiffness under load. It is commonly used for rigid plastics, flat panels, housing enclosures, structural sheets, electrical insulation materials, and components that may flex during use, often through a three-point bend test. 

When Do You Need Both Tests?

ASTM D638 and ASTM D790 are not interchangeable, but some projects require both tests to understand how a plastic performs under different loading conditions. 

You need both tests when a plastic part experiences more than one type of force in the field. What we haven’t mentioned in this ASTM D638 vs ASTM D790 discussion yet is that many finished parts may be exposed to both bending and tensile forces during operation. 

In such cases, you’ll need both tests to get a clear picture of the strength, stiffness, and stress-strain behavior of the material. 

Need Expert Material Testing Support?

Selecting the right protocol is necessary for the sake of product safety, compliance, and performance. This is what we do at VTEC Laboratories. We provide materials testing for plastics, polymers, and other materials across a wide range of industries. 

If you need help choosing the right method between ASTM D638 vs ASTM D790, or need reliable custom testing services with quick turnaround times, contact us today.

 

Frequently Asked Questions

What’s the difference between tensile and flexural testing when measuring modulus?

Tensile modulus measures a material’s resistance to uniform stretching across its entire cross-section. Flexural modulus measures resistance to bending, which creates a gradient of simultaneous forces: compression on the top surface, zero stress at the center neutral axis, and tension at the bottom surface. Because plastics react differently to compressive versus tensile loads, a material’s flexural modulus is frequently higher than its tensile modulus. 

In ASTM D638 vs ASTM D790 or plastic tensile vs flexural testing comparisons, can finished plastic parts be tested, or are standard specimens required?

Both tests usually require specific specimen shapes and dimensions to produce valid, repeatable data. In some cases, specimens can be molded, machined, or cut from finished parts or sheets. However, the sample preparation method can affect results. Therefore, confirm the right specimen approach before testing.