ASTM D5023 is a test to study the dynamic mechanical behavior of plastic materials under three-point bending conditions. A rectangular bar is supported on two supports. At the same time, a probe applies an oscillatory load at the center, and the test machine measures the energy stored and lost as the temperature varies. MatX Lab conducts the test for both filled and unfilled plastics.
What Is ASTM D5023?
ASTM D5023 covers the determination of dynamic mechanical properties of plastics in flexure using three-point bending. The instrument applies a sinusoidal deformation at a fixed frequency and measures the resulting force along with the phase lag between the two.
That phase lag splits the response into three reported quantities. The storage modulus is responsible for the elastic property of the material, while the loss modulus indicates the viscous portion that is transformed into heat. These three curves, plotted as a function of temperature, indicate the polymer’s softening point, the presence of secondary transitions, and the material’s damping effect.
Three-point bending suits stiff specimens. A glass-filled polyamide bar or a cured epoxy laminate is far easier to test in flexure than in tension, and the geometry needs no gripping.
ASTM D5023 Scope and Applications
The method applies to rigid plastics capable of carrying their own weight across a span. Rigid thermoplastics, thermosetting polymers, and fiber-reinforced laminates are common types of plastics that would be investigated.
Applications:
Glass transition determination. Locating Tg from the storage modulus onset, the loss modulus peak, or the tan delta peak, with the basis stated because the three values differ.
Cure verification. Confirming that a thermoset reached full cure, since undercured material shows a Tg that rises on a second heating run.
Service temperature limits. Mapping how much stiffness a part retains at operating temperature, which a room-temperature modulus number will not tell you.
Filler and formulation comparison. Ranking glass loading, impact modifier content, or resin systems against one another under identical conditions.
Soft films and elastomers usually run better in tension under ASTM D5026 or in torsion under ASTM D5279. Where you need Tg from a thermal signal rather than a mechanical one, ASTM D7426 covers the DSC route on polymers and elastomers. Aerospace work under this method is limited to commercial programs, and MatX Lab analyzes materials at the specimen level.
ASTM D5023 Test Procedure
Specimen geometry and clamping control the numbers more than the instrument settings do.
| Step | What happens |
| Specimen preparation | Bars are machined or cut to a uniform rectangular section with parallel faces. Dimensions are measured at three points and averaged. |
| Conditioning | Specimens are conditioned at standard laboratory conditions, and hygroscopic materials such as polyamide are dried or conditioned to a stated moisture level. |
| Span setup | Set the support span to give a span-to-thickness ratio of at least 16:1 so shear contribution stays small. |
| Mounting | The bar is mounted centrally on the supports, and the probe is engaged under a static preload to ensure contact throughout the entire oscillation. |
| Temperature program | Specimens are matched to an initial temperature and then heated up at a controlled heating rate, usually 2 to 3 °C/min. |
| Strain check | The amplitude is selected in the linear viscoelastic regime, confirmed using a strain sweep in case of an unknown material |
| Data collection | Storage and loss moduli, and tan delta are recorded continuously against temperature at the selected frequency, usually 1 Hz. |
| Reporting | Transition temperatures are extracted from each curve, and the basis for every reported value is stated. |
Limitations: Tg from DMA is not a single value. Storage modulus onset, loss modulus peak, and tan delta peak on the same specimen can sit 10 °C to 20 °C apart, so quoting a Tg without the basis invites an argument with your customer. The heating rate shifts all transitions up at high speeds, and frequency does too; that’s why laboratory comparisons work only when both are identical. Materials that deform easily will droop between the supports,s and the modulus will fail due to geometrical considerations rather than the nature of the material itself. Thinner, more flexible materials show low values in three-point flexure due to indentation at the supports. Moisture can shift the Tg of polyamides by tens of degrees.
ASTM D5023 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Specimen form | Rectangular bar, machined or cut with parallel faces |
| Length | 50 mm to 60 mm, sized to the support span plus overhang |
| Width | 10 mm to 13 mm |
| Thickness | 2 mm to 4 mm, uniform along the span |
| Span-to-thickness | 16:1 minimum |
| Replicates | Three per material condition |
| Frequency | 1 Hz standard, multi-frequency runs available on request |
| Heating rate | 2 °C/min to 3 °C/min for transition work |
Send more material than the test needs. Machining rejects happen, and a warped or tapered bar has to be recut rather than tested.
Tell us the transition you are chasing and the temperature window that matters. A run that stops at 150 °C will miss a thermoset Tg at 165 °C, and repeating it costs a day.
ASTM D5023 Test Results and Reporting
The report includes storage modulus, loss modulus, and tan delta versus temperature for each sample, and the data will be provided on request.
Transition temperatures are listed with the basis of each noted as: storage modulus onset, loss modulus peak, tan delta peak. The test conditions include frequency, heating rate, range, sample size, static load, and strain. These conditions define the reproducibility of your results elsewhere.
Where several specimens were run, the mean and the spread appear together. Specimen-to-specimen variation in filled materials is often real material variation, and we describe what the specimens looked like rather than averaging the observation away.
ASTM D5023 FAQs
Can you test a molded part instead of a bar?
Only if we can machine a suitable bar from it. Tell us where in the part the specimen should come from, because orientation and fiber alignment change the result.