# Determining Deflection Temperature Under Load (DTUL) with the TMA (ASTM E2092)

Keywords: Heat Deflection Temperature (HDT), Thermomechanical Analysis (TMA), Deflection Temperature Under Load (DTUL), ASTM E2092

TA421

### Introduction

Heat Deflection Temperature (HDT) and Deflection Temperature Under Load (DTUL) are equivalent terms that reflect the temperature at which a material subjected to a three-point bending load deforms to a pre-determined position. The actual force applied to the sample and the amount of deflection required depend upon the sample geometry. ASTM E2092 and a related standard D648, defines a standard set of these parameters. The TMA flexure probe configuration (Figure 1) is utilized in this method.

ASTM E2092 defines DTUL as the temperature at which a precise strain (either 0.25 mm deflection or 0.20% strain as defined by sample dimensions in the procedure*) occurs under a specific stress (either 455 or 1820 kPa). With the TMA, the load (force) needed to achieve these stresses can be determined using equation (1).

where:

F = TMA Force (N)
S = Stress (MPa)
= 0.455 MPa (66 psi) or 1.82 MPa (264 psi)
b = Sample width (mm)
d = Sample thickness (mm)
L = Sample length = 5.08 mm (as defined by the flexure probe geometry, see Figure 1)

The TMA deflection equivalent to the ASTM specified % strain can be determined using the relationship shown in equation (2).

where:

D = TMA dimension change at center span (mm)
r = Sample strain = 0.0020 or 0.20%*

*[Sample strain computed based on sample dimensions and deflection change defined by ASTM E2092.

### TMA Experimental Guidelines

• Thin samples (1 mm thick or less) are cut to appropriate dimensions (10 mm long x 3 mm wide).
• Samples are placed on the bending fixture and the knife-edged probe is lowered onto the sample with the predetermined force (F).
• The sample is heated at 2 ºC/minute until the required deformation is achieved as defined by equation (2).
• The temperature at the required deformation is recorded as the DTUL for the corresponding stress.
• An example of a DTUL experiment is shown in Figure 2 for Polyvinyl Chloride.