**Active Standard: D4535 - 21**

Developed by Subcommittee: D18.12 | Committee D18 | Contact Staff Manager

**Keywords**

**Rationale**

The standard was approved in 2013 and requires revision as it has been 7 years since the last approval. In addition, the following updates are proposed: (1) In Section 12.2, the standard lists the stability criterion for Schedule A as point-to-point change in displacement less than 2% of the displacement between any two temperature steps. When testing materials with small thermal expansions or when using small temperature steps, the fluctuation or noise in displacement readings at steady state may exceed this threshold even if the measurement system meets the accuracy requirements listed in section 7.2. An alternative stability criterion based on rate of change of displacement is proposed in the revised standard. (2) Section 12.4 specifies that at least two complete heating and cooling cycles be completed to record changes caused by heating. Section 12.4 further states that if large hysteresis is observed, additional cycles may be required. A definition of large hysteresis has been proposed in the revised standard. Furthermore, the end user of the thermal expansion test data may not have a use for the additional heating and cooling cycles. In this scenario, the additional heating cycles requirement causes undue expense and labor without clear benefit. Additional heating cycles are now optional in the revised standard for the case where changes in properties with successive heating cycles are desired. (3) Section 12.2 lists the maximum heating and cooling rate of the dilatometer assembly as 1C per minute when following Schedule A. Unconfined dilatometers usually comprise heated and unheated subassemblies, and the revised standard now specifies that the temperature ramp rate is to be measured adjacent to the specimen. For Schedule A, the maximum acceptable heating rate listed in the current standard is 1C per minute. Experience has shown that this heating rate limit may be excessively conservative and that faster heating rates do not affect the steady state thermal strain values. The revised standard specifies a faster allowable heating rate developed from the results of a series of thermal-mechanical calculations. (4) Section 13 of the current standard provides a method to calculate the coefficient of thermal expansion using a point-to-point method. While the point-to-point method is common, it is not suitable for rocks with nonlinear thermal responses, and the resulting coefficient can be strongly influenced by random error in the temperature and displacement readings. In addition to the point-to-point calculation method, an alternative calculation has been added that fits an appropriate function to the strain versus temperature data and takes the derivative of the fitted function. This alternative approach is suitable for all thermal responses, and the coefficient of thermal expansion is less influenced by random point-to-point error.

Back to Top**Date Initiated:**

09-15-2020

**Technical Contact:**

Evan Keffeler

**Item:**

004

**Ballot:**

D18 (21-03)

**Status:**

Ballot Item Approved as D4535-2021
and Pending Publication