GB/T 1634.1 and GB/T 1634.2-1979 (first edition) described three methods (A, B and C) using different test loads and two specimen positions, edgewise and flatwise. For testing in the flatwise position, test specimens with dimensions 80 mm × 10 mm × 4 mm were required. These can be moulded directly or machined from the central section of the multipurpose test specimen (see ISO 20753).
GB/T 1634.1 and GB/T 1634.2-2004 (second edition) specified the flatwise test position as preferred, while still allowing testing in the edgewise position with the test conditions. Therefore, with this revision, the edgewise test position will be removed.
Technical development of testing instruments made instruments based on a fluidized bed or air ovens available. These are especially advantageous for use at temperatures at which the common silicone oil-based heat transfer fluids reach their limit of thermal stability. The fluidized bed and air oven methods of heat transfer are introduced in this part.
Plastics — Determination of Temperature of Deflection under Load — Part 1: General Test Method
1 Scope
This part of GB/T 1634 specifies a general test method for the determination of the temperature of deflection under load (flexural stress under three-point loading) of plastics. Different types of test specimen and different constant loads are defined to suit different types of material.
GB/T 1634.2 specifies specific requirements for plastics (including filled plastics and fibre-reinforced plastics in which the fibre length, prior to processing, is up to 7.5 mm) and ebonite, while GB/T 1634.3 specifies specific requirements for high-strength thermosetting laminates and long-fibre-reinforced plastics in which the fibre length is greater than 7.5 mm.
This part is applicable to the assessing the relative behaviour of different types of material at elevated temperature under load at a specified rate of temperature increase. The results obtained do not necessarily represent maximum applicable temperatures because in practice essential factors, such as time, loading conditions and nominal surface stress, can differ from the test conditions. True comparability of data can only be achieved for materials having the same room-temperature flexural modulus.
This part specifies preferred dimensions for the test specimens. This part is commonly known as the HDT test (heat deflection test or heat distortion test), although there is no official document using this designation.
2 Normative References
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 1634.2 Plastics — Determination of Temperature of Deflection under Load — Part 2: Plastics, Ebonite and Long-fibre-reinforced (GB/T 1634.2-2019, ISO 75-2:2013, MOD)
GB/T 1634.3 Plastics — Determination of Temperature of Deflection under Load — Part 3: High-strength Thermosetting Laminates (GB/T 1634.2-2004, ISO 75-3:2003, IDT)
GB/T 2918 Plastics — Standard Atmospheres for Conditioning and Testing (GB/T 2918-2018, ISO 291:2008, MOD)
3 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
3.1
flexural strain
εf
nominal fractional change in length of an element of the outer surface of the test specimen at midspan
Note: It is expressed as a dimensionless ratio or a percentage (%).
3.2
flexural strain increase
Δεf
specified increase in flexural strain that takes place during heating
Note 1: It is expressed as a percentage (%).
Note 2: This quantity is introduced to highlight the fact that the initial deflection caused by application of the test load is not measured and that therefore the criterion for the end of the test does not constitute an absolute strain value. Only the deflection increase is monitored (see 3.4).
3.3
deflection
s
distance over which the top or bottom surface of the test specimen at midspan deviates during flexure from its original position
Note: It is expressed in millimetres (mm).
3.4
standard deflection
Δs
increase in deflection corresponding to the flexural strain increase, Δε f , at the surface of the test specimen, and which is specified in GB/T 1634.2 or GB/T 1634.3
Note: It is expressed in millimetres (mm) [see 8.3, Formula (4)].
3.5
flexural stress
σf
nominal stress at the outer surface of the test specimen at midspan
Note: It is expressed in megapascals (MPa).
3.6
load
F
force, applied to the test specimen at midspan, which results in a defined flexural stress
Note: It is expressed in newtons (N) [see 8.1, Formulae (1) to (3)].
3.7
temperature of deflection under load
Tf
temperature at which the deflection of the test specimen reaches the standard deflection as the temperature is increased
Note: It is expressed in degrees Celsius (°C).
4 Principle
A standard test specimen is subjected to three-point bending under a constant load in the flatwise position to produce one of the flexural stresses given in the relevant part of GB/T 1634. The temperature is raised at a uniform rate, and the temperature at which the standard deflection, corresponding to the specified increase in flexural strain, occurs is measured.
5 Apparatus
5.1 Means of producing a flexural stress
The apparatus shall be constructed essentially as shown in Figure 1. It consists of a rigid metal frame in which a rod can move freely in the vertical direction. The rod is fitted with a weight-carrying plate and a loading edge. The base of the frame is fitted with test-specimen supports; these and the vertical members of the frame are made of a material having the same coefficient of linear expansion as the rod.
The test-specimen supports consist of metal pieces that are cylindrical in the contact area and with their lines of contact with the specimen in a horizontal plane. The size of the span, i.e. of the distance between the contact lines, is given in GB/T 1634.2 or GB/T 1634.3. The supports are fitted to the base of the frame in such a way that the vertical force applied to the test specimen by the loading edge is midway (±1 mm) between them. The contact edges of the supports are parallel to the loading edge and at right angles to the length direction of the test specimen placed symmetrically across them. The contact edges of the supports and loading edge have a radius of (3.0 ± 0.2) mm and shall be longer than the width of the test specimen.
Unless vertical parts of the apparatus have the same coefficient of linear thermal expansion, the difference in change of length of these parts introduces an error in the reading of the apparent deflection of the test specimen. A blank test shall be made on each apparatus using a test specimen made of rigid material having a low coefficient of expansion and a thickness comparable to that of the specimen under test. The blank test shall cover the temperature ranges to be used in the actual determination, and a correction term shall be determined for each temperature. If the correction term is 0.01 mm or greater, its value and algebraic sign shall be recorded and the term applied to each test result by adding it algebraically to the reading of the apparent deflection of the test specimen.
Note: Invar and borosilicate glass have been found suitable as materials for the test specimen in the blank test.
Foreword II
Introduction IV
1 Scope
2 Normative References
3 Terms and Definitions
4 Principle
5 Apparatus
6 Test Specimens
7 Conditioning
8 Procedure
9 Expression of Results
10 Precision
11 Test Report
Bibliography
