ASTM C336 Standard Test Method for Annealing Point and Strain Point of Glass by Fiber Elongation

This is a standard test used in glass manufacturing, research and development, process optimization, and quality assurance. Such thermal parameters are useful for manufacturers to program the appropriate annealing schedule, optimize production, and avoid thermal-stress failures.

Get Certified ASTM C336 Testing for Reliable Glass Thermal Property Evaluation

The ASTM C336 test enables manufacturers to precisely measure the annealing point and strain point of glass by the standardized fiber elongation technique. Accurate thermal property testing can aid in process optimization, control of stresses during manufacture, testing for glass composition consistency, and providing high-quality glass products to ensure dimensional stability and mechanical reliability. 

• Finds the annealing point and strain point of glass.
• Measures the elongation rate of the glass fiber under a constant tensile load as a function of temperature.
• Determines the temperatures corresponding to viscosities of approximately 10¹³ poise for the annealing point and 10¹⁴·⁵ poise for the strain point.
• Helps to develop and validate annealing schedules.
• Makes precise and consistent measurements for quality control and material analysis.
• ASTM C336 can be used for any type of glass that can be formed into fiber, such as soda-lime, borosilicate, aluminosilicate, optical, and specialty glass. 

What are the Uses of ASTM C336 Test Standard?

ASTM C336 is widely used by glass manufacturers and testing laboratories to determine the annealing and strain points of glass and to measure the thermal processing properties. Common applications include:

• Development of annealing schedules for flat, container, and specialty glass.
• Thermal and Residual stress analysis of glass products.
• Supports the evaluation of optical glass used for precision lenses and prisms.
• Assists with quality control during glass manufacturing.
• Comparing properties of various types of glass.
• Optimization of heat treatment, tempering, and annealing operations.
• Supporting research and development for new types of glass.
• Investigating failures due to thermal stress. 

What is Annealing and Strain Point Testing in ASTM C336?

Annealing and strain point testing determine the temperatures corresponding to specific viscosity values of glass. The annealing point is the temperature at which, within a practical time frame, the internal stresses can be relieved, and the strain point is the lowest temperature at which the internal stresses are permanently locked into the glass. ASTM C336 is a method for measuring the temperatures based on the elongation rate of a glass fiber under a constant load. 

Why is Annealing and Strain Point Testing Important for Glass?

Thermal properties such as annealing and strain points are important for the strength, dimensional stability, and durability of glass products. Excessive residual stress, distortion, cracking, or failure on service use may result because of incorrect annealing temperatures. These thermal reference points help manufacturers establish appropriate annealing schedules, improve production consistency, and enhance product reliability. 

How Does ASTM C336 Measure Annealing and Strain Points?

A standard dead weight is placed on a uniform glass fiber that is suspended inside a Furnace with a controlled temperature. As the furnace temperature changes, the elongation rate of the fiber is measured continuously. The temperatures corresponding to the specified viscosity values are determined from the elongation data and reported as the annealing point and strain point of the glass. 

Common Challenges and Troubleshooting

Factors that may cause errors in ASTM C336 measurements include non-uniform fiber diameter, specimen preparation, inaccurate temperature control, unstable loading conditions, furnace temperature variations, and errors in measuring elongation. The uniformity of the glass fibers, the calibration of the furnace temperature, the application of the specified constant load, and the use of precision elongation measuring equipment enhance the repeatability and reliability. Good specimen preparation and regular test procedures will increase the accuracy of the determination of the annealing and strain points.

ASTM C336 Testing Technique, Process, and Data Collection Guide

The uniform glass fiber is taken from the bulk glass sample and then suspended vertically in a calibrated tube furnace under a standardized dead-weight load. The furnace temperature is set as per the desired heating program, and the elongation of the fiber is recorded continuously. Elongation data are used to calculate the annealing point temperature and the strain point temperature. Test reports contain information such as specimen identification, glass composition, fiber diameter, applied load, furnace conditions, elongation rate versus temperature, annealing point temperature, strain point temperature, and testing equipment calibration information.