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

What is ASTM C336?

ASTM C336 is a standard method for determining the annealing point and strain point of glass by fiber elongation. The method is useful for estimating the thermal performance of glass by measuring the elongation rate of a glass fiber subjected to a constant load as it is heated in a controlled furnace. The annealing point is the temperature at which internal stresses in glass can be substantially relieved within a practical time, while the strain point is the temperature below which residual stresses become essentially permanent. They give important information about the temperatures at which the residual stresses are relieved or permanently set in the glass.

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. 

ASTM C336 Equipment and Sample Preparation Guide

The key points for accurate determination of the annealing point and strain point of glass are proper specimen preparation and precise temperature control. ASTM C336 calls for a standard specimen consisting of a uniform type of glass fiber, loading, and measurement equipment to ensure repeatable and reliable results.

Testing Procedures and Requirements

ASTM C336 provides a standard procedure for measuring the elongation of the fiber, which results in the annealing point and the strain point for the glass. It consists of preparing a glass fiber specimen, loading it under a constant stress, measuring the elongation during controlled heating, and calculating from the measured viscosity values the annealing point and the strain point.

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. 

What is the Scope of ASTM C336?

The ASTM C336 standard test method specifies the method for measuring the annealing point and strain point of glass using the fiber elongation technique. The test determines the temperatures corresponding to specified viscosity values by measuring the elongation rate of a glass fiber under a constant tensile load. The standard:

• 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.

Analysis Results and Interpretation

• The annealing point and the strain point of glass are defined by ASTM C336, which are usually expressed in degrees Celsius (°C).
• The annealing point is at a viscosity of ~ 10¹³ poise, while the strain point is at a viscosity of ~ 10¹⁴·⁵ poise.
• Higher annealing point and strain point temperatures generally indicate that the glass maintains higher viscosity at elevated temperatures before stress relaxation or permanent strain occurs.
• There is a standard procedure (ASTM C336) that permits the determination of these thermal reference points, but acceptable values vary with glass composition and use.
• These measurements are used by engineers to optimize the annealing schedules, to enhance thermal processing, to assess the glass composition, and to guarantee the dimensional stability and durability of glass products. 

What are the Factors to Consider for ASTM C336 Testing?

• Speed: The testing efficiency can be improved, and the reliability of the thermal property measurement can be gained by the appropriate preparation of fibers and the controlled operation of the furnace. 
• Expertise: Skilled laboratory personnel provide expertise for accurate drawing of fibers, temperature control, elongation measurement, and interpreting test results. 
• Cost: Routine annealing and strain point testing reduce product failures, optimize furnace operation, improve quality control, and reduce manufacturing costs associated with thermal stresses. 

Link to ASTM C336

By Gokula Srinivasan Selvam

Gokula Srinivasan Selvam is a Material Testing Associate at MaTestLab Inc. and holds a postgraduate degree in Ceramic Engineering from IIT (BHU),...