ASTM C978 calculates residual stress in a transparent glass matrix by utilizing a polarizing microscope and optical retardation compensation techniques. This test method determines the magnitudes and local distributions of residual stress systems within glass substrates. Additionally, it evaluates the compatibility of the coefficient of thermal expansion for enamel or applied color labels (ACL) against that of the glass substrate. The ASTM C978 (Standard Test Method for Photoelastic Determination of Residual Stress in a Transparent Glass Matrix Using a Polarizing Microscope and Optical Retardation Compensation Procedures) approach is effective and quantitatively acceptable, particularly when optical stress coefficients and significant ion exchange are unknown.
ASTM C978 Photoelastic Determination of Residual Stress in a Glass Matrix
What is ASTM C978?
ASTM C978 Equipment and Sample Preparation Guide
The equipment used in this test is a polarizing microscope and optical retardation compensators. Ring section, bar section, or any other correctly shaped section of the test specimens with suitable annealing are used in ASTM C978 testing.
The length of the optical path in the segment of the test specimen shall not be less than 2.0 mm and not more than 30.0 mm
What are the Testing Procedure and Requirements for ASTM C978?
The ASTM C978 test method utilizes photoelastic retardation compensation procedures to determine residual stresses in clear glass matrices quantitatively. To produce a retardation null or extinction in the test specimen, compensators such as rotating compensators (covering a range of one order of retardation), birefringent quartz wedge compensators (covering a range of four to six orders of retardation), or tilting optical retardation compensators (covering a range of five orders of retardation) are used.
ASTM C978 Analysis Results and Interpretation
ASTM C978 determines residual stress in a transparent glass matrix using a polarizing microscope and optical retardation compensation procedures.
For ASTM C978, the optical retardation value, R, is calculated as :
R = (L × a) /180°
where,
L = wavelength
a = angle of rotation
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