The raw and auxiliary materials for the meter were melted at high temperatures, poured into molds, cooled, cut, and tempered to form the glass bottle. Initially, the bottle was made of quartz. Glass has a unique ability to return to its actual state. Consequently, it produces a force that is termed internal stress. Stress concentration at a particular spot will result in excessive fragility of the glass bottle in that area. Adopting different cooling speeds in accordance with the thickness of the wall is essential. The fast-cooling approach can reduce the chances of temporary stress generated during cooling from exceeding the strength limit of the glass and causing rupture.


Glass is exceptionally stable and is rarely influenced by atmospheric pressed. Decomposition by solid or liquid substances is not commonly seen. Therefore, glass bottles are primarily used for a variety of dosage forms in pharmaceutical companies.


The chemical characteristics and radiation resistance properties of the glass may be altered by changing the chemical composition of the Glass. Stress on glass bottles can be determined by employing a glass polarizing stress meter.


What is Stress?

Stress is described as an internal force that corresponds to or interacts with every part of that subject when it is subjected to deformation by external factors to the pre-deformed position. The internal force per unit area at a certain point under consideration is called Stress.

The geometry and size of an object will change when it’s unable to create displacement in response to an external force. This deformation is termed a strain. When a material deforms, an equal but opposing reaction force is generated internally to counteract the external force. This reaction force per unit area is referred to as stress.


Deformation due to external factors is primarily due to the production of an internal force interacting with the various parts of the object to resist the influence of external factors. It tries to bring the thing back to its original position.


There are different types of Stress:

  • Normal Stress: It is parallel to the direction of strain
  • Shear Stress: It is perpendicular to the direction of strain.
  • Compressive Stress: Compression of two ends of the cylinder results in Stress along its axis.


The established characteristics of Stress include:

  • The sum of all the Stress on (or in) an object can be directly accumulated.
  • The Stress can either be temporary or instantaneous (or both)
  • The Stress exists only while an external force operates upon it. The external force must be removed to remove the Stress from the object.
  • Long-term Stress manages to impact the object permanently.

Why test glass bottles for Stress?

The strength of the Glass is directly associated with the Stress on it. Excessive thermal stress causes a thermal explosion, whereas excessive mechanical Stress causes impact rupture. Self-explosion is primarily due to extreme internal Stress. Any increase in the strength of the Glass due to thermal or chemical tempering is because of an increase in the competitive surface stress. Thermal Stress is the primary type of Stress detected by the polarizing stress meter. Temperature gradients give rise to thermal stress as glass is an amorphous material melting at high temperatures and rapidly solidifies. As a result, the temperature variations of various sections of the glass plate surface during the manufacturing process will not be consistent, resulting in thermal Stress.


What is the principle of a glass bottle stress tester?

The principle of the glass bottle internal stress tester is based on the polarized light interference to determine the internal Stress of the Glass or the birefringence effect of the crystals. The instrument can qualitatively and quantitatively measure the internal Stress of the Glass because of the sensitive color plate and 1/4 wave plate compensation method.


Qualitative Measurement Principles:

  1. Placement of birefringent material between the orthogonal polarizer and analyzer results in interference colors in the field of view. A birefringence optical path difference correlates to a specific interference color.


The full-wave plate is present between the polarizer and analyzer, which is a thin sheet prepared from crystal. Its birefringence optical path difference is 565nm. According to the comparison table, the interference color in the field of view is purplish red. Placement of the test piece between the crossed polariser addition to the full-wave plate will result in a combined optical path difference between the two to be >< 565nm.


  • When the fast axis of the specimen is parallel to the fast axis of the full-wave plate: &’= & -565 (nm) — — -A
  • When the fast axis of the additional specimen is parallel to the slow axis of the full-wave plate: &’= & -565 (nm) — — -B


  1. Glass specimens that have been stressed are likewise birefringent. Stress inspection optical path of such specimens can produce changes in interference colors. Since the stress distribution is not constant, the birefringence optical path difference of each point is also different. This causes variation of the interference color in each store in the field of view to be different.
  2. When the glass specimen’s birefringence optical path difference is significant, the measurement can be done without a full-wave plate. A test piece is positioned in the orthogonal polarization field, and distinct interference colors are visible.


Quantitative Measurement Principle:

  1. The polarizer works to linearly polarize the white light coming directly from the light source. An angle Q rotates the vibration direction of the linearly polarized light as it passes through the test piece with a birefringence optical path different and 1/4 wave plate. The angle’s value is proportional to the test piece’s birefringence optical path difference. The relationship
  2. The stress distribution inside the Glass is inconsistent. Therefore, the birefringence optical path difference of each part of the specimen is also unstable. Different parts will obtain different optical path difference values.


How to use a polarizing stress meter to detect?

  1. Inspection of colorless test products


  1. A quarter-wave plate should be placed in the field of view for inspection of the bottom of the colorless test product. The zero point of the polarizing stress meter must be adjusted to make it appear as a dark field of view.


  1. Once the test product is placed into the field of view, observe it from the bottom. A dark cross will appear in the field of view. When the Stress of the test product is minimal, the dark cross appears blurred. The analyzer is rotated to differentiate the dark cross into two arcs moving against each other. A blue-gray area on the concave and a brown area on the convex side will be visible as the dark area moves outward. Rotate the analyzer until the blue-gray color at that spot is just replaced by brown to measure the stress value at that position. Rotate the test sample to find the area of maximum Stress, record the rotation angle of the analyzer for the shift from blue-gray to brow and determine the thickness of the point.



For the inspection of the side wall of the colorless test article, a quarter-wave plate is placed into the view field. The zero point is adjusted similarly to the previous method. The test product must be placed in the view field, so the angle is 45° to the polarization plane. Keep rotating the analyzer until the dark areas correspond with the side walls. Replace the bright regions completely. Rotate the test article about its axis to determine the ​​maximum stress area. Also, record the placement angle of the analyzer determine the original thickness of the side walls.


2) Inspection of non-ferrous test articles

The steps for inspection are identical to colorless test products. The rotation endpoint of the analyzer cannot be determined when there’s a lack of blue and brown color followed by low glass transmittance, especially in the case of dark samples. The standard average method can be used to measure the end point accurately.



What is end stress?

The inevitable temperature gradient induces the tension in the thickness direction during cooling.


What is plane stress?

The Stress induced by an uneven temperature distribution in the width direction during cooling,


What is Temporary Stress?

Temporary Stress is the tension that occurs in the Glass while the temperature gradient is there and vanishes when the temperature gradient is no longer present.


What is permanent Stress?

The Stress that remains after the temperature gradient in the Glass has vanished. The unequal structure of the Glass creates persistent tension due to the variable chemical composition of the Glass. Annealing will not be able to remove this stress.


Is the stress depth of tempered Glass the higher, the better?

The Stress of the tempered glass is based on two things:

  • The compressive stress of the surface.
  • Tensile Stress of the intermediate layer.

The tensile strength is considered insufficient when the compressive stress on the surface is significant. Therefore, the possibility of self-explosion increases.


What is the relationship between glass stress and strength?

Stress and strength of the Glass are directly associated. High thermal Stress equates to the thermal explosion, excessive mechanical stress results in impact rupture, and considerable internal Stress causes self-explosion.