Minimization of annealing costs, by automatic measurement of the residual stress

23 Pages·2009·1.66 MB·English

by  Henning Katte

Minimization of Annealing Costs
by Automatic Measurement of
the Residual Stress Distribution
Glassman Europe 2009
Centre de Congrès, Lyon, France
13th May 2009
Henning Katte, ilis gmbh
ilis gmbh | Konrad-Zuse-Str. 12 | D-91052 Erlangen | [phone] | [email protected] | www.ilis.de
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Overview
Motivation for stress measurement
(cid:132)
Measuring principle
(cid:132)
Automatic measurement
(cid:132)
Optimization of the annealing process
(cid:132)
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ilis gmbh
Products and services for the
(cid:132)
glass and packaging industry
Founded 1998 in Clausthal-Zellerfeld,
(cid:132)
based in Erlangen, Germany since 2001
Privately owned and financed
(cid:132)
Main activities and products:
(cid:132)
(cid:133) Measurement of glass color:
PRISMA Software Package & SmartSpec Spectrometers
(cid:133) Batch calculation/glass property prediction:
BatchMaker® Software Package
(cid:133) Measurement of residual stresses birefringence:
StrainMatic® Polarimeter Systems
(cid:133) Corporation-wide operational data management:
GLASDATA (developed for Saint-Gobain Oberland)
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Motivation for Stress Measurement
The strength of glass products and their ability to be processed is
(cid:132)
influenced strongly by mechanical stresses
If at all, residual stresses are often determined
(cid:132)
using simple polariscopes (acc. to ASTM C148-00)
(cid:47) imprecise, subjective measurement
(cid:47) problematic for colored glasses
(cid:47) no automatic documentation of results
Advantages of automatic measurement:
(cid:132)
☺ objective (i.e. operator-independent) and reliable results
☺ measurement independent of product characteristics such as color
☺ a precise measurement is a prerequisite for optimization of the
annealing process (no control without measuring)
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Fundamentals of Photoelasticity
Light wave?
Polarized light?
Refraction?
Birefringence?
Temper Polarimeter?
Optical
number? Polariscope?
isotropic?
Quarter-wave
plate?
Optical
Polarization
retardation?
angle?
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What is Stress Birefringence?
(cid:132) Light is an electromagnetic wave that activates
the oscillation of atom electron shells in a
wavelength
material; this in turn produces light.
(cid:132) The velocity of light in a material depends on
the particle density.
(cid:132) The refractive index is the ratio of the
zero-crossing
velocity of light in vacuum and in material:
n=c/v (for CaNaSi-Glass: n~1,5).
(cid:132) Mechanical stress leads to deformation of the
material structure and therefore changes the
distance between the particles
(cid:132) If the velocity of light (and therefore the
refractive index) differs in different directions in
space the material is called birefringent.
(cid:132) Glass normally is optically isotropic but
becomes birefringent when put under stress.
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Polarized Light
(cid:132) If the electric field of the light oscillates only within
one plane the light is called linearly polarized.
(cid:132) A (linear) polarizer only lets the light waves that are
orientated parallel to its optical axis pass and thus
creates linearly polarized light.
(cid:132) In a birefringent material the light waves spread with
different speed in the horizontal and vertical direction
which results in optical retardation.
(cid:132) Linearly polarized light leaves a birefringent material
as a superposition of two light waves at right angles
to one another with different phase.
(cid:132) If the optical retardation is exactly a quarter of the
wavelength the light is called circularly polarized.
(cid:132) In general, the departing light is elliptically
polarized. The ellipticity is a measure of the optical
birefringence.
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Polarized Light - Visualization
linear circular elliptical
polarization polarization polarization
0 λ 1/8 λ 1/4 λ 3/8 λ 1/2 λ
(linear) (elliptical) (circular) (elliptical) (circular)
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Polarimeter Setup
detector
analyzer
λ/4 plate
specimen
polarizer
light source
Intensity image of a strain Principle of a polarimeter for optical measurement of
disc at analyzer angles of birefringence according to the photoelastic principle
0° and 8°
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Measuring Units
(cid:132) From the polarization angle α the optical retardation R in nm can be calculated
using a simple formula:
R = α · λ / 180°
(cid:132) When the birefringence is homogeneous along the optical path the optical retardation
can be normalized to a standard thickness of 1 cm (for a given thickness d in mm):
N = R · (10 / d)
(cid:132) In case of membrane stresses the optical retardation can be converted to stress in
MPa, if the photoelastic coefficient C of the material is known:
S = R / (d · C)
(cid:132) For describing the residual stress state in container glass, ASTM C 148-00 defines the
unit apparent temper number, referring to the nominal retardation of a Strain Disc
standard (which was originally used for visual comparison):
T = R / 22.8 nm
A
(cid:132) Taking the thickness d into consideration the apparent temper number can be
converted into a so-called real temper number:
T = T · (4.06 mm / d)
R A
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