Thermal Tempering Research Articles

The catastrophic failure of thermally tempered glass caused by small-particle impact

One of the extensively used practical methods of strengthening soda–lime–silica glass against external stresses is to put its surface in a state of compression by thermal tempering or ion exchange1–3. These processes place the subsurface regions under tension. The stress distribution in thermally tempered glass can be represented by a parabola, with the magnitude of the surface compression stress equal to twice the centre tension. A consequence of the bulk tensile stresses is that once a crack (generated by, for example, particle impact) reaches this tensile zone, the entire glass sample breaks up. In this situation, the energy required for producing cracks is elastically stored in the sample because of the strengthening procedure. There has been little work published on the manner in which a crack enters the tensile region, causing a catastrophic failure of the entire sample. Moreover, the velocities at which the cracks move through the different regions of a tempered sample are not known in detail, although a few measurements of the maximum crack velocities have been made3,4. We now report the results of our high-speed photographic investigations, which provide clear answers to these questions. We also indicate the conditions under which the catastrophic failure of thermally tempered glass, caused by particle impact, is likely to occur.

Strong glass

Factors that govern the strength of bulk glass are reviewed, along with some processes by which that strength may be increased. Most involve compressive pre-stressing of surfaces, and some — notably thermal tempering and ion stuffing — are highly developed for certain applications. Strengths attained in the laboratory and in industrial practice are contrasted. More fundamentally, the usable strength of glass is limited by its brittleness, and the question is raised whether composites with glass-like properties can be made less brittle.

Effect of tempering on quasi-static and impact fracture toughness and mechanical properties for 5140 H steel

The effects of various thermal treatments,i.e., oil quench and different tempering conditions, on quasi-static and impact fracture toughness, stress-strain characteristics, hardness, and Charpy energy of 5140 H steel were examined. During quasi-static and impact loading notched round tensile specimens were used with a prefatigued crack. A specially designed device together with a pendulum hammer and electronic measuring system was used enabling testing of the opening mode fracture toughness at loading rates up to K1= 3 x 106 MPa√m per second. It has been found that within the region of the lower tempering temperatures, 500 K≤ 650 K, the critical stress intensity factor KIc determined from impact testing is lower than that obtained during slow loading, whereas at the higher tempering temperatures, 650 K ≤T* ≤ 900 K, dynamic KIu values show a tendency to be higher than their quasi-static counterparts. This behavior was analyzed quantitatively using the Hahn-Rosenfield model which relates tensile properties to fracture toughness. A good agreement was found between quasi-static experimental results and the model. The relation between Charpy energy Kv and the critical stress intensity factor KIc was also evaluated. Changes of the fracture toughness are discussed within the framework of SEM fractographs taken after quasi-static and impact tests.

Fatigue of Autofrettaged Thick Tubes: Effect of Thermal Shock, Electropolishing, and Tempering

Thick tubes of 32.3 mm (1.27 in.) ID and wall ratio of 2.5 for SAE 4333 steel were subjected to fatigue testing after the following treatments: normal autofrettage followed by thermal shock; electropolishing; electropolishing then normal autofrettage; electropolishing then normal autofrettage then thermal shock; electropolishing then open-ended autofrettage; tempering from Rc 36 down to Rc 29. Results showed a thermal shock of 600°C for 3 s reduced the fatigue strength of autofrettaged specimens 6 to 8 percent. Electropolishing reduced the fatigue strength 13 percent for as-received bores and 6 percent for autofrettaged bores. Double tempering to Rc 29 reduced the fatigue strength 57 percent compared to Rc 36.

Strong composite glasses

Glass strength can be enhanced by several methods including thermal tempering and ion exchange which put the surface in compression. Overlaying a higher expansion core with a lower expansion surface cladding to achieve a compressive surface is also known. A new modification of the overlay technique involves pairs of glasses which are melted separately and delivered to a single compound orifice that overlays one glass on the other as they leave the orifice. This is in contrast to most of the previous overlay methods in that the surfaces of the two glasses which form the interface are fluid and flaw-free. Physical property and composition constraints are presented for glasses which achieve a modulus of rupture as high as 42 kg/mm 2. Stress relationships and profiles are also shown.

Strengthening of Glasses by Partial Leaching

A technique for strengthening certain alkali borosilicate glasses is described. Porous surface layers of a desired thickness are created on an article, taking advantage of the phase separation and leaching phenomena in such glasses. The semiporous composite is then heated to sinter the high‐silica clad about the borosilicate core. On cooling, residual compressive stresses develop within the cladding. The mechanical properties of cylindrical specimens prestressed by this method were characterized by photoelastic measurements and moduius‐of‐rupture tests. Residual stresses as high as —410 MPa were observed and found to be in reasonable agreement with the form of a previously developed thermal‐stress model. The roles played by thermal tempering and stress‐relaxation effects are also discussed.

The Effects of Low Temperature Magnetic Cycling on the Properties of Ferromagnetic Materials

The interaction of dislocations with magnetic domain walls and the low energy response of dislocations at cryogenic temperatures have been investigated as a means of improving the structure sensitive properties of steel. Both cutting tools and carburized surfaces have shown an improvement when subjected to a cyclic magnetic field in the temperature range of 77 K to room temperature (RT). An annealing effect of the treatment suggests it may be a low energy substitution for conventional thermal tempering in specific applications.

Rain erosion mechanisms in brittle materials

The rain erosion behavior is reported for a series of glasses exposed to a 2.5 cm h −1 rainfall of 1.8 mm water drops at an impact velocity of 222 m s −1. The test specimens include arsenic trisulfide, borosilicate, soda lime and calcium aluminosilicate glasses and two glass surfaces modified by thermal tempering and ion-exchange strengthening. A qualitative ranking of the erosion rates correlates with the fracture thresholds for microsphere indentation. Attempts to correlate erosion rates with conventional mechanical properties have been less successful.

Stress state in tempered glass plate and determination of heat-transfer rate

Thermal tempering is widely used to manufacture safety glass for economic as well as for certain safety measures. Laboratory investigations of the tempering process and the resultant strengthening effect are generally limited to rectangular specimens. Results are, therefore, appropriate for this particular geometry. This paper describes a simple stress-state model of a tempered flat glass specimen. the model, developed using photoelastic equations to determine the three-dimensional stress components, was used to predict the transient birefringence in a rectangular glass specimen subjected to uniform and symmetrical heat-transfer conditions, at a temperature where glass behaves as a perfect elastic material with no stress relaxation within the experimental time. A method of determining the coefficient of heat-transfer rate was then developed based on the analysis of the transient birefringence. This technique uses the glass specimen as an optical transducer, and does not affect, in any way, the natural flow of heat by forced convection or contact cooling.

Thermal stress calculations based on a linear viscoelastic deviatoric response and a fictive temperature component for the volumetric response

A temperature dependent thermal expansion coefficient is introduced into the Lee, Rogers and Woo linear viscoelastic model for calculating thermal stresses in glass. Volumetric strain, is calculated as follows: ϱ = α liq ( T f - T O) + α gl ( T - T f) where α liq and α gl are the thermal expansion coefficients for the liquid and glassy states respectively, T is the glass temperature, T O is the initial temperature, and T f is the fictive temperature as defined by Tool. Fictive temperatures are calculatedfrom theory and measurements independent of thermal stress experiments. Comparison of calculated with experimentally measured stresses shows that the modified model is useful for predicting thermal tempering propensity for different glass compositions as well as different processing parameters.

熱強化ガラス板の残留応力

熱強化ガラス板の残留応力