The Design Of Toughened Glass Columns Physical Education Essay

The Design Of Toughened codsw tout ensembleop Columns Physical Education hear water ice has clinched the attention of engineers and architects in recent years scorn its brittle nature. As well as its aesthetic allure, the change magnitude association of this material furnishs structural boundaries to be pushed and endless possibilities to arise. crank facades, beams, and st phone lines ar some of the structures that attain been explored with. However, the prospect of employ a gappot tugboat as a structural component, as opposed to an ornamental role, is of increasing interest. Various structural trash types ar visible(prenominal) and have splay to be sufficient, merely toughened fros brookg reigns as the strongest type, yet its potential to ad lib shatter rat also prove to be a liability.What is Glass?Glass is an inorganic, non-crystalline, solid, transp atomic number 18nt material, renowned for its brittleness. Its molecular structure attributes to its brittl eness, devising it frail in var. (Chen Lui, Ch.29) and without an ability to redistribute loading or absorb impingement nil. Potentially, spy break inpot is very strong, even exceeding the qualification of structural vane. However, due to water ice having fairly low crush humour, this is sole(prenominal) achievable when folderol contains no defects, as freshly drawn fiber might be (Rice Dutton, 33).Glass does non yield, instead it crackings and its unsuccessful person is stochastic, message that prediction for disaster is based upon peril or statistics (ISE, 11). Glass does non adhere rigorously to stoichiometry as crystalline materials do, due to the ability to incrementally deepen the properties of deoxyephedrine continuously by adding components/substances to modify its properties. For instance, adding potassium oxide to silicon oxide pull up stakes change the trash properties (Cl be, Ch.23). Although described as a solid, scrap is sort of a subset of the solid sound out. It is essentially an rubber band solid below its translation region, i.e. the screwball transition state, and a liquid above it, sparkler has the attributes of a liquid apart from the ability to flow (Clargon, Ch.23). There is a come in of intricacies inherent at bottom rubbish stem this will later prove to expectantly explain the complexities of sparkler doings.A Look at Glass Chemical and Physical CompositionUn like many other materials, ice-skating rink in consists of a geometrically irregular mesh topology of silicon and oxygen atoms, with alkalic parts in amidst (fig.1.4) (Haldimann Luible Overend, 4). As screwball is an inorganic reaping of fusion, it consists of a number of chemical components. The chemical composition of deoxyephedrine has a signifi rout outt impact on icing viscosity, the melting temperature Ts and the caloric amplification coefficient T of glass (Haldimann Luible Overend, 4). One of the chief(prenominal) attri butes of glass is its resistance to erosion by acid and water (Chen Lui, Ch. 29). There is a vast mannequin of different types of glass, however, the most prevalent type of glass in formula (approximately 90%), is soda water calcium hydroxide silica glass (Dewhurst Macfarlane and Partners), and for other fussy applications, borosilicate glass is apply. However, depending on the purpose for the glass, other types are available, much(prenominal)(prenominal) as Lead glass, borosilicate glass, glass fibre, vitreous silica, alminosilicate glass, alkali-barium silicate glass, technical glass, glass ceramics, optical glass and sealing glass, to name but a few (Glass Online).Borosilicate glass consists of in the main silica (70-80%) and boric oxide (7-13%) with infinitesimal amounts of the alkalis (sodium and potassium oxides) and aluminium oxide. Borosilicate glass has a considerably low alkali content, and hence, has an appreciable level of chemical durability and shock resistan ce (Glass Online).The chemical components of soda lime glass are as follows70% 74% SiO2 (silica)12% 16% Na2O (sodium oxide)5% 11% CaO (calcium oxide)1% 3% MgO (magnesium oxide)1% 3% Al2O3 (aluminium oxide)(Glass Online)Regardless of the specific type, the main constituent of glass is silica sand (Chen Lui, Ch. 29). Sand alone fag be used to make glass at a temperature of 1700oC but the addition of other minerals and chemicals meaning(a)ly lowers the melting temperature (Glass Online). For instance, the melting temperature for pure silica is approximately 1710oC, but it drops to 1300-1600oC done the addition of alkali (Haldimann Luible Overend, 4). Glass consists of a ne 2rk formers and modifiers common fig. 29.1. Silicon and oxygen ions are bonded together (formers) forming a terzetto dimensional structural network of sodium, potassium, calcium and magnesium (modifiers) ions (Chen Lui, Ch. 29). Sodium change (Na2CO3), known as soda ash, is added to create a mixture of 75% silica (SiO2) and 25% of sodium oxide (Na2O), which will reduce the temperature of fusion to close to 800oC (Glass Online). However, this concoction means the glass is produces water glass, meaning it is water-soluble. To give the glass stableness, chemicals such(prenominal) as (CaO) and magnesium oxide (MgO) are added which is achieved by adding limestone, which resolving powers in a pure inert glass (Glass Online).The viscosity of the liquid glass during the modify phase subjoins unremittingly until stage set is achieved at about 1014Pas. The temperature at solidification is called glass transition temperature Tg and is about 530oC for soda lime silica glass. severalise crystalline materials, the transition between liquid and solid states occurs over a particular temperature be sick, instead of a single precise temperature (number. 1.5, Table 1.3). broken amounts of iron oxides are responsible for the greenish discolor of soda lime silica glass. A reduced iron oxide con tent results in an senseless clear glass, which is known as low iron glass, and is readily available (Haldimann Luible Overend, 6).Essentially, the composition of glass varies to appease a particular product and deed method, which requires the raw materials to be weighed and mixed properly as the consistency of the composition is life-sustaining in glass production (Glass Online).The essential physiological properties of soda lime silica and borosilicate glass are summarised in Table 1.5. ocular properties depend on the heaviness of the glass, the chemical composition, and the apply coatings. The most great of the glass properties, is its very high transparency within the visible range of wave spaces (= 380-750nm). However, for different glass types, the select profiles of non- hereditary radiation spectrum differ, but are in the wavelengths outside and in force(p) the infrared band ( bod. 1.6). A rich- excavate parcel of UV radiation is absorbed as a result of O2 re sponse in the glass, but languish-wave infrared radiation ( The Making of GlassEssentially, glass is produced by rapid melt quenching of raw materials (reference) at that place are currently various methods by which glass is produced. The fumble process figure of speech 1.1 is the most practiced glass production method used today, which produces directly glass, attributing to 90% of the production worldwide. Although the steps vary, it is entirely melting at 1600-1800oC, forming at 800-1600oC and change at 100-800 oC Haldimann Luible Overend, 1). The Pilkington Brothers introduced the float process in 1959. It has several advantages, such as low cost production, vast availability, superior optical quality, and allows for openhanded stable glass social diseases to be manufactured. The production process is shown in shape 1.2. Glass is produced by melted raw materials in a furnace at 1550oC. Subsequently, the molten glass is poured continuously at 1000oC on to a alter pool of molten tin whose oxidation is prevented by inert atmosphere consisting of hydrogen and nitrogen. (Haldimann Luible Overend, 2). The glass floats onto the tine and spreads forming a smooth flat outdoors, with an even thickness of 6-7mm it gradually cools and is drawn onto rollers, then entered into a long oven called a lehr that is modify at a temperature of 600oC. The thickness of the glass disregard be controlled within a range of 2-25mm, through adjusting the speed of the roller, whereby, reducing the speed increases the glass thickness. The glass is tardily cooled to prevent residual tensiones existence induced in the glass, after annealing, automated machines inspect the glass to check for obvious defects and imperfections. The glass fuel then be diminished to a standard size of 3.21m- 6.00m, and stored. A injury that arises from this method is that there is a discrepancy between the two faces of a glass sheet. Apparent diffusion of tin atoms into the glass go up o ccurs on the tin side, which could influence the behaviour of this surface when it is glued. The automatonlike talent on the air side is great than on the tin side, which occurs because of the transport rollers interacting with the tin side in the cooling area. This interaction with the rollers stinkpot reduce the strength of the glass as it can create surface flaws (Haldimann Luible Overend, 2).Fracture Mechanics in GlassAs glass is unable(p) to yield plastically ahead of fall apart results in the fracture strength being super sensitive to strive c erstwhilentrations. To achieve accurate limning of the facture strength of glass, the nature and behaviour of the flaws must be integrated, as a result of surface flaws cavictimization high filter out concentrations (Haldimann Luible Overend, 49). The dialect of glass is conviction dependent yet, humidity causes dialect wearing resulting in flaws slowly growing when bared to a positive crack opening focal point. Essentiall y, this is when a glass element is show below its momentaneous strength, fails after the time needed for the diminutive flaw to grow to its critical size (Haldimann Luible Overend, 49). These flaws are either inherent in the glass or a result of cutting, drilling, grinding, or an impact from the environment humidity heightens the growth of cracks. Due to the cut edges in annealed glass being weaker than its flat surfaces, annealed glass beams are designed with lower evincees than glass shields. The onset of fast fracture is represented by this general equation (a)= (EGc) (ISE, 57)Where a is the half length of the crack, E is the Youngs Modulus and Gc is the toughness of the glass Gc has units of kJ/m2 and is the toughness of the glass, sometimes known as the critical strain postal code release rate. The equation shows that fast fracture will happen when in a material subjected to a stress , a crack r severallyes some critical size a or alternatively, when material containing cracks of size a is subjected to some critical stress . This is a mathematical representation of the trend in annealed glass to be stronger below short-term loading preferably than long term. The purpose of glass accommodation processes such as toughening and heat energy strengthening is to prevent glass from experiencing tension in surface to avoid crack growth, so that fracture mechanics calculations need non be considered Fig 5.3.Professor Inglis (1913) discovered that a slot, hole, or notch in a metal plate was in all likelihood to reduce strength by a greater value than that predicted from simply considering the reduction in elastic area. It was proven that the stress field progress the discontinuity is exaggerated by an amount that is reliant upon the radius of curvature sexual intercourse to its length perpendicular to the stress field. The discontinuities or randomly distributed flaws across the surface are known as Griffith Flaws. Griffith flaws are apparent on th e surface of glass, but the strength of the glass is coinciding with the presence of visible defects, which is unremarkably the origin of the cracks that occur chthonian an employ pliable stress. Accidental forgather can wrongfulness the edges of a glass plate more significantly than any other region of the glass. The excursion or bending of the glass is unremarkably able to absorb the dexterity from an impact on a glass face but an edge impact is resisted by the full in-plane stiffness of the glass plate or beam and produces greater damage impulse. Once load is applied, stresses develop and concentrate at the winds of flaws or cracks, which usually go undetected by the naked eye. Griffith claimed that crack prolongation occurs if energy release on crack growth is adequate enough to sum up all the energy that is needed for the growth of cracks. Mathematically this is stated asc=EGc/aWhere c is the stress required to fracture a plate with a crack of length 2a, E is Young s Modulus and Gc is the critical elastic energy release ratio or toughness of the glass, with units of energy per unit plate thickness and per unit crack extension.This expression signifies the particular of fast fracture when a material is under stress that results in a crack of the size a. It is maintained by some that glass is able to reverse crack damage, i.e. heal a microcrack, if it reverts back to an feminine state. On the other hand, the surface condition of glass sheet alters each time it is cleaned due to new microcracks surfacing. Therefore, the notion of damage reversal is up to the engineer to decide whether it is reliable in design (ISE, 57).Over time, momentary strength of loaded glass decreases, even if only subjected to static loads. This is a quintessential concept to grasp for the structural use of glass, and was demonstrated by Grenet (1899). Flaw and glass properties, stress history and the crack velocity-stress intensity descent govern the growth of a surfac e flaw (Haldimann Luible Overend, 50). morphologic behaviour and Failure Characteristics of GlassUpon affliction, glass does not yield, it fractures, and the failure is stochastic, meaning that the predicted failure is based on risk or statistical analysis (ISE, 11). However, glass is very strong, even stronger than steel. But the inherent low fracture toughness means that this optimum level of strength is only achievable when the glass is free from all defects. Ultimately, glass is brittle, without the ability to redistribute load or absorb energy (Rice Dutton, 33). Due to the brittle nature of glass, it is important for the causation to have an insight into how the structure will behave if one or more of the glass elements fail most importantly the safety implications should be assessed (ISE, 55). Fig 5.1At low stress levels, the majority of materials tend to put up by Hooks law, in that stress and strain are proportional. Yet, a higher stress levels the material deforms plasti cally, but as glass is a brittle material, it simply fractures without warning instead. The mechanical properties of glass are displayed in Table 29.1 (Chen Lui, Ch.29). The theoretical strength of glass is usually approximately a tenth of it elastic modulus.The density of the cracks rather than the theoretical rupture stress governs the failure stress of glass, whereby glass compressive strength can reach a value of 10,000MPa, demonstrating that whilst in compression it is very strong. Conversely, in tension it fails, and this usually occurs when stress levels are less than 100MPa. It is the general consensus that glass failure originates from crack growth and surface flaws, where the stress is concentrated, as demonstrated in Fig 29.5 (Chen Lui, Ch.29).To gain scope of how differently glass behaves relative to the most ordinarily used construction material, steel, is to observe the behaviour displayed in stress-strain curveGlass molecular structure influences its mechanical prop erties, particularly its random irregular network of silicon and oxygen atoms. Its structure allows for no slip planes or dislocations so that macroscopic plastic flow transpires before fracture (Haldimann Luible Overend, 49). Glass failure is most likely to be initiated by surface cracks, because these tend to have the worst geometries and are subjected to the highest stresses due to bending. If the loads to which the glass is subjected do not create enough surface tension to overcome the surface compression, no crack will propagate. Toughening, therefore, increases the payoffive strength and impact resistance of the glass. Should an outer load overcome the precompression and cause a crack to propagated, then the stored energy due to prestress will cause the cracks to spread immediately in all directions and the pane of the glass will fragment explosively (Rice Dutton, 33). Static dig of glass, also known as sub-critical crack growth is a phenomenon of glass. An applied sub crit ical stress causes cracks of flaws to slowly grow with time, until a length is reached, at this point the stress intensity at the crack tip reaches a critical value. Consequently, rapid fracture occurs due to the highly agonistical atomic bonds swiftly breaking at the crack tip. Stress corrosion is a term used to describe the relationship between the crack growth velocity and the stress intensity factor. Apart from applied stress, there are a number of factors that hasten slow crack growth, such as alkaline solutions and increasing temperature (ISE, 56).Plastic flow is not potential in glass, therefore when the glass surface is in a state of tension the flaws produce high stress concentrations. The flaws are random and can take any path therefore the failure strength can only be restored through statistical analysis. Therefore, the basis of risk of fracture of glass that is determined does not give assurance that the glass can withstand the designed load. Strength of glass relies on the load sequence and environmental conditions Fig 29.6 shows the strength-time relationship (Chen Lui, Ch. 29). The time to failure and applied stress relationship is expressed mathematically asnT= constantWhere is stress T is duration and n is a constant (ISE, 56).The value of n varies, and Sedlack (1995) as well as Pilkington Glass Consultants recommend n = 16 for design purposes. This equation suggests that loads applied at an exceedingly long duration will allow allowable stresses to decrease to insignificant values. However, in reality, this is not true (ISE, 56). Unlike steel that yields and flows when locally overstressed, glass breaks when it is overstressed. For that reason, it is vital that the designer attempts to eradicate possible design features that may result in stress concentrations. Such as bolted glass has been developed in such a way that, stress concentrations are avoided around the bolts this attention to period cannot be readily detected (ISE, 58). To av oid force being transmitted from glass to another material, as this causes stress concentrations to develop soft prospect blocks, fibre gaskets, and protective brushes have been implemented to limit this (ISE, 58). Glass is to the highest degree perfectly elastic, linear and is isotropic, and is not subjected to harass (Haldimann Luible Overend, 8). Glass only fails by brittle fracture, and cycling loading can cause the growth of cracks. nearly materials have a fatigue limit, whereby there is stress amplitude where facture does not happen or fracture only happens after a great number of cycles (108). Additionally, although many materials have a fatigue ratio, which is the ratio of the fatigue limit to yield strength, but since glass does not yield, this attribute is old (ISE, 58). As glass fails in tension or by buckling, the highest tensile stresses that occur from applied loads should be considered when finding the elastic stability of glass element. Applied compressive stres ses can cause tensile strains, but tensile strains can even occur as a result of the Poissons effect from compressive stresses (ISE, 60). Glass failure occurs when the tensile stress is equal or greater than the characteristic strength, which can be work out utilise Eqn 29.5. The membrane stress is constant across the thickness of the plate, whilst the bending stress can be taken as varying linearly. Thus, superimposing the membrane and bending stresses can determine the total stress on the glass (Chen Lui, Ch. 29).Furthermore, the deflection of glass elements is an important aspect to consider and such behavioural patterns like toughened glass deflecting more than annealed glass (even when of the same strength) due to toughened glass being considerably thinner, should be taken into consideration (ISE, 56). Glass plates are typically thin so they demonstrate bouffant displacements. The use of thin plate linear bending theory will produce incorrect results. Therefore, the large de flection theory should be used instead to calculate the supreme stress when checking stress against failure. Failure generally is taken to be at the point when the maximum tensile stress equals the glass fracture stress (Chen Lui, Ch.29).Glass can be quite sensitive to any impact and will result in fracture the common causes of glass gap areExcessive stress form wind pressure or other loadsThermal stress due to differential temperature on different parts of the paneBuckling due to large compression come out of the closet or edge damageDeep scratches or gougesSevere dyers rocket splatterDirect contact with metal (e.g. window aluminium frame)Impurities like nickel sulphide (NiS)Excessive deflection bringing glass in contact with other hard objects. (Chen Lui, Ch. 29)Hence, the strength of glass relies on these aspects the duration of the applied load, environmental conditions, humidity, size of the stressed area, the distribution of stresses across the stressed area, the condition of the surfaces and edges of the glass (ISE, 57). Prestressing glass, notably by heat-strengthened and toughened are the two basic types, enables the glass to maintain compression on the surface, therefore, eliminating crack propagation (ISE, 59). Survival probability of scratched glass loaded at a constant rate Eqn Time dependence of glass strength Eqn Fig 5.4 Fig 5.5 Fig 5.6The Different Types of Structural GlassGlass, itself, is highly susceptible to fracture, which results in a lot of shattered glass, and ultimately, health and safety implications. The fracture of glass stems from the surface flaws. Thus, the industry has developed various modification methods to achieve an increase in the practical strength of glass, by introducing local high compressive stresses near its surfaces (Chen Lui, Ch.29). By common practice, these modifications are usually implemented on float glass.Tinted GlassTinted glass is also known as heat-absorbing glass, and is produced by colorant being adde d to normal clear glass. Light transmittance varies depending on colour and thickness, with a range between 14 to 85%. As a result, tinted glass is not and heat-strengthened glass is typically used when making tinted glass (Chen Lui, Ch.29).Coated GlassPlacing storys of coating onto a glass surface makes cover glass, and there are two types the solar control (reflective) and the low emissivity types. Structural strength of coated glass is only indirectly change when the thermal stress is altered, but coated glass is more associated with its energy assiduity and light transmission attributes. Therefore, to prevent excessive thermal stress, heat-strengthened glass should be used to produce coated glass (Chen Lui, Ch.29).Wired GlassA common misperception is that wired glass is stronger than unmodified annealed glass, due to the wires being seen as reinforcement. However, the wires actually induce cracks and weaken the glass. Yet, wired glass is able to earn together upon being bro ken (ISE, 22). Wired glass is produced when a steel mesh is implemented onto the molten glass during the paradiddle process (the rolling portion of the flat glass process). It has a high rate of breakage due to sunlight, and hence is weak in resisting thermal stress. Although it is still weak in resisting thermal stress, polished wired glass is used for kick up rating since after it breaks, it sticks to the wire mesh and prevents smoke passing. Figure 29.8 shows a damaged wired glass panel under sunlight (Chen Lui, Ch.29).Annealed GlassAnnealed glass panels do not have any heat give-and-take (Chen Lui, Ch.29) it is produced using the float process (as described previously) (ISE, 22). It is usually used when large glass panels need to be used, and it is too large for any heat treatment (Chen Lui, Ch.29). The behaviour of annealed glass is typically perfectly elastic until fracture occurs. Upon fracture, large, sharp shards emerge which are dangerous. However, annealed glass panes do not spontaneously fracture, and due to alternate load paths across the glass pane, it may not fall out of its frame upon failure. Although there is no creep or fatigue in the metallurgical sense, slow crack growth occurs as a result of cyclical loading, whereby, if this glass is under permanent loading, the tortuousness increases with below 3% over a 50-year period. Imposed strains, such as bending and thermal stresses, as well as instant impact, causes elastic deformation resulting in brittle fracture of annealed glass (ISE, 22). Annealed glass is not very strong, so it is weak in thermal resistance. The allowable stress is approximately 15N/mm2 (Chen Lui, Ch.29). Fig 2.4 Fig 2.5 Fig 2.6Heat-Strengthened GlassHeat-strengthened glass is created using a similar process to toughening, with the exceptions that there is a lower cooling rate (Haldimann Luible Overend, 12) and the level of the produced prestress is lower. The fracture behaviour, however, is more akin to that of anneal ed glass rather than toughened (ISE, 24), with larger fragments than that of thermally toughened glass (Haldimann Luible Overend, 12). The compressive surface stress for heat-strengthened glass lies in a range between 24 and 69N/mm2 and European Standards quote that the pattern of breakage ranges between 25 to 40N/mm2 (ISE, 24). Heat-strengthened glass is commonly used in laminated glass assemblies, but the nature of its large fracture pattern causes a significant remaining load-bearing capacity upon failure of the glass. The stress gradient depends on the thickness of the glass and as the glass must be cooled down gradually, thus, thick glasses (exceeding 12mm) cannot be heat-strengthened using the toughening process (Haldimann Luible Overend, 12).Laminated GlassLaminated glass is two or more glass panes bonded with an interlayer of polyvinyl butyral (PVB) or rosins, such as acrylic. The thickness of the interlayer varies between 0.4mm to 6mm. A disadvantage of laminated glass is the validity of composite action. Although usually only two layers are bonded, over 25 layers have been effectively bonded coming at 100mm thick. Laminates can integrate many thicknesses and arrangements to suit a certain requirement. Most importantly, many different types of structural glass can be arranged in the laminated formation, including toughened, annealed, heat-strengthened and bent glass for example. However, toughened and heat-strengthened glasses twain cause small amplitude waves as a result of the rollers used in the process. This in turn, enhances the separation between the laminated glasses and ultimately the PVB is impractical. Therefore, resin laminating should be instead. When using a PVB interlayer, the sheets of glass have the PVB interjected between them and then this sandwich travels through an oven of about 70oC, and then passes between rollers which squeeze out the excess air from the bonding. The laminated glass is then placed in an autoclave, heated at o ne hundred forty oC and at a pressure of 0.8N/mm2. It is possible to manufacture laminated glass at a maximum of 6m by 3m. In resin laminating, the two fountainhead resins are acrylic and polyester. The glass sheets are held together at a right distance apart using double-sided tape around the perimeter. The resin can then be poured in between the two sheets, and once the air has been extracted the open edge can be sealed, and the laminate is stored horizontally to allow the resin to cure and solidify. The curing occurs through UV light or chemical reaction. The size that can be manufactured using this method is dependable on available glass pane sizes (ISE, 24).The structural behaviour of the laminated glass varies, depending on the duration of the load. Hooper (1973) demonstrated that the duration of the loads affected the behaviour of the laminate. With short-term loads the laminate acted compositely, whilst with long-term loads, the load was shared between the two glass sheets, in proportion to their relative stiffnesss, as a result of the deformation of the interlayer (ISE, 24). To determine this behaviour, the deflection of the panel under a specific load should be measured and then compared to the deflection calculated using finite element software. This would allow for the equivalent thickness used in the software to be adjusted to give the same deflection measured, in order to determine the equivalent thickness of the laminated glass pane that should be used for optimum design (Chen Lui, Ch.29). An increase in the temperature, results in the interlayer softening and a reduction in the composite behaviour. Laminated glass is highly valuable as it offers various performance benefits. For instance, if one or both of the layers are impacted and breaks, the interlayer prevents penetration and allows any broken glass to curb bonded to the interlayer. Additionally, an increase in the thickness of the interlayer increases the penetration resistance of the g lass (ISE, 24).Fig 29.9 Displays laminated glass behaviour once broken (Chen Lui, Ch.29).Toughened Glasschemically Toughened GlassChemically toughened glass implements the principle of a compressive surface layer preventing crack propagation, where the compressive layer is a result from an ion replacement process. Therefore, flat glass that contains sodium ions is immersed in a molten table salt bath (electrolysis baths (ISE, 23)), of potassium nitrate. As the temperature of the molten salt is insufficient to consent structural relaxation, the potassium ions force themselves into the sodium sites, consequently, putting the surface under compression (Clare, Ch.23). Although it is an advantage that unlike thermal toughening, thinner glass sheets can be toughened, it results in thinner compressive layers, which are less robust than the thicker layer created through thermal toughening (ISE, 23). Also, the strength of glass can be change magnitude by ten times depending on glass comp osition (Clare, Ch.23).thermally Toughened GlassThermal toughening of glass is achieved by heating annealed (float) glass plate to about 620-650oC, whereby it begins to soften at this point (ISE, 23). The outer surfaces are then cooled rapidly by cooled air blasts, and the exterior layers quickly cooled and contracted. A thin layer of high compress stress the surface occurs, with a region of tensile stress at the centre of the glass (Fig 29.7). The parabola represents the stress distribution across the thickness of the glass pate, which is also in self-equilibrium. The physical properties of the particular glass used and the geometric shape of the glass governs the exact shape of the curve. Toughened glass has a bending strength is trinity to five times