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READING PASSAGE 2

Flawed Beauty: the problem with toughened glass

On 2nd August 1999, a particularly hot day in the town of Cirencester in the UK, a large pane of toughened glass in the roof of a shopping centre at Bishops Walk shattered without warning and fell from its frame. When fragments were analysed by experts at the giant glass manufacturer Pilkington, which had made the pane, they found that minute crystals of nickel sulphide trapped inside the glass had almost certainly caused the failure

The glass industry is aware of the issue, says Brian Waldron, chairman of the standards committee at the Glass and Glazing Federation, a British trade association, and standards development officer at Pilkington. But he insists that cases are few and far between. It’s a very rare phenomenon,’ he says

Others disagree. ‘On average I see about one or two buildings a month suffering from nickel sulphide related failures,’ says Barrie Josie, a consultant engineer involved in the Bishops Walk investigation. Other experts tell of similar experiences Tony Wilmott of London-based consulting engineers Sandberg, and Simon Armstrong at CladTech Associates in Hampshire both say they know of hundreds of cases. ‘What you hear is only the tip of the iceberg’ says Trevor Ford, a glass expert at Resolve Engineering in Brisbane. Queensland. He believes the reason is simples: ‘No-one wants bad press’

Toughened glass is found everywhere, from cars and bus shelters to the windows, walls and roofs of thousands of buildings around the world. It’s easy to see why. This glass has five times the strength of standard glass, and when it does break it shatters into tiny cubes rather than large, razor-sharp shards. Architects love it because large panels can be bolted together to make transparent walls, and turning it into ceilings and floors is almost as easy

It is made by heating a sheet of ordinary glass to about 620 °C soften is slightly, allowing its structure to expand, and then cooling it rapidly with jets of cold air. This causes the outer layer of the pane to contract and solidify before the interior. When the interior finally solidifies and shrinks, it exerts a pull on the outer layer that leaves it in permanent compression and produces a tensile force inside the glass. As cracks propagate best in materials under tension, the compressive force on the surface must be overcome before the pane will break, making it more resistant to cracking

The problem starts when the glass contains nickel sulphide impurities. Trace amounts of nickel and sulphur are usually present in the raw materials used to make glass, and nickel can also be introduced by fragments of nickel alloys falling into the molten glass. As the glass is heated these atoms react to form tiny crystals of nickel sulphide. Just a tenth of a gram of nickel I the furnace can create up to 50,000 crystals

These crystals can exist in two forms: a dense form called the alpha phase, which is stable at high temperatures, and a less dense form called the beta phase, which is stable at room temperatures. The high temperatures used in the toughening process convert all the crystals to the dense, compact alpha form. But the subsequent cooling is so rapid that the crystals don’t have time to change back to the beta phase. This leaves unstable alpha crystals in the glass, primed like a coiled spring, ready to revert to the beta phase without warning

When this happens, the crystals expand by up 4%. And if they are within the central, tensile region of the pane, the stresses this unleashes can shatter the whole sheet. The time that elapses before failure occurs is unpredictable. It could happen just months after manufacture, or decades later, although if the glass is heated – by sunlight, for example – the process is speeded up. Ironically, says Graham Dodd, of consulting engineers Arup in London, the oldest pane of toughened glass known to have failed due to nickel sulphide inclusions wars in Pilkington’s glass research building in Lathom, Lancashire. The pane was 27 years old

Data showing the scale of the nickel sulphide problem is almost impossible to find. The picture is made more complicated by the fact that these crystals occur in batches. So even if, on average, there is only one inclusion in 7 tonnes of glass, if you experience one nickel sulphide failure in your building, that probably means you’ve got a problem in more than one pane. Josie says that in the last decade he was worked in over 15 buildings with the number of failures into double figures

One of the worst examples of this is Waterfront Place, which was completed in 1990. Over the following decade, the 40 storey Brisbane block suffered a rash of failures, Eighty panes of its toughened glass shattered due to inclusions before experts were finally called in. John Barry, an expert in nickel sulphide contamination at the University of Queensland, analysed every glass pane in the building. Using a studio camera, a photographer went up in a cradle to take photos of every pane. These were scanned under a modified microfiche reader for signs of nickel crystals. ‘We discovered at least another 120 panes with potentially dangerous inclusions which were then replaced,’ says Barry. ‘It was a very expensive and time-consuming process that took around six months to complete.’

Though the project cost A$1.6 million ( nearly 700,000), the alternative – re-cladding the entire building – would have cost ten times as much

Question 1-5

Complete the summary with the list of words A-P below

Write down answers in boxes 1-5 on your answer sheet

Toughened Glass

Toughened glass is favoured by architects because it is much stronger than ordinary glass, and the fragments are not as 1………………………………………….. when it breaks. However, it has one disadvantage: it can shatter 2…………………………………………… This fault is a result of the manufacturing process. Ordinary glass is first heated, then cooled very 3…………………………………………..The outer layer 4………………………………………….. before the inner layer, and the tension between the two layers which is created because of this makes the glass stronger. However, if the glass contains nickel sulphide impurities, crystals of nickel sulphide are formed. These are unstable, and can expand suddenly, particularly if the weather is 5………………………………………….., If this happed, the pane of glass may break. The frequency with which such problems occur is disputed by glass experts. Furthermore, the crystals cannot be detected without sophisticated equipment

Nguồn: Cambridge ielts 5

ĐÁP ÁN

1 F - sharp

2 I - unexpectedly

3 C-quickly

4 K- contracts

5 E -warm

GIẢI THÍCH

1 Bạn có thể tìm thấy đáp án trong câu ‘ This glass has five times the strength of standard glass, and when it does break it shatters into tiny cubes rather than large, razor-sharp shards.’

2 Bạn có thể tìm thấy đáp án trong câu ‘ Just a tenth of a gram of nickel I the furnace can create up to 50,000 crystals’

3 Bạn có thể tìm thấy đáp án trong câu ‘ It is made by heating a sheet of ordinary glass to about 620 °C soften is slightly, allowing its structure to expand, and then cooling it rapidly with jets of cold air.’’ Quickly = Rapidly’’

4 Bạn có thể tìm thấy đáp án trong câu ‘ This causes the outer layer of the pane to contract and solidify before the interior’

5 Bạn có thể tìm thấy đáp án trong câu ‘As the glass is heated these atoms react to form tiny crystals of nickel sulphide. Just a tenth of a gram of nickel I the furnace can create up to 50,000 crystals’

HIGHLIGHT VOCAB

Toughened glass: kính cường lực

Manufacturer /ˌmænjuˈfæktʃərə(r)/: người chế tạo, hãng chế tạo

Fragment /ˈfræɡmənt/: mảnh vỡ

Sulphide /ˈsʌlfaɪd/ sunfua

Phenomenon /fəˈnɒmɪnən/: hiện tượng, vật kỳ dị

Bus shelters: /ˈbʌs ʃeltə(r)/ trạm chờ xe buýt

Shatter /ˈʃætə(r)/: vỡ thành mảnh

Transparent /trænsˈpærənt/: trong suốt, nhìn qua được

Solidify /səˈlɪdɪfaɪ/ làm cho đặc lại

Shrink /ʃrɪŋk/: co lại, quắt lại

Exert /ɪɡˈzɜːt/ cố gắng, nỗ lực, dùng, sử dụng sức mạnh

Compression: /kəmˈpreʃn/: sự nén, ép

Permanent: /ˈpɜːmənənt/: lâu dài

Tensile /ˈtensaɪl/: căng, dãn

Impurity /ɪmˈpjʊərəti/: tạp chất

Subsequent /ˈsʌbsɪkwənt/: theo sau

Prime /praɪm/: thời kỳ sẵn sàng, thời kỳ đẹp nhất

Coiled spring: cuộn lò xo

Unleash /ʌnˈliːʃ/: thả ra, buông lỏng ra

Contamination /kənˌtæmɪˈneɪʃn/ sự ô nhiễm

Cradle /ˈkreɪdl/: cái nôi

Modify: /ˈmɒdɪfaɪ/ thay đổi, sửa đổi

Microfiche /ˈmaɪkrəʊfiːʃ/ vi chiếu, tờ vi phim

Cladding: /ˈklædɪŋ/ lớp bảo vệ bên ngoài

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