【词汇】雅思阅读常用词(011)- Flawed Beauty: the problem with toughened glass

Cambridge 05 Test 04 -Passage2: 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 simple: ‘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℃ to soften it 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 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 in 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 rystals 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 to 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 was 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 has worked on 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 sulphide 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.




“玻璃行业已经意识到了这个问题。”英国贸易协会之一——玻璃及玻璃装配协会标准委员会主席,同时也是Pilkington公司标准发展官员的Brian Waldron宣称。但是他坚持认为这种情况只不过是沧海一粟罢了。“这是非常罕见的现象。”他说。

但不同意见也同时存在。参与主教街事件调查的咨询工程师Barrie Josie宣称,“平均每个月我都会看到一两幢建筑物遭遇类似的硫化镍问题。”其他专家也讲述了类似经历。位于伦敦的Sanberg公司的咨询工程师Tony Wilmott以及来自汉普西尔的CladTech协会的 Simon Armstrong均声称自己了解大量此类情况。“公众所知道的只不过是冰山一角罢了,”昆士兰州布里斯班市Resolve工程公司的玻璃专家Trevor Ford说道。他认为原因很简单:“没人想要坏新闻。”钢化玻璃被广泛应用于汽车、候车亭以及世界各地数以千计的建筑物上的窗子、墙面和屋顶。原因很简单,钢化玻璃的强度是普通玻璃的5倍,破碎时裂成细小颗粒而不是锋利的大块碎片。大块的钢化玻璃可以拼成透明的玻璃墙,用它做屋顶和地面也很方便,所以建筑师对钢化玻璃情有独钟。




当硫化镍晶体由α相转化成β相时,体积膨胀4%。如果α相晶体位于张力最大的玻璃中央,膨胀产生的压力可以使整块玻璃破裂。破裂时间无法预测,可能是生产出来的几个月后也可能是几十年后,尽管玻璃被日光加热会加快晶体的转化速度。Graham Dodd,伦敦Arup顾问建筑师说:“具有讽刺意味的是,因为含有硫化镍而导致破裂的‘历史最悠久’的钢化玻璃是兰开夏郡拉苏的Pilkington公司的玻璃研究大厦的玻璃,使用时间是27年。”


一个最糟糕的例子就是建干1990年的Waterfront Place。在建成后的10年间,这个位于布里斯班的40层的大楼经历了一系列的毁坏。在专家最终到来之前,因为硫化镍杂质的问题,有80块钢化玻璃破碎。昆士兰大学的硫化镍专家Barry分析了大厦所有的钢化玻璃。摄影师乘吊车用室内摄像机拍下了每一块钢化玻璃,之后这些照片被放在改良缩微胶片阅读机上扫描以检测硫化镍的痕迹。“我们发现至少还有120块钢化玻璃存在可能导致危险的杂质,这些玻璃都需要更换,”Barry说:“这是一个昂贵的、非常耗赞时间的过程,大约需要6个月的时间才能完成。”虽然这项工程花费了160万澳币(相当于70万英镑),但重新修复整个大厦所需的花费将会是这个费用的10倍。