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April 13, 2005

強さの不思議

tuyosanohushigi
 【今週の一冊】
 ●『強さの不思議』
  ものづくりで遊ぶ材料力学

  編:日本機械学会(技報堂出版)
   2005.02 / ¥1,995

-----------------------------------
 ◆ 燃える一言 ◆
 『「少ない材料で、強い構造物をつくるにはどのような条件を
      満足しなければならないのか」を人々は研究するようになり、
            その結果まとめられた学問が“材料力学”です。』
-----------------------------------
 機械系のエンジニアや、建築家にとって逃れられないのが「材料力学」。
 
 今年大学に入学した、若きエンジニア達にとっても、必修でしょう。
 
 しかしながら、流体力学や熱工学と並んで、数式のオンパレードに悩まされ
 るのも間違いありません。
 
 どうせ離れられないサダメなら、本質をイメージで理解して、楽しく付き合
 おう、というのが本書です。
 
 
 ちから君とひずみちゃんの会話や、「燃焼系~、燃焼系~」のCMを髣髴と
 させる「人間鯉のぼりに必要な筋力は?」といった、非常にとっつきやすい
 話題から、トラスやラーメン構造の解説が展開されます。
 
 その上で、材料力学の数式も丁寧に解説されており、おまけにインターネッ
 トの練習問題にまでリンクしており、物腰のやわらかさからよりも骨太な中
 身です。
 
 
 座屈の説明として、自転車のスポークの例が挙げられています。
 
 思えば、あの細長い棒が、子供と買い物袋を満載したママチャリの重量を支
 えているのはフシギであり、よくぞ座屈しないものです。
 
 しかし、スポークを半分抜いて、下半分にスポークを配置すると、実は荷重
 に耐えられず、やっぱり座屈してしまいます。
 
 実は、自転車の荷重は、スポークで「吊る」ことで支えているのであり、材
 料力学的に言えば、不安定な座屈が起こる圧縮荷重ではなく、強い引張荷重
 により支えているのです。
 
 
 薄肉円筒や薄肉球の計算問題は、試験前に丸暗記しようとしても分かりにく
 く、苦労した覚えがあります。
 
 その事例として、「紙コップの上にのってみよう!」という、これまた分か
 りやすい実験が登場します。
 
 簡単に潰れる紙コップも、伏せて5つ並べると、一人くらい楽に支えられる
 のですが、その強度は、紙コップに密閉された空気の圧力がミソです。
 
 直径5cm、高さ8sm、紙厚0.28mm、紙の引張強度が15.43MPaとすると、理論上
 の紙コップの強度は35kgf!
 実験でも26kgfの荷重に耐えられるそうです。
 
 こうした感覚をつかんでおくと、無味乾燥な薄肉円筒の計算問題も、驚きを
 もって深く理解ができます。
 
 
 最近は、シミュレーションによる解析で、モデルの作成から計算まで、ほと
 んどブラックボックス化してしまい、材力を知らなくても設計ができてしま
 います。
 
 効率が上がった反面、桁や単位を間違えたままでジェットコースターの車軸
 を設計し、事故を起こした、信じられないようなミスも発生しています。
 
 基本に立ち返り、理論と実感で材料力学を体得したいものです。
 ----------------------------------
 ◇ カンドコロ! ◇
 
 A4の1/4のコピー紙で、橋を作るなら、どんな形が丈夫だろうか。
 
 多くの人は、ギザギザにたくさんの山を作る。
 
 確かにある程度強くなるが、より強くしようとギザギザを多くすると、かえ
 って弱くなる。
 
 強いブリッジにするときは、できるだけ「高く」するのがポイントだ。
 
 曲げに対する剛性を上げるために、断面二次モーメントを大きくする。
 
 毛利元就の3本の矢も、曲げる方向に縦に並べると、3倍ではなく11倍以上
 の対荷重となるのだ。
───────────────────────────────────
 ◆ 熱い行動 ◆
 数式やモデルの意味するところを、身近な例に置き換えてイメージしよう。
 
 富士山はなぜ、三角錐ではなく、末広がりなのか。
 材料力学的に証明してみよう。
-----------------------------------
 ◆ 燃えるゲージ ◆ | 炎 | 炎 | 火 | (炎3つが満点)
-----------------------------------
 ◎ 目 次 ◎
 第1話 ものが変形するパターンは?
 第2話 どうしてハサミで紙が切れるの?
 第3話 どうしてモビールは一点で支えられるの?
 第4話 弾性と塑性(そせい)の性質とは?
 第5話 富士山の形は何できまるの?
 第6話 人間鯉のぼりに必要な腕力は?
 第7話 ガラスを割ってみよう
 第8話 斜面にはどんな力が働くかな?
 第9話 チョークをねじってみよう
 第10話 つり橋を渡るときどうして真ん中で落ちそうになるの?
 第11話 強いペーパーブリッジをつくろう
 第12話 板はどうして曲がるの?
 第13話 自転車は宙づりになって走っているってホント?
 第14話 ばねをつくっておもちゃをつくろう
 第15話 紙コップに乗ってみよう

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