Construction material (15 points)
2006-11-30 4:59 am
Why concrete is strong in compression but weak in tension ??? Explain in detail.
回答 (2)
2006-12-06 5:24 am
✔ 最佳答案
Why concrete is strong in compression but weak in tension ??? 任何material 都會有void 係入面,係tension 時候, 呢d void 會成為一個stress concentration point令到係個void 附近的stress 增加. 但係compression 時候, 呢d void 會合埋咗, 因為個contract area 大咗咁樣stress 就唔會被增加. 所以concrete 係tension 就會差過compression la.
why is brittle materials weak in tension??
就tensile stress黎講brittle material 比dutile material 差係因為dutile material 可以令到stress concentration 減少. 因為dutile material 會有一定程度的strain, 係d void 附近的material 會plastic deformation. 呢d deformation 會令到void 嘅radius 增加, 令到個stress concentration factor 下降. 但brittle 得好少strain, 唔可以好似dutile material 咁relief stress, 所以唔可以抵受咁多stress.
furthermore, brittle material 嘅tensile strength 係可以assume 0 ga. tensile/compressive strength 個定義係stress vs strain curve 同x-axis(即係strain) bound 住嘅area. brittle material 係tension 個邊只有好小面積,但compression 個邊就大好多. 因為個差距咁大,所以可以assume 為0.
希望可以解答到你la.
參考: some mechanical engineering cources
2006-11-30 7:42 pm
Concrete is considered as brittle material that does not have a yield point, and do not strain-harden which means that the ultimate strength and breaking strength are the same. A stress-strain curve for a typical brittle material is shown in the figure below.
圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Stress vs. Strain curve typical of a brittle material
1. Ultimate Strength
2. Rupture. Tensile strength is measured in units of force per unit area. In the SI system, the units are newtons per square metre (N/m²) or pascals (Pa), with prefixes as appropriate. The non-metric units are pounds-force per square inch (lbf/in² or PSI). Engineers in North America usually use units of ksi which is a thousand psi.
In brittle materials such as rock, concrete, cast iron, or soil, tensile strength is negligible compared to the compressive strength and it is assumed zero for most engineering applications. Glass fibers have a tensile strength stronger than steel[1], but bulk glass usually does not. This is due to the Stress Intensity Factor associated with defects in the material.
That's why concrete is high in compression strength and low in tensile strength.
2006-11-30 15:33:09 補充：
because brittle material has zero tensile strength. refer to the diagram above.
2006-12-01 12:47:27 補充：
bacause concrete has no yield strength.......that's the nature of concrete...
圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
Stress vs. Strain curve typical of a brittle material
1. Ultimate Strength
2. Rupture. Tensile strength is measured in units of force per unit area. In the SI system, the units are newtons per square metre (N/m²) or pascals (Pa), with prefixes as appropriate. The non-metric units are pounds-force per square inch (lbf/in² or PSI). Engineers in North America usually use units of ksi which is a thousand psi.
In brittle materials such as rock, concrete, cast iron, or soil, tensile strength is negligible compared to the compressive strength and it is assumed zero for most engineering applications. Glass fibers have a tensile strength stronger than steel[1], but bulk glass usually does not. This is due to the Stress Intensity Factor associated with defects in the material.
That's why concrete is high in compression strength and low in tensile strength.
2006-11-30 15:33:09 補充：
because brittle material has zero tensile strength. refer to the diagram above.
2006-12-01 12:47:27 補充：
bacause concrete has no yield strength.......that's the nature of concrete...
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