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四、材料性質和特性 4.1 工程材料性質: 4.1.1 抗拉強度 (Tensile strength) ˙ Tension test 是最常用於決定材料機械性質的一種測試 . 該試驗需要一測試試樣 (test specimen) 大都為實心且為圓形 . Gage length l 0 = 50mm (usually) A 0 → D 0 = 12.5mm Engineering stress = σ = P/A 0 Engineering strain = e = (l - l 0 ) ∕ l 2
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四、材料性質和特性 4.1工程材料性質: 4.1.1抗拉強度(Tensile strength) ˙Tension test是最常用於決定材料機械性質的一種測試. 該試驗需要一測試試樣(test specimen)大都為實心且為圓形. Gage length l0=50mm (usually) A0→D0=12.5mm Engineering stress=σ=P/A0 Engineering strain=e=(l-l0)∕l2 Modulus of elastistic,E =σ/e (表示材料的剛性). 4.1.2材料可分類為: A.金屬:(非鐵:1.強度不高但抗蝕性好 2.貴) 1.鐵:灰鑄鐵、展性鑄鐵、steel、白鑄鐵、熟鐵。 2.非鐵:Al、Copper、Mn鎂、Ni、Ti、Zin鋅。 B.非金屬:(difference:有機通常可溶解於有機液中,ex:乙醇、四氯化碳、But waker.無機則可溶於水)
1.有機:塑膠、石油產品、木材、紙、橡膠、皮革。1.有機:塑膠、石油產品、木材、紙、橡膠、皮革。 2.無機:礦物、水泥、陶瓷、玻璃、石墨。 ˙非鐵金屬強度大都不高,但抗蝕性較鐵材好,but大都較貴。 ˙當其他元素加入金屬中,以加強他的特性時,此種組合體稱為合金(Alloy) ex:黃銅=純銅+鋅,青銅=純銅+錫,鋼=純鐵+碳。 ˙純金屬會在恆溫下凝固,但合金則否。 ex:1.鐵→礦物在鼓風爐中產生。 2.鋁→鋁礬土礦→氧化物→電解法產生高熱→還原成鋁。 4.2 Structure of Alloy ˙不定形or非晶體(amorphous):材料由液體固化,但沒有明確的原子晶格結構,如液體一樣,以隨機排列。(ex:玻璃) ˙結晶性(crystalline):即材料在凝固時,原子會自動排列成幾何模型,此原子模型會順應材料的空晶格子(space lattice)
Principles for complete form the substitutional solid solution. 2 metals must have similar caystal structure difference in their atomic radii should be less than 15%. ˙Inserstitial solid solution(插入固溶體):所加元素的原子插入溶劑原子的空間(空隙),此合金稱之。(Solute atom size is much smaller than that of solrent atom ) ex:鐵(Iron)中的碳(Carbon)即屬此種→鋼 ☆Principles for the complete interstitial solid solution. 1. Solvent atom has more than one valence(原子價) 2. atomic radius of the solute atom is less than 59% of the solvent atom. ˙Intermetallic compounds金屬間化合物:(solute atoms are present among soluvent atom in certain proportion)其結晶結構非常複雜,溶解溫度一定,強度、硬度較bcc、fcc、hcp大,但傳導性,延性低。(ex:鋁銅合金,錫-銻)
4.2.2延性ductility a. percent elongation Elongation = (lf-l0)∕l0 ×100 b. reduction of Area reduction of Area = (A0-Af)∕A0×100 4.2.3 True stress and true strain True stress,σ= P/A (A: instantaneous area) True strain,ε= ln(l∕l0) 當e很小時,engineering and true strain are approximately equal.但當e↗,difference between then is getting largen true strain is more real to the physical phenomena.(example in textbook) 4.2.4 Construction of stress-strain curues. 4.2.5 Strain at necking in a tension. 4.2.6 Compression Many manufacturing processes are doing under pressure, thus compression test is very important. 通常取抗拉強度的50%,對脆性材料而言係將其壓至破裂.For延性材料,則壓至某個變形程度為準。
4.2.7 Torsion shear stress,τ=T∕2πr2 t shear strain,γ=rψ∕l 4.2.8 Hardness resistance to pormanent indentalion . 測試試樣抵抗小球體,角錐體,圓錐體等壓痕器壓入的能力。 1.硬度和抗拉強度成正比。 2.硬度和磨耗成反比。 Process: 壓痕器接觸預加負載的試樣,then adding load to壓痕器,記錄最後負載及預加負載間的壓痕深度差,便換算成硬度讀數。 Steel>aluminum>lead 硬度不是基本的property,∵ the resistance to indentation depends on the shapeof the indenter. A. Brinell test(勃氏硬度):用dia=10mm的steel ball with aload of 500、1500、3000kg. (軟化高硬度的鋼) HB or BHN=P∕curred surface area of indentation
˙通常退火件annealing metals generally have rounded profile. ˙Cold-worked metals have a shape profile. B. Rockwell test:measures the depth of penstration instead of diameter of indentation. 量深度取代直徑。 用不同壓痕器:鑽石、steel ball、Diamond、cone. C. Vickers test:use diamond pyramid,load:1kg~120kg. unit:HV or DHP. D. Knoop test:diamond pyramid,load:25g~5kg.量測depth. E. Scleroscope:diamond eipped indenter enclosed in a glass tube,then dropped forma castain hight,measure the rebound of the indentor. F. Mohs hardness:one material to scratch another scale form 1~10 diamond. G. Durometer:for rubbers,plasties,and soft and elastic material. 快速以constant load against surface → measure depth after 1 second,hardness is imersaly related to the depth. H. Hot hardness:for cutting took in machining or die in hot working. (將hardness Indentor and sqecimen於jurnace.
4.2.9 Fatigue:(疲勞)由於repeated loads. ˙The part fails at a stress level below which failure would occur under Static loading mechanical component常發生。 ˙Use various states of stress:combination of tension,compression And torsion. ˙試驗canied out at various stress ainplitnele and No. of cycles. 4.2.10 Creep 4.2.11 Impact:衝擊,金屬may have high yield stress and hardness,but not necessaryfor impact. 4.2.11 Residual stresses:當workpieces被deformed,但又非均勻的變形,就會產生殘留應力。 4.3 Iron-Carbon System pure iron = 0.008﹪C (up to) steel = 2.11% C cast iron = 6.67% C (most are less than 4.5%) → commescially雖可增加至100%成graphit(石墨) 鐵-碳平衡圖:提供information how to modofy and evaluate the properties of these important material for specific application. ˙Ferrite(Alpha Ferrite): relatively soft and ductile,其中含固溶的碳。當temp.於室溫至768℃間時具磁性。
ex:體心立方格子 (bcc)body centered cubic. 面心立方格子 (fcc)face centered cubic. (較佳延展性) have mast density 面密方格子 hexagonal close paclced. (延展性差) (鈹,鎘,鎂,鈦) →彎曲或加工時會變得更脆。 ˙若干固體金屬如鐵,當溫度達到臨界溫度,就會改變其晶子結構的型式,這種改變稱為”同素異形”(allotropic),(每種格子結構有不同的特性)。 ex: 鐵→體心立方→稱α鐵(鉻、鉬、釩、鎢,have the same structure) →室溫極高於1394℃(2541℉) 鐵→面心立方→γ鐵(鋁、銀、銅、金、鎳、鉛、鉑) →910℃~1394℃(1679℉) ˙純金屬的性質可藉由晶格型式來判斷,but合金的晶格結構則不易估定。 ˙Substitutional Solid Solution(置換型固溶體): 任純金屬加入元素會改變其晶格大小,形成合金不同,晶格的型式亦不同,所加入元素的原子可代替溶劑or金屬中的某些原子的原來位置,此合金則called substitutional solid solution. Add solute to solvent→合金alloy ex: Zinc→Lattice of copper,改變Zinc的含量可改變黃銅合金的性質。