材料的机械抗逆性

材料的机械抗逆性材料的机械抗逆性 译文 材料的机械性能反常现象 材料的机械性能可以被分成三个方面物理性能化学性能机械性能 物理性能密度或比重 温度等可以归为这一类 化学性能这一种类包括很多化学性能 其中包括酸碱性 化学反应性 腐蚀性 其中最重要的是腐蚀性 在外行人看来腐蚀性被解释为在某处的零件抵抗腐蚀的能力 机械性能机械性能包括拉伸性能 压缩性能 剪切性能 扭转性能 冲击性能 疲劳性能和蠕变 材料的拉伸强度可以通过试件的横截面积出试件承受的最大载荷得 到 这是在拉伸试验中应力沿Y轴 应边沿X轴变化的曲线 一种材料加载时开始发生变化的初值取决于负载的大小 当负载去掉时可以看到变形消失 对于很多材料而言 在达到弹性极限的一定应力值A之前 一直表现 为这样 在应力 应变图中 这是可以用线性关系来描述的 这之后又一个小的偏移 在弹性范围内 达到应力的极限之前 应力和应变是成比例的 这 被称为比例极限Ap 在这个区域 零件符合胡克定律 即应力与应变是成比例的 在弹性范围内 材料能完全恢复到最初的尺寸 这可以认为是材料恢复初值时落后于前者 这种影响在不含铁的材料中经常提到 实际上 能否清楚地分辩弹性极限和比例极限取决于测量设备的灵 敏度 常理中曲线应该反映被破坏的情况 但是当超出弹性极限后 应力在纪录曲线上又增大 这种变化主要有两个原因 材料的应力硬化 由于塑性变形而引起 的试件横截面积的变小由于加工硬化 金属塑性变化越大 硬化越 严重 金属拉伸越长 他的直径 横截面积就越小 直到到达s点为止 S点之后 减少的速率开始变化超过了应力增加的速率 应变很大以至于在局部的某些点的面积减少 这个现象被称为颈缩 The neweststudy material properties The materialproperties can be classifiedinto threemajor headings 1 Physical 2 Chemical 3 Mechanical Physicalproperties Density orspecific gravity moisture content and soon Them can be classifiedunder thiscategory Chemical properties Many chemicalproperties eunder thiscategory These includeacidity oralkalinity reactivity andcorrosion The mostimportant of these iscorrosion Which canbe explainedin layman s termsas theresistance of the materialto decaywhile incontinuous usein aparticular atmosphere Mechanical properties Mechanical propertiesinclude in the strengthproperties liketensile pression shear torsion impact fatigue andcreep The tensilestrength ofa materialis obtainedby dividingthe maximumload which the specimen bearsby thearea ofcross section of thespecimen This isa curveplotted betweenthe stressalong theY axis ordinate and thestrain alongthe X axis abscissa in atensile test A materialtends tochange orchanges itsdimensions When it is loaded depending uponthe magnitudeof theload When theload isremoved itcanbeseen that the deformationdisappears For manymaterials thisours opto acertain value of the stress calledthe elastic limit Ap This isdepicted bythe straightline relationshipand asmall deviationthereafter inthestress strain curve Within the elastic range the limitingvalueofthestressup towhich thestress andstrain areproportional is calledthe limitof proportionalityAp In thisregion the metalobeys hooke s law which statesthat thestress isproportional tostrain inthe elasticrange ofloading In theactual plottingofthecurve the proportionalitylimit isobtained ata slightlylower valueoftheload thantheelastic limit This maybe attributedto thetime lagin theregaining ofthe originaldimensions ofthe material This effectis veryfrequently noticedin somenon ferrous metals Logically speaking once theelasticlimitis exceeded the metalshould startto yield and finallybreak without anyincrease inthe valueof stress The curveused toshow thatitisbeing destoryed But thecurve recordsan increasedstress evenafter theelasticlimitis exceeded Two reasonscanbegiven forthis behavior The strainhardening ofthe material The diminishingcross sectional areaofthespecimen suffered onaount ofthe plastic deformation The moreplasticdeformationthe metalundergoes the harderit bees due towork hardening The morethe metalgets elongatedthe moreits diameter and hence cross sectional area is decreased This continuesuntil thepoint Sis reached After S the rate at which the reduction in areat