一、实验目的
1. 用电测法测定叠梁,复合梁在弯曲受力状态下,沿其横截面高度的正应变(正应力)分布规律。
2. 推导叠梁,复合梁的正应力计算公式。
二、实验仪器和设备
1. 纯弯曲梁实验装置(纯弯曲梁复合梁)。
2. 静态数字电阻应变仪。
三、实验原理及步骤
1. 实验原理
复合梁的材料为铝梁和钢梁,上层是铝梁,其弹性模量分别为E=70GN/m
2和E=210GN/m
2。复合梁上总过贴上了12各应变片,每个梁各6个。
1. 几何、物理和静力学关系
以材料尺寸相同的两层矩形截面复合梁在纯弯 曲情况下为计算模型,在其纵向对称面内,承受弯矩
M的作用。上梁的弹性模量为
E1,横截面面积为
A1,下梁的弹性模量为
E2,横截面面积为
A2,且
A1=
A2,单梁的梁宽和梁高分别为
b和
h,在其纵向对称面内,承受弯矩
M的作用。两种不同的材料由胶粘合制成。下面建立横截面上的弯曲正应力公式,平面假设与单向假设均成立。
设中性层的曲率半径为
![](/uploads/allimg/20240307/1-24030FU443E9.jpg)
,并沿截面纵向对称轴与中性轴分别建立坐标轴
y与
z,中性层离交界面的距离为
e。
(1)变形几何关系。根据平面假设可知,横截面上y处的纵向正应变为:
![](/uploads/allimg/20240307/1-24030FU444939.jpg)
(6-1)
(2)物理关系。依胡克定律
![](/uploads/allimg/20240307/1-24030FU444U7.jpg)
,而
![](/uploads/allimg/20240307/1-24030FU4454Q.jpg)
由式(1)带入,可以得出不同材料区的弯曲正应力分别为:
![](/uploads/allimg/20240307/1-24030FU445N6.jpg)
,
![](/uploads/allimg/20240307/1-24030FU4463F.jpg)
(6-2)
(3)静力学平衡关系。根据受力分析,由静力学平衡关系,考虑横截面上内力的平衡,
![](/uploads/allimg/20240307/1-24030FU44BN.jpg)
,可以得出:
![](/uploads/allimg/20240307/1-24030FU44Nc.jpg)
(6-3)
由
![](/uploads/allimg/20240307/1-24030FU44H35.jpg)
组成的内力系,在横截面上形成一个内力偶矩
M,即为横截面上的弯矩
M,即:
![](/uploads/allimg/20240307/1-24030FU44X15.jpg)
(6-4)
2. 确定中性层位置
将式(2)代入式(3)中,得
![](/uploads/allimg/20240307/1-24030FU44YI.jpg)
令
![](/uploads/allimg/20240307/1-24030FU449334.jpg)
则
![](/uploads/allimg/20240307/1-24030FU4504D.jpg)
(6-5)
则
![](/uploads/allimg/20240307/1-24030FU451C6.jpg)
并且,令
![](/uploads/allimg/20240307/1-24030FU451529.jpg)
将
![](/uploads/allimg/20240307/1-24030FU452W0.jpg)
和
![](/uploads/allimg/20240307/1-24030FU452236.jpg)
带入式(5)得:
![](/uploads/allimg/20240307/1-24030FU453W2.jpg)
(6-6)
3. 推导弯曲正应力计算公式
将式(2)代入式(4),得:
![](/uploads/allimg/20240307/1-24030FU453934.jpg)
令
![](/uploads/allimg/20240307/1-24030FU454N3.jpg)
,
![](/uploads/allimg/20240307/1-24030FU454939.jpg)
其中
![](/uploads/allimg/20240307/1-24030FU4552L.jpg)
、
![](/uploads/allimg/20240307/1-24030FU455B9.jpg)
分别为截面
![](/uploads/allimg/20240307/1-24030FU45D41.jpg)
、
![](/uploads/allimg/20240307/1-24030FU45E21.jpg)
对中性轴的惯性矩。由于各梁曲率相同,经变化得:
![](/uploads/allimg/20240307/1-24030FU45N31.jpg)
(6-7)
再将式(7)代入式(2),得:
![](/uploads/allimg/20240307/1-24030FU45OT.jpg)
,
![](/uploads/allimg/20240307/1-24030FU45X16.jpg)
(6-8)
式中
![](/uploads/allimg/20240307/1-24030FU4552L.jpg)
、
![](/uploads/allimg/20240307/1-24030FU455B9.jpg)
分别为
![](/uploads/allimg/20240307/1-24030FU45D41.jpg)
、
![](/uploads/allimg/20240307/1-24030FU45T43.jpg)
对共同中性轴
Z的惯性矩。
由单向应力状态的虎克定律公式
![](/uploads/allimg/20240307/1-24030FU45T33.jpg)
,可求出应力实验值。应力实验值与应力理论值进行比较,以验证复合梁的正应力计算公式。
2. 实验步骤
1. 首先确定单梁的物理参数,得到
h=20mm,
b=20mm,
c=150mm。
2. 启动实验装置,将各应变片分别接到各个AB通道之间,同时把公共补偿片接到
![](/uploads/allimg/20240307/1-24030FU4593R.jpg)
上,并且把C通道与短接片短接。
3. 进行实验:
a.取初载荷0.5kN,每次逐级加1.0kN,直至4.5kN,总共分4次加载。
b.接完线路以及加初载荷之前都要重复置零。
c.每次加载完毕都要记录下数据。
四、实验数据
表6-1 Ⅰ梁应变数据表
![](/uploads/allimg/20240307/1-24030FU459204.jpg)
表6-2 Ⅱ梁应变数据表
五、数据处理
1. 根据实验数据计算各点的平均应变,求出各点的实验应力值,并计算出各点的理论应力值;计算实验应力值与理论应力值的相对误差。
答:根据上面实验数据,结合材料力学相关知识计算如下:
(1)由实验数据可知,各应变片处的平均应变值为:
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU501111.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU502S8.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU502Y7.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU503R0.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU503430.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504338.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU504457.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU505449.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU505364.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU50A15.jpg)
Ⅱ梁上处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU506357.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU502S8.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU50O91.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU50W02.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU504338.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU50aR.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU5093S.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU510345.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU510X3.jpg)
处的平均应变为:
![](/uploads/allimg/20240307/1-24030FU511426.jpg)
(2)各应变片处的实验应力值为:
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU511106.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU512V0.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU512J5.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU513C2.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU513Y6.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU5142J.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU514J5.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU515605.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU515620.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU51D49.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU512V0.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU5164B.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU51B28.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU51N17.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU513Y6.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU51K62.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU51X26.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU515605.jpg)
处的应力值为:
![](/uploads/allimg/20240307/1-24030FU51Q62.jpg)
(3)理论计算试件上个应变片处的理论应力值
加载的力的大小为
F=1000N。复合梁的单梁截面厚度b=20mm,高度h=20mm,截面作用点到梁支点的距离为c=150mm。设n=E
2/E
1=210/70=3,中性轴位置的偏移量为:
![](/uploads/allimg/20240307/1-24030FU519344.jpg)
因此,可得到复合梁Ⅰ和复合梁Ⅱ正应力计算公式分别为
![](/uploads/allimg/20240307/1-24030FU5203E.jpg)
其中
![](/uploads/allimg/20240307/1-24030FU520B1.jpg)
![](/uploads/allimg/20240307/1-24030FU521494.jpg)
根据材料力学知识分析如下:
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU521K8.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU502S8.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU522538.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU522T8.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504338.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU523458.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU523W9.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU510X3.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU524492.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU524Y2.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU502S8.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU525c6.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU525633.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU504338.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU525462.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU52E09.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU515605.jpg)
处计算应力值为:
![](/uploads/allimg/20240307/1-24030FU526409.jpg)
(4)计算实验应力值与理论应力值的相对误差
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU52K31.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU502S8.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU52JP.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU52W51.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504338.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU52U08.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU5295D.jpg)
Ⅰ梁上
![](/uploads/allimg/20240307/1-24030FU515605.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU529628.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU501c0.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU5302A.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU530J1.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU531148.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU50U49.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU5314C.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU513Y6.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU532511.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU504A5.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU532950.jpg)
Ⅱ梁上
![](/uploads/allimg/20240307/1-24030FU510X3.jpg)
处应力值的相对误差为:
![](/uploads/allimg/20240307/1-24030FU533B2.jpg)
2. 上述的各点应力的实验值与应力的理论值,将两者进行比较。可以得出:实验值与理论值的结果十分接近,说明复合梁的正应力计算公式成立。
六、实验结果
根据前面计算可以看出,实验计算的结果和理论计算的结果基本吻合。现列表如下:
表6-2 结果对比表