## A NEW PHOTOELASTIC EQUIPMENT FOR CLASSROOM TEACHING1)

SU Fei,2), LUO Qian

School of Aeronautical Science and Engineering, Beihang University, Beijing 100191, China

 基金资助: 1)国家自然科学基金资助项目(12072008)

Abstract

Photoelastic experiment is a full field and real-time optical experiment. It can vividly show some abstract mechanical phenomena to students in the form of images, which greatly improves the teaching efficiency and students interest. However, the reality is that the teaching time for basic mechanics theory is compressed, and the time for experimental class is very limited. The advantages of photoelastic experiment in teaching cannot be brought into full play. This paper introduces a new type of photoelastic equipment which can be used in classroom teaching to demonstrate its working principle. It is composed of only LED light source integrated with circular polarizer, loading frame and pixel polarizing camera. It has simple structure and can be folded. It weighs about 5kg together with suitcase. The equipment can be easily carried to the classroom to accomplish photoelastic teaching experiments in ten minutes. The experimental results can be displayed on the large screen of the classroom, and the underlying theories can be explained to students with a well prepared teaching plan. Through the above teaching reform, we give full play to the advantages of photoelastic experiment in teaching without increasing class hours.

Keywords： photoelasticity test; compact and portable teaching equipment; foundamental mechanics

SU Fei, LUO Qian. A NEW PHOTOELASTIC EQUIPMENT FOR CLASSROOM TEACHING1). Mechanics in Engineering, 2022, 44(2): 419-422 DOI:10.6052/1000-0879-21-352

## 1 新型光弹系统的工作原理

### 图1

Fig.1

$\left.\begin{array}{l}S_{0}=I(0)+I(90) \\S_{1}=I(0)-I(90) \\S_{2}=I(45)-I(135) \\D_{\mathrm{lp}}=\frac{\sqrt{S_{1}^{2}+S_{2}^{2}}}{S_{0}}\end{array}\right\}$

$I=A^{2}\sin^{2}(2\alpha)\sin^{2}(\delta /2)$

## 2 随堂教学设备软硬件的开发

### 2.3 试件材质的选择

(1) 玻璃材料弹性好,不像传统的环氧试件易出现塑性变形导致的残余应力,影响演示效果。

(2) 光弹效应弱,加载时最大相位差一般不超过$\pi$/2,定量计算时不用做解包裹处理。当然做定性实验时,也可采用传统的环氧树脂试件。

(3) 价格便宜,一个试件的价格不超过10元。

### 2.4 关于系统的测试精度问题

(1) 膜材光学器件产生的非理想圆偏振光;

(2) 相机镜头内的残余应力;

(3) LED光源较大的带宽;

(4) 相机内置偏振元件较低的消光比;

(5) 试件对相机过大的张角导致的光线偏振态事实上的改变。

## 3 系统的应用实例

(1) 不同大小的圆孔周围的应力集中现象及应力集中系数测试 (材料力学、弹性力学);圆孔聚集对应力集中行为的影响(弹性力学)。

(2) 圣维南原理(材料力学、弹性力学)。

(3) 纯弯曲梁的应力分布特点(沿高度线性分布,见图4,材料力学、弹性力学),三点弯曲梁的应力分布特点(弹性力学)。

### 图4

(4) 受载的杆和梁上外凸和内凹两个点处的应力状态的不同(见图5(a),一处为应力集中状态,另一处为零应力状态,弹性力学)。

### 图5

(5) 裂尖的应力场分布(断裂力学)。

(6) 对径压缩圆盘的应力场分布和主应力方向场分布形式(见图5(b),弹性力学)。

(7) 玻璃、石英器件内的残余应力及其随温度变化的在线观察(光测力学)。

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