螺栓强度8.8但是直径不同，比如M10和M20，抗拉强度分别多少？
The bolt strength is 8.8, but the diameter is different, such as M10 and M20. What are the tensile strengths respectively?
首先，需要纠正一点，抗拉强度是在拉伸过程中，材料经过屈服阶段后进入强化阶段后随着横向截面尺寸明显缩小在拉断时所承受的大力（Fb），除以试样原横截面积（So）所得的应力（σ），称为抗拉强度或者强度极限（σb），单位为N/mm2（MPa）。抗拉强度是单位面积内能够承受的拉力。
First of all, it needs to be corrected. The tensile strength is the stress obtained by dividing the strength (FB) borne by the material at the time of tensile fracture by the original cross-sectional area (so) of the sample after the material enters the strengthening stage after the yield stage（ σ）， It is called tensile strength or strength limit（ σ b) , in N / mm2 (MPA). Tensile strength is the tensile force that can be borne per unit area.
因此，螺栓强度为8.8，无论螺栓直径如何变化，其抗拉强度是不变的，题主问的应该是小拉力载荷。
Therefore, the bolt strength is 8.8. No matter how the bolt diameter changes, its tensile strength remains unchanged. The main problem should be small tensile load.
螺栓分类
Bolt classification
螺栓按照性能等级分为：4.8、4.8、5.6、5.8、6.8、8.8、9.8、10.9、12.9共九个等级，其中8.8级以上(含8.8级)螺栓材质为低碳合金钢或中碳钢并经热处理(淬火+回火)，通称高强度螺栓，8.8级以下(不含8.8级)通称普通螺栓。
Bolts are divided into 9 grades according to performance grades: 4.8, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9 and 12.9. Bolts above grade 8.8 (including grade 8.8) are made of low-carbon alloy steel or medium carbon steel and heat treated (quenched + tempered), which are generally referred to as high-strength bolts, and bolts below grade 8.8 (excluding grade 8.8) are generally referred to as ordinary bolts.
按照制作精度可分为：A、B、C三个等级，A、B级为精制螺栓，C级为粗制螺栓。对于钢结构用连接螺栓，除特别注明外，一般为普通粗制C级螺栓。不同的级次通常对应加工方式如下：①A、B级螺栓的栓杆由车床加工而成，表面光滑，尺寸精 确，其材料性能等级为8.8级，制作安装复杂，价格较高，很少采用;②C级螺栓用未加工的圆钢制成，尺寸不够精
According to the manufacturing accuracy, it can be divided into three grades: A, B and C. grade A and B are refined bolts and grade C are rough bolts. For connecting bolts for steel structure, unless otherwise specified, they are generally ordinary rough grade C bolts. The corresponding processing methods of different grades are as follows: ① the bolt rods of Grade A and B bolts are machined by lathe, with smooth surface and accurate size, and their material performance grade is grade 8.8 , the fabrication and installation are complex, the price is high, and it is rarely used; ② grade C bolts are made of unprocessed round steel, and the size is not precise enough
确，其材料性能等级为4.6级或4.8级。
Indeed, the material performance grade is 4.6 or 4.8.
螺栓性能等级的标记制度
Marking system of bolt performance grade
螺栓、螺钉和螺柱性能等级的代号，由点隔开的两部分数字组成：
The code of performance grade of bolts, screws and studs consists of two parts separated by points:
点左边的一或二位数字表示公称抗拉强度的1/100，以MPa计；
One or two digits on the left of the point represent 1 / 100 of the nominal tensile strength, in MPa;
点右边的数字表示公称屈服强度或规定非比例延伸0.2%的公称应力与公称抗拉轻度比值的10倍
The number to the right of the point represents 10 times the nominal yield strength or the ratio of the nominal stress with a specified non proportional extension of 0.2% to the nominal tensile strength
题主描述的螺栓8.8，表示公称抗拉强度σb=800MPa（d＞16mm时，σb=830MPa），公称屈服强度σs=640。
The bolt 8.8 described in the main question represents the nominal tensile strength σ B = 800MPa (when D ＞ 16mm, σ B = 830mpa), nominal yield strength σ s=640。
螺栓性能等级8.8的小拉力载荷
Small tensile load of bolt performance grade 8.8
计算其小拉力载荷可以通过计算公式和查表得出。
The small tensile load can be calculated through the calculation formula and look-up table.
1.计算公式：F=σs*A，其中F为拉力（许用载荷），σs为材料抗拉强度，A为有效面积。
1. Calculation formula: F= σ S * a, where f is tensile force (allowable load), σ S is the tensile strength of the material and a is the effective area.
M10的有效横截面积为58mm^2。8.8级M20小抗拉强度σb=800MPa（d＜16mm），F=800*58=46400N=46.4KN
The effective cross-sectional area of M10 is 58mm ^ 2. Grade 8.8 M20 small tensile strength σ b=800MPa（d＜16mm），F=800*58=46400N=46.4KN
M20的有效横截面积为245mm^2。8.8级M20小抗拉强度σb=830MPa（d＞16mm），F=830*245=203350N=203.35KN
The effective cross-sectional area of M20 is 245mm ^ 2. Grade 8.8 M20 small tensile strength σ b=830MPa（d＞16mm），F=830*245=203350N=203.35KN
2.通过查表法，也可得到同样数值，更加方便快捷。
2. The same value can be obtained by looking up the table, which is more convenient and fast.
