如图所示圆截面杆件，承受轴向拉力P作用，设拉杆的直径为d，端部墩头的直径为D，厚度为<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016310361073.bmp" />，已知许用应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016291116745.jpg" />=120MPa，许用剪应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016292922575.jpg" />=90MPa，许用挤压应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016294683432.jpg" />=240MPa。试根据强度方面要求，则D，d，<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016310361073.bmp" />三者间的合理比值为（）<br /> <img src="https://img.ppkao.com/2019-04/xiaotian/2019041016312654713.jpg" />&ensp;&ensp;<br /> &nbsp;<p> 如图所示圆截面杆件，承受轴向拉力P作用，设拉杆的直径为d，端部墩头的直径为D，厚度为<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016310361073.bmp" />，已知许用应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016291116745.jpg" />=120MPa，许用剪应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016292922575.jpg" />=90MPa，许用挤压应力<img src="https://img.ppkao.com/2019-04/xiaotian/2019041016294683432.jpg" />=240MPa。试根据强度方面要求，则D