# Finding Stress Concentration Factor when (D/d) ratio is not on a table?

So we have this homework problem, it is a rectangular filleted bar and we have to find the minimum thickness. I go and calculated the ratios but here's the problem: (D/d) = 1.375. What may the problem be? Well for this intro to Civil Engr. Course we are taught to find the stress concentration factors using a graph. This graph only has the lines plotted for (D/d) = [ 1.1 1.2 1.3 1.5 ]. How do you guys deal with this when the (D/d) ratio is not listed on a table? Is there some formula to derive the function?

I am a Mechanical Engineering Major taking an intro Civil Engineering Course (Mechanics Materials) , so I have limited knowledge in Civil Engineering terms.

• Can you interpolate between values? – Solar Mike Feb 7 '19 at 22:33
• I don't think we are allowed. But besides interpolation is there any other method? – Prandals Feb 7 '19 at 22:35

So in this case, get the stress concentration factor for $$D/d = 1.3$$ and for $$D/d = 1.5$$, and then just interpolate:
$$f_{1.375} = f_{1.3}+\dfrac{f_{1.5}-f_{1.3}}{1.5-1.3}(1.375-1.3)$$