You can construct an therotical SN curve if you have the Ultimate tensile strength of your material. the ordinates of that curve will be as followsnFor an four point curve( mostly helpful)nat 0 Cycle = SUTnat 1E3 Cycle = n x SUT where n =0.8 and changes based on Manufacturing ( refer to shigley textbook)nat 1E6 Cycle = 0.5 x SUT for SUT <= 1400MPa or 700 MPa if less than 1400MPa nand at infinte cucle it just continues .n use the followign python script to visulise the plot nimport matplotlib.pyplot as pltnimport numpy as npnfrom scipy.interpolate import interp1dnSUT=650nSYT=510nSE=118.4ndef sn(SUT,SYT,R,a,m):n plt.subplot(2, 1, 1)n X=np.array([0,1E3, 1E6,1E7] )n Y=np.array([SUT, 0.8*SUT, 0.5*SUT, 0.5*SUT])n plt.xlabel(Cycles)n plt.ylabel(Strength (MPa))n plt.grid()n plt.plot(X,Y)n plt.subplot(2, 1, 2)n X1 = np.array([SYT, 0])n X2 = np.array([SUT, 0])n Y1 = np.array([0, SYT])n Y2 = np.array([0, 0.5*SUT])n plt.plot(X1, Y1, label='Yield Line')n plt.plot(X2, Y2, label='Modified GoodMan')n yu = R * X2n plt.plot(X2, yu, label='Ratio=%f' % (R))n plt.axis([0, SUT, 0, SYT])n plt.hlines(y=a,xmin=0,xmax=m)n plt.vlines(x=m,ymin=0,ymax=a)n plt.xlabel(Mean stress (MPa))n plt.ylabel(Alternating Stress (MPa))n plt.legend(loc='upper right')nn plt.show()nThis is theroy,you can extend this idea as you want and fit your results.