Question 6.4: Design of a Cantilever Bracket for Fully Reversed Bending Pr...

See Figure 6-41 and Tables 6-9 and 6-10 (pp. 358, 359).

1     This is a typical design problem. Very little data are given except for the required performance of the device, some limitations on size, and the required cycle life. We will have to make some basic assumptions about part geometry, materials, and other factors as we go. Some iteration should be expected.

2    The first two steps of the process suggested above, finding the load amplitude and the number of cycles, are defined in the problem statement. We will begin at the third step, creating a tentative part-geometry design.

3    Figure 6-41a shows a tentative design configuration. A rectangular cross section is chosen to provide ease of mounting and clamping. A piece of cold-rolled bar stock from the mill could simply be cut to length and drilled to provide the needed holes, then clamped into the frame structure. This approach appears attractive in its simplicity because very little machining is required. The mill-finish on the sides could be adequate for this application. This design has some disadvantages, however. The mill tolerances on the thickness are not tight enough to give the required accuracy on thickness, so the top and bottom would have to be machined or ground flat to dimension. Also, the sharp corners at the frame where it is clamped provide stress concentrations of about K_{t} = 2 and also create a condition called fretting fatigue due to the slight motions that will occur between the two parts as the bracket deflects. This motion continuously breaks down the protective oxide coating, exposing new metal to oxidation and speeding up the fatigue-failure process. The fretting could be a problem even if the edges of the frame pieces were radiused.

4    Figure 6-41b shows a better design in which the mill stock is purchased thicker than the desired final dimension and machined top and bottom to dimension D, then machined to thickness d over the length l. A fillet radius r is provided at the clamp point to reduce fretting fatigue and achieve a lower K_{t}. Figure 4-36 (p. 190) shows that with suitable control of the  r/d and D/d ratios for a stepped flat bar in bending, the geometric stress-concentration factor K_{t} can be kept under about 1.5.

5    Some trial dimensions must be assumed for b, d, D, r, a, and l. We will assume (guess) values of b = 1 in, d = 0.75 in, D = 0.94 in, r = 0.25, a = 5.0, and l = 6.0 in to the applied load for our first calculation. This length will leave some material around the hole and still fit within the 6-in-length constraint.

6    A material must also be chosen. For infinite life, low cost, and ease of fabrication, it is desirable to use a carbon steel if possible and if environmental conditions permit. Since this is used in a controlled, indoor environment, carbon steel is acceptable on the latter point. The fact that the deflection is of concern is also a good reason to choose a material with a large E. Low- to medium-carbon, ductile steels have the requisite endurance-limit knee for the infinite life required in this case and also have low notch sensitivities. An SAE 1040 normalized steel with S_{ut} = 80 000 psi is selected for the first trial.

7    The reaction force and reaction moment at the support are found using equations h from Example 4-5. Next the area moment of inertia of the cross section, the distance to the outer fiber, and the nominal alternating bending stress at the root are found using the alternating load’s 500-lb amplitude.

\begin{array}{l} R=F=500  lb \\ M=R l-F(l-a)=500(6)-500(6-5)=2500  lb -\text { in } \end{array}              (a)

\begin{array}{l} I=\frac{b d^3}{12}=\frac{1(0.75)^3}{12}=0.0352  in ^4 \\ c=\frac{d}{2}=\frac{0.75}{2}=0.375  in \end{array}               (b)

\sigma_{a_{\text {nom }}}=\frac{M c}{I}=\frac{2500(0.375)}{0.0352}=26667  psi          (c)

8    Two ratios must be calculated for use in Figure 4-36 (p. 190) in order to find the geometric stress-concentration factor K_{t} for the assumed part dimensions.

\frac{D}{d}=\frac{0.938}{0.75}=1.25 \quad \frac{r}{d}=\frac{0.25}{0.75}=0.333      (d)

\begin{array}{lll} \text { interpolating } & A=0.9658 & b=-0.266 \end{array}      (e)

K_t=A\left\lgroup\frac{r}{d}\right\rgroup^b=0.9658(0.333)^{-0.266}=1.29       (f)

9    The notch sensitivity q of the chosen material is calculated based on its ultimate strength and the notch radius using equation 6.13 (p. 344) and the data for Neuber’s constant from Table 6-6 (p. 346).

q=\frac{1}{1+\frac{\sqrt{a}}{\sqrt{r}}}      (6.13)

From the Table for         S_{u t}=80 kpsi : \quad \sqrt{a}=0.08           (g)

q=\frac{1}{1+\frac{\sqrt{a}}{\sqrt{r}}}=\frac{1}{1+\frac{0.08}{\sqrt{0.25}}}=0.862                (h)

10    The values of q and K_{t} are used to find the fatigue stress-concentration factor K_{f}, which is in turn used to find the local alternating stress \sigma _{a} in the notch. Because we have the simplest case of a uniaxial tensile stress, the largest alternating principal stress \sigma _{1a} for this case is equal to the alternating tensile stress, as is the von Mises alternating stress \sigma_a^{\prime} . See equations 4.6 (p. 145) and 5.7c (p. 249).

\begin{aligned} \sigma_a, \sigma_b &=\frac{\sigma_x+\sigma_y}{2} \pm \sqrt{\left\lgroup\frac{\sigma_x-\sigma_y}{2}\right\rgroup ^2+\tau_{x y}^2} \\ \sigma_c=0 \end{aligned}      (4.6a)

\tau_{\max }=\tau_{13}=\frac{\left|\sigma_1-\sigma_3\right|}{2}        (4.6b)

\sigma^{\prime}=\sqrt{\sigma_1^2-\sigma_1 \sigma_3+\sigma_3^2}      (5.7c)

K_f=1+q\left(K_t-1\right)=1+0.862(1.29-1)=1.25      (i)

\sigma_a=K_f \sigma_{a_{\text {nom }}}=1.25(26667)=33343  psi        (j)

\begin{aligned} \tau_{a b} &=\pm \sqrt{\left(\frac{\sigma_x-\sigma_y}{2}\right)^2+\tau_{x y}^2}=\sqrt{\left(\frac{33343-0}{2}\right)^2+0}=16672  psi \\ \sigma_{1_a}, \sigma_{3_a} &=\frac{\sigma_x+\sigma_y}{2} \pm \tau_{a b}=33343  psi , 0  psi \\ \sigma^{\prime} &=\sqrt{\sigma_1^2-\sigma_1 \sigma_2+\sigma_2^2}=\sqrt{33343^2-33343(0)+0}=33343  psi \end{aligned}             (k)

11    The uncorrected endurance limit S_{e^{\prime}} is found from equation 6.5a (p. 330). The size factor for this rectangular part is determined by calculating the cross-sectional area stressed above 95% of its maximum stress (see Figure 6-25c, p. 332) and using that value in equation 6.7d (p. 331) to find an equivalent diameter test specimen for use in equation 6.7b (p. 331) to find C_{size}.

steels : \left\{\begin{array}{ll} S_{e^e} \cong 0.5 S_{u t} & \text { for } S_{u t}<200 kpsi (1400 MPa ) \\ S_{e^{\prime}} \cong 100 kpsi (700 MPa ) & \text { for } S_{u t} \geq 200 kpsi (1400 MPa ) \end{array}\right\}      (6.5a)

d_{\text {equiv }}=\sqrt{\frac{A_{95}}{0.0766}}      (6.7d)

\text {for}   d \leq 0.3  \text {in}  (8  mm) : \quad C_{\text {size }}=1 \\ \text {for}   0.3  \text {in} \lt d \leq 10  \text {in} : \quad C_{\text {size }}=0.869 d^{-0.097} \\ \text {for}  8  mm \lt d \leq 250  mm : \quad C_{\text {size }}=1.189 d^{-0.097}    (6.7b)

S_{e^{\prime}}=0.5 S_{u t}=0.5(80000)=40000  psi       (l)

\begin{aligned} A_{95} &=0.05 db =0.05(0.75)(1)=0.04  in ^2 \\ d_{\text {equiv }} &=\sqrt{\frac{A_{95}}{0.0766}}=0.700  in \\ C_{\text {size }} &=0.869\left(d_{\text {equiv }}\right)^{-0.097}=0.900 \end{aligned}        (m)

12    Calculation of the corrected endurance limit S_{e} requires that several factors be computed. C_{load} is found from equation 6.7a (p. 330). C_{surf} for a machined finish is found from equation 6.7e (p. 333). C_{temp} is found from equation 6.7f (p. 335) and C_{reliab} is chosen from Table 6-4 (p. 335) for a 99.9% reliability level.

\begin{array}{ll} \text { bending: } & C_{\text {load }}=1 \\ \text { axial loading: } & C_{\text {load }}=0.70 \end{array}       (6.7a)

C_{\text {surf }} \equiv A\left(S_{\text {ut }}\right)^b \quad \text { if } C_{\text {surf }}>1.0 \text {, set } C_{\text {surf }}=1.0      (6.7e)

\text {for}  T \leq 450^\circ C (840^\circ F) : \quad C_{\text {temp }}=1\\ \text {for}  450^\circ C \lt T \leq 550^\circ C : \quad C_{\text {temp }}=1-0.0058(T-450)\\ \text {for}  840^\circ F \lt T\leq 1020^\circ F : C_{\text {temp }}=1-0.0032(T-840)     (6.7f)

\begin{aligned} C_{\text {load }} &=1: & & \text { for bending } \\ C_{\text {surf }} &=A\left(\text { Sut }_{k p s i}\right)^b=2.7(80)^{-0.265}=0.845: & & \text { machined } \\ C_{\text {temp }} &=1: & & \text { room temperature } \\ C_{\text {reliab }} &=0.753: & & \text { for } 99.9 \% \text { reliab. } \end{aligned}            (n)

The corrected endurance limit is found from equation 6.6 (p. 330).

\begin{array}{l} S_e=C_{\text {load }} C_{\text {size }} C_{\text {surf }} C_{\text {temp }} C_{\text {reliab }} S_{e^{\prime}} \\ S_f=C_{\text {load }} C_{\text {size }} C_{\text {surf }} C_{\text {temp }} C_{\text {reliab }} S_f^{\prime} \end{array}     (6.6)

\begin{aligned} S_e &=C_{\text {load }} C_{\text {size }} C_{\text {surf }} C_{\text {temp }} C_{\text {reliab }} S_{e^{\prime}} \\ &=1(0.900)(0.845)(1)(0.753) 40000=22907  psi \end{aligned}           (o)

Note that the corrected S_{e} is only about 29% of S_{ut}.

13    The safety factor is calculated using equation 6.14 (p. 354) and the beam deflection y is computed using equation (j) from Example 4-5 (p. 168).

N_f=\frac{S_n}{\sigma^{\prime}}       (6.14)

N_f=\frac{S_n}{\sigma^{\prime}}=\frac{22907}{33343}=0.69         (p)

\begin{aligned} y &=\frac{F}{6 E I}\left[x^3-3 a x^2-\langle x-a\rangle^3\right] \\ y_{@ x=l} &=\frac{500}{6(3 E 7)(0.0352)}\left[6^3-3(5)(6)^2-(6-5)^3\right]=-0.026  \text { in } \end{aligned}        (q)

14    The results of all these computations for the first assumed design are seen in Table 6-9 (p. 358). The deflection of 0.026 in is not within the stated specification, and the design fails with a safety factor of less than one. So, more iterations are needed, as was expected. Any of the dimensions can be changed, as can the material. The material was left unchanged but the beam cross-sectional dimensions and the notch radius were increased and the model rerun (this took only a few minutes) until the results shown in Table 6-10 (p. 359) were achieved.

15    The final dimensions are b = 2 in, d = 1 in, D = 1.125 in, r = 0.5, a = 5.0, and l = 6.0 in. The safety factor is now 2.5 and the maximum deflection is 0.005 in. These are both satisfactory. Note how low the fatigue stress-concentration factor is at K_{f} = 1.16. The dimension D was deliberately chosen to be slightly less than a stock mill size so that material would be available for the cleanup and truing of the mounting surfaces. Also, with this design, hot-rolled steel (HRS) could be used, rather than the cold-rolled steel (CRS) initially assumed (Figure 6-41a, p. 355). Hotrolled steel is less expensive than CRS and, if normalized, has less residual stress, but its rough, decarburized surface needs to be removed by machining all over, or to be treated with shot peening to strengthen it.

16    The files EX06-04 are on the CD-ROM.