
The assembly consists of two A992 steel bolts AB and EF and an 6061-T6 aluminum rod CD. When the temperature is at 30°C, the gap between the rod and rigid member AE is 0.1 mm. Determine the normal stress developed in the bolts and the rod if the temperature rises to 130°C. Assume BF is also rigid.
Prob. R9-1

Find the normal stress developed in the bolts and rod.
Answer to Problem 1RP
The normal stress developed in the bolts and rod are 33.5 MPa_ and 16.8 MPa_.
Explanation of Solution
Given information:
The two bolts AB and EF are made of A992 steel.
The rod CD is made of 6061-T6 aluminum.
The Young’s modulus of the steel is (Est) 200×103 N/mm2.
The Young’s modulus of the aluminum (Eal) is 68.9×103 N/mm2.
The coefficient of thermal expansion of the steel (αst) is 12 ×10−6 m/m°C.
The coefficient of thermal expansion of the aluminum (αal) is 24 ×10−6 m/m°C.
The initial temperature (T1) is 30°C.
The finial temperature (T2) is 130°C.
The gap between the rod and rigid member AE is 0.1 mm.
The diameter of the bolts AB and EF (db) is 25 mm..
The diameter of the rod CD (dr) is 50 mm.
The length of the bolts AB and EF (Lb) is 400 mm..
The length of the rod CD (Lr) is 300 mm.
Calculation:
Calculate the area of the bolts AB and EF (Ab) using the formula:
Ab=π4d2b
Substitute 25 mm for db.
Ab=π4×252=490.874 mm2
Calculate the area of the rod CD (Ar) using the formula:
Ar=π4d2r
Substitute 50 mm for dr.
Ar=π4×502=1,963.495 mm2
Calculate the difference of temperature (ΔT) using the formula:
ΔT=T2−T1
Substitute 30°C for T1 and 130°C for T2.
ΔT=130−30=100°C
Show the free body diagram of the rigid cap as in Figure 1.
Calculate the vertical forces by applying the equation of equilibrium:
Sum of vertical forces is equal to 0.
∑Fy=0−Fb+Fr−Fb=0Fr−2Fb=0Fr=2Fb (1)
Here, Fb is force at the bolts AB and EF and Fr is force at the rod CD.
Show the initial and final position of the assembly as in Figure 2.
Here (δT)r is deformation at rod due to temperature, (δT)b is deformation at bolts due to temperature, (δF)r is deformation at rod due to force, and (δF)b is deformation at bolts due to force.
The deformation is as follows:
(δT)r−(δF)r−0.1=(δT)b+(δF)bαalΔTLr−FrLrArEal−0.1=αstΔTLb+FbLbAbEst
Substitute 24 ×10−6 m/m°C for σal, 12 ×10−6 m/m°C for αst, 100°C for ΔT, 300 mm for Lr, 400 mm for Lb, 1,963.495 mm2 for Ar, 490.874 mm2 for Ab, 68.9×103 N/mm2 for Eal, and 200×103 N/mm2 for Est.
[(24×10−6×100×300)−Fr×3001,963.495×68.9×103−0.1]=[(12×10−6×100×400)+Fb×400490.874×200×103]0.72−2.2175×10−6Fr−0.1=0.48+4.0743×10−6Fb4.0743×10−6Fb+2.2175×10−6Fr=0.72−0.1−0.48
4.0743×10−6Fb+2.2175×10−6Fr=0.14Fb+0.5443Fr=34,361.73
Calculate the force at the bolts AB and EF (Fb):
Substitute 2Fb for Fr.
Fb+0.5443(2Fb)=34,361.732.0886Fb=34,361.73Fb=34,361.732.0886Fb=16,452 N
Calculate the force at the rod CD (Fr);
Substitute 16,452 N for Fb in Equation (1).
Fr=2(16,452)=32,904 N
Calculate the normal stress developed in the bolts AB and EF (σb) using the formula:
σb=FbAb
Substitute 16,452 N for Fb and 490.874 mm2 for Ab.
σb=16,452490.874=33.5 N/mm2×1 MPa1 N/mm2=33.5 MPa
Calculate the normal stress developed in the rod CD (σr) using the formula:
σr=FrAr
Substitute 32,904 N for Fb and 1,963.495 mm2 for Ar.
σr=3,29041,963.495=16.8 N/mm2×1 MPa1 N/mm2=16.8 MPa
Hence, the normal stress developed in the bolts and rod are 33.5 MPa_ and 16.8 MPa_.
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Chapter 9 Solutions
Statics and Mechanics of Materials (5th Edition)
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