ME 323: Mechanics of Materials Name Last quiz December 8 ... · Problem: For the stress state...
Transcript of ME 323: Mechanics of Materials Name Last quiz December 8 ... · Problem: For the stress state...
ME 323: Mechanics of Materials Name Last quiz December 8, 2017 Table # For this quiz, you are presented with ten questions from a hypothetical ME 323 exam, along with answers that were provided by one student in the class. Unfortunately, all ten answers by the student are incorrect. As a grader for the exam, you are asked to provide a concise explanation to the student as to why each question is incorrect. You are to be graded on the quality of your explanation. Question 0 Problem: For the loading on the beam shown, determine the
maximum normal stress on the cross section shown for two values of Young’s modulus: i) E, and ii) 2E.
Student’s answer:
The student’s answer indicated that σ
max,ii= 2σ
max,i.
Why is that answer wrong? Question 1 Problem: For the beam shown loaded by its weight,
determine the deflection at end C, δC , for two values of beam length: i) L, and ii) 2L.
Student’s answer:
The student’s answer indicated that δC ,ii = 2δC ,i .
Why is that answer wrong? Question 2 Problem: The rod shown is made up of steel (a ductile
material). Determine the load P that will cause the member to fail.
Student’s answer:
The rod cannot fail due to this loading since ductile materials fail due to shear stress, and a rod with an axial load experiences only normal stress.
Why is that answer wrong?
C B
C B
P
τ
σx
y
σ x
τ xy
C B
C B
P
τ
σx
y
σ x
τ xy
C B
C B
P
τ
σx
y
σ x
τ xy
Question 3 Problem: For the stress state shown, draw Mohr’s circle.
Student’s answer:
The student’s Mohr’s circle is as shown.
Why is that answer wrong? Question 4 Problem: Consider the loaded truss shown. Member (1) is
made of steel. Determine the stress σ1 experienced in member (1) if member (2): i) is made up of steel, ii) is made up of aluminum.
Student’s answer:
The student’s answer indicated that σ1,i >σ1,ii .
Why is that answer wrong? Question 5 Problem: Consider the state of plane stress provided. The
material of the body experiencing this stress is ductile. Determine the factor of safety (FS) against failure using: i) maximum shear stress theory, and ii) maximum distortional energy theory.
Student’s answer:
The student’s answer indicated that FSi > FS ii .
Why is that answer wrong?
C B
C B
P
τ
σx
y
σ x
τ xy
C B
C B
P
τ
σx
y
σ x
τ xy
x
y
σ x
τ xy
L
2( )
1( )
2( )1( )
P
x
y
σ x
τ xy
L
2( )
1( )
2( )1( )
P
Question 6 Problem: Consider the truss shown. There are no applied
loads; however, the temperature of member (1) is increased with the temperature of (2) being held constant. Determine the stress in member (1), σ1 , if the length of member (1) is: i) L, and ii) 2L.
Student’s answer:
The student’s answer indicated that σ1,i <σ1,ii .
Why is that answer wrong? Question 7 Problem: Consider the clamped-free beam loaded as shown.
Make a sketch of the deflection of the beam.
Student’s answer:
Shown above.
Why is that answer wrong? Question 8 Problem: Consider shaft shown with axial force P and axial
torque T loadings. Draw the components of stress for the element at point “a” on the surface of the shaft.
Student’s answer:
As shown to the right.
Why is that answer wrong?
x
y
σ x
τ xy
L
2( )
1( )
2( )1( )
P
x
C B
P T
y
x
y
z x
C B
P T
y
x
y
z
x
C B
P T
y
x
y
z
a
Question 9 Problem: Consider the cantilevered beam with a point load
at end C. Determine the shear stress at point “a” on the beam for two values of point load: i) P, and ii) 2P.
Student’s answer:
The student’s answer indicated that τ
ii> τ
i.
Why is that answer wrong? Question 10 Problem: Consider columns (i) and (ii) shown for which we
will consider in-plane buckling (buckling about an axis perpendicular to the page). These columns have radii of gyration related by rii / ri = 2 .
Determine the critical buckling load Pcr for each column.
Student’s answer:
The student’s answer indicated that Pcr ,ii > Pcr ,i .
Why is that answer wrong? POST-QUIZ SURVEY- On a scale of 1-10, how effective was this quiz at identifying (or re-identifying) gaps in your knowledge of ME 323 concepts (1 = not useful, 10 = very useful)? 1 2 3 4 5 6 7 8 9 10 Do you think offering regular quizzes like this would improve your understanding and prevent you from making these common mistakes in the future? Yes or No
Additional comments/suggestions:
C B
a
P
L
(i)
P
L
(ii)
C B
a
P
L
(i)
P
L
(ii)