# MECH 401 Mechanical Design Applications

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1 MECH 401 Mechanical Design Applications Dr. M. O Malley Master Notes Spring 008 Dr. D. M. McStravick Rice University

2 Updates HW 1 due Thursday ( ) Last time Introduction Units Reliability engineering Materials This week Load and stress analysis Quiz #1 is Jan 9 (in class) Covers material through Chapter 3 (first weeks of class)

3 Equilibrium Basic equations of equilibrium enable determination of unknown loads on a body For a body at rest, (recalling from statics) ΣF 0 ΣM 0 For a body in motion, (recalling from dynamics) ΣF ma ΣM Iα

4 Determining loads Machine and structural components are load-carrying members We need to be able to analyze these loads in order to design components for the proper conditions Determining loads Engines/compressors operate at known torques and speeds (easy!) Airplane structure loads depend on air turbulence, pilot decisions (not so easy) Experimental methods / past performance Often we can determine loads by using a free body diagram (FBD) Gives a concise view of all the forces acting on a rigid body

5 Steps for drawing FBD s 1. Choose your body and detach it from all other bodies and the ground sketch the contour. Show all external forces From ground From other bodies Include the weight of the body acting at the center of gravity (CG) 3. Be sure to properly indicate magnitude and direction Forces acting on the body, not by the body 4. Draw unknown external forces Typically reaction forces at ground contacts Recall that reaction forces constrain the body and occur at supports and connections 5. Include important dimensions

6 Example Drawing FBD s Fixed crane has mass of 1000 kg Used to lift a 400 kg crate Find: Determine the reaction forces at A and B CG 1.5 m A 400 kg B m 4m A-Pin-Rxforces B - Rocker - 1 Rx force

7 Crane example (FBD s) cont. CG 1.5 m A B m 4m 400 kg A-Pin-Rxforces B - Rocker - 1 Rx force 1. Choose your body and detach it from all other bodies and the ground sketch the contour. Show all external forces (from ground, from other bodies). Include weight of the body acting at the center of gravity (CG) 3. Be sure to properly indicate magnitude and direction (acting ON the body, not BY the body)

8 Crane example (FBD s) cont. CG 1.5 m A 400 kg Ay 400 kg B m 4m A-Pin-Rxforces B - Rocker - 1 Rx force Ax 1000 kg 4. Draw unknown external forces (typically reaction forces at ground contacts). Recall that reaction forces constrain the body and occur at supports and connections 5. Include important dimensions 1.5 m B Ay Ax B m (400 kg)(9.81 m/s ) 3.5 kn W (1000 kg)(9.81 m/s ) 9.81 kn 4m

9 Find A x, A y, and B ΣF x 0 ΣF y 0 ΣM 0 Find B: ΣM A 0 B(1.5) (9.81)() (3.5)(6) 0 B kn Which forces contribute to ΣM A? B, 9.81, 3.5 Which forces contribute to ΣF X? A x, B Which forces contribute to ΣF y? A y, 9.81, 3.5 Find A x : ΣFx 0 A x + B 0 A x kn A x kn 1.5 m Ax B Ay (400 kg)(9.81 m/s ) 3.5 kn W (1000 kg)(9.81 m/s )9.81kN Find A y : ΣFy 0 A y A y 33.3 kn m 4m

10 3-D Equilibrium example transmission belts pass over sheaves welded to an axle supported by bearings at B and D A radius.5 C radius Rotates at constant speed Find T and the reaction forces at B, D Assumptions: Bearing at D exerts no axial thrust Neglect weights of sheaves and axle

11 Draw FBD Detach the body from ground (bearings at B and D) Insert appropriate reaction forces 4 lb 18 lb B z B y D z 30 lb T D y

12 Solve for reaction forces for each axis If we sum moments about x (along the shaft), what forces are involved? 4lb, 18lb, 30 lb, T If we sum moments about y, what forces are involved? 4lb, 18lb, D z 4 lb If we sum moments about z, what forces are involved? 30lb, T, D y 18 lb B z B y D z If we sum forces in y, what forces will we need to consider? B y, 30lb, T, D y 30 lb T D y If we sum forces in Z, what forces do we need to consider? 4lb, 18lb, B z, D z

13 Solve for reaction forces for each axis ΣM x 0 (4)(.5) 18(.5) + 30() T() T 37.5 lb ΣM y 0 (4)(8) + (18)(8) D z (1) D z 8 lb ΣM z 0 -(30)(6) (37.5)(6) + D y (1) D y lb 4 lb ΣF y 0 B y D y B y + D y 67.5 B y lb 18 lb B z B y D z ΣF z B z + D z 4 + D z B z B z 70 lb 30 lb T D y B (33.75 lb)j (70 lb)k D (33.75 lb)j + (8 lb)k

14 Linearly Elastic Material Behavior Some linearly elastic materials: Metals Wood Concrete Plastic Ceramic and glass Linearly elastic materials obey Hooke s Laws!

15 Uniaxial Stress State Hooke s Law in uniaxial tension-compression: σ x Eε x Also, for isotropic and homogenous material Poisson s Ratio y x x x ν -ε y / ε x 0 (cork) < ν < 0.5 (rubber)

16 Thermal Stresses Expansion of Parts due to temperature without constraint no stresses with constraint stress buildup Expansion of a rod vs. a hole Differential Thermal Expansion Two material with differential thermal expansion rates that are bound together Brass and steel Metals vs. plastic

17 Hooke s Law for Shear τ Gγ G? Shear Modulus Modulus of Rigidity Note: No equivalent to Poisson for shear (no coupling between axes) yx xy

18 Relating E and G G E (1 + ν ) For linearly elastic, homogenous, isotropic material characterized by TWO independent parameters

19 Hooke s Law for Biaxial Stress State y ε x σ x E σ y ν E x xy x ε y σ E y σ x ν E yx y τ γ xy G xy

20 Hooke s Law for triaxial state of stress Most general case of static loading Coupled: zy yz zx y yx xy xz x ( ) σ x ν ε x σ y + σ z E E σ y ν ε y x + E E σ E ν E ( σ σ ) z ( σ σ ) z ε z x + y z Decoupled: Note: G E (1 + ν ) τ γ xy G xy τ xz γ xz G τ γ yz G yz

21 More on Hooke s Law Used to relate loading to stress state through geometry Analytical solutions for classical forms of loading Axial cases: Column in tension (trivial) Column in compression (non-trivial) We ll do this later Other cases: Beam in pure bending Beam in bending and shear Shaft in torsion

22 Column in tension Uniaxial tension σ F A Hooke s Law ε ε x ε z y σ E y ν F EA F EA

23 Beam in pure bending What does PURE mean? Moment is constant along the beam Other assumptions Symmetric cross-section Uniform along the length of the beam Linearly elastic Homogenous Constant properties throughout Isotropic Equal physical properties along each axis Enable geometric arguments Enable the use of Hooke s Law to relate geometry (ε) to stress (σ)

24 Beam in pure bending Result Mc σ x I z I Z c I is the area moment of inertia: M is the applied bending moment c is the point of interest for stress analysis, a distance (usually y max ) from the neutral axis (at y 0) If homogenous (E constant), neutral axis passes through the centroid Uniaxial tension: ε ε x y ε My EI z ν My EI M Z (section modulus) I z A y da

25 Example Beam with rectangular cross-section

26 Beam in pure bending example, cont.

27 Example Find the maximum tensile and compressive stresses in the I-beam Beam in pure bending -- Mc σ I

28 Review of centroids Recall Parallel Axis Theorem I I + c Ad I is the moment of inertia about any point I c is moment of inertia about the centroid A is area of section d is distance from section centroid to axis of I

29 Example, cont. Recall: c and I defined with respect to the neutral axis (NA) First, we ll need to find I So We need to find the neutral axis Assume the I-beam is homogenous NA passes through the centroid of the cross-section

30 Finding the centroid Neutral axis passes through the centroid of the cross-sectional area Divide into simple-shaped sections to find the centroid of a composite area y y i A A i i y ( h )( ) ( h hb + + h)( hb) 4 3 3hb + hb 3 4 h

31 Finding I Moment of Inertia Red dot shows centroid of the composite area that we just found Similarly, we can find the moment of inertia of a composite area I I z I I i yellow z + I blue Use parallel axis theorem to find I for the blue and yellow areas Moment of inertia about a different point is the moment of inertia of the section about its centroid plus the area of the section times square of the distance to the point I Ic + Ad

32 Finding I Moment of Inertia, cont. I ( 6b)( h ) 3 yellow + 1 ()( b h) ( )( h ) 3 3hb bh 43 4 ( )( 3 ) 3 hb h bh I blue I I I blue + yellow 15 bh 48 3

33 Finding the bending stresses Maximum tensile stress occurs at the base 3 Mc M ( 4 h) 36 M σ base 3 15 I bh 15 bh Maximum compressive stress occurs at the top 48 7 ( h) Mc M 4 15 bh 84 M 15 bh σ top 3 I 48 Note, y is the distance from the point of interest (top or base) to the neutral axis Total height of cross-section h + h/ 10h/4 Neutral axis is at 3h/4

34 Beams in bending and shear Assumptions for the analytical solution: σ x Mc / I holds even when moment is not constant along the length of the beam τ xy is constant across the width

35 Calculating the shear stress for beams in bending V(x) shear force I I z area moment of inertia about NA (neutral axis) b(y) width of beam Q ( y) A yda τ xy VQ Ib Where A is the area between yy and the top (or bottom) of the beam cross-section General observations about Q: Q is 0 at the top and bottom of the beam Q is maximum at the neutral axis τ 0 at top and bottom of cross-section τ max at neutral axis Note, V and b can be functions of y

36 Usually we have common cross-sections τ max for common shapes on page 136 Example: Rectangular cross-section Q b h y Shear and normal stress distributions across the crosssection

37 Relative magnitudes of normal and shear stresses Rectangular cross-section τ 3 max σ V 4 A max My I P 3 4 bh 3 PL bh 3P 8bh τ σ max max 1 4 h l For THIS loading, if h << L, then τ max << σ max and τ can be neglected

38 Shafts in torsion T Assumptions Constant moment along length No lengthening or shortening of shaft Linearly elastic Homogenous Where J is the polar moment of inertia Note: J A r Circular shaft Hollow shaft da τ z θ 4 πd J 3 π J d o d i [ ] Tr J T T

39 Recap: Primary forms of loading Axial Pure bending σ σ F A Mc I Bending and shear Torsion Mc σ I VQ τ Ib τ Tr J

40 Questions So, when I load a beam in pure bending, is there any shear stress in the material? What about uniaxial tension? Yes, there is! The equations on the previous slide don t tell the whole story Recall: When we derived the equations above, we always sliced the beam (or shaft) perpendicular to the long axis If we make some other cut, we will in general get a different stress state

41 General case of planar stress Infinitesimal piece of material: A general state of planar stress is called a biaxial stress state Three components of stress are necessary to specify the stress at any point σ x σ y τ xy

42 Changing orientation Now let s slice this element at any arbitrary angle to look at how the stress components vary with orientation We can define a normal stress (σ) and shear stress (τ) Adding in some dimensions, we can now solve a static equilibrium problem

43 Static equilibrium equations y x l cosφ l sinφ σ σ ( cosφ ) dl τ ( sinφ ) dl σ xdy τ xydx ( sinφ ) dl + τ ( cosφ ) dl σ dx τ dy y xy

44 From equilibrium We can find the stresses at any arbitrary orientation (σ x, σ y, τ xy ) ) cos( ) sin( ) sin( ) cos( ) sin( ) cos( φ τ φ σ σ τ φ τ φ σ σ σ σ σ φ τ φ σ σ σ σ σ xy y x xy xy y x y x y xy y x y x x

45 Mohr s Circle These equations can be represented geometrically by Mohr s Circle Stress state in a known orientation: Draw Mohr s circle for stress state: φ is our orientation angle, which can be found by measuring FROM the line XY to the orientation axis we are interested in

46 Question from before Is a beam in pure bending subjected to any shear stress? Take an element Draw Mohr s Circle τ max occurs at the orientation φ 90º φ 45º σ My I

47 Special points on Mohr s Circle σ 1, Principal stresses At this orientation, one normal stress is maximum and shear is zero Note, σ 1 > σ τ max Maximum shear stress (in plane) At this orientation, normal stresses are equal and shear is at a maximum Why are we interested in Mohr s Circle?

48 Mohr s Circle, cont. A shaft in torsion has a shear stress distribution: τ Tr J Why does chalk break like this? Look at an element and its stress state: τ Tr J

49 Mohr s circle for our element: σ 1 and σ are at φ 90º Therefore φ 45 º This is the angle of maximum shear! The angle of maximum shear indicates how the chalk will fail in torsion

50 Example #1 σ x -4 σ y -81 τ xy 30 cw x at (σ x, τ xy ) x at ( -4, 30) y at (σ y, τ yx ) y at ( -81, -30) Center σ + x σ y Radius R τ xy,0 σ x σ ,0 y 30 + ( 61.5,0) 4 ( 81) 35.8

51 Example #1, cont. Now we have: x at ( -4, 30) y at ( -81, -30) C at (-61.5, 0) R 35.8 Find principal stresses: σ 1 C x + R -5.7 σ C x R τ max R 35.8 Orientation: φ tan 1 τ xy σ x σ y tan o cw Recall, φ is measured from the line XY to the principal axis. This is the same rotation direction you use to draw the PRINCIPAL ORIENTATION ELEMENT

52 Example #1, cont. Orientation of maximum shear At what orientation is our element when we have the case of max shear? From before, we have: σ 1 C x + R -5.7 σ C x R τ max R 35.8 φ 8.5 º CW φ max φ 1, + 45º CCW φ max 8.5 º CW + 45º CCW 16.5 º CCW

53 Example # σ x 10 σ y -40 τ xy 50 ccw x at (σ x, τ xy ) x at ( 10, -50) y at (σ y, τ yx ) y at ( -40, 50) Center σ + x σ y Radius, ,0 ( 40,0) R τ xy σ x + σ y ( 40) 94.3

54 Example #, cont. Now we have: x at ( 10, -50) y at ( -40, 50) C at (40, 0) R 94.3 Find principal stresses: σ 1 C x + R σ C x R τ max R 94.3 Orientation: φ tan 1 τ xy σ x σ y tan 1 10 ( 50) ( 40) 3.0 Recall, φ is measured from the line XY to the principal axis. This is the same rotation direction you use to draw the PRINCIPAL ORIENTATION ELEMENT o ccw

55 Example #, cont. Orientation of maximum shear At what orientation is our element when we have the case of max shear? From before, we have: σ 1 C x + R σ C x R τ max R 94.3 φ 16.0 º CCW φ max φ 1, + 45º CCW φ max 16.0 º CCW + 45º CCW 61.0 º CCW º CW 9.0 º CW

56 3-D Mohr s Circle and Max Shear Max shear in a plane vs. Absolute Max shear Biaxial State of Stress Still biaxial, but consider the 3-D element

57 3-D Mohr s Circle τ max is oriented in a plane 45º from the x-y plane (φ 90º) When using max shear, you must consider τ max (Not τ x-y max )

58 Out of Plane Maximum Shear for Biaxial State of Stress Case 1 σ 1, > 0 σ 3 0 σ 1 τ max Case σ,3 < 0 σ 1 0 σ 3 τ max Case 3 σ 1 >0, σ 3 < 0 σ 0 τ max σ 1 σ 3

59 Additional topics we will cover 3-13 stress concentration 3-14 pressurized cylinders 3-18 curved beams in bending 3-19 contact stresses

60 Stress concentrations We had assumed no geometric irregularities Shoulders, holes, etc are called discontinuities Will cause stress raisers Region where they occur stress concentration Usually ignore them for ductile materials in static loading Plastic strain in the region of the stress is localized Usually has a strengthening effect Must consider them for brittle materials in static loading Multiply nominal stress (theoretical stress without SC) by K t, the stress concentration factor. Find them for variety of geometries in Tables A-15 and A-16 We will revisit SC s

61 Stresses in pressurized cylinders Pressure vessels, hydraulic cylinders, gun barrels, pipes Develop radial and tangential stresses Dependent on radius 0 for o o i o i i r o i o i i t p r r r r p r r r r r p r σ σ

62 Stresses in pressurized cylinders, cont. Longtudincal stresses exist when the end reactions to the internal pressure are taken by the pressure vessel itself ri pi σ l r r These equations only apply to sections taken a significant distance from the ends and away from any SCs o i

63 Thin-walled vessels If wall thickness is 1/0 th or less of its radius, the radial stress is quite small compared to tangential stress ( σ ) ( σ ) σ l t t avg max pd 4t i pd t p i ( d + t) i t

64 Curved-surface contact stresses Theoretically, contact between curved surfaces is a point or a line When curved elastic bodies are pressed together, finite contact areas arise Due to deflections Areas tend to be small Corresponding compressive stresses tend to be very high Applied cyclically Ball bearings Roller bearings Gears Cams and followers Result fatigue failures caused by minute cracks surface fatigue

65 Contact stresses Contact between spheres Area is circular Contact between cylinders (parallel) Area is rectangular Define maximum contact pressure (p 0 ) Exists on the load axis Define area of contact a for spheres b and L for cylinders

66 Contact stresses - equations First, introduce quantity Δ, a function of Young s modulus (E) and Poisson s ratio (ν) for the contacting bodies Then, for two spheres, p ν ν Δ + E E F ( 1/ R + 1/ R ) 1 Δ a / R FΔ / R For two parallel cylinders, F ( 1/ R + 1/ R ) 1 p0 LΔ b 1.13 L FΔ ( 1/ R + ) 1 1/ R

67 Contact stresses Contact pressure (p 0 ) is also the value of the surface compressive stress (σ z ) at the load axis Original analysis of elastic contact 1881 Heinrich Hertz of Germany Stresses at the mating surfaces of curved bodies in compression: Hertz contact stresses

68 Contact stresses Assumptions for those equations Contact is frictionless Contacting bodies are Elastic Isotropic Homogenous Smooth Radii of curvature R 1 and R are very large in comparison with the dimensions of the boundary of the contact surface

69 Elastic stresses below the surface along load axis (Figures4-43 and 4-45 in JMB) Surface Cylinders Below surface Spheres

70 Mohr s Circle for Spherical Contact Stress

71 Mohr s Circle for Roller Contact Stress

72 Bearing Failure Below Surface

73 Contact stresses Most rolling members also tend to slide Mating gear teeth Cam and follower Ball and roller bearings Resulting friction forces cause other stresses Tangential normal and shear stresses Superimposed on stresses caused by normal loading

74 Curved beams in bending Must use following assumptions Cross section has axis of symmetry in a plane along the length of the beam Plane cross sections remain plane after bending Modulus of elasticity is same in tension and compression

75 Curved beams in bending, cont. r n σ σ σ i o Ae A da r My Mci Aer ( r y) i n Mc Aer o o

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