MEE 320 Chapter 1, Spring 2018

MEE 320: Strength of Materials Chapter 1 Concept of Stress MEE 320: Strength of Materials Concept of Stress Objectives The main objective of the study of mechanics of materials is to provide the future engineer with the means of analyzing and designing various machines and load bearing structures. Both the analysis and design of a given structure involve the determination of stresses and deformations. This chapter is devoted to the concept of stress. 2 MEE 320: Strength of Materials Statics Review Example: Consider the structure shown below and assume that it is designed to support a 30 kN load. The structure consists of a boom and rod joined by pins (zero moment connections) at the junctions and supports. Plan: Perform a static analysis to determine the internal force in each structural member and the reaction forces at the supports 3 MEE 320: Strength of Materials Statics Review FBD: free body diagram Structure is detached from supports loads and reaction forces are indicated Ay and Cy cannot be determined from these equations need to find new equations      kN 30 kN 0 30 0 kN 40 0 kN 40 kN 8.0 m 30 6.0 m 0                     y y y y y x x x x x x x C C A C A F A C C A F A A M Conditions for static equilibrium 4 MEE 320: Strength of Materials Component Free-Body Diagram 5 In addition to the complete structure, each component must satisfy the conditions for static equilibrium       kN 30 kN 40 kN 40 y x x C C A Reaction forces are directed along boom and rod   0 8.0 m 0     y y B A A M Consider the FBD for the boom: Now the structure equilibrium equation yields MEE 320: Strength of Materials Method of Joints 6 The boom and rod are 2-force members, i.e., the members are subjected to only two forces which are applied at member ends kN 50 kN 40 3 kN 30 5 4 0       BC AB BC AB B F F F F F Joints must satisfy the conditions for static equilibrium which may be expressed in the form of a force triangle: For equilibrium, the forces must be parallel to an axis passing through the force application points, equal in magnitude, and in opposite directions MEE 320: Strength of Materials Definition: Stress 7 Stress : in a member, stress is defined as force per unit area, or intensity of the forces distributed over a given section. A P  Positive stress: member in tension Negative stress: member in compression Units: P in N A in m2 σ in N/m2 = Pa (Pascal) MEE 320: Strength of Materials Stress Analysis 8 Conclusion: the strength of member BC is adequate MPa all165  From the material properties of steel, the allowable stress is Can the structure safely support the 30 kN load ? MPa 159 10 m 314 10N 50 2 6- 3      A BC P  At any section through member BC, the internal force is 50 kN with a force intensity or stress of FAB = 40 kN (compression) From a statics analysis: FBC = 50 kN (tension) MEE 320: Strength of Materials Design 9 Design of new structures requires selection of appropriate materials and component dimensions to meet performance requirements For reasons based on cost, weight, availability, etc., the choice is made to construct the rod from aluminum (σall = 100 MPa). What is an appropriate choice for the rod diameter?   52 10 m 252. mm .2 450010 m 4 4 10Pa 50010 m 100 10N 50 2 2 6 2 2 6 6 3                  d A d d A P A A P all all      An aluminum rod 26 mm or more in diameter is adequate MEE 320: Strength of Materials Axial Loading: Normal Stress 10 The normal stress at a particular point may not be equal to the average stress but the resultant of the stress distribution must satisfy      A ave dA dF A P   The resultant of the internal forces for an axially loaded member is normal to a section cut perpendicular to the member axis. A P A F ave A         0 lim The force intensity on that section is defined as the normal stress. The detailed distribution of stress is statically indeterminate, i.e., cannot be found from statics alone. MEE 320: Strength of Materials Centric & Eccentric Loading 11 If a two-force member is eccentrically loaded, then the resultant of the stress distribution in a section must yield an axial force and a moment. The stress distributions in eccentrically loaded members cannot be uniform or symmetric. A uniform distribution of stress in a section infers that the line of action for the resultant of the internal forces passes through the centroid of the section. A uniform distribution of stress is only possible if the concentrated loads on the end sections of two-force members are applied at the section centroids. This is referred to as centric loading. MEE 320: Strength of Materials Shearing Stress 12 Forces P and P’ are applied transversely to the member AB. A aveP  The corresponding average shear stress is, Corresponding internal forces act in the plane of section C and are called shearing forces. The resultant of the internal shear force distribution is defined as the shear of the section and is equal to the load P. Shear stress distribution varies from zero at the member surfaces to maximum values that may be much larger than the average value. The shear stress distribution cannot be assumed to be uniform. MEE 320: Strength of Materials Shearing Stress Examples 13 A F A aveP  Single Shear A F A P 2 ave    Double Shear MEE 320: Strength of Materials Bearing Stress in Connections 14 Bolts, rivets, and pins create stresses on the points of contact or bearing surfaces of the members they connect. td P A b P  Corresponding average force intensity is called the bearing stress, The resultant of the force distribution on the surface is equal and opposite to the force exerted on the pin. MEE 320: Strength of Materials Procedure for Analysis 15 The statement of the problem should be clear and precise. It should contain the given data and indicate what information is required. A simplified drawing showing all essential quantities involved should be included. 1. Write the equilibrium equations of the system, this will require the drawing of one or several FBDs. 2. Determine the reactions at supports and internal forces and couples. 3. Compute the required stresses and deformations. 4. After the answer has been obtained, it should be carefully checked. It can be done by substituting the numerical values obtained into an equation which has not yet been used and verifying that the equation is satisfied. MEE 320: Strength of Materials Stress Analysis & Design Example 16 Example: determine the stresses in the members and connections of the structure shown. Must consider maximum normal stresses in AB and BC, and the shearing stress and bearing stress at each pinned connection From a statics analysis: FAB = 40 kN (compression) FBC = 50 kN (tension) MEE 320: Strength of Materials Rod & Boom Normal Stresses 17 The rod is in tension with an axial force of P = 50 kN and circular cross-sectional area ABC = 314×10-6m2 The boom is in compression with an axial force of 40 kN and average normal stress of –26.7 MPa. The minimum area sections at the boom ends are unstressed since the boom is in compression.    MPa 167 10 m 300 10 50 mm 30010 m mm 25 mm40 20 2 6 3 , 2 6            N A P A BCend  At the flattened rod ends, the smallest cross- sectional area occurs at the pin centerline, At the rod center, the average normal stress in the circular cross-section is σBC = +159 MPa. MEE 320: Strength of Materials Pin Shearing Stresses 18 The cross-sectional area for pins at A, B, and C, 6 2 2 2 49110 m 2 mm 25           r A MPa 102 10 m 491 10N 50 6 2 3 ,       A Cave P  The force on the pin at C is equal to the force exerted by the rod BC, The pin at A is in double shear with a total force equal to the force exerted by the boom AB, 407. MPa 10 m 491 kN 20 6 2 ,      A Aave P MEE 320: Strength of Materials Pin Shearing Stresses 19 Divide the pin at B into sections to determine the section with the largest shear force kN 15 EP 509. MPa 10 m 491 kN 25 6 2 ,      A Bave PG  Evaluate the corresponding average shearing stress, (larges kN GP 25 MEE 320: Strength of Materials Pin Bearing Stresses 20 To determine the bearing stress at A in the boom AB, we have t = 30 mm and d = 25 mm,    mm 533. MPa mm25 30 kN 40   td P b  To determine the bearing stress at A in the bracket, we have t = 2(25 mm) = 50 mm and d = 25 mm,    mm 320. MPa mm25 50 kN 40   td P b MEE 320: Strength of Materials Example 1 21 1-4: Two solid cylindrical rods AB and BC are welded together at B and loaded as shown. Knowing that d1=50 mm and d2=30 mm, find the average normal stress at the midsection of (a) rod AB, (b) rod BC MEE 320: Strength of Materials Example 2 22 1-7: Each of the four vertical links has an 8×36-mm uniform rectangular cross section and each of the four pins has a 16-mm diameter. Determine the maximum value of the average normal stress in the links connecting (a) points B and D, (b) points C and E. MEE 320: Strength of Materials Example 2 23 MEE 320: Strength of Materials Example 3 24 1-17: When the force P reaches 8 kN, the wooden specimen shown failed in shear along the surface indicated by the dashed line. Determine the average shearing stress along that surface at the time of failure. MEE 320: Strength of Materials Example 3 25 MEE 320: Strength of Materials Example 4 26 1-25: Knowing that θ = 40° and P = 9 kN, determine (a) the smallest allowable diameter of the pin at B if the average shearing stress in the pin is not to exceed 120 MPa, (b) the corresponding average bearing stress in member AB at B, (c) the corresponding average bearing stress in each of the support brackets at B. MEE 320: Strength of Materials Example 4 27 MEE 320: Strength of Materials Example 4 28 MEE 320: Strength of Materials Stress in Two Force Members 29 We will show that either axial or transverse forces may produce both normal and shear stresses with respect to a plane other than one cut perpendicular to the member axis. Axial forces on a two force member result in only normal stresses on a plane cut perpendicular to the member axis. Transverse forces on bolts and pins result in only shear stresses on the plane perpendicular to bolt or pin axis. MEE 320: Strength of Materials Stress on an Oblique Plane 30 Pass a section through the member forming an angle θ with the normal plane.            sin cos cos sin cos cos cos 0 0 2 0 0 A P A P A V A P A P A F       The average normal and shear stresses on the oblique plane are   sin cos V P F P   Resolve P into components normal and tangential to the oblique section, From equilibrium conditions, the distributed forces (stresses) on the plane must be equivalent to the force P. MEE 320: Strength of Materials Maximum Stresses 31 The maximum normal stress occurs when the reference plane is perpendicular to the member axis, 0 , 0 m     A P The maximum shear stress occurs for a plane at ±45o with respect to the axis,       0 0 45 2 sin45cos A P A P m      sin cos cos 0 2 0 A P A P   Normal and shearing stresses on an oblique plane MEE 320: Strength of Materials Example 5 32 1-30: Two wooden members of uniform rectangular cross section are joined by the simple glued scarf splice shown. Knowing that the maximum allowable shearing stress in the glued splice is 620 kPa, determine (a) the largest load P that can be safely applied, (b) the corresponding tensile stress in the splice. MEE 320: Strength of Materials Example 5 33 MEE 320: Strength of Materials Stress Under General Loadings 34 A member subjected to a general combination of loads is cut into two segments by a plane passing through Q. The distribution of internal force components may be defined as, normal force acting on a small area ΔA shearing force acting on a small area ΔA Fx  x z x y x V V V    Vx  yVx  zVx  component parallel to the y-axis component parallel to the z-axis MEE 320: Strength of Materials Stress Under General Loadings 35 For equilibrium, an equal and opposite internal force and stress distribution must be exerted on the other segment of the member. A V A V A F xz A xz xy A xy x A x                lim lim lim 0 0 0    The distribution of internal stress components may be defined as, xz  xy The y component of the shear stress exerted on the face perpendicular to the x-axis The z component of the shear stress exerted on the face perpendicular to the x-axis MEE 320: Strength of Materials State of Stress 36 Stress components are defined for the planes cut parallel to the x, y and z axes. For equilibrium, equal and opposite stresses are exerted on the hidden planes. The combination of forces generated by the stresses must satisfy the conditions for equilibrium: 0 0             z y x z y x M M M F F F Stress components are defined for the planes cut parallel to the x, y and z axes. For equilibrium, equal and opposite stresses are exerted on the hidden planes. MEE 320: Strength of Materials State of Stress 37 It follows that only 6 components of stress are required to define the complete state of stress     yx xy yx xy z Aa Aa M            0 zx xz zy yz       Consider the moments about the z axis: zx yz z xy y x       and , , , , Similarly MEE 320: Strength of Materials Factor of Safety 38 stress allowable stress ultimate of safety Factor all u      FS FS Structural members or machines must be designed such that the working stresses are less than the ultimate strength of the material. Factor of safety considerations: • Uncertainty in material properties • Uncertainty of loadings • Uncertainty of analyses • Number of loading cycles • Types of failure • Maintenance requirements • Deterioration effects MEE 320: Strength of Materials Example 6 39 1-38: Link BC is 6 mm thick, has a width w = 25 mm, and is made of a steel with a 480 MPa ultimate strength in tension. What was the safety factor used if the structure shown was designed to support a 16 kN load P? MEE 320: Strength of Materials Example 6 40 MEE 320: Strength of Materials Example 7 41 1-46: Three steel bolts are to be used to attach the steel plate shown to a wooden beam. Knowing that the plate will support a 110 kN load, that the ultimate shearing stress for the steel used is 360 MPa, and that a factor of safety of 3.35 is desired, determine the required diameter of the bolts. MEE 320: Strength of Materials Example 8 42 1-55: In the structure shown, an 8 mm diameter pin is used at A, and 12-mm- diameter pins are used at B and D. Knowing that the ultimate shearing stress is 100 MPa at all connections and that the ultimate normal stress is 250 MPa in each of the two links joining B and D, determine the allowable load P if an overall factor of safety of 3.0 is desired. MEE 320: Strength of Materials Example 8 43 MEE 320: Strength of Materials Example 8 44