Kiến trúc xây dựng - Chương 11: Mechanics of materials

MECHANICS OF MATERIALS Third Edition Ferdinand P. Beer E. Russell Johnston, Jr. John T. DeWolf Lecture Notes: J. Walt Oler Texas Tech University CHAPTER © 2002 The McGraw-Hill Companies, Inc. All rights reserved. Energy Methods © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 2 Energy Methods Strain Energy Strain Energy Density Elastic Strain Energy for Normal Stresses Strain Energy For Shearing Stresses Sample Problem 11.2 Strain Energy for a General State of Stress Impact Loading Example 11.06 Example 11.07 Design for Impact Loads Work and Energy Under a Single Load Deflection Under a Single Load Sample Problem 11.4 Work and Energy Under Several Loads Castigliano’s Theorem Deflections by Castigliano’s Theorem Sample Problem 11.5 © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 3 • A uniform rod is subjected to a slowly increasing load • The elementary work done by the load P as the rod elongates by a small dx is which is equal to the area of width dx under the load- deformation diagram. workelementarydxPdU == • The total work done by the load for a deformation x1, which results in an increase of strain energy in the rod. energystrainworktotaldxPU x === ∫1 0 112 12 12 1 0 1 xPkxdxkxU x === ∫ • In the case of a linear elastic deformation, Strain Energy © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 4 Strain Energy Density • To eliminate the effects of size, evaluate the strain- energy per unit volume, densityenergy straindu L dx A P V U x x == = ∫ ∫ 1 1 0 0 ε εσ • The total strain energy density resulting from the deformation is equal to the area under the curve to ε1. • As the material is unloaded, the stress returns to zero but there is a permanent deformation. Only the strain energy represented by the triangular area is recovered. • Remainder of the energy spent in deforming the material is dissipated as heat. © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 5 Strain-Energy Density • The strain energy density resulting from setting ε1 = εR is the modulus of toughness. • The energy per unit volume required to cause the material to rupture is related to its ductility as well as its ultimate strength. • If the stress remains within the proportional limit, E EdEu x 22 2 1 2 1 0 1 1 σεεε ε === ∫ • The strain energy density resulting from setting σ1 = σY is the modulus of resilience. resilience of modulus E u YY == 2 2σ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 6 Elastic Strain Energy for Normal Stresses • In an element with a nonuniform stress distribution, energystrain totallim 0 ===∆ ∆= ∫→∆ dVuUdVdUVUu V • For values of u < uY , i.e., below the proportional limit, energy strainelasticdV E U x 2 2∫ == σ • Under axial loading, dxAdVAPx ==σ ∫= L dx AE PU 0 2 2 AE LPU 2 2 = • For a rod of uniform cross-section, © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 7 Elastic Strain Energy for Normal Stresses I yM x =σ • For a beam subjected to a bending load, ∫∫ == dVEIyMdVEU x 2 222 22 σ • Setting dV = dA dx, dx EI M dxdAy EI MdxdA EI yMU L L A L A ∫ ∫ ∫∫ ∫ = ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛== 0 2 0 2 2 2 0 2 22 2 22 • For an end-loaded cantilever beam, EI LPdx EI xPU PxM L 62 32 0 22 == −= ∫ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 8 Strain Energy For Shearing Stresses • For a material subjected to plane shearing stresses, ∫= xy xyxy du γ γτ 0 • For values of τxy within the proportional limit, G Gu xyxyxyxy 2 2 2 12 2 1 τγτγ === • The total strain energy is found from ∫ ∫ = = dV G dVuU xy 2 2τ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 9 Strain Energy For Shearing Stresses J T xy ρτ = ∫∫ == dVGJTdVGU xy 2 222 22 ρτ • For a shaft subjected to a torsional load, • Setting dV = dA dx, ∫ ∫ ∫∫ ∫ = ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛== L L A L A dx GJ T dxdA GJ TdxdA GJ TU 0 2 0 2 2 2 0 2 22 2 22 ρρ • In the case of a uniform shaft, GJ LTU 2 2 = © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 10 Sample Problem 11.2 SOLUTION: • Determine the reactions at A and B from a free-body diagram of the complete beam. • Develop a diagram of the bending moment distribution. a) Taking into account only the normal stresses due to bending, determine the strain energy of the beam for the loading shown. b) Evaluate the strain energy knowing that the beam is a W10x45, P = 40 kips, L = 12 ft, a = 3 ft, b = 9 ft, and E = 29x106 psi. • Integrate over the volume of the beam to find the strain energy. • Apply the particular given conditions to evaluate the strain energy. © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 11 Sample Problem 11.2 SOLUTION: • Determine the reactions at A and B from a free-body diagram of the complete beam. L PaR L PbR BA == • Develop a diagram of the bending moment distribution. v L PaMx L PbM == 21 © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 12 Sample Problem 11.2 • Integrate over the volume of the beam to find the strain energy. ( )ba EIL baPbaab L P EI dxx L Pa EI dxx L Pb EI dv EI Mdx EI MU ba ba +=⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛ += ⎟⎠ ⎞⎜⎝ ⎛+⎟⎠ ⎞⎜⎝ ⎛= += ∫∫ ∫∫ 2 2223232 2 2 0 2 0 2 0 2 2 0 2 1 6332 1 2 1 2 1 22 EIL baPU 6 222 =vL PaM x L PbM = = 2 1 BD,portion Over the AD,portion Over the 43 in 248ksi1029 in. 108in. 36a in. 144kips45 =×= == == IE b LP ( ) ( ) ( )( )( )( )in 144in 248ksi 10296 in 108in 36kips40 43 222 ×=U kipsin 89.3 ⋅=U © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 13 Strain Energy for a General State of Stress • Previously found strain energy due to uniaxial stress and plane shearing stress. For a general state of stress, ( )zxzxyzyzxyxyzzyyxxu γτγτγτεσεσεσ +++++= 21 • With respect to the principal axes for an elastic, isotropic body, ( )[ ] ( ) ( ) ( ) ( )[ ] distortion todue 12 1 change volume todue 6 21 2 2 1 222 2 222 =−+−+−= =++−= += ++−++= accbbad cbav dv accbbacba G u E vu uu E u σσσσσσ σσσ σσσσσσνσσσ • Basis for the maximum distortion energy failure criteria, ( ) specimen test tensileafor 6 2 G uu YYdd σ=< © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 14 Impact Loading • To determine the maximum stress σm - Assume that the kinetic energy is transferred entirely to the structure, 2 02 1mvUm = - Assume that the stress-strain diagram obtained from a static test is also valid under impact loading. • Consider a rod which is hit at its end with a body of mass m moving with a velocity v0. ∫= dVEU mm 2 2σ • Maximum value of the strain energy, • Rod deforms under impact. Stresses reach a maximum value σm and then disappear. • For the case of a uniform rod, V Emv V EUm m 2 02 ==σ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 15 Example 11.06 SOLUTION: • Due to the change in diameter, the normal stress distribution is nonuniform. • Find the static load Pm which produces the same strain energy as the impact. • Evaluate the maximum stress resulting from the static load Pm Body of mass m with velocity v0 hits the end of the nonuniform rod BCD. Knowing that the diameter of the portion BC is twice the diameter of portion CD, determine the maximum value of the normal stress in the rod. © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 16 Example 11.06 • Find the static load Pm which produces the same strain energy as the impact. ( ) ( ) L AEUP AE LP AE LP AE LPU m m mmm m 5 16 16 5 4 22 222 = =+= • Evaluate the maximum stress resulting from the static load Pm AL Emv AL EU A P m m m 2 0 5 8 5 16 = = =σ SOLUTION: • Due to the change in diameter, the normal stress distribution is nonuniform. E VdV E mvU mm m 22 22 2 02 1 σσ ≠= = ∫ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 17 Example 11.07 A block of weight W is dropped from a height h onto the free end of the cantilever beam. Determine the maximum value of the stresses in the beam. SOLUTION: • The normal stress varies linearly along the length of the beam as across a transverse section. • Find the static load Pm which produces the same strain energy as the impact. • Evaluate the maximum stress resulting from the static load Pm © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 18 Example 11.07 • Find the static load Pm which produces the same strain energy as the impact. For an end-loaded cantilever beam, 3 32 6 6 L EIUP EI LPU m m m m = = SOLUTION: • The normal stress varies linearly along the length of the beam as across a transverse section. E VdV E WhU mm m 22 22 σσ ≠= = ∫ • Evaluate the maximum stress resulting from the static load Pm ( ) ( )22 66 cILWhEcIL EU I LcP I cM m mm m == ==σ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 19 Design for Impact Loads • For the case of a uniform rod, V EUm m 2=σ ( ) ( ) ( ) ( ) V EU VLcccLcIL cIL EU m m m m 24 // 6 4 12 4 124 4 12 2 = === = σ ππ σ • For the case of the cantilever beam Maximum stress reduced by: • uniformity of stress • low modulus of elasticity with high yield strength • high volume • For the case of the nonuniform rod, ( ) ( ) V EU ALLALAV AL EU m m m m 8 2/52/2/4 5 16 = =+= = σ σ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 20 Work and Energy Under a Single Load • Previously, we found the strain energy by integrating the energy density over the volume. For a uniform rod, ( ) AE LPdxA E AP dV E dVuU L 22 2 2 1 0 2 1 2 == == ∫ ∫ ∫σ • Strain energy may also be found from the work of the single load P1, ∫= 1 0 x dxPU • For an elastic deformation, 112 12 12 1 00 11 xPxkdxkxdxPU xx ==== ∫∫ • Knowing the relationship between force and displacement, AE LP AE LPPU AE LPx 2 2 11 12 1 1 1 =⎟⎠ ⎞⎜⎝ ⎛= = © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 21 Work and Energy Under a Single Load • Strain energy may be found from the work of other types of single concentrated loads. EI LP EI LPP yPdyPU y 63 32 1 3 1 12 1 112 1 0 1 =⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛= == ∫ • Transverse load EI LM EI LMM MdMU 2 2 11 12 1 112 1 0 1 =⎟⎠ ⎞⎜⎝ ⎛= == ∫ θθ θ • Bending couple JG LT JG LTT TdTU 2 2 11 12 1 112 1 0 1 =⎟⎠ ⎞⎜⎝ ⎛= == ∫ φφ φ • Torsional couple © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 22 Deflection Under a Single Load • If the strain energy of a structure due to a single concentrated load is known, then the equality between the work of the load and energy may be used to find the deflection. lLlL BDBC 8.06.0 == From statics, PFPF BDBC 8.06.0 −=+= From the given geometry, • Strain energy of the structure, ( ) ( )[ ] AE lP AE lP AE LF AE LFU BDBDBCBC 2332 22 364.0 2 8.06.0 22 =+= += • Equating work and strain energy, AE Ply yP AE LPU B B 728.0 364.0 2 1 2 = == © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 23 Sample Problem 11.4 Members of the truss shown consist of sections of aluminum pipe with the cross-sectional areas indicated. Using E = 73 GPa, determine the vertical deflection of the point E caused by the load P. SOLUTION: • Find the reactions at A and B from a free-body diagram of the entire truss. • Apply the method of joints to determine the axial force in each member. • Evaluate the strain energy of the truss due to the load P. • Equate the strain energy to the work of P and solve for the displacement. © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 24 Sample Problem 11.4 SOLUTION: • Find the reactions at A and B from a free- body diagram of the entire truss. 821821 PBPAPA yx ==−= • Apply the method of joints to determine the axial force in each member. PF PF CE DE 8 15 8 17 += −= 0 8 15 = += CD AC F PF PF PF CE DE 8 21 4 5 −= = 0=ABF © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 25 Sample Problem 11.4 • Evaluate the strain energy of the truss due to the load P. ( )2 22 29700 2 1 2 1 2 P E A LF EEA LFU i ii i ii = == ∑∑ • Equate the strain energy to the work by P and solve for the displacement. ( )( ) 9 33 2 2 1 1073 1040107.29 2 2970022 × ××= ⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛== = E E E y E P PP Uy UPy ↓= mm27.16Ey © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 26 Work and Energy Under Several Loads • Deflections of an elastic beam subjected to two concentrated loads, 22212122212 21211112111 PPxxx PPxxx αα αα +=+= +=+= • Reversing the application sequence yields ( )21111221222221 2 PPPPU ααα ++= • Strain energy expressions must be equivalent. It follows that α12=α21 (Maxwell’s reciprocal theorem). ( )22222112211121 2 PPPPU ααα ++= • Compute the strain energy in the beam by evaluating the work done by slowly applying P1 followed by P2, © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 27 Castigliano’s Theorem ( )22222112211121 2 PPPPU ααα ++= • Strain energy for any elastic structure subjected to two concentrated loads, • Differentiating with respect to the loads, 2222112 2 1212111 1 xPP P U xPP P U =+=∂ ∂ =+=∂ ∂ αα αα • Castigliano’s theorem: For an elastic structure subjected to n loads, the deflection xj of the point of application of Pj can be expressed as and j j j j j j T U M U P Ux ∂ ∂=∂ ∂=∂ ∂= φθ © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 28 Deflections by Castigliano’s Theorem • Application of Castigliano’s theorem is simplified if the differentiation with respect to the load Pj is performed before the integration or summation to obtain the strain energy U. • In the case of a beam, ∫∫ ∂∂=∂∂== L jj j L dx P M EI M P Uxdx EI MU 00 2 2 • For a truss, j i n i i ii j j n i i ii P F EA LF P Ux EA LFU ∂ ∂=∂ ∂== ∑∑ == 11 2 2 © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 29 Sample Problem 11.5 Members of the truss shown consist of sections of aluminum pipe with the cross-sectional areas indicated. Using E = 73 GPa, determine the vertical deflection of the joint C caused by the load P. • Apply the method of joints to determine the axial force in each member due to Q. • Combine with the results of Sample Problem 11.4 to evaluate the derivative with respect to Q of the strain energy of the truss due to the loads P and Q. • Setting Q = 0, evaluate the derivative which is equivalent to the desired displacement at C. SOLUTION: • For application of Castigliano’s theorem, introduce a dummy vertical load Q at C. Find the reactions at A and B due to the dummy load from a free-body diagram of the entire truss. © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 30 Sample Problem 11.5 SOLUTION: • Find the reactions at A and B due to a dummy load Q at C from a free-body diagram of the entire truss. QBQAQA yx 4 3 4 3 ==−= • Apply the method of joints to determine the axial force in each member due to Q. QFF QFF FF BDAB CDAC DECE 4 3;0 ;0 0 −== −== == © 2002 The McGraw-Hill Companies, Inc. All rights reserved. MECHANICS OF MATERIALS Third Edition Beer • Johnston • DeWolf 11 - 31 Sample Problem 11.5 • Combine with the results of Sample Problem 11.4 to evaluate the derivative with respect to Q of the strain energy of the truss due to the loads P and Q. ( )QP EQ F EA LFy i i ii C 42634306 1 +=∂ ∂⎟⎟⎠ ⎞ ⎜⎜⎝ ⎛=∑ • Setting Q = 0, evaluate the derivative which is equivalent to the desired displacement at C. ( ) Pa1073 10404306 9 3 × ×= NyC ↓= mm 36.2Cy