2 Body Under External Loading 實驗力學研究室First PrinciplesBody Under External LoadingFirst Law: A body will remain at rest or will continue its straight line motion with constant velocity if there is no unbalanced force acting on it.Second Law: the acceleration of a body will be proportional to the resultant of all forces acting on it and in the direction of the resultant.Third Law: Action and reaction forces between interacting bodies will be equal in magnitude, collinear, and opposite in direction.
3 G, the center of gravity of the body. 實驗力學研究室G, the center of gravity of the body.
4 G= mv constitutes the linear momentum vector of a body. 實驗力學研究室G= mv constitutes the linear momentum vector of a body.H is the angular momentum vector of the body.
6 Constraining the body to uniaxial motion, 實驗力學研究室Constraining the body to uniaxial motion,Constraining the body to planar motion,aG is the vectorial acceleration of the center of gravity (c.g.) of the body. IG is the mass moment of inertia of the body about an axis normal to the plane of motion through the c.g., and α is the body’s angular acceleration.
7 Stress and Strain What Is Stress? 實驗力學研究室Stress and StrainWhat Is Stress?For static equilibrium, τxy=τyx, τyz=τzy, and τzx=τxz.
8 實驗力學研究室Principal StressesIn a given loaded structure, a particular element orientation exists for which all the shear stress components are zero. The normals to the faces of an element in this orientation are called principal directions and the stresses along these normals are the principal stresses.when one of the principal stresses is zero, the stress state is considered to be biaxial or plane stress. These problems can be deconstructed into planar approximations in which the loading and boundary conditions are in that plane and identical on any parallel plane.
10 Maximum/minimum Principal Stresses 實驗力學研究室Plane Stress Case:Maximum/minimum Principal StressesMaximum/minimum Shear Stresses (45° away from the orientation of principal stress)
11 實驗力學研究室Triaxial Case:whereI1, I2, and I3 are called stress invariants. I1, or the first invariant, is the internal hydrostatic pressure.The maximum shear stress is given in terms of the maximum and minimum principal stresses as follows.
21 實驗力學研究室where σt is also known as the hoop stress in the cylinder, σr is radial stress.The longitudinal stress due to pressure on the end caps is constant throughout the cylinder and is given by the equation.
22 實驗力學研究室If the pressure created due to an equal-length cylindrical press-fit was known, above equations could be used for obtaining the stress state on both the outside and inside cylinders.where ri is the inside cylinder’s internal radius, ro is the outside cylinder’s external radius, R is the transition radius, and Ei, Eo, vi, and vo are the inside and outside material Young’s moduli and Poisson’s ratios, respectively. δ is the radial interference.
23 Stress in Contact (Hertzian stresses ) 實驗力學研究室Stress in Contact (Hertzian stresses )Contact circular area resulting from the forced contact of two spheres will be
24 實驗力學研究室At the center of this area, a maximum pressure pmax will occur of the following magnitude.For two cylinders of equal length l and diameters d1 and d2, the resulting contact surface is a rectangle of length l and width 2b, whereThe maximum pressure occurs along the long center line of the rectangle.
25 Stress in thermal Expansion 實驗力學研究室Stress in thermal ExpansionThe constant of proportionality α is know as the material’s coefficient of thermal expansion. For a straight beam constrained at both ends, the resulting compressive stress at a distance from the ends is given by the next equation.
26 Stress Concentration Factors 實驗力學研究室Stress Concentration FactorsThe stress concentrations will also appear close to unplanned irregularities in the part, such as cracks and pits.σo and τo are the nominal stresses found in the part without the feature.
27 Material Properties Types of Materials 實驗力學研究室Material PropertiesTypes of MaterialsIsotropic. Properties are the same in any direction or at any cross section.Anisotropic. Properties differ in two or more directions.Orthotropic. Specific type of anisotropic in which planes of extreme values are orthogonal(i.e.,perpendicular to one another).
28 實驗力學研究室Common Material Properties (modulus of elasticity (E), modulus of rigidity (G), and Poisson’s ratio (v))
29 實驗力學研究室Point P, known as the proportional limit. Point E, the elastic limit. Point Y is the yield point of the material, corresponding to its yield strength (Sy). Point U indicates the maximum stress that can be achieved by the material. This corresponds to its ultimate or tensile strength. Fracture point (F), which marks the fracture strength (SF) of the material.
30 Ductile versus Brittle Material Behavior 實驗力學研究室Ductile versus Brittle Material BehaviorIf permanent set (plastic deformation) is obtainable, the material is said to exhibit ductility. For ductile materials , the ultimate tensile and compressive strengths have approximately the same absolute value. Brittle materials on the other hand are stronger in compression than in tension.
31 Brittle materials exhibit the behavior described below. 實驗力學研究室Brittle materials exhibit the behavior described below.A graph of stress versus strain is a smooth, elastic curve until failure which manifests as fracture. Materials behaving in this manner do not have a “yield strength.”Compressive strength is usually many times greater than tensile strength.Modulus of rupture is approximately the same as tensile strength.Rapid crack propagation along cleavage planes occurs with no noticeable plastic deformation.
32 If the percent elongation is at or below 5%, assume brittle behavior. 實驗力學研究室Rules of thumb used to determine if brittle or ductile behavior should be expected are summarized below.If the percent elongation is at or below 5%, assume brittle behavior.If the published ultimate compressive strength is greater than the ultimate tensile strength, assume brittle behavior.If no yield strength is published, suspect brittle behavior.
33 Failure Modes Typical Failure Modes 實驗力學研究室Failure ModesTypical Failure ModesFracture. Fracture is said to occur when new cracks appear or existing cracks are extended. A brittle fracture is one that exhibits little or no permanent (plastic) deformation.Yielding. A body which experiences stresses in excess of the yield strength is said to have failed only when this yielding compromises the integrity or function of the part. Yielding near stress concentrations is not considered a failure if it produces localized strains which merely redistribute the stress, whereupon yielding ceases.
34 實驗力學研究室Insufficient stiffness. Parts must be stiff enough to hold tolerances and support required loads. Moving parts may have undesirable resonant frequencies if they are too flexible.Buckling. The sudden loss of stability or stiffness under applied load. Stress levels need not be high for buckling to occur.Fatigue. Parts that are subject to variable loading will lose strength with time and may fail after a certain number of cycles.Creep. Bodies under load gradually deform over time. The apparent modulus property is derived form empirical creep data for various materials and may be used to compensate for the effects of creep.
35 Classic Failure theories 實驗力學研究室Classic Failure theoriesDuctile Failure theoryMaximum normal stress theory. Failure occurs whenever σ1 or σ3 equals the failure strength of the material in tension or compression, respectively.Maximum shear stress theory ( Tresca criterion ). Yielding begins when the maximum shear stress becomes equal to one-half the yield strength. Failure in tension of ductile materials occurs on one of the 45°maximum shear planes. Annealed ductile materials tend to fail according to this theory.
36 實驗力學研究室Distortion energy ( Von Mises-Hencky ) theory (Suitable for entire stress state). Probably the most widely used, this theory predicts that failure by yielding will occur whenever the von Mises, or effective stress (σ’ ), equals the yield strength of the materials.
38 Brittle Failure Theory 實驗力學研究室Brittle Failure TheoryMaximum normal stress. Similar to that defined for ductile materials. Failure occurs when the ultimate strength, not yield, is reached.Coulomb-Mohr theory. Fracture occurs when the maximum and minimum principal stresses combine for a condition which satisfies the following:where Sut and Suc represent the ultimate tensile and compressive strengths, and both σ3 and Suc are always negative, or in compression.
39 實驗力學研究室Modified Mohr theory. Fracture occurs as defined in the Coulomb-Mohr theory except in the fourth quadrant condition where σ1 is in tension and σ2 is in compression.
40 Other Failure Theories 實驗力學研究室Other Failure TheoriesBucklingwhere E is the modulus of elasticity of the column’s materials, I is the smallest or least moment of inertia of its cross-sectional area, and Le is its effective length. The last term, Le=KL, depends on the actual length L of the column and an effective length factor K, which is assigned according to the constraint conditions of the column ends.
42 If a slenderness ratio (Le/r) is defined as 實驗力學研究室r as the smallest radius of gyration of the column’s cross-sectional area (A).A corresponding critical stress(σcr) may be calculated as seen in the next equation.If a slenderness ratio (Le/r) is defined asthe column is considered Euler, and a critical load must be calculated and recorded.
43 實驗力學研究室For nonEuler columns,a tangent modulus variable (Et) has taken the place of the elastic modulus (function of location), below the yield point, Et=E as expected.
46 實驗力學研究室Endurance or fatigue limit (Se) is defined as the maximum cyclic stress which a part can sustain for an “ infinite” number for cycles. The endurance limit of the actual rotating beam speciman is designated as Se'. The correlation between Se and Se' isHere, ka is a surface factor, kb is a size factor, kc is a load factor, kd is a temperature factor, and ke is an all encompassing, other miscellaneous effects factor.
47 實驗力學研究室Because nenferrous metals and alloys lack an endurance limit ( the strength of material never stablizes but keep decreasing with time). A fatigue strength (Sf’) is usually reported for 50(107) cycles of reversed stress. This strength is often as low as 1/4 Sut for some aluminum alloys.
48 where a and b are provided by 實驗力學研究室To obtain the fatigue strength at N cycles for a part experiencing alternating or completely reversed stress, you can curve-fit the S-N curve using the following equation:where a and b are provided byNote that Se' may be substituted for Se in above equation to predict Sf '.
49 實驗力學研究室If the completely reversed stress has an amplitude (σa), the corresponding number of cycles of life is calculated via the next equation.
50 實驗力學研究室When the mean stress (σm) is at a level other than zero, the cyclic loading is classified as fluctuating stress case. One of the most accepted equations that provides a solution to this scenario is the modified Goodman relation:where Sut is the ultimate tensile strength of the material and n is the safety factor used in the design.
51 Crack initiation. A crack begins to form within the material. 實驗力學研究室When a ductile material is subjected to a fatigue-type loading, there are basic structural changes that occur. In chronological order, the changes are summarized below.Crack initiation. A crack begins to form within the material.Localized crack growth. Local extrusions and intrusions occur at the surface of the part because plastic deformations are not completely reversible.Crack growth on planes of high tensile stress. The crack proceeds across the section at those points of greatest tensile stress.
52 實驗力學研究室Ultimate ductile failure. When the crack reduces the effective cross section to a size that cannot sustain the applied loads, the sample ruptures by ductile failure.All the modifying factors that affect the endurance life of a part, are summarized below.Stress concentrators. General part features as described in the “Stress and Strain” section, which cause high local stresses and thus decrease fatigue life.Surface roughness. Smooth surfaces are more crack resistant because roughness creates stress concentrators.
53 實驗力學研究室Surface conditioning. Hardening processes tend to increase fatigue strength while plating and corrosion protection tend to diminish fatigue strength.Environment. A corrosive environment greatly reduces fatigue strength. A combination of corrosive attack and cyclic stresses is called corrosion fatigue.
55 The stress state for a viscoelastic material can be expressed as 實驗力學研究室The stress state for a viscoelastic material can be expressed asThe four stages of creep failure, as shown in figure, are described below.Instantaneous elongation. Normal deformation under applied load.Primary creep. Material strain hardens under load to decrease creep rate.Secondary creep. Material elongates at a steady rate, called minimum creep rate.Tertiary creep. Due to necking and formation of voids, elongation proceeds at an increasing rate until fracture.
56 實驗力學研究室The secondary phase is of significant interest to engineers because it dominates the actual creep process from a time standpoint. Creep strength is defined as the stress which produces a minimum creep rate of 10-5 % per hour.
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