CE 221: MECHANICS OF SOLIDS I CHAPTER 1: STRESS By Dr. Krisada Chaiyasarn Department of Civil Engineering, Faculty of Engineering Thammasat university
Agenda Introduction to your lecturer Introduction to the Module Chapter 1 Lecture coverage
Course Instructor Dr. Krisada Chaiyasarn(PhD, Cambridge) Lecturer Thammasat University Engineering School, Thailand Work Experience J.P.Morgan, UK, IB Technology, Electronic Market Maker Developer Ministry of Mobility and Public Works Belgium, Engineering Consultant ITM Soil, Engineering Consultant Contact Email: ckrisada@engr.tu.ac.th Website: www.krisadachaiyasarn.org
Introduction to the Module Overview of Module What can you expect? Learning Objectives How will you benefit? Learning Strategy How you will learn? Assessment Overview How you will be tested?
Overview of Module Mechanics of Solids Introduction to mechanics of deformable bodies. Relations among loads and deformations, Material testing, Stress-strain relationship, Axial loading, Torsion, Bending in elastic range. Bending and shearing stresses in beams, Transformation of stress, Mohr s circles, Introduction to failure theory, Deflection of beams by integration, Eccentric loading, Bucking of compression members.
Learning Objectives Students are expected to: be able to explain mechanics of a deformable body loaded by applied forces, such as axial load, torsion, bending, and shear. be able to determine the responses of the deformable body, for examples, support reactions, internal forces in the body, and deformation, and be able to solve practical problems. be able to determine load-carrying capacity of simple structures, such as axially loaded members, shaft, and column, and be able to make a proper design of these structures as well.
Learning Strategy Taught over 1 semester Total 15 weeks of class covering 10 chapters Each chapter consists of Lectures: Knowledge, theory and example Additional examples in supplementary Homeworks Total of 2 quiz
Course Overview CHAPTER 1 STRESS CHAPTER 2 STRAIN CHAPTER 3 MECHANICAL PROPERTIES OF MATERIALS CHAPTER 4 AXIAL LOAD CHAPTER 5 TORSION CHAPTER 6 BENDING CHAPTER 7 TRANSVERSE SHEAR CHAPTER 8 COMBINED LOADINGS CHAPTER 9 DEFLECTIONS OF BEAMS CHAPTER 10 BUCKLING OF COLUMNS
Prerequisite Free-body diagrams, Equilibrium of forces, Bending moment diagram and Shear force diagram, Cross-sectional properties of members such as Centroid and Moment of inertia. You have already learned this in CE202
References Main Textbook Hibbeler, R.C., (2004/200x). Mechanics of Materials (SI Edition). Prentice Hall, Singapore. Third/xx Edition. Any edition will do All slides and diagrams are taken from this book.
Assessment Details 4 Components from 100% Quiz 5% 2 quiz will be given Attendance 5% Random check Mid-term 35% Chapter 1-5 Final 55%, Chapter 6-10 Chapter 6-10
Grading Range of Marks Grade 80-100 A 70-79 B/B+ 60-69 C/C+ 50-59 D/D+ > 40 F Remarks Excellent: outstanding performance with only minor errors Very Good: above the average standard but with some errors Good: generally sound work with a number of notable errors Satisfactory: fair but with significant shortcomings Fail: performance does not meet the minimum criteria and considerable further work is required
Further Information All details are summarized in Course Outline All materials can be download on www.krisadadachaiyasarn.org
Any Questions
Outline Introduction to simple stresses Average normal stress in axial bar Average shear stress Bearing stress Allowable stress Designs of simple connections
Introduction Mechanics of solids Studies the internal effects of stress and strain in a solid body subjected to an external loading Stress is the strengths of material Strain is a measure of deformation All engineering designs are based on this subject. Historically, 17 th century Galileo experimented the effects of loads on rods and beams on various materials. 18 th century, more material testing, notably, Saint-Venant, Poisson, Lame and Navier Most fundamental problems have been solved, more advanced and complex problems, ongoingly Theory of elasticity Theory of plasticity
Equilibrium of a Deformable Body This is the review of CE202 Statics The knowledge of Free-body diagram is essential Free body diagram consists of External loads Support Reactions Then, use equilibrium equations to solve for all unknown forces Then, we will look at equilibrium using the internal forces
External Loads Surface Forces Forces applied directly on the object, i.e. wind load Forces due to contact with another surface Two types, concentrated force force applied over a small area, e.g. a force acting from a ground on wheels distributed force force applied over a larger area, can be idealised as linear distributed load, equivalent resultant force is used for analysis, e.g. loading along the length of a beam E.g. friction, reaction force Body Forces Force exerting on another body without a direct physical contact between bodies, e.g. gravity (i.e. weight), electromagnetic field
Support Reactions Surface forces at the supports
Equilibrium equations
Internal Forces Internal forces must exist within a body for equilibrium Found by making a cut through a body Exact distribution of internal force is unknown But the internal force can be represented by equivalent resultant force F R and equivalent resultant moment M R acting on point O (normally a centroid) The area along the cut is normally referred as cross-section
Internal Forces 3D Normal force, N, acts perpendicular to the area Shear Force, V, acts along a plane, causing two segments to slide over one another Torsional Moment or torque, T, tend to twist one segment of the body about an axis perpendicular to the area Bending moment, M, tend to bend the body about an axis lying within the plane of the area
Internal Forces 2D Normal force, N, acts perpendicular to the area Shear Force, V, acts along a plane, causing two segments to slide over one another, just one force Bending moment, M, tend to bend the body about an axis lying within the plane of the area
Example
Example
Stress M Ro and F R are the resultant effects acting at O on the sectioned area We want to find the actual distribution of loading of the resultant effects in order to find stress To find a distribution, we reduce the size of an area to ΔA towards zeros, two assumptions, Material is continuous, i.e. no voids Cohesive, i.e. all portions are connected, no cracks, or separations A finite force ΔF is acting on the area, and have x, y and z force components as shown, ΔF x, ΔF y, and ΔF z respectively As ΔA approaches zero, so as ΔF, the quotient is called stress
Normal Stress Stress is the intensity of the internal force acting on a specific plane Normal Stress, σ, is the intensity of the force acting normal to ΔA Pulling force generates tensile stress, pushing force generate compressive stress
Shear Stress The intensity of force acting tangent to ΔA, use symbol τ The subscript indicates the axes along which each shear stress acts
General State of Stress If the body is further sectioned by planes x-z and y-z, then a cubic volume element of material represents the state of stress Stress has unit N/m 2 or Pa (Pascal),
Average Normal Stress in axially loaded bar Assuming the bar is homogeneous and isotropic, then the bar will deform uniformly Homogeneous material has the same physical and mechanical properties throughout the volume Isotropic material has the same properties in all directions, e.g. steel Since the material is uniform, it is subjected to a constant normal stress distribution
Average Normal Stress in axially loaded bar P passes through the centroid, hence zero moments Only a normal stress exists on any small volume of material located at each point on the cross section, consider equilibrium Hence two normal stress components on an element are equal but opposite in direction, this is termed uniaxial stress P is equivalent to the volume under the stress diagram, P = σa
Maximum Average Normal Stress When there are changes in cross-section or change of normal force, the stress will change Hence, we need to find the ratio P/A, to determine the location of the maximum normal stress. Hence we need to plot the normal force diagram, i.e. the diagram to show variations of P along the bar at any point x The sign convention is + for tension and - for compression
Example
Example
Average Shear Stress Stress component acts in the plane of the sectioned area From the diagram, if F is large enough, it will fail along the plane AB and CD From equilibrium V = F/2 The average shear stress distributed over each sectioned area is The shear stress has the same direction as V Actual distribution of the shear stress is higher, but this value is generally acceptable.
Shear Stress Equilibrium Shear stress in an element must be in equilibrium, in both force and moment equilibrium. Hence it requires all other shear stress to meet the conditions.
Shear Stress Equilibrium All four shear stresses must has equal magnitude and be directed either toward or away from each other at opposite edges of the element, this is pure shear
Example
Allowable Stress Factor of safety is a ratio of the failure load and the allowable load to ensure a margin to prevent structural failure
Design of a Simple Connections The area of the member are used to help achieve the allowable stress to comply with the safety factor
Example
Example