|Politecnico di Torino|
|Academic Year 2017/18|
Fundamentals of strength of materials
1st degree and Bachelor-level of the Bologna process in Mechanical Engineering - Torino
1st degree and Bachelor-level of the Bologna process in Automotive Engineering - Torino
This course is aimed at providing the students some fundamentals of solid mechanics needed to perform at least a preliminary operation of either design or verification of structural and mechanical systems undergoing some static loading conditions. Paradigm of this analysis is the beam, whose elementary approaches to compute the stress resultants and the occurring stress and strain are given together with a preliminary description of strength of materials in static behavior and related testing techniques. Course starts with the static equilibrium and shows how beamlike structures are constrained, loaded and to compute the external and internal reactions, together with the distribution of stress resultants along the beam line axis. This analysis is deeply performed at least in case of statically determined structure. Two and three dimensional examples will be proposed as well as a short description of rules for truss structures. A deep description of basic concepts of solid continuous mechanics is then proposed, by including definitions and computations of stress, strain and constitutive laws of materials under the assumption of linear elastic behaviour. Elastic and mechanical properties of material and static strength are then defined according to the standard tensile test. The elementary theory of beam is then described to allow the student computing both the stresses and the strains occurring in a one-dimensional structural element. De St Venant principle and related elaborations are then developed and applied to several examples to investigate the axial, flexural, torsional and shear behaviors. Some additional topics are proposed, concerning the computation of displacements and rotations in beam under a defined combined set of loading conditions. Elastic stability of slender beam under compression is evenly discussed and buckling phenomenon investigated. The last part of the course is aimed at showing the students how static structural analysis can be performed through the matrix form as an alternate approach to the energy theorems and virtual works principle. Discretization of static equilibrium equations and basic approaches to write them in matrix form are developed through the definition of stiffness matrices for all the typical loads and the solution of the static problem once the assembly operation is performed. Some examples of computation of reactions and stresses occurring into statically indeterminate structures are finally given to complete the scenario of static analysis used in mechanical design. Concept of static safety factor for design against yielding, rupture or buckling of beamlike structures is defined and applied to some examples of structures built in ductile and brittle materials as well as computation of equivalent (ideal) stress in multi-axial loading conditions.
Expected learning outcomes
At the end of this course it is required that student easily handle some typical tools of analytical methods for the static behavior prediction at least of beamlike structures.
Fundamental goals of the discipline are:
•a comprehensive knowledge, understanding and distinguishing of mechanical properties and strength of ductile and brittle materials; linear and nonlinear elastic behaviour conditions; concepts of stress, strain, displacement and rotation, described in both principal and non principal reference frames;
static failure criteria and safety factor; geometrical properties of plane figures, interpreted as cross section of beams; theory of beam and relations between stress and load for each static behavior foreseen by De St Venant. In addition student will get acquainted with the performing of static analysis through the matrix calculus, applied to system of assembled beam elements.
•providing some skills as the basic tools to:
1) simplify to a level of elementary scheme the layout of a beamlike mechanical component and perform a complete static analysis;
2) evaluate the degree of indeterminacy of the system;
3) calculate reaction forces of statically determinate structures (by analytical method) and indeterminate structures (at least by the matrix calculus);
4) calculate the internal stress resultant diagrams,
stresses, strains, displacements and rotations of each cross section of one-dimensional elements;
5) identify the critical points of the structure and compute the equivalent stress to be compared to the strength of material or even to buckling threshold;
6) handle the matrix form to analyse the structure, being able of writing the equations which describe the static equilibrium, by assembling the whole system and its matrices, then of solving for a straight computation of the safety factor and a prediction of the deformed shape of the whole structure.
Prerequisites / Assumed knowledge
Some typical mathematical tools (study of functions, computation of derivatives and integrals, matrix algebra, solution of eigenvalue/eigenvectors problems) and physics (basic concepts of kinematics and statics) and some basics of materials sciences (materials classes and properties).
Topics dealt within this course are herein listed.
1. Statics : Basic concepts of static behavior of structures (force, moment, rigid and deformable bodies), loading conditions, constraints, static and kinematic determinacy, equilibrium conditions and equations. Computations of reactions, internal forces, diagrams. Beams, bars, trusses. Outlines of Virtual Work Principle and application to undetermined structures.
2. Stress : Stress vector, tensor, components. Principal stresses and direction, related computation. Mohr circles. Equivalent stress definition and computation.
3. Strain : Rigid body motion and strain definition in elastic body. Strain components, principal strain and direction. Stress-strain relations, Hooke’s law. Elastic properties of materials. Elastic energy storage.
4. Strength of materials : Tensile test, material behaviour and properties. Elastic coefficients. Yielding phenomenon, brittle and ductile materials. Safety factors in statics.
5. Beam theory : De Saint Venant principle, beam definition, loading conditions, axial, flexural, shear, torsional behaviors. Approximated solutions for torsion of rectangular cross sections, multiple rectangles and thin walled structures. Computation of stresses, strains, displacements and rotations. Shear centre. Coupled behavior. Buckling and elastic instability.
6. Matrix calculus applied to solid mechanics: Matrix form. Static equilibrium equations. Definition of stiffness matrix: computation in case of axial load, bending and shear, torsion, distributed load, thermal load. Reference frames: local and global. Assembly of the whole structure. Constraints application. Solution and computation of reactions, displacements, rotations, stresses and strains.
This course is organized in two parts. Lectures will give a straight presentation of relevant topics to be studied to perform a complete structural static analysis of some mechanical structures.
Practice hours will be offered to solve examples, numerical exercises and practical cases and even an exam simulation.
Texts, readings, handouts and other learning resources
Textbooks: Some notes directly taken from the classes will be shared with students through the website.
Theoretical aspects presented during the lectures can be found on the following textbooks:
1.D.Gross, W.Hauger, J.Schroder, W.A.Wall, N. Rajapakse - "Engineering Mechanics 1: Statics", Springer.
2.V. Da Silva - "Mechanics and strength of materials", Springer.
3.J.D. Renton, "Applied elasticity : matrix and tensor analysis of elastic continua", Chichester, Horwood, New York: Wiley, 1987
or evenly, but textbooks are just partially dedicated to the above topics:
1.J. Beer, S.Johnston - "Solid mechanics", McGraw-Hill.
2.R.D. Cook – "Finite Element Modeling for Stress Analysis", John Wiley & Sons, 1995
3.K.J. Bathe – "Finite element procedures", Prentice Hall, 1995 e succ.
Assessment and grading criteria
The final exam consists of two written tests, taken in two different dates.
The 1st test is an open-book test and it consists of 3 (three) exercises to be numerically solved.
The test lasts 2 hours. The difficulty level of the exercises is comparable to those solved during the tutorials of the course.
Exercise #1 focuses on the calculation of reactions and internal actions of either a beam assembly or a truss.
Exercise #2 focuses either on the stress field in beams sections or on stress-strain relationships and Mohr circles. Static verification at the maximum stressed points might be asked.
Exercise #3 focuses on matrix calculus to solve 2D structures (either a rod’s or a beam’s assembly) by computing nodal displacements, reaction forces and stress components in elements.
To take the exam, the student has to show a valid identity document with a clear picture. In absence of this evidence, the candidate will not be allowed to take the exam.
Any student found with any communication device (phones, tablets, laptops, ect...) switched on during the test will be expelled out of the room and his/her test will be invalidated.
The maximum score is 30/30. The minimum score to access the 2nd test is 18/30.
The 2nd test is a closed-book exam and it consists of 3 questions about the contents of the course, requiring a written demonstration, response, calculation or a graphical solution.
Students who attended DexPiLab during the course, will only have to answer to 2 questions.
The test will last 45 minutes.
Any student caught cheating during the exam (i.e. copying solutions from notes or from other candidates) will be expelled out of the room and his/her exam will be invalidated.
The 2 tests must be taken in the same exam session, strictly and only on the official dates published through the website of Politecnico di Torino.
For the student to pass the exam, both tests must be evaluated sufficient (minimum score of 18/30). Even if the student only fails the 2nd test the whole exam (1st and 2nd test) shall be completely repeated.
The final mark will be a weighted average of the marks of the 2 tests (final mark = 2/3*A + 1/3*B, with A: mark of the 1st test; B: mark of the 2nd test).
Programma definitivo per l'A.A.2017/18