THE 2010 ASME STUDENT MECHANISM AND ROBOT DESIGN COMPETITION
More details here: https://forum.solidworks.com/message/143698#143698
Archive for the ‘FEA’ Category
Mohr-Coulomb Stress Criterion
10/24/2009The Mohr-Coulomb stress criterion is based on the Mohr-Coulomb theory also known as the Internal Friction theory. This criterion is used for brittle materials with different tensile and compressive properties. Brittle materials do not have a specific yield point and hence it is not recommended to use the yield strength to define the limit stress for this criterion. This theory predicts failure to occur when:
s1 ≥ sTensileLimit if s1 > 0 and s3 > 0
s3 ≥ – sCompressiveLimit if s1 < 0 and s3 < 0
s1 / sTensileLimit + s3 / sCompressiveLimit < 1 if s1 ≥ 0 and s3 ≤ 0
The factor of safety is given by:
Factor of Safety (FOS) = {s1 / sTensileLimit + s3 / sCompressiveLimit} (-1)
von Mises and Tresca Stress Criteria comparison
10/24/2009The maximum shear stress criterion is more conservative than the von Mises stress criterion since the hexagon representing the shear stress criterion is enclosed within the ellipse representing the von Mises stress criterion. For a condition of pure shear, von Mises stress criterion predicts failure at (0.577*yield strength) whereas the shear stress criterion predicts failure at 0.5 yield strength. Actual torsion tests used to develop pure shear have shown that the von Mises stress criterion gives more accurate results than the maximum shear stress theory.
Maximum von Mises Stress Criterion
10/24/2009The maximum von Mises stress criterion is based on the von Mises-Hencky theory, also known as the Shear-energy theory or the Maximum distortion energy theory.
In terms of the principal stresses s1, s2, and s3, the von Mises stress is expressed as:
svonMises = {[(s1 - s2)2 + (s2 - s3)2 + (s1 - s3)2]/2}(1/2)
The theory states that a ductile material starts to yield at a location when the von Mises stress becomes equal to the stress limit. In most cases, the yield strength is used as the stress limit. However, the software allows you to use the ultimate tensile or set your own stress limit.
svonMises ≥ slimit
Yield strength is a temperature-dependent property. This specified value of the yield strength should consider the temperature of the component. The factor of safety at a location is calculated from:
Factor of Safety (FOS) = slimit / svonMises
What is Meshing?
10/21/2009Finite Element Analysis (FEA) provides a reliable numerical technique for analyzing engineering designs. The process starts with the creation of a geometric model. Then, the program subdivides the model into small pieces of simple shapes (elements) connected at common points (nodes). The representation of a given region by a set of elements (i.e., discretization or mesh generation) is an important step in finite element analysis. Meshing the model is the heart of any FEA analysis. The choice of element type, number of elements, and density of elements depends on the geometry of the domain, the problem to be analyzed, and the degree of accuracy desired. Local mesh refinement tools are very important to have good mesh with gradual transitions between the mesh densities. One should have a finer mesh in the areas of high stress gradient to ensure accuracy of the solution.
What is nonlinear analysis?
10/21/2009What are the types of nonlinearities that can occur?In linear analysis, the response of a structure is directly proportional to the load. We assume that:
- the displacements and rotations are small
- stress is directly proportional to strain
- loads maintain their original directions as the structure deforms.
However any of the convenient assumptions that are made during a linear analysis may not hold good in real life situations. For example:
- A contact area may change as the load changes
- A material may no longer exhibit an elastic behavior especially after it starts to yield and flows into the plasticity region.
- The stiffness of the structure may decrease because of buckling or the material may even fracture!
- The displacements and rotations may become too large and thus there is a need to develop equations describing the equilibrium at various intervals instead of one single configuration.
The direction and magnitude of the applied force can change in large rotation problems.
What is Linear Static Analysis?
10/21/2009When loads are applied to a body, the body will deform and the effect of the loads will be transmitted throughout the body. To absorb the effect of loads, the body generates internal forces and reactions at the supports to balance the applied external loads. Linear static analysis refers to the calculation of displacements, strains, and stresses under the effect of external loads.
What is Static Analysis?
10/21/2009When loads are applied to a body, the body will deform and the effect of the loads will be transmitted throughout the body. To absorb the effect of loads, the body generates internal forces and reactions at the supports to balance the applied external loads. Linear static analysis refers to the calculation of displacements, strains, and stresses under the effect of external loads based on two basic assumptions:
Static Assumption All loads are applied slowly and gradually until they reach their full magnitudes. After reaching their full magnitudes, loads will remain constant (time-invariant). This assumption allows us to disregard insignificant inertial and damping forces due to negligibly small accelerations and velocities. Time-invariant loads that induce considerable inertial and/or damping forces may warrant dynamic analysis. Dynamic loads change with time and, in many cases, induce considerable inertial and damping forces that cannot be neglected.
Linearity Assumption The relationship between loads and induced responses is linear. If you double the magnitude of loads, for example, the response of the model (displacements, strains, and stresses), will also double. You can assume that the linearity assumption is valid if:
- All the materials in the model comply with Hook’s law, that is stress is directly proportional to strain.
- The induced displacements are small enough to ignore the change in stiffness caused by loading.
- Boundary conditions do not vary during the application of loads. Loads must be constant in magnitude, direction, and distribution. They should not change while the model is deforming.
What is Conduction?
10/21/2009Thermal energy transfers from one point to another through the interaction between the atoms or molecules of the matter. Conduction occurs in solids, liquids, and gasses. For example, a hot cup of coffee on your desk will eventually cool down to the room-temperature mainly by conduction from the coffee directly to the air and through the body of the cup. There is no bulk motion of matter when heat transfers by conduction.
What is Convection?
10/21/2009Convection is the heat transfer mode in which heat transfers between a solid face and an adjacent moving fluid (or gas). Convection involves the combined effects of conduction and the moving fluid. The fluid particles act as carriers of thermal energy. The rate of heat exchange between the fluid of temperature Tf and the face of a solid of area A and temperature Ts can be expressed as:
Q convection= hA(Ts-Tf)
where h is the convection heat transfer coefficient, Tf is the temperature of the fluid away from the face of the solid. The units of h are: W/m2.°C or Btu/s.in2.°F Convection processes can be divided into two main classes:
Free (Natural) Convection The motion of the fluid adjacent to a solid face is caused by the buoyancy forces induced by changes in the density of the fluid due to the presence of the solid. When a hot plate is left to cool down in the air, the particles of air adjacent to the face of the plate get warmer, their density decreases and hence they move.
Forced Convection An external means such as a fan or a pump is used to accelerate the flow of the fluid over the face of the solid. The rapid motion of the fluid particles over the face of the solid maximizes the temperature gradient and results in increasing the rate of heat exchange.
What is Radiation?
10/21/2009Thermal radiation is the thermal energy emitted by bodies in the form of electromagnetic waves because of their temperature. All bodies with temperatures above the absolute zero emit thermal energy. Because electromagnetic waves travel in a vacuum, no medium is necessary for radiation to take place. The thermal energy of the sun reaches earth by radiation. Because electromagnetic waves travel at the speed of light, radiation is the fastest heat transfer mechanism.
What is a steady state thermal analysis?
10/21/2009In this type of analysis, we are only interested in the thermal conditions of the body when it reaches thermal equilibrium, but we are not interested in the time it takes to reach this status. The temperature of each point in the model will remain unchanged until a change occurs in the system. At equilibrium, the thermal energy entering the system is equal to the thermal energy leaving it. Generally, the only material property that is needed for steady state analysis is the thermal conductivity.
What is a transient thermal analysis?
10/21/2009In this type of analysis, we are interested in knowing the thermal status of the model at different instances of time. A thermos designer, for example, knows that the temperature of the fluid inside will eventually be equal to the room temperature (steady state), but he or she is interested in finding out the temperature of the fluid as a function of time. In addition to the thermal conductivity (only material property needed for steady-state thermal analysis), we also need to specify density, specific heat, initial temperature profile, and the period of time for which solutions are desired.
3dcadware
fredvierheller
bob92003
Deepak Gupta