實驗力學研究室 1 Introduction of FEA in the Product Design Process

實驗力學研究室 2 1.Lord John William Strutt Rayleigh (late 1800s), developed a method for predicting the first natural frequency of simple structures. It assumed a deformed shape for a structure and then quantified this shape by minimizing the distributed energy in the structure. 2.Walter Ritz then expanded this into a method, now known as the Rayleigh-Ritz method, for predicting the stress and displacement behavior of structures. A Brief History of Computer-aided Engineering

實驗力學研究室 3 3.In 1943, Richard Courant proposed breaking a continuous system into triangular segments. (The unveiling of ENIAC at the University of Pennsylvania.) 4.In the 1950s, a team form Boeing demonstrated that complex surfaces could be analyzed with a matrix of triangular shapes. 5.Dr. Ray Clough coined the term “finite element” in The 1960s saw the true beginning of commercial FEA as digital computers replaced analog ones with the capability of thousands of operations per second.

實驗力學研究室 4 6.In the early 1960s, the MacNeal-Schwendler Corporation (MSC) develop a general purpose FEA code. This original code had a limit of 68,000 degrees of freedom. When the NASA contract was complete, MSC continued development of its own version called MSC/NASTRAN, while the original NASTRAN become available to the public and formed the basis of dozens of the FEA packages available today. Around the time MSC/NASTRAN was released, ANSYS, MARC, and SAP were introduced. 7.By the 1970s, Computer-aided design, or CAD, was introduced later in the decade.

實驗力學研究室 5 8.In the 1980s, the use of FEA and CAD on the same workstation with developing geometry standards such as IGES and DXF. Permitted limited geometry transfer between the systems. 9.In the 1980s,CAD progressed from a 2D drafting tool to a 3D surfacing tool, and then to a 3D solid modeling system. Design engineers began to seriously consider incorporating FEA into the general product design process. 10.As the 1990s draw to a place, the PC platform has become a major force in high end analysis. The technology has become to accessible that it is actually being “hidden” inside CAD packages.

實驗力學研究室 6 Rapid Product Development Process (1) communication, (2) visualization, and (3) simulation

實驗力學研究室 7 There enabling technologies have emerged to provide the communication, visualization, and simulation capabilities required by RPD. These technologies are 3D solid modeling, finite element analysis, and rapid prototyping.

實驗力學研究室 8 Traditional product development process.

實驗力學研究室 9 Relative cost of product change at the different stages of the design.

實驗力學研究室 10 Cost versus knowledge dilemma.

實驗力學研究室 11 Product development using predictive engineering

實驗力學研究室 12 Improved tracking of cost versus product knowledge with simulation.

實驗力學研究室 13 Who Should Use FEA? The champion or the designate. The champion was integral in the acquisition of the technology. The designate, on the other hand, was selected to be the FEA guy or gal once management made the decision to bring in FEA. Many champions lose sight of their limitations in their enthusiasm to validate their tool while some designates proceed with methodical caution to ensure that results are accurate. Also common to both types is isolation from peer interaction to talk about modeling techniques and results interpretation. While some do not know where to look for this support, others do not know they should.

實驗力學研究室 14 Pointing FEA in the right Direction Process 1.Establish a clearly defined goal. 2.Compile and qualify the inputs. 3.Solve the problem with the most appropriate means. 4.Verify and document the results.

實驗力學研究室 15 Substances in the Process What is the goal of the analysis? Predictive engineering versus failure verification Trend analysis versus absolute data Selecting required output data What input is required for the solution and what level of uncertainty does it introduce? What is the most efficient means to solve the problems?

實驗力學研究室 16 Common Misconceptions About FEA 1.Meshing is Everything 2.FEA Replaces Testing 3.Finite Element Analysis is Easy 4.Finite Element Analysis is Hard 5.Learning the Interface Equals Learning FEA

實驗力學研究室 17 FEA Capabilities and Limitations

實驗力學研究室 18 Actual Performance versus FEA Results “How accurate are the results?” Every variable, or price of data, that you are required to the system is an assumption and source of error.

實驗力學研究室 19 Source of Errors 1.Material properties. 2.Geometry 3.Loading condition

實驗力學研究室 20 How FEA Calculates Data Basic Equation Equilibrium Assembly

實驗力學研究室 21 Correctness versus Accuracy The Correct Answer Correct results are results expected by those observing or envisioning the parts or systems working in the field.

實驗力學研究室 22 The accurate answer An accurate answer in FEA is considered the best result obtainable for the properties, geometry, and boundary conditions specified, that is, the best answer to the question posed. Typically, the degree of accuracy refers, in large part, to convergence or the refinement of the mesh necessary to reduce error. H-elements versus P-elements Elements that can assume higher edge orders are called p- elements. H-element which typically limits the element order to quadratic, and convergence requires mesh refinement.

實驗力學研究室 23 Key Assumptions in FEA for Design Four Primary Assumptions Geometry Properties Mesh Boundary conditions

實驗力學研究室 24 Linear Static Assumption Material Properties A material is said to be linear if its stress-strain relationship is or can be assumed to be linear. Geometry Concerns(geometric stiffening ) The primary result of this condition is decreasing displacement under increasing load.The primary cause of this stiffening is increased tensile stresses in the areas being deformed.As the axial tensile stress in a self-stiffen. This is often called stress stiffening

實驗力學研究室 25 Boundary Conditions The boundary conditions do not change from the point of load application to the final deformed shape. Loading must be constant in magnitude, orientation, and distribution.

實驗力學研究室 26 Static Assumption A good way to interpret the static assumption is as that of steady state and constant magnitude.

實驗力學研究室 27 Dynamic Simulation There are three primary type of dynamic loading in FEA ： Transient response or time- dependent loading Frequency response or sinusoidal loading Random response

實驗力學研究室 28 Transient Response If the duration or period of the event is so small that the system can respond quickly enough to fully deform before the load reported by a static analysis. Frequency Response A frequency response analysis is also steady state,m but differs form a static analysis in that both the magnitude and orientation of the load vary sinusoidally. As the frequency of the input approaches any natural frequency of the system, the difference between the static and dynamic responses diverges.

實驗力學研究室 29 Random Response While loading in a frequency,random response input typically in pounds or pounds versus frequency,random response input is in the form of acceleration squared (G 2 )versus frequency. These data are usually compiled in the form of a power spectral density (PSD) curve.Random response input rally is not random at all but a preapproved spectrum of excitations and frequencies

實驗力學研究室 30 Other commonly Used Assumptions Geometry The supplied CAD geometry adequately represents the physical part. Nonlinear geometric stiffening will not affect the behavior of the system. Stress behavior outside the area of interest is not important to this project such that geometric simplifications in those areas will not affect the outcome.

實驗力學研究室 31 Only internal fillets in the area of interest will be included in the solution. The thickness of the part is small enough relative to its width and length such that shell idealization is valid. The thicknesses of the walls are sufficiently constant to justify constant thickness shell element. Local behavior at the corners and intersection of thin surfaces is not of primary interest such that no special modeling of these areas is required.

實驗力學研究室 32 The primary members of the structure are long and thin such that a beam idealization iv valid. Local behavior at the joints of beams or other discontinuities are not of primary interest such that no special modeling of these areas is required. Decorative external features will be assumed insignificant to the stiffness and performance of the part and will be omitted form the model. The variation in mass due to suppressed features in negligible.

實驗力學研究室 33 A 2D(plane stress or plane strain) Solution will be used.It is assumed that any part features that violate the planar assumption will have no impact on the behavior of interest. Reflective symmetry will be used. The geometry and boundary conditions are,or can be assumed to be,equivalent across once or more planes.

實驗力學研究室 34 Material Properties Material properties will remain in the linear regime.It is understood that either stress levels exceeding yield or excessive displacements will constitute a component failure, that is,nonlinear behavior can not be accepted. Nominal material properties adequately represent the physical system. Material properties are not affected by the load rate. Material properties can be assumed isotropic (or orthotropic)an homogenous.

實驗力學研究室 35 The part is free of voids or surface imperfections that can produce stress risers and skew local results. Actual nonlinear behavior of the system can be extrapolated form the linear material results. Weld material and the heat affected zone will be assumed to have the same properties as the base metal. All simulations will assume room temperature although temperature variation may have a significant impact on the properties of the materials used.Unless otherwise specified, this change in properties will be neglected.

實驗力學研究室 36 The effects of relative humidity(RH) or water absorption on the materials used with be neglected. The material properties used assume dry, 50% RH, or 100% RH properties as specified in the manufacturer’s date sheets. No compensation will be made to account for the effects of UV,chemicals, corrosives, wear, or other factors which may have an impact on the long-term structural integrity of the components. Material damping will be assumed negligible, constant across all frequencies of interest, and/or a published value or one determined form testing.

實驗力學研究室 37 Boundary Conditions Displacements will be small such that the magnitude, orientation, and distribution of loading remain constant throughout the deformation. A static solution will be used. Loading rates are expected to be sufficiently low as to make this assumption valid. Frictional losses in the system will be considered negligible. All interfacing components will be assumed rigid.

實驗力學研究室 38 The portion of the structure being studied can be assumed Decoupled from the rest of the system such that any reactions or inputs form the adjacent features can be neglected. Symmetry may be assumed to minimized model sized and complexity. Load is to be assumed purely compressive, tensile, torsional, or thermal.No other load components are to be included in the study. Pressure loading will be assumed uniform across all loaded surfaces.

實驗力學研究室 39 The components modeled as pure forces will impart no additional rigidity in the actual system.

實驗力學研究室 40 Fasteners Residual stress due to fabrication, preloading on bolts, welding, and/or other manufacturing or assembly processes will be neglected. Bolt loading is primarily axial in nature. Bolt head or washer surface torque loading is primarily axial in nature. Surface torque loading due to friction will produce only local effects. Stress relaxation of fasteners or other assembly components will not be considered.Failure is assumed to be early in the service life of the assembly.

實驗力學研究室 41 Load on the threaded portion of the part is evenly distributed On engaged threads. The bolts, spot welds, welds, rivets, and/or fasteners are numerous and stiff such that the bound between the two components can be considered prefect. All welds between components will be considered ideal and continuous. The failure of fasteners will not be considered.

實驗力學研究室 42 General Only the results in the area of interest are important and mesh convergence will be limited to shit area. No slippage between interfacing components will be assumed. Any sliding contact interfaces will be assumed frictionless. System damping will be assumed negligible, constant across all frequencies of interest, and/ or a published value or one determined from testing.

實驗力學研究室 43 Stiffness of bearings, radically or axially, will be considered infinite or rigid. Elements with poor, or less than optimal geometry, are only allowed in areas not expected to be of concern and do not affect the overall performance of the model.