Three methods of calculating total life, crack initiation, and crack growth
By Adarsh Pun, Senior Product Manager, MSC.Software Corp. -- Design News, December 16, 2001
Purely static loading is rarely observed in modern engineering components or structures. By far, the majority of structures involve parts subjected to fluctuating or cyclic loads, often resulting in fatigue-caused structural failure. In fact, 80% to 95% of all structural failures occur through a fatigue mechanism. For this reason, design analysts must address the implications of repeated loads, fluctuating loads, and rapidly applied loads. As a result, fatigue analysis has become an early driver in the product development processes of a growing number of companies.
What is fatigue? Fatigue is failure under a repeated or varying load, never reaching a high enough level to cause failure in a single application. The fatigue process embraces two basic domains of cyclic stressing or straining, differing distinctly in character. In each domain, failure occurs by different physical mechanisms:
Low-cycle fatigue—where significant plastic straining occurs. Low-cycle fatigue involves large cycles with significant amounts of plastic deformation and relatively short life. The analytical procedure used to address strain-controlled fatigue is commonly referred to as the Strain-Life, Crack-Initiation, or Critical Location approach.
High-cycle fatigue—where stresses and strains are largely confined to the elastic region. High-cycle fatigue is associated with low loads and long life. The Stress-Life (S-N) or Total Life method is widely used for high-cycle fatigue applications—here the applied stress is within the elastic range of the material and the number of cycles to failure is large. While low-cycle fatigue is typically associated with fatigue life between 10 to 100,000 cycles, high-cycle fatigue is associated with life greater than 100,000 cycles.
Fatigue analysis refers to one of three methodologies: local strain or strain life, commonly referred to as the crack initiation method, which is concerned only with crack initiation (E-N, or sigma nominal); stress life, commonly referred to as total life (S-N, or nominal stress); and crack growth or damage tolerance analysis, which is concerned with the number of cycles until fracture.
Saturday, October 16, 2010
Monday, August 2, 2010
hyper
1) why geometry cleanup operations r required?
ans: The gaps, overlaps and misalignment's that can occur when surfaces are
imported into HyperMesh can prevent the automesher from creating quality
meshes. By eliminating misalignment's and holes and by suppressing the
boundaries between adjacent surfaces, you can automesh across larger,
more logical regions of the model and improve overall meshing speed and
quality.
2) how to connect shell and solid elements?
ANS: Two options one has in connecting a solid to a shell
first option: Overlap some rows/columns of shell elements with that of
solid elements .with the fact in mind that only there are overlapping
DOFS corresponding to the three displacement components. There is no transfer
of rotational components. This is justified in that the displacement DOFS
being apart by a distance of length of element account for rotation components
coming in.
Second option: Is by penalty function approach, although u can have a rigid
spring with high stiffness to transfer the displacements.The solution all
depends on what kind of load transfer that is expected between the solid
and shell. If join is assumed to be rigid and with the external loading
causing no much predominant twist at the joint the use of above two options
is valid.
ans: The gaps, overlaps and misalignment's that can occur when surfaces are
imported into HyperMesh can prevent the automesher from creating quality
meshes. By eliminating misalignment's and holes and by suppressing the
boundaries between adjacent surfaces, you can automesh across larger,
more logical regions of the model and improve overall meshing speed and
quality.
2) how to connect shell and solid elements?
ANS: Two options one has in connecting a solid to a shell
first option: Overlap some rows/columns of shell elements with that of
solid elements .with the fact in mind that only there are overlapping
DOFS corresponding to the three displacement components. There is no transfer
of rotational components. This is justified in that the displacement DOFS
being apart by a distance of length of element account for rotation components
coming in.
Second option: Is by penalty function approach, although u can have a rigid
spring with high stiffness to transfer the displacements.The solution all
depends on what kind of load transfer that is expected between the solid
and shell. If join is assumed to be rigid and with the external loading
causing no much predominant twist at the joint the use of above two options
is valid.
FEA + SOM
1.what is moment ?
Ans: m=f*x
2.relation in pure bending case
m/i=sigma/y=e/r
3.if u want to calculate stress do u want material properties???
Ans: in simple cases like beam problems we don't want material property to calculate the stress
. the complex problems like other than beam problems,moment m can be calculated from the relation m=eid^2y/dx^2,we can not able to calculate the moment at any point by the applied force for complex problems.
4.what are nonlinear fem methods?
Ans:nonlinear geometry,material,contact. if the stiffness changes by geometry it is the non linear geometry. if the stiffness changes by material it is the non linear material case. if the stiffness changes due to the contact it is nonlinear contact case.
5.what is nonlinear geometry case??
Ans: if the force applied continuously for example wind turbine blades by the application of wind loads the deformation is high,in that case we must not ignore the higher order terms in strain-deflection relation strain=dy/dx+d^2y/dx^2+d^3y/dx^3+---- considering higher order terms are nonlinear geometry case. these higher order terms are called as green's function.
6.what are the theories of failures??
Ans: 1.max normal stress2.max shear stress3.vonmisses----etc
7.why u consider vonmisses stress theory only?
Ans:there is a limitations on max normal stress and max shear stress theories---etc. The vonmisses stress theory is derived from energy formulation so it is suitable for all cases and the stresses developed in the real time environment is near to the vonmisses stress.
GM
1.prrove that m=ei d^2y/dx^2
2. what is the constituency relation ship???
the relation ship between stress starin in 3 dimensional
3.what is the slope of stress strain curve after exceeding proportionality limit??
Ans: tagential modulus is the slope of the curve in the plastic region of stress strain curve
4.
MSC NASTRAN
1.shell,plate are having How many degress of freedom ??/
Ans: shell is having 6 degrees of freedom 3- translational 3 rotational degress of freedom
Plate is having 2 rotational 1- traslation degrees of freedm
solid is having 3 traslational degress pf freedom
2.what is the shape function??
Ans: read the definition from any text book
3.
4. how can you solve a cantilever beam problem in fea software
Ans: cantilever beam problem can be solved in 3 ways
1. By using beam element having 3 rotational 3 translational degress of freedom-- we will give geometric dimensions in the form as moment of inertia I
2. By using shell element---mesh with shell elements and give the thickness in the direction
then solve it for analysis
3.By using solid element --mesh with solid elements and apply the boundary conditions.. solve for analysis
Q).5. in which method u will get good results from the above 3??
Ans:1. By using beam method we can get exact results but this is valid only for beam structures, not valid for complex structures.
2. in the case of irregular sections, complex strcutures the shell element method is the best method because it allows 6 degress of freedom
3. using solid elements we can get results but the results are not correct. because it allows only 3 degress of freedom.
Q).6.in the case of shell element procedur how the stress is calculated??
Ans:in the finite element formulation the stress is calculated throught out the thickness at a node point .i.e integration from -t/2 to t/2 it will calculate
Ans: m=f*x
2.relation in pure bending case
m/i=sigma/y=e/r
3.if u want to calculate stress do u want material properties???
Ans: in simple cases like beam problems we don't want material property to calculate the stress
. the complex problems like other than beam problems,moment m can be calculated from the relation m=eid^2y/dx^2,we can not able to calculate the moment at any point by the applied force for complex problems.
4.what are nonlinear fem methods?
Ans:nonlinear geometry,material,contact. if the stiffness changes by geometry it is the non linear geometry. if the stiffness changes by material it is the non linear material case. if the stiffness changes due to the contact it is nonlinear contact case.
5.what is nonlinear geometry case??
Ans: if the force applied continuously for example wind turbine blades by the application of wind loads the deformation is high,in that case we must not ignore the higher order terms in strain-deflection relation strain=dy/dx+d^2y/dx^2+d^3y/dx^3+---- considering higher order terms are nonlinear geometry case. these higher order terms are called as green's function.
6.what are the theories of failures??
Ans: 1.max normal stress2.max shear stress3.vonmisses----etc
7.why u consider vonmisses stress theory only?
Ans:there is a limitations on max normal stress and max shear stress theories---etc. The vonmisses stress theory is derived from energy formulation so it is suitable for all cases and the stresses developed in the real time environment is near to the vonmisses stress.
GM
1.prrove that m=ei d^2y/dx^2
2. what is the constituency relation ship???
the relation ship between stress starin in 3 dimensional
3.what is the slope of stress strain curve after exceeding proportionality limit??
Ans: tagential modulus is the slope of the curve in the plastic region of stress strain curve
4.
MSC NASTRAN
1.shell,plate are having How many degress of freedom ??/
Ans: shell is having 6 degrees of freedom 3- translational 3 rotational degress of freedom
Plate is having 2 rotational 1- traslation degrees of freedm
solid is having 3 traslational degress pf freedom
2.what is the shape function??
Ans: read the definition from any text book
3.
4. how can you solve a cantilever beam problem in fea software
Ans: cantilever beam problem can be solved in 3 ways
1. By using beam element having 3 rotational 3 translational degress of freedom-- we will give geometric dimensions in the form as moment of inertia I
2. By using shell element---mesh with shell elements and give the thickness in the direction
then solve it for analysis
3.By using solid element --mesh with solid elements and apply the boundary conditions.. solve for analysis
Q).5. in which method u will get good results from the above 3??
Ans:1. By using beam method we can get exact results but this is valid only for beam structures, not valid for complex structures.
2. in the case of irregular sections, complex strcutures the shell element method is the best method because it allows 6 degress of freedom
3. using solid elements we can get results but the results are not correct. because it allows only 3 degress of freedom.
Q).6.in the case of shell element procedur how the stress is calculated??
Ans:in the finite element formulation the stress is calculated throught out the thickness at a node point .i.e integration from -t/2 to t/2 it will calculate
Softwares for mechanical students
There are so many institutes that are providing training on software's like ANSYS,HYPERMESH.
They will teach you to some extent only and they will collect more money.
.
For designing : Auto cad, Solidworks,catia v6,Pro-e
Finite element mesh : HYPERMESH
Finite element analysis : ANSYS
Explicit finite element analysis :LSDYNA
i will write the brief discussion about these software's----------------
procedure to execute ls dyna input files
You don't want lsdyna solver if u have ansys version installed in ur computer to run the input files of lsdyna
input files for lsdyna are in the extension of *.k,*.key------etc
download any input file from website and follow the procedure
1. go to the product launcher of ansys
2. select lsdyna solver
3. give the input file bath
4. give te name for output file
5. click on run
u will see the running command prompt.
go to the output file folder and verify for the results
websites to do crash analysis for different models using LSDYNA
crash analysis for different car models
http://www.ncac.gwu.edu/vml/models.html
http://www.ncac.gwu.edu/vml/models.html
Difference between IT softwares and Mechanical softwares
IT software's:1). like java, data-warehousing deals with data , main goals are data manipulation
data is to be extracted and transformed in to meaning full way.Ex: if u with draw money from ur account that data has to be sent to servers by using software's.
2).Any one can learn a software and can get a job
data is to be extracted and transformed in to meaning full way.Ex: if u with draw money from ur account that data has to be sent to servers by using software's.
2).Any one can learn a software and can get a job
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