2008 Int ANSYS Conf Trailer Hitch System

© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary

2008 International ANSYS Conference

Fatigue analysis and design optimization of a trailer hitch system

Erke Wang, Rainer RauchCADFEM GmbHGermany



ContentDescription of customer needs

Some remarks on the fatigue assessment of welded parts

Workbench work flow to analyze welded structures

Fatigue analysis and comparison to test results

Return on Invest

DXplorer study based on parameterized DM-model

Summary


Customer needs• Every part has to pass a fatigue test with 2.E6 cycles• Testing time approximately 4 days• Increased complexity of component – removable trailer hitch• Increased loading due to increasing performance of towing vehicle (SUV)

Tests facility


Location of failure


German SUVs


VW Touareg 850 Nm BMW X6 300 kW (407 HP)

Audi Q7 368 kW (500 HP)Porsche Cayenne 368 kW (500 HP)


Stress in welded structures

What stress component can we use as a reference?

Measure and calculate stresses at locations adjacent to the weld

define reference weld geometries

Geometry of weld cannot be modeled in detail


Material properties vary over weld cross section



Approaches to assess welded structures welded structures

• Nominal stress approach

• Structural hot spot stress approach

• Notch stress approach

• Linear elastic fracture mechanics

Not valid for the approval of automotive components

During the product development cycle, the following approaches are widely used to assess the welded structures life cycle, depending on the nature of the problem, the appreciated approach will be used.


Nominal stress approach:

Structural hot spot approach:

Measure and calculate stresses atlocations adjacent to the weld

Notch stress approach:

define reference weld geometries


Way too conservative – useful for structures like cranes

AFS /=

sSSREF DD= *



Structural hot spot approach: stresses can be calculated using shell or solid elements

Comparison of allowable stress component for welded detail


According to the design detail, there are N° of welded details in catalogue available(IIW-Recommendations), which describe where the stress could be measured.



MPaFAT 6363 ∂Δ≡

Approaches to assess welded structures:

According to the catalogue, one reference SN curve can be chosen to compare with calculated stress results.




• There are several approaches to assess welded structures

• Nominal stress approach

• Structural hot spot stress approach

• Notch stress approach

• Linear elastic fracture mechanics Not valid for the approval of automotive components

Way too conservative – useful for structures like cranes

N° of welded details in catalogue (IIW-Recommendations) is limited



Approaches to assess welded structures:Notch stress approach – R1MS - Radius 1 Max Stress (acc. to IIW-Recommendations)

Geometric detail of the weld seam

Pros and Cons:

+ concept for suitable for the FE-process

+ no structural detail list necessary

+ straightforward in result interpretation

- high number of elements

IIW-Recommendations FE Model

Radius=1mm




„Fatigue assessment of a welded component in a construction machinery based on different methods” J. Martinsson, J. Samuelsson IIW DOCUMENT No. XIII-1905-01

Notch stress approach

• With this approach, the FE model must be modeled in weld region with very fine mesh,

• this leaded to a very large FE model. • Because of the hardware performance

and software efficiency issues, this approach was not widely used in product development process

Accuracy


Geometric detail of the weld seam

IIW-Recommendations FE Model


Notch stress approach – an economic way based on ANSYS Workbench technology



A short revive on the submodel - technique

Global modelPhysical Model Submodel

1.

1. Generate Submodel (Design Modeler!)

2.

2. Transfer Displacements as Boundary Conditions on Submodel



Submodel examplesGlobal model without welded detailShell element

Submodel with welded detailSolid element

Global model

Local model 1

Local model 3

Local model 2



Submodel examples


Submodel

Global model

Submodel with different mesh


Work flow1. Analyze global structure using a

uniform element size

2. Identify hot spots

3. Prepare Submodel -geometry feasible for the R1MS-assessment

4. Calculate local results with different Submodels

5. Use Workbench Fatigue for Life time estimation



(1) Analyze global model – (2) identify potentially critical location

/copy,file,rst,,..\voll,rst save,file,db /copy, file,db,,..\voll,db

critical location



(3) Prepare Submodel(s) acc. to IIW-Recommendations

/prep7 cmsel,s,CUTnwrite,cut,nodeallssave fini /clear /post1 resu,..\voll,db /post1 file,..\voll,rst set,last alls cbdo,cut,node,,cbdo,cbdo fini resu /prep7 /inp,cbdo,cbdo alls /solu



Check results for all defined submodelsN° of cycles estimated

N° of cycles in test

Test 1 600.000 790.000Test 2 600.000 880.000

N° of cycles estimated


Test 1 870.000 1.3 Mill.Test 2 870.000 1.1 Mill.

N° of cycles estimated


Test 1 > 2 Mill. @ 2 Mill. test stop

Test 2 > 2 Mill. 1.75 Mill.

Model 1

Model 2

Model 3



Estimated life at maximum load amplitude (realistic test scenario?)

Reviewing results with ANSYS Fatigue (Model 1)

log [N]

log Δδ[MPa]

2E6

225



Reviewing results with ANSYS Fatigue

log [N]

log Δδ[MPa]

2E6

225

Estimated life at a varying load amplitude (more realistic test scenario)



ROI for the analyzed structures



Do the simulations pay off?

Cost for prototype: 400 – 1.200.- Euro

Test rig 3 days: 2.500.- Euro

Preparation/Docu: 500.- Euro

Cost per test: 3.400 – 4.200.- Euro

TECS Professional: 4.100.- Euro

Engineer: 60.000.- EuroCost per year: 64.100.- Euro

Preventing 16 unsucessful tests each year pays off all cost for simulation!



Do the simulations pay off?

Time to analyze a complete trailer hitch system: 1 – 5 days

Uncounted numbers:

- Costs for re-design (CAD, PDM, Customer discussions)

- Delay in project

- Reputation and acknowledge technical know how

- Knowledge about the product



Knowledge about the product – or how to optimize products



Parameterize geometry – generate dependent parameters to guarantee feasible models during optimization run



Define Boundary Conditions and depedent parameters in welded detail



Check sensitivity of geometry parameters on loading of the critical welds

Select pipe diameter and wall thickness (integers)

Check all relevant stresses and displacements

Review sensitivities



DXplorer – Simulation Driven Design

Freeze design by direct use of simulation results

Extended product knowledge


Simulation as applied Science to replace the try-and-errorprocess


Accuracy

Summary• ANSYS Workbench delivers the possibility to assess

even complicated welded details with local models.

• The submodel approach is extremely efficient – all documentation is done via the structured tree.

• ANSYS Fatigue Module provides the opportunity to account for fatigue effects like mean stresses and load history.

• With the ANSYS DesignXplorer design decisions are based on knowledge not on presumptions.



Thanks!


2008 Int ANSYS Conf Trailer Hitch System

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