Vorhersage der Ermüdungslebensdauer und Test-CAE-Korrelation

Strain-Life (EN) for low-cycle fatigue

The Strain-Life method is applicable to a wide range of problems including low-cycle fatigue with the local elastic-plastic strain controls the fatigue life. The standard EN method uses the Coffin-Manson-Basquin formula, defining the relationship between strain amplitude εª and the number of cycles to failure N^f. Material models can also be defined using general look-up curves. This enables the ability to interpolate multiple material data curves for factors such as mean stress or temperature.

Predicting endurance limit

Dang Van is a multi-axial fatigue limit criterion and is a method of predicting the endurance limit under complex loading situations. The output from the analysis is expressed as a safety factor rather than fatigue life.

• Uses specific material parameters calculated from tensile and torsion tests.
• Manufacturing effects can be accounted for by using equivalent plastic strain in the unloaded component.
• Can take into account the cut edge effect on thin sheet material
• Includes new optimised and spot weld methods.

Calculating stress-based factors of safety

Safety Factor enables the calculation of stress based factors of safety. This method is widely used as a key design criteria for engine and powertrain components like crankshafts, camshafts and pistons.

• Inputs are linear stress or strain for this SN based technique.
• Material inputs are standard mean stress corrections or user-specified Haigh diagrams to assess durability.
• Stresses from a complete finite element model are analyzed in a single analysis process.

Fatigue analysis of spot welds in thin sheets

The Spot Weld option enables the fatigue analysis of spot welds in thin sheets. The approach is based on the LBF method (see SAE paper 950711) and is well-suited to vehicle structure applications.

• Spot welds are modelled by:
  • Stiff beam elements (e.g., NASTRAN CBAR), as supported by many leading FE pre-processors
  • Supports CWELD and ACM formulations using solid element representation
• Cross sectional forces and moments are used to calculate structural stress around the edge of the weld
• Life calculations are made around spot weld at multiple angle increments and the total life reported includes the worst case
• Python scripting enables modelling of other joining methods such as rivets or bolts

Improve accuracy of thick weld analysis

The methods used in the WholeLife option improves the accuracy of analysis of thick welds. It uses an integrated approach for modeling fatigue over the entire lifetime of a component - from the very early stages to final fracture - to give more accurate determination of weld lives particularly for complex geometries. The same structural stress technique used for seam welds is used in WholeLife to determine the structural bending and membrane stresses at the weld.

WholeLife uses the through thickness stress distribution for the geometry and can include the effect of a known residual stress profile. Although this is primarily a CAE based analysis, the same method may also be applied to measured stress data.

High temperature fatigue and creep

The Thermo-Mechanical Fatigue (TMF) option provides solvers for high temperature fatigue and creep by using stress and temperature results from finite element simulations. Mechanical loads that vary at a different rate to the temperature variations can also be combined. Applications include components that are both mechanically and thermally loaded such as vehicle exhaust systems and manifolds.

Stress-life fatigue of anisotropic materials

The Short Fibre Composite option uses stress-life fatigue calculations for anisotropic materials such as glass fibre filled thermoplastics. The stress tensor for each layer and section integration point through the thickness is read by DesignLife from FE results. The material orientation tensor describing the “fibre share” at each calculation point is provided by mapping a manufacturing simulation to the finite element model. This orientation tensor can be read from the FE-results file or supplied from an ASCII file.

Short Fibre Composite module features: 

• Simulate complex loading scenarios using any time domain method (static or modal superposition, duty cycles, etc.)
• Simulate vibration tests driven by random (PSD), swept sine, sine dwell or sine-on-random loading
• Predict damage and life per layer and integration point
• Incorporate results of manufacturing simulation including fibre orientation tensors or residual stresses
• Model local fatigue properties based on microstructure (orientation tensor) and stress state
• Calculate fatigue based on principal stresses or critical plane — including stresses calculated from FE-Digimat and multiaxial stress states
• Choice of fatigue property model - SN curve interpolation or interface to Digimat
• Use of homogenized matrix or fibre stresses as well as typical composite ones

Calculate composite static failure criteria

The Composite Analysis option allows users to evaluate the strength of a structure against industry standard composite failure criteria. Rather than limiting this evaluation to a small number of load cases or steps, stresses can be assessed by using the chosen failure criteria throughout realistic duty cycles (quasi-static or dynamic). This allows critical locations, load combinations and associated design reserve factors to be readily identified. In addition, selected location loading paths may be visually compared with the material failure envelope.

The following methods can be used individually or combined to give the most conservative result:

Durability calculations on adhesive joints

nCode DesignLife uses a fracture mechanics-based method to assess which joints in the structure are most critically loaded. The Adhesive Bonds option enables durability calculations on adhesive joints in metallic structures. 

• Adhesive bonds are modeled with beam elements and grid point forces are used to determine line forces and moments at the edge of the glued flange.
• Approximate calculations of the strain energy release rate are made at the edge of the adhesive and, by comparison to the crack growth threshold, a safety factor is calculated.
• The theoretical basis of the method was developed by the Volvo Group and the testing and software implementation was carried out as part of a collaborative research project with partners including Jaguar Land Rover, Coventry University and Warwick University.

Distributing jobs on remote or clustered computers

Distributed Processing enables a DesignLife analysis running in batch mode to be distributed across multiple computers or nodes of a computer cluster. 

• Uses MPI standard that is common in high performance computing (HPC) environments so that even the largest of finite element simulations can be completed efficiently.
• Enables you to rapidly solve jobs by using the combined processors of many machines.
• Includes a batch interface program to simplify the running of distributed jobs.