Tensile failure in Low Density Foams

When modeling low-density foams, it is important to consider potential failure under tensile loading. In LS-DYNA, the most popular constitutive material model is MAT_LOW_DENSITY_FOAM (MAT_057), in which the default treatment of foams under tensile loading is linear with no failure. The Elastic Modulus (Emax) in tension is computed to be the max( max(slope of the […]

Mat224 Keyword Manual Pages

Several of you requested the manual pages for the newly developed tabulated Johnson-Cook constitutive law now labeled as MAT_224. Here are the three pages of the keyword manual. MAT224 All Pages [ Click the image to enlarge] MAT224 Page 1 [ Click the image to enlarge] MAT224 Page 2 [ Click the image to enlarge] […]

Shell Local Material Axis Definition for Orthotropic Material

Most orthotropic material rely on the parameter AOPT to define the change/update the default material axes defined by LS-DYNA. The default axis for every shell element is based on the order of the node numbers if INN=0 which is then based on N1-N2 and the cross-product of N1-N2 and N1-N4. The default material axes may […]

LS-DYNA Material Selector

Finding a good material law can sometimes be a daunting task even for experienced users. To facilitate a quick review of possible material laws for a given material criteria, a LS-DYNA Material Law Browser was created for personal use and I hope its useful for others. Please send any feedback you may have at info […]

Mass scaling for MAT_SPOTWELD elements

Deformable spotwelds modeled using beams or solids elements can invariably limit the global timestep due to its small dimensions. LS-DYNA provides two methods to scale the spotweld timestep. The first method is the well-known global mass-scaling using DT2MS in CONTROL_TIMSESTEP. When DT2MS is non-zero (usually a negative number), the density of the spotwelds are modified […]

Modified Crushable Foam

LS-DYNA now offers a modified version of *MAT_CRUSHABLE_FOAM in the form of *MAT_MODIFIED_CRUSHABLE_FOAM . In the new material model, the strain rate effects can be included in the foam a *DEFINE_TABLE . To reduce or eliminate the noise in the evaulation of the new yield stress as a function of strain and strain-rate, LS-DYNA offers […]

Unloading Behavior in MAT_083

There are three different ways to model unloading behavior when using material model MAT_083. They are graphically depicted below (these figures may appear in the next release of the LS-DYNA keyword and theory manual). Option 1 – Default (Click image to enlarge) In the default option (1), HU=0, and the table is positive which then […]

Stress Relaxation in Viscoelastic Material Models

LS-DYNA allows several ways to model of stress relaxation often seen in viscoelastic materials when subjected to a sudden constant strain. The methods to model the stress relaxation is briefly discussed here. 1. Curve Input When using MAT_GENERAL_VISCOELASTIC material model, one can directly input the time log dependence of the relaxation modulus using LCID parameter. […]

Modeling Off-Axis Dependent Yield Stress for Honeycombs using MAT_126

Honeycomb cell structures are popular among weight conscious designers due to their high strength to weight ratio. In the automotive space, aluminum based honeycomb structures are widely used to represent barriers to simulate a controlled energy absorption. Honeycomb structures are highly anisotropic and requires adequate testing to characterize them. This post focusses on the characterization […]

Curve Extrapolation

Curves are used widely to define a XY data that are used by several entities in LS-DYNA. They help us to define either a time-dependent function, used in loads and boundary conditions, and/or a strain/strain-rate dependent function used frequently in constitutive models. Three most salient features of curve representation, that occur internally in LS-DYNA and […]

Limitations of Using LCSR for Strain-Rate Inclusion in MAT_024

The widely popular material model *MAT_PIECEWISE_LINEAR_PLASTICITY (MAT_024) supports various methods to include the strain-rate effects. One of the methods is the scaling of the quasi-static stress-strain curves using a load-curve, LCSR, that defines a scale-factor as a function of strain-rate. This method works by first finding the yield-stress, SIG_QUASI_STATIC, as a function of the effective […]

Modeling Loading and Unloading Behavior in Seatbelt Materials

Seatbelt constitutive model, invoked by using *MAT_SEATBELT, in LS-DYNA provides features to model the loading and unloading characteristics from a uni-axial test. Parameter LLCID provides ability to model the loading curve which allows the definition of force as a function of engineering strain. Parameter ULCID, provides ability to model the unloading curve again allowing the […]

Identifying Necking in Metals and Plastics

When characterizing materials such as Metals and Plastics in LS-DYNA, most constitutive models provide a yield criteria that accounts for a 3D state of stress which reduces to a uniaxial yield stress in 1D. This allows us to directly input the true stress-strain curve from a one-dimensional state of stress testing such as in uniaxial […]

Modeling Symmetric/Unsymmetric NonLinear Discrete Springs

Discrete springs provide a easy way to model complicated systems by using their responses in the material definitions. This post brings attention to the way LS-DYNA handles the default behavior in tension or compress when the material input does not pass through the origin (0,0) but simply begins from origin. When only one of either […]