Simulation Wrench Tool Stress Analysis

Here we present a simulation of a wrench tool (stress analysis). The combination wrench in this case study is modeled as a linear elastic material and Hooke's law applies. The computation is carried out statically with the geometry created in the CAD software. The inside of the spanner is statically fixed and the ring is given a small angle of rotation. This creates stresses that are computed here (see figure 4).

Although we used a linear elastic material in this case study, the NOGRID software can also compute viscoelastic materials. In contrast to purely elastic bodies, a viscoelastic material has an elastic and a viscous part. The viscosity of a viscoelastic material gives the body a strain rate dependence over time. Purely elastic materials do not dissipate energy when a load is applied and then removed. However, a viscoelastic material dissipates energy when a load is applied and then removed. Viscoelastic behavior has elastic and viscous components modeled as linear combinations of springs and dampers, respectively. Each model differs in the arrangement of these elements. The elastic components can be modeled as springs with elastic constant E given the formula:

σ = Eε

where σ is the stress, E is the elastic modulus of the material, and ε is the strain that occurs under the given stress, similar to Hooke's law. The viscous components can be modeled as dampers, so the relationship between stress and strain rate can be given as:

σ = η dε/dt

where σ is the stress, η is the viscosity of the material, and dε/dt is the time derivative of the strain.

The implemented Maxwell model can be represented by a purely viscous damper and a purely elastic spring connected in series. Under this model, if the material is put under a constant strain, the stresses gradually relax. When a material is put under a constant stress, the strain has two components. First, an elastic component occurs instantaneously, corresponding to the spring, and relaxes immediately upon release of the stress. The second is a viscous component that grows with time as long as the stress is applied.

The implemented Kelvin–Voigt model consists of a Newtonian damper and Hookean elastic spring connected in parallel. This model represents a solid undergoing reversible, viscoelastic strain. Upon application of a constant stress, the material deforms at a decreasing rate, asymptotically approaching the steady-state strain. When the stress is released, the material gradually relaxes to its undeformed state. Without viscous stress (η=0), the material is fully elastic, and only Hooke's law is valid.

In addition, the Generalized Maxwell model, also known as the Wiechert model, is implemented as well and it is the most general form of the linear model for viscoelasticity. It takes into account that the relaxation does not occur at a single time, but at a distribution of times. The Generalized Maxwell model consists of one or more Maxwell elements (viscous damper and elastic spring connected in series), an optional pure viscous and an optional pure elastic element, all assembled in parallel. One special deduction of the Generalized Maxwell model implemented in NOGRID software is the Tool-Narayanaswamy-Moynihan model.

NOGRID points can be used for designing and problem solving for all kinds of stress related tasks.

Capabilities of NOGRID points CFD software

NOGRID combines the capability to handle stress computations for large deformations and various viscoelastic materials, and it allows the simulation of any conceivable geometry and operating mode, such as

• fully 3D computation solving viscoelastic material behavior
• easy and intuitive setup
• freely definable material properties by user equations or user-defined curves
• Structure-Structure Interaction with viscoelastic materials
• Fluid-Structure Interaction using all implemented viscoelastic models 
• computation of moving parts or moving boundaries in combination with viscoelastic materials

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