Nonlinear FEA Structural Analysis

The main goal of structural FEA analysis is to determine the response of a system to some form of external or internal stimuli. Finite element technology is a numerical method that allows the user to approximate this response. The quality of the response strongly depends upon the approach the user decides to take in modeling the system.

The structural response of all engineering materials to loading is nonlinear, the scope of the nonlinearity depends upon the material and geometry of the situation. For most metals, when the loading is sufficiently low, a linear response is often assumed by the user and the  material behavior is modeled using a constant tensile modulus.

 If the resulting deformation or the system is sufficiently small, the use of linear or small displacement approximation can be effectively used.

However, when studying the physical response of materials like plastic, the assumption of linearity must be carefully evaluated. Failure to do so may result the false prediction of failure resulting in wasted product over design or abandonment of the project all together.

MCS has the tools, experience and expertise in the method of performing nonlinear analyses of all sorts of materials. As previously stated, CAE is just a tool, the proper usage requires that the user understand the mechanical nature of the material used within the system and must recognize the proper way of simulating this behavior. After 23 years of working with polymers, MCS understands the relationship between the rheological ramifications of manufacturing to the structural performance of the molded material. 

The following image is from a project performed by MCS in order to determine the cause of failure of a new industrial power tool. The end user originally contracted a large design house to design the tool using CAE software.

The design house created the design and passed the design models to their flow and FEA structural experts. The flow expert following what he had no doubt learned from basic Moldflow training placed the gating into an area that would insure that both ends of the housing fill at the same point in time therefore minimizing the potential for product  overpack and molded in residual stresses. The FEA expert looked up the tensile modulus book values for the resin used and performed a linear structural analysis. The end result, the tool failed. 

The failure of the tool was the direct result of experts failure to look at the big pitcher. As stated before within this document, the act of injection molding dramatically alters the material behavior of the molded product. Failure to recognize this fact has results in the failure of many products, this housing is just one of many projects that MCS has been brought in for postmortem analyses.

The product was molded using a glass filled nylon material, which is extremely anisotropic.

The flow expert recommended that the gating be placed into the trigger opening, this resulted in fiber orientation running perpendicular to the principal stresses at the weakest section of the housing.

 The FEA expert ignored the fact that the book value for the tensile modulus does not represent the true behavior of the resin at the strain levels predicted within his own analyses let alone that the isotropic single value of tensile modulus does not represent the material's behavior other then with flow direction.

MCS performed flow simulations placing the gating into the end of the tool, this resulted in fiber orientation parallel to the main principal stress within the weakest section, thereby maximizing the strength of that section.

MCS then created an anisotropic material model for the housing using information processed from the flow simulations. This anisotropic model was then used to better model the actual material performance throughout the housing. 

The end result was that the tool was modified based upon MCS recommendations and met design performance specifications.

The images represents one set of stress results using the anisotropic housing model, each element within the model was assigned orthotropic properties based upon local flow conditions within that element. The analysis represented below accurately predicted the location of the points of failure missed by the original linear simulations.

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