Predicting Composite Failure: First Ply Failure or Progressive Failure?

Engineers need to choose their progressive failure parameters. Image courtesy of MAYA HTT.

Predicting the failure of your composites can be a challenge in the world of simulation. First, what is considered a failure with respect to composites?

“Answering this question is not straightforward. It really depends on your application,” said Elias Ghossein, application engineer and software developer at MAYA Heat Transfer Technologies Ltd.

“Historically, analysts have wanted to know if and where the first ply will fail. Today, if you can afford to be conservative, you stop there. It’s generally the case in the aerospace industry,” he added. “But in other applications, you might look further. You can continue to increase the load and see how the other plies handle it. Progressive failure analysis goes beyond the rupture of the first ply.”

There are benefits and drawbacks to each model and it is up to the engineer to determine which model is best for his or her system.

For instance, prediction of first ply failure is more computationally efficient than progressive failure analysis. “Indeed, progressive failure analysis involves an iterative process and therefore needs more computational resources,” Ghossein said.

Progressive Failure Analysis: How it really works

Progressive failure analysis iterative process. Image courtesy of MAYA HTT.

Progressive failure is an iterative analysis where the first step is to distribute the load between each laminate ply and perform a stress analysis.

“If one or more plies fail, their mechanical properties are degraded and the stress field distribution in the laminate is recomputed,” said Ghossein. “Ply failure is then checked again and the procedure is repeated until no new intact ply fails.”

At this point, the total load is increased and the cycle starts again until all the plies are destroyed. At this point the maximum load is determined.

“Therefore, progressive failure analysis can be viewed as the inner of two [iterative] loops,” explained Ghossein. “In the inner loop, failure is checked for each laminate ply. Once a failure is detected, the properties of the corresponding ply are degraded and the stress field is computed again. In the outer loop, the load is incremented.”

Progressive Failure Models

Composites can fail in a handful of ways. For instance, cohesion between laminates can break down causing an inter-laminar failure such as delamination.

Additionally, intra-laminar failures are possible. These failures happen inside the layer and include matrix breakage, fiber breakage and cohesion failure between the fibres and matrix.

Failure models assess a series of criteria to help engineers determine if any of the aforementioned laminar failures have occurred within the composite simulation.

Intra and inter laminar failures of composites. Image courtesy of Maya HTT.

As Ghossein stated above, there isn’t one failure model that stands out above the others. Each model has its own benefits and drawbacks. The engineer will need to determine which model best fits their materials and application.

Users can choose from the following, classical failure criteria in the literature, such as:

Hill
Hoffman
Tsai-Wu
Max Stress
Max Strain
Puck
LaRC02

It is important to note that a progressive failure model is a combination of two models: the first assesses the failure criterion and the second assesses the degradation of the material’s mechanical properties.

Sudden vs. Gradual Degradation Models

Degradation models state how the mechanical properties of a ply are degraded once it fails. These models comes in two flavors: sudden and gradual degradation.

Simply put, sudden degradation models will set the properties of a ply to zero once it fails, so it can no longer carry any loads. Once that happens, the load is redistributed to the remaining plies.

The gradual degradation model works similarly to the sudden degradation model, except that the mechanical properties of the plies are progressively reduced.

At each degradation step, the load is redistributed and failure is checked again. In this model, the degraded plies can still carry the load, unlike the sudden degradation model.

Depending on the properties of the material, the engineer can choose between sudden or gradual degradation. Furthermore, two plies can be modeled by a gradual degradation model but with different degradation factors. This creates freedom for the engineer to simulate various types of composites, but it also means he will have to intimately understand the ply material properties.

Multi-Continuum Theory Looks into the Fiber and Matrix Stress Fields

Microscopic image of a composite perpendicular to fiber orientation. Image courtesy of MAYA HTT.

One drawback of typical progressive failure analysis is that it treats each laminate ply as a homogeneous material.

Therefore, the fibers are not treated separately from the matrix, despite having different properties.

This approach makes it difficult to determine whether the failure originated within the matrix or the fibers.

Using Multi-Continuum Theory (MCT), however, engineers can glimpse deeper into each ply. “We don’t have information about what happened in the fibers and what happened in the matrix,” said Ghossein. “But using micromechanics and homogenization theory, we can, from the ply stress field, determine the stress field in the matrix and fibers. For that, we will need to know the mechanical properties of the matrix and fibers, as well as the fiber volume fraction, orientation and distribution.

MCT is a way to look inside the ply using micromechanics to gain information on the fibers and the matrix. This method can allow engineers to better understand where failures in the ply might originate.

Predicting Composite Failure in FEA

Ghossein explains that MAYA HTT offer NX Laminate Composites, a laminate modelling add-on within the Siemens NX FEA software. Using this modeller, users can define the thickness, the orientation and the material for each ply and then apply the composite definition to a complex 3D geometry mesh. NX will then write the ABAQUS input file and solve the model.

NX Laminate Composite screenshot. Image courtesy of MAYA HTT.

“If you are using MCT,” Ghossein added, “you will also have to provide the properties of the matrix and fibers, including the fiber orientation and distribution.”

For each ply, users can select a material out of the NX library or create one of their own. In both cases, the user should define a progressive damage model and appropriate degradation factors for each ply.

Different progressive damage models have already been implemented, Ghossein explained. “Some models we implemented are for unidirectional fibers, some are for woven fibers. The users have to choose the suitable model. Each damage model has its own ABAQUS UMAT subroutine. If the user wants something specific, he can develop his UMAT subroutine and add it to the existing library. There are no limitations when creating new user-defined models in the NX/ABAQUS environment.”

Using NX Laminate Composites, engineers will be able to assess the viability of their simple or complex composite designs.

To learn more about composites, you can watch Maya’s webinar about progressive failure analysis here: http://www.mayahtt.com/events/progressive-failure-analysis-in-laminate-composites-webinar

MAYA HTT Ltd. has sponsored this post. They have no editorial input. All opinions are mine. —Shawn Wasserman