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What’s So Big about Progressive Failure Analysis? July 2011

Composite Analysis Tips

Using Progressive Failure to Determine More Accurate Laminate Strengths

When designing with composite laminates, the engineer has many tough questions facing them. Among them: When will the composite laminate really fail? How do I determine this?

A traditional approach used by many to predict the strength of a laminate is to determine the “first-ply- failure” (FPF). This is done by taking the loads applied to the laminate and, using Classical Laminate Theory (CLT), determining the stresses in each ply of the laminate. Once the stresses in each ply is known, failure of that ply is determined based upon the tested material properties (strength) of the individual ply. With FPF, the laminate is assumed to fail when the first ply fails. Is this approach a good estimate of failure? Let’s take a look...

Is First-Ply-Failure Good Enough?

Here, FPF is used to determine the strength of a typical composite laminate and see how it compares to test data.

Using ply data for a unidirectional carbon/epoxy lamina (AS4/8552) and a [45/0/-45/90]2s layup, a First-Ply-Failure is determined in the 0° direction at 56.6 ksi.

In comparison, experimental data from material testing at the National Institute for Aviation Research (NIAR) laboratory indicates a 0° direction tensile strength of this laminate to be 88.6 ksi (56% above the FPF value!):

Using an FPF approach resulted in underestimating laminate strength by over 50%. Such an under prediction, resulting in overdesigned composite laminates, just doesn’t cut it for engineers trying to minimize part weight. First-ply-failure is often just too conservative when it comes to predicting laminate strengths.

Looking Beyond First Ply Failure

So what is an engineer to do? There is another way!

A more thorough method would take into consideration the continued load-carrying capability of the surrounding plies in a laminate beyond failure of the first ply. This is done by allowing the weakest plies in a particular direction to fail first and redistributing loads into the adjacent plies - in turn causing those plies to fail and so on until the composite laminate is severely damaged and can no longer support an increase in loading. This method is referred to as progressive ply failure (PPF).

Let’s see how this method works for predicting laminate strength...

Using the same lamina data and layup, a progressive failure analysis is run in Helius:CompositePro resulting in the following stress-strain curve:

What can be seen from the progressive failure analysis curve is that FPF does affect the composite laminate, but load can continue to accumulate until an ultimate failure of 80.0 ksi is reached. At this load value, a large load reduction is seen and the composite laminate can be considered to have failed.

Helius:CompositePro’s Progressive Failure analysis can also show the order in which each ply in the layup fails until there is absolutely no load carrying capability left:

With an under prediction of only 10%, a progressive failure analysis comes much closer to predicting the laminate strength and gives the engineer the insight and confidence to pursue a lighter, more efficient design.

Helius:CompositePro’s Laminate module includes several laminate analyisis features, including First Ply Failure, Progressive Failure and Failure Envelope.

View a demo of the Progressive Failure feature available in Helius:CompositePro.

To learn more about the many composite analysis features available in Helius:CompositePro, visit www.firehole.com/compositepro/ or email compositepro@firehole.com.