7.1 Intact Strength

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Like most pages in CADEC Chapters, Strength pages can be used as a powerful calculator. That means you can change the values of the objects that were loaded to study their effect on the results but the modified values are not saved on the objects. We are talking about the values of loads, environment, partial safety factors, and damage factor $d_ f$.

In the Intact Material page, the damage factor is fixed to zero, because that is what intact means: no damage. In the Degraded Material page, you load the damage factor from the Laminate object, and you can change it locally to whatever you want.

There are 3 things to load:

Laminate. Is an object that contains the definition of a laminate, as explained in My Laminates. Although the complete definition of the Laminate used by CADEC is loaded, the only values shown on the page are the two partial safety factors $\alpha ,\phi $, and the degradation factor $d_ f$, which were entered when the laminate was defined in My Laminates.

Shell Load. Is an object that contains a set of membrane, flexural, and transverse shear loads per unit length, also called stress resultants, as explained in My Shell Loads. The values appear under the headings Membrane Forces, Moments, and Transverse Shear Force.

Environment. Is an object that contains a temperature deviation and a moisture intake, as explained in My Environments.

Then, you select the Failure Criterion that you want to use. There is a lot to say about this choice, as discussed in the textbook and elsewhere.

In-situ effect should always be checked–for accurate results, but CADEC provides the option to calculate without in-situ effect. Why? Mainly for you to be able to reproduce other people’s results, which most likely do not account for in-situ effect. If you know of any software that accounts for in-situ effect, please let me know! Popular software such as Abaqus$^{\rm TM}$ 6.13 and ANSYS$^{\rm R}$ 14.0 do not provide this feature.

The results are shown towards the bottom of the page. Just above the ToExcel button you see the overall first ply failure (FPF) result. If all the loads and environment are multiplied by FPF, one lamina will fail, or at lest it is expected to fail according to the failure criterion chosen. Many times, different criteria yield different values of FPF.

You can see what lamina fails by looking at the table and identifying the lamina with the lowest strength ratio R. By the way, the lowest R is the FPF. In this example, all the laminas at 90 deg. fail simultaneously when the loads are increased by a factor of 38.34.

The strength ratio is like the FPF of a lamina, while the FPF is a value for the laminate. This is better explained in the textbook. In the table, k is the lamina number. The computations are done twice, at top and bottom of each lamina. There is a reason for this. Computing (or visualizing in FEA) anything at the center of a lamina does not make sense. The stresses and strains are largest at the interfaces between laminas.

You can find out the mode of failure by looking at the icons on the last column on the table. In this case all the laminas at 90 deg. fail simultaneously with transverse failure mode. You can find out the name of the mode by hovering the cursor over the icon until the tooltip appears. Try it out on a live session of CADEC !

If the 90 deg. laminas would not fail, the 0 deg. laminas would fail with R=258.8, but obviously this will not happen because the 90 deg fail at a lower load.

When the 90 deg laminas fail, it does not imply that the laminate fails. In fact, the 0 deg laminas can carry the load. Computation of the load multiplier that would break the remaining laminas is tantamount to finding the Last Ply Failure (LPF). This is done on the next page by first degrading the laminate.