JEC COMPOSITES MAGAZINE - Issue #72 - April/May 2012 - 60

damage
Plan B is needed to complete Plan A. - Plan A results in a laminate optimized for pristine properties. For damagetolerant structures, pristine laminates are not in the design space. Plan B results in a laminate that will be optimized for damage tolerance. In reference to the second item mentioned as a drawback to the original approach, by using a blanket knockdown factor a part can be under-designed. Suppose a quasi-isotropic laminate is used to establish CAI allowables for a given material system and establish knockdown factors. Now suppose the lay-up is changed to obtain more strength and stiffness in the 0° direction by replacing the -45° plies with 0° plies. Actual material data generated at MSFC is presented in Figure 2.

Damage tolerance
Fig. 1: Two methods to generate strength allowables for damage-tolerant composite design

fibre waviness in the laminate near the honeycomb core. These sandwich-level strength allowables would then have to be further reduced to strength allowables with a certain level of damage. Since the structure being designed experienced compressive loads an order of magnitude greater than any other form of loading, this made the only data of interest the CAI strength allowable of the sandwich structure that the interstage was comprised of. The Damage Tolerance team developed this data early in the programme. By following the method proposed in this article, many of the intermediate steps in the original (and more traditional) method of determining strength allowables could be eliminated. The general flow of this scenario is shown in Figure 1 with “Plan A” indicating the original plan and “Plan B” indicating the work of the Damage Tolerance team. It should be noted that this figure is based on the methodology of the ARES I composite interstage programme and is very general in nature. Some key points from Figure 1 are: - Both plans end with the same data. - Plan A will take more time and expense. In addition, data from

According to the damage tolerance testing on the quasi-isotropic lay-up, the knockdown factor due to the prescribed level of impact is 52%. Thus, throughout the programme, a 52% knockdown factor would be imposed upon the pristine material compression strength values to account for impact damage. Adding a higher percentage of 0° plies will increase the pristine compression strength from 684 MPa to 846 MPa, an increase of 24%. Applying the damage tolerance knockdown gives a damage tolerance compression strength value of 406 MPa. However, the higher stiffness in the 0° direction leads to a higher stress concentration factor and in actuality, the CAI strength of the laminate with 50% 0° plies is slightly lower (323 MPa) than that of the quasi-isotropic laminate. The 24% increase in damaged laminate strength as dictated by applying a knockdown

Fig. 2. Pristine and compression after impact data for two lay-ups of T40-800/5276-1

60 jec composites magazine / No72 April - May 2012

JEC COMPOSITES MAGAZINE - Issue #72 - April/May 2012

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