However, with the development of major primary aeronautical and space composite structures, one may reverse that perspective and consider the hybrid concept of FMLs as the reinforcement of composites by the addition of metallic constituents.
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This thickness increase can be limited further by replacing specific composite plies by metallic layers. The combination with the ductility of metallic layers substantially improves the bearing strength required for joining panels. Take the impact damage tolerance of layered composites. Hence, the presence of delaminations is not or barely visible in composites after impact.
Beyond certain impact energies, residual dents may be visible to aid inspection, but at lower impact energies, these dents may not be observed, while delaminations can occur referred to as nonvisible damage. Here, FMLs have the benefit of ductile metal layers at the outside that leave a residual dent after impact. Experimental studies have demonstrated that delaminations within the laminate are constraint by the dent size. The ductile layers effectively act as sensor to aid visual inspections.
In addition, the plastic deformation together with the local residual stresses introduced significantly retards the propagation of cracks created under impact. Scientifically, the question here remains whether the selected allowable stresses create a stress condition below a fatigue limit or below a damage growth threshold?
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For fatigue in metallic structures, it is known that under certain conditions cracks may nucleate at a notch root but subsequently retard. Similarly, no growth could imply that damage that has been formed still grows but that the growth is limited such that it does not reveal apparent and obvious macroscopic growth within the tested lifetime. This mechanism can be illustrated with a specific case for FMLs. Scientifically, one could study this case further though. This explains the observation of no apparent macroscopic crack growth.
Another example, possibly connected to the previous one, relates to the physical interpretation of fatigue threshold. This appears not always to be obvious. Physically, the fatigue threshold may constitute a minimum amount of strain energy required in the crack tip vicinity to impose a crack increment, analogue to exceeding static friction in moving a box over a floor.
A third case in which the concept of FMLs could be exploited to study fatigue crack growth in metals relates to crack growth under the uniaxial versus biaxial loading. Originally, the Westergaard stress distribution in the crack tip vicinity was developed for a biaxial load condition as occurs in a pressure vessel.
The mean stress levels of the effective stress cycles in both material constituents may be further tuned with changing the residual curing stresses through altering the curing temperatures. Constraining transverse contraction, while loading in the axial direction effectively imposes a transverse load component.
A fourth example relates to the application of geometry correction factors in the equations for SIFs. A physical explanation could not be given for this observation. When the crack grows, the panel compliance increases yielding larger displacements under the application of load P , hence 6 and 7. However, because of the presence of intact bridging fibres, the compliance increase of FML panels is substantially limited to the elongation of only the fibre layers under the load P. The latter can be varied in the applied fatigue load spectra but can also be tuned with the magnitude of residual curing stress through modifying the curing temperature and cure cycle.
In that respect, one may learn that the standard finite width corrections may all become incorrect, once an ambient temperature cycle occurs while fatigue testing a monolithic metallic specimen containing a centre crack.
Depending on the magnitude of the temperature variation, the corrections may be inaccurate by several percent. With all the past research on fatigue damage growth in FMLs, the fracture mechanisms and their interplay are rather well understood.
Academically, the concept of FMLs could be further exploited to develop scientific test methods and procedures to better understand the physics of fatigue fracture in both metals and composites. This has been illustrated in this paper through four examples. Volume 42 , Issue The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.
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If the address matches an existing account you will receive an email with instructions to retrieve your username. Tools Request permission Export citation Add to favorites Track citation. Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Abstract With the introduction of fibre metal laminates FMLs as a fatigue damage tolerant material concept in aeronautics, an interesting field emerged where fatigue damage interaction plays a dominant role.
Figure 1 Open in figure viewer PowerPoint. Illustration of determining the inspection threshold based on fatigue life evaluation for monolithic aluminium and fibre metal laminate FML not to scale ; UL and LL are ultimate load and limit load, respectively 27 [Colour figure can be viewed at wileyonlinelibrary.
Figure 2 Open in figure viewer PowerPoint. Figure 3 Open in figure viewer PowerPoint. Figure 4 Open in figure viewer PowerPoint. Energy release rate calculated from the experiments reported in Vries et al 39 for delamination onset and final failure 38 [Colour figure can be viewed at wileyonlinelibrary.
Figure 5 Open in figure viewer PowerPoint.
Comparison between the crack growth rates predicted for the cases tested in Hooijmeijer 37 with the delamination resistance curve for the interface determined in Alderliesten et al 40 [Colour figure can be viewed at wileyonlinelibrary. Link Either by signing into your account or linking your membership details before your order is placed.
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