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If you previously purchased this article, Log in to Readcube. Log out of Readcube. Click on an option below to access. Log out of ReadCube. A brief overview of this technology is provided in this article, addressing most significant issues including design, choice of materials, repair implementation, and, for repairs to primary structure, certification. 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. If the address matches an existing account you will receive an email with instructions to retrieve your username.
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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. Get access to the full version of this article. View access options below. You previously purchased this article through ReadCube. Institutional Login. Log in to Wiley Online Library. The size of the crack tip yielding zone can be estimated by the Irwin approach, where the elastic stress analysis is used to estimate the elastic plastic boundary.
If this plastic zone is small compared to the crack size, then the linear elastic assumptions are correct. The plate, without patch, was meshed using standard six node-isoparametric elements with triangular shape, as showed in Fig.
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The strain energy release rate is calculated using the modified crack closure method and then the stress intensity factor from the Eq. The values are reported in the Table 5. The numerical values are in good agreement with the analytical ones but they are always higher. The finite element model of the half plate geometry: detail of crack tip in pure mode I The Fig.
Analytical and numerical evaluation of the SIF vs. The crack grows up in a stable way until the SIF reaches the fracture toughness, i. But the rate of the crack growth increases more rapidly when the critical condition of fracture is reached. In order to model the contribution of the static fracture the Eq. The number of cycles to failure is based on the Kmax criterion, i. Fatigue life prediction of the cracked un-patched panel The Fig. The details of the experimental measurements are reported in Section 5.
The higher experimental crack growth rate at the number of cycles close to the failure is due to an asymmetric growth of the crack experienced during the tests. Panel with patch 4. EA 8. FM73 8. AFK 9. EA FM73 AFK Analytical and numerical SIF for the repaired plate in Test 3 4. The patched plate is meshed using standard two-dimensional six node isoparametric elements with triangular shape. The repaired structure is modeled as three layer structure plate, patch and adhesive. Due to the symmetry of the problem, only half plate is modeled using nodes and elements.
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Tables from 7. The configuration with the adhesive AFK usually demonstrates the highest difference between numerical and analytical values. This performance is due to improved atomic bond of the glue. The adhesive AFK has the lowest elastic and shear moduli. The patch type 2 always provides a higher SIF and among these configurations those using the adhesive AF 2K are the highest. Experimental procedure 5.
The tests are performed at room temperature in a kN Metrocom Engineering SpA servo-hydraulic test machine.
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Each test is performed up to the complete fracture of the panel. In each test the number of the cycles, the crack growth and the SIF are evaluated. In particular the SIF is measured at the crack tip using three measurement grids strain-gauges. A National Instrument data acquisition system is used to acquire the load and strain measurements during the tests. In the configuration A4 reported in the Table 8, the surface is subjected to a treatment of pickling sulfur chromium for aluminum alloys according to UNI EN - Aerospace Division.
Finite element model of the plate repaired with the composite patch 5. The configurations B1 e B2 are characterized by an higher load amplitudes.
Moreover for the configuration B1 the patch is bonded using the technique of mechanical compression; on the contrary for the configuration B2 it is bonded using the vacuum bag technique. Therefore the results of the configurations B1 and B2 are presented separately in order to show the scatter of the experimental results, summarized in the Tables 9 and The configurations A1 showed the best performances in terms of stress level and fatigue life.
In particular the configuration A4 provides a fatigue life comparable with configuration A1 although the measured SIF for A4 is the highest among the configurations A. In Fig. In other configurations the crack propagates in the adhesive layer and the patch slid over the panel surface as showed in Fig. The configuration A4: the crack propagated through the composite patch Fig. The crack propagated in the adhesive layer that leads to the sliding of the full patch The configurations B demonstrate again the role of the curing procedure and a small variation in the SIF provided a larger difference in the fatigue life.
Therefore the same numerical value is reported in the Tables 9 and The comparison of the SIF shows a good agreement between experimental and numerical results. On the contrary, the numerical results, in general, overestimated the experimental fatigue life. The small differences in terms of SIF values lead to a larger overestimation of the fatigue life. The curing procedure of the patch on the metallic panel introduces random factors that cannot be easily accounted for in the finite element model reducing its predictive capabilities.
Final considerations and conclusions The main purpose of this study has been the determination of the effectiveness of bonded composite patches to repair cracked thin aluminum panels. The repair is realized by patching only one side of the panel in order to reduce the associated costs and time required. In most of the cases, in fact, the other side of the panels is not easily accessible.
Moreover, this type of repair might be applied locally without the need of disassembling a complex structure. Several experimental tests are carried out to obtain a set of data useful to estimate the state of stress and the fatigue behavior of cracked and bonded repaired panels and to define the effectiveness of the repairing patch. The comparison of the data obtained from the cracked and patch repaired panels have pointed out a significative reduction of the stress intensity factor at the crack tips in the repaired configurations.
To support the experimental activities and reduce the associated costs, theoretical and numerical models have been developed and the mechanisms of the damage propagation by using fracture mechanics and fatigue strength analyzed. The capabilities of numerical and analytical methods are compared with a set of experimental results. The numerical and experimental results show that the bonded patch highly reduces the stress intensity factor and increases the fatigue life.
Moreover, it has been shown that the effectiveness of the repair strongly depends on the patch stiffness, adhesive characteristics, surface treatments and in general curing procedure. The effectiveness of the composite patches is shown in the next results. The finite element model takes into account a progressive patch cracking as far as the crack grows up in the plate, even though the effect of the patch, including the SIF growth rate, might be different as explained in the Section 5.
When the crack tips overtake the edges of the patch, the crack growth rate quickly increases as showed in Fig. In this figure the crack length is plotted as function of the number of loading cycles for the patched and unpatched aluminum cracked plate used in the experiments Test 2.
For the patched panel, it is evident as the growth rate increases when the crack becomes larger than the patch width. Table 13 summarizes the fatigue lifes for various specimen configurations. The bonded patch can significantly increase the fatigue life of the cracked panel, depending on the stiffness of the patch, the adhesive characteristics, the surface treatments and, in general, the procedure used to apply the patch. Comparison of the fatigue life of the panel with and without patch 70 Without Patch With Patch 60 50 Crack Length mm 40 30 20 10 0 0 Number of cycles Fig.
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Comparison the crack length vs. References Baker A. Swenson D. Version 1. Handbook of the stress analysis of cracks, Paris Productions Incorporated, St.
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