Laminate fracture mechanics

Adhesion, laminates, fracture mechanics, adhesive fracture toughness, polymers, metal substrates, T-peel, fixed arm peel... 

Fracture is caused by higher stresses around flaws or cracks than in the surrounding material. However, fracture mechanics is much more than the study of stress concentration factors. Such factors are useful in determining the influence of relatively large holes in bodies (see Section 6.3, Holes in Laminates), but are not particularly helpful when the body has sharp notches or crack-like flaws. For composite materials, fracture has a new dimension as opposed to homogeneous isotropic materials because of the presence of two or more constituents. Fracture can be a fracture of the individual constituents or a separation of the interface between the constituents. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Leong, K.H.. Herszberg, I. and Bannister, M.K. (1996). An investigation of fracture mechanisms of carbon epoxy laminates subjected to impact and compression-after-impact loading. In Proc. 1st Australasian Congress on Applied Mechanics, Melbourne, pp, 315-320. 

The important case of specimens with a fixed total thickness was considered in Ref. [40]. There are certain features of crack bifurcation under these conditions, such as that if the sample with a fixed total thickness has too large a number of layers there will be no bifurcation. Layer thickness and composition are important and efficient parameters to control the bifurcation in laminates. The effect is comparable with a crack bridging phenomenon [21], The bifurcation mechanism increases the laminate fracture toughness by approximately 1.5-2 times. 

Another aspect to be considered is the difficulty in producing curved structures with the same fibre content as flat laboratory panels. This effect is shown in Figure 16, at the comer the laminate thickness is larger than at the flat section and fibre content is rather lower. This will affect the bending stiffness of the arm and the predicted failure load. This figure also shows the fillet, which is critical to initiation in the specimens without implanted defects. It is well known that fillets can significantly alter the load path in lap shear joints and increase the failure loads (see [1] and Figure 3 for example). If a fracture mechanics approach is to be applied this effect must be considered. Some recent studies on stress intensity factors for such cases may allow this to be addressed [22]. 

Lau, C. C. (1993) A Fracture Mechanics Approach to the Adhesion of Packaging Laminates , Doc. Thesis, Imperial College of Science, UK. 

Laminated Composite, Residual stress, transverse cracking, fracture mechanics... 

Tranj-laminar fracture of composites with a certain amount of fibres in the throughthickness direction will lead to fibre breaking with significant effects on delamination resistance (see e.g. Refs [24,37] for details). Woven fibre mats, 3D fibre performs or additional 3D reinforcement (pins and stitching) of FRP composites with fibres ahgned in one plane (see e.g. Refs [84,85]) have been developed and can be tested for their fracture mechanics properties. The typical approach for testing these is to apply a standard test method developed for unidirectionally reinforced FRP composites and to assess the difference in delamination resistance compared to the standard laminate. So far, that approach has yielded (nominal) numbers, but their interpretation is not... 

Currently available data indicate that 3D reinforcements typically result in high values of Gc compared with the unidirectional laminate made from the same fibre and matrix. Whether the high delamination resistance of 3D FRP laminates does represent material values or apparent values, for example depending on specific test and specimen parameters, is still debated (see e.g. Ref. [92]). More generally, the applicability of linear elastic fracture mechanics to through-thickness reinforced FRP composites is questionable [98]. 

Wimmer G, Schuecker C, Pettermann H. Numerical simulation of delamination in laminated composite components — a combination of a strength criterion and fracture mechanics. Compos Part B Eng 2009 40 158-65. http //dx.doi.0rg/lO.lOl6/ j.compositesb.2008.10.006. 

Vlot AD. GLARE — a history of development of a new aircraft material. Springer 2001. Compston P, Cantwell WJ, Jones C, Jones N. Impact perforation resistance and fracture mechanisms of a thermoplastic based fiber-metal laminate. J Mater Sci Lett 2001 20 597-9. 

Excellent texts on fracture mechanics are available [9-11]. Related topics are failure criteria for the onset of cracking in laminated systems [55], dynamic fracture mechanics [56], and nonlinear fracture mechanics [9,11]. 

Few types of composites such as laminates and polymer concretes in which the non-polymer component predominates the polymeric one are not included in the following collections of data. This is partly also due to that interfacial fracture mechanics approaches being often used to analyse the crack propagation performance of such composites adequately are not subject of this chapter. 

---