Toughness of composites

It is critical that surface treatment conditions be optimized to composite properties since overtreatment as well as undertreatment will degrade composite properties. Typically composite interlaminar shear strength (ILSS), in-plane shear, and transverse tension ate used to assess the effectiveness of surface treatment. More recently damage tolerance properties such as edge delamination strength, open hole compression, and compression after impact have become more important in evaluating the toughness of composite parts. 

Tsai, H.C., Arocho, A.M. and Cause, L.W. (1990). Prediction of fiber-matrix interphase properties and their influence on interface stress, displacement and fracture toughness of composite materials. Mater. Sci. Eng. A126, 295-304. 

O Brien, T.K. and Martin, R.H. (1992). Results of ASTM round robin testing for mode I interlaminar fracture toughness of composite materials, NASA TM 104222. 

Williams, J.G., Davies, P. and Brunner, A.J. (1995). Standard tests for the toughness of composite laminates-some bones of contention. In Proceedings of Intern. Conf. on Composite Materials (ICCM-10), Whistler, Canada. Vol. I, pp. 71-75. 

Cooper, G.A. (1970) The fracture toughness of composites reinforced with weakened fibers. J. Mater. Sci. 5, 645-654. 

Further details of the above techniques to improve the transverse fracture toughness of composites by controlled interfaces of various nature and modifying materials are discussed in the following sections. The effectiveness of these toughening methods on transverse fracture toughness and strength in relation to the controls are summarized in Table 7.1. 

Vf, fiber volume fraction neat resin fracture toughness mode 1 interlaminar fracture toughness of composite G),., mode II interlaminar fracture toughness of composite. 

Hirschbuehler, K.R. (1987). A comparison of several mechanical tests used to evaluate the toughness of composites. In Toughened Composites, ASTM STP 937 (N.J. Johnston ed.), ASTM, Philadelphia, PA, pp. 61-73. 

The evaluations of short fiber reinforced composites for fracture behavior includes studies with respect to material parameters such as the fiber content [42,103,120], fiber length and orientation [69,97,103,115], and fiber bundling [115.120] testing conditions such as the temperature [24,42,121,122] and the loading rate [121,123,124] and fractography [42,43,103,123], The influence of some of these parameters, e.g fiber content, rate of loading, and the test temperature on the fracture toughness of composites can be presented in the form of property maps [125,126],... 

Andersons J, Konig M. Dependence of liactute toughness of composite laminates on interface ply orientations and delamination growth direction. Compos Sci Technol 2004 64 2139—52. http //dx.doi.org/10.1016fj.compscitech.2004.03.007. 

Schwartz HS, Hartness JT, Effect of fibre coatings on interlaminar fracture toughness of composites, Johnston NJ ed., ASTM STP 937, Philadelphia, ASTM, 150-178, 1987. 

Unlike the case of toughening binary blends, where the primary variables controlling the fracture resistance are elastomer volume fraction and the size and distribution of elastomer inclusions (26-28), the toughness of composites with EILs is extremely sensitive to the thickness and elastic moduli of the interphase (6,7,31). A lower limit of strain energy release rate, Gc (or fracture toughness. 

Many matrix choices are available, and each type has an impact on the processing techniques, physical and mechanical properties, and enviromnental resistance of the finished part. Thermoplastic and thermoset materials can be resin matrices. Thermoplastic matrices have been developed to increase hot/wet use temperature and the fracture toughness of composites. Thermosetting resins, however, are more common. The common thermoset matrices for composites include polyester and vinyl esters, epoxy, bismaleimide, polyimide, and cyanate ester and phenolic triazine resins. 

Fig. 5.49 Toughness of composites as a function of total zirconia content [31], With kind permission of Elsevier...

These processes gradually decrease the strength and fracture toughness of composite elements. The rate of the processes is a function of environmental conditions humidity, temperature, and fibre and matrix properties that is, fibre sensitivity to corrosion and pH of the matrix. The main problem consists of identifying appropriate ways of prevention, which are ... 

If the breaking strain of the fibre is above a critical value, which depends upon the nature of both the fibre and the matrix, the impact strength and toughness of composite structures based on brittle fibres... 

Fracture toughness depends on matrix and fiber properties, fiber orientations, and stacking sequence. The emphasis on thermoplastic composites by the U.S. Air Force has been directed toward increasing the fracture toughness of composites. 

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