Fatigue crack growth in epoxy polymer nanocomposites

Abstract

The present paper describes detailed analyses of experimental data for the cyclic-fatigue behaviour of epoxy nanocomposite polymers. It has been shown that the data may be interpreted using the Hartman–Schijve relationship to yield a unique, ‘master’, linear relationship for each epoxy nanocomposite polymer. By fitting the experimental data to the Hartman–Schijve relationship, two key materials parameters may be deduced: (i) the term A , which may be thought of as the fatigue equivalent to the quasi-static value of the fracture energy, G c , and (ii) the fatigue threshold value, Δ G thr , below which no significant fatigue crack growth (FCG) occurs. It has then been established that the values of these parameters, together with the slope, n , and intercept, D , of the Hartman–Schijve master relationship, may be used (i) to compute the experimental results measured for the fatigue behaviour of the epoxy nanocomposite polymers, (ii) to understand the observed fracture and fatigue behaviour of these materials with respect to the structure of the epoxy nanocomposite polymers, and (iii) to deduce the ‘upper-bound’, i.e. ‘worst-case’, FCG rate curve which may be used by industry as a material development, material selection, design and service-life prediction tool when these epoxy nanocomposite polymers are used in engineering applications such as structural adhesives and/or as matrices in fibre-reinforced composites. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

Crack growth in adhesives: Similitude and the Hartman-Schijve equation

“…Notwithstanding this intriguing finding, that the da/dN versus Δκ master-relationships obtained for the 'FM94' adhesive, the 'FM73' adhesive and the automotive adhesive are all very similar, it should be noted that previous work [15] has shown that when a far wider range of formulations of toughened epoxy polymers is studied the overall picture is more complex, i.e. not all adhesives have similar Hartman-Schijve master-relationships.…”

“…This solution revealed that the near-tip stress field for rectilinearly orthotropic composites was indeed uniquely described by √𝐺𝐺. Thus, as explained in [9][10][11][12][13][14][15], the logical and corresponding extension of the Paris crack growth equation to FCG in adhesives, composites or nanocomposites, is to express da/dN as a function of ∆√𝐺𝐺, or of �𝐺𝐺 𝑚𝑚𝑚𝑚𝑚𝑚 ; and not of ∆𝐺𝐺, nor of 𝐺𝐺 𝑚𝑚𝑚𝑚𝑚𝑚 . An additional reason for using the square root of the energy release rate is that general graphical description of the data between using ∆√𝐺𝐺 and ∆𝐾𝐾 is homomorphic.…”

“…The realisation that the logical extension of the Paris Law equation to FCG in adhesives, composites and nanocomposites is to express da/dN as a function of ∆√𝐺𝐺 , or �𝐺𝐺 𝑚𝑚𝑚𝑚𝑚𝑚 , subsequently led to the development of the Hartman-Schijve crack growth equation [11][12][13][14][15][16][17] in the form given below in Equations ( 4) and ( 5) below. These equations, which are a variant of the NASGRO equation [18,19], are given by:…”

On the Link between Plastic Wake Induced Crack Closure and the Fatigue Threshold

“…Indeed, variants of this approach have also been shown to be able to model delamination growth in composites as well as cohesive crack growth in adhesives and nanocomposites, see [31][32][33][34][35][36][37].…”

Effects of Dynamic Disulfide Bonds on Mechanical Behavior in Glassy Epoxy Thermosets

“…Glassy thermoset polymers have been demonstrated as highly useful materials, especially in structural, adhesive, and coatings applications, due to their robust mechanical properties, dimensional stability, and corrosion resistance. − Their ability to serve as matrices for multifunctional composites also expands their design space into an even greater diversity of applications; − however, the high cross-linking density, which can impart strength, typically inflicts the materials with brittleness and decreased toughness and prevents them from being effectively reprocessed or recycled. ,,, Some of this loss in toughness can be mitigated through extrinsic toughening mechanisms like the addition of core–shell rubber particles, copolymerization with thermoplastics, , or nano-/micro-scale structural heterogeneity, − but, ideally, a neat, homogeneous thermoset matrix can be engineered to maintain its desirable properties without significant loss of toughness or reprocessability.…”