Int J Fract https://doi.org/10.1007/s10704-019-00388-4 ORIGINAL PAPER Multi-directional composite laminates: fatigue delamination propagation in mode I—a comparison Leslie Banks-Sills · Ido Simon · Tomer Chocron Received: 23 June 2019 / Accepted: 20 August 2019 © Springer Nature B.V. 2019 Abstract Double cantilever beam (DCB) specimens composed of carbon fiber reinforced polymer lami- nate composites were tested. Two material systems were investigated. One consisted of plies from a woven prepreg alternating with tows in the 0 ◦ /90 ◦ -directions and the +45 ◦ / − 45 ◦ -directions. The second was fab- ricated by means of a wet-layup process with the same multi-directions as the prepreg. In addition, for the second material system, a unidirectional (UD) fabric ply was added. The delamination for this laminate was between the UD fabric and the woven ply with tows in the +45 ◦ / −45 ◦ -directions. Both fracture resistance R- curve and fatigue delamination propagation tests were carried out. It is found that the initiation value of the interface energy release rate is substantially lower for the wet-layup; whereas, their steady state values are quite similar. The fatigue delamination propagation tests were performed at various cyclic R-ratios. The delamination propagation rate da/d N was calculated from the experimental data and plotted using a modi- fied Paris equation with different functions of the mode I energy release rate. As expected, the da/d N curves L. Banks-Sills (B ) · I. Simon · T. Chocron Dreszer Fracture Mechanics Laboratory, School of Mechanical Engineering, Tel Aviv University, 6997801 Ramat Aviv, Israel e-mail: banks@tau.ac.il I. Simon e-mail: idosimon@gmail.com T. Chocron e-mail: tomerchocron@mail.tau.ac.il depend upon the R-ratio. By using another parameter based on the Hartman–Schijve equation for metals, it is possible to obtain a master-curve for all R-ratios. It is seen that the propagation rate for the prepreg is faster than that of the wet-layup. Keywords Constant amplitude · Fatigue delamination growth rate · Fiber reinforced composites · Fracture toughness · Resistance curve · R-ratio 1 Introduction Composite laminates are being used extensively in aerospace, ship building, medical and sport applica- tions. One of the main disadvantages of these mate- rials is their sensitivity to delamination; that is the separation of adjacent plies (Bolotin 1996; Raju and O’Brien 2008). This is particulary observed for multi- directional (MD) laminates in which adjacent plies have fibers in different directions. There has been much work in characterizing the fracture and fatigue delamination propagation behavior of such materials (Gustafson and Hojo 1987; Hojo et al. 1987, 1994; Sela and Ishai 1989; Hashemi et al. 1990; Martin and Murri 1990; Atodaria et al. 1999; Asp et al. 2001; Mar- tin 2003; Tay 2003; Andersons et al. 2004; Matsub- ara et al. 2006; Shindo et al. 2006; Argüelles et al. 2008; Shivakumar et al. 2006; Peng et al. 2011; Rans et al. 2011; Jones et al. 2012, 2014, 2015, 2016, 2017; Shahverdi et al. 2012; Banks-Sills et al. 2013; Pas- 123