Fracture Mechanics Analysis of Composite Microcracking: Experimental Results in Fatigue

A recent variational mechanics analysis gives the energy release rate due to the formation of a new microcrack between two exisiting microcracks (John A. Nairn, J. Comp. Mat., 23, 1106 (1989)). This analysis has been useful in providing fracture mechanics interpretation of matrix microcracking in cross-ply laminates. This paper describes using the new energy relase rate analysis for a fracture mechanics based interpretation of microcrack formation during fatigue loading. Fatigue experiments were run of three layups of Avimid © K Polymer/IM6 laminates and on four layups of Fiberite 934/T300 laminates. A modifed Paris-law was used and the data from all layups of a single material system were found to fall on a single master Paris-law plot. We claim that the master Paris-law plot gives a complete characterization of a given material system’s resistance to microcrack formation during fatigue laoding. INTRODUCTION Many observations have confirmed that the initiation of damage in multidirectional laminates is often by microcracks in the off-axis plies that run parallel to the fibers in those plies [1–8]. These microcracks have typically been studied in cross-ply laminates in which the cracks form in the 90◦ plies [1–8]. Microcracks form during static testing [1–8], during fatigue testing [3,9,10], and during thermal cycling [11]. Because microcracks cause a reduction in stiffness [3], a change in the thermal expansion coefficient [12,13], and provide sites for the initiation of delaminations, it is important to gain a quantitative understand of the formation and propagation of microcracks during both monotonic loading (static tests) and during cyclic loading (fatigue tests). Some attempts at analysis of microcracking have been based on ply strength theories [2,14]. As pointed out by Flaggs and Kural [5], however, strength based theories are fundamentally inappropriate and most recent work has been based on energy release rate calculations [4,6,7,15–17]. Early energy release rate analyses were based on the shear-lag model [4,6,7,15] or on shear-lag type assumptions [16]. These types of analyses are probably too qualitative to be useful. A more recent energy release rate analysis [17] uses the improved stress analysis technique developed by Hashin [18,19]. The improved stress analysis is based on variational mechanics principles and has been shown to accurately predict stiffness reduction [18,19]. In Ref. [17] the variational approach was modifed to include thermal stresses and used to calculate the energy release rate due to the formation of microcracks. The new energy release rate analysis has been sucessuful in predicting the microcrack density as a function of applied load during static testing in a variety of composite material systems [17,20]. In this paper, we make further use of the new fracture analysis described in Ref. [17] to give a fracture mechanics interpretation of the propagation of microcracks during fatigue testing. The fracture mechanics interpretation is based on a modifed Paris-law approach. † Graduate Student and Associate Professor, respectively, Materials Science and Engineering Department, University of Utah, Salt Lake City, Utah 84112