A comparison of total fatigue life models for composite laminates

Abstract

A B S T R A C TA total fatigue life model proposed previously by the same authors for laminated composite structures subjected to mode-I fracture loading is evaluated by two newer models. The new models differ from the original model by normalization factor and approximation of stable delamination growth rate equation. All three models include the delamination growth in subcritical (delamination initiation), linear (stable growth) and final (unstable growth) fracture domains. All three models were evaluated by comparing the total life predictions for two different block loadings. One is a normal operation of a rotor crafttype structure and the other is an aggressive loading. Predicted results of the three models were compared with each other and with the experimental data. The material considered was woven-roving E-glass fibre and vinyl ester matrix and the laminate was made by vacuum assisted resin transfer molding.Keywords delamination growth rate; fatigue; glass/vinyl ester; resistance curve. N O M E N C L A T U R Ea = delamination length da/dN = delamination growth rate G C = critical energy release rate G IC = opening Mode I interlaminar fracture toughness G Imax = maximum cyclic mode I energy release rate G IR = opening Mode I interlaminar fracture toughness resistance G Ith = threshold value of mode I energy release rate P Imax = maximum mode I cyclic load R = ratio of minimum to maximum cyclic displacement δ Imin = minimum value of cyclic displacement δ Imax = maximum value of cyclic displacement = delamination length correction factor for modified beam theory I N T R O D U C T I O NSusceptibility to delamination is a major weakness of composite laminates. Knowledge of material's resistance to interlaminar fracture and fatigue is essential to establish design-allowable and damage-tolerance guidelines for structures. Fracture mechanics-based delamination growth models are required to predict fatigue life and establish suitable inspection intervals so that a delamination can be found and repaired long before it becomes critical or when the applied stress exceeds the residual strength of the component. Fatigue delamination growth laws that Correspondence: Kunigal Shivakumar. cover the threshold, the stable growth and the unstable fracture domains are needed for total life estimation. Such growth laws were proposed in the past for metallic materials 1,2 and are now becoming accepted in damage-tolerant designs. Similar methodology is needed for composite laminates. Hypothetically, we can assume that the delamination growth rate has three domains, namely, subcritical (slow growth or delamination initiation), linear (stable growth) and unstable growth rate domains (see Fig. 1).The growth rate depends on microscope details of fibre architectures and resin properties in domain 1; on crack driving force (energy release rate G or G) in domain 2; and on the fracture characteristics in the unstable or the third domain. In composite laminates substantial research 3-13

Methods for the prediction of fatigue delamination growth in composites and adhesive bonds – A critical review

“…This approach was also adopted by Shivakumar, Chen, and co-workers [116,117] and Zhang, Peng, and co-workers [118,119]. The resistance to delamination growth is caused by a number of different mechanisms, such as: "matrix cracking and fiber bridging in the case of unidirectional composites; tow cracking, multiple delaminations, tow bridging, and tow breaking in the case of woven/braided fiber composites [116]."…”

“…Abdel Wahab et al [144] also used a relation similar to equation 37 to find the fatigue life of adhesive bonds, but included a mode-mix dependence. Shivakumar, Chen, and co-workers [116,117] have proposed a number of variations of equation 37, making use of the length dependent delamination resistance G IR as proposed by Poursartip [115]. This model was applied to a T700 carbon / vinyl ester composite [145].…”

A novel interpretation of fatigue delamination growth behavior in CFRP multidirectional laminates

“…However, an obvious R-curve exhibiting the relationship between the fracture toughness and delamination length shows that the fracture toughness was actually variable [4,23]. Shivakumar et al [24], Chen et al [25] and Murri [26] proposed that G should be normalized by Gc(a) in the Paris relation. They supposed that the quasi-static delamination curve presented the changing resistance against delamination growth, which accounted for the fiber bridging during mode I fatigue loading, and they further used the curve to determine the variable fracture toughness Gc(a).…”

A novel understanding of the normalized fatigue delamination model for composite multidirectional laminates

“…Normalized similitude parameters including G max /G c [11,12] and G max /G c (a) [13,14] are used, where G c and G c (a) denote the constant fracture toughness and the R-curve of the fracture toughness, respectively. However, the intrinsic physical mechanism of this normalization method remains unclear.…”