On The Prediction of Induced Residual Stresses in Fibre Metal Laminates

Residual stresses induced via a net difference in the coefficient of thermal expansion of the constituent materials in fibre metal laminates (FMLs) were investigated. These stresses are a consequence of the curing process and are invariably present in all FML variants. Two analytical approaches were employed, the first being an idealized case in which the S-2-glass, fibrous composite prepreg layers were reduced to isotropic conditions; the second introduced full orthotropic material properties for the glass layers. Following this, finite element (FE) analyses were conducted on a GLARE2-2/1-0.3 variant, employing the same orthotropic conditions for the glass layers. Detailed, layer by layer models were generated and included isotropic strain hardening in the aluminum layers. To model delamination of the cohesive interfaces, a mixed mode, quadratic nominal strain damage initiation criterion, with a Benzeggagh-Kenane (B-K) criterion for damage evolution and exponential softening was also included. Finally, a Hashin based damage initiation criterion with linear damage evolution for the composite layers was also enforced. A distinct analysis step was implemented into the models to develop the residual stress state as a function of a predefined temperature field with a net change equal to 100 degrees C - typical for FML fabrication. The results of the idealized analytical calculations produced a net tensile residual stress of 32.83 MPa in the aluminum layers with a corresponding compressive residual stress of -78.79 MPa in the glass. layers. Similarly, the orthotropic analysis predicts stress values of 26.59 MPa and -63.84 MPa, respectively. The FE analyses revealed a 27.6 MPa, net tensile residual stress in the aluminum layers coupled with a -65.81 MPa compressive stress in the glass layers. A comparison of these results yielded a relative error of 16% for the idealized case and 3% for the orthotropic case. In addition, all predicted results are nearly tantamount to empirically measured values in the literature and the FE analyses suggest that there is a near constancy of induced residual stresses though the thickness of the aluminum layers, though their non-zero values are significant and must be considered in designs employing FML structures.