The material failure strains were assumed to be the tensile strength ( Table 7.9) divided by Young's modulus. If any of these criteria were exceeded, subcell failure was activated.
The maximum stress, maximum strain, and Tsai-Hill failure criteria were all active for each subcell. The predicted laminate tensile strength is 306 MPa, compared with a measured value of 310 MPa (Zenkert and Burman, 2011). Several additional failure events occur within the laminate before a large event that causes a significant decrease in stress and stiffness signals failure of the panel. Because the simulation was performed in strain control (i.e., the axial midplane strain component on laminate was monotonically increased, while all force and moment resultants were kept at zero), each loss of stiffness results in a decrease in stress, followed by continued loading with a reduced slope. Damage, in the form of subcell failures, initiates at 112 MPa. The GMC RUC shown in Figure 7.43 was used for each ply, along with the subcell elimination method (see Chapter 2). The results of a progressive failure analysis of the tensile response of the s 42% E-glass/vinylester laminate (described in Zenkert and Burman, 2011) is shown in Figure 7.42. To predict the fatigue life of the foam sandwich beam, the static and cyclic failure behavior of the facesheet laminates must first be understood. The inability to mold the long foam rails to a precise length would likely result in resin flow variations due to the changes in the fit of the preform in the molding tool. At higher production volumes the long cycle times for the foam core production would require multiple tools adding a substantial cost penalty to the process. The dimensional variability of the large foam cores and the long cycle time required for molding (estimated at 20 min for this core) were identified as significant technical and cost issues. As shown in Figure 16, the attachment point reinforcements were added to the foam during the core assembly and bonded into the proper location. This allowed some limited adjustment to achieve a consistent fit to the preform and tool. Following molding in closed epoxy tools, the foam sections were assembled with a fixture-using adhesive. The foam was molded in three sections in an effort to minimize the effect of shrinkage and distortion.
The foam core required for the center area of the preform was manufactured from a water blown 128 kg m −3 urethane.