Three Dimensional Analysis and Resulting Design Recommendations for Unidirectional and Multidirectional End-Notched Flexure Tests

Abstract

Results are presented from a theoretical investigation of the effects of stacking sequence on the energy release rate in laminated composite end-notched flexure test specimens. Deflections and energy release rates of unidirectional and multidirectional ENF specimens are obtained by classical laminated plate theory, shear deformable plate theory, and three dimensional finite element analyses. It is shown that the distribution of energy release rate varies across the front of an initially straight delamination. The percentage of mode I1 and mode 111 energy release rates for the specimen, as well as the local peak values of the mode 11. mode 111 and total energy release rates that occur at the specimen's free edges are shown to correlate with a nondimensional ratio comprised of the specimen's flexural rigidities. The results of the study are used as a basis for a proposed "ENF test design procedure" that may be used for the determination of appropriate specimen stacking sequences and test geometries for studying delamination growth at interfaces between plies at various orientations. The test design procedure minimizes the contributions to the energy release rate from residual thermal stresses, geometric nonlinearities, local mode I1 concentrations at the specimen's free edges and local mode III effects.

Fracture Mechanics Analyses for Interface Crack Problems - A Review

“…Similar results were also obtained in a related investigation, in which mixed-mode energy release rates were computed for an ENF specimen with the same layup and interface location as the SLB specimen [80,82]. This study indicated that the computed energy release rates, G II and G III , did not exhibit a significant variation when ∆a/h was kept within the suggested range.…”

Simulation and experimental validation of mixed mode delamination in multidirectional CF/PEEK laminates under fatigue loading

“…In order to reduce the inevitable thermally induced residual contractions that occur during cooling after consolidation, coupling stiffness B ij of the chosen laminates is desired to be zero or very close to zero (Daniel andIshai, 1994 or Jones, 1975). The specimen stacking sequence is also chosen to minimize and keep the non-dimensional ratio of the specimens flexural rigidities, D c ; ðD c ¼ D 2 12 =D 11 D 22 Þ smaller than 0.25 in each delamination arm in order to minimize non-uniform toughness value distribution and the errors in the perceived values of critical fracture toughness (Davidson et al, 1995). To obtain a defined delamination according to the standard test method for mixed mode delamination (ASTM D6671, 2002) a 50 mm wide polyimide film (Kapton Ò ) is placed in the mid-plane of each lay-up as a delamination starter.…”

Mode II interlaminar fracture of carbon/epoxy multidirectional laminates

“…For example, anticlastic deformations resulting from bending-bending couplings may generate highly curved delamination fronts and a considerable mode III component at the specimen edges [7,12]. Davidson et al [12] suggested a 0.25 upper bound for the parameter D c ¼ ðD 12 Þ 2 =ðD 11 D 22 Þ, where D ij represent the Classical Lamination Theory bending stiffness coefficients of each specimen arm. On the other hand, bending-twisting and membrane-bending couplings are generally present in 0=h and h/Àh specimens.…”

“…a 1 ¼ À0:1 Â 10 À6 =°C and a 2 ¼ 26 Â 10 À6 =°C, and the temperature variation was 100°C. In the next paragraphs, we present the results for 0/0, 0/45, 0/90 and 45/ )45 specimens, since they represent the limit cases of elastic couplings [12]. Fig.…”