A single leg bending test for interfacial fracture toughness determination

Abstract

A single leg bending test is described and its suitability for interfacial fracture toughness testing is evaluated. The test specimen consists of a beam-type geometry comprised of two materials, one 'top' and one 'bottom,' with a split at one end along the bimaterial interface. A portion of the bottom material in the cracked section of the beam is removed and the geometry is loaded in three-point bending. Thus, the reaction force of the support at the cracked end is transmitted only into the material comprising the top portion of the beam. The test is analyzed by a crack tip element analysis and the resulting expressions for energy release rate and mode mixity are verified by comparison with finite element results. It is shown that, by varying the thicknesses of the two materials, the single leg bending test can be used to determine the fracture toughness of'most bimaterial interfaces over a reasonably wide range of mode mixities. I n t r o d u c t i o nInterfacial fracture is a problem of significant importance with wide-ranging practical applications. A few examples where interfacial fracture is of concern are delamination of laminated materials, adhesive failure of bonded structural assemblies, decohesion of coated surfaces, and fiber/matrix debonding in advanced composites. However, despite the large body of theoretical work that has been performed on this subject (for example, see the reviews in [1] and [2]), experimental investigations are, in many respects, still in their early stages.For those cases where the singular field is oscillatory, the slow progress of experimental work is likely due to the difficulty of designing a specimen and load fixture that:( 1 ) can accommodate a wide variety of material pairs, (2) can produce a wide range of mode mixities, (3) are both relatively easy and inexpensive to manufacture, and (4) preferably do not involve an excessively laborious analytical or numerical pi'ocedure to extract results.As reviewed in recent papers [1-4], a number of excellent test methods have been developed, however, none of these approaches fully satisfy all of the above 'requirements'. For those cases where the singular field is non-oscillatory, the development of test methods has primarily been driven by the need to characterize the delamination toughness of laminated materials. The double cantilever beam [5,6] and end-notched flexure tests [7,8] are wellestablished for use with these materials under pure mode I and mode II conditions, respectively. A number of mixed-mode tests have been used over the years, e.g. [9]; the relatively recent mixed-mode bending (MMB) test is perhaps the most widely accepted of these methods [10, 1 1]. However, by comparison with the results of [12], the MMB test will necessitate geometrically nonlinear analyses for most bimaterial specimen geometries.

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

“…(A.14) to zero yields the normalised crosshead displacements [5,6]; b) cracked lap shear (CLS) [7]; c) un-symmetric double cantilever beam (U-DCB) [8]; d) single leg bend (SLB) [9]; e) four point bending (FPB) [10][11][12][13][14]; (f) single leg four point bending (SLFPB) [15]; (g) over-leg bending (OLB) [16]. …”

Fracture Mechanics Analyses for Interface Crack Problems - A Review

“…The SLB specimen was designed for mixed-mode fracture toughness testing and exhibits a nearly constant mode ratio of G II /G T =0.4 (40% mode II) [79]. A specimen with multidirectional layup was analyzed, where the delaminated interface was located between a +30˚ and -30˚ ply [80,81].…”

“…The proposed mixed-mode fracture criterion is similar to the criterion shown in Figure 1. The Single Leg Bending (SLB) specimen, for mixed-mode testing at approximately constant 40% mode II was also introduced [79]. The methodology provides a straightforward practical solution for engineering applications.…”

On the calculation of energy release rates for cracked laminates with residual stresses

“…A particularly simple example, because the two arms are homogeneous and residual stresses do not cause contact (at least in beam theory approximations), is interface fracture in the single-leg bending specimen 17 as illustrated in Fig. 8.…”

“…[17] that ignored residual stresses. In the absence of mechanical loads and substituting the two-layer results in Appendix A for α (3) κ and C (3) κ , the energy release rate for spontaneous adhesive failure is G = 1 2…”