<p>Sandwich beams are highly susceptible to debonding at the interface between face and core. These debonds can grow and eventually lead to complete failure of the structure. To analyze this, there is a need for a simple approach to obtain the energy release rate and mode partitioning. A comprehensive Elastic Foundation Analysis (EFA) that includes the deformation of the substrate part can be used to obtain analytical expressions for fracture parameters. The sandwich beam is first divided into two parts along its thickness direction - the debonded face sheet and the substrate (core and the bottom face sheet). In Mode - II/III fracture, the effects of crack face contact are significant. The proposed model extends EFA by introducing a tensionless spring foundation in the cracked region. This is a novel approach, where tensionless springs are used to capture the compressive stresses across the interface between the debonded face sheet and the substrate. The absence of tensile stresses in the foundation is because when there is tension the debonded face sheet lifts away from the substrate. To study these effects, an End Notched Flexure sandwich specimen is used. It is a widely used test specimen for mode II fracture. It is a simply supported beam with a shear load at the middle of the beam. The ENF sandwich beam is divided into 4 regions along its length. Region I &amp; II form the cracked portion of the beam and Region III &amp; IV together form the bonded portion of the beam. For an ENF specimen, it has been shown in experiments that there is a significant contact zone near the end support in the cracked section.&nbsp;&nbsp;Region I is the contact zone and is modeled with tensionless spring foundation. Here the length of the contact zone is unknown. Region II is the non-contact region in the cracked portion of the beam. Region III &amp; IV have the elastic foundation to incorporate the effects of the substrate part on the top debonded face sheet. Governing equations for the beam in all the regions are obtained based on the Euler-Bernoulli beam theory. Appropriate boundary and continuity conditions are used to obtain a system of nonlinear equations. It is then solved using a numerical code to obtain the general solutions for transverse displacements in the debonded part and the substrate. Expressions for the energy release rates are obtained using the J-Integral and solutions for the mode partitioning are obtained using the axial and transverse displacements at the crack tip. The results are compared with a finite element model, in which the crack is pre-meshed with singular elements. In the cracked portion of the beam, a frictionless surface-to-surface contact is introduced. Results obtained from the tensionless foundation approach are compared with results from the finite element model. The estimation of energy release rate values including the effects of crack face contact are consistently better than the values obtained with no crack face contact. This approach also estimates the size of the contact zone near the end support and the mode partitioning measure values are in good agreement with the exact mode mixity phase angles from the finite element solution.</p>

Crack Face Contact Modeling Using Tensionless Foundation for Face/core Debonds in Sandwich Beams

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.