Investigating the effects of loading system on the fracture behavior of DCB specimens considering T-stress

Double cantilever beam specimen is a standard specimen for assessment of mode I failure and separation in adhesively bonded joints and also composite materials. Among the several load introduction systems, the piano hinges and end loading blocks are more common. It has been accepted that the fracture toughness results of the two cases are different, but the effect of the loading system on the load-displacement data and fracture mechanisms is not entirely known yet. Therefore, in this study, the two loading concepts are compared both by precise finite element simulations and experimental tests. The adhesive layer is modeled with its own material properties, and the failure of adhesive is investigated by known LEFM procedures. The results reveal that the load block makes the double cantilever beam specimen stiffer and exhibit more non-linear behavior. Moreover, double cantilever beam with the load block system fails in higher loads and lower crack opening displacements compared with the same specimen loaded by the hinges. To study the effect of loading arrangement in more details and including the softening phase, cohesive zone model was utilized. A correction for strain energy release rate based on the parameter T was proposed, and the role of the T-stress on the traction-separation law of the cohesive zone model and the load-displacement behavior were investigated. It was concluded that the T-stress as a crack tip constraint parameter can successfully justify the difference between the two cases. Applying the correction to the traction-separation law of CZM proves the validity of proposed correction in justifying the experimental results.

Mixed-mode fracture evaluation of aerospace grade honeycomb core sandwich specimens using the Double Cantilever Beam–Uneven Bending Moment test method

Fracture testing of aerospace grade honeycomb core sandwich composites is carried out using the Double Cantilever Beam specimen loaded with Uneven Bending Moments, and a Double Cantilever Beam–Uneven Bending Moment test rig capable of applying pure moments is utilized. Specimens with carbon fiber-reinforced plastic face sheets are employed with a range of honeycomb core grades comprising of Nomex® and Kevlar paper. The sandwich specimens are reinforced with steel doublers to reduce excessive rotation of the face sheets. The mode mixity phase angle pertaining to a particular ratio of moments between the two arms of the Double Cantilever Beam specimen is determined using the numerical mode mixity method—Crack Surface Displacement Extrapolation method. For Nomex® honeycomb core sandwich specimens, it is observed that the mode I interface fracture toughness increases with increase in core density. The interface fracture toughnesses for Nomex®-based honeycomb cores are also compared against specimens with Kevlar paper-based honeycomb cores. Crack propagation is observed at the interface just beneath the meniscus layer for the majority of the tested specimen configurations. The Double Cantilever Beam–Uneven Bending Moment test methodology with the concept of direct application of moments on both crack flanks has proven to have a significant potential for mixed mode face/core fracture characterization of aerospace grade sandwich composites.

Displacement-controlled crack growth in double cantilever beam specimen: A comparative study of different models

This paper presents, discusses, and compares different techniques to model fracture initiation and static crack growth in double cantilever beam specimen under displacement-controlled loading. Energy release rate, critical displacement for the onset of crack growth, and critical load were determined by analytical solution, standard, and extended finite element method. The crack growth was also examined, and the advantages of each method were described as well. In addition, the compliance technique was used in the analytical method. In this regard, the crack growth relations were formulated based on four models including simple Euler–Bernoulli model, Euler–Bernoulli on the elastic foundation, simple Timoshenko beam, and the beam on the elastic foundation considering shear effects. Closed-form relations were extracted for the fracture parameters. Afterward, the Abaqus software was utilized to simulate the crack growth by the standard finite element method. Since the extended finite element has the ability to model the discontinuities inside the elements, the problem was also simulated by this method. Cohesive fracture of double cantilever beam specimen was performed using a closed-form solution and using a finite element model. Results of different modeling techniques were determined and compared.

Experimental Study On Fracture Property Of Double Cantilever Beam Specimen With Aluminum Foam

This study aims to investigate double cantilever beam specimen with aluminum foam bonded by spray adhesive to investigate the fracture strength of the adhesive joint experimentally. The fracture energy at opening mode is calculated by the formulae of British Engineering Standard (BS 7991) and International Standard (ISO 11343). For the static experiment, four types of specimens with the heights (h) of 25 mm, 30 mm, 35 mm and 40 mm are manufactured and the experimental results are compared with each other. As the height becomes greater, the fracture energy becomes higher. After the length of crack reaches 150 mm, the fracture energy of the specimen (h=35 mm) is greater than that of the specimen (h=40 mm). Fatigue test is also performed with DCB test specimen. As the height decreases, the fracture energy becomes higher. By the result obtained from this study, aluminum foam with adhesive joint can be applied to actual composite structure and its fracture property can possibly be anticipated.