Double Cantilever Beam Fracture Specimens Research Articles

Cohesive zone modeling in load – unload situations

In some circumstances, loss of uniqueness of the stress/displacement solution may arise in cohesive modeling, preventing the use of cohesive elements in simulations involving loading and unloading of solids containing cracks. In these cases, it may be erroneously predicted that the damage of cohesive cracks increases when the external forces acting on the solid containing the cracks are reduced due to a prescribed “load reversal”. This issue is illustrated with various practical examples, including a finite element model of a double cantilever beam fracture specimen. Further numerical experiments are presented, that help to explain the causes of the observed phenomenon and shed some light on possible solutions. It is shown that lack of thermodynamic consistency of the cohesive model is not the cause of the observed behavior. The cause of the unphysical response is finally identified in its relationship with features of the nonlinear solution procedure and material stress update routines, by means of the formulation of a custom one-dimensional cohesive element. Two solutions are proposed based on a redesign of the loading/unloading criterion and a custom material model using one of them is presented. It is used as the material definition for Abaqus cohesive elements via UMAT subroutine, and it is shown that it solves the problem found during unloadings for all the considered numerical examples.

Analysis of Fracture in Aluminum Joints Bonded with a Bi-Component Epoxy Adhesive

Abstract Adhesive bonding is a viable alternative to traditional joining systems (e.g., riveting or welding) for a wide class of components belonging to electronic, automotive, and aerospace industries. However, adhesive joints often contain flaws; therefore, the development of such technology requires reliable knowledge of the corresponding fracture properties. In the present paper, the candidate mode I fracture toughness of aluminum/epoxy joints is determined using a double cantilever beam fracture specimen. A proper data reduction scheme for fracture energy calculation has been selected based on the results of a sensitivity analysis. Furthermore, a scanning electron microscope is used in order to explore the locus of failure. Finally, the experimental findings are assessed by means of numerical simulations of crack growth carried out using a cohesive zone model.

Self-healing structural composite materials

A self-healing fiber-reinforced structural polymer matrix composite material is demonstrated. In the composite, a microencapsulated healing agent and a solid chemical catalyst are dispersed within the polymer matrix phase. Healing is triggered by crack propagation through the microcapsules, which then release the healing agent into the crack plane. Subsequent exposure of the healing agent to the chemical catalyst initiates polymerization and bonding of the crack faces. Self-healing (autonomic healing) is demonstrated on width-tapered double cantilever beam fracture specimens in which a mid-plane delamination is introduced and then allowed to heal. Autonomic healing at room temperature yields as much as 45% recovery of virgin interlaminar fracture toughness, while healing at 80 °C increases the recovery to over 80%. The in situ kinetics of healing in structural composites is investigated in comparison to that of neat epoxy resin.

Time Dependent Crack Growth and Loading Rate Effects on Interfacial and Cohesive Fracture of Adhesive Joints

Abstract Double cantilever beam fracture specimens were used to investigate rate dependent failures of model epoxy/steel adhesively bonded systems. Quasi-static tests exhibited time dependent crack growth and the maximum fracture energies consistently decreased with debond length for constant crosshead rate loading. It was also possible to cause debonding to switch between interfacial and cohesive failure modes by simply altering the loading rate. These rate dependent observations were characterized using the concepts of fracture mechanics. The time rate of change of the strain energy release rate, dG/dt, is introduced to model and predict failure properties of different adhesive systems over a range of testing rates. An emphasis is placed on the interfacial failure process and how rate dependent interfacial properties can lead to cohesive failures in the same adhesive system. Specific applications of the resulting model are presented and found to be in good agreement when compared with the experimental data. Finally, a failure envelope is identified which may be useful in predicting whether failures will be interfacial or cohesive depending on the rate of testing for the model adhesive systems.

The Influence of Grain Size on the Toughness of Monolithic Ceramics

Experiments have shown that there may be an optimal grain size which maximizes the toughness of polycrystalline ceramics. In this paper, we attempt to develop a theoretical model which can predict the effect of grain size on the toughness of ceramics. We assume that three principal mechanisms affect the toughness of the material: distributed microcracking; crack trapping by tough grains; and frictional energy dissipation as grains are pulled out in the wake of the crack. The grain size influences these mechanisms in several ways. The energy dissipated due to frictional crack bridging increases with the size of the bridging grains, tending to improve toughness. However, as the grain size increases, the density of microcracks in the solid also increases, which eventually weakens the material. In addition, the level of inter-granular residual stress is also reduced by microcracking, which as a detrimental effect on the toughening due to bridging. We have developed a simple model to quantify these effects. However, the model does not predict the dramatic loss of strength which has been observed to occur beyond a critical grain size. We have therefore proposed an alternative explanation for the apparent decrease in toughness in coarse grained ceramics. Calculations indicate that in a coarse grained material, the main contribution to toughness is due to frictional crack bridging. However, to produce this toughening, the bridging zone must be over 500 grains long. In practice, the length of the bridging zone in a coarse grained solid may be comparable to the dimensions of the specimen used to measure its toughness. Under these conditions, it is not appropriate to use the concept of a geometry independent toughness to characterize the strength of the specimen. We have therefore developed a simple model of a double cantilever beam fracture specimen, which accounts for the effects of large scale bridging. Using this model, we are able to predict the apparent decrease in toughness measured in coarse grained specimens.

The effect of compressive stress on the fracture energy of Sioux quartzite

The fracture energy of Sioux quartzite in air and water measured with the wedge‐loaded double cantilever beam fracture specimen is found to depend on the angle of the wedge used. The measured fracture energy is greater when the wedge is sharper. This is ascribed to the decrease in the compressive stress acting on the specimen. It is proposed that the compressive stress aids in the formation of the microcracks necessary for crack extension. Measurements of the width of the microcrack zone (visible with the scanning electron microscope) adjacent to the primary crack support this interpretation.

An augmented double cantilever beam model for studying crack propagation and arrest

An improved analytical model for the double cantilever beam fracture specimen is developed by treating a finite length beam which is partly free and partly supported by an elastic foundation. The results obtained are shown to be in excellent agreement with established data for initial crack extension. Some preliminary computational results for unstable crack propagation are also presented.