Fracture Energy Model Research Articles

Fracture in quasi-brittle materials: a review of continuum damage-based approaches

Starting from the early ideas on smeared-crack modelling of quasi-brittle materials (concrete, rock) as pioneered in the late 1960s various refinements are discussed such as the shear retention factor, the tension-stiffening concept and the tension-softening concept. Next, smeared-crack models are put in the unified context of damage mechanics together with the multiple-smeared crack approach and the (closely) related microplane damage models. The concept of fracture energy is introduced for tensile and compressive loadings and is also elaborated for reinforced concrete. A concise summary is given of recent finite element concepts for cohesive-zone models (fracture energy models).

Influence of the adhesion force crystal/heat exchanger surface on fouling mitigation

The accumulation of unwanted crystalline deposits (fouling) reduces the efficiency of heat exchangers considerably. In order to decrease the cost of fouling two strategies have been developed. The first fouling mitigation strategy is based on the modification of energy and geometry related characteristics of the heat transfer surface to realize an increased duration of the induction period. By means of a DSA (drop shape analysis) measurement device the interaction at the interface crystal/heat transfer surface is determined. The deployment of the fracture energy model and the interfacial defect model relates wetting characteristics to the adhesion phenomenon. Hence, a first estimation of the optimal choice of surface material is realized. Furthermore, the influence of surface topography on interfacial interactions has been analyzed. The second fouling mitigation strategy is based on the adjustment of the hydrodynamic flow conditions using a pulsation technique. Here, single strokes of higher velocity are superimposed on the stationary flow. These strokes shift the equilibrium of forces to an improved removal process. Fouling experiments have proved that pulsation is a powerful tool to mitigate the built-up of fouling layers on heat transfer surfaces.

Prediction of Cracking Response of Reinforced Concrete Tensile Members

This paper presents the results of an experimental and analytical study .that ~as c~ed out. to improve the understanding of the response of cracked concrete from a fracture mechanlc~ p~mt of ~Iew. Rem­ forced concrete panels made of normal- and high-strength concrete were tested under umaxlal tensIOn. A pre­ viously proposed fracture energy model was modified to predict the obtained tensile response of both normal­ and high-strength concrete panels. Steady crack gro~ was des.cribed with energy eq~ilibriu~ during cracking. A new method is proposed to approximate the effective comphance for a member With multiple cracks. For a given cracking stage, the model can predict concrete and steel strains and entire load-deformation curves. The proposed fracture energy model is also capable of predicting the minimum reinforcement ratio of normal- and high-strength concrete tensile members of different sizes.

Fracture Modeling of Concrete and Steel

A new relationship between the fracture energy of concrete, the tensile strength, and the ultimate crack width has been developed by considering the postcracking strength or softening of concrete. Linear, power, and exponential mathematical models were used to describe the descending branch of the stress-strain diagram. A nonlinear finite-element model was developed, using smeared cracks for a layered model approach. This facilitated the representation of the reinforced concrete and composite (steel and concrete) members that were analyzed to verify the modeling of the materials and a range of structures. The predictions of the load-deflection behavior of a reinforced concrete slab and a continuous composite beam in the elastic and inelastic range incorporated the fracture energy model, and the correlation between the theoretical and experimental results was very good. In a final evaluation, the fracture criteria of concrete and metals have been compared, and excellent correlation has been established bet...

New model to predict static strength of tapered laminates

Tapered laminates have wide applications in engineering structures. However, the problem of predicting static strength accurately has still not been satisfactorily resolved. Many models are available but all have limitations. In this paper, a new model called the variable fracture energy model is proposed. This model is established based on a comprehensive experimental and analytical study on constant thickness specimens with the central plies cut across the complete width. Many experimental results on tapered specimens indicate that this model can be used to predict the static strength of tapered laminates satisfactorily.

Interfacial modification of Nb/MoSi 2 composites and its effects on fracture toughness

Niobium-reinforced MoSi 2 composites have shown a large improvement in fracture toughness over the MoSi 2 matrix. However, the chemical incompatibility of niobium with MoSi 2 has to be solved for high-temperature structural applications of the composite. In addition, the effects of interfacial coatings on the fracture toughness of ductile-phase-reinforced composites need to be investigated to find the optimum toughening effect of niobium reinforcement. In the present study different oxide coatings, Al 2O 3 and ZrO 2, were applied to niobium reinforcement and effectiveness of the coatings as diffusion barriers was evaluated. The mechanical behavior and the fractographic characteristics of constrained niobium were also examined. Finally, the effect of the coatings on fracture toughness of the composites was studied and compared with predictions based upon tensile tests on a single constrained niobium reinforcement. The results are discussed in terms of the interfacial fracture energy and micromechanical models of ductile-phase-reinforced composites.

Giant vesicle bilayers composed of mixtures of lipids, cholesterol and polypeptides. Thermomechanical and (mutual) adherence properties.

Micromechanical tests of giant vesicle bilayer elasticity and bilayer–bilayer adhesivity have been carried out on vesicles made from mixtures of lipids, cholesterol and polypeptides. Mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) exhibited ideal solution behaviour over a temperature range that covered both liquid-to-gel phase transitions. Addition of cholesterol (CHOL) to a saturated chain lecithin (DMPC) reduced, broadened and shifted to higher temperature the main crystalline-to-liquid acyl chain transition. Cholesterol greatly reduced the membrane area compressibility and increased membrane cohesion to levels exhibited by frozen acyl chain bilayers, but maintained the bilayer in a liquid state. Addition of amphiphilic polypeptides to PC and PC–CHOL mixtures slightly increased bilayer compressibility at high temperatures; when the temperature was lowered, bilayer compressibility for DMPC–CHOL–peptide mixtures was greatly reduced to below that of the single component lipid. Cholesterol appeared to change from an association with the lipid at low temperatures to an association with the protein at high temperatures. Membrane cohesion correlated with a simple fracture energy model where the level of tension required to lyse vesicles is proportional to the square root of the elastic area compressibility modulus. Free-energy potentials for adhesion of mixed lipid (PC and PE) bilayers showed a dramatic increase as the mole fraction of PE approached unity. Based on published values of separation distance between lamellae at full hydration for each pure component, the free energy potentials for adhesion of mixed-lipid bilayers were calculated from theoretical models of the van der Waals attraction and hydration repulsion then compared with the measured values of adhesion energy. The correlation provides strong evidence that the hydration repulsion can be represented by a surface-potential theory.