Applied Mechanics and Materials Vol. 784

Abstract: In this paper we present a family of gradient-enhanced continuum damage models which can be viewed as a regularization of the variational approach to fracture capable of predicting in a unified framework the onset and space-time dynamic propagation (growth, kinking, branching, arrest) of complex cracks in quasi-brittle materials under severe dynamic loading. The dynamic evolution problem for a general class of such damage models is formulated as a variational inequality involving the action integral of a generalized Lagrangian and its physical interpretation is given. Finite-element based implementation is then detailed and mathematical optimization methods are directly used at the structural scale exploiting fully the variational nature of the formulation. Finally, the link with the classical dynamic Griffith theory and with the original quasi-static model as well as various dynamic fracture phenomena are illustrated by representative numerical examples in quantitative accordance with theoretical or experimental results.

Abstract: The iterative Return Mapping Algorithm (RMA) is widely used for the plastic integration owing to its accuracy and efficiency, but it is CPU time consuming and may cause divergence problems in case of large strain increments. This paper presents a fast plastic integration method called Direct Scalar Algorithm (DSA) for the damage prediction in forming process simulations. A simplified three-dimensional (3D) strain-based damage model is coupled with the plasticity and implemented into the DSA which does not need iterative solution to make the plastic integration very fast and robust even for very large strain increments. The basic idea is to transform the constitutive equations in terms of the unknown stress vectors into a scalar equation in terms of the equivalent stresses which can be determined by using the experimental tensile curve; thus, the plastic multiplier ∆λ can be directly calculated. The DSA is as accurate as RMA but much faster for the plastic integration.

Abstract: In this study, a planar spring lattice model is used to study the evolution of damage variable d_L in disordered media. An elastoplastic softening damage constitutive law is implemented which introduces a cohesive length scale in addition to the disorder-induced one. The cohesive length scale affects the macroscopic response of the lattice with the limiting cases of perfectly brittle and perfectly plastic responses. The cohesive length scale is shown to affect the strength-size scaling such that the strength increases with increasing cohesive length scale for a given size. The formation and interaction of the microcracks is easily captured by the inherent discrete nature of the model and governs the evolution of d_L . The proposed method provides a way to extract a mesoscale dependent damage evolution rule that is linked directly to the microstructural disorder.

Abstract: This work investigates the effect of residual stresses on the damage of composite laminate. The incremental hole-drilling method is applied to determine residual stresses in composite laminates [0₂/θ₂]_s and then acoustic emission technique is used for the identification of damage appearance during the tensile testing. The samples with different residual stress distribution are prepared through curing and post curing in order to study the role of residual stress on the damage of composite laminates. Besides the experimental method, the theoretical approach is applied to illustrate the role of residual stress on the damage of composite laminates.

Abstract: A Continuum Damage Mechanics model, that incorporates a crack closure parameter and a cut-off on negative triaxiality, is employed to simulate the damage growth in upsetting problem using the finite element package ABAQUS. Parametric studies are carried out to find the effect of the crack closure parameter and the cut-off. It is shown that the correct location of the maximum damage in upsetting problem (i.e., the location reported in the experimental literature) is predicted only if these parameters are incorporated.

Abstract: This research aimed to reveal the bond behavior and failure mechanism of varying size rebar embedded in concrete with inside pre-crack through a series of quasi-static/cyclic pull-out tests and real-time acoustic emissions monitoring (AE). The obtained AE signal density records of the tests provide potential usage for evaluate the safety of reinforced concrete (RC) structure subjected to cyclic load, as well as quantitative study in the remaining life of RC after earthquake or shake damage.

Abstract: More high-rise structures are currently being constructed and correspondingly, the compressive strength of concrete has been increased. However, compared to conventional strength concrete the high strength concrete (HSC) exhibits coarse inner pore structure which blocks escape routes of vapour generated in the event of fire. This results in spalling and subsequently, are responsible for fire vulnerability of the structure. In addition, spalling phenomena is also affected by the section dimensions of HSC which is also another crucial factor from socio-economic considerations. Thus, this study was carried out to evaluate the fire resistance performance of hybrid fiber (i.e. steel-polypropylene-fibre)-reinforced HSC columns with different cross-section dimensions. The result of the fire resistance performance testing using 100MPa concrete showed that delay to failure was observed by approximately 76 per cent.

Abstract: In case the structure is exposed to the high temperature, the lateral shear crack is the major cause exposing the steel bar to the high temperature and also the major cause of the collapse of buildings. In the study, in order to control the lateral shear crack, the presterssing using strands in a reverse direction of the lateral shear crack has applied and the fire resistance test of the PS corrugated web composite beam has been carried out, produced in a corrugated type of the steel to increase the efficiency of the prestressing. As a result of the test, the lateral shear crack introducing the PS has been reduced but it has been concluded that in case the strands are failed, the brittle failure can be taking place.

Abstract: This research is to show the microstructure of fracture parts of structural steels by welding at the high temperature. Discontinuity of mechanical and chemical property at HAZ of welding parts is the cause of decreasing structure safety. Therefore, this study was determined the effect of the welding of steels through a high temperature tensile tests and fracture portion of the microstructure. The results showed that does not cause destruction until temperature reached 600°C.

Abstract: The influence of the local state of stress on the predictability of the Beremin model is investigated. For this purpose, the Beremin model is used on a series of tensile experiments with varying constraints. One specimen series is used to calibrate the Beremin parameters. These are afterwards applied to all specimen series. For all series the test results are compared to the predicted 5% to 95% failure probability corridor calculated by the Beremin model. The results are then discussed in concern of the influence of the stress state on the predictability of the Beremin model.