Papers by Keyword: Damage

Abstract: Concrete is multi-phase composites. Due to the inhomogeneity of mechanical properties and complexity of physical properties, constitutive relations of concrete are more complicated. Starting from irreversible thermodynamics theory, internal state variable theory and nonlocal field theory, non-local damage constitutive model of concrete under freeze-thaw action is established in this paper. In the model, non-local influence functions are discussed which are used to describe interplay of damage between adjacent point.

Abstract: The use of structural adhesive joints to join carbon fibre reinforced polymer (CFRP) adherends is now well established in the aerospace industry. These joints are subjected to varied load spectra, of which one of the most damaging forms of loading is fatigue with intermittent low energy impacts, which is termed combined standard and impact fatigue (CISF) in this paper. It is seen that the rate of crack growth in impact fatigue is greater than that in standard fatigue for a given value of the strain energy release rate, moreover, it is seen that the fatigue crack growth rate (FCGR) in standard fatigue (SF) increases after a block of impact fatigue. In this paper a model is proposed to predict crack growth in bonded joints subjected to CISF. The model is based on numerical crack growth integration (NCGI) with a method of accounting for the accelerated crack growth in SF following IF. The model was seen to provide a good prediction of the fatigue crack growth in CISF.

Abstract: This paper presents the comparative study on three types of fatigue data editing technique for summarising long records of fatigue data. Two of techniques were developed based on timefrequency domain (continous wavelet and discrete wavelet) and another one technique was developed based on time domain. These three techniques are used to extract fatigue damaging events in the record that cause the majority of fatigue damage, whilst preserving the load cycle sequence in the data. The objective of this study is to observe the capability of each technique in summarising long records of fatigue data. For the purpose of this study, two set of nonstationary data that exhibits random behaviour was used. This random data was measured in microstrain unit on the SAE1045 material that were used as a lower suspension arm of a car. Experimentally, the data was collected for 60 seconds at sampling rate of 500Hz, which gave 30, 000 discrete data points. The result of the study indicates that all techniques are applicable in detecting and extracts fatigue damaging events that exist in the fatigue data. However, it was found that continous wavelet can extract the data better than the other technique based the shorten signals, retention of damage and statistical parameter.

Abstract: The crack tip region in an elastic composite can be separated into three different regions. based on a simple rate-independent phenomenological constitutive model, the path independence of the J-integral and the concept of cracktip shielding, the maximum radii R of the damage saturation zone is obtained. Damage isotropy in the largest saturated damage zone is expressed by utilizing the Monte-Carlo technique to create the uniform distributions of microcrack location and orientation. With the assumption of dilute microcrack concentration, interaction among microcracks are neglected, and the stress intensity factor produced by interaction between main-crack and each microcrack can be superposed. Two sources of loading are analyzed, one is for the main-crack microcrack interaction under an applied remote load, and the other is for the main-crack microcrack interaction accompanied by the relief of residual stresses on the microcrack surfaces. The results show that two sources of loading can shield the main-crack tip, and microcracks behind the main-crack tip can make the most shielding whereas microcracks ahead of the main-crack tip play no role in shielding.

Abstract: This paper deals with the characterization of the thermomechanical behavior of monolithic refractory castables in a wide temperature range, up to 1200°C. Different test types are considered: tensile tests, compression tests, bending tests and tests on more complex shapes and geometries. A particular attention is paid to the detailed characterization and interpretation of the non-linear behaviors of these materials. Monotonic, cyclic and creep tests are considered. In some cases, digital image correlation (DIC) methods can be coupled to mechanical tests to obtain strain fields. Such results are particularly interesting to observe and to understand damage processes. As damage is a major characteristic of the monolithic castable behaviors, links are established between the thermomechanical behavior and damage mechanisms. Two main scales are taken into account for damage characterization: the macroscopic and the microscopic ones. Main mechanisms that are considered deal with microcracking, macrocracking, debonding and cavitation. Two types of materials are considered: non-reinforced and fiber reinforced refractory castables.

Abstract: Damage is a crucial characteristic of refractory castables and has to be considered to simulate correctly the behaviour of refractory structures. But, damage modelling by finite element simulations remains difficult. Indeed, the use of a continuum damage model with softening leads to strain localization phenomena. Numerical results depend on the mesh. Several numerical methods allow solving this meshing dependence by introducing an internal length in the material constitutive laws. In this paper, a regularization method has been applied with the damage plasticity model, considering a scalar value for damage. This model enables to take into account permanent strains due to plasticity and damage before and after the peak stress in tension and compression. Thermomechanical simulations are performed with this model to predict damage in a gas-burner. The damage level is evaluated after a thermal simulation generating high temperature gradients. Interests to take into account damage in the refractory structures are discussed. Sensitivity of results to material properties is studied. This work gives an example of using thermomechanical simulations to improve the design of refractory castable structures and to help in the material choice.

Abstract: Concerning the 3-roll skew rolling process of round billets, a 3-D thermo-mechanical coupled simulation is performed with the aid of commercial FE code MSC.SuperForm. The influence of the feed angle and inlet cone angle of the roll on the strain and damage filed are investigated. It is found that the distribution pattern of the equivalent plastic strain in the longitudinal section of the billet presents U-shape, and that the feed angle and inlet cone angle of the roll make difference in the gradient of deformation intensity in radial direction of round billet. The ring-shaped distribution of the damage field in transverse cross section of the billet indicates a high tendency for the ring-shaped inner crack to occur, instead of the center crack appearing in 2-roll skew rolling of round billet. The results show that the critical parameters to control the ring-shaped crack during the 3-roll skew rolling process are the feed angle and inlet surface cone angle of the roll. It should be avoided to use too large feed angles so as to reduce the tendency of annular inner crack in the actual rolling of high-alloy steels with poor hot workability.

Abstract: Each earthquake represents a particular moment in the history of the affected region. The seismicity in Italy reaches frequently high values, what makes it a country particularly affected by this kind of natural disasters. Historical constructions (in particular masonry ones) are structures that show a high vulnerability to the type of loads introduced in the sequence of a seismic event. This paper focus on the effects of the 6th of April 2009 earthquake, that affected the region of Abruzzo (Italy), over the historical buildings of the region, in particular churches, by establishing a set of different objectives directed to the understanding of their structural behavior and to the assessment of the extent of post seismic damage in this structures. During the emergency period that followed the earthquake, many churches and other historical constructions (towers, walls, palaces, etc…) were surveyed, according to the official 1st level damage survey forms for Cultural Heritage (C.H.), by the workgroup of the University of Padova (UNIPD). The information collected from these surveys was later inserted and organized in a database. A statistical work is presented, illustrating the referred information and focusing on the data related with the damage assessment of the considered monuments. This work presents an intuitive overview of the seismic effects over the surveyed churches, allowing not only to better understand the response of these structures to this particular action, but also correlating the earthquake data with its effects on the churches.

Abstract: to study oil shale creep properties, we use rock SJ-1B three-axis creep instrument on oil shale to carry three axis creep test. Through test results, the creep of oil shale is nonlinear, and in accelerating creep stage oil shale injury increased dramatically. Using Burgers creep model introduced damage variable to analyse the oil shale surrounding stress and displacement field and Systematically analyse the creep properties under various support intensity. So it has important theoretical significance and value for the oil shale resource development and roadway stability control.

Abstract: Capabilities to locate damage, and applicability to simply supported bridges are considered as basic characteristics of the method to be investigated in this paper. A new method which employs support reaction and mid-span displacement as indicators is presented to detect a certain type of damage. Unlike the existing study, this work proposes the direct relationship between the change in local mass caused by damage and the measured support reaction and mid-span displacement values. With the cooperation of inspection means, this method is capable of successfully identifying the location of local potential damage. The feasibility of this method is demonstrated by numerical simulation and model experiment.