Research On The Damage Identification And Detection Of The Local Ship Structures

Working condition of the ship structures in sea is very serious. Local damage often occurs in ship structures duo to the seawater corrosion and heavy wave load. If the damage can't be discovered and dealt with in time, maybe it will extend to the degree that affecting the global strength of the ship structures. Then the ship structures will be in the danger of collapse. Collapse of ships in sea can bring not only great loss of life and property but also pollution for the surrounding sea environment. Serious accidents even create huge ecological disaster. It is significant to research the damage detection of the ship structures and construct effective detection methods. We can use the methods to detect the damage at its beginning. Then the damage can be dealt with timely and the safety of the ship can be improved.Damage detection of some typical local ship structures is researched in this paper, especially beam, plate and panel. Considering the application of the composite material in ship structures is become more and more extensive, damage detection of the FRP sandwich structures is also discussed. Damage detection methods based on the wavelet transform and mode analysis are research emphasis in this paper, along with several other methods.Crack detection method based on the static analysis for the beam structure is studied first. The model of the cracked cantilever beam is constructed. After the crack is defined as an additional flexibility by theoretical analysis, the deflection curve equation of the cracked beam with a concentrate force acting on the cantilever end is gotten. The concept of the coefficient of the moment of inertia is introduced. The relationship between the additional flexibility and the coefficient of the moment of inertia is constructed with the help of the finite element method. A method to detecting the crack location and evaluating the damage extension by measuring deflections of two nodes is presented. An example about a trapezium cross-section cantilever beam is given to validate the equation and the method. Then the damage detection method based on the wavelet transform and mode analysis for cracked beam is researched. Continuous wavelet transform is used to analyze the cracked beam mode. Lipschitz exponents of the mode can be calculated from the wavelet coefficients. Modulus of the wavelet coefficients and Lipschitz exponents give the information about the crack location. Detail signals after discrete wavelet decomposition can also be used to define the location of the crack. Other problems are also discussed in this paper, such as the disturbance of the zero point to the Lipschitz exponents, the definition of the maximum order for the discrete wavelet decomposition and the selection of the maximum sampling distance for mode signals.Local corrosion and local penetrated crack are discussed for the wavelet-based damage detection of the plate structure. A two-step method is proposed in the paper. Coarse mode signal is gotten by applying large distance 2-dimension sampling to the damage displacement mode of the plate. 2-dimension wavelet decomposition is used to analyze the coarse mode signal and the preliminary damage area is defined according to the diagonal detail signal. Then 1-dimension dense sampling along the x-direction and y-direction is applied to the mode in the preliminary damage area and the fine 1-dimension components are extracted. Lipschitz exponents of the fine 1-dimension components are used to determine the type and location of the damage. Otherwise, corrosion damage detection methods based on the strain mode are discussed. Change ratio of the strain energy and residual bending moment index are constructed using the strain and used to locate the corrosion damage.Panel is a kind of common structure in ship and its damage detection problem is discussed in this paper. Considering the complexity of the panel structure, determination of the damage location is the research focus. Welding crack and stiffener corrosion are investigated for simple panel structure. Continuous wavelet transform is used to analyze the mode of the ribband on the stiffener location, and the location is located according to the distribution of the modulus of the wavelet coefficients. Damage type is determined by analyzing the distribution of the strain in the damage segment. Panel segment corrosion and welding fatigue crack are researched for the cross panel structure. Frequency response function of the acceleration is used to judge the existence of the damage. Considering the arrangement of the sampling control points, relative change ratio of the strain energy is used to search the damage area of the two cases. For fatigue crack damage case, 1-dimension sampling is applied to the displacement mode of the preliminary damage area. Accurate location of the crack is determined by the discrete wavelet analysis of the 1-dimension sampling signal.Detection of the crack damage and special disbond damage of the FRP sandwich structure is researched on the basis of the steel beam and plate. Wavelet tools are used to analyze the mode of the FRP sandwich beam and plate. Location and area of the damage are determined. Upper disbond case and lower disbond case are compared. It is discovered that the damage detection method based on the discrete wavelet transform is more accurate than the damage detection method based on the continuous wavelet transform for the FRP sandwich structure.