Jialai Wang

Interface crack between two shear deformable elastic layers

Abstract An improved method based on the first-order shear deformable plate theory is developed to calculate the energy release rate and stress intensity factor for a crack at the interface of a bi-layer structure. By modeling the uncracked region of the structure as two separate Reissner–Mindlin plates bonded perfectly along the interface, this method is able not only to take into account the shear deformation in the cracked region, but also to capture the shear deformation in the uncracked region of the structure. A closed form solution of energy release rate and mode decomposition at the interface crack is obtained for a general loading condition, and it indicates that the energy release rate and stress intensity factor are determined by two independent loading parameters. Compared to the approach based on the classical plate theory, the proposed method provides a more accurate prediction of energy release rate as well as mode decomposition. The computational procedures introduced are relatively straightforward, and the closed form solution can be used to predict crack growth along the layered structures.

Novel beam analysis of end notched flexure specimen for mode-II fracture

Abstract A novel beam model of end notched flexure (ENF) specimen for mode-II fracture testing is presented. By applying the principle of superposition, the ENF specimen is modeled as two sub-problems: (1) an un-cracked beam under three-point bending; and (2) a skew symmetric cracked beam under shear traction on the crack surface. Due to skew-symmetry of sub-problem two, only the upper half of the beam is analysed, and based on compatibility of deformation, a shear compliance coefficient is introduced to establish beam deformation equation. Explicit and simple closed form solutions of compliance and strain energy release rate are obtained, and they compare well with existing finite element analyses. Compared to other available analytical methods of the ENF specimen, the present beam model is relatively simple and easy to use; further, it can be applied to other beam fracture specimen analysis (e.g., mixed mode fracture and bi-material interface specimen).

Improved Damage Detection for Beam-type Structures using a Uniform Load Surface:

A combined analytical and experimental study is conducted to develop efficient and effective damage detection techniques for beam-type structures. Unlike many other vibration-based damage detection methods, in which the mode shapes are often chosen to retrieve damage information, the uniform load surface (ULS) is employed in this study due to its less sensitivity to ambient noise. In combination with the ULS, two new damage detection algorithms, i.e., the generalized fractal dimension (GFD) and simplified gapped-smoothing (SGS) methods, are proposed. The GFD method is developed by modifying the conventional definition of fractal dimension. By using a moving window, the GFD of ULS can be obtained for each sampling point, and due to the irregularity of ULS introduced by the damage, a peak exists on the GFD curve indicating the location of the damage. Not only does such a peak at the GFD curve locate the damage, but also it reveals the relative size of the damage. The SGS method is also proposed to take advantage of the simple deformation shape of ULS. Both methods are then applied to the ULS of cracked and delaminated beams obtained analytically, from which the damage location and size are determined successfully. Based on the experimentally measured curvature mode shapes, both the GFD and SGS methods are further applied to detect three different types of damage in carbon/epoxy composite beams. The successful detection of damage in the composite beams demonstrates that the new techniques developed in this study can be used efficiently and effectively in damage identification and health monitoring of beam-type structures.

Analysis of tapered ENF specimen and characterization of bonded interface fracture under Mode-II loading

Abstract An engineering approach for evaluating the shear-mode (Mode-II) fracture toughness of wood–wood and wood-composite bonded interfaces is presented. A tapered beam on elastic foundation model is developed to analyze and design a linear tapered end-notched flexure (TENF) specimen for fracture tests of bonded interfaces. The elastic foundation model is verified numerically by finite element analysis and experimentally by compliance calibration tests, which demonstrate that the present model can accurately predict the compliance and compliance rate-change of the specimen, and with proper design, an approximate constant rate of compliance change with respect to crack length can be achieved. The proposed TENF specimen can be used for Mode-II fracture toughness evaluations with reasonable confidence in the linearity of compliance crack-length relationship. The fracture of wood–wood and wood-composite bonded interfaces under Mode-II loading is experimentally evaluated using the proposed TENF specimen, and the corresponding values of critical strain energy release rate are obtained. The modeling technique and testing method presented can be efficiently used for characterization of Mode-II fracture of bonded bimaterial interfaces.

Tapered beam on elastic foundation model for compliance rate change of TDCB specimen

Abstract A modified beam theory is developed to predict compliance rate change of tapered double cantilever beam (TDCB) specimens for mode-I fracture of hybrid interface bonds, such as polymer composites bonded to wood. The analytical model treats the uncracked region of the specimen as a tapered beam on generalized elastic foundation (TBEF), and the effect of crack tip deformation is incorporated in the formulation. A closed-form solution is obtained to compute the compliance and compliance vs. crack length rate change. The present TBEF model is verified with finite element analyses and experimental calibration data of compliance for wood–wood and wood–composite bonded interfaces. The compliance rate change can be used with experimental critical fracture loads to determine the respective critical strain energy release rates or fracture toughness of interface bonds. The present analytical model, which accounts for the crack tip deformation, can be efficiently and accurately used for compliance and compliance rate-change predictions of TDCB specimens and reduce the experimental calibration effort that is often necessary in fracture studies. Moreover, the constant compliance rate change obtained for linear-slope TDCB specimens can be applied with confidence in mode-I fracture tests of hybrid material interface bonds.

Dynamics-based Damage Detection of Composite Laminated Beams using Contact and Noncontact Measurement Systems

A reliable and effective damage detection technique is one of the significant tools to maintain the safety and integrity of structures. A dynamic response offers viable information for the identification of damage in the structures. However, the performance of such dynamics-based damage detection depends on the quality of measured data and the effectiveness of data processing algorithms. In this article, the experimentally measured data of two sensor systems, i.e., a surface-bonded piezoelectric sensor system and a noncontact scanning laser vibrometer (SLV) system, are studied, and their effectiveness in damage identification of composite laminated beams is compared. Three dynamics-based damage detection algorithms are evaluated using the data acquired from these two measurement systems. The curvature mode shape is selected as a parameter to locate damage due to its sensitivity. The piezoelectric sensors directly acquire the curvature mode shapes of the structures, while the SLV measures the displacement mode shapes. The difference in the measurement characteristics of these systems and their influence in the damage identification performance are addressed. The beam specimens are made of E-glass/epoxy composites, and several different types of damages are introduced in the beams (i.e., delaminations, and impact and saw-cut damages). This study provides a thorough assessment of the two sensor systems in damage detection of composite laminated beams and verifies the validity of dynamics-based damage detection methodology in locating the local defects in composite structures.

Shear Moduli of Structural Composites from Torsion Tests

A combined experimental/analytical approach for effective evaluation of in-plane and out-of-plane shear moduli of structural composite laminates from torsion is presented. The test samples are produced by pultrusion and consist of E-glass fiber systems and vinylester resin. Three types of rectangular samples are used: unidirectional samples cut from plates; angle-ply and angle/cross ply samples cut from wide-flange beams. The shear moduli are obtained from the experimental torsional stiffnesses (T/) and data reduction techniques based on torsion solutions by Lekhnitskii and Whitney. The classical approach of usingpaired samples of different widths but same material orientation can grossly underestimate out-of-plane shear moduli. Thus, to overcome this problem, samples with material orientations normal to each other are used, and to obtain samples of larger dimensions than the available thicknesses of the material, the pultruded laminates are bonded flat-wise, from which transverse strips of different widths are cut. Consistent results are obtained by the two data reduction techniques, and the experimental values are compared to predictions by micro/macro-mechanics models.

Fracture Toughness of Wood-Wood and Wood-FRP Bonded Interfaces under Mode-II Loading

In this paper, the shear-mode (Mode-II) fracture toughness of wood-wood and wood-fiber reinforced plastic (FRP) bonded interfaces using unique linear tapered end-notched flexure (TENF) specimens is evaluated. A tapered beam on elastic foundation (TBEF) model developed for design and analysis of the TENF specimens is briefly introduced. Based on the TBEF model, the TENF specimens are designed to achieve an approximate constant compliance rate change for fracture tests, and design procedures for Mode-II fracture of bonded interface are accordingly developed. The linear compliance rate changes of the designed TENF specimens are verified experimentally by the compliance calibration tests and numerically by finite element modeling. Two types of TENF specimens, namely for wood-wood and wood-FRP bonded interfaces, are designed and tested to determine the fracture toughness values of adhesively bonded interfaces under Mode-II loading. The critical loads for crack initiation and arrest are measured during the testing, in which the critical strain energy release rates are then easily evaluated by making use of the approximate constant compliance rate change of the designed specimens over defined crack lengths. The wood-wood and wood-FRP bonded interfaces tested in this study exhibit a stable crack propagation behavior. The unique TENF specimen and experimental-design program presented in this paper can be efficiently used to evaluate the Model-II fracture toughness of hybrid material interface bonds, and the fracture toughness values obtained can be applied to detect the potential delamination and crack growth of the adhesively bonded joints.

Transverse Shear Stiffness of Composite Honeycomb Cores and Efficiency of Material

Abstract Effective transverse shear moduli of composite honeycomb cores are important material properties in analysis and design of sandwich structures. An analytical approach using a two-scale homogenization technique is presented to predict the effective transverse shear stiffnesses of thin-walled composite honeycomb cores with general configurations. To improve the performance of core transverse shear behavior, a nondimensional index, the so-called efficiency of material (EOM), is introduced to evaluate the optimal design of periodic cellular cores. Explicit formulas of effective transverse shear stiffnesses and the corresponding efficiencies of material are provided for three typical honeycomb cores consisting of reinforced sinusoidal, elliptical, and reinforced hexagonal geometries. It has been shown that the efficiency of material is only determined by nondimensional core geometric ratios. The effects of these ratios on the EOM of effective transverse shear stiffnesses, which are examined in detail in this study, offer insight and guidelines for optimal design and selection of honeycomb cores. The explicit formulas for the effective transverse shear stiffness properties of thin-walled composite honeycomb cores and their related EOMs can be used effectively to predict and optimize the core transverse shear behavior in sandwich structures.

Mechanics of bimaterial interface : Shear deformable split bilayer beam theory and fracture

A novel split beam model is introduced to account for the local effects at the crack tip of bi-material interface by modeling a bi-layer composite beam as two separate shear deformable beams bonded perfectly along their interface. In comparisons with analytical two-dimensional continuum solutions and finite element analysis, better agreements are achieved for the present model, which is capable of capturing the local deformation at the crack tip in contrast to the conventional composite beam theory. New solutions of two important issues of cracked beams, i.e., local buckling and interface fracture, are then presented based on the proposed split bi-layer shear deformable beam model. Local buckling load of a delaminated beam considering the root rotation at the delamination tip is first obtained. By considering the root rotation at the crack tip, the buckling load is lower than the existing solution neglecting the local deformation at the delamination tip. New expressions of energy release rate and stress intensity factor considering the transverse shear effect are obtained by the solution of local deformation based on the novel split beam model, of which several new terms associated with the transverse shear force are present, and they represent an improved solution compared to the one from the classical beam model. Two specimens are analysed with the present model, and the corresponding refined fracture parameters are provided, which are in better agreement with finite element analysis compared to the available classical solutions.