Double Cantilever Beam Fracture Research Articles

Using an embedded Buckypaper to monitor mode I crack growth in bonded joints

ABSTRACT Structural Health Monitoring (SHM) systems are commonly integrated in bonded joints for different applications in order to detect and predict potential fatigue and damages caused by internal and external agents they are subjected to during their operation life. However, each of the current SHM systems presents some drawbacks, as the reduction of structural properties or the difficulty in its implementation. This work deals with the use of a Carbon Nanotube film (Bukypaper) as a sensor in double cantilever beam (DCB) bonded joint specimens for crack growth monitoring purposes. Buckypaper (BP) sheets were manufactured and integrated in adhesive film layers and its integration quality was studied by Scanning Electron Microscopy (SEM). The effect of the integration of different thickness BPs in the mechanical performance of the bonded joints was also assessed performing DCB fracture tests. Also, electro-mechanical tests were conducted using the BP as the sensor, monitoring the output in the electrical resistance during the crack propagation. It was observed the capability of BP to detect and locate the damage during the test with a linear dependence between the electrical resistance measured in the BP and the deduced crack growth, proving the potential of the BP to monitor self-sensing bonded joints.

Toughening of carbon fibre/polybenzoxazine composites by incorporating polyethersulfone into the interlaminar region

Two grades of hydroxyl-terminated polyethersulfone (PES), one low and one high molecular weight, were incorporated into carbon fibre/polybenzoxazine (CF/PolyBZ) composites as interlayer toughening agents. A preliminary study on the PES–benzoxazine (BZ) resin mixtures was carried out via hot stage microscopy to observe the dissolution behaviour of the PES in the BZ during the curing cycle. CF/PolyBZ composites with PES in the interlaminar regions were produced by a vacuum resin infusion process and characterised via double cantilever beam (DCB) tests to determine the mode-I type interlaminar fracture toughness (GIC). The DCB fracture surfaces were examined under an optical microscope and scanning electron microscope (SEM). The thermomechanical properties of the composites were also studied via dynamic mechanical analysis (DMA). The incorporation of both grades of PES resulted in a significantly improved GIC and marginally increased glass transition temperature of the composite.

Interface debonding characterization by image correlation integrated with Double Cantilever Beam kinematics

A procedure is proposed for the identification of spatial interfacial traction profiles of peel loaded Double Cantilever Beam (DCB) samples, from which the corresponding traction–separation relation is extracted. The procedure draws upon recent developments in the area of non-contact optical techniques and makes use of so-called Integrated Digital Image Correlation (I-DIC) concepts. The distinctive feature of the I-DIC approach proposed herein is that the unknown degrees of freedom are not displacements or rotations, but the set of interfacial fracture properties describing the traction profile. A closed-form theoretical model is developed to reconstruct a mechanically admissible displacement field representing the deformation of the adhering layers during debonding in the DCB fracture test. The proposed modeling accounts for the spatial traction profile along the interface between the adherends using few degrees of freedom, i.e. crack tip position, maximum stress and size of the process zone. By minimizing the correlation residual with respect to the degrees of freedom, the full set of interfacial fracture properties is obtained through a one-step algorithm, revealing a substantial gain in terms of computational efficiency and robustness. It is shown that the identified traction profile can be effectively combined with the crack opening displacement to extract the corresponding traction–separation relation, i.e. the key input data for any cohesive zone model (CZM). The proposed procedure is validated by post-processing virtually deformed images generated through the finite element method. The robustness with respect to noisy data, as well as the low sensitivity to the initial guess, are demonstrated.

Fracture test for double cantilever beam of honeycomb sandwich panels

A modified double cantilever beam (DCB) test geometry is designed to investigate the fracture behavior of honeycomb sandwich panels containing embedded artificial pre-crack, and measure the strain energy release rate of the laminate facesheet/honeycomb cores interface. However, in terms of our DCB fracture test, owing to that the pre-crack does not propagate expectedly along the interface of facesheet/honeycomb core, a new fracture mode, namely IKP (initiation of interlaminar delamination, kinking into facesheet and propagation of interlaminar delamination), has been found.

Mode I fracture toughness estimation of wood by DCB test

Abstract The mode I fracture toughness of wood obtained by the double cantilever beam (DCB) test was analyzed by six conventional analysis methods: two methods based on elementary beam theory, Williams’s correction method, modified beam theory method, and two compliance calibration methods. In addition to these methods, the compliance combination method, which was originally proposed by the author for obtaining the mode II fracture toughness by the end notched flexure (ENF) test, was applied to the DCB test. In the compliance combination method, the strain at a certain point of the cracked portion of the specimen was measured, as was the loading-line displacement and critical load for crack propagation. Similarly to the mode II fracture toughness analysis for wood by the ENF test, the compliance combination method is more advantageous in practice than the other ones examined here in that the fracture toughness can be determined by the DCB fracture test alone without requiring separate tests, which should be conducted in the modified beam theory and compliance calibration methods.

Digital image processing of moire´ fringe patterns with an application to fractures in bovine dentin

A single fringe phase shifting technique is presented for ex- tracting the whole-field displacement distribution from single fringe pat- terns. By making a small modification in the traditional Fourier transform (FFT) method for fringe processing, the proposed method is capable of maintaining the fringe sign information. An additional approach is intro- duced that utilizes knowledge of the fringe sign in identifying fringe skel- etons with high precision. A combination of these methods of fringe pat- tern processing was used to examine the whole-field displacement distribution in double cantilever beam (DCB) fracture specimens pre- pared from bovine maxillary molars. Phase maps and precise fringe skel- etons were generated using the proposed techniques from fringe pat- terns obtained using moireinterferometry. Both aspects of the displacement measurements were used to determine the local displace- ment distribution near the crack tip, and to study the mechanics of frac- ture. © 2002 Society of Photo-Optical Instrumentation Engineers.

On the Possibility of Discrimination of Mixed Mode Fatigue Fracture Mechanisms in Adhesive Composite Joints by Advanced Acoustic Emission Analysis

Acoustic emission (AE) from pure and mixed mode fatigue fracture of adhesive composite joints was recorded and analyzed. Transient AE signal classification was attempted by a computational pattern recognition analysis. Good separation was achieved for pure mode AE signals from the double cantilever beam (mode I) and end notch fracture (mode II) tests. AE signals from the mixed mode lap joint test were found to be closer to the mode II fracture signals. The results may provide insight into the fracture mechanisms in joints and can be used for the development of mechanism-based predictive models of fracture and life.

Processing and fracture behavior of a polyethylene-based thermoplastic adhesive and a glass-fiber filled epoxy adhesive

The processing and fracture performance of a high-density polyethylene (HDPE)-based thermoplastic adhesive and an epoxy-based thermosetting adhesive with a glass fiber additive were studied. Differential scanning calorimetry (DSC) was used to characterize the processing parameters. Double cantilever beam (DCB) fracture testing and butt-joint tensile testing were used to characterize the adhesive joint fracture toughness and strength. Although the thermoplastic adhesive had low strength, it exhibited high DCB fracture toughness because of a large plastic deformation zone in the fracture process. The fiber additive in the thermosetting adhesive stabilized the DCB fracture. Loading speed, adhesive thickness, pre-crack length, and curing temperature were all considered in the study.

Stress Corrosion Cracking of ASTM A516 Steel in Hot Caustic Sulfide Solutions—Potential and Weld Effects

Abstract A combination of slow strain rate (tensile) testing (SSRT) and double cantilever beam (DCB) fracture mechanics testing techniques have been used to study the stress corrosion cracking (SCC) behavior of welded and unwelded pressure vessel quality steel plate (ASTM A516 Grade 70) in hot (92 C) caustic sulfide solutions (3.35 m NaOH + 0.42 m Na2S) of general relevance to the pulp and paper industry. Tests were conducted over a range of potentials (E) and stress intensities (KI). The most susceptible SCC situation occurred at potentials near the activepassive transition, where crack propagation rates were slightly lower in the fusion zone (FZ) of the weld than in the base material (BM). However, crack initiation on smooth specimens was far more frequent in the FZ. More noble potentials did not prevent SCC, but decreased the crack propagation rate and decreased crack initiation (i.e., crack density), particularly in the FZ. A fracture mechanics analysis of partial surface cracks and through thickness cracks was used to assess the cracking of kraft digesters in light of the results. It was concluded that anodic protection could play a beneficial role by decreasing the probability of crack coalescence and the resulting formation of a long critical (catastrophic) crack in the weld region. Instead, only small isolated cracks should form, which penetrate the vessel wall more slowly and lead to leakage before catastrophic failure.