Width-tapered Double Cantilever Beam Research Articles

A combined experimental and computational analysis of failure mechanisms in open-hole cross-ply laminates under flexural loading

In this study, integrated experimental tests and computational modeling are proposed to investigate the failure mechanisms of open-hole cross-ply carbon fiber reinforced polymer (CFRP) laminated composites. In particular, we propose two effective methods, which include width-tapered double cantilever beam (WTDCB) and fixed-ratio mixed-mode end load split (FRMMELS) tests, to obtain the experimental data more reliably. We then calibrate the traction-separation laws of cohesive zone model (CZM) used among laminas of the composites by leveraging these two methods. The experimental results of fracture energy, i.e. GIc and GTc, obtained from WTDCB and FRMMELS tests are generally insensitive to the crack length thus requiring no effort to accurately measure the crack tip. Moreover, FRMMELS sample contains a fixed mixed-mode ratio of GIIc/GTc depending on the width taper ratio. Examining comparisons between experimental results of FRMMELS tests and failure surface of B–K failure criterion predicted from a curve fitting, good agreement between the predictions and experimental data has been found, indicating that FRMMELS tests are an effective method to determine mixed-mode fracture criterion. In addition, a coupled experimental-computational modeling of WTDCB, edge notched flexure, and FRMMELS tests are adopted to calibrate and validate the interfacial strengths. Finally, failure mechanisms of open-hole cross-ply CFRP laminates under flexural loading have been studied systematically using experimental and multi-scale computational analyses based on the developed CZM model. The initiation and propagation of delamination, the failure of laminated layers as well as load-displacement curves predicted from computational analyses are in good agreement with what we have observed experimentally.

Bonded composite repair of metallic pipeline using energy release rate method

A methodology of repair in pipes involving bonded joints, manufactured through the wrappage of a composite around the pipe, is becoming a common practice in the industry for a series of associated advantages. The prediction of failure pressure in bonded repaired pipes is vital to guarantee a good quality of adhesion in this type of repair. Nowadays, the failure pressure is usually evaluated using hydrostatic tests that demand complex preparation to pressurize a section of the pipe and that are also dangerous for possible debris during rupture. With this fact in mind, this research purpose is to verify a simpler and more efficient methodology to predict the failure pressure applying energy release rate concepts. Double Cantilever Beam (DCB) and Width Tapered Double Cantilever Beam (WTDCB) tests were used to obtain the energy release rate of dissimilar specimens constituted with the same material as the most common repairs (steel/glass fiber reinforced polymer). Through a design equation that relates the energy release rate and the failure pressure presented in Standards ISO/PDTS 24817 and ASME PCC-2, it is possible to predict a value of failure pressure using DCB and WTDCB energy release rate results. The failure pressure predicted value was compared with the experimental hydrostatic tests results and analyzed. Results showed a good agreement between these two values, indicating that the methodology developed in this research might be used, in the future, to predict the failure pressure in bonded repaired pipes.

Interlaminar fracture of CF/EP composite containing a dual-component microencapsulated self-healant

We present an experimental study of the self-healing ability of carbon fibre/epoxy (CF/EP) composite laminates with microencapsulated epoxy and its hardener (mercaptan) as a healing agent. Epoxy- and hardener-loaded microcapsules (average size large: 123μm; small: 65μm) were prepared by in situ polymerisation in an oil-in-water emulsion and were dry-dispersed at the ratio 1:1 on the surface of unidirectional carbon fabric layer. The CF/EP laminates were fabricated using a vacuum-assisted resin infusion (VARI) process. Width-tapered double cantilever beam (WTDCB) specimens were used to measure mode-I interlaminar fracture toughness of the CF/EP composites with a pre-crack in the centre plane where the microcapsules were placed. Incorporation of the dual-component healant stored in the fragile microcapsules provided the laminates with healing capability on delamination damage by recovering as much as 80% of its fracture toughness. It was also observed that the recovery of fracture toughness was directly correlated with the amount of healant covering the fracture plane, with the highest healing efficiency obtained for the laminate with large capsules.

Fracture behavior of a self-healing, toughened epoxy adhesive

A self-healing, toughened epoxy adhesive is demonstrated based on a commercial structural adhesive film. Self-healing is achieved via embedded microcapsules containing dicyclopentadiene monomer and solid particles of bis(tricyclohexylphosphine)-benzylidine ruthenium (IV) dichloride (Grubbs') catalyst. Recovery of fracture toughness is assessed through fracture testing of width tapered double cantilever beam (WTDCB) specimens. Healing efficiencies as high as 58% were achieved for 6.6wt% DCPD microcapsules and 10mg Grubbs’ catalyst. However, virgin fracture toughness is reduced with the addition of ca. 117μm diameter microcapsules as a result of suppression of the damage zone as revealed by transmission optical micrographs. The uniform dispersal of microcapsules throughout a rubber toughened epoxy adhesive formulated using EPON 828, piperidine and CTBN alleviated the suppression effect and demonstrated retention of virgin fracture toughness of adhesives.

Experimental studies of Mode I energy release rate in adhesively bonded width tapered composite DCB specimens

The Mode I energy release rate, G I, derived from conventional uniform width double cantilever beam (DCB) test data depends on crack length. Data acquisition and analysis from such experiments can be simplified if G I is independent of crack length, particularly at high loading rates. Previous research by others has shown that width tapered double cantilever beam (WTDCB) specimens made from aerospace grade carbon/epoxy composite laminates without adhesive layers exhibit critical energy release rates which are independent of crack length. The purpose of the present investigation was to determine whether or not adhesively bonded WTDCB specimens of automotive type sheet molding compound (SMC) composites show similar behavior. It was observed that the critical energy release rate was practically independent of crack length for unidirectional carbon/epoxy WTDCB specimens without adhesive layers but not for unidirectional carbon/epoxy, unidirectional E-glass/epoxy or automotive SMC WTDCB specimens with adhesive layers, and that self-similar crack growth only occurred in unidirectional carbon/epoxy WTDCB specimens without adhesive layers.

Self-healing structural composite materials

A self-healing fiber-reinforced structural polymer matrix composite material is demonstrated. In the composite, a microencapsulated healing agent and a solid chemical catalyst are dispersed within the polymer matrix phase. Healing is triggered by crack propagation through the microcapsules, which then release the healing agent into the crack plane. Subsequent exposure of the healing agent to the chemical catalyst initiates polymerization and bonding of the crack faces. Self-healing (autonomic healing) is demonstrated on width-tapered double cantilever beam fracture specimens in which a mid-plane delamination is introduced and then allowed to heal. Autonomic healing at room temperature yields as much as 45% recovery of virgin interlaminar fracture toughness, while healing at 80 °C increases the recovery to over 80%. The in situ kinetics of healing in structural composites is investigated in comparison to that of neat epoxy resin.

Effect of Crack Propagation Directions on the Interlaminar Fracture Toughness of Carbon/Epoxy Composite Materials

The interlaminar fracture toughness of carbon/epoxy composite materials has been studied under tensile and flexural loading using width-tapered double cantilever beam (WTDCB) and end-notched flexure (ENF) specimens. This study experimentally examines the effect of various interfacial ply orientations, α (0°, 45° and 90°) and crack propagation directions, θ (0°, 15°, 30° and 45°) in terms of the critical strain energy release rate. Twelve differently layered laminates were investigated. The fracture energy is deduced from the data according to the compliance method and beam theory. Beam theory is used to analyze the effect of crack propagation direction. The geometry and lay-up sequence of specimens are designed to probe various conditions such as skewness parameter and beam volume. Results show that fiber bridging occurred due to non-midplane crack propagation; this causes the difference in fracture energy calculated by both methods. For the construction of safer and more reliable composite structures, we obtain the optimal stacking sequence from the initial fracture energy in each mode.

Interlaminar fracture and low-velocity impact of carbon/epoxy composite materials

The interlaminar fracture and the low-velocity impact behavior of carbon/epoxy composite materials have been studied using width-tapered double cantilever beam (WTDCB), end-notched flexure (ENF), and Boeing impact specimens. The objectives of this research are to determine the essential parameters governing interlaminar fracture and damage of realistic laminated composites and to characterize a correlation between the critical strain energy release rates measured by interlaminar fracture and by low-velocity impact tests. The geometry and the lay-up sequence of specimens are designed to probe various conditions such as the skewness parameter, beam volume, and test fixture. The effect of interfacial ply orientations and crack propagation directions on interlaminar fracture toughness and the effect of ply orientations and thickness on impact behavior are examined. The critical strain energy release rate was calculated from the respective tests: in the interlaminar fracture test, the compliance method and linear beam theory are used; the residual energy calculated from the impact test and the total delamination area estimated by ultrasonic inspection are used in the low-velocity impact test. Results show that the critical strain energy release rate is affected mainly by ply orientations. The critical strain energy release rate measured by the low-velocity impact test lies between the mode I and mode II critical strain energy release rates obtained by the interlaminar fracture test.

Effect of Crack Propagation Directions on the Interlaminar Fracture Toughness of Carbon/Epoxy Composite Materials

Interlaminar fracture toughness of carbon/epoxy composite materials has been studied under tensile and flexural loading by the use of width tapered double cantilever beam(WTDCB) and end notched flexure(ENF) specimens. This study has significantly examined the effect of various interfacial ply orientation, and crack propagation direction, in terms of critical strain energy release rate through experiments. Twelve differently layered laminates were investigated. The data reduction for evaluating the fracture energy is based on compliance method and beam theory. Beam theory is used to analyze the effect of crack propagation direction. The geometry and lay-up sequence of specimens are considered various conditions such as skewness parameter, beam volume, and so on. The results show that the fiber bridging occurred due to the non-midplane crack propagation and causes the difference of fracture energy evaluated by both methods. For safer and more reliable composite structures, we obtain the optimal stacking sequence from initial fracture energy in each mode.

Laminar fracture behaviour of (carbon/glass) hybrid fibre reinforced laminates—II. Laminar fracture criterion

Double cantilever beam (DCB) and width tapered double cantilever beam (WTDCB) specimens of hybrid laminates are used to determine the values of the laminar fracture energy, and the results are compared. Based on the test characteristics of the WTDCB specimen, the model for evaluating G IC (as k = 3) is modified by the three components of deflection over the whole crack propagation; in addition, conditional cracking is discussed and recommended. A compliance experiment for DCB is conducted to further prove the relationship between compliance and crack length. Finally, we reassess the criterion of laminar fracture energy.

The Interlaminar Fracture Energy of Glass Fiber Reinforced Polyester Composites

This study deals with the interlaminar fracture energy of a glass fiber reinforced polyester composite. Width tapered double cantilever beam (WTDCB) specimens are used for the test. The compliance and fracture behavior of the specimen as well as the effect of specimen geometry on fracture energy are studied. The fracture energy at liquid nitrogen temperature (77°K) is also studied. It is found that the fracture energy obtained decreases with increasing height of specimens, finally reaching a constant value. The minimum height requirement for the fracture test of Extren is one half inch. The fracture energy slightly decreases with a decrease in specimen taper. A load-deflection hysteresis is observed to increase with specimen width. Hysteresis is dependent on the beam volume, i.e., height, taper and crack length. The fracture energy at liquid nitrogen temperature (77°K) is about 3 times higher than that at room temperature and the fracture energy transverse to the fiber direction is slightly higher than in the fiber direction.