Modified Double Cantilever Beam Research Articles

Comparative study of fracture appearance in crack tip opening angle testing of gas pipeline steels

The crack tip opening angle (CTOA) experiments were conducted on API X65, X70, X80 and X100 steels using a modified double cantilever beam (MDCB) geometry. The fracture surfaces of the test specimens were examined after fracture tests by optical and scanning electron microscopy. The fracture surfaces of the lower grade X65 and X70 steels had fully ductile features including classic flat and slant fracture characteristics. However, repetitive propagation and arrest of quasi-cleavage in the form of chevron arrow-head fractures was observed on the fracture surfaces of higher grade X80 and X100 steels. Interestingly, the analysis of load-displacement records from the latter steels demonstrated local ragged patterns after the test peak load, associated with the repetitive low energy quasi-cleavage mechanism. Detailed discussion on the micro-fractographs of the tested steels and comparison of the test observations with the only newly published work in this filed conclude the paper.

Measurement and analysis of crack tip opening angle in pipeline steels

The quasi-static tearing tests were conducted to analyze the crack propagation process and fracture mechanism in X70 and X80 pipeline steels. The crack tip opening angle (CTOA) was measured by a single specimen method using modified double-cantilever beam (MDCB) specimen. Some influence factors of CTOA toughness were discussed including specimen thickness, precrack types, loading rate and crack tunneling. Experimental results showed that critical CTOA values increase with increasing specimen thickness, which indicated an opposite trend in comparison with the earlier aerospace aluminum alloy studies. The crack tunneling transformed from “triangular shape” to “parabolic shape”, as specimen thickness increased.

Effect of Pressure Cycling on Fracture Energy of Polyurethane/Aluminum Adhesive Bonds

An experimental study was conducted to investigate the effect of pressure-cycling on adhesive bond fracture energy of polyurethane/aluminum adhesive bond joints. Initially, two types of peel tests were conducted to characterize adhesive bond strength and challenges associated with pre-mature polyurethane cracking and failure during these tests are discussed. A modified double cantilever beam (MDCB) specimen configuration was specially designed and opening-mode loading conditions were employed to determine the interfacial adhesive bond energy (GC). The test specimens were pressure-cycled in water-filled tanks for 1 to 4 weeks with an increment of 1 week. The GC of pressure-cycled specimens was compared with both control and water-soaked samples (without pressure-cycling). The results indicated that pressure-cycling decreased GC values to those of the control and water-soaked samples: hence, prolonged pressure-cycling could be problematic to polymer/metal adhesive bonds of hardware installed outboard of submarine pressure hulls.

An Analytical Model of FRP-Concrete Bond Deterioration in Moist Environment

Debonding of FRP from concrete substrate can lead to premature failure of the FRP repaired or retrofitted concrete structures. Therefore, FRP-concrete bond is a major factor affecting the performance of FRP strengthened concrete members. Experimental studies show that moisture plays an important role in the reliability of the bond between FRP and concrete. In this study, a cohesive zone model (CZM) of debonding along the FRP–concrete interface under peel load was analytically established by using a linear elastic separation-stress law. The interface region relative humidity (IRRH) was selected as the primary factor to study the bond durability of FRP/concrete bond. Based on a nonlinear deterioration model, the analytical solution of the CZM was used to determine critical load vs. crack length relations ( P-a curves) of Modified Double Cantilever Beam (MDCB) specimens at different moisture exposure durations. The good agreement with experimental data indicates that the CZM for FRP-concrete joint is an efficient way to analyze bond behavior in moist environments.

Nonlinear Deterioration Model for Bond Interfacial Fracture Energy of FRP-Concrete Joints in Moist Environments

This study proposed a bond mechanism based deterioration model of bond interfacial fracture energy for fiber-reinforced polymer (FRP)-concrete joints in moist environments. The bond interface region relative humidity (IRRH) in a moist environment was correlated to the bond fracture energy in this deterioration model. The IRRH-dependent interface separation tractions were derived in the frame of a cohesive zone model. Such an IRRH-dependent interface separation-traction law was simulated by a series of nonlinear interface elements attached to the bond interface to calculate the macroscopic load-displacement curves for the modified double cantilever beam (MDCB) specimens. Through moisture diffusion analysis, IRRH was determined as a function of the moisture exposure time for given specimen dimension and environmental RH. Using IRRH as the bridge, the time-dependent load-displacement curves of the MDCB specimens were obtained. The good agreement with the experimental data indicated that the model worked well. The approach developed in this study can be used to simulate and predict the durability of the bond between the FRP and concrete in moist environments.

Crack tip opening angle measurement through a girth weld in an X100 steel pipeline section*

ABSTRACTCrack tip opening angle (CTOA) is becoming one of the most accepted methods for characterizing fully plastic fracture. It provides a measure of the resistance to fracture for a material in cases where there is a large degree of stable‐tearing crack extension during the fracture process. Our current pipeline research uses the CTOA test as an alternative, or addition, to the CTOD (crack tip opening displacement) and the fracture energy characterization provided by the J‐integral approach. A test technique was developed for measurement of CTOA that uses a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate the CTOA. In this article, CTOA data on crack growth orientations perpendicular to pipeline girth welds are presented. The CTOA for X100 high strength bainitic gas pipeline steel is reported. Two different specimen gauge sections, 3 mm and 8 mm, were used and the effect of the specimen thickness on the CTOA is discussed. The results show a change in the CTOA as the crack grows into the heat affected zone (HAZ). A slight improvement in the fracture resistance is measured, and through the weld, a slight decrease in fracture resistance is observed.

Experimental and Numerical Study of Moisture Effects on the Bond Fracture Energy of FRP/Concrete Joints

This study experimentally investigates the mechanism of deterioration for the bond between CFRP plate and concrete due to moisture attack. The debonding failure in the concrete structures with externally bonded FRP has two major modes: cohesive failure in the concrete surface layer and adhesion failure at the adhesive/concrete interface. A modified double cantilever beam (MDCB) test was used to measure the interfacial fracture energy for the CFRP plate debonding from concrete substrate under Mode I loading. A simple method was developed to measure the value of interface region relative humidity (IRRH). The relation between IRRH and the bond fracture energy was then obtained. By quantitatively measuring the residual thickness of concrete (RTC) on the detached plate, it was found that RTC was directly related to the value of IRRH. Based on the experimental data, a constitutive relation between RTC and IRRH was obtained. Using this relation and virtual crack closure technique (VCCT), the bond fracture energy was numerically determined as a function of IRRH for the FRP/concrete bond joints. It was found that when the IRRH value was greater than 55%, the deterioration of fracture energy was accelerated. When the IRRH was greater than 75%, the fracture energy tended to become steady, and did not change significantly with the increases of IRRH.

Crack tip opening angle optical measurement methods in five pipeline steels

Crack tip opening angle (CTOA) is becoming one of the more widely accepted properties for characterizing fully plastic fracture. In fact, it has been recognized as a measure of the resistance of a material to fracture in cases where there is a large degree of stable-tearing crack extension during the fracture process. Our current pipeline research uses the CTOA concept as an alternative or an addition to the fracture energy characterizations provided by the Charpy V-notch (CVN) and drop weight tear test (DWTT). A test technique for direct measurement of CTOA was developed by use of a modified double cantilever beam (MDCB) specimen. A digital camera and image analysis software were used to record the progression of the crack tip and to estimate CTOA. In this report, different optical measurement methods are compared, three using the crack edges adjacent to the crack tip (defined in the ISO draft standard and ASTM standard) and one using the specimen surface grid lines. Differences in CTOA resulting from the various measurement methods are evaluated. The CTOAs for five different grades of gas pipeline steel are reported, and the effect of microstructure on CTOA is discussed.

The Effect of the Presence of Water on the Durability of Bond between CFRP and Concrete

This research assesses the effects of the presence of water during the carbon fiber reinforced polymer (CFRP) application and after the CFRP cure on the bond between the CFRP and the concrete. Modified double cantilever beam (MDCB) specimens are used to determine the interfacial energy release rate, G, of the CFRP—concrete bond. A CFRP fabric is applied to the specimens with three different initial levels of water/moisture presence. Other specimens, with a CFRP fabric applied in dry conditions and allowed to cure, are conditioned in a saturated environment for different lengths of time. The test results indicate that the presence of water during the CFRP application decreases the bond quality significantly and most of the resulting failures are adhesive failures along the primer/concrete interface. The use of a specially formulated primer results in a slightly higher bond capacity but the same undesirable failure. High quality CFRP applications, conditioned in a saturated environment for a relatively short period of time, from 3 to 8 weeks, demonstrate that exposure to water degrades the bond between CFRP and concrete after the epoxy has cured.

Finite Element Analysis of Test Specimens to Assess Composite-Concrete Bond Durability

A modified double cantilever beam (MDCB)-hinged specimen that has been used to measure the toughness of the bond between concrete and composite overlays experimentally is modeled by finite element analysis to validate the equations used to analyze the results. Results show that the MDCB-hinged specimen gives good results for Mode I loading but not for Mode II loading. Therefore, a MDCB-tabbed specimen is proposed and evaluated. Equations for calculating the strain energy release rate of the new sample are proposed. Results show that the MDCB-tabbed specimen may be used to determine the Mode II strain energy release rate of the composite-to-concrete bond. In addition, several parameters that may affect the interface bond between the FRP overlay and the concrete are studied.