A Numerical Study on Crack Growth Resistance Curve of X70 Steels

In this work a phenomenological approach is employed for fracture initation and propagation in SENT specimens. Modified Mohr-Coulomb (MMC) fracture criterion establish the initiation of damage/fracture at the critical material point and the post-initation softening controls its rate of propagation. By means of a tridimensional mixed stress state (equivalent plastic strain, εp; Triaxiality, η; and Lode angle, θ) the nature of the growing crack front can be explained.

Strength prediction of composite ceramic under the non-ellipsoidal particles bridging

Purpose This paper aims to promote a strength model for TiC-TiB2 composite ceramic with non-ellipsoidal particles bridging. Based on the microstructure of TiC-TiB2 composite ceramic, equivalent average residual stress under particles interaction is calculated with the interact direct derivative estimate. Supposing the crack opening displacement keeps ellipsoidal under the TiB2 particles bridging, crack growth resistance curve is obtained. Design/methodology/approach Composite strength under R-cure with crack unstable propagation is calculated. Based on this model, influences of particles volume fraction, shape, size and other parameters on strength are analyzed. Findings Results indicated that calculated values are consistent to the tested data. Crack growth resistance increases with crack propagation and TiB2 volume fraction. The TiB2 particle does not pull-out entirely even ceramic fracture. Ceramic strength increases with the TiB2 particle volume fraction, the ratio of platelet diameter and thickness, and it reduces with particle thickness. Originality/value Supposing the crack open displacement keeps ellipsoidal under the TiB2 particles bridging, crack growth resistance curve is obtained.

Generation of Crack Growth Resistance Curve from ENF Specimens Made of Glass/Epoxy: An Experimental Study

This article examines the fracture toughness of end notched flexure (ENF) composite specimens of three different lay-ups. Experiments were conducted on these glass/epoxy specimens and the critical fracture energy,, was evaluated based on compliance based beam method (CBBM). Classical methods require crack length measurements, which are not easy to obtain as propagation occurs rapidly without a clear opening. The CBBM is based on crack equivalent concept, which does not require crack length monitoring during propagation and hence the crack growth resistance curve (R-curve) can be generated in a much easier way. Moreover, the CBBM accounts the non-negligible energy dissipation in the fracture process zone (FPZ) in addition to stress concentrations near crack tip, contact between specimen arms at the pre-crack region and root rotation effects. Hence, the complete R-curves of ENF specimens of different lay-ups were obtained using the CBBM with higher degree of accuracy. It was observed that the unidirectional specimen did show higher propagation toughness value than the angle-ply and cross-ply specimens.

Further Insights on the Relationship Between J and CTOD for SE(B) and SE(T) Specimens Including Ductile Crack Propagation

In structural assessment procedures the crack driving force is usually estimated numerically based on the J -Integral definition because its determination is well established in many finite element codes. The nuclear industry has extensive fracture toughness data expressed in terms of J-Integral and huge experience with its applications and limitations. On the other hand, material fracture toughness is typically measured by Crack Tip Opening Displacement (CTOD) parameter using the hinge plastic model or double clip gauge technique. The parameter CTOD has a wide acceptance in the Oil and Gas Industry (OGI). Also, the OGI has a lot of past data expressed in terms of CTOD and the people involved are very familiar with this parameter. Furthermore, the CTOD parameter is based on the physical deformation of the crack faces and can be visualized and understood in an easy way. There is a unique relationship between J and CTOD beyond the validity limits of Linear Elastic Fracture Mechanics (LEFM) for stationary cracks. However, if ductile crack propagation occurs, the crack tip deformation profile and stress-strain fields ahead of the crack tip will change significantly when compared to the static case. Thus, the stable crack propagation may change the well established relationship between J and CTOD for stationary cracks compromising the construction of resistance curves J-Δa from CTOD-Δa data or vice versa. This investigation is a complementary study on the relationship between J-Integral and CTOD under ductile crack propagation of a previous work. The theoretical definition of CTOD using the 90° method and the empirical expression used in the standard ASTM E1820 are analyzed under stable crack growth. Plane-strain finite element computations including stationary and growth analysis are conducted for 3P SE(B) and clamped SE(T) specimens having different notch length to specimen width ratios in the range of 0.1–0.5. For the growth analysis, the models are loaded to levels of J consistent to a crack growth resistance curve representative of a typical pipeline steel. A computational cell methodology to model Mode I crack extension in ductile materials is utilized to describe the evolution of J with a. Laboratory testing of an API 5L X70 steel at room temperature using standard, deeply cracked C(T) specimens is used to measure the crack growth resistance curve for the material and to calibrate the key cell parameter defined by the initial void fraction, f 0. The presented results provide additional understanding of the effects of ductile crack growth on the relationship between J-Integral and CTOD for standard and non-standard fracture specimens. Specific procedures for evaluation of CTOD-R curves using SE(T) and SE(B) specimens with direct application to structural integrity assessment and defect analysis in pipelines and risers will be proposed, yielding accurate and robust relations between J-Integral and CTOD.

Generation of R-Curve from 4ENF Specimens: An Experimental Study

The experimental determination of the resistance to delamination is very important in aerospace applications as composite materials have superior properties only in the fiber direction. To measure the interlaminar fracture toughness of composite materials, different kinds of specimens and experimental methods are available. This article examines the fracture energy of four-point end-notched flexure (4ENF) composite specimens made of carbon/epoxy and glass/epoxy. Experiments were conducted on these laminates and the mode II fracture energy, GIIC, was evaluated using compliance method and was compared with beam theory solution. The crack growth resistance curve (R-curve) for these specimens was generated and the found glass/epoxy shows higher toughness values than carbon/epoxy composite. From this study, it was observed that R-curve effect in 4ENF specimens is quite mild, which means that the measured delamination toughness, GIIC, is more accurate.