Crack Mouth Opening Displacement Data Research Articles

Conceptualization of Three-Stage Fatigue Failure in Asphalt-Rubber Gap-Graded Mixtures using Dynamic Semi-Circular Bending Test

The objective of this study was to qualitatively measure the cracking mechanism of asphalt-rubber gap-graded (AR-Gap) mixtures and compare the methodical approach proposed in this research with the conventional fatigue process. As part of experimentation plan, dynamic a semi-circular bending (SCB) test was conducted on 27 AR-Gap mixtures with varying mix parameters, including, binder type, binder content, and aggregate gradation. Fatigue life ( Nf) obtained from the dynamic SCB test was analyzed from a statistical viewpoint, and key relationships that potentially contribute to fatigue performance were identified. Later, crack mouth opening displacement (CMOD) was used to study the cracking mechanism of AR-Gap mixtures. CMOD data were analyzed using the Francken model that theorizes the accumulated damage as a three-stage failure. Further, fatigue tertiary life ( Nft) was determined on the premise of structural deterioration obtained from the three-stage failure process. The fatigue disparity factor (ξ), the ratio of Nf to Nft for each asphalt mix was estimated to compare fatigue performance indices. The score of ξ for all the mixtures exceeded 50%, which was indicative of longer crack initiation and crack propagation phase over the third stage of the fatigue cracking mechanism. Overall, the fatigue mechanism was explained through the conceptualization of the three-stage fatigue process through various intrinsic properties of AR-Gap mixtures.

Effect of specimen crack lengths on stress intensity factor for Al6061-TiC composites using experimental and 3D numerical methods

PurposeThe purpose of this paper is to present the determination of critical stress intensity factor (KC) both by experimental method and three-dimensional (3D) finite element simulations.Design/methodology/approachCT specimens of different compositions of Al6061-TiC composites (3wt%, 5wt% and 7wt% TiC) with variable crack length to width (a/W=0.3-0.6) ratios are machined from as-cast composite block. After fatigue pre-cracking the specimens to a required crack length, experimental load vs crack mouth opening displacement data are plotted to calculate the KC value. Elastic 3D finite element simulations have been conducted for CT specimens of various compositions and a/W ratios to compute KC. The experimental results indicate that the magnitude of KC depends on a/W ratios, and significantly decreases with increase in a/W ratios of the specimen.FindingsFrom 3D finite element simulation, the KC results at the centre of CT specimens for various Al6061-TiC composites and a/W ratios show satisfactory agreement with experimental results compared to the surface.Originality/valueThe research work contained in this manuscript was conducted during 2015-2016. It is original work except where due reference is made. The authors confirm that the research in their work is original, and that all the data given in the article are real and authentic. If necessary, the paper can be recalled, and errors corrected.

Estimation of J-resistance Curves for CT Specimen Based on Unloading Compliance Method and CMOD Data

Abstract The unloading compliance method is one of the most widely used methods in laboratory testing for J-resistance estimation. A specimen configuration including compact tension (CT) is recommended by almost all the standard test procedures for evaluating fracture toughness, such as ASTM E1820-11, ISO 12135-2002, and GB/T 21143-2007. In the unloading compliance method, key techniques including instantaneous crack size measurement, correction of specimen rotation, and J-integral computation can be used for a CT specimen when using load line displacement (LLD) but not when using crack mouth opening displacement (CMOD) from those standard test procedures. However, a CT specimen with CMOD measurement is also used for J-resistance estimation under certain severe environments because of the convenience of measuring displacement. This study proposes a simplified linear formula for crack size calculation for a CT specimen with CMOD data, discusses the rotation correction in detail based on the equivalence of work, and develops reasonable incremental expressions for J-integral computation and a CMOD-to-LLD conversion equation. By measuring the LLD and CMOD data of a CT specimen simultaneously, we carried out experiments on J-resistance curve estimation for four different types of materials. According to the developed procedure of the unloading compliance method in this study, J-resistance curves for all materials determined from LLD and CMOD data of the CT specimen are clearly exhibited.

Deflection–crack mouth opening displacement relationship for concrete beams with and without fibres

The three-point bending (TPB) test of a notched beam is a method widely used to characterise the toughness of fibre reinforced concrete (FRC) in terms of fracture energy. However, since the measurement of deflection is significantly dependent on the test set-up, spurious deflections are sometimes obtained. Therefore, it is necessary to correlate deflection with crack mouth opening displacement (CMOD) so that the fracture energy can be evaluated from load–CMOD data. In this study, TPB tests were carried out on a series of beam specimens prepared with plain concrete, FRC and ultrahigh-performance concrete (UHPC). Load, deflection and CMOD were recorded. The results showed that a trilinear relationship could be established between deflection and CMOD. The FRC beams showed slightly higher deflection/CMOD ratios than the plain concrete beams, while a dependency of deflection/CMOD ratios on the fibre type or fibre content was not observed. Moreover, the trilinear relationship cannot apply to UHPC due to deflection-hardening and multiple-cracking behaviour.

Stress Intensity at the Initiation of Instability by R Curve

The life expectancy or failure of aerospace pressure vessels is evaluated by the critical stress intensity determined by the crack growth resistance curve of a material. Load versus crack mouth opening displacement data is generated from the Compact Tension specimens made from the weld joints of maraging steel rocket motor segments. The steps involved to generate critical stress intensity factor is explained. A power law is adopted to model the crack extension in terms of stress intensity factor and determine the maximum failure load of weld specimens. Maximum failure loads of CT specimens obtained by test and analysis are presented.

On the fracture toughness evaluation in weldments of a maraging steel rocket motor case

Fracture toughness of the weld joints of the maraging steel rocket motor case obtained from the limited Compact Tension (CT) specimens are found to be invalid due to the measured crack lengths through the thickness of the specimens exceeding 10% of the initial crack length. This paper proposes an indirect method to evaluate the pre-crack length of the CT specimen from the recorded load versus crack mouth opening displacement data. The Chauvenet’s criterion is applied. After confirming the initial crack size and other validity conditions, the evaluated fracture toughness is designated as the plane-strain fracture toughness ( K IC ).

Revised Incremental J-Integral Equations for ASTM E1820 Using the Crack Mouth Opening Displacement

Abstract The incremental J-integral equations used in ASTM E1820 were developed utilizing the work input to the test specimen, and utilize the load versus load-point/line displacement (LLD) data obtained from the fracture test. The analysis of Ernst et al. [Fracture Machanics: Thirteenth Conference, ASTM STP 743] was then used to obtain crack growth corrected estimates of the J-integral as needed for an accurate J-R curve evaluation. For the compact type specimens, the crack mouth opening displacement (CMOD) gage was moved to the load line so that an adequate estimate of LLD could be obtained from the clip gage and utilized for compliance-based crack extension and incremental J-integral estimates. For the single edge-notched bend [SE(B)] specimens, however, it has been necessary to measure both CMOD for compliance-based crack length estimates and LLD for the crack growth corrected J-integral calculations. Additional experimental complexity and expense are thus added. ASTM E1820 includes a “basic” procedure which allows evaluating the J-integral using CMOD without crack growth corrections if the result is used only for evaluation of initiation toughness. Recently, Annex A16 based on work of Wallin and Laukkanen [Engineering Fracture Mechanics, Vol. 71, 2004, pp. 1601–1614] has been added to ASTM E1820 that presents a multiple-step procedure to obtain a crack growth corrected J-R curve from the basic method data. A more direct approach was proposed recently by Zhu et al. [Journal of ASTM International, Vol. 5, No. 5, 2008, paper ID: JAI 101532] using an incremental procedure to obtain the desired, crack growth corrected J-R curve from the CMOD data of SE(B) specimens with two geometry factors ηCMOD and γCMOD similar to the parameters presently used in the existing LLD-based J-integral analysis. Using fracture toughness test data for SE(B) specimens, detailed comparisons and discussions are made between the proposed method and the ASTM E1820 CMOD methods in determination of J-R curves for HY80 steels. As the basis of comparison, the “accurate” solutions obtained from the standard LLD-based incremental J-integral method are provided for all specimens considered. The difference between the CMOD and LLD methods are then quantified. The results show that the proposed CMOD incremental J-integral equations are simple and effective, and thus are recommended here for use in ASTM E1820 for testing J-R curves and for reducing test costs and complexity.

Further developments in J evaluation procedure for growing cracks based on LLD and CMOD data

Laboratory testing of fracture specimens to measure resistance curves (J − Δa) have focused primarily on the unloading compliance method using a single specimen. Current estimation procedures (which form the basis of ASTM E1820 standard) employ load line displacement (LLD) records to measure fracture toughness resistance data incorporating a crack growth correction for J. An alternative method which potentially simplifies the test procedure involves the use of crack mouth opening displacement (CMOD) to determine both crack growth and J. However, while the J-correction for crack growth effects adopted by ASTM standard holds true for resistance curves measured using load line displacement (LLD) data, it becomes unsuitable for J-resistance measurements based upon the specimen response defined in terms of load-crack mouth opening displacement (CMOD). Consequently, direct application of the evaluation procedure for J derived from LLD records in laboratory measurements of resistance curves using CMOD data becomes questionable. This study provides further developments of the evaluation procedure for J in cracked bodies that experience ductile crack growth based upon the eta-method and CMOD data. The introduction of a constant relationship between the plastic components of LLD (Δp) and CMOD (Vp) drives the development of a convenient crack growth correction for J with increased loading when using laboratory measurements of P-CMOD data. The methodology broadens the applicability of current standards adopting the unloading compliance technique in laboratory measurements of fracture toughness resistance data (J resistance curves). The developed J evaluation formulation for growing cracks based on CMOD data provides a viable and simpler test technique to measure crack growth resistance data for ductile materials.

Composite Beam Analogy Fracture Model for Concrete

A new model for the determination of fracture-mechanics parameters in concrete is developed. Application of this method requires merely knowledge of peak load and peak crack-mouth–opening displacement (CMOD) values. The interrelationship between the damaged and undamaged sections within the microcracked zone is drawn through an analogous composite beam section. The extent of damage material within the process zone is assumed to vary linearly from its maximum level at the crack tip all the way to the boundary of the undamaged section. Hence, the fracture-process zone is defined as a composite material with a linearly variable modular ratio within its boundary. By using this approach, it is possible to determine a number of important fracture parameters from the knowledge of peak load and CMOD at peak load alone. These parameters include the fracture-process–zone size ( r p ), critical fracture-energy release rate ( G IC ), fracture toughness ( K IC ), and fracture energy ( G F ). Furthermore, use of the proposed methodology makes it possible to construct the entire load CMOD, deflection diagrams from the peak load, and CMOD data alone. Fracture-mechanics parameters and load-displacement relationships obtained through analysis by the present approach compare very well with a number of available experimental results.