Load-load Line Displacement Research Articles

Fracture properties of asphalt concrete under various displacement conditions and temperatures

Fracture mechanics has been widely applied to asphalt concrete (AC) materials to evaluate the cracking potential of AC pavements under traffic and thermal loadings. While AC materials behave as quasi-brittle at low temperature and high loading rate, its behavior becomes more viscous as temperature increases and/or loading rate decreases. Considerable fracture tests have been developed to capture the fracture properties of AC materials; however, fracture properties may be misinterpreted when using various fracture tests, especially in bending-mode. This would result in an erroneous characterization of AC cracking potential, which leads to a misestimate of pavement service life. The objective of this study is to investigate common fracture test methods for the AC materials, compare the fracture energies from various displacement measurements to capture true fracture property, and examine the effect of temperature on fracture energy and Flexibility Index (FI). Fracture energy from the semi-circular bending (SCB) geometry was compared to that from the disk-shaped compact tension (DCT) test. The effect of misinterpretation of test results on fracture energies was investigated. This study also suggests a proper method to capture fracture properties from the SCB test using the basic understanding of work and energy concepts in physics. The effects of test temperature on (1) fracture energy values, (2) FI, and (3) the overall ability to discriminate the cracking potential of AC mixtures are presented. Results show that while the crack characteristics can be captured more reasonably with a Load-Crack Mouth Opening Displacement (CMOD) curve, the fracture energy is better obtained using load-Load Line Displacement (LLD) curve. Moreover, performing fracture tests at a relatively higher temperature increases the ability to distinguish between various AC mixtures’ cracking potential.

Fracture Performance of Type 304 Stainless Steel Reinforcement Belt from Cryogenic to Elevated Temperatures

Effects of temperature on fracture characteristics of type 304 stainless steel at cryogenic to elevated temperatures are investigated. Three point bending fracture tests according to ASTM E1820 guidelines are conducted on single edge notched bending specimens made of cold drawn flat bars in a temperature range of −195 °C to 500 °C. The test setup includes a thermal chamber, a temperature display and control unit capable of maintaining desired thermal conditions and a remote displacement recording device. Crack lengths are estimated by using load-load line displacement curves with proper calibration equations. The temperature dependency of tensile properties of the test material is also considered. Resistance curves (J-Δa) are then determined for ten values of test temperature. Finally, fracture parameters of test specimens such as critical J values and tearing modulus are extracted. Results reveal that harsh thermal conditions have significant effects on ductile fracture performance of type 304 stainless steel.

EFFECT OF LOADING RATE ON FRACTURE BEHAVIOUR OF Mg-Al-Zn ALLOYS

In recent years, magnesium alloys are widely used for automotive applications as structural components due to its lightweight property and high specific strength. In this regards, magnesium alloys are subjected to high velocity and impact loads during accident. Hence, understanding the impact and dynamic behaviours of magnesium alloys are essential. In this study, the effect of loading rates on the fracture behaviour of Mg-Al-Zn alloys was investigated using pre-cracked single-edge notched bending (SENB) specimens. Three-point bending tests were conducted at different loading rates of 5, 50 and 500 mm/min. The Mg-Al-Zn alloys that used in the present study were extruded AZ31 and AZ61 magnesium alloys. From the load-load line displacement results, both alloys exhibited nonlinear fracture behaviour. The maximum load (Pmax) of these two alloys increased with increasing loading rate. Comparing both alloys, AZ61 exhibited higher Pmax than that of AZ31 due to the higher volume of b-phase and smaller grain size in AZ61. Fracture surface observation revealed that both alloys fractured in ductile manner with large scale yielding and high shear lips ratio at all loading rates.

Studies on the effect of compliance on fracture behaviour of carbon steel pipes with circumferential through-wall crack

The design of Nuclear Power Plant piping is based on Leak-Before-Break (LBB) design philosophy. The integrity of cracked nuclear piping system in LBB is often examined from conventional fracture mechanics. Conventionally the fracture analysis is performed by assuming infinite compliance of the piping system, which means that the stiffening effect of the connected uncracked piping system is fully neglected. However, in reality the ends of the pipe are connected to the rigid pressure vessels, primary coolant pipes, etc. In order to study the effect of compliance on the fracture behaviour of pipes, fracture studies were carried out on fourteen specimens of 219 mm outer diameter carbon steel pipes having circumferential through-wall crack. The fracture tests were conducted under four point bending. Out of the fourteen specimens, twelve specimens were tested in fixed end condition (finite compliance) and the remaining two specimens were tested in simply supported condition (infinite compliance). These pipes were made of SA 333 Gr. 6 carbon steel. Extensive instrumentation was carried out on these pipes to collect various data, e.g., load, load-line displacement (LLD), crack mouth opening displacement (CMOD), rotation at the ends (flanges), deflection at critical locations and surface crack growth. Surface crack growth was monitored using Image Processing Technique (IPT). The investigations have shown that the load carrying capacity of the pipes under finite compliance condition (fixed end condition) increases when compared to infinite compliance (simply supported condition). The load carrying capacity of the pipes under finite compliance weakly depends on the initial crack angle where as the load carrying capacity of the pipes under infinite compliance strongly depends on the initial crack angle. The safety margin is quite large particularly with a stiff piping system. This benefit in terms of increase in safety margin from LBB point of view is due to the moment redistribution. This paper discusses the effects of notch angle, span and the support conditions on the fracture behaviour of pipes with circumferential through-wall crack.

Characterization of stable crack growth through AISI 4340 steel using cohesive zone modeling and CTOD/CTOA criterion

The criteria based on cohesive zone model (CZM) and CTOD/CTOA have been employed for analysis of stable crack growth (SCG) through AISI 4340 steel. Investigations have been reported first concerning characterization of mode I SCG using the CZM and a particular crack size in a CT specimen 8 mm thick. The characteristic data is then verified considering other mode I crack sizes and the same specimen geometry. Similar verifications have also been done considering cracks under mixed mode (I and II) loading. The same mode I cases have been studied later considering some variations of crack tip opening displacement/angle (CTOD/CTOA) with crack extension. Load–load-line displacement (LLD) predictions based on these two approaches have been compared with experimental results reported in the literature. The predictions based on the CZM are found to be closer to the experimental results. The results include the traction-separation law suitable for characterization of SCG through AISI 4340 steel under mode I and mixed mode (I and II). Similarly a CTOD/CTOA variation with crack extension for the similar purpose under only mode I loadings has been reported. Other observations on modeling of crack tip constraint effects by mixed (plane strain core plus plane stress outer domain) discretisation scheme, crack tip element size and J integral, are presented.

응집영역모델을 이용한 다공질 재료의 파괴 거동 연구

The effect of porosity on the crack propagation is studied by using the cohesive zone model. Standard mode Ⅰ fracture test were done by using compact tension specimens with various porosities. Load-load line displacement curves and δ5-crack resistance curves for various porosities were obtained from experiments. The cohesive zone model proposed by Xu and Needleman was employed to describe the crack propagation in porous media, and the Gurson model is used for constitutive relation of porous materials. These models were implemented into user subroutines of a finite element program ABAQUS. The fracture mode changes from ductile fracture to brittle fracture as the porosity increases. Numerical calculations agree well with experimental results.

Determination of J-Integral Using the Load-COD Record for Circumferential Through-Wall Cracked Pipes

The present paper provides experimental J estimation equation based on the load-crack opening displacement (COD) record for testing the circumferential through-wall cracked pipe under combined tension and bending. Based on the limit analysis and the kinematically admissible rigid-body rotation field, the plastic η-factor for the load-COD record is derived and is compared with that for the load-load line displacement record. Comparison with the J results from detailed elastic-plastic finite element analysis shows that the proposed method based on the load-COD record provides reliable J estimates even for shallow cracks (small crack angle), whereas the conventional approach based on the load-load line displacement record gives erroneous results for shallow cracks. Thus, the proposed J estimation method could be recommended for testing the circumferential through-wall cracked pipe, particularly with shallow cracks.

Fracture analysis of a high-strength concrete and a high-strength steel-fiber-reinforced concrete

This paper addresses the use of R-curves to study the fracture behavior of high-strength concrete and steel-fiber-reinforced concrete subjected to crack ing in a three-point bending configuration. The R-curves are modeled through an effective approach based on the equations of linear-elastic fracture mechanics (LEFM), which relates the applied load to the fundamental displacements of notched-through beams loaded monotonically. It is initially shown that, for quasi-brittle materials, the R-curves responses can be evaluated in a quasi-analytical way, using the load-crack mouth opening, the load-load line displacement, or exclusively the displacement responses obtained experimentally. Afterward, the methodology is used to obtain the fracture responses of high-strength and fiber-reinforced concretes, up to the final stages of rupture.

A Method of J Testing Based on Crack Opening Displacement for Circumferential Through-Wall Cracked Pipes under Combined Tension and Bending

Application of the leak-before-break concept to nuclear piping requires accurate fracture mechanics assessment on pipes with postulated circumferential through-wall crack subject to combined tension and bending. One important element is determination of relevant J-resistance curve for pipes under combined loading. This paper provides experimental J estimation method for the circumferential through-wall cracked pipe under combined tension and bending, based on the load-crack opening displacement (COD) record to estimate J-resistance curve. To give confidence in the proposed method, the J results from detailed elastic-plastic finite element (FE) analysis are also compared with estimated J based not only on proposed method but also on conventional method using load-load line displacement, which shows that estimated J based on the proposed method provides reliable J estimates regardless of analysis condition, on the other hand, the conventional method using the load-load line displacement record gives erroneous results for shallow cracks.

하중-균열열림변위를 이용한 굽힘하중이 작용하는 원주방향 관통균열 배관의 새로운 J 실험법

The present paper provides experimental J estimation equation for the circumferential through-wall cracked pipe under four-point bending, based on the load-crack opening displacement (COD) record. Based on the limit analysis and the kinematically admissible rigid-body rotation field, the plastic η-factor for the load-COD record is derived and is compared with that for the load-load line displacement record. Comparison with the J results from detailed elastic-plastic finite element (FE) analysis shows that the proposed method based on the load-COD record provides reliable J estimates even for shallow cracks, whereas the conventional approach based on the load-load line displacement record gives erroneous results for shallow cracks. Thus, the proposed J estimation method could be recommended for testing the circumferential through-wall cracked pipe, particularly with shallow cracks.

In Situ Fracture Toughness Measurement Using Scanning Electron Microscopy

Abstract In situ tests were developed to measure fracture toughness (KIc, JIc, CTOD, R-curves) in small samples, simultaneously observing details of the crack blunting, initiation, and propagation by using scanning electron microscopy (SEM). A small load frame was employed for this purpose and in situ tests were performed in a Philips 515 SEM with a small chamber. A load-load line displacement record could be obtained from the different types of tests in order to calculate any fracture mechanics parameter, determine the initiation and amount of stable crack growth, measure directly the CTOD or Schwalbe's δ5 and correlate any of these parameters with the observed micromechanisms. The applicability of the proposed in situ fracture toughness measurement technique has been exemplified making use of a wide set of materials, and its limitations were also evaluated.

On the Sensitivity of J Estimation to Materials' Stress-Strain Curves in Fracture Toughness Testing Using the Finite Element Method

Abstract The present paper reports the accuracy of the J estimation equations for three typical homogeneous fracture toughness testing specimens: M(T), C(T), and SE(B) specimens, based on systematic 2D FE analyses. A wide range of crack lengths is considered, except for C(T) specimens where only deep cracks are considered, a/W = 0.45 to 0.7. By carefully selecting the properties of the materials in the analysis, particular emphasis is placed on the sensitivity of such equations to the strain hardening as well as the type of stress strain curve, such as continuously hardening materials or materials with Lüders strain. Two different J estimation schemes are considered, one based on load-load line displacement records and the other on load-crack mouth opening displacement (CMOD) records. For all cases considered, the J estimation based on load-CMOD records gives more accurate results, compared to those based on load-load line displacement records. Moreover, the J estimation is not sensitive to the material's strain hardening and to the type of the stress strain curve, except for shallow-cracked M(T) specimens with a/W < 0.2. For shallow-cracked M(T) specimens with a/W < 0.2, the J estimation equation is not sensitive to the material, for materials with Lüders strain, but is sensitive for continuously hardening materials. The error increases with the hardening, i.e., a higher hardening material gives a larger error in J.

Net shape manufacturing and the performance of polymer composites under dynamic loads

An experimental investigation was conducted to determine the influence of postmold trimming and resultant edge quality on the performance of fiber-reinforced plastics (FRP) under dynamic loads. Graphite/epoxy and graphite/bismaleimide laminates were machined using three state-of-the-art industrial techniques and subjected to three-point bend impact to failure. The load load-line displacement records were used to obtain the load, bend deflection and energy absorbed to fracture. High-speed photography was also employed to identify the initiation and progression of failure and record the time dependent fracture process. From a comparison of edge quality and subsequent material performance for both polymeric composites, it was found that the impact response of FRPs is highly process dependent. In general, the load and energy absorbed to fracture decreases with increasing surface roughness. Reductions in the load and energy to the onset of fracture with degrading surface quality were as high as 20 percent. The dynamic response was also found to be dependent on the constituents, stacking sequence and impact velocity.

Analysis of stable crack growth across welded fusion line: Japanese round-robin

Bimaterial CT specimens are numerically analyzed in the elastic-plastic states. By changing the material constants and the distance between the crack tip and the phase boundary, the parametric analyses are conducted. J integral evaluated by the line integration is compared with that obtained by the empirical formula by Merkel-Corten. The effects of the inhomogeneities on the accuracy of the J integral evaluation are discussed. Next the stable crack growth analyses are conducted. Using the relation between a and load-load line displacement obtained experimentally, the generation phase analyses are carried out. J integral, T ∗ integral and CTOA are evaluated and the effects of the fusion line are discussed.

Evaluation of conventional and microfilled composite resins using an acoustic emission technique.

Load-load line displacement diagrams and acoustic emission characteristics were measured in the transverse test of conventional and microfilled composite resins in order to develop a method for the evaluation of strength of dental restorative materials and the non-destructive inspection of restorations in oral cavity. Both conventional and microfilled composite resins exhibited burst type acoustic emission. Conventional composite resins generated the abrupt acoustic emission, but microfilled composite resins did not showed an appreciable increase in acoustic emission before fracture for a low acoustic emission activity.

Applicability of the EPRI estimation scheme for the prediction of load-load line displacement behaviour and for single specimen J-R-curve determination

Applicability of the EPRI estimation scheme for the prediction of load-load line displacement behaviour and for single specimen J-R-curve determination

Ａ５３３Ｂ‐１鋼の弾塑性破壊じん性および破壊抵抗の評価

The elastic-plastic fracture toughness JIc value and fracture resistance of A533B-1 steel were evaluated using compact type (1/2CT, 1CT and 2CT) specimens. The results obtained are summarized as follows:(1) According to the standard method of the Japan Society of Mechanical Engineers (JSME), the JIc value can be evaluated accurately by using the smaller size of specimens compared with the ASTM method.(2) The valid JIc value becomes constant regardless of stress states or yielding conditions, when J integral is evaluated from a load-load line displacement curve.(3) As long as the flat fracture preceeds in the midthickness of the specimen, the tearing modulus, TJ(Δamax), evaluated from the J-Δamax resistance curve, becomes a material constant for a wide range of Δamax.(4) Before the initiation of shear fracture, the following equation can stand;Δaave=0.38ΔamaxWithin this region, TJ(Δaave) evaluated from the J-Δaave resiatance curve becomes constant in appearance. However, as the area of shear lips increases, TJ(Δaave) decreases rapidly.