Fracture Toughness Test Specimens Research Articles

Study on fracture behaviour of pipeline girth welds based on curved wide plate specimens: Experimental and numerical analysis

Understanding the fracture characteristics and strain capacity of pipelines with girth welds under external loads is crucial for evaluating pipeline safety. The use of curved wide plate (CWP) specimens closely resembling full-scale (FS) pipelines in geometry provides a more realistic representation of crack tip constraint states compared to small-size fracture toughness test specimens in laboratory settings. This study aims to investigate the ductile fracture characteristics of center cracks on the outer surface of girth welds in high-grade steel pipelines under external loads through large-scale CWP tensile tests and advanced numerical simulations. Tensile tests were conducted on CWP specimens using a kiloton tensile testing machine, with double crack opening displacement (COD) gauge monitoring crack mouth opening displacement (CMOD), and high-precision displacement sensors and strain gauges tracking deformation and strains at various positions. The study yielded significant experimental results, and a finite element (FE) model of the CWP was developed using the nonlinear finite element method (FEM) to validate the experimental data against simulation results. The FE model was then used to investigate the influence of material properties on crack propagation resistance and driving force curves. A method for determining the ultimate tensile strain capacity (UTSC) of pipelines based on actual material fracture toughness was proposed. This research contributes valuable experimental data for further exploration of fracture behaviour in large-scale pipeline girth welds and offers insights for safety evaluations of such welds.

Functionalization of calcium-deficient nanohydroxyapatite improves the mechanical properties of 3D printed biopolymer nanocomposites

Agglomerations of nanoparticles in a polymer matrix can drastically reduce the mechanical properties of a polymer nanocomposite, especially its strength. The grafting of nanoparticle surfaces with suitable functional groups can provide improved dispersion and stronger interfacial bonding, improving the fracture resistance of the nanocomposite. In this study, calcium-deficient nanohydroxyapatite (nHA) particles were functionalized with an amino acid-based urethane methacrylate (lysine urethane methacrylate, LUM) and subsequently reacted with hydroxyethyl methacrylate. We mixed these functionalized nHA particles with resin, composed of methacrylated acrylated epoxidized soybean oil, methacrylated isosorbide, and triethylene glycol dimethacrylate, and 3D-printed nanocomposites using masked stereolithography. We hypothesized that the functionalized nanoparticles would enhance the mechanical performance of the 3D-printed nanocomposites due to the greater dispersion and stronger interface. Flexural, tensile, compression and Mode-I fracture toughness test specimens were fabricated using a mSLA printer and tested following ASTM standards. The LUM functionalization of nHA improved the dispersion and increased the viscosity of the uncured nanocomposite ink. The flexural fracture strength, yield strength, and mode-I fracture toughness values were increased by 10 %, 30 %, and 11 %, respectively. The LUM improved the strength and fracture toughness by providing a stronger, more stable interface, resisting debonding between the matrix and particles, allowing for greater plastic deformation.

Solutions and applications of 3D elastic–plastic constraint parameters for clamped single edge notched tension (SENT) specimens

Single edge notched tension (SENT) specimen is a common fracture toughness test specimen now widely used in the pipeline and pressure vessels industries. In this paper, an extensive study has been conducted by three-dimensional (3D) finite element analyses (FEAs) to investigate the in-plane constraint parameters (namely A and QSSY) and out-of-plane constraint parameter, that is, Eε33 for clamped SENT specimens with side-grooves. A wide range of crack depth to specimen width ratios a/W = 0.2, 0.3, 0.4, 0.5, 0.6 and 0.7, specimen thickness to specimen width ratios B/W = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0 were included. The various strain hardening exponent (n = 4, 7, 10) and applied loading levels (from small-scale yielding to large-scale yielding) were also investigated. The results show that the effects of crack depth, specimen thickness, loading level and strain hardening index on the 3D constraint parameters show a significant coupling effect. With the increase of load level, A value increases monotonically, while QSSY and the normalized Eε33 (called V33) decrease, and the influence of specimen geometry on the constraint levels will become greater. The values of A and V33 obviously decrease with the increase of hardening exponent, but the change of QSSY is relatively small. Moreover, it can be observed that both A and QSSY can well describe the in-plane constraint effect, and V33 can better quantify the out-of-plane constraint effect under varying loading levels and n. In addition, elastic–plastic constraint parameters were also calculated for various experimental test specimens made of 16MND5 steel, with n = 9.2. It was demonstrated these constraint parameters can be applied to quantify and predict the J-R curves of 16MND5 steel with good agreements for specimens with different in-plane and out-of-plane constraint conditions.

A split-Chevron-Notched-Beam sandwich specimen for fracture toughness testing of bonded interfaces

The realization that the use of strength tests as a means to probe the bonding performance of adhesive systems to dental restorative materials suffers from inherent drawbacks has shifted efforts towards devising viable and valid fracture toughness test for that purpose. Here we introduce a new procedure to produce split-Chevron-Notched-Beam specimens that need not undergo sawing or notching after bonding, thus sparing the interface from pre-stresses. We evaluate the formal geometric factor considering the influence of the use of different materials having different elastic properties, and show that the results obtained fall within reasonable ranges measured by other established compliance calibrated methods. We demonstrate the application of this new approach for dentin-luting composite and zirconia-luting composite interfaces for different adhesive systems and pre-treatment procedures in order to probe the sensitivity of the method to different bond qualities.

Mechanical degradation of contemporary CAD/CAM resin composite materials after water ageing

ObjectiveThe aim of the present study was to assess the effect of water storage on the quasi-static properties and cyclic fatigue behavior of four contemporary CAD/CAM resin composite materials. MethodsThe CAD/CAM resin composites Grandio Blocs, LavaTM Ultimate, CerasmartTM and Brilliant Crios, as well as the direct resin composite Grandio SO, were evaluated. Rectangular plates were cut from the blocks or fabricated using a silicon mold to obtain specimens for fracture toughness (KIc, n = 10), biaxial strength (σ0, n = 30) and cyclic fatigue testing (n = 30). Half of the specimens was stored for 24 h in dry conditions and the other half was aged for 60 days in distilled water at 37 °C. KIc was determined using the Compact-Tension (C(T)) method and σ0 and cyclic fatigue were tested using the Ball-on-Three-Balls assembly. Additional disc-shaped specimens (n = 5) were produced to obtain water sorption curves of the materials. Weibull statistics and two-way ANOVA with Tukey’s post-hoc test were used for data assessment. ResultsThe highest water sorption was observed for LavaTM Ultimate (42.6 μg/mm3), whereas Grandio SO displayed the lowest uptake (14 μg/mm3). A statistically significant drop in KIc and σ0 was measured for all materials after water storage, except for the σ0 of CerasmartTM. Water ageing had a dissimilar effect on the cyclic fatigue behavior, increasing the slow crack growth susceptibility of LavaTM Ultimate, but decreasing it for CerasmartTM and Brilliant Crios. SignificanceContemporary CAD/CAM resin composites are susceptible to water driven degradative processes, although differences in filler content and resin matrix constitution play an important role in how it impacts their mechanical properties.

The Effect of Single-Walled Carbon Nanotube (SWCNT) Concentration on the Mechanical and Rheological Behavior of Epoxy Matrix

The improvement of Mode I fracture toughness of epoxy by the addition of single-walled carbon nanotubes (SWCNTs) is considered. To prepare nanocomposites, chemical-vapor-deposition-grown SWCNTs noncovalently functionalized with an ethoxylated alcohol was used as the additive and a diglycidyl ether of bisphenol-A-based epoxy as the matrix material. The SWCNTs were dispersed in the epoxy matrix via a mechanical stirrer and a 3-roll mill. The effect of their concentration (0.0125, 0.025, 0.05, 0.1, 0.3, and 0.5 wt.%) on the mechanical properties of the nanocomposites was investigated, and the optimum concentration was determined. Mode I fracture toughness (single-edge-notch 3-point bending) and tensile tests were carried out on neat epoxy and SWCNT-reinforced epoxy nanocomposites. The fractured surfaces of fracture toughness and tensile test specimens were examined by the SEM to reveal the effect of SWCNTs on their failure modes.

On the use of edge cracked short bend beam specimen for PMMA fracture toughness testing under mixed-mode I/II

In this paper an inclined edge cracked short beam specimen subjected to symmetric three-point bend loading was designed and examined for conducting mixed-mode I/II fracture toughness experiments. The aspect ratio (i.e. length to width ratio) and the loading span distance are considered much lower than the other conventional cracked bend beam samples. Crack tip parameters such as stress intensity factors and T-stress were computed numerically for this specimen by several finite element analyses and it was demonstrated that the specimen is able to produce full combinations of mode I and II including pure mode II. The practical capability of the short bend beam specimen was studied experimentally by conducting a set of mixed-mode fracture tests on PolymethylMethacrylate (PMMA) as a well-known model brittle material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using two stress and strain-based fracture criteria. The fracture toughness of tested PMMA was decreased by moving towards mode II case due to the effect of T-stress on the fracture mechanism of the short bend beam specimen.

Fatigue Precracking Time Estimates for Three-Point Bending Specimens

Specimens containing sharp cracks are needed in certain types of mechanical tests, first and foremost for fracture toughness measurement of materials. Their use, however, is not just limited to this type of test. Another category of experiments deals with characterizing nonlinear vibrations of beams containing breathing cracks. To produce cracked beam specimens, fatigue cracks can be grown ahead of sharp notches under controlled loading. ASTM E399 and ASTM E1820 standards provide guidance on such procedures for preparation of fracture toughness test specimens. However, certain issues which might become important in testing of vibrations of cracked beams, such as the time required for specimen preparation is not addressed in these standards. In this article, both low cycle fatigue methods and linear elastic fracture mechanics methods are used to estimate the number of loading cycles required to have a crack of desired length at the notch tip. Calculation results are compared with experimental ones, and the effects of various factors influencing the required number of loading cycles for a certain crack size are discussed.

A crack-location correction for T0 analysis of an alloy 52 dissimilar metal weld

For narrow-gap Alloy 52 dissimilar metal welds, the fusion boundary between the ferritic pressure vessel steel and austenitic Ni-base alloy weld metal is the weakest region from fracture mechanical perspective. Producing of the pre-crack for fracture toughness test specimens in the fusion boundary region to determine T0 is challenging. For pre-cracks further away from the fusion boundary, the brittle fracture initiation does not necessarily occur on the initial crack plane, but rather in the fusion boundary region. This offset in initiation site can increase the measured fracture toughness leading to a nonconservative T0 estimate. In this study, a method is proposed for correcting the fracture toughness for crack location and obtaining a lower boundary fracture toughness curve. The crack-location correction shows promise for analysing T0 of specimens with pre-cracks systematically at a significant distance from the weakest region, but with brittle fracture initiation still occurring at the weakest location.

Stress intensity factor and T-stress solutions for three-dimensional clamped single edge notched tension (SENT) specimens

In the present paper, a crack compliance analysis approach and extensive three-dimensional finite element analysis were conducted for clamped SENT specimens, which is one of the most widely used low-constraint specimens for less-conservative fracture toughness testing. A wide range of geometrical parameters variations are considered including in-plane crack depth to plate width ratio (a/W), out-of-plane plate thickness to width ratio (B/W), and daylight to width ratio (H/W). Complete solutions of stress intensity factor (K), in-plane T-stress (T11) and out-of-plane T-stress (T33) are obtained. The results obtained from above two methods are in good agreement. Furthermore, the combination of the effects of crack depths (a/W), plate thickness (B/W), and daylight to width ratio (H/W) on the stress intensity factor, T11 and T33 stress are thus illustrated. To better understand the difference between the derived stress intensity factor solutions and 2D plane strain results, fracture experiments were conducted for clamped SENT specimens with different crack depths. Solutions obtained will be very useful for analyzing fracture toughness test data and determining fatigue crack growth rate for clamped SENT specimens with different crack depths (a/W), plate thickness (B/W), and daylight to width ratio (H/W).

Forming mechanism of delamination cracks observed during tensile and fracture toughness testing of X70 pipeline steel

In this study, we have investigated microstructures of the delamination cracks observed from tensile and fracture toughness test specimens using an API X70 pipeline steel. It is found that the delaminations observed from both tensile and toughness specimens are intergranular fractures. At the occurrence, characteristics of brittle fracture were observed, but it was found not to be a brittle fracture as the delaminations were induced by plastic deformation. It is shown that severe plastic deformation produced strain concentration around particles located along grain boundaries and caused decohesion between adjacent grains, resulting in intergranular fracture.

Numerical simulation of residual stress modification by reverse bending of notched fracture toughness test specimens of multipass welds

In the fracture toughness tests of multipass welds of thick steel plates, the fatigue precrack front initiating from a machined notch root frequently be non-uniform affected by the welding residual stress distribution. Reverse bending before fatigue precracking could be an effective measure of residual stress modification. However, the effect of reverse bending is not well understood from the viewpoint of the variation of the residual stress distribution at the notch root. Therefore, the transition of the welding residual stress throughout the multipass welding, notch machining, and reverse bending processes is calculated using a numerical simulation model developed in this study. The compressive plastic deformation at the notch root caused by the reverse bending and successive tensile loading during unloading are the major mechanisms of residual stress modification by reverse bending. A flattened residual stress distribution along the thickness can be achieved if the reverse bending condition is properly controlled.

Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

AbstractThe cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (KIc) of rock, and it has also been applied to mode II fracture toughness (KIIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the KIc value measured using the CCNBD specimen is usually conservative, and the assumptions made in the mode II test are rarely assessed. In this study, both laboratory experiments and numerical modeling are performed to study the modes I and II CCNBD tests, and an acoustic emission technique is used to monitor the fracture processes of the specimens. A large fracture process zone and a length of subcritical crack growth are found to be key factors affecting the KIc measurement using the CCNBD specimen. For the mode II CCNBD test, the crack growth process is actually quite different from the assumptions often made for determining the fracture toughness. The experimental and numerical results call for more attention on the realistic crack growth processes in rock fracture toughness specimens.

Residual stress state in pipe cut ring specimens for fracture toughness testing

Abstract Thin-walled pipes are not suitable for measuring fracture toughness parameters of vital importance because longitudinal crack failure is the most common failure mode in pipes. This is due to the impossibility to manufacture standard specimens for measuring fracture toughness, such as SENB or CT specimens, from the thin wall of the pipe. Previous works noticed this problem, but until now, a good and convenient solution has not been found or developed. To overcome this problem, very good alternative solution was proposed, the so-called pipe ring notched bend specimen (PRNB) [1–5]. Until now, only the idealized geometry PRNB specimen is analyzed, i. e., a specimen which is not cut out from an actual pipe but produced from steel plate. Based on that, residual stresses are neglected along with the imperfections in geometry (elliptical and eccentricity). The aim of this research is to estimate the residual stress state(s) in real pipes used in the boiler industry produced by hot rolling technique. These types of pipes are delivered only in normalized condition, but not stress relieved. Therefore, there are residual stresses present due to the manufacturing technique, but also due to uneven cooling after the production process. Within this paper, residual stresses are estimated by three methods: the incremental hole drilling method (IHMD), X-ray diffraction (XRD) and the splitting method (SM). Knowing the residual stress state in the ring specimen, it is possible to assess their impact on fracture toughness measured on the corresponding PRNB specimen(s).

Modification of fracture toughness Master Curve considering the crack-tip Q-constraint

Engineering structures may have significant lower crack-tip constraint compared with those in the standard fracture toughness test specimens. This paper considers a methodology to extend the application of fracture toughness master curve approach for structures with low crack-tip constraint. This is conducted by considering the Q-parameter as a suitable crack-tip constraint parameter, and study its effects on the reference temperature T0 in the master curve approach. The experimental fracture toughness data from the literature as well as supplemental experiments are used for this purpose. The relation between Q and T0 is investigated by using numerical analyses of experiments covering various types of geometry, loading condition, thickness and crack depth. The effects of loading amplitudes and material properties on the relation between Q and T0 are also studied. Based on this study, a relationship between Q and T0 is derived and experimentally verified. It is shown that the proposed relationship has a considerable ability to predict the apparent fracture toughness behavior in the ductile to brittle region of complex structures without need of conducting additional experimental fracture toughness tests.

Correlation of Pipe Ring Notched Bend (PRNB) specimen and Single Edge Notch Bend (SENB) specimen in determination of fracture toughness of pipe material

AbstractIt is well known that pipes or tubes due to their geometry are not suitable for producing standard specimens for fracture toughness testing (e. g. Single Edge Notched Bend (SENB) specimens or Compact Tension specimens. This is especially evident in thin walled pipes or tubes. We propose a new type of specimen, called Pipe Ring Notched Bend (PRNB) specimen, which is ideal for pipes or tubes due to its ring geometry. In the recent years, the authors have made a lot of research on this type of specimen, but cut out from a plate, not from pipes, where some imperfections in the geometry, material, residual stresses, etc. are present. Within this paper, two PRNB specimens (designations PRNB1 and PRNB2) are cut out from pipe which is used in boilers and made from 16Mo3 steel. From the same pipe, four SENB specimens (designations SENB1, SENB2, SENB3 and SENB4) are also made in order to compare the fracture behaviour of standard SENB and newly proposed PRNB specimens.

Evaluation of rotational deformation in compact specimens for CTOD fracture toughness testing

In Crack Tip Opening Displacement (CTOD) fracture toughness tests, the plastic hinge model is used for calculating the plastic component of CTOD, but the accuracy of the plastic hinge model has been a matter of controversy. In this study, 3-D elastic-plastic finite element analysis was conducted by using a stepped notch 1T compact specimen model, in which the ratio of crack length to specimen was set at 0.5. Two steel models, the low yield to tensile ratio, Y/T, of 0.6 and the high Y/T of 0.9, were employed for the analysis. The plastic rotational center was determined by using distributions of the plastic strain increment and the total strain increment in the crack opening direction along the ligament on the mid-thickness plane. The effect of Y/T on the plastic rotational factor was quite small in the C(T) specimen model, and the average of their plastic rotational factors was 0.52 in both Y/T steel models. This plastic rotational factor is consistent with conventional ones, and is reasonably used for CTOD calculation.

Experimental investigation of local tensile and fracture resistance behaviour of dissimilar metal weld joint: SA508 Gr.3 Cl.1 and SA312 Type 304LN

AbstractThe aim of the paper is to evaluate the local tensile and fracture toughness properties of the dissimilar metal weld joints between SA508Gr.3 Cl.1 and SA312 Type 304LN pipe. Weld joints have been prepared by manual gas tungsten arc welding (GTAW) process with conventional V‐groove and automatic hot wire gas tungsten arc welding with narrow gap using different filler wires/electrode such as Inconel 82/Inconel 182 and ER309L/ER308L. The tensile and fracture toughness test specimens have been machined from different regions of dissimilar metal weld such as heat affected zones, fusion lines, buttering layer, weld metal and both base metals. Tensile and fracture toughness tests have been carried out as per the ASTM standard E8 and E1820 respectively. Tensile and fracture toughness results of all the regions of dissimilar metal weld joints have been discussed in this paper. Metallurgical and fracture surface examinations have also been reported to substantiate the tensile and fracture toughness results. Need for the local properties for integrity assessment of the dissimilar metal weld joints has also been brought out.

A novel test specimen for investigating the mixed mode I+III fracture toughness of hot mix asphalt composites – Experimental and theoretical study

Asphalt concrete pavements containing top-down cracks can be subjected to combined opening-out of plane sliding (i.e. mixed mode I/III) deformations due to traffic loads. However, lack of a suitable laboratory test specimen for fracture toughness testing of asphalt composites under mixed mode I/III, is one of the main shortcomings of this field. Hence, a new and simple test configuration is proposed in this research for determining mixed mode I/III fracture toughness of these composite materials. The specimen which is called the Edge Notched Disc Bend (ENDB), is a disc containing an edge crack created through one side of specimen along the diameter and is loaded by a three-point bend fixture. It was demonstrated that the complete range of mode mixity from pure mode I to pure mode III can be achieved by this specimen simply by changing the crack inclination angle relative to the loading support direction. The practical ability of the suggested ENDB specimen, was then examined for obtaining the mixed mode I/III fracture toughness of a typical hot mix asphalt (HMA) composite for different mode mixities including pure modes I and III. Corresponding values of KIc and KIIIc were determined for each mode mixity and it is shown that the obtained results are in quite well agreement with the predictions of a mixed mode I/III fracture theory. Consequently, the ENDB specimen can be used as a favorite candidate specimen for mixed mode fracture toughness study of asphalt concrete materials.

1120 延性-脆性遷移温度域材ミニチュア破壊靭性試験片の開発(OS8-4 材料力学と計算力学の新展開)

1120 延性-脆性遷移温度域材ミニチュア破壊靭性試験片の開発(OS8-4 材料力学と計算力学の新展開)