Crack Tip Opening Displacement Research Articles

Fracture testing of adhesive joints in mixed-mode I+III

The Mixed-Mode Controlled Double Cantilever Beam (MC-DCB) test has been developed to test samples under mixed-mode loading in a combination of mode I and mode III. The mixed-mode ratio is defined based on J-integral and the principle of superimposition holds even in the case of nonlinear fracture behavior. This mixed-mode ratio is externally controlled to be constant during the entire test duration by realizing the MC-DCB test setup as a dual-actuator test in a biaxial testing machine.In this work, test results at constant mode-mixity on two different kinds of adhesive joints are presented: elastic-plastic adhesive joints made of SikaPower®-498 and rather brittle adhesive joints made of Henkel Teroson® EP4510GB. These recently achieved test results give more detailed information on the fracture behavior of the particular adhesive joints. With regard to cohesive zone modelling of possible joint failure, the traction-separation behavior at the initial crack tip has been evaluated by considering the crack-tip opening displacement, which has been measured by digital image correlation in the experiments.

Additively manufactured tensile ring-shaped specimens for pipeline material fracture examination - influence of geometry

In order to define a procedure for integrity assessment of pipelines and determining the fracture mechanics parameters, a new type of specimen with a sharp notch, Pipe Ring Notched Tension (PRNT) specimen, is tested. The aim of the study is to determine the influence of the specimen geometry (cylindrical shape, as well as number and length of stress concentrators) on parameters such as force, Crack Mouth Opening Displacement CMOD, Crack Tip Opening Displacement CTOD and J integral. The specimens were fabricated by an additive production method - selective laser sintering (SLS). EOS Formiga P100 machine is used, and the material is polyamide PA12. In addition to Pipe Ring Notched Tension specimens, Single Edge Notched Tension (SENT) specimens with identical cross section were also considered. All specimens were tested on a universal tensile testing machine. A tool specially designed to apply contact pressure to the inner walls in the tension direction was used, simulating the internal pressure. For determining the field of displacement and strain on the surface of the tested samples that occur during the loading, Aramis GOM 2M optical measurement system was used. Aramis is applied for determination of geometry fracture mechanics parameters: CMOD and CTOD (based on δ5 concept). In addition to the examination of fracture properties of additively manufactured PA12, the main topic of this work is the development of the non-standard testing procedure, which will be subsequently applied to the specimens cut from metallic or non-metallic pipes. As an important part of this procedure, the calculation of fracture parameters will be conducted based on the results presented here.

Pneumatic Fracture Computations with Peridynamics

In this study, a peridynamic approach to pneumatic fracture is presented. The examined peridynamic model relies on the classical state-based formulation, where a critical bond-strain damage is assumed for a linear solid material. A penny-shaped initial crack is subjected to internal pressure, and it is found that that the crack tip opening displacement agrees well with the analytical solution. Therefore, the peridynamic model to pneumatic fracture captures the crack evolution quantitatively correctly. With the results at hand, the pneumatic cracking inside a concrete cylinder is simulated. It is shown that the shape of the cracks and fragments is in good qualitative agreement with the results of our supporting experiments.

Investigation of S960QL type high strength steel and its welded joints applying absorbed specific fracture energy and notch opening displacement

Classical techniques of fracture toughness evaluation, such as determination of the plain-strain fracture toughness, or the critical value of the crack-tip opening displacement are complex methods. The determination of the absorbed specific fracture energy and the notch opening displacement is basically simpler; notched cylindrical tensile specimens characterized by different notch radii can be applied. S960QL type high strength steel and its welded joints without and with preheating were examined; notch opening displacement and absorbed specific fracture energy values were determined. Conclusions belong to the effect of the preheating and the sensitivity of the high strength steel, as well as the reliability of the applied material characteristics were drawn.

Novel method for measurement of pipeline materials fracture resistance-examination on selective laser sintered cylindrical specimens

This paper presents a part of development of a non-standard method for testing of cylindrical test specimens for measurement of fracture properties of pipeline materials. This method for testing of cylindrical structures working under pressure is based on determining of fracture mechanics parameters on SENT (Single Edge Notched Tension) specimens and new PRNT (Pipe Ring Notched Tension) specimens. In this work, both types of specimens required for this testing were manufactured from polyamide PA12 by using SLS (selective laser sintering) additive manufacturing method. Testing of the specimens is performed on the universal device for testing of mechanical properties of materials Shimadzu, AGS-X 100 kN. The tensile testing is accompanied by GOM Aramis 2M system, used for digital image correlation. By using these two systems, test results are obtained for ring-shaped and SENT specimens, including forces, displacements and fracture mechanics parameters CMOD (Crack Mouth Opening Displacement) and CTOD-?5 (Crack Tip Opening Displacement obtained by ?5 technique), as well as crack growth. Repeatability of this process, along with valid result consistency, represent the basis for further development of the new method, including the determining of energy-based fracture mechanics parameters: J integral and stress intensity factor.

Fracture Mechanical Analysis of Gleeble Simulated Heat Affected Zones in High Strength Steels

During the research work the fracture mechanical investigation of heat affected zones of thermomechanical rolled high strength steels (Voestalpine Alform 960M) were carried out. For production of appropriate heat affected zones Gleeble 3500 physical simulator was applied, with different heating cycles and specific cooling times. Following the simulation, fracture mechanical investigations were performed, in favor of determination crack tip opening displacement (CTOD or δ) values.

Effect of residual stress modification technique on CTOD test results in heavy thick welded joints

ABSTRACT In order to investigate the effects of residual stress modification on the results of crack-tip opening displacement (CTOD) tests in heavy thick welded joints, experiments using a submerged-arc welding (SAW) joint of an EH47 steel plate with the thickness of 70 mm and residual stress analyses by the finite element method (FEM) were performed. It was confirmed that appropriate reversed bend conditions improved fatigue precrack front shapes, and the low ratio of the fatigue precrack length to the size of the compressive plastic zone induced by reversed bending contributed to the fatigue crack shape improvement. Although the local compression with a large diameter platen was effective in improving the fatigue precrack front shape, small diameter platens were not effective even if they applied to wide ranges of specimens. In addition, specimens treated by the reversed bending showed higher critical CTOD values than those treated by local compression. According to the FEM analysis, the reversed bending introduced tensile residual stress at the mechanical notch root, although the initial residual stress distribution of the welded joint still remained a little away from the notch root. On the other hand, the initial residual stress was almost removed by the local compression with a large diameter platen. It was presumed that the initial residual stress distribution affected fatigue precrack front shapes and the critical CTOD in welded joints. The reversed bending is very useful if appropriate conditions can be used.

Brittleness of Concrete under Different Curing Conditions.

In order to shorten construction periods, concrete is often cured using steam and is loaded at an early age. This changes the performance and even the durability of the concrete compared to concrete that has been cured under normal conditions. Thus, the pattern and the mechanism of concrete performance change under different curing conditions, and loading ages are of great significance. The development of brittleness under different curing conditions and loading ages was studied. The evaluation methods that were used to determine concrete brittleness were expounded. Steam, standard, and natural curing conditions were carried out on single-side notched concrete beams as well as on a concrete prism and cubic blocks. The compressive strength and splitting tensile strength of the concrete blocks along with the fracture performance of the concrete beams were tested after 3, 7, 28, and 90 days. The steam curing condition significantly improved the strength of concrete before 28 days had passed, and the standard curing condition improved the strength of concrete after 28 days. Based on the experimental fracture parameters, a two-parameter fracture model was applied to study the development of fracture toughness , critical crack tip opening displacement , and critical strain energy release rate with hydration age under different curing conditions. With respect to long-term performance, the standard curing condition was better at resisting concrete crack propagations than the steam curing condition was. The characteristic length and the material length under the three curing conditions and the long-term development of brittleness in the concrete indicated that steam curing increased the concrete brittleness. Considering the effects of the curing condition and the loading age, a time-dependent concrete fracture toughness model was established, and the predicted value of the model was verified against the measured value. The results indicated that the model was able to accurately predict the fracture toughness with an error rate of less than 16%.

Effect of Sigma Phase in Wire Arc Additive Manufacturing of Superduplex Stainless Steel

In the present study, the thermal program in wire and arc additive manufacturing has been varied in terms of heat input and interpass temperature. Three walls were completed with subsequent Charpy V impact toughness and crack-tip opening displacement fracture toughness, together with a detailed microstructure characterization using light microscopy and scanning and transmission electron microscopy. The results clearly demonstrate that the formation of sigma phase may deteriorate the toughness of superduplex components. Such formation may take place under prolonged cooling time, which may occur when subsequent passes are deposited with too high interpass temperatures. This transformation behavior may limit the productivity in additive manufacturing of such steels and care must be taken in selection of proper combination of arc energy and interpass temperature.

Stage-II fatigue crack growth model from 3D dislocation dynamics simulations

ABSTRACT Stage-II fatigue crack propagation in FCC metals is investigated using 3D discrete dislocation dynamics (DD) simulations. The calculations show that dislocation/grain–boundary interaction affects slip dispersion everywhere in the grain, including in the crack tip region. The results are analysed and drawn in the form of simple, quantitative mathematical expressions, depending on a set of physical variables. This approach helps in developing a comprehensive crack propagation model, where the crack growth rate da/dN depends on both the crack tip opening displacements and the tip-related nucleation of nano-cavities. It is shown that the modelled crack response is compatible with the well-known Paris expression, which we further interpreted as a ratio between the energy stored in the tip region over the energy spent extending the crack surfaces. The model validity is finally evaluated by a comparison with relevant experimental evidence.

Fracture of severely plastically deformed titanium

The mechanical properties with emphasis on the fracture behaviour of ultrafine-grained Ti processed by high pressure torsion were evaluated. Through this process a substantial refinement of the microstructure was induced. As a consequence, a pronounced increase of tensile strength was achieved compared to conventional Ti while the ductility exhibited only a minor reduction after processing. The fracture toughness in terms of the critical stress intensity factor and crack tip opening displacement was evaluated in different crack growth directions and showed a pronounced orientation dependency, which can be related to the typically elongated microstructure induced by this deformation method.

The impact of water to cement ratio on the fracture behavior of rubberized concrete

In the failure mechanism of concrete, its cracking behavior is an essential issue. Application of light and recycled materials such as crumb rubber can change the cracking behavior of concrete. The fracture behavior of rubberized concrete beams with varying water to cement ratios (WCR) of 0.35 to 0.55 was assessed in this investigation. Fracture parameters were achieved through WFM (work of fracture method) and SEM (size effect method). The findings revealed that with the reduction of the WCR, effective crack tip opening displacement at the peak load and the length of the fracture process zone in SEM and characteristic length in WFM decreased, while brittleness number, initial fracture energy and fracture toughness in SEM and the total fracture energy in WFM increased. Furthermore, for rubberized concrete containing different WCRs, the GF/Gf ratio was approximately 2.44.

Experimental investigations of the critical values of J-integral and crack tip opening displacement (CTOD) of high-density poly-ethylene PE100 elbow

The present study aims to characterize the cracking behaviour of high-density poly-ethylene (HDPE) pipe elbow. Experimental studies were performed to determine the mechanical behaviour of HDPE and study the anisotropy of the material. Tension tests were performed on compact tension (CT) specimens to identify the toughness value, expressed by the critical value of the J-integral JIc. These tests were carried out on specimens cut from a PE100 plate with a different orientation angle with respect to the extrusion direction. The testing results have shown that HDPE can be considered as an isotropic material. The methods used to calculate the JIc value resulted in different value. Due to these uncertainties, a study of a failure criteria using crack tip opening displacement (CTOD) was proposed and conducted. The combination of the J-integral and “CTOD” method gave the possibility to determine for the critical value of the J-integral as a value of the fracture toughness of the HDPE. The obtained results suggest that the value of 8.4 kJ/m2 can be considered as the JIc fracture toughness required to initiate the crack for the HDPE.

FRACTURE PROPERTIE ANALYSIS OF RECYCLED AGGREGATE CONCRETE BASED ON DIGITAL IMAGE CORRELATION TECHNIQUE

Three point bending fracture tests of recycled aggregate concrete beams with different recycled coarse aggregate replacement ratio (0%, 30%, 50%, 70%, 100%) have been carried out, with results analyzed by digital image correlation (DIC) method and the traditional electrometric method. The double-K fracture parameters, crack-mouth-opening displacement (CMOD), crack-tip-opening displacement (CTOD), crack extension length ( \begin{document}$\Delta a$\end{document} ) and fracture growth path were obtained, and the validity of DIC method was verified. The results show that when the replacement rate is 100%, the crack initiation fracture toughness and unstable fracture toughness of recycled aggregate concrete decrease by 15.81% and 15.39%, respectively. With the increase of the replacement rate of recycled aggregate, the slopes of the descending sections of P-CMOD, P-CTOD and P- \begin{document}$\Delta a$\end{document} curves increase. DIC method can effectively capture the fracture propagation process. The CMOD values measured by DIC method and clip type extenometer are in good agreement. The variation trends of DIC measured value and formula-calculated value of CTOD are relatively consistent. Based on the test results, the estimated values, \begin{document}$\Delta a$\end{document} , of specimens with different replacement rates of recycled aggregate under different loads can be obtained by fitting the curve between \begin{document}$P$\end{document} / \begin{document}${P_{\max }}$\end{document} and \begin{document}$\Delta a$\end{document} / \begin{document}$\Delta {a_{\text{c}}}$\end{document} .

Interaction of shear waves with semi-infinite moving crack inside of a orthotropic media

The present work investigates the motion of a semi-infinite moving crack inside a semi-infinite half-space of orthotropic medium subjected to anti-plane shear wave. The crack is located at a finite depth from the surface of semi-infinite orthotropic medium. Our aim is to examine how such anisotropy and geometric parameters can be adjusted to reduce the magnitude of stress intensity factor (SIF) to control the crack propagation near the crack tip region. As mathematical tools, Fourier transformation and inverse Fourier transformation techniques are employed to convert the governing mixed boundary value problem to the well-known Weiner-Hopf equation with suitable boundary conditions. Some physical quantities such as SIF at the crack tip and crack opening displacement (COD) around the crack tip have been derived. Graphical exhibition has been carried out to show the impact of relevant parameters such as crack velocity, layer depth from the surface to crack and orthotropic material properties on SIF and COD. The numerical results show that SIF decay with crack depth from the layer. It is also observed that SIF decreases with an increase in crack velocity and finally tends to zero as crack velocity approches near SH-wave velocity. Also, the value of COD decays as we move along the damage near the crack tip along negative x-axis and finally tends to zero at the crack tip. This behavior of COD is consistent with the physical nature of the semi-infinite crack of the problem. The results are validated for isotropic material with some reported work and are well in agreement. The study of these physical quantities (SIF, COD) ensures the arrest of onset of crack expansion by monitoring geometric parameters and wave velocity to avoid fracture.

A methodology for assessing short fatigue crack growth in DCI materials affected by intergranular embrittlement at temperatures nearby 400 °C

A crack growth model for assessing the lifetime and short fatigue crack growth of ductile cast iron materials is proposed. The model bases on the assumption that the crack advance per cycle correlates with the cyclic crack tip opening displacement. Intergranular embrittlement at medium temperatures is accounted for as a temperature and strain rate dependent crack accelerating mechanism. The model is calibrated and successfully applied to predict temperature dependent fatigue lifetime and short crack growth of a ductile cast iron material.

Effects of build orientation and heat treatments on the tensile and fracture toughness properties of additively manufactured AlSi10Mg

The purpose of this study aimed to examine how build orientation and heat treatments affect microstructure, and consequently, the mechanical properties obtained from tensile and fracture toughness tests of additively manufactured AlSi10Mg samples. Samples were manufactured in several orientations using a laser-based powder bed fusion (L-PBF) additive manufacturing (AM) process, following which they were subjected to three separate heat treatments: (i) stress relief at 300 °C for 2 h (SR); (ii) homogenization at 540 °C for 2 h followed by artificial aging at 170 °C for 8 h (HA-1); (iii) homogenization at 540 °C for 6 h followed by artificial aging at 170 °C for 14 h (HA-2). Tensile tests showed that the AB samples presented the highest mechanical strength; however, low ductility was also observed. Therefore, a model for crack propagation during tensile testing was proposed for L-PBF typical microstructure. Samples subjected to fracture toughness tests showed sensitivity to porosity and microstructure. The crack-tip opening displacement (CTOD) of the AB and HA-2 samples showed similar average values of approximately 10 µm in all three orientations. The SR samples showed the best fracture toughness behavior with average values ranging from 19 µm to 32 µm. The HA-1 samples presented average values between 13 µm and 22 µm. The fracture toughness values reported in the J-integral ranged from 6.0 to 8.4 kJ.m−2, 10.4 to 15.5 kJ.m−2, 5.5 to 7.8 kJ.m−2, and 4.6 to 5.7 kJ.m−2, respectively, for the AB, SR, HA-1, and HA-2 heat treatments.

Experimental investigation on the concrete fracture process zone using electronic speckle pattern interferometry

AbstractIn order to advance the understanding of fracture failure mechanisms in concrete, a series of three‐point bend tests of pre‐notched specimens were conducted to study the characteristics of the fracture process zone. Electronic speckle pattern interferometry (ESPI) was used to determine the location and size of the cracks. The experimental results have shown that crack in the pre‐notched concrete beams will initiate when the load increases to 40% of the peak load (Pmax) and expands unstably after reaching the peak load. When the load drops to about 25% Pmax in the load drop section, a complete fracture process zone (FPZ) was developed, and the corresponding crack tip opening displacement is about 7.1 times to the critical crack tip opening displacement. The analysis of surface strain of concrete specimen showed that microcracks will initiate when the surface tensile strain of concrete reaches about 2 × 10−4; when the surface tensile strain value of concrete reaches 4 × 10−2, the FPZ will move forward; the length of the FPZ of concrete essentially remains the same in the entire fracture process, which is about 71% of the ligament height. The analysis of experimental data also shows that the size of the FPZ is not significantly affected by the cubic compressive strength of concrete. In addition, the tension‐softening curves of the specimens were finally determined by using the incremental displacement collocation method, and it is concluded that the critical width of the FPZ is related to the cubic compressive strength of the concrete.

A simplified estimation method for tensile softening curve of quasi-brittle materials

Based on cohesive zone model (CZM), a simplified method for estimating the tensile softening curve of quasi-brittle materials is proposed. First, assume that the virtual crack opening displacement is linearly distributed. According to Castigliano's theorem, a cohesive displacement proportional function is proposed, which can obtain Crack Tip Opening Displacement (CTOD) directly from Crack Mouth Opening Displacement (CMOD). This simplifies the governing equations of CZM and the tensile softening curve can be obtained quickly. Finally, the effectiveness of the method is verified by experiment and finite element. Highlights A simplified method for estimating the tensile softening curve of quasi-brittle materials is proposed. The proposed simplified method does not require sophisticated mathematical techniques or finite element modeling. The effectiveness of this method is verified by previous experimental results and finite element simulations. It is reasonable to assume that the crack opening displacement in the fracture process zone conforms to the linear distribution. The cohesive displacement proportional function is proposed. CTOD can be directly obtained from CMOD using the function.

J and CTOD-Based Tensile Strain Capacity Prediction for Pipelines With a Surface Crack in Girth Weld

Abstract Long-distance pipelines for the transport of oil and natural gas to onshore facilities are mainly fabricated by girth welding, which has been considered as a weak location for cracking. Pipeline rupture due to crack initiation and propagation in girth welding is one of the main issues of structural integrity for a stable supply of energy resources. The crack assessment should be performed by comparing the crack driving force with fracture toughness to determine the critical point of fracture. For this reason, accurate estimation of the crack driving force for pipelines with a crack in girth weld is highly required. This paper gives the newly developed J-integral and crack-tip opening displacement (CTOD) estimation in a strain-based scheme for pipelines with an internal surface crack in girth weld under axial displacement and internal pressure. For this purpose, parametric finite element analyses have been systematically carried out for a set of pipe thicknesses, crack sizes, strain hardening, overmatch, and internal pressure conditions. Using the proposed solutions, tensile strain capacities (TSCs) were quantified by performing crack assessment based on crack initiation and ductile instability and compared with TSCs from curved wide plate tests to confirm their validity.