Compact Tension Test Specimens Research Articles

Hydrogen-accelerated fatigue crack growth of equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy evaluated by compact tension testing

We investigated the effect of hydrogen on the resistance to mechanically long fatigue crack growth in an equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy using compact tension (CT) tests at a frequency of 1 Hz and room temperature (20 °C). The CT test specimens were hydrogen charged using 100 MPa of hydrogen gas prior to testing. Fatigue crack growth progressed three times faster after exposure to hydrogen gas. The major change caused by hydrogen charging was the occurrence of intergranular crack growth. Striation features were observed in the intergranular fracture region, indicating that intergranular crack growth occurred via a plasticity-driven cycle-by-cycle mechanism. More specifically, we propose that the dislocation emission from the grain boundaries at a crack tip controls the intergranular fatigue crack growth.

Influence of 3D Printing Topology by DMLS Method on Crack Propagation.

The presented text deals with research into the influence of the printing layers’ orientation on crack propagation in an AlSi10Mg material specimen, produced by additive technology, using the Direct Metal Laser Sintering (DMLS) method. It is a method based on sintering and melting layers of powder material using a laser beam. The material specimen is presented as a Compact Tension test specimen and is printed in four different defined orientations (topology) of the printing layers—0°, 45°, 90°, and twice 90°. The normalized specimen is loaded cyclically, where the crack length is measured and recorded, and at the same time, the crack growth rate is determined. The evaluation of the experiment shows an apparent influence of the topology, which is essential especially for possible use in the design and technical preparation of the production of real machine parts in industrial practice. Simultaneously with the measurement results, other influencing factors are listed, especially product postprocessing and the measurement method used. The hypothesis of crack propagation using Computer Aided Engineering/Finite Element Method (CAE/FEM) simulation is also stated here based on the achieved results.

Stress Distribution and Fracture Toughness of Underground Reinforced Plastic Pipe Composite.

Reinforced composite materials have many applications in the aerospace, marine, and petroleum industries. Glass fiber-reinforced pipes are of considerable importance as pressurized vessels, infrastructure materials, and petroleum wastewater pipelines. The stress intensity factor due to through-thickness discontinuities is a major parameter in fracture mechanics to understand the failure mechanisms in glass fiber-composite pipes. The stress intensity factor is calculated for a composite cylinder subjected to internal pressure using the linear extended finite element method based on the law of energy release evaluation of surface damage. The analytical model needs two material properties; they are the tensile strength and the fracture toughness; therefore, a standard tensile test was carried out on a standard specimen taken from the pipe’s wall thickness. Moreover, the compact tension test specimen was manufactured from the pipe’s wall thickness to obtain the fracture toughness. The average tensile strength was measured as 21.5 MPa with a standard deviation of 5.59 MPa, moreover, the average Young’s modulus was measured as 32.75 GPa with a standard deviation of 6.64 GPa. The fracture toughness was measured as 2322 () with a standard deviation of 142.5 (), whereas the average surface release energy () was 153.6 kJ/m2 with a standard deviation of 22.53 kJ/m2. A valuable design equation was extracted from the finite element model to measure the effect of cracks on the hoop stress of the cylinder wall thickness based on a nonlinear model. Moreover, an acceptable equation was used to calculate the correction and shape factor of a cylinder with movable and unmovable through-thickness cracks. This study provides useful tools and guidance for the design and analysis of composite cylinders.

Determination of GTN Model Parameters Based on Artificial Neutral Network for a Ductile Failure

The Gurson – Tvergaard – Needleman (GTN) mechanical model is widely used to predict the failure of materials based on laboratory specimens, direct identification of Gurson – Tvergaard – Needleman parameters is not easy and time-consuming, and the most used method to determine them is the combination between the experimental results and those of the finite elements, the process consists of repeating the simulations several times until the simulation data matches the experimental data obtained at the specimen level.This article aims to find GTN parameters for the Compact Tension (CT) and Single Edge Tensile Test (SENT) specimen based on the Notch Specimen (NT) using the Artificial Neural Network (ANN) approach. . This work presents how the ANN could help us determine the parameters of GTN in a very short period of time. The results obtained show that ANN is an excellent tool for determining GTN parameters.

Fatigue Crack Growth and Fracture Toughness in a Dual Phase Steel: Effect of Increasing Martensite Volume Fraction

Crack growth resistance in dual-phase steel was studied. The dual phase steel microstructure was modified through heat treatments to increase the martensite volume fraction from 10% to 40%. The as-received and heat-treated samples were evaluated using a uniaxial tensile test, fatigue crack growth test, and fracture toughness test. Extended Finite Element Method (XFEM) was used to simulate the crack growth in compact tension test specimens. The results showed that an increase in martensite volume fraction is an effective way to increase the fracture resistance under different load conditions, quasistatics and dynamic, increasing the fracture toughness, tensile strength and fatigue resistance of the heat-treated material. Presence of a highest content of martensite results in formation of an important number of secondary cracks during the fatigue crack growth, which slow down the crack propagation. Moreover, martensite generates a crack closure over the crack tip, making the propagation difficult due to the irregularities caused by the crack growth on the martensite. Finally, the computational load-displacement curves are in good agreement with the experimental data.

A two-dimensional ordinary state-based peridynamic model for elastic and fracture analysis

A framework of the two-dimensional (2D) ordinary state-based peridynamic model is established for quantitative elastic and fracture analysis, in which the peridynamic force state considering the thermomechanical loading (elastic model), bond failure criteria, and numerical implementation method are presented. The critical time step for explicit time integration in the state-based peridynamics is given. Then, a plate under combined tensile and thermal expansion loads and a compact tension (CT) test specimen are analyzed using the present 2D peridynamic model, and the results are compared well with the available analytical and finite element modeling solutions. The present 2D peridynamic model can quantitatively capture the elastic and fracture behavior of 2D problems.

Fracture mechanics analysis of generalized compact tension specimen geometry using the mechanics of net-section

Fracture mechanics analysis of a generalized compact tension specimen, on the basis of the mechanics of deformation of the net-section, is shown to provide a simple and a broadly useful expression for stress intensity factor calculations. A single analytical expression is found to be sufficient for the characterization of crack behavior in compact tension, extended compact tension and wedge splitting test specimens without any restriction on the specimen length (or height) and width. The analysis is enabled by the concept of the change in net-section energy, which is determined by summing the changes in strain energies of the net-section of the generalized specimen for tension and bending deformation modes, which result from the introduction of the crack. This is equivalent in concept to the increase in strain energy upon the introduction of the crack, as in the Griffith’s fracture theory. The square-root of the change in net-section strain energy parameter multiplied by the elastic modulus provides an expression that is equivalent to the conventional stress intensity factor expression. The application of the net-section based expression to the standard compact tension, the extended compact tension and the wedge-splitting test specimens shows very good agreements with the crack behaviors as expressed by the stress intensity factor expressions for the respective geometries. The proposed method enables easy analytical determination of stress intensity factors for any asymmetrically-loaded mode-I crack problem.

Solder joint fatigue life prediction using peridynamic approach

Solder joint fatigue life prediction using peridynamic approach is presented for the first time. The underlying premise is that material degradation through energy dissipative mechanisms play a central role in crack initiation and propagation, and that fatigue cracks follow paths similar to cracks form under quasi-static loading. The new method uses peridynamic approach in simulating crack initiation as well as propagation. Dissipated energy in the vicinity of newly generated fracture surfaces are calculated as the cracks propagate. Once failure starts taking place, load rebalance leads to further propagation. The method is first demonstrated using controlled experimental measurements of compact tension test specimens that were subjected to mechanical loading only. The fatigue life predicted by simulations of compact tension test specimens is within 12% of the measurements. The methodology is then applied to solder joint fatigue problems. The nonlinear thermo-mechanical problem is first solved by finite element analysis for four thermal cycles. The most severe deformation configuration during the fourth thermal cycle at the most critical solder joint is identified and applied as the static loading in the peridynamic simulation. Number of cycles to crack initiation and propagation on five distinct packages are predicted. The combined finite element and peridynamic approach yielded predictions within 25% of the experimentally measured fatigue life values for four packages. In the fifth package, a close match is not achieved. Overall, the results point to the conclusion that the peridynamic approach has great potential to accurately predict solder joint fatigue life.

Effect of Cooling Medium on Fracture Toughness of Rotomoulded Product

In day-to-day life, usage of plastics is numerous. It offers variety of benefits compared to other materials in various sectors like house hold applications, agricultural industry, and packaging, etc. There are numerous methods for processing plastics. These include: blow moulding, injection moulding, rotational moulding, transfer moulding and thermoforming. Rotational moulding is a competitive alternative to other plastic manufacturing process, since it offers designers an opportunity to achieve an economic production of stress free products. Many products made by rotational moulding process using linear low density polyethylene (LLDPE) are widely used in outdoor applications such as boats, over head tanks, and car body parts etc. In such applications, fracture properties are considered to be critical from the quality characterization point of view. Selection of appropriate cooling medium plays vital role to enhance the quality of rotomolded products. In this paper, an attempt has been made to investigate the effect of cooling medium on fracture toughness of the rotationally moulded products. Fracture tests are carried out on a compact tension (CT) test specimens prepared as per the ASTM D 6068 (2012). The tests are performed on a universal testing machine. R-curve method is used to determine the fracture toughness (JIC) of rotomoulded products. From the experimental results it is found that rapid cooling method favours better fracture toughness of rotomoulded products. Therefore, it is recommended to use faster cooling aids like water cooling in rotational moulding process to achieve highest fracture toughness.

Evaluating Fracture Toughness of Rolled Zircaloy-2 at Different Temperatures Using XFEM

Fracture toughness and mechanical properties of the zircaloy-2 processed by rolling at different temperatures have been investigated, and simulations have been performed using extended finite element method (XFEM). The solutionized alloy was rolled at different temperatures for different thickness reductions (25–85%). Fracture toughness has been investigated by compact tension test. The improved fracture toughness of the rolled zircaloy-2 samples is due to high dislocation density. SEM image of the fractured surface shows the reduction in dimple sizes with the increase in dislocation density due to the formation of microvoids as a result of severe strain induced during rolling. Compact tension test, edge crack, center crack and three-point bend specimen simulations have been performed by XFEM. In XFEM, the cracks are not a part of finite element mesh and are modeled by adding enrichment function in the standard finite element displacement approximation. The XFEM results obtained for compact tension test have been found to be in good agreement with the experiment.

Fatigue crack growth behavior of DLC coated AISI 4140 steel under constant and variable amplitude loading conditions

Abstract In this study, the effect of DLC (diamond-like carbon) coating on fatigue crack growth (FCG) behavior of AISI 4140 alloy was investigated under constant and variable loading conditions. For this purpose, DLC films were deposited on compact tension (CT) test specimens made of AISI 4140 steel using physical vapor deposition (PVD) method. The influence of DLC coating on structural properties of material was studied by means of XRD, SEM, scratch and microhardness tester. Afterwards, untreated and DLC coated CT specimens were subjected to fatigue loads by a servo-hydraulic fatigue testing system. The FCG behavior was monitored and correctly evaluated via digital monitoring system. Constant amplitude load, overload (OL), underload (UL) and their sequences were applied for tests. The results showed that DLC coating extended the FCG life of the material under constant amplitude loading due to increasing compressive residual stress and microhardness. FCG life of both untreated and DLC coated specimens increased after the application of OL. However, after OL application, a reduction in the increased FCG life of DLC coated material was observed because of decreasing crack tip plasticity. Also, FCG rate of untreated material decreased after the application of UL and DLC coating almost suppressed this behavior.

CTOD‐R curves of the metal‐clad interface of API X52 pipes cladded with an Inconel 625 alloy by welding overlay

AbstractCrack‐tip opening displacement resistance curves (CTOD‐R) of the substrate/cladding interface of an API 5 L X52 steel pipe internally coated with Inconel 625 applied by TIG (GTAW) welding were experimentally evaluated. A small pipe section with 168 mm of outer diameter and 22.5 mm of thickness was internally coated with a 15 mm thick layer of Inconel 625 corrosion resistant alloy. Tension test specimens were obtained from both substrate and cladding, as well as compact tension test specimens (C(T)) for the evaluation of CTOD‐R curves. The fracture testing specimens were notched at the interface in RL orientation. In addition to fracture and tensile testing, microstructural characterization was conducted at the interface using optical microscopy (OM) and scanning electron microscopy (SEM). Qualitative chemical composition scanning and microhardness determination were also performed. The results indicated high fracture toughness for the substrate/cladding interface and the absence of low toughness regions at the interface of the tested samples.

Fracture toughness of chairside CAD/CAM materials – Alternative loading approach for compact tension test

ObjectiveThis in-vitro study determined plane-strain fracture toughness (KIC) of five different chairside CAD/CAM materials used for crown fabrication, following alternative innovative loading approach of compact tension test specimens. MethodsRectangular-shaped specimens were cut from CAD/CAM blocks (n=10): Vita Mark II (Vident) (VMII); Lava-Ultimate (3M/ESPE) (LU); Vita Enamic (Vident) (VE); IPS e.max CAD (Ivoclar Vivadent); crystallized and un-crystallized (E-max and E-max-U, respectively); and Celtra Duo (Dentsply) fired and unfired (CD and CD-U, respectively).Specimens were notched with thin diamond disk prior to testing. Instead of applying tensile loading through drilled holes, a specially-made wedge-shaped steel loading-bar was used to apply compressive load at the notch area in Instron universal testing machine. The bar engaged the top ¼ of the notch before compressive load was applied at a cross-head speed of 0.5mm/min. Fracture load was recorded and KIC calculated. Data was statistically-analyzed with one-way ANOVA at 95% confidence level and Tukey's tests. ResultsMeans and SDs of KIC in MPam1/2 for VMII, LU, VE, E-max, E-max-U, CD and CD-U were: 0.73 (0.13), 0.85 (0.21), 1.02 (0.19), 1.88 (0.62), 0.81 (0.25), 2.65 (0.32) and 1.01 (0.15), respectively. ANOVA revealed significant difference among the groups (p<0.001). CD and E-max had significantly highest mean KIC values. SignificanceMean KIC values of the tested materials varied considerably, however, none of them reached mean KIC of dentin (3.08MPam1/2) previously reported. For E-max and CD, specimens firing significantly increased mean KIC. The modified test arrangement was found to be easy to follow and simplified specimen preparation process.

Peridynamics for unguided crack growth prediction under mixed-mode loading

Peridynamics enables complex crack path prediction in structures without resorting to a special treatment of mathematical singularities as in the case of classical continuum mechanics. The peridynamic equations of motion is valid everywhere even in the presence cracks. Thus, failure process is included as part of the constitutive model for crack initiation and propagation in an unguided manner. Also, peridynamics correctly redistributes the load in the presence of complex and multiple cracks for accurate prediction of residual strength of structures. This study concerns the prediction of crack propagation paths in isotropic materials under complex loading conditions by using peridynamics. Crack growth process is controlled by a single failure parameter referred to as critical stretch which can be expressed in terms of the fracture toughness of the material. The demonstration problems concern crack growth in a four-point shear specimen and a compact tension test specimens with a sink and a miss hole. All of these applications include a pre-existing crack under mixed-mode loading conditions. The simulations capture experimentally observed non-self-similar crack propagation paths.

Pressurized hole as a Stress Reducing feature

In many structures, cracks may appear during manufacturing process or in service. Stress intensity factor is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. In this work a rectangular plate with central hole & two cracks emanating from his hole is analyzed for remote tensile loading. Stress Intensity Factor is determined for this configuration using finite element based software ANSYS. This value is compared with the one obtained by analytical method. Then hole as a stress reducing feature is studied. The effect of hole location, hole size on the stress intensity factor is studied. Feasibility of pressurized hole as a stress reducing feature is studied for the best hole location & size obtained earlier. Further variation in the pressure was done to study effect of pressurized hole. Fracture Mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material resistance to fracture. Stress intensity factor, K, is used in fracture mechanics to more accurately predict the stress state (stress intensity) near the tip of the crack caused by remote load or residual stresses. It is theoretical construct applicable to a homogeneous elastic material and is useful for providing failure criteria for brittle materials. The stress intensity factor remains finite and provides a basis for determining the critical load. If the value of SIF is less than the toughness of the material, the structure is safe. If SIF value is more than the toughness of material, the structure fails. Typical Stress Intensity Factor Reduction Techniques used by earlier researchers are: Strip or patch repair method, Artificial crack closure by crack filling, Welding repair, Hole drilling. In this work, finite element analysis of a cracked plate is done. Initially a hole is used as a stress reducing feature. Then the effect of pressure applied in this hole is studied. II. LITERATURE REVIEW Miloud Souiyah, Abdulnaser Alshoaibi. et al (1) considered a rectangular plate with crack starting from a circular hole and double edge notched plate. Both geometries were in tensile loading and under mode-I conditions. In this paper a displacement extrapolation technique was employed particularly to predict the crack propagations direction and to calculate SIFs and validated with the relevant numerical and analytical results obtained by other researcher. Xiangqiao Yan (2) studied the stress intensity factors (SIFs) of cracks emanating from a rhombus hole in a rectangular plate subjected to internal pressure by means of the displacement discontinuity method with crack-tip elements (a boundary element method) proposed by the author. Moreover, an empirical formula of the SIFs of the crack problem was presented and examined. It was found that the empirical formula is very accurate for evaluating the SIFs of the crack problem. Junping Pu (3) studied a Boundary value problem of a plate with crack and defect such as the circular and/or elliptical holes. This kind of problem was suitable for solving by boundary element method with its higher precision. The sub-region method was used in this work. A center cracked plate subjected to remote tensile and shear loading was studied numerically. Levend Parnas and Omer G. Bilir (4) calculated Stress Intensity Factor by experimental procedures using strain gages. In this method, cracks were opened at the tip of crack starter slot on the standard compact tension test specimens by using Electrical Discharge Machining (EDM) and Wiring Discharge Machining (WDM). Strain gage data from the crack tip region were used to calculate stress intensity factors. But the values obtained were on lower side. This was partly due to unavoidable deformations in the adhesives used to fasten the strain gages to the specimen. This might be also due to the use of the relatively long strain gages, local yielding and three-dimensional effects and limited regions for strain gauge.

Engineering Framework To Transfer The Minimum Fracture Toughness In The DBTT Region Between SE(B) And 1T CT Specimens

The test specimen size effect of fracture toughness Jc of the material in the ductile-to-brittle transition temperature (DBTT) region is known to not be ignored. One practical method was to use the 1-inch thickness (1T) compact tension (CT) test specimen to integrity assessment of cracked structure. To support this practice, a method to convert Jc obtained with other sized CT specimens is provided in ASTM E1921. On the other hand, a large number of fracture toughness test data have been collected for the single-edge notched bend (SE(B)) specimens. In order to practically use the SE(B) test data in engineering application, it is essential for converting Jc obtained from different sized SE(B) specimen to the 1T CT specimen Jc. In this paper, an engineering framework is proposed to convert minimum Jc obtained from various sized SE(B) specimens to the minimum 1T CT specimen Jc. The approach applies the modified Ritchie-Knott-Rice failure criterion, which predicts the onset of cleavage fracture when the crack-opening σ22 measured at a distance from the crack-tip equal to four times the crack-tip opening displacement σt, denoted as σ22d, exceeds a critical value σ22c, to the elastic-plastic finite element results. In addition, this framework utilizes our recent finding that the J when σ22d reaches the critical value σ22c has a possibility to correspond to the minimum Jc for a specified specimen configuration and material. The proposed framework has an advantage on the point that it requires only stress-strain curve as an experimental data. The validity of the framework was demonstrated through the experimental results.

Mode I Fracture Analysis of Polymethylmetacrylate Using Modified Energy-Based Models

The paper presents two energy-based approaches to predict the fracture trajectory and the fracture load in components containing a mode I crack. The fracture behavior of polymethylmetacrylate (PMMA) samples was investigated experimentally and theoretically for compact tension and double cantilever beam test specimens. The crack growth trajectories and the values of apparent fracture resistance in these two specimens were considerably different although both were under pure mode I loading. Two energy-based methods, i.e., the strain energy density and the averaged strain energy density criteria were modified to estimate the fracture trajectory and the fracture load in brittle materials respectively by considering the T-stress effects. The difference between the crack trajectories and the fracture resistances of different cracked specimens of the same material (PMMA) was found to be related to the magnitude and the sign of T-stress.

Corrosion fatigue load frequency sensitivity analysis

This paper presents experimental assessment of crack growth rates of S355J2+N steel in a corrosion fatigue environment similar to what is experienced on offshore wind farm monopile structures under various cyclic load frequencies in order to assess the effect of cyclic frequency of the applied loading within a frequency range pertinent to the structure. Fatigue crack propagation behaviour in this test programme is evaluated through fatigue tests on six compact tension test specimens in air and in laboratory simulated seawater under free corrosion condition. Fatigue crack lengths were monitored by back face strain (BFS), DCPD and ACPD. A regression model was derived through the BFS method to express strain values as a function of crack length to width ratio. The effectiveness of BFS method is particularly demonstrated in the simulated marine environment. Within the range of test frequencies, crack growth rates in simulated seawater when compared to the equivalent air test revealed environmental reduction factors of 2 and 4 at lower and higher values of stress intensity factors respectively. Significant difference in the results of the seawater test frequencies is discussed.

Determination of the mode I crack resistance curve of polymer composites using the size-effect law

This paper presents a new method to measure the crack resistance curve associated with the longitudinal failure of polymer composites reinforced by unidirectional fibres. Rather than using compact tension test specimens, the identification of the size-effect law of double edge notched specimens is used to obtain the crack resistance curve. Special emphasis is placed on the appropriate calculation of the stress intensity factor of the specimens when using quasi-isotropic or cross-ply laminates. For this purpose, both analytical closed-form solutions and numerical methods are investigated. Four different carbon-epoxy material systems, T800/M21, IM7/8552, T700/AR-2527, and T700/ACE are tested and the corresponding size effect laws and R-curves are measured. A good correlation between the crack resistance curve obtained using the size effect law and that previously measured for one of the material systems using the compact tension test is obtained. The highest value of the longitudinal fracture toughness was obtained for the T800/M21 material.

Comparison of intralaminar and interlaminar mode I fracture toughnesses of a unidirectional IM7/8552 carbon/epoxy composite

Abstract The intralaminar and interlaminar mode I fracture toughnesses of a unidirectional IM7/8552 carbon/epoxy composite were measured using compact tension (CT) and double cantilever beam (DCB) test specimens, respectively. Two starter crack geometries were considered for both the CT and DCB specimen configurations. In the first case, starter cracks were produced by 12.5 μm thick, Teflon film inserts. In the second case, considerably sharper starter cracks were produced by fatigue precracking. For each specimen configuration, use of the Teflon film starter cracks resulted in initially unstable crack growth and artificially high initiation fracture-toughness values. Conversely, specimens with fatigue precracks exhibited stable growth onset and lower initiation fracture toughnesses. For CT and DCB specimens with fatigue precracks, the intralaminar and interlaminar initiation fracture toughnesses were essentially equal. However, during propagation, the CT specimens exhibited more extensive fiber bridging and rapidly increasing R-curve behavior as compared to the DCB specimens. Observations of initiation and propagation of intralaminar and interlaminar fracture, and the measurements of fracture toughness, were consistent with fractographic analysis using scanning electron microscopy.