Fracture Toughness Experiments Research Articles

Thickness Effect of 2195 Al–Li Alloy Friction Stir Weld Fracture Toughness

For damage tolerance design in engineering components, the fracture toughness value, KIC, of the material is essential. However, obtaining specimens of sufficient thickness from stir friction welded plates is challenging, and often, the experimental test values do not meet the necessary criteria, preventing the experimental fracture toughness, Kq, from being recognized as plane strain fracture toughness KIC. The fracture toughness Kq of 2195 Al–Li alloy welding seams with different thicknesses was measured on the forward and backward sides. Microstructure characterization was conducted by scanning electron microscope (SEM). The results indicated minimal significant differences in grain size between the advancing and retreating sides of the weld nugget zone. In specimens of the same thickness, fracture toughness measurements along the normal direction of the joint cross-section showed a high similarity between the advancing and retreating sides of the weld nugget zone. Utilizing the quantitative relationships between fracture toughness and sample thickness derived from both the fracture K and G criteria, it is possible to predict the fracture toughness of thick plates using thin plates. This study employs these relationships to calculate the fracture toughness KIC of 2195 aluminum–lithium alloy friction stir welds. The KIC values obtained are 41.65 MPa·m1/2 from the fracture K criterion and 43.54 MPa·m1/2 from the fracture G criterion.

Mechanical and biological behavior of double network hydrogels reinforced with alginate versus gellan gum

Alginate and gellan gum have both been used by researchers as reinforcing networks to create tough and biocompatible polyethylene glycol (PEG) based double network (DN) hydrogels; however, the relative advantages and disadvantages of each approach are not understood. This study directly compares the mechanical and biological properties of polyethylene glycol di-methacrylate (PEGDMA) hybrid DN hydrogels reinforced with either gellan gum or sodium alginate using PEGDMA concentrations from 10 to 20 wt% and reinforcing network concentrations of 1 and 2 wt%. The findings demonstrate that gellan gum reinforcement is more effective at increasing the strength, stiffness, and toughness of PEGDMA DN hydrogels. In contrast, alginate reinforcement yields DN hydrogels with greater stretchability compared to gellan gum reinforced PEGDMA. Furthermore, separate measurements of toughness via unnotched work of rupture testing and notched fracture toughness testing showed a strong correlation of these two properties for a single reinforcing network type, but not across the two types of reinforcing networks. This suggests that additional notched fracture toughness experiments are important for understanding the full mechanical response when comparing different tough DN hydrogel systems. Regarding the biological response, after conjugation of matrix protein to the surface of both materials robust cell attachment and spreading was supported with higher yes associated protein (YAP) nuclear expression observed in populations adhering to the stiffer gellan gum-PEGDMA material. This study provides valuable insights regarding how to design double network hydrogels for specific property requirements, e.g., for use in biomedical devices, as scaffolding for tissue engineering, or in soft robotic applications.

A new method for predicting reference temperature of ferritic steel using finite element analysis and master curve method

Fracture toughness is an important parameter for measuring material properties in industry. In order to obtain data on the fracture toughness of a material it is often necessary to carry out a large number of fracture toughness experiments. The cost of the experiments is high and the experimental process is cumbersome. Moreover, in practice, there are often not a large number of materials that meet the requirements for the experiment. The relationship between different experimental ambient temperatures and fracture toughness can be determined by determining the reference temperature T0 in the master curve method. In this paper the load-displacement curves of the specimens are obtained using finite element analysis. Determine the J integrals corresponding to the initiating load Pint and the maximum load Pc on the load-displacement curve using the Load Slope Compliance Changing Rate method. Determine the reference temperature T0 in conjunction with the single temperature method. On the one hand, in order to validate the method, finite element analyses were performed on specimens of different thicknesses, temperatures, materials and types. This method was also applied to obtain fracture toughness data and to determine the reference temperature T0. On the other hand, combining the master curve method and combining the experimental data of 16MnDR, 16MnR and 20MnMoNi55 ferritic steels were verified. The experimental results show that the results predicted by this method are in good agreement with the actual results. However, it has a certain conservative type. It provides a good solution for practical engineering. The accuracy of fracture toughness and reference temperature of ferritic steels of different thicknesses can be verified by few experiments.

Fracture toughness and fatigue crack growth resistance of precipitate-free and precipitation hardened NiTiHf shape memory alloys

The present work investigates fracture toughness, and actuation and mechanical fatigue crack growth responses of Ni50.3Ti29.7Hf20 HTSMAs across martensitic transformation with two different microstructures, one with H-phase nanoprecipitates and one without. H-phase precipitation is known to stabilize the actuation cycling response of NiTiHf HTSMAs and notably impacts transformation-induced plasticity. The fracture toughness tests performed reveal that precipitate-free NiTiHf has a higher fracture toughness and undergoes significantly more inelastic deformation than the one with the precipitates resulting in toughness enhancement, i.e., stable crack advance during fracture toughness experiments, which is not observed in the precipitated NiTiHf for the crack configuration and loading conditions tested. Furthermore, the precipitate free NiTiHf has higher actuation and mechanical fatigue crack growth resistance than the precipitation-hardened microstructure. This is attributed to plasticity buildup, which exacerbates the manifestation of retained martensite upon repeated transformations. The fatigue crack growth rates obtained from both actuation and mechanical fatigue experiments align to a single Paris Law Curve for the precipitation-hardened NiTiHf. This work aims to determine if unified Paris Law curves can be generated from mechanical and actuation fatigue experiments, irrespective of composition and microstructure, to estimate actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Designing a simple and suitable laboratory test specimen for investigating the general mixed mode I/II/III fracture problem

A novel test specimen was designed for experimental investigations of pure and mixed mode fracture. This novel test configuration is called “modified edge notched disc bend” (M-ENDB) specimen. The M-ENDB specimen has a circular disc shape with an edge crack along its diameter that is loaded using asymmetric three-point bending. Different fracture parameters including: stress intensity factors (SIFs), geometry factors, T-stress, biaxiality ratio (BR), and out-of-plane constraint factor (Tz) were determined for the suggested disc bend specimen using the extensive finite element (FE) analyses. According to the obtained results, the proposed specimen can present all combinations of mode mixities, including pure modes I, II, and III as well as mixed-mode I/II, I/III, and I/II/III deformations. Afterwards, a large number of pure and mixed-mode fracture toughness experiments were carried out on polyurethane (PUR) foam to examine the capabilities of the suggested M-ENDB specimen. Fracture path, fracture surface, fracture toughness, work of fracture, and critical T-stress of broken foams under general mixed mode conditions were investigated. Mixed mode I/II/III fracture toughness value was smaller than the pure mode I fracture toughness (KIc) value showing the weakness of brittle materials against multi-axial fracture compared to pure tensile type fracture.

A comparative study into the fracture toughness properties of duplex stainless steels

Beside their high mechanical strength, duplex stainless steels are a suitable choice in highly corrosive environments. These types of steels are used in steel bridges more and more frequently exposed to low temperatures and fatigue loads. However, for low temperature applications, it must be guaranteed that brittle fracture is avoided since duplex stainless steel shows a toughness-temperature relationship similar to that of carbon steel. For this reason, in the frame of the German national FOSTA research project “P 1390”, comprehensive investigations have been started into the material selection of duplex stainless steel to avoid brittle fracture considering the fracture mechanic based background of EN 1993-1-10. For this purpose, Charpy-V impact tests and fracture toughness tests have been systematically carried out for various duplex stainless steels in order to create the basis for the development of toughness requirements for new duplex classes. The validity of the already existing Master Curve concept and the applicability of the transition temperature correlation for duplex stainless steels based on experimental fracture toughness and Charpy-V impact tests have been investigated. The aim of this contribution is to present first results of these investigations.

Experimental and Numerical Fracture Toughness Evaluation of Spiral Seam Weld of API X65 Steel

In this research fracture toughness of spiral seam weld of API X65 is measured through the ASTM E1820 standard recommendations. According to thin-wall pipe of API X65 in this research, specimen sizes could not satisfy the plain strain condition of ASTM E399 therefore, indirect method is accomplished by evaluating the crack tip opening displacement. According to the nature of welding procedure and inhomogeneous probable defects in the different regions of seam weld, δ-R curve with multi-specimen technique of single-edge notched bend (SENB) specimens is applied for test procedure in this research. The measured CTOD and K1C are 0.23 mm and 265 MPam3/2 respectively. Furthermore, Three-dimensional finite element method numerical evaluation of crack propagation with GTN model has been performed. Good agreement with the other researches was observed for CTOD and fracture toughness of tested material. The difference was about 10 percent.

Composition optimization of PMMA base denture reinforced with different percentages of Nano Hydroxyapatite and alumina particles to obtain highest KIc and KIIc values using hybrid IWO/PSO algorithm

Composition of three-phase dental denture (Polymethyl methacrylate or PMMA + Nano Hydroxyapatite + Nano Alumina) was optimized using hybrid IWO/PSO algorithm to obtain the highest KIc and KIIc. First, initial fracture toughness tests were performed on nine groups of PMMA base dentures with different percentages of ingredients. Symmetric single edge notch beam and asymmetric three-point bend beam with inclined crack were used for KIc and KIIc testing, respectively. The initial design parameters were considered as: {75–100% PMMA, 0–10% Nano Hydroxyapatite and, 0–15% Nano Alumina}. As the initial guess values for starting the hybrid optimization algorithm, B-spline curves (as the cost function) were fitted to the experimental results of both modes I and II. Then the IWO/PSO algorithm was run to find the optimum composition and maximum fracture toughness value of each mode. The best KIc was found for the composition of {78.5% PMMA + 9% Nano Hydroxyapatide + 12.5% Nano Alumina}. Similarly, using the hybrid IWO/PSO algorithm the optimum composition of {83.4% PMMA + 8.7% Nano Hydroxyapatide + 7.9 % Nano Alumina} was obtained to gain the highest KIIc. The experimental fracture toughness values obtained from the proposed compositions via employing the hybrid algorithm were approximately 40% higher than the all tested nine groups of PMMA base dentures under both modes I and II. The fracture toughness ratio KIIc/KIc of the optimum mix-designs predicted by the hybrid algorithm was equal to 0.78 and showed good agreement with the experimental KIIc/KIc ratio. The fracture toughness for any desired mixed mode condition can also be predicted in-terms of the KIc and stress intensity factors and T-stress of the desired mode mixity.

Coupled Effects of Temperature and Humidity on Fracture Toughness of Al-Mg-Si-Mn Alloy.

The combined effect of temperature and humidity on the fracture toughness of aluminium alloys has not been extensively studied, and little attention has been paid due to its complexity, understanding of its behaviour, and difficulty in predicting the effect of the combined factors. Therefore, the present study aims to address this knowledge gap and improve the understanding of the interdependencies between the coupled effects of temperature and humidity on the fracture toughness of Al-Mg-Si-Mn alloy, which can have practical implications for the selection and design of materials in coastal environments. Fracture toughness experiments were carried out by simulating the coastal environments, such as localised corrosion, temperature, and humidity, using compact tension specimens. The fracture toughness increased with varying temperatures from 20 to 80 °C and decreased with variable humidity levels between 40% and 90%, revealing Al-Mg-Si-Mn alloy is susceptible to corrosive environments. Using a curve-fitting approach that mapped the micrographs to temperature and humidity conditions, an empirical model was developed, which revealed that the interaction between temperature and humidity was complex and followed a nonlinear interaction supported by microstructure images of SEM and collected empirical data.

A general correlation between ductile fracture toughness and unified constraint parameter for different materials

In structural integrity assessment based on the two-parameter approach with considering constraint, it is an important step to establish a correlation between fracture toughness and constraint parameter for a specific material. But it is difficult and costly. In this work, based on a large number of ductile fracture toughness JC data measured from specimens with different geometries and crack sizes for different materials and finite element calculation of a newly developed unified constraint parameter Ad* for each specimen, a general correlation equation between JC and Ad* has been established. This correlation equation may be applied to predict constraint-dependent fracture toughness for different materials and structures, and its prediction capability has been validated by a great quantity of experimental fracture toughness data of various specimen geometries and materials (including welded joints). This correlation equation has a wide application range and may provide a simple and convenient engineering method to estimate constraint-dependent ductile fracture toughness.

Comparison of mixed mode I/III fracture toughness data obtained from the ENDB specimens manufactured by gyratory cylinders with two diameters of 100 and 150 mm

ABSTRACT Different specimens have been developed to characterise the fracture parameters. Recently, a disc shape specimen entitled ‘edge notched disc bend’ (ENDB) sample was introduced; it is used for conducting pure mode I, III, and related fracture mode mixities. In this paper, to extend the application of ENDB specimen, a large number of mode I, III, and I/III fracture toughness experiments were conducted on HMA at three different low-temperatures (−5, −15, and −25°C) and by applying two rates of loading (1 and 5 mm/min). The effect of ENDB radius (i.e. 50 and 75 mm) for a constant thickness of 30 mm was investigated experimentally. Based on the data, both ENDB diameters result in nearly the same mixed I/III fracture toughness values,. The difference between the fracture toughness of both ENDB sizes was less than 8%. Hence smaller size of ENDB sample with a diameter of 100 mm can be employed successfully instead of 150 mm (that is commonly used in many asphalt fracture toughness studies). The results also indicated that the fracture toughness values in the full ranges of mode I/III case increase as the test temperature decreases (about −7% and −15% by increase from −25 to −15°C and −5°C).

R-Curve Behavior of Polyhedral Oligomeric Silsesquioxane (POSS)–Epoxy Nanocomposites

Polyhedral oligomeric silsesquioxane (POSS) is a suitable nanoscale reinforcement for thermosetting polymers, such as epoxy resin systems in order to modify its mechanical, thermal and chemical properties. The inclusion of POSS in the epoxy resin at higher loading (greater than 1 wt.%); however, it introduces the ductility during the fracture behavior of these nanocomposites. Consequently, the J-integral is used to quantify the fracture behavior of these materials and characterize the crack growth resistance curve against stable crack growth. A range of nanocomposites is prepared by adding 0.5, 1, 3, 5, and 8 wt.% of glycidyl POSS into DGEBF epoxy resin cured with an amine-based curing agent. From fracture toughness experiments, the load-displacement result confirms that when the POSS reinforcement is greater than 1 wt.%, the fracture behavior of the nanocomposite changes from brittle to ductile. For both brittle and ductile nanocomposites, the addition of POSS molecules improves the crack initiation toughness. The development of POSS–POSS compliant domains, reported previously, is responsible for this change in the failure behavior. The fractured images of POSS–epoxy nanocomposites, obtained by using scanning electron microscopy, show that the increase in fracture resistance at higher values of POSS loading occurs due to the extensive shear yielding. Meanwhile, the increased fracture toughness at lower values of POSS loading occurs due to crack pinning and crack deflection.

Functionally graded ultra-high temperature ceramics: From thermo-elastic numerical analysis to damage tolerant composites

To maximize the toughening contributions due to fiber bridging and residual stresses upon layering, ultra-high temperature ceramics containing variable amounts of short carbon fiber in functionally graded stacking sequences were designed and characterized. Stress fields evaluated by finite element model on (AB)nA and more complex asymmetric architectures were compared to the experimental fracture toughness pointing to an effective toughness increment in those structures where the notch fell in zones of residual compression. For the best composite, toughness at room temperature achieved 7 MPa·m0.5 and further increased to 10 MPa·m0.5 when tested at 1500 °C within a light ZrB2-based composite with density below 4 g/cm3.According to the numerical simulations and the effective microstructural features of the composites, the main guidelines for the realization of ceramics with simultaneous failure tolerance and ablation resistance were established.

Correlation between fracture toughness and constraint of bi‐material interfaces with different mechanical mismatches

AbstractIn this work, the correlation between ductile fracture toughness and a new unified constraint parameter Ad* for bi‐material interfaces with different mechanical mismatches (strength and hardening mismatches) has been investigated based on finite element analysis and experimental data validation. The results show that with increasing crack depth (geometric constraint) and mechanical mismatch level (increasing undermatch or overmatch level), the J–R curves and fracture toughness JC of bi‐material interfaces decrease. The constraint parameter Ad* values of bi‐material interface cracks are basically independent of the strength and hardening mismatches. A general correlation equation between fracture toughness JC and constraint parameter Ad* has been obtained, which may be applied to prediction of fracture toughness of any specimens or components with different geometric constraints and material properties mismatches (including homogeneous materials). The prediction capability of the correlation equation has been validated by experimental fracture toughness data of cracks with different strength and hardening mismatches in dissimilar metal welded joints.

Development of a design method for the fracture performance of steel fiber-reinforced concrete by considering steel fiber characteristics and aggregate gradation

In this study, a design method has been developed, which considers the characteristics of steel fiber and aggregate gradation, for the fracture performance of steel fiber-reinforced concrete. This design method requires only one specimen type and test specification, and input of the characteristic parameters of the steel fiber, the characteristic particle size of the concrete aggregate, the peak load of specimen under static loading, and the sizes and types of specimens. According to the calculation process of this design method, the fracture toughness, and the tensile strength, of SFRC can be determined simultaneously. This design method can be used to determine the fracture performance of SFRC without size effect. By introducing the characteristic parameters of steel fiber and the characteristic particle size of concrete aggregate have been introduced into the specific calculation formula of fictitious crack growth of SFRC, a design method has been developed, which considers the coupling effect of steel fiber characteristics and aggregate gradation, for a simultaneous determination of the fracture toughness and the tensile strength of a SFRC. By applying the design method developed in this study to 15 different proportions of SFRC three-point bending specimens with different volumes of steel fiber, aggregate gradations, and water-cement ratios, the fracture toughness and the tensile strength of the SFRC have been determined simultaneously. The determined values are in good agreement with the experimental tensile strength and fracture toughness calculated by the size effect model. Using the fracture toughness and the tensile strength obtained by the normal distribution method, a complete failure curve for SFRC with 95% reliability has been constructed, and the size meeting the requirements of linear elastic fracture mechanics has been obtained. Simultaneously, the prediction curves of fracture toughness and tensile strength determined by the design method developed in this study are basically consistent with those determined by the normal distribution method. This further verifies the correctness and rationality of the fracture performance design method for SFRC developed in this study. The simplified analytical expressions between the peak load and the fracture toughness, and between the peak load and the tensile strength of SFRC have been developed. Based on the simplified analytical expressions, the peak load of the minimum specimen size satisfying linear elastic fracture has been predicted.

Test method for fracture toughness GIC based on ubiquitiform theory

A new fracture toughness test method for obtaining the crack initiation fracture toughness GIC and crack propagation fracture toughness GC was proposed. This method was based on the basic idea of ubiquitiform geometry and experimental results showing that material fracture surfaces possess ubiquitiform characteristics. A uniaxial tensile test was performed on plate specimens with cracks made by ductilecast iron to generate Mode I fracture failure. Loading–displacement curves were recorded in accordance with the experimental process, and the crack-initiation work W1 and the crack propagation work W2 were recorded. The morphology of the fracture surface was observed through electron microscopy, and the ubiquitiformal complexity D of the fracture surface was calculated on the basis of ubiquitiform theory. The ubiquitiform fracture area of the fracture surface was further obtained. The formula for ubiquitiform fracture energy was provided and used to calculate the ubiquitiform fracture energies Guf1 and Guf2 of the tested material. By analyzing the relationship between the above ubiquitiform fracture energy and fracture toughness GIC, a test method for determining fracture toughness based on ubiquitiform fracture energy was proposed. The experimental fracture toughness test results for several cracks of different sizes were consistent within a certain range of crack lengths.

Design of Fe2B-based ductile high temperature ceramics: First-principles calculations and experimental validation

Iron Boride (Fe2B) has significant potential to be used as a wear- and corrosion-resistant ceramic because of its high hardness, good corrosion resistance, and low cost. However, the toughness and elastic heat resistance of Fe2B can be further improved under high-temperature wear conditions. First-principles energetics and phonon calculations showed that Ti, Cr, Mn, Co, Ni, Nb, Mo, Ta, and W can dissolve in Fe2B. Among these, Cr, Mn, and Ni can form a continuous solid solution in (Fe, M)2B. Furthermore, Ti, Cr, Mn, Co, Ni, Cu, Y, Nb, Mo, Ta, and W improved the toughness of Fe2B. The results were validated by the experimental results. It was found that the experimental fracture toughness of Fe2B doped with Cr and Ni reached 4.16 and 4.27 MPa m1/2, respectively, which are higher than 3.64 MPa m1/2 of pure Fe2B. Chemical bond analyses showed that the toughness was improved owing to the stiffening of the B–B bond and the invariability of the Fe–B bond in Fe2B by doping Cr and Ni with increasing pressure. The elastic heat resistance of alloyed Fe2B was related to the degree to which the B–B and Fe–B bonds weakened with temperature. The less these bonds strength weakened with increasing temperature, the better the elastic heat resistance. The bond strength of Fe–B and B–B bonds in Fe2B doped with Cr showed a minimum decrease with increasing temperature, whereas the bond strength of Fe–B and B–B bonds in Fe2B doped with Ta showed a maximum decrease with increasing temperature. This indicates that the alloying element Cr can improve the elastic heat resistance of Fe2B, while Ta decreases the elastic heat resistance of Fe2B.

An R-curve effect-included delamination growth criterion for mixed-mode I/II delamination predictions of composite laminates

Most existing mixed-mode I/II delamination growth criteria are purely empirical or semi-empirical fitting functions of the experimental fracture toughness, which limit their wide range applications. In this work, a novel delamination growth criterion is proposed for mixed-mode I/II delamination predictions considering the significant R-curve effects in composite laminates. Its adequacy and accuracy are validated by the available experimental data for two different composite materials from literature. Both the trend and the magnitude of the predicted failure loci show fairly good agreements with the experimental ones. Only two experimentally obtained parameters, i.e. the pure mode I and mode II fracture toughness curves are required in the proposed criterion, which makes the proposed criterion easy to use. The proposed criterion can be applied for the evaluation of the mixed-mode I/II delamination growth behavior and thus the prediction of the structural strength, which may have important theoretical significance and engineering guidance value for the damage-tolerant design of composite materials and structures.

Size effect in true mode [formula omitted] fracturing of rocks: Theory and experiment

The influence of specimen size on the measured values of true mode II fracture toughness (KIIc) of rocks have been investigated both experimentally and theoretically. In the experimental study, the double-edge notched Brazilian disk (DNBD) test is utilised to perform fracture toughness experiments with various specimen radii ranging from 27 to 101 mm. Test results demonstrate a significant increase of KIIc with the specimen size. The reason for this trend is explained by a novel mathematical model developed based on the fracture process zone and the energy release rate (ERR) concepts. Comparing the predictions of the theoretical model with the experimental data reveals that the proposed theory can accurately predict the size effect phenomenon in the DNBD test data. To compare the results of KIIc with KIc, similar experimental and theoretical analyses are also performed for mode I fracture toughness tests using the semi-circular bend specimens. Lastly, we present a set of correction factors as functions of specimen size, whereby making it possible to estimate the size independent value of KIIc from the test results obtained from laboratory size DNBD specimens.

Toughening and Healing of CFRPs by Diels-Alder-Based Nano-Modified Resin through Melt Electro-Writing Process Technique.

In the current study, a novel approach in terms of the incorporation of self-healing agent (SHA) into unidirectional (UD) carbon fiber reinforced plastics (CFRPs) has been demonstrated. More precisely, Diels–Alder (DA) mechanism-based resin (Bis-maleimide type) containing or not four layered graphene nanoplatelets (GNPs) at the amount of 1 wt% was integrated locally in the mid-thickness area of CFRPs by melt electro-writing process (MEP). Based on that, CFRPs containing or not SHA were fabricated and further tested under Mode I interlaminar fracture toughness experiments. According to experimental results, modified CFRPs exhibited a considerable enhancement in the interlaminar fracture toughness properties (peak load (Pmax) and fracture toughness energy I (GIC) values). After Mode I interlaminar fracture toughness testing, the damaged samples followed the healing process and then were tested again under identical experimental conditions. The repeating of the tests revealed moderate healing efficiency (H.E.) since part of the interlaminar fracture toughness properties were restored. Furthermore, three-point bending (3PB) experiments were conducted, with the aim of assessing the effect of the incorporated SHA on the in-plane mechanical properties of the final CFRPs. Finally, optical microscopy (OM) examinations were performed to investigate the activated/involved damage mechanisms.