Toughness Tests Research Articles

Temperature Dependence of Cleavage Fracture Toughness for an Ultrahigh Strength Martensitic Steel

Abstract This work conducts an exploratory evaluation of the brittle fracture behavior for an ultrahigh strength martensitic steel using conventional three-point bend SE(B) and precracked Charpy V-notch (PCVN) specimens. A primary purpose of this study is to verify the effectiveness of the Master Curve methodology in providing a reliable estimate of the reference temperature (T0) derived from fracture toughness data sets measured in the ductile-to-brittle transition (DBT) region of an ultrahigh strength, low alloy martensitic steel. Fracture toughness testing conducted on three-point bend SE(B) specimens and PCVN configurations at different test temperatures in the DBT region provides the cleavage fracture resistance data in terms of the J-integral at cleavage instability, Jc, and its corresponding KJc-values for the tested material. Although this class of ultrahigh strength steel having a martensitic microstructure is currently beyond the reach of ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, the analyses described here show that the predicted normalized curves of median fracture toughness versus temperature are in good agreement with the experimental measurements.

Loading Rate of Fracture Toughness Tests in Transition Regime

Abstract ASTM E1921-21, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, contains a limitation on the stress intensity factor loading rate (K˙) for the quasistatic loading. The specimens should be loaded at a rate such that K˙ during the initial elastic portion is between 0.1 and 2 MPa√m/s. However, based on recent improvements in the test technique, it should be possible to narrow the loading rate range in order to reduce variability and have more comparable results between laboratories. For all users to be able to achieve the reduced range, clear guidelines are needed, which are not yet available. The Belgian Nuclear Research Centre (SCK CEN) has extensive experience in fracture toughness testing and has constructed a large database on which statistical studies were performed. This paper illustrates, using the SCK CEN database, that Wallin’s theoretical displacement formulas can only be used in the case in which the compliance of the testing setup is taken into account. An improved equation is proposed, guidelines for implementing the equation are drawn up, and these are verified using the database. It is clear that the methodology is sufficiently robust to provide an average loading rate within the suggested range of 0.5 and 2 MPa√m/s. This methodology could thus be integrated into the testing standard ASTM E1921-21.

Prediction of hydrogen-assisted fracture under coexistence of hydrogen-enhanced plasticity and decohesion

This work presents a novel strategy to model the coexistence of HELP & HEDE mechanisms. The proposed method accounts for ductile and brittle damage with a competitive fracture criterion to describe the competition effect between HELP & HEDE mechanisms. The cooperation aspect is addressed by a dislocation-enhanced hydrogen accumulation model, which agrees with the HELP-mediated HEDE mechanism. The tensile, fracture toughness, and threshold stress intensity factor tests are simulated in different hydrogen environments. The results demonstrate that the proposed method reproduces the hydrogen-dependent ductile to brittle transition phenomenon. Under different hydrogen conditions, the dominating damage mechanism and degradation trends agree with experimental observations. Furthermore, for the investigated 4130 steel, the predicted value of fracture toughness and threshold stress intensity factor match the experiment results. The difference in damage kinetics inherent in the two test methods is captured, which supports the rationality of the proposed strategy for HELP and HEDE interaction.

Structural Anisotropy Parameters’ Effect on the Low-Temperature Impact Strength of Alloy Steels in Rolled Products

The present work is devoted to the study of the influence of the parameters of the structural anisotropy of rolled products on the low-temperature impact strength of alloyed steels. A quantitative metallographic analysis of the microstructure of rolled steel samples obtained after testing for low-temperature toughness was carried out. It was established that the main reason for the decrease in the low-temperature impact strength of rolled steel samples is a highly developed segregation band enriched with carbon films formed at the stage of steelmaking conversion in violation of the technology of continuous casting of steel. The microstructural analysis of rolled stock samples was used in the work, and studies of the fracture surface of rolled stock samples were carried out with a scanning electron microscope using X-ray microanalysis methods. The studies carried out showed that the metallurgical quality of sheets of one heat, as well as individual samples within one sheet, varied over a wide range, from satisfactory to unacceptably low. It was established that the main reason for the decreasing low-temperature impact strength of rolled products was a highly developed segregation band enriched with carbon films, formed at the stage of steelmaking in case of violation of the continuous casting of steel technology. The multivariate statistical analysis carried out showed that only the size of the segregation band has an effect on the low-temperature impact strength of 10 mm thick rolled coil samples.

Determination of fracture toughness of 2.25Cr1Mo0.25V steel based on acoustic emission technique

The correct determination of fracture toughness is dependent on the accurate measurement of the point of crack initiation. In this research work, the AE technique is used to accurately detect the point of crack initiation and determine the initiation fracture toughness value JAE of 2.25Cr1Mo0.25V steel. Moreover, conventional standard procedures such as the J-R curve and stretch zone methods are also used to determine the fracture toughness JQ and JSZW values, respectively. Multiple AE parameters involving the entropy, peak amplitude, count, and energy are extracted from AE signals to identify the damage progression during fracture toughness tests. The results show several critical points associated with the point of plastic yielding, the onset of crack initiation, and the rapid crack growth and final fracture can be accurately identified by AE characteristics. Furthermore, the point of crack initiation recognized by AE is used to determine the JAE value. The fracture toughness JAE values of different specimens are also compared with the corresponding JQ and JSZW values. The JAE values are found to be about 20% lower than the JQ values but are significantly larger than the JSZW(2D) and JSZW(3D) values. It is believed that the fracture toughness parameter JAE determined by the AE technique can provide an important reference for evaluating the fracture toughness of engineering materials.

Comparative Study of the Relationship between Microstructure and Mechanical Properties of Aluminum Alloy 5056 Fabricated by Additive Manufacturing and Rolling Techniques.

In recent years, additive manufacturing of products made from 5000 series alloys has grown in popularity for marine and automotive applications. At the same time, little research has been aimed at determining the permissible load ranges and areas of application, especially in comparison with materials obtained by traditional methods. In this work, we compared the mechanical properties of aluminum alloy 5056 produced by wire-arc additive technology and rolling. Structural analysis of the material was carried out using EBSD and EDX. Tensile tests under quasi-static loading and impact toughness tests under impact loading were also carried out. SEM was used to examine the fracture surface of the materials during these tests. The mechanical properties of the materials under quasi-static loading conditions exhibit a striking similarity. Specifically, the yield stress σ0.2 was measured at 128 MPa for the industrially manufactured AA5056_IM and 111 MPa for the AA5056_AM. In contrast, impact toughness tests showed that AA5056_AM KCVfull was 190 kJ/m2, half that of AA5056_IM KCVfull, which was 395 kJ/m2.

Characterization and optimization of weld properties due to the effect of different pin profiles for AA8011 – based friction stir welding

PurposeThis Friction stir welding study aims to weld thick AA8011 aluminium plates, and the interface joints created with a variety of tool pin profiles were examined for their effects on the welding process.Design/methodology/approachScanning electron microscopy and optical microscopy and X-ray diffraction were used to examine the macro and micro-structural characteristics, as well as the fracture surfaces, of tensile specimens. The mechanical properties (tensile, hardness tests) of the base metal and the welded specimens under a variety of situations being tested. Additionally, a fracture toughness test was used to analyse the resilience of the base metal and the best weldments to crack formation. Using a response surface methodology with a Box–Behnken design, the optimum values for the three key parameters (rotational speed, welding speed and tool pin profile) positively affecting the weld quality were established.FindingsThe results demonstrate that a defect-free junction can be obtained by using a cylindrical tool pin profile, increasing the rotational speed while decreasing the welding speeds. The high temperature and compressive residual stress generated during welding leads to the increase in grain size. The grain size of the welded zone for optimal conditions is significantly smaller and the hardness of the stir zone is higher than the other experimental run parameters.Originality/valueThe work focuses on the careful examination of microstructures behaviour under various tool pin profile responsible for the change in mechanical properties. The mathematical model generated using Taguchi approach and parameters was optimized by using multi-objectives response surface methodology techniques.

Simple inverse FEM based procedure of flow curve extraction applied to unirradiated and irradiated 73 W weld material

The H2020 FRACTESUS project is aimed to validate miniaturized compact tension specimens usage for fracture toughness characterization of irradiated materials. One of the tasks in the project is devoted to the numerical simulations of the 0.16T and 1T-CT specimens and comparison of their stress/strain fields in. For this purpose accurate material flow curves are necessary to derive comparable mechanical response between the numerical models and experiments. This paper presents a simple technique of flow curve extraction from the tensile test using engineering stress–strain data. The proposed method utilizes a coarse meshed Finite Element Model and a manual flow curve data optimization procedure without additional scripting. The robustness of the method was shown on the example of the 73 W weld material in the unirradiated and irradiated states, tested at various temperatures from −100 °C to 125 °C. The obtained numerical flow curves are in a very good agreement with the experimental results. Moreover, the advantages of the proposed method were shown over either the methods based on extrapolation of the predictions of popular models with parameters fitted to prior necking or the method based on fracture diameter measurements. The final validation of the proposed methodology was demonstrated by numerical simulations of fracture toughness tests on precracked Charpy specimens.

Effect of pre-crack non-uniformity for mini-CT geometry in ductile tearing regime

The mini-CT specimen attracts the attention from all over the world for application in fracture toughness measurement. However, one of the shortcomings of this geometry is related to the required tight accuracy of the specimen dimensions, in particular the fatigue pre-crack curvature which often violates the requirements of the ASTM standards. Given the limited thickness of mini-CT geometry, non-uniform pre-crack tends to develop during fatigue pre-cracking, resulting in a large proportion of mini-CT specimens being considered invalid.Previous investigations have demonstrated that mini-CT specimens with excessive crack front curvature can still provide meaningful fracture toughness results. In this paper, the effect of pre-crack front non-uniformity on ductile fracture is studied: first, the difference of macro parameters such as the applied load, J-integral and crack tip stress–strain field are investigated to illustrate the varying fracture behavior related to non-uniform pre-crack. Next, two micro-mechanics-based approaches, the Rice-Tracey void growth model and Thomason void coalescence model, are integrated to compare the ductile fracture initiation conditions associated with uniform and non-uniform fatigue pre-crack. Finally, the experimental verification of the ductile fracture simulations is performed for mini-CT specimens with uniform and 30° tilted initial cracks. The results indicate that the pre-crack non-uniformity plays a major role in the redistribution of local J-integral and stress–strain state, further affects the position of crack initiation and the early stages of crack propagation. Nevertheless, the pre-crack non-uniformity has limited effect on the global properties that are usually expected from fracture toughness tests, such as applied load, J-R curve and critical fracture toughness. The requirements in the ASTM E1820 regarding pre-crack front curvature need thus to be relaxed.

Synthesis and characterization of bulk mechanical properties of a bio-based resin filled by graphene nanoplatelets and cellulose nanocrystals

In the present paper, a novel bio-based resin derived from epichlorohydrin was reinforced by Graphene NanoPlatelets (GNPs) and Cellulose NanoCrystals (CNCs) in different weight ratios and characterized experimentally through tension tests and fracture toughness tests on bulk specimens. The nanofillers were applied separately. The experimental results showed that the neat resin has a Young’s modulus of 3.29 GPa, a tensile strength of 45.15 MPa, a stress intensity factor of 0.61 MPa m1/2 and a critical strain energy release rate of 0.091 kJ/m2. The level of the properties reveal that the bio-based adhesive can be used for cosmetic and for some structural applications. The addition of cellulose nanocrystals in the epoxy resin didn’t improve the mechanical properties of the neat resin mainly due to the development of intense aggregation of cellulose nanocrystals. On the other hand, the addition of graphene has led to the increase of the Young’s modulus and the fracture toughness and to the decrease of the tension strength of the resin. Development of agglomerations of graphene were also present in this case. The contradictory findings on the mechanical properties of the reinforced resin gives a clear message about the need for optimizing the manufacturing process. Both nanocomposites have undergone a complete life-cycle analysis which has shown that they are far more environmentally friendly than a conventional epoxy resin.

Design of damage-resistant hybrid lay-up structures for fiber-reinforced composites based on interface properties

In this work, the poor impact damage resistance of carbon fiber-reinforced plastic (CFRP) composites was improved by mixing carbon fibers with ductile fibers (glass fibers/Kevlar fibers). The damage behavior of five interfaces of the prepared hybrid fiber-reinforced plastic (HFRP) composites was investigated through mode-I and mode-II interlaminar fracture toughness tests and micro-droplet debonding tests. On this basis, a numerical model for predicting the impact damage of HFRP composites was established. Through a comprehensive evaluation of the impact damage resistance indexes obtained from the numerical simulations, the hybrid fiber type, hybrid ratio between different fibers, and hybrid lay-up sequence could be optimized. The specimens with optimized lay-ups were prepared by 3D printing, and the effectiveness of the numerical model was verified through low-velocity impact and compression after impact (CAI) tests. The results show that the introduction of glass fiber layers in CFRP composites can significantly improve the damage resistance of the composites. Compared with CFRP composites, when the volume fraction of the glass fibers is 25%, HFRP composites have a smaller delamination damage area (reduced by 74.08%), furthermore, the pit depth is reduced by 43.01%, the CAI is increased by 17.29%, and the strength retention rate is increased by 31.94%.

Numerical prediction of warm pre-stressing effects for a steam turbine steel

In warm pre-stressing (WPS), the fracture resistance of cracked steel components is raised when subjected to certain temperature-load histories. WPS’s beneficial effects enhance safety margins and potentially prolong fatigue life. However, understanding and predicting the WPS effects is crucial for employing such benefits. This study utilised pre-cracked compact tension specimens made from steam turbine steel for WPS and baseline fracture toughness testing. Two typical WPS cycles were investigated (L-C-F and L-U-C-F), and an increase in fracture resistance was observed for both cycles. The WPS tests were simulated using finite element analysis to understand its effects and predict the increase in fracture resistance. A local approach was followed based on accumulative plastic strain magnitude ahead of the crack tip. Since cleavage fracture is triggered by active plasticity, the WPS fracture is assumed when accumulated plasticity exceeds the residual plastic zone formed at the crack tip due to the initial pre-load.

Evaluation of the frictional effect on the energy release rate in doubly end-notched tension test for mode II delamination

Fracture of composite structures is frequently caused by delamination, and evaluation of the interlaminar fracture toughness is thus important. A new double-lap joint-like fracture toughness test, that is, the doubly end-notched tension (DENT) test, achieves stable mode II delamination growth in a simple tensile test. This study numerically investigated the influence of friction between delamination surfaces on the perceived energy release rate in the DENT test. The virtual crack closure technique was adapted with considering friction for the DENT models, as well as the three-point-bend and four-point-bend end-notched-flexure models for comparison. Despite the tensile loading in the DENT test, the frictional effect was significant when the thickness ratio of the surface to the middle layers was high. A corrected equation for evaluating the energy release rate successfully eliminated the frictional effect in the DENT test. An analysis of the full DENT model revealed that the crack length difference alleviated the contact and frictional effects.

Microstructure and Mechanical and Impact Behaviors of WC-Particle-Reinforced Nickel-Based Alloy Surfacing Layers at Evaluated Temperatures

A WC-particle-reinforced nickel-based alloy surfacing layer was fabricated on 42CrMo ultra-high-strength steel. The microstructure and the mechanical and impact-damage behaviors of the surfacing layers at the evaluated temperatures were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and the Vickers hardness tester. Results showed that these WC particles gradually changed from elongated and crisscross needle-like phases to blocks with the increase in impact temperature. Numerous carbide phases (e.g., (Cr,Ni,Fe)23C6) and γ-Ni phases were formed in the substrate matrix. The surfacing layer showed a typical brittle fracture, and the impact energy decreased with the increase in temperature. Moreover, the surfacing layer showed a clear quasi-cleavage fracture morphology without dimples after a 600 °C impact test but exhibited a mixture of dimple fractures and cleavage fractures after the 200 °C and 400 °C impact tests. The Vickers fracture toughness test showed that the average hardness of the surfacing layer after a 600 °C impact test was 383 HV1.0, which is about 0.8 times that after the 200 °C impact test. In addition, the WC particles in the surfacing layer after the 600 °C impact test showed the highest fracture toughness, but the corresponding Ni40A binder phase possessed the lowest fracture toughness.

Study on the Performance of FSW Joint Welded of Aluminum Alloy by S-Type Track

The article adopts S-Type track friction stir joining to butt joining of 10 mm thick 6061-T6 aluminum alloy, and conducts impact toughness test and 24h salt spray corrosion on the joint area. On the premise of ensuring the tensile strength, by changing the swing radius of different S-shaped trajectories, and comparing with the conventional linear joining trajectory, it is obtained that at a speed of 500r/min, a joining equivalent speed of 130 mm/min, and a swing radius of 0.5 mm Under the parameters, the maximum impact toughness is 80.74 J/mm2, and the corrosion resistance is better than the base material and the straight joining track. The S-shaped trajectory of friction stir joining area with a swing radius of 0.5 mm forms a “thousand-layer cake” structure with high bonding strength, which can effectively improve the impact resistance and corrosion resistance of the joining area.

Effects of freeze–thaw cycles on mode I fracture toughness of adhesively bonded CFRP joints

The effects of freeze–thaw (FT) cycles on the mode I fracture toughness of adhesively bonded carbon fiber reinforced plastic joints were studied. Fracture toughness specimens were immersed in water and exposed to thermal cycles consisting of thawing (+60 °C) and freezing (−50 °C). Two types of fracture toughness specimen were prepared: those without mechanical loading (non-stressed) and those with mechanical loading using a wedge (stressed) during the environmental conditioning. Fracture toughness of the non-stressed specimens gradually decreased with increasing number of FT cycles, whereas that of the stressed specimens first increased and then gradually decreased. Fracture mechanisms are discussed on the basis of the results of fracture toughness tests, the tensile properties of bulk adhesives, and fracture morphologies.

Study of mechanical properties and brittle fracture resistance for weld metal of WWER RPV

The results of the study of mechanical properties and brittle fracture resistance (BFR) are presented for weld metal of WWER RPV performed by automatic arc welding with use welding wire Sv-15CrNiMoTiA and ceramic flux 48AF-71. Mechanical properties are determined on the basis of test results of tensile smooth round bar. BFR are determined from impact strength tests and fracture toughness tests. The anisotropy of mechanical properties and BFR is investigated by testing the specimens with different orientations. Tests are conducted for specimens of two orientations: first orientation corresponds to the position of the specimen, in which the fracture surface is perpendicular to the axis of the weld; second orientation corresponds fracture surface parallel to the axis of t he weld. It i s shown that the weld metal performed according to above mentioned technology has no anisotropy both in mechanical properties and in BFR. An explanation of the significant scatter of BFR on the basis of the results of metallographic studies is proposed. The obtained experimental results on mechanical properties for investigated weld metal allow to use tensile smooth round bar with 3 mm diameter with transverse orientation instead of specimens with 6 mm diameter with longitudinal orientation as the scale factor and anisotropy are negligible. The correlation dependence between the values of reference temperature T0 determined by the Master Curve method and reference temperature T100 determined by the Advanced Unified Curve method and the value of critical brittleness temperature TK0 for the studied weld metal in the initial state is established.

Dynamic Fracture and Crack Arrest Toughness Evaluation of High-Performance Steel Used in Highway Bridges.

Impact energy tests are an efficient method of verifying adequate toughness of steel prior to it being put into service. Based on a multitude of historical correlations between impact energy and fracture toughness, minimum impact energy requirements that correspond to desired levels of fracture toughness are prescribed by steel bridge design specifications. Research characterizing the fracture behavior of grade 485 and 690 (70 and 100) high-performance steel utilized impact, fracture toughness, and crack arrest testing to verify adequate performance for bridge applications. Fracture toughness results from both quasi-static and dynamic stress intensity rate tests were analyzed using the most recently adopted master curve methodology. Both impact and fracture toughness tests indicated performance significantly greater than the minimum required by material specifications. Even at the AASHTO Zone III service temperature, which is significantly colder than prescribed test temperatures, minimum average impact energy requirements were greatly exceeded. All master curve reference temperatures, both for quasi-static and dynamic loading rates, were found to be colder than the Zone III minimum service temperature. Three correlations between impact energy and fracture toughness were evaluated and found to estimate reference temperatures that are conservative by 12 to 50 °C (22 to 90 °F) on average for the grades and specimen types tested. The evaluation of two reference temperature shifts intended to account for the loading rate was also performed and the results are discussed.

A discrete element analysis for general failure behavior of basalt

Discrete element method is a commonly used numerical method in rock mechanics and rock engineering. In this research, the flat-joint contact bond model is used to reproduce the macro-properties of basalt on an assembly of particles. A series of laboratory tests, including uniaxial compression test, Brazilian tension test, and fracture toughness test are used to simulate progressive degradation of rock mass strength in a size-independent model with a unique set of micro-properties. The work involves a detailed analysis for estimating compressive elastic modulus, Poisson's ratio, bimodularity, mode I fracture toughness, crack closure, crack initiation, and crack damage thresholds, as well as the tensile and compressive strengths. The constructed flat-jointed material yields a realistic uniaxial compression to tensile strength ratio and nonlinear stress-strain response through sustaining adequate particle interlocking and embedded pre-existing microcracks. The role of sample size on the compressive elastic modulus, crack initiation and crack damage stress limits is demonstrated in detail. A model size-independency analysis was also performed to examine the effect of particle size on the results. The numerical results presented in this study are within 10% of the laboratory results.

A novel damage evaluation of CFRPs under mode-I loading by using multi-instrument structural health monitoring methods

Mode-I fracture toughness test provides valuable information in the thickness direction of fiber reinforced polymer matrix composites. However, damage propagation under mode-I loading is dependent on the configuration of reinforcing material of the laminate. Understanding the damage types and their growth rate in mode-I fracture toughness test is a vital factor to obtain material allowable for safe design. In this study, mode-I tests are conducted on unidirectional, and twill woven carbon fiber reinforced polymer composite laminates. A new approach is proposed to interpret passive infrared thermography results based on correlating acoustic emission and thermography results in time whereby thermal activities can be classified into two main groups corresponding to matrix and fiber dominant failure types. It is demonstrated that matrix and fiber dominant failures lead to thermal activities with line-wise and point-wise form, respectively. Results show that four different damage types can be seen for mode-I fracture of both laminates. The temporal observations during thermoelastic cooling of the materials show that twill woven laminate releases relatively higher energies due to matrix dominant damage developments which means this configuration type is more prone for delamination failures.