Unnotched Specimens Research Articles

Effect of welding processes on high cycle fatigue behavior for naval grade HSLA joints: A fatigue strength prediction

Naval grade high strength low alloy steel is designed especially for shipbuilding applications due to its high strength-to-weight ratio. The fatigue failure of welded structures is susceptible to cyclic loading through heterogeneities of consumables, microstructure deformation, and complicated local stress-strain conditions. In this investigation, the influence of welding processes on high cycle fatigue behavior and fatigue strength prediction of DMR249 A grade steel. The fatigue test was conducted for five different stress levels from 50 to 90 % yield strength with a stress ratio R = −1. The evaluated fatigue properties are correlated with mechanical properties and microstructural characteristics. The maximum obtained fatigue strength of SMAW and GMAW joints is about 342 MPa and 355 MPa for the unnotched specimens. From the experimental results, it is found that the fatigue strength of the fusion welded joints relative to 55 % (SMAW) and 59 % (GMAW) of tensile strength of the parent metal. Therefore, the fatigue strength of GMAW joints is appreciably better than SMAW joints. Finally, the constructed SN curves of base metal and welded joints were lying above the IIW standard curve.

Fracture toughness determination of porcine muscle tissue based on AQLV model derived viscous dissipated energy

The ability of soft collagenous tissue (SCT) to withstand propagation of a defect in the presence of a macroscopic crack is termed the ’fracture toughness parameter’. In soft tissues not undergoing significant plastic deformation, it is purported that a considerable amount of additional energy is dissipated during failure processes, due to viscoelasticity. Hence the total work, measured experimentally during failure, is the sum of fracture and viscoelastic energies. Previous authors have aimed to apply constitutive modeling to describe viscoelastic hysteresis for fracture toughness determination with a tendency of models to either over or underestimate the viscous energy. In this study, the fracture toughness of porcine muscle tissue is determined using two strategies. Firstly, it was determined experimentally by calculation of the difference in dissipated energy of notched and unnotched tissue specimens undergoing cyclic ’triangular wave’ excitation with increasing strain levels in uniaxial tension. The second strategy involved the extension and use of the adaptive quasi-linear viscoelastic model (AQLV) to model cyclic loading (model parameters were obtained from a previous study) and sequentially the dissipated energy was calculated. The mean value of the dissipated energy based on the AQLV approach was then subtracted from the total dissipated energy of notched porcine muscle tissue samples to determine the fracture toughness. The mean experimental viscous dissipated energy ratio was 0.24 ± 0.04 in the experimental approach, compared to 0.28 ± 0.03 for the AQLV model. Fracture toughness determined experimentally yielded 0.84 ± 0.80 kJ/m2, and 0.71 ± 0.76 kJ/m2 for the AQLV model, without a significant difference (p = 0.87). Hence, the AQLV model enables a reasonable estimation of viscous dissipated energy in porcine muscle tissue with the advantage to perform tests only on notched specimens, instead of testing additional unnotched samples. Moreover, the AQLV model will help to better understand the constitutive viscoelastic behaviour of SCTs and might also serve as a basis for future fracture toughness determination with constitutive model simulations.

Determination of tensile strength of cementitious composites using fracture parameters of two-parameter model for concrete fracture

In this study, the formulation of the two-parameter model based on the size effect proposed by previous researchers was initially improved to be much more practical. Subsequently, six series of notched and unnotched beams with a size ratio of 1:4 and notched and unnotched square prismatic specimens with a size ratio of 1:6 were tested to verify this modified formulation. The peak loads obtained from the fracture tests of the notched specimens were examined using the two-parameter model. Then, the test results of the unnotched bending and splitting specimens were simulated according to the modified formulation. Furthermore, this formulation was applied to extensive test data obtained from the direct tension specimens with double edge notches and from the unnotched specimens in the literature to derive the direct tensile strength of concrete. Finally, when comparing the results of this study to the previous studies, there were slight differences among the other formulas, except for the one that covered splitting specimens.

Effect of the stress concentration factor on the final fracture zone of aluminium AW 6063 T6 for rotating bending specimens

The paper presents fracture surfaces of the polycrystalline material - AW 6063 T6 aluminium alloy - under a rotating bending load. The fatigue fracture surfaces were analysed for high-cycle fatigue at approximately 1 × 105, 8 × 105 cycles and stress amplitude level equal to 140 MPa. The specimens with four different stress concentration factors Kt (with values ~1, 1.4, 2 and 2.6) were made from a drawn bar. The fatigue tests were performed by rotating bending stand with a frequency of 50 Hz. Thirty (30) tests were carried out for each geometry of the specimen. Final fracture zones for unnotched specimens were 39.9%, 35.8% and 43.3% of the total cross-sectional area for 1 × 105, 8 × 105 cycles and 140 MPa stress amplitude, respectively, However, the final fracture zone for the notched specimen with Kt equal to 2.6 was less and had a value of 13.7%, 13.8% and 18.8% of the total cross-sectional area for 1 × 105, 8 × 105 cycles and 140 MPa stress amplitude, respectively. The research aimed to compare the ratio of the area of the final fracture zone to the total cross-sectional area. The authors have found a relationship that could be used to estimate the stress concentration factor based on the area of the final fracture zone. Based on this, the stress concentration factor can be determined. The estimates calculated using the equation and the test results show a good correlation between the stress concentration factor and the ratio of the area of the final fracture zone to the total cross-sectional area. The proposed method of determining the stress concentration factor can be used to verify different manufacturing processes (e.g. surface finish, notch radius) and the loading history (e.g. existing overload conditions).

Dynamic Fracture Toughness Behaviour of CFRP-Foam-CFRP Sandwich Composite and Particles Filled Hybrid Glass Fiber Cloth, Graphene Nanoplates Coated Glass Fiber Strand Composite Materials under Low Impact Velocity

The main objective of the present study is to investigate the dynamic fracture toughness behaviors of CFRP-Foam-CFRP sandwich composite of V-notched through -thickness, surface, and un-notched specimens under Izod, and Charpy impact tests.  The sandwich composite structures are made of cross-plied carbon fiber reinforced plastic (CFRP) composite faces with polyurethane foam core. CFRP composites are used to combine the upper face and the lower face through the core in stitched sandwich structures. Compressive strength of weight drop impact perforated and un-perforated sandwich composite specimens are measured from a universal testing machine. Also, particles (Al2O3, CNTs, and cement) filled glass fiber cloth and graphene nanoplates coated glass fiber strands reinforced polymer hybrid composite are fabricated for V-notched, un-notched Izod impact and Charpy impact tests. The results show that weight drop impact energy is lower than the Izod impact energy but higher than the Charpy impact energy, whereas the dynamic fracture toughness of Izod impact energy is more than the Charpy and weight drop impact energy due to geometry of impactor and sandwich specimen. However energy and dynamic fracture toughness of Al2O3, CNTs, and Cement filled un-notched hybrid composites higher than the notched hybrid composites under Izod Impact. The dynamic fracture toughness and energy of CNTs filled hybrid composites is higher than the sandwich composites, Al2O3, and Cement filled hybrid composites under Charpy Impact.

Assessment of fatigue resistance of concrete: S-N curves to the Paris’ law curves

Fatigue behaviour of concrete materials is often investigated on un-notched specimens under the compressive or bending loads. In this experimental study, a notched three-point bending (TPB) specimen is used in high-cycle fatigue experiments to obtain the Wohler’s curve. Based on this approach, a novel, yet relatively simple transition from the traditional Wohler’s curve to the Paris’ law curve is proposed. Such a methodology allows one to obtain the Paris’ law material constants, which are used to determine the fatigue failure of the structure or a component. The constants of the experimentally determined material, measured in four different concrete mixtures, have been verified by recalculating the number of cycles until the fatigue failure Nf by the integration of the Paris’ law equation. The back-calculated number of cycles and the approximation of the S-N curve allowed for a comparison with the experimental data. Furthermore, the initial notch tip was extended in this approximation by the value of the critical distance. Such an extension allowed us to cover a wide range of the experimental data and provided a better prediction of fatigue life. The proposed method was verified on all four studied materials and showed satisfactory results.

Local fracture criterion for quasi-cleavage hydrogen-assisted cracking of tempered martensitic steels

Electrochemical permeation cell built on a Instron tensile testing machine allows fracturing notched specimens under hydrogen flux while monitoring simultaneously the flow stress and the permeation anodic current. Analysis of the fracture surfaces reveals that cracking initiates at the hydrogen-entry surfaces as quasi-cleavage regions followed by ductile propagation. Quasi-cleavage zones were fully developed at the maximum engineering stress and often initiate around inclusions for unnotched specimens. The observation of multiple cracks at the hydrogen-entry surfaces and correlations between quasi-cleavage features and martensitic boundaries suggest that cracking is related to decohesion of cropping out and inner boundaries. The local mechanical states and diffusible hydrogen concentration and flux at quasi-cleavage failure were calculated by finite element method (FEM). The results predict local failure criteria in terms of decrease of local maximum principal stress and equivalent plastic strain with local diffusible hydrogen concentration, revealing that the inclusion of hydrogen-mechanical-structural interactions at the charging surfaces is necessary to complete fracture analysis.

Effect of notch severity and crystallographic texture on local deformation and damage in commercially pure titanium

The evolution of local deformation and damage in commercially pure titanium was studied for flat un-notched, U-notch and V-notch tensile specimens with two distinct initial textures namely off-basal (BA) and prismatic-pyramidal (PP) orientations to establish the micro-mechanisms of anisotropic deformation and damage in titanium for different notch severity. BA orientation showed higher apparent yield stress, greater plastic deformation, better notch strength ratio (NSR) value than PP orientation for all the cases. Electron backscatter diffraction investigation showed an increase in the number of twins per grain and the thickness of twins with notch severity irrespective of orientation with BA samples showing higher twin activity than corresponding PP samples. In addition, extensive slip activity was observed within twins and parent grains in BA orientation unlike PP orientation that showed slip activity near grain boundaries. These microstructural observations correlated well with strain measurement from digital image correlation which also showed that the local maximum longitudinal strain for BA orientation increased with notch severity unlike PP orientation. Viscoplastic self-consistent crystal plasticity simulations at the mesoscale indicated that the activity of basal, pyramidal <c+a> slip systems and twinning increased at the expense of prismatic slip with an increase in notch severity for both the orientations. The experimental observations show extensive diffuse necking in BA orientation, while higher fraction of fluted voids with c-axis aligned along the major axis of the void are observed in the PP orientation. Prismatic slip promotes nucleation and growth of voids in the prism plane which is perpendicular to the tensile axis for PP orientation contributing to void growth along c-axis forming fluted voids, while higher twinning in BA orientation leads to fewer fluted voids, thus highlighting the significant role of crystallographic texture in controlling anisotropic deformation and damage mechanisms in titanium.

Stress crack resistance of unaged high-density polyethylene geomembrane fusion seams

The stress crack resistance (SCR) of high density polyethylene (HDPE) geomembrane (GMB) fusion seams is examined for two 1.5 mm HDPE GMBs and a range of welding parameters. Results are reported for both unnotched and notched seams as well as their corresponding sheet material. Unnotched seam SCR specimens are shown to preferentially initiate craze formation at the terminating edge of the squeeze-out bead, while incorporating potentially degraded areas, such as the seams heat-affected zone (HAZ), within the slow crack growth region of the specimen. In the short-term, little variation was observed between the majority of seams for the nine welding parameter combinations examined, with an average normalized seam SCR value (normalized with respect to the unnotched sheet SCR) of 0.3 ± 0.1, or about 30% of the SCR of the unnotched sheet. It is shown that squeeze-out geometry plays an important role in the SCR of fusion seams. Seams with weld track rippling, a known qualitative indication of overheating, were found to have average unnotched SCR values 45% lower than smooth weld track seams. Deleterious squeeze-out geometries are identified to provide a framework through which CQA engineers and researchers can more readily identify ‘higher risk’ seams with respect to stress cracking.

Effects of voids and raster orientations on fatigue life of notched additively manufactured PLA components

In this study, the fatigue life of notched polylactic acid (PLA) samples fabricated through the fused deposition modeling (FDM) technique was studied experimentally and numerically. The volumetric method based on the theory of critical distance was employed for fatigue life predictions. The effects of influential process parameters including raster orientation and FDM-induced defects such as voids or gaps inside the parts were examined on the fatigue strength reduction factors and fatigue lives. Circular and elliptical-shaped notch geometries were considered with various dimensions. Fatigue tests were conducted on notched and un-notched samples at the load ratio of 0.1. Predicted results were compared with experimental fatigue test data. Results revealed that the raster orientation parameter had a substantial impact on fatigue strength reduction factors and fatigue lives. The stress concentrations induced by the FDM process on the surfaces and inside the parts for the samples with 90° raster angles acted similar to the sharp notches, resulting in no substantial difference in fatigue life of notched and un-notched 3D-printed samples. In contrast, un-notched 3D-printed specimens with 0° raster orientations possessed higher fatigue lives as compared to the notched samples. While the volumetric approach efficiently predicted the fatigue lives of the samples with 90° raster orientations, it moderately underpredicted the fatigue lives of the samples with 0° raster angles.

Highly Dissipative Fiber-Reinforced Concrete for Structural Screeds

Synthetic fibers, especially polypropylene (PP) fibers, are emerging as a viable reinforcement for concrete, on account of their excellent durability, affordability, anti-spalling capability, low density, and magnetic transparency. Yet, the chemical nature of PP hinders the development of strong bonds at the fiber-to-matrix interface, with negative effects on the mechanical performance. To overcome this difficulty, in this research fibers are either chemically attacked (etched) or coated through sol-gel nanosilica deposition in order to promote their affinity to the hydration products in the binder. Three-point bending tests at different scales are carried out on unnotched specimens, including large-scale beams consisting of PP-reinforced concrete for structural screeds. Functionalization, especially in the form of silica coating, improves the binder-fiber interaction, which is responsible for a remarkable increment in the specific energy dissipated at failure, with respect to untreated fibers. Most importantly, both surface treatments induce a substantial hardening response as opposed to the softening behavior that is characteristic of low-dosage fiber-reinforced concrete. We conclude that surface functionalization, and especially nanosilica coating, offers significant advantages for better exploiting the reinforcing effect of PP fibers, and these carry over at different scales. In particular, results appear promising for screeds, which advocate optimal mechanical performance and durability while keeping the fiber content to a minimum.

Failure Behaviour of Carbon-Epoxy Composite in Crack Arrester and Crack Divider Mode

Failure behaviour of a carbon-epoxy composite is studied both in crack arrester (CA) and crack divider(CD) mode by conducting 3-point bend test of un-notched (UN) and single edge pre-crack notched (N) (notch depth varied from 0.5 to 3.3 mm) specimens at various cross head speeds(CHS) such as 0.2, 50, 100, 200 and 500 mm/min. Influence of notch depth (ND) and CHS on the conditional fracture toughness(CFT) is understood using ANOVA. In both the mode, while the CFT values varies linearly with CHS, its values are observed to be lower at short ND, higher at medium ND and again lower at high ND. In both the modes at high ND (~3.3mm) opening mode/fibre breaking (mode I) failure occurs because of high crack tip stress concentration (as analysed from ANSYS 15.0) which cause fibre failure at the tip and corresponding Mode I fracture toughness was observed to be~20 MPa.m0.5. The composite also shows similar flexural strength both in CA mode (641+48) and in CD mode (634+49 MPa). In CD mode N specimen predominantly fails by mode I. While in CD mode the delamination (mode II) is only observed in UN specimens, in CA mode UN or small N specimen shows mode II failure and medium notched specimen shows mixed mode failure. In CA mode, number of delaminated layer is higher for the specimen tested at high CHS. In CA mode during charpy test (strain rate 103/s), the composite shows numerous delamination and absorbed much higher energy(~40J/cm2) than that in CD (~10J/cm2) mode where mode I failure is predominant. During impact test, lowering of test temperature to -40oC has very little effect on the impact energy since it does not significantly affect the mode of failure.

Experimental evaluation of fracture properties of bovine hip cortical bone using elastic-plastic fracture mechanics.

Understanding the fracture mechanics of bone is very important in both the medical and bioengineering field. Bone is a hierarchical natural composite material of nanoscale collagen fibers and inorganic material. This study investigates and presents the fracture toughness of bovine cortical bone by using elastic plastic fracture mechanics. The J-integral was used as a parameter to calculate the energies utilized in both elastic deformation (Jel) and plastic deformation (Jpl) of the hipbone fracture. Twenty four different types of specimens, i.e. longitudinal compact tension (CT) specimens, transverse CT specimens, and also rectangular unnotched specimens for tension in longitudinal and transverse orientation, were cut from the bovine hip bone of the middle diaphysis. All CT specimens were prepared according to the American Society for Testing and Materials (ASTM) E1820 standard and were tested at room temperature. The results showed that the average total J-integral in transverse CT fracture specimens is 26% greater than that of longitudinal CT fracture specimens. For longitudinal-fractured and transverse-fractured cortical specimens, the energy used in the elastic deformation was found to be 2.8-3 times less than the energy used in the plastic deformation. The findings indicate that the overall fracture toughness measured using the J-integral is significantly higher than the toughness calculated by the stress intensity factor. Therefore, J-integral should be employ to compute the fracture toughness of cortical bone.

A Comparison of the Surface and Mechanical Properties of 3D Printable Denture-Base Resin Material and Conventional Polymethylmethacrylate (PMMA).

To study the surface and mechanical properties of 3D printed denture-base resin materials and compare them with conventional heat-cured polymethylmethacrylate (PMMA). Three brands of 3D printed denture-base resin materials and one conventional heat-cured PMMA were tested in this study: NextDent 3D printed resin, Dentona 3D printed resin, ASIGA 3D printed resin, and Meliodent conventional PMMA. Sixty specimens (25 × 25 × 3 mm) were fabricated (n=15 per group) to perform the following tests: wettability, surface roughness, and microhardness. One hundred twenty specimens (65 × 10 × 3 mm) were fabricated (n=30 per group) and stored in distilled water at (37 ±1°C) for 7 days. Specimens (N = 15) in each group were subjected to the three-point bending test and impact strength test, employing the Charpy configuration on un-notched specimens. The morphology of the fractured specimens was studied under scanning electron microscope (SEM). Statistical analysis was performed using one-way ANOVA and Tukey-pairwise multiple comparisons with 95% confidence interval. P-values of ≤0.05 were considered significant. The conventional heat-cured specimens demonstrated the highest means of surface roughness (0.23 ± 0.07 μm), Vickers hardness number (18.11 ±0.65) and flexural strength (92.44 ±7.91 MPa), and the lowest mean of contact angle (66.71° ±3.38°). ASIGA group showed the highest mean of contact angle (73.44° ±2.74°) and the lowest mean of surface roughness (0.19 ±0.03 μm). The highest mean of impact strength was recorded in the Dentona group (17.98 ±1.76 kg/m2 ). NextDent specimens showed the lowest means of Vickers hardness number (16.20 ±0.93), flexural strength (74.89 ±8.44 MPa), impact strength (15.20 ±0.69 kg/m2 ), and recorded the highest mean of bending modulus (2,115.80 ±178.95 MPa). 3D printed resin exhibited noticeable differences in surface and mechanical properties between different brands and with conventional heat-polymerized PMMA.

Mean stress sensitivity of sintered iron and steel

Abstract There is a well-based correlation between the pulsating amplitude σ A(R = 0) and the alternating amplitude σ A(R = – 1) in fatigue testing of sintered iron and steel which permits to predict the mean stress sensitivity and the pulsating amplitude if the alternating endurance limit σ A(R = – 1) has been measured. Without loss of precision the relationship can be extended from smooth unnotched specimen geometries to notched geometries if only the stress concentration factor is known. The coefficients of the predictive equation depend on the brittleness of the material, a systematic approach to take the brittleness into account is still missing. In Haigh diagrams, the slope of the σ A–σ m diagram with tensile mean stresses can be linearly extrapolated also into the compressive mean stress range σ m &lt; 0.

Exploring thermography technique to validate multiscale procedure for notched CFRP plates

Stress concentrations induced by notches is an engineering problem, presented in most real structural failures. Due to the importance of this topic and considering the large amount of variables related to composite design, a novel multiscale procedure is proposed to evaluate damage onset and propagation in notched composite plates, minimizing the amount of experimental test required. The micromechanical modeling combines Tsai's modulus with VSPK model, while the macromechanical modeling uses the finite element method and the Puck failure criterion. Experimentally, tensile tests of notched plates are carried out monitored by thermographic camera. The thermography technique is a fundamental part of the proposed methodology, due to its capability to capture damage onset related to significative heating events. Just 3 experimental tests are necessary: 1 unnotched specimen for tensile test and 2 notched specimens monitored with thermography. 3 constituents’ properties from supplier datasheet are necessary to obtain others 15 properties: 8 lamina properties, 2 matrix properties and 5 fiber properties. Comparing the experimental results with the numerical-analytical methodology, a mean square error of 1.42% is obtained.

Deformation failure behavior and fracture model of twin-roll casting AZ31 alloy under multiaxial stress state

The influence of temperature and strain rate on the deformation behavior of twin-roll casting AZ31 alloy has been examined as a function of stress state using notched and un-notched tensile specimens. Failure behavior during the physical tension was carried out using FEM simulations with different fracture criteria. Using a combined experimental-FEM investigation, the applicability of each fracture criterion was verified to serve the prediction for the crack initiation under multiaxial stress state. Results show that the critical fracture strain presents a clear exponential relationship with stress triaxiality and strain rate, and a linear relationship with temperature. Through the quantitative analysis of the relationship, a mathematical model can be well established. After comparison and verification, the fracture model can accurately predict the forming limit during the plastic deformation of twin-roll casted AZ31 magnesium alloy under complex stress state, and the classical Cockcroft & Latham criterion is the most suitable criterion for predicting the related crack initiation.

Designing carbon nanotube and nanofiber-polypropylene hybrid cementitious composites with improved pre- and post- crack load carrying and energy absorption capacity

This study reports recent developments on carbon nanotube (CNT), nanofiber (CNF), and polypropylene microfiber (PP) hybrid reinforced cementitious composites. Notable characteristics are (i) significantly enhanced load carrying capacity; and (ii) increased energy absorption capability at both elastic and post-first cracking stages. Three point bending tests on notched and unnotched specimens have shown that the addition of 0.3 vol% CNTs and CNFs in PP reinforced mortars increases the modulus of elasticity and first crack strength and toughness by 93%, 64% and 49% respectively. The experimental values perfectly agree with the theoretically determined modulus using a micromechanics-based model. Hybrid mixes showed considerable increase in toughness, exhibited toughness index values I5 of 4 or more, and 4 times higher residual strength. Relative to the PP microfiber reinforced mortar, the developed hybrid material is able to sustain a much higher load and successfully transfer tensile stresses across cracks, without losing its tensile load carrying capacity. Such exceptional mechanical behavior is an important performance factor for serviceability and is interpreted in terms of the mechanism of a synergistic interaction between the nano and micro scale fiber reinforcement.

Hydrogen diffusivity and tensile properties degradation in SLMed Inconel 718

Besides elevated temperature applications, Inconel 718 is widely chosen for components subjected to heavy loads operating in aggressive environments, which can promote the production of hydrogen on the metal surface. Selective Laser Melting (SLM) is an emerging technology for the production of structural components in Inconel 718 or Inconel 625 alloys, allowing to create functionally optimized shapes and overcome the traditional manufacturing limitations. However, the SLM process introduces a peculiar microstructure and severe residual stresses that can affect hydrogen migration and accumulation. While the mechanical properties were deeply investigated in recent years, the resistance of SLMed Inconel 718 to the Hydrogen Embrittlement (HE) is still an open issue. The present work deals with a preliminary assessment of the hydrogen embrittlement susceptibility of the SLMed In718 in the as-built condition. Slow strain rate tests were carried out on unnotched specimens that were pre-charged by electrochemical cathodic charging. Fractographic analyses, carried out with a Scanning Electron Microscope (SEM), were used to identify the effects produced by the hydrogen intake on the material microstructure. The material resulted to be susceptible to hydrogen embrittlement, featuring a significant ductility reduction in presence of extremely elevated hydrogen concentrations. The hydrogen content was measured by employing the Hot Extraction Method (HEM), for each mechanical test. Due to a low diffusivity value, hydrogen penetrates only within a thin external layer of the specimen. Therefore, to identify the concentration profile, the effective hydrogen diffusion coefficient was identified and the hydrogen diffusion was simulated using Fick’s equation. The loss in mechanical resistance was then correlated to the hydrogen concentration near the fracture onset.

Flexural and fracture behaviour of a cement-based material reinforced with GO nanoplates

In the present research work, the mechanical properties of a cement-based material reinforced with Graphene Oxide (GO) nanoplates are experimentally investigated. In particular, a detail experimental campaign, consisting of three-point bending tests on both unnotched and edge-notched specimens, is performed in order to determine flexural strength and fracture toughness. More precisely, the flexural strength is computed as a function of the experimental values of the peak load according to UNI EN Recommendation, whereas the fracture toughness is analytically determined according to the Modified Two-Parameter Model.