Repetitive hot rolling followed by hot pressing or hybrid severe plastic deformation (HSPD) of Mg-4Zn-4Gd was performed at 450 °C to produce ultrafine-grains (UFG) in the alloy. The processed ...specimens are compared with conventional 75% hot rolled, solutionized (ST) and as-cast (AC) specimens to study the mechanical and microstructural evolution. Prior to HSPD process, the pre-processed blocks were solutionized at 400 °C for 24 h to obtain the equiaxed and residual stress free alloy for easy plastic deformation up to the high absolute true strain value of 1.39. The hot rolling (R) and hot pressing (P) methods were combined to deform the material in three different combinations in terms of true strains to achieve the final deformation with true strain 1.39 (75% reduction). The HSPDed specimens were subjected to tensile, hardness and fracture toughness (KQ and JⅠC) tests. The 50R50P specimen has shown better improvement in yield strength (σYS = 268 MPa), tensile strength (σTS = 284 MPa)), and hardness (2.21 GPa) amongst other HSPDed specimen. The improvement in mechanical properties of 50R50P alloy is nearly 9%, 6.5%, and 81%, respectively over the 75% rolled specimen and 154%, 65%, and 176%, respectively against ST specimens. However, the elongation of 50R50P specimen has improved only up to ~ 27% than ST specimen. The highest JⅠC fracture toughness of the processed alloy is ~ 23.25 kJ/m2 when pre-crack is normal to rolling direction (RD) and ~ 18.51 kJ/m2 when pre-crack is parallel to RD analysed in 50R50P and 70R16P specimens, respectively. The strengthening mechanisms operating in the processed alloy is due to solid solution strengthening and deformation slip based failure mechanism in fine grained alloys, which were elucidated with the help of high resolution transmission electron microscopy (HRTEM). The HRTEM results are correlated with X-ray diffraction results of processed alloy. Fracture phenomena for different HSPDed specimens were analysed through FE-SEM to understand the failure characteristics of the alloy under static load.
To study the fracture toughness of sandstone, this study investigated the influence of temperature on the mixed-mode (I+II) and mode-II fracture toughness of sandstone using semi-circular bend (SCB) ...specimens. The results showed that temperatures above 500°C significantly influenced mixed-mode (I+II) and mode-II fracture behavior. A comparison of experimental results with pure mode-I fracture toughness showed that the impact of temperature variations on the mixed-mode (I+II) and mode-II fracture toughness was similar to the impact of temperature variations on pure mode-I fracture toughness. The mixed-mode (I+II) and mode-II fracture toughness of sandstone increased slowly from 20 to 100°C and decreased slowly from 100 to 500°C. The mixed-mode (I+II) and mode-II fracture toughness of sandstone decreased sharply from 500 to 600°C. The effective stress intensity factor of mixed-mode (I+II) at artificial pre-crack angles of 15°and 30° decreased by 41.02% and 42.55%, respectively. Mode-II fracture toughness decreased by 54.30%. From 500°C to 600°C, the change of the sandstone structure is the main reason for the sharp decrease of the mixed-mode (I+II) and mode-II fracture toughness. The threshold temperature range for this sharp decrease is 500–600°C for the mixed-mode (I+II) and mode-II fracture toughness are identical to pure mode-I fracture toughness.
The paper presents a correlation between dynamic and static fracture toughness of polyurethane rigid foams. Static three point bend tests and instrumented impact tests were performed using single ...edge notch specimens. The obtained results show that for all foam densities the dynamic fracture toughness is higher than the static toughness. Density appears to have the main influence on both static and dynamic fracture toughness. A quasi brittle fracture without plastic deformations and cushioning was observed for all foam densities.
•Crack initiation load Pi has been predicted using the maximum fracture load Pmax and FPZ length.•Analytical predictions of Pi have been confirmed by experiments reported in literature.•The ...corresponding toughness Ki and Kun at Pi and Pmax are obtained analytically.•The size-independent fracture toughness KIC are compared with Ki and Kun.•Simple approximation of the tensile stress at the notch tip is provided.
The initial cracking load (Pi) of a notched concrete beam, difficult to measure, has been predicted from the maximum fracture load (Pmax) together with the notch-tip fracture process zone (FPZ) at Pmax. A simple analytical model is used to link Pi and Pmax together through the FPZ. The corresponding initiation and unstable fracture toughness, Ki and Kun, have also been determined. The model assumes that the lower limit of Pmax from a group of identical notched concrete beams corresponds to the lower limit of the FPZ length FPZL, so that the lowest limit Pmax = Pi is established for FPZL = 0. Detailed FPZ measurements before and at Pmax using a digital image correlation technique were analysed and used to predict the initial cracking load Pi. Six different specimens with various sizes (W = 40, 60 and 80 mm), initial notches (a0 = 12 to 48 mm) and FPZL (7 to 13 mm) at Pmax showed the average ratio of FPZL/(W – a0) was around 0.25, indicating the Pi/Pmax ratio was around 0.67 based on the analytical model. The crack growth resistance KR-curve between the crack initiation toughness Ki at Pi and the unstable fracture toughness Kun at Pmax was also established approximately by the simple model. Estimated intrinsic fracture toughness KIC was compared with Ki and Kun. The influence of average aggregate size dav on Pi and FPZL at Pmax was also discussed.
Asphalt concrete pavements containing top-down cracks can be subjected to combined opening-out of plane sliding (i.e. mixed mode I/III) deformations due to traffic loads. However, lack of a suitable ...laboratory test specimen for fracture toughness testing of asphalt composites under mixed mode I/III, is one of the main shortcomings of this field. Hence, a new and simple test configuration is proposed in this research for determining mixed mode I/III fracture toughness of these composite materials. The specimen which is called the Edge Notched Disc Bend (ENDB), is a disc containing an edge crack created through one side of specimen along the diameter and is loaded by a three-point bend fixture. It was demonstrated that the complete range of mode mixity from pure mode I to pure mode III can be achieved by this specimen simply by changing the crack inclination angle relative to the loading support direction. The practical ability of the suggested ENDB specimen, was then examined for obtaining the mixed mode I/III fracture toughness of a typical hot mix asphalt (HMA) composite for different mode mixities including pure modes I and III. Corresponding values of KIc and KIIIc were determined for each mode mixity and it is shown that the obtained results are in quite well agreement with the predictions of a mixed mode I/III fracture theory. Consequently, the ENDB specimen can be used as a favorite candidate specimen for mixed mode fracture toughness study of asphalt concrete materials.
•True mode II fracturing is compared to the conventional mode II fracturing.•A criterion is suggested to predict the fracturing type (tensile- or shear-based).•The superiority of the DNBD test over ...the existing ones is discussed.•True mode II DNBD tests are conducted on three different rock types.•True mode II values of the fracture toughness are compared to the mode I ones.
This paper discusses the use of the double-edge notched Brazilian disk test (DNBD) for measuring true mode II fracture toughness of rocks. The term true emphasises that in this test, not only is the crack tip loading shear-based, but also the material failure is shear-induced. Conventional mode II tests typically experience dominantly tensile failure. We introduce a fracture growth criterion that explains where and how a shear-based fracture extension occurs. Our theoretical analysis demonstrates that large values of compressive T-stress in the DNBD specimen significantly help inducing a true mode II fracturing. Crack tip parameters are computed by finite element analyses for various notch lengths and loading angles. These values are then employed to determine the geometry and loading condition for the optimal performance of the test. We also compare the DNBD with two other available tests for measuring true mode II fracture toughness, and show that the DNBD test typically has a lower contribution of mode I loading than the two alternative approaches and, therefore, better approximates the true mode II condition while at the same time being the experimentally simplest. Three types of rocks (limestone, marble and granite) were tested using the new approach and their true mode II fracture toughness is reported for two different crack lengths. The measured true mode II fracture toughness is compared with the mode I fracture toughness obtained from the semi-circular bending test.
•Investigating the effect of the number of wetting- drying cycles on the fracture toughness of sandstone in natural environment.•Investigating the effect of the number of wetting- drying cycles on ...the fracture toughness of sandstone in acidic environment.•Presenting the relationship between the fracture toughness and the effective porosity for sandstone in natural environment.•Presenting the relationship between the fracture toughness and the effective porosity for sandstone in acidic environment.
Factors such as rainfall and its diffusion into the ground and the subsequent drying cause alternating water–stone interactions leading to loosening of stones in long periods and catastrophic events such as landslides. Accordingly, the effect of these factors on the mode I and mode II fracture toughness of sandstone was investigated. Experiments were conducted on centrally cracked Brazilian disc (CCBD) specimens (chevron notch). The specimens were tested after 0, 1, 4, 8, 16 and 20 wetting–drying cycles (in the neutral (pH = 7) and acidic (pH = 3) environments). In addition to the effect of wetting–drying cycles in the natural environment (pH = 7), the effects of salt weathering and chemical weathering (caused by acidic environment) on the fracture toughness were also investigated. The acidic solution consisted of water, hydrochloric acid and sodium sulfate. The mode I and mode II fracture toughness of the sandstone decreased in both natural and acidic environments with increasing the number of wetting–drying cycles. The sandstone was also evaluated in terms of effective porosity and mineralogy to compare the natural and acidic environments. The results showed the insignificant impact of the acidic environment on the sandstone specimen. The effective porosity of the sandstone specimens also increased with increasing the number of cycles.
The production of continuous carbon fibre composites using a fused deposition modelling (FDM) method has addressed the problem of low mechanical performance of raw- or short-fibre reinforced polymer ...parts fabricated by the same process, due to the excellent specific strength and stiffness of continuous fibres. However, one key issue of 3D printed polymers or fibre-reinforced polymers is the formation of microscopic voids between individual filaments and within the filaments during the FDM process. This study aims to quantify the adverse effects of voids on 3D printed continuous fibre-reinforced polymer composites. Optical microscopy and micro-CT are used to quantify the void content in continuous CF/PA6 composites fabricated on a 3D printing platform. As a benchmark, 3D printed CF/PA6 composites with the same printing configurations were further processed by compression moulding (CM) with thickness controlled to achieve the minimum void content. Apart from tensile and three-point bending tests in the longitudinal and transverse directions, the study also evaluated the Mode I interlaminar fracture toughness of CF/PA6 composites. By revealing the substantial adverse effects of the microscopic voids in 3D printed composites, this study articulates the critical importance of developing in-process techniques during 3D printing to decrease the void content within the continuous fibre reinforced composites, for the sake of expanding practical applications of 3D printed continuous fibre composites.
•Scaling effect on mode I notch fracture toughness of graphite specimens is studied.•Original fracture tests are performed on notched SCB specimens at different scales.•PS criterion with two various ...FPZ lengths is employed to estimate the test results.•Notch fracture toughness increases significantly by increasing the specimen scale.•PS criterion with both FPZ lengths can estimate the test results well.
In this paper, the mode I notch fracture toughness of graphite specimens weakened by notches of different shapes is investigated experimentally and theoretically. For this purpose, first, by conducting experiments, the apparent fracture toughness values of cracked semi-circular bending (SCB) graphite specimens with four various radii are determined. Next, numerous fracture experiments are performed on the notched SCB graphite specimens in four various scales. Then, the critical radius of the fracture process zone (FPZ) for the notched graphite specimens is obtained using two various equations already proposed in literature for cracked bodies. A well-known stress-based failure model, namely the point stress (PS) criterion, is employed for estimating the notch fracture toughness values of the specimens tested at different scales, taking advantages of two different critical distance formulas. The results show that the fracture toughness values of various notches obtained from PS criterion are in good agreement with the experimental results. Also, found in this research that the notch fracture toughness is considerably dependent on the nominal dimension (radius) of the specimen, such that, as the specimen radius increases, the mode I notch fracture toughness increases. Furthermore, it is revealed that for the present specimens tested, both the critical distance formulas provide almost identical predictions, suggesting that each of them can be arbitrarily selected for prediction.
Carboxyl terminated butadiene acrylonitrile (CTBN) was added to epoxy resins to improve the fracture toughness, and then two different lateral dimensions of graphene nanoplatelets (GnPs), nominally ...<1μm (GnP-C750) and 5μm (GnP-5) in diameter, were individually incorporated into the CTBN/epoxy to fabricate multi-phase composites. The study showed that GnP-5 is more favorable for enhancing the properties of CTBN/epoxy. GnPs/CTBN/epoxy ternary composites with significant toughness and thermal conductivity enhancements combined with comparable stiffness to that of the neat resin were successfully achieved by incorporating 3wt.% GnP-5 into 10wt.% CTBN modified epoxy resins. According to the SEM investigations, GnP-5 debonding from the matrix is suppressed due to the presence of CTBN. Nevertheless, apart from rubber cavitation and matrix shear banding, additional active toughening mechanisms induced by GnP-5, such as crack deflection, layer breakage and separation/delamination of GnP-5 layers contributed to the enhanced fracture toughness of the hybrid composites.
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