Pre-cracked Specimens Research Articles

Influence of crack aperture and orientation on the uniaxial compressive behavior of rock

The pre-existing cracks with various geometric and physical properties significantly affect the mechanical behaviors of rock mass. In this study, uniaxial compressive tests are performed on numerical models of specimens containing a single pre-existing crack with different apertures and orientations based on PFC 2D to shed light on the impact of different crack apertures and orientations. The results reveal that the uniaxial peak strength and elastic modulus tend to decrease with increasing apertures, but they tend to increase with increasing inclination angles. Crack apertures can affect the stress intensity factor in the vicinity of the pre-existing crack tips, which further influences the crack initiation and crack type. Furthermore, shear cracks are more prone to occur in a pre-cracked specimen with a larger crack aperture and larger crack orientation. The crack initiation stress and strain initially show a slight decrease until the inclination angle reaches 45°, and then significantly increase with the further increase of angle from 45° to 90°. The microcracks occur earlier and easier when the inclination angle is 45°. As the inclination angle of the pre-existing crack increases, shear-sliding along the crack becomes more prominent, resulting in the formation of coplanar secondary cracks. The macro failure patterns switch from tensile failure to shear failure with increasing crack inclination angle, which is in good agreement with trend lines of displacement in the displacement field.

Hydrogen-assisted cracking and damage tolerance of high-strength lath-martensite steel bars for structural applications

The paper addresses the hydrogen-assisted cracking and damage tolerance of high-strength lath martensite steel bars assessed from slow strain rate tensile tests. These were carried out on fatigue precracked specimens simultaneously charged with hydrogen in a standardized 20 % aqueous solution of ammonium thiocyanate at 50 °C and in air, at room temperature, for comparison. The fractographic analysis of the tested specimens showed the steel sensitivity to hydrogen uptake, which gives rise to the sub-critical extension of the pre-existing crack in two phases, initiation and growth, with both hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP) being synergically involved as damage mechanisms. Despite this, the damage tolerance behavior of the steel is highly satisfactory because the crack size causing the steel failure in tension slightly exceeds the theoretical limit of plastic collapse under combined tension and bending loading, regardless of the environment aggressiveness. Although H-assisted failure loads fit the theoretical limit loads, the fast propagation of the crack under constant load beyond the small-scale yielding regime could represent a real threat for the structural use of these steel bars in aggressive media unless the service loads are limited in accordance with such behavior.

Damage tolerance analysis of double strap joints with composite adherends

ABSTRACT Damages in composite structures are typically repaired using external bonded patches. In this work, the damage tolerance within external patches is studied using double strap joint (DSJ) with carbon fibre reinforced plastic (CFRP) specimens. The damage tolerance in this work refers to bond-line manufacturing-related defects such as debond or dry area. Both intact and pre-cracked DSJ specimens are numerically analysed using experiments for comparisons. DSJ specimens’ adherends were CFRP laminates with AS4/3501–6 plies and bonded using adhesive film FM 300–2. The pre-cracked specimens are monitored during experiments using high-speed digital image correlation to visualize damage propagation. The unsymmetric pre-crack influences in a clear way as shown by experiments. The numerical analyses reveal the effect of failure mechanisms on final failure and the maximum force during a test. The included parametric sensitivity analysis is performed using statistical methods. The normally distributed parameters are sampled using separate distributions for each analysed interface and fracture mode. UNCLASSIFIED Based on Foreground Information under EDA Contract No B.PRJ.RT.670 covering the Ad Hoc Project entitled ”PATCHBOND II”.

Compressive Fracture Behavior and Acoustic Emission Characteristics of Sandstone under Constant Crack Water Pressure

Abstract Engineering rock containing flaws or defects under a large water source is frequently subject to the couple influence of constant crack water pressure and geostress. To investigate the fracture behavior of precracked rock under hydromechanical coupling with constant crack water pressure, compression tests were conducted on red sandstone specimens containing a single crack of different angles using a device to realize the constant crack water pressure during loading, and the failure process of rock specimens was monitored by acoustic emission (AE) technique. The results show that the presence of constant crack water pressure has a significant promotion effect on the development of shear wing cracks, and the promotion effect is influenced by the prefabricated crack angle and water pressure. As the constant crack water pressure increases, the failure mode of the 0° precrack specimen changes from “X”- shear failure to the single oblique shear failure along the shear wing crack direction, the main failure crack of the inclined precracked specimens (precrack angles of 15°, 45°, and 60°) changes from a small acute angle with the prefabricated crack to a direction along the shear wing crack, and irregular cracks occur at the chipped prefabricated crack in the 90° precracked specimen. With an increase in the constant crack water pressure, the average energy for a single hit, cumulative AE energy, and cumulative AE hits decrease, and the proportion of the tensile cracks increases and that of the shear cracks decreases.

Experimental study of the damage characteristics of rocks containing non-penetrating cracks under cyclic loading

The damage evolution process of non-penetrating cracks often causes some unexpected engineering disasters. Gypsum specimens containing non-penetrating crack(s) are used to study the damage evolution and characteristics under cyclic loading. The results show that under cyclic loading, the relationship between the number of non-penetrating crack(s) and the characteristic parameters (cyclic number, peak stress, peak strain, failure stress, and failure strain) of the pre-cracked specimens can be represented by a decreasing linear function. The damage evolution equation is fitted by calibrating the accumulative plastic strain for each cycle, and the damage constitutive equation is proposed by the concept of effective stress. Additionally, non-penetrating cracks are more likely to cause uneven stress distribution, damage accumulation, and local failure of specimen. The local failure can change the stress distribution and relieve the inhibition of non-penetrating crack extension and eventually cause a dramatic destruction of the specimen. Therefore, the evolution process caused by non-penetrating cracks can be regarded as one of the important reasons for inducing rockburst. These results are expected to improve the understanding of the process of spalling formation and rockburst and can be used to analyze the stability of rocks or rock structures.

Assessment of self-healing behavior of polypropylene fiber-reinforced cement mortar with crystalline admixture: the effects of crack widths, cracking ages, and external conditions

Crystalline admixture (CA) is an effective self-healing agent for mortar. However, the effects of crack parameters (i.e. crack width and cracking age) and the service environment on the self-healing behavior of CA-containing mortar are not well understood. Herein, the self-healing behavior of mortar containing a self-developed CA was assessed by testing strength recovery, impermeability recovery, and crack closure in pre-cracked specimens. Three initial crack widths (0.2, 0.3, and 0.4 mm), five cracking ages (3, 7, 14, 28, and 56 days), and four external exposure conditions (humidity chamber, air exposure, water immersion, and wet-dry cycles) are investigated. Furthermore, the influence of different external conditions on the healing products at the region of crack and the pore structure of hardened paste containing CA are studied. The results show that adding 4.54% CA into mortar allows rapid healing of 300 μm-wide cracks. Although wider cracks (400 μm) are more difficult to heal, the sorptivity coefficients of the mortars with 400 μm-wide cracks after healing decrease. When the cracks are produced at an earlier age, the pre-cracked specimens have higher recovery ratios of strength and impermeability after healing, and the specimens pe-cracked at a later age still have acceptable compressive strengths after healing. The analysis shows that the strengths and impermeabilities of pre-cracked mortars containing CA exposed to the four external conditions are all recovered. The best self-healing performance is observed for the specimens exposed to water immersion and wet-dry cycles conditions. Somewhat less good self-healing was observed in the specimens exposed to humid chamber condition, while the worst self-healing performance was in the specimens exposed to air exposure condition. This study provides a theoretical basis for the application of novel CAs in cement-based materials.

Determination of Ductile Tearing Resistance parameter of Interstitial-Free Steel Sheet

The present work deals with the characterization of ductile tearing resistance of 1 mm thick interstitial free steel sheet using crack tip opening angle (φ), and δ5 concepts with some experimental modification. CTOA (φ) measurements on the specimen surface at the growing crack tip have been performed using light microscope on both pre-cracked DENT and SENT specimens at three ramp rates following the essence of ASTM E 2472. In order to determine the effect of pre-cracking, CTOA of the notched (ρ:0.1 mm) SENT specimen has also been measured at 10-4 s-1. The transferability of φ−Δα and δ5−Δα between two geometries has been verified. It is concluded that for a specified thickness both φ−Δα and δ5−Δα plots can be used for crack growth characterisation of sheet metals even without using pre-cracked specimens. However, the angle φ(δ5) determined from δ5−Δα curve is not the true measure of optical CTOA,φ as the δ5 gauge position does not follow the tip position of the growing crack.

Self-healing evaluation of bacteria grouted light weight aggregate concrete containing rice husk ash and steel fibers

Utilization of microbiologically induced calcite precipitation along with fiber composite have great influence on improving strength and durable properties of concrete. The concept of mechanical properties of grouted concrete added with bacteria, steel fibers (SF), rice husk ash (RHA) and light weight aggregate (LWA) has been focused on this work. In the fabrication of concrete specimens, concentration of bacteria, combination of steel fibers and LWA was placed in the formwork, and to fill the voids flowable grout was injected. The variables studied in this work are two different sizes of LWA viz., 10 mm and 12.5 mm with constant dosage of 2% hooked end steel fibers by volume of concrete, 10% RHA was used as cement replacement for preparation of grout and bacteria was incorporated in cement grout by direct application. The properties such as compressive strength (CS), compressive strength regain (CSR), crack width healing, impact strength for first crack and final failure, rate of healing was studied for pre-cracked specimens using visual and microscopic observation. In addition, microstructure was studied for grouted concrete without bacteria and with bacteria under immersed curing conditions. From the experimental results, performance of bacteria added grouted concrete properties such as CS, CSR, cracking healing capacity, and impact strength has improved with the addition of fibers.Utilization of microbiologically induced calcite precipitation along with fiber composite have great influence on improving strength and durable properties of concrete. The concept of mechanical properties of grouted concrete added with bacteria, steel fibers (SF), rice husk ash (RHA) and light weight aggregate (LWA) has been focused on this work. In the fabrication of concrete specimens, concentration of bacteria, combination of steel fibers and LWA was placed in the formwork, and to fill the voids flowable grout was injected. The variables studied in this work are two different sizes of LWA viz., 10 mm and 12.5 mm with constant dosage of 2% hooked end steel fibers by volume of concrete, 10% RHA was used as cement replacement for preparation of grout and bacteria was incorporated in cement grout by direct application. The properties such as compressive strength (CS), compressive strength regain (CSR), crack width healing, impact strength for first crack and final failure, rate of healing was studied for pre-cracked specimens using visual and microscopic observation. In addition, microstructure was studied for grouted concrete without bacteria and with bacteria under immersed curing conditions. From the experimental results, performance of bacteria added grouted concrete properties such as CS, CSR, cracking healing capacity, and impact strength has improved with the addition of fibers.

Fracture toughness determination methods of WC-Co cemented carbide material at micro-scale from micro-bending method using nanoindentation

Determining toughness properties is crucial for the development of components made of brittle materials, especially at the microscale. Numerous methods rely on establishing a limit load that triggers the propagation of a pre-cracked specimen. The primary challenge lies in accurately and repeatably determining this limit load value. In this study, we introduce a coupled numerical-experimental approach to determine the stress intensity factor at the microscale using a micro-bending test on pre-notched cemented carbide micro-cantilevers. Specimens are fabricated through micro-electrical-discharge milling, and the initial crack is generated using a focused ion beam process to ensure a repeatable crack shape. Geometries are defined to validate beam theories.Cycling bending tests, employing nanoindentation with three types of loading (fatigue-like, progressive repeated, and ramping sinusoidal loadings), are utilized to ascertain fracture toughness properties of tungsten carbide material at the microscale. Micro-indentation is also conducted conventionally to compare hardness at meso and micro-scales. Analytical methods and finite element analysis are employed to extract the critical stress intensity factor. The cyclic bending test with the proposed ramping sinusoidal loading demonstrates a time shift between the applied force and the displacement, enabling accurate and repeatable detection of the initiation of crack propagation. The primary contributions are associated with the development of a micro-bending test on precracked microcantilevers with ramping sinusoidal loading applied by a nanoindentor device. The miniaturization of specimens and tests, for an equivalent grain size, does not result in a scale effect on toughness properties.

Study on the semi-circular bend method for characterizing the mixed mode I/II fracture toughness of sandstone: A micro-perspective

Manufacturing and precracking specimens for semi-circular bend (SCB) tests are simple processes and do not require complicated loading devices. They have been used to conduct mode I, mode II and mixed mode I/II fracture tests on brittle materials. However, characterizing mode I and mode II fractures from a microcosmic and quantitative perspective has never been thoroughly evaluated. In this paper, high-resolution digital image correlation (DIC) and moment tensor inversion are first used to quantitatively measure micron-level displacement and analyse microfailure types. The DIC results indicate that SCB specimens with a notch inclination angle exhibit both opening displacement and sliding displacement, resulting in a mixed mode I/II fracture. A quantitative analysis of three typical classification methods based on moment tensor inversion suggests that SCB specimens with mixed loads are mainly tensile mechanisms. Some tensile sources are not pure tensile cracks, and shear components also play an important role. It is also observed that the moment tensor components, nature of sources and Hudson k value are associated with the notch inclination angle, which sheds new light on the mechanism of mixed mode I/II fractures.

Assessment of impact resistance recovery in Ultra High-Performance Concrete through stimulated autogenous self-healing in various healing environments

Ultra High-Performance Concrete (UHPC) is widely acknowledged for its remarkable mechanical properties, owing to its compact microstructure. The response of UHPC to impact forces plays a vital role in ensuring the safety and longevity of structures, specifically in protective buildings, high-performance pavements and offshore concrete structures. In this context, this paper reports on an experimental investigation aimed at assessing the effects of stimulated autogenous self-healing of UHPC on the recovery of its performance under impact loadings. Drop weight tests were performed on UHPC slabs, with a 10 kg heavy impactor dropped from the height of 1 m on the centre of the specimens. Specimens were pre-cracked by repeated impacts up to 40% of their predetermined capacity. Pre-cracked specimens were exposed to different healing conditions, water submersion, 95% ± 5% RH, and wet/dry cycling (12/12 h) either in water or in a NaCl solution. Self-healing was evaluated through rebound height, elastic stiffness recovery, natural frequency, and laser displacement measurements. High-speed cameras and Digital Image Correlation were used to capture rebound height and crack formation. Performance was assessed at time 0, pre-damaging, 1, 2, and 4 months. After the healing period, all specimens were tested to failure. Specimens exhibited an increasing healing efficiency when moving from 95% ± 5% RH, over wet/dry cycling, to submerged conditions. Specimens healed continuously under submerged conditions exhibited a complete closure of surface cracks (50–150 μm) and an 80% recovery in natural frequency. Furthermore, they showed a more than 10% increase in stiffness and energy dissipation capacity after four months of healing.

A logarithmic model for predicting fracture trajectory of pre-cracked rock specimen under compression

Researches on the fracture trajectory of pre-cracked specimen are mostly focused on experiments and numerical simulations, and theoretical studies are usually limited to the fracture criterion, crack initiation angle and load, with few theoretical studies on the prediction of the complete fracture trajectory. This work first derives stress intensity factor (SIF) and T-stress expressions related to crack geometry, friction and deformation parameters, and physical properties of the rock. Then, the classical maximum tangential stress (MTS) fracture criterion is modified, followed by the development and validation of a logarithmic model for predicting the fracture trajectory of pre-cracked specimen made of diverse brittle or quasi-brittle rock materials under compression. The results shown that the crack geometry, friction and deformation parameters have significant effects on SIF and T-stress under compression. The present study demonstrates a strong concurrence between the anticipated fracture trajectory of pre-cracked specimens subjected to compressive loading and diverse experimental outcomes. Furthermore, the fracture trajectory is predominantly influenced by the crack geometry, friction and deformation parameters, and the mechanical characteristics of the specimen.

Effects of copper additives on load carrying capacity and micro mechanisms of fracture in 3D-printed PLA specimens

This study deals with the effects of employing copper additives on the fracture resistance and fracture micro-mechanisms of 3D-printed Polylactic Acid (PLA) specimens under mixed mode I/II loading. Several comparative fracture tests are conducted on pre-cracked Semi-Circular Bend (SCB) specimens made of pure PLA and particulate copper-reinforced PLA (PLA/Cu) printed using the Fused Deposition Modeling (FDM) technique. The SCB specimens are fabricated in three different layer orientations. The comparison of the experimental results reveals that employing copper additives increases the load carrying capacities (LCCs) of the pre-cracked SCB specimens with the flat, on-edge, and upright layer orientations by 2.9%, 15.5%, and 27.7%, respectively. In addition, to determine the tensile properties of two desired materials, dogbone specimens made of PLA and PLA/Cu in three different layer orientations are fabricated and tested. The experimental results obtained from the characterization tests similarly indicate that the presence of copper additives in 3D printed PLA specimens increases ultimate tensile strength by 15.9%, 14%, and, 10.4% in the three flat, on-edge, and upright layer orientations, respectively. Aside from performing experimental tests, the LCCs of pre-cracked specimens are predicted theoretically by utilizing Equivalent Material Concept (EMC) combined with the well-known Average Strain Energy Density (ASED) criterion. It is shown that the EMC-ASED criterion can estimate accurately the experimental LCCs of both FDM-PLA and FDM-PLA/Cu specimens. Finally, the Scanning Electron Microscope (SEM) technique is employed to provide a better understanding of the effects of copper additives on the micro-mechanisms of fracture.

Fatigue Threshold R-Curves for Dental Lithium Disilicate Glass–Ceramics

Chemical and mechanical fatigue degradation in ceramic materials is generally inconspicuous yet ubiquitous, to the effect that clinical fractures still consist of the main cause of failure in all-ceramic restorations. Implications of this span wide, from a reduced survival prognosis for the affected teeth, including more frequent and increasingly invasive procedural interventions, to the financial burden borne by individuals and health care systems. To suffice as an effective corrective, restoration lifetimes need only to be extended so to outlive the patient. That opens a box of problems from a materials science standpoint, entailing inherent deficiencies of brittle materials to resist mechanical and environmental challenges. Efforts in developing more damage-tolerant and fatigue-resistant restoratives go thus hand in hand with understanding intrinsic mechanisms of crack growth behavior under conditions that simulate the oral environment. Here we developed experiments using size-relevant sharp precracked specimens with controlled size and geometry (truncated semielliptical crack in the surface-crack-in-biaxial-flexure method) to establish a relationship between crack size and strength. The tangent method was used to construct envelopes for the quasi-static resistance curves (R-curves), which served as template for deriving residual cyclic R-curve analogs. By means of experimentally obtained stress–cycle curves, lifetime and fatigue parameters were employed within a mechanistic framework to reveal constitutive toughening mechanisms during subcritical growth under cyclic loading in a wet environment. Using 3 modern dental lithium disilicate glass–ceramics, we demonstrate the extent of R-curve degradation up to a threshold of 10 million cycles (~30 y in service) and draw parallels between the scope of fatigue degradation and the size of the microstructural units responsible for toughening mechanisms in glass–ceramic materials. Our results indicate that larger microstructural elements endow glass–ceramics with a higher reaching quasi-static R-curve at the onset but degrading more rapidly to comparable levels of lithium disilicates having submicrometric and nanometric crystal phases.

The Effect of Crack-Initiating Feature on the Environment-Assisted Cracking Behavior of Sensitized AA5456-H116 in Marine Environments

The influence of crack-initiating feature on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 exposed to marine environments is assessed via fracture mechanics-based testing. Specimens that contained either a traditional fatigue precrack or purposefully introduced intergranular corrosion fissures were immersed in 0.6 M NaCl and polarized to select electrochemical potentials while held at a constant force. The measured crack length vs. time relationships from these experiments reveal that the two specimen geometries yield similar crack growth rates at −900 mVSCE and after the onset of accelerated crack propagation at −800 mVSCE. However, precorroded specimens exhibit significantly shorter times to failure than the precracked specimens at −800 mVSCE due to increased crack growth rates at the start of the experiment. The mechanical, environmental, and material factors that could contribute to the initially increased EAC susceptibility of the precorroded specimens are identified using a generalized model for EAC. Analysis of these possible causal factors suggests that the increased susceptibility is due to a residual, initially more deleterious crack chemistry at the occluded corrosion fissure tip from the aggressive galvanostatic polarizations used to accelerate fissure growth. The implications of these results on the efficacy of traditional fracture mechanics-based methods for quantifying EAC susceptibility are discussed.

Offset bending for interfacial toughness of plasma sprayed ceramic coatings

A modified three-point bending technique has been proposed to measure the interfacial toughness of thick plasma sprayed ceramic coatings. Four different ceramic materials (titania, yttria stabilized zirconia, alumina, and yttria) on steel substrate were investigated resulting in a range of toughness values. Plasma sprayed coatings were prepared on grit blasted substrates with two different roughnesses and subsequently a notch in the centre of the coated beam was fabricated using a SiC saw. A two-step three-point bend test was conducted: first, the notched sample was loaded with a small offset to create pre-crack. The pre-cracked specimens were further loaded with offset 0.5 to measure interfacial toughness. Crack propagation manifested as a pop-in in the load-deflection plot. Subsequently, finite element analysis was used to calculate interfacial toughness, Gc, corresponding to the pop-in load. The visualization of crack propagation after pop-in load suggested that the crack had propagated along the interface in all four coatings. The widely accepted tensile adhesion test (ASTM C633) was separately performed on the same four coatings to compare the results obtained from the offset bending test. A correlation between interfacial toughness measured using offset bending and adhesion strength measured using the tensile adhesion test has been proposed.

Mode I fracturing behavior of Malan loess and implications for the toppling failure of loess slopes

Loess-covered areas are susceptible to a variety of geohazards, among which toppling is one of the typical slope failures causing deadly and destructive threat on human life and properties. Loess toppling usually involves slab-like or columnar blocks being detached from the loess mass by vertical or sub-vertical cracks that develop in the manner of Mode I fracturing. Thus, understanding the static fracture behavior of loess is the key to deciphering the mechanism of such failures. This study carried out Mode I fracturing tests on pre-cracked specimens of undisturbed Malan loess following the semi-circular bend test method suggested by ISRM. Through digital image correlation and numerical simulation, the fracturing process (including development of surficial strain and internal stress) was examined, and it is found Malan loess experiences a four-stage fracturing, i.e., compaction, linear deformation, yield and post-failure. The fracturing exhibits fragile or ductile behavior depending on the physical properties and loading conditions. Low water content, high dry density, and/or fast loading rate favor the absorption of elastic strain energy, which tends to lead a fragile failure and a relatively high fracture toughness. The opposite conditions induce gradual consumption of plasto-damage strain energy, promoting a progressive propagation of crack and, therefore a ductile failure and a relatively low fracture toughness. Among these three factors, water content was found to have the first priority when influencing the fracture behavior of Malan loess. These findings depict the development of vertical cracks and toppling failure in loess area, and have practical implication in designing disaster risk management plan.

Tensile mechanical properties and fracture evolution characteristics of sandstone containing parallel pre-cracks under dynamic loading

Parallel cracks have a significant influence on the dynamic tensile failure behavior of rock. The systematic investigation of dynamic tensile mechanical properties and fracture evolution characteristics of fractured rocks are significant for guiding the safe excavation and stability sustaining of rock engineering. Two groups of dynamic Brazilian splitting tests were conducted for the sandstone specimens containing parallel pre-cracks. The influences of crack inclination angle (Group A) and rock bridge length (Group B) on the tensile mechanical properties and fracture evolution characteristics were analyzed. Results showed: the dynamic peak load of pre-cracked specimens is significantly lower than that of intact specimens. The peak load presents a V-shaped evolution trend with the growth of crack inclination angle. Differently, the dynamic peak load increased monotonically with the increasing of rock bridge length. In addition, the crack initiation points move from crack tips to crack walls when the crack inclination angle is increasing. When rock bridge length is large enough, the crack propagation paths are observed near the central axis of specimens. By utilizing the DIC method, six different failure modes are identified to explain the crack initiation and propagation mechanisms under dynamic loading. The tensile-shear fracture modes are mainly presented for specimens with different crack inclinations. Differently, the ultimate failure modes change from tensile-shear mode to tensile mode when the rock bridge length is increasing. In addition, numerical results (ANSYS/LS-DYNA) are in good agreement with the experimental results as a whole, which can verify and explain the experimental results. This study is expected to provide guidance for controlling fractured rock masses.

Investigating the effect of raster orientation on fracture behavior of 3D-printed ABS specimens under tension-tear loading

Most of the engineering components are servicing under complex loading conditions or have complex geometries that lead to a multiaxial stress state in the component. Thus, the fracture behavior of parts under mixed-mode loading conditions should be investigated. The Fused Deposition Modeling (FDM) is a subcategory of the Additive Manufacturing (AM) techniques that has some important manufacturing parameters. The raster orientation (or raster angle) is one of fabricating parameters that significantly affect the mechanical behavior of fabricated parts. Therefore, this study aims to investigate the fracture behavior of FDM specimens made of Acrylonitrile Butadiene Styrene (ABS) under mixed-mode I/III loading conditions. To this goal, four different raster configurations of ψR = 0/90°, 15/-75°, 30/-60°, and 45/-45° were selected for FDM fabrication of edge cracked rectangular specimens, and five loading angles of β = 0° (mode I), 40°, 65°, 72°, and 90° (mode III) were used for performing mixed-mode I/III fracture experiments on the pre-cracked specimens. Besides, some tensile experiments were performed on dog-bone specimens fabricated with the mentioned raster configurations. In order to theoretically predict the failure loads in the tested cracked specimens, a theoretical model named Equivalent Material Concept (EMC) was utilized for equating the ductile behavior of FDM-ABS material with a brittle one. The mentioned concept was coupled with J-integral (EMC-J) and Maximum Tangential Stress (EMC-MTS) criteria to predict the failure loads under mixed-mode loading conditions. The results showed that both EMC-J and EMC-MTS criteria could predict the experimental failure loads well. Additionally, Scanning Electron Microscopy (SEM) was performed on the fracture surfaces to analyze the failure mechanisms of the tested specimens. SEM analysis confirmed the presence of three failure features namely air gaps, filament pullouts, and hackle regions that were discussed comprehensively.

Notch sensitivity of combined mode I–II fracture behaviour of a rolled magnesium alloy

ABSTRACTFatigue pre-cracked four-point bend specimens of a rolled AZ31 Mg alloy are loaded asymmetrically and their mixed-mode (I and II) fracture behaviour is contrasted with pre-notched samples. In-situ mapping of displacement and strain fields is performed through optical imaging coupled with digital image correlation analysis. The fracture surface morphology and the deformed microstructure are also studied. The fracture toughness Jc for both types of specimens is highest for mode I and drops steeply with increase in mode II component. Furthermore, at the same level of mode-mixity, Jc for the notched samples is between two to three times the value pertaining to pre-cracked specimens. This is primarily attributed to quasi-brittle failure, irrespective of mode-mixity, for the latter, whereas the former display a combination of ductile and brittle fracture surface features. At the crack-initiation stage, more profuse tensile twinning is perceived at the far-edge of the ligament in the notched specimens, which also contributes to the observed toughness enhancement.