Cracked Specimens Research Articles

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.

Repair efficiency evaluation for cracked asphalt mixture pavement in different ambient temperatures using bitumen and polymer concrete as repair materials

This paper studies the effect of crack repairing process on the behavior of pavement at four ambient low and medium temperatures ranging from −20 °C to 30 °C. Asphalt mixture samples in the shape of semi-circular bend (SCB) were tested in intact, cracked, and repaired conditions under tensile or opening mode of loading. Two adhesive types: namely pure bitumen and polymer concrete were used to repair the crack. To investigate the efficiency of the repair process, some indexes such as failure load, initial and post-failure energies, and failure patterns were evaluated using the intact and cracked SCB specimens. Failure patterns showed that in the repaired specimens with pure bitumen, the failure initiates from the mid-section (i.e., from the repaired zone), while in polymer concrete repaired specimens, the failure initiates far from the repaired zone. Experimental results also revealed that the temperature has a noticeable effect on the repair efficiency using bitumen adhesive. Indeed, for sub-zero temperature conditions, such repairing material partially provided acceptable integrity for the pavement compared to the intact asphalt mixture. But, for service temperatures greater than + 10 °C, repair with bitumen was inefficient. However, repair with polymer concrete was efficient in all temperatures, and the strength of specimens repaired by the polymer concrete was even higher than the intact specimen. Similar trends and results were also obtained for the work of failure indexes.

Effect of cyclic loading on mechanical properties and microstructure of die cast magnesium alloy AZ91D

Abstract Magnesium alloy materials are often affected by cyclic stress, resulting in fatigue fracture due to changes in mechanical properties. In this study, external cyclic loads were introduced to simulate the loading of AZ91D (Mg-9Al-0.5Zn) parts. Tensile tests under static and external circulation loads were performed on the same specimens, and the effect of loading conditions on the mechanical properties of the specimens (tensile strength, yield strength and elongation) were studied. The internal microstructure of the test specimens was analyzed, and the influence of cyclic loading on the microstructure change of the specimens was discussed. Compared with static load, under the condition of cyclic loading, the tensile strength and yield strength of the specimens are increased by more than 15%, and the elongation rate is increased by 3 times. In addition, the microstructure images showed that many twins and the second phase β-Mg17Al12 were generated inside the AZ91D microstructure under cyclic loading. The former results in specimens crack initiation and it is the root cause of fatigue fracture. And the latter accounts for the strengthening of the mechanical properties of specimens.

Degradation Mechanism of Silver Sintering Die Attach Based on Thermal and Mechanical Reliability Testing

This article investigates the degradation mechanism of sintered silver (s-Ag) layer in silicon carbide (SiC) die attach by means of thermal shocked test (TST) and nine-point bending test (NBT). TST can thermally provide out-of-plane deformation with s-Ag die layer, whereas NBT developed by the authors can mechanically provide the out-of-plane deformation. Two types of Ag paste (NP: nanopaste and NMP: nano- and micropaste) are employed, which are sintered with 300 °C for 10 min under 60-MPa pressure for SiC die attach. TSTs are conducted under thermal cyclic loading between −40 °C and 150 °C with trapezoidal waveform for 60 min per a cycle. NBTs are conducted under cyclic mechanical loading between 0 and 300 N with triangle waveform for 3 min per a cycle at 150 °C. Scanning acoustic tomography (SAT) evaluates the die-attach delamination during TST and NBT, which starts from the edge of the die. Cross-sectional scanning electron microscopy (SEM) observation demonstrates that mechanical cracking and material aging in s-Ag layer coexist after TST under the −40 °C to 150 °C conditions only. A damage parameter (DP) proposed in this study works well to fit the delamination area ratio in the cracking and cracking-aging specimens separately after 1000 cycles. Molecular dynamics (MD) simulation and classical pore growth discussion suggest that pores can grow under only tensile stress state at high temperature, which would have given rise to the difference between the two degradation mechanisms.

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Semi-analytical models for J-integral and load–displacement relation of surface-cracked body and their applications for J-R curves

Based on the energy density equivalence principle, this paper proposed the semi-analytical models to describe the relationships between the J-integral and load versus displacement for three types of surface-cracked specimens. The proposed models were validated using multiple finite element models over a wide range of crack sizes and materials. The results demonstrated the accuracy and practicality of the semi-analytical model. Fracture toughness tests with monotonic tension and bending loads were implemented subsequently. Combined with the measured load versus displacement curves, the J-R curves for the three types of surface cracked specimens were estimated.

Failure mechanism and static bearing capacity on circular RC members under asymmetrical lateral impact train collision

This paper aims to validate circular reinforcement concrete (RC) members' responses to the effect of an asymmetrical lateral impact span train collision. It was determined that the RC members' failure mechanism and dynamic response characteristics were important. Numerical analyses were conducted on four specimens. The specimen's crack development pattern, failure mode, impact force, and deflection time history curves are verified. Due to the difference in impact velocity, longitudinal reinforcement, and stirrups ratios, shear cracks indicate two types. A finite element (FE) modeling method has been proposed and successfully demonstrated. Using the control variables to study the failure process and mode, which were affected by reinforcement ratio, impact velocity, and slenderness ratio, by analyzing impact response characteristics. It is found that parameters greatly influence shear cracks' type and development range. Simultaneously, the changes in impact velocity and slenderness ratio affect the failure mode of the members. RC members' mechanical properties and failure mechanisms are studied using test and FE analysis. Members' failure modes, bending moment distribution and development, shear forces, reinforcement strain, and energy consumption are also investigated. The development of type I and type II shear cracks is discussed. A method to determine the shear cracks was proposed through regression analysis by supplementing other scholars' test data and combining it with the study test data.

Experimental and analytical investigations on tension stiffening of reinforced engineered cementitious composites members

Experimental and analytical investigations on the performance of reinforced ECC members in tension were studied in this paper through direct tension load. The applied load and elongation of members were measured during testing. The number and width of major cracks in specimens were determined at failure. The influences of the cross-sectional dimension and steel reinforcement diameter on the tension stiffening of members were investigated. Comparisons were made among reinforced ECC members with different cross-sectional dimensions or reinforcement diameters in terms of the cracking load, ultimate load and ultimate strain. Experimental results showed that the load-deformation relationship of reinforced ECC members could be divided into three stages, namely, initial elastic stage, multiple cracking stage and major cracking stage. In addition to experimental tests, an analytical model for the tension stiffening behaviour of reinforced ECC members was proposed. The interaction between ECC and steel reinforcement was considered at different stages in this model. Comparisons with experimental data suggest that the analytical model could accurately predict the relationship between load and deformation of reinforced ECC members. The cracking load obtained by this model is conservative as compared with test data. Based on the validated analytical model, a parametric study was carried out on the effects of reinforcement ratio and bond stress on tension stiffening of reinforced ECC. The direct tension tests and analytical results in this study can be used to predict the load and deformation behaviour of reinforced ECC members in tension.

Influencing factors to the capillary water uptake of (un)cracked cementitious materials

Capillary water absorption tests are widely used in uncracked cementitious materials to assess the quality and durability. Due to the easy execution of the test, it is also frequently used to assess the self-healing efficiency of self-healing concrete and mortar. It is established that the presence of a crack significantly increases the water uptake by a specimen. However, it is not known how the crack width, healing agents and mix composition influence the capillary water absorption. In this research, for cylindrical mortar specimens with four different crack widths, both a capillary water absorption test and water permeability were test were executed in order to investigate the relation between these two test methods. After the first round of testing, cracked specimens were healed manually with polyurethane and methyl methacrylate and the capillary absorption test was performed again to investigate the sensitivity of the test method to different degrees of crack healing. Furthermore, prismatic specimens were cast to investigate the influence of crack creation and geometry. It was found that the crack width does not have an influence on the capillary absorption rate. However, the crack width has a significant influence on the water flow through the crack. As expected, manual healing with polyurethane is better in comparison to the sealing of the crack mouth with methyl methacrylate.

Assessment of autonomous and autogenous healing on cementitious grouts promoted by additions of microcapsules and crystalline admixtures

The demand for more sustainable building materials has led to the development of systems with self-repairing properties. The self-healing technology has been shown to be effective in concrete and mortars, however, this technology is not often studied in grouts. Cementitious systems can show an autogenous healing, i.e., an intrinsic ability to repair microcracks by themselves. This type of healing can be improved by the addition of crystalline admixtures. In addition, the crack healing can also be enhanced by adding other materials, e.g., through the incorporation of polymeric microcapsules into the cementitious matrix that will promote a healing effect but, in this case, an autonomous healing. Thus, the main objective of this work is to assess the effect of the addition of microcapsules and crystalline admixture on viscosity and water capillary absorption of cementitious grouts. Cementitious grouts (w/b = 0.46 and w/b = 0.39) were prepared containing microcapsules (3% by weight of binder) and crystalline admixture (3% by weight of binder). Rheological measurements and water sorptivity tests were made. Viscosity measurements were taken at 3, 20 and 60 minutes. Sorptivity tests were performed on cracked specimens in order to quantify the healing efficiency. Cracks were created 7 and 28 days after casting and the water absorption was measured for 7, 14 and 28 days after cracking. The results showed that the viscosity changed considerably depending on the w/b ratio and the healing agent type. Among all grouts, reference grout presented the highest viscosity and grout with microcapsules and crystalline admixture the lowest. The water absorption of the grouts with microcapsules was the lowest regardless of curing age and w/b ratio. Regarding crystalline admixture, at both curing ages the water absorption was quite high.

Crack initiation mechanism of laser powder bed fusion additive manufactured Al-Zn-Mg-Cu alloy

Crack is a serious defect limiting the additive manufacturing of metal materials. The morphologies and crystallographic orientation of cracks for Al-Zn-Mg-Cu alloy prepared by laser powder bed fusion (LPBF) were clarified. The mechanisms of crack initiation and propagation were expounded. Results show that the cracks were crisscrossed horizontally and propagated through multilayers along the building direction in the as-printed specimens. The cracks were classified as solidification cracks, which occurred in the final stage during solidification when the liquid fraction was <20%. Al-Zn-Mg-Cu alloy has a wide temperature range between the liquidus and solidus, leading to high cracking susceptibility. Solidification cracks were prone to initiate at high-angle grain boundaries (HAGBs) and terminate at low-angle grain boundaries (LAGBs) or within grains. LPBF process could affect crack initiation and propagation by refining the grain size and inhibiting the growth orientation. Solidification and thermal shrinkage of adjacent grains during solidification provided subjective factors for cracking initiation. Directional preferential growth of dendrites along the 〈001〉 crystal orientation and the untimely filling of the liquid phase provided secondary factors. This study can provide theoretical guidance for crack suppression in additive manufacturing of 7075 aluminum alloy and other metals with high cracking susceptibility. • Crack initiation and propagation mechanisms of Al-Zn-Mg-Cu alloy in LPBF were investigated. • Internal cracks of as-printed specimens are solidification cracks. • Cracks occur in the final stage during solidification. • Oriented crystallization and texture affect the initiation and propagation of cracks. • Cracks initiate at straight HAGBs and terminate at LAGBs or within grains.

Manufacturing of WC-17Co 3D-objects by Laser Powder Bed Fusion followed by heat-treatment

Conventional tungsten carbide (WC) cermet parts containing a cobalt matrix phase are generally produced via powder sintering. In this study it is shown that cermet parts can also be produced by laser powder bed fusion from WC-17Co powder precursor. Optimal process parameters were found for manufacturing without preheated building plate. Micro-structural analysis revealed noteworthy porosity fraction (1.41%) as well as the presence of small-scale cracks in the as-built specimens. It is further shown that most of these defects can be eliminated by heat treatment at ambient pressure or by hot isostatic pressing. Heat treatment also led to the dissolution of the fragile W2C phase. Hardness test results indicate that the performance of the AM parts was comparable to that of a reference produced via powder sintering. The work is a successful demonstration of manufacturing of cermet parts using laser powder bed fusion.

Kinetic characteristics of cracks in polymers and composites based on mechanical and thermal loads

Theoretical ratios of important kinetic characteristics for brittle polymers with cracks under mechanical and thermal effects are obtained, which underlie the study of the thermokinetics of the process of polymer fracture in terms of the theory of the time dependence of strength-durability. Rectilinear (internal and surface) cracks in plate-type specimens and internal circular (disk-shaped) cracks in polymer fibers are considered. Two test modes are considered successively: constant tensile stress, constant absolute temperature, unchanging structure, inactive medium, as well as a more complex mode of purely thermal loading - the case least developed in the theory of destruction. The calculated ratios of a number of limiting characteristics and parameters of the destruction process are given: safe and critical stress; initial length of a microcrack and its relative critical length; safe and critical voltage; local stress at the crack tip (in the fluctuation volume); the value of the free surface energy. The above relations are the basis for the development of the theory of the time dependence of strength-durability.

Experimental Study of Mode-I and Mode-II Interlaminar Fracture Characteristics of Poplar LVL

Fracture is a common failure form of poplar laminated veneer lumber (LVL). In the present work, we performed an experimental study on the mode-I along-grain interlaminar fracture, mode-I cross-grain interlaminar fracture, and mode-II interlaminar fracture of poplar LVL. We investigated stress mechanisms, failure modes, and fracture toughness values of the different fracture types. The experimental results revealed that the crack in the mode-I along-grain interlaminar fracture specimen propagated along the prefabricated crack direction, and the crack tip broke. The mode-I cross-grain interlaminar fracture specimen had cracks in the vertical direction near the prefabricated crack. In the mode-II interlaminar fracture specimen, cracks appeared along the initial prefabricated crack direction. The load–displacement curves of these three specimens were linear in the early stage of loading. With the increase in the load, a nonlinear segment appeared before crack propagation and a descending segment appeared after crack propagation. The nonlinear segments of the mode-I along-grain interlaminar fracture and mode-II interlaminar fracture were very short, and cracks expanded quickly after their initiation, resulting in brittle fracture. The nonlinear segment of the mode-I cross-grain interlaminar fracture was long, resulting in plastic failure. The average toughness values of the mode-I along-grain interlaminar fracture, mode-I cross-grain interlaminar fracture, and mode-II interlaminar fracture were 15.43, 270.15, and 39.72 MPa·mm1/2, respectively.

Corrosion Rate and Weight Loss of Geopolymer Mortar Coated Concrete Specimens with Different Thicknesses

This study is about the use of geopolymer mortar as an alternative repair material to reduce the corrosion rate of building structures. The experiment was conducted by comparing the corrosion rate and weight loss of concrete specimens with different thicknesses of geopolymer mortar coating with concrete specimens coated with Sikagrout 215M. Observations showed that the specimens with 2.5 cm thick geopolymer mortar coating cracked faster and had higher current values than the others. Corrosion rate measurements showed that the specimen with 4 cm thickness geopolymer mortar coating had the lowest corrosion rate, while the specimen with Sikagrout 215M coating had a higher corrosion rate. Visualization of the cracked specimens shows that the geopolymer mortar layer’s thickness affects the reinforcement’s rust formation. Geopolymer concrete with proper coating thickness can reduce the corrosion rate and weight loss of concrete structures. This research further explains the effect of coating thickness and coating type in protecting steel in concrete from corrosion.

Three-point bending of eccentric edge crack specimens under mixed mode loading

Three-point bending of eccentric edge crack specimens under mixed mode loading

Investigation of the Acoustic Emission Behavior and Crack Evolution Characteristics of Limestone Broken by Static Crushing Agent

In order to investigate the effect of crack and acoustic emission evolution behaviour during the deformation and damage of rock specimens under the action of SCA, static crushing tests were systematically carried out on tuff specimens in the laboratory. Based on AE real-time monitoring techniques and macroscopic crack observations, the crack evolution mechanism of the cracked specimens during deformation and damage was investigated. The results show that there are three stages of microcrack incubation, macroscopic crack extension and cracking damage during SCA crushing. The tensile stress is 0.06-0.1 times of the compressive stress when brittle materials such as rocks are subjected to SCA leading to cracking damage. A large number of AE events with small amplitudes and a small number of AE events with large amplitudes represent that macroscopic cracks are generated by the rapid aggregation of a large number of microcracks before they are slowly formed. The cracking is usually done by first connecting adjacent holes into lines and then extending to the free surface. The direction of static crushing and the cracking effect can be controlled by designing the borehole location and free surface distribution.

Influence of printing direction on 3D-printed vascular based self-healing cementitious composites

Compared with other self-healing mechanisms, embedding vascular networks in cementitious matrix enables repairing wider cracks and performing multiple healing cycles. For vascular-based self-healing cementitious composites, additive manufacturing (AM) allows fabricating vascular structures with complex geometry. For Fused Filament Fabrication (FFF), printing direction has great influences on the properties of the 3D-printed vascular network. To timely release the healing agent when cracks occur, selecting the proper printing direction for the vascular network in self-healing concretes is necessary. In this work, two commercial printing filaments, i.e., polylactic acid (PLA) and polyvinyl alcohol (PVA), are used to create the vascular structures. The influence of printing direction on the dissolution of 3D-printed PVA structure is investigated. Besides, the flexural strength and water tightness of samples with PLA vascular printed in different printing directions are compared. After the 4-point bending tests, epoxy resin is manually injected into the vascular networks to seal the cracks. It is found that the strength recovery and the permeability of the cracked specimens are influenced by the printing direction due to the different crack closure after unloading.

The Effect of Stress Ratio on Fatigue Cracks Growth Rate in Aluminum Alloy

The financial and societal consequences of the sudden failure of various mechanical structures and components can occasionally be severe. As a result, there has been extensive research done on fatigue failure in both the scientific and industrial fields. The impact of stress ratio adjustments on the rates of fatigue fracture propagation for an aluminum alloy has been investigated in the current study. This aluminum alloy was extracted from a Russian MI25 helicopter's primary blade. The stress intensity factor was calculated and employed by creating an initial crack in the test specimens after a specimen of MI25 blade was cut into testing specimens with standard dimensions. A special program was written specifically for this purpose to draw a graph of the stress intensity factor range (k) with value of the propagation of fatigue cracks (da / dN) at each stress ratio (R); Propagation of fatigue cracks has been studied. Finally, the relationship between the two different stress ratios has been discovered. Between the two, a comparison has been established. The findings indicated that when the stress ratio rises, so too does the rate at which fatigue cracks form. Conversely, with a negative stress ratio, the negative increase leads to a reduction in the rate at which fatigue cracks grow. The data have been examined, and during the second stage of crack propagation, where (R = 0.0), pertinent equations that are compatible with the Paris equation have been discovered.

The Effect of Stress Ratio on Fatigue Cracks Growth Rate in Aluminum Alloy

The financial and societal consequences of the sudden failure of various mechanical structures and components can occasionally be severe. As a result, there has been extensive research done on fatigue failure in both the scientific and industrial fields. The impact of stress ratio adjustments on the rates of fatigue fracture propagation for an aluminum alloy has been investigated in the current study. This aluminum alloy was extracted from a Russian MI25 helicopter's primary blade. The stress intensity factor was calculated and employed by creating an initial crack in the test specimens after a specimen of MI25 blade was cut into testing specimens with standard dimensions. A special program was written specifically for this purpose to draw a graph of the stress intensity factor range (k) with value of the propagation of fatigue cracks (da / dN) at each stress ratio (R); Propagation of fatigue cracks has been studied. Finally, the relationship between the two different stress ratios has been discovered. Between the two, a comparison has been established. The findings indicated that when the stress ratio rises, so too does the rate at which fatigue cracks form. Conversely, with a negative stress ratio, the negative increase leads to a reduction in the rate at which fatigue cracks grow. The data have been examined, and during the second stage of crack propagation, where (R = 0.0), pertinent equations that are compatible with the Paris equation have been discovered.