Crack Propagation Properties Research Articles

Effect of weld reinforcement on tensile and fatigue properties of 5083 aluminum metal inert gas (MIG) welded joint: Experiments and numerical simulations

The weld reinforcement has a strong impact on mechanical properties of MIG welded joints. This paper investigates the effect of weld reinforcement on tensile, high cycle fatigue and fatigue crack propagation properties of 5083 MIG welded joint through experimentation and numerical simulations. The results show that the existence of reinforcement significantly increases the tensile strength of the welded joint, even exceeding that of base material. But the weld reinforcement is detrimental to fatigue properties. Another significant discovery is that the weld reinforcement alters the mechanism of fatigue crack initiation and rupture relative to the smooth specimens.

Mechanical and Crack-Sensing Capabilities of Mode-I Joints with Carbon-Nanotube-Reinforced Adhesive Films under Hydrothermal Aging Conditions.

The fracture behavior and crack sensing of mode-I joints with carbon nanotube (CNT)-reinforced adhesive films were explored in this paper under hydrothermal aging conditions. The measured fracture energy of CNT-reinforced joints in grit blasting conditions is higher for non-aged samples than for neat adhesive joints (around 20%) due to the nanofiller toughening and crack bridging effects. However, in the case of brushed surface-treated adherents, a drastic decrease is observed with the addition of CNTs (around 70%) due to the enhanced tribological properties of the nanofillers. Hydrothermal aging has a greater effect in the CNT-reinforced samples, showing a more prevalent plasticization effect, which is confirmed by the R-curves of the specimens. The effects of surface treatment on the crack propagation properties was observed by electrical resistance monitoring, where brushed samples showed a more unstable electrical response, explained by more unstable crack propagation and reflected by sharp increases of the electrical resistance. Aged specimens showed a very uniform increase of electrical resistance due to slower crack propagation, as induced by the plasticization effect of water. Therefore, the proposed adhesive shows a high applicability for crack detection and propagation without decreasing the mechanical properties.

Effect of strain localization on fatigue properties of precipitation-hardened steel with an arbitrarily length crack

The present study deals with fatigue crack propagation and fatigue limit properties of two precipitation-hardened steels. One steel is more susceptible to strain localization than other. Both steels have similar tensile and micro-hardness properties. The fatigue properties are compared for a plain specimen, a specimen with a short crack and with a long crack. Effect of strain localization on fatigue properties of precipitation-hardened steel with an arbitrarily length crack is discussed with the help of plasticity-induced crack closure. The superiority of fatigue limit for precipitation-hardened steel depends on the initial crack size, and the strain localization influences notch sensitivity.

Crack propagation and electronic properties of semiconducting polymer and siloxane-urea copolymer blends

A persistent challenge in the field of organic electronics is balancing the optoelectronic properties of π-conjugated semiconducting polymers with their thermomechanical properties. A popular and effective approach to resolve this dichotomy is to blend π-conjugated polymers with amorphous, stretchy elastomers. In this work, poly(diketopyrrolopyrrole-co-thienovinylthiophene) was blended with an easily-prepared poly(dimethylsiloxane)-based phenylurea copolymer (PDMS-PU) to further explore this approach. Interestingly, the differing surface energy and polarity of this soft amorphous copolymer in comparison to other common siloxane-based polymers blended with conjugated polymers showed little impact on the solid-state morphology. Various techniques were used to evaluate the properties of the polymer blends, including atomic force microscopy, UV–vis spectroscopy, and x-ray diffraction. An in-depth morphological evaluation was performed on the blend at varying strain, elucidating the formation of cracks at the nanoscale. The results show a significant decrease in crystallinity and increase in crack onset with increased PDMS-PU content. Fabrication of organic field-effect transistors (OFETs) utilizing the new polymer blends exhibited charge mobility up to 8.2 × 10−2 cm2 V−1 s−1, and charge transfer characteristics up to 75 wt.% PDMS-PU content. The study shows the promise of PDMS-PU/conjugated polymer blends for use in OFETs, and towards the large-scale preparation of mechanically robust and stretchable electronic devices.

Study on crack propagation and shear behavior of weak muddy intercalations submitted to wetting-drying cycles

This study addresses the crack evolution law and shear behavior of weak muddy intercalations (WMIs) during wetting-drying cycles. The original crack images of WMI were processed; adjustments were made to overcome the reflective differences of crack on the surface of the WMI samples caused by the uneven surface and impurities, effectively distinguishing the crack and non-crack areas in the images of the WMI sample surface. In addition, the total length, average width, area ratio, and number of cracks were extracted as the index represents the crack condition of the WMI sample surface. Shear testing on surface of the WMI sample was used to analyze the shear properties for different wetting-drying cycles, and the relationships between crack propagation and shear properties in the wetting-drying cycles were considered. The results show that the wetting-drying cycles elongate crack on the surface of the WMI sample. The average widths of the cracks also increase, and a large number of new tensile cracks are generated. As a result, the crack degree increases with the wetting-drying cycles. Crack propagation could also weaken the WMI structure as the development of the crack may reduce the tight contact points between WMI particles, leading to decreased cohesion in the WMI. As for the shear behavior of the WMI, the shear strength may decline with the wetting-drying cycles but tends to stabilize after a certain number of cycles.

Effects of Carbon Content and Hardening Treatment on Fracture Toughness and Fatigue Crack Propagation Properties in 17mass%Cr Cast Iron

Effects of Carbon Content and Hardening Treatment on Fracture Toughness and Fatigue Crack Propagation Properties in 17mass%Cr Cast Iron

Effects of surface oxide layer on fatigue crack propagation properties of submicrometre-thick freestanding copper films

To investigate the effects of oxide layer on fatigue crack propagation properties, fatigue tests in about 500-nm-thick copper films with a thickness-controlled oxide layer and without an oxide layer were conducted. Specimens with native oxide layer with a thickness of about 2 nm, those with thermal oxide layer with a thickness of about 20 nm and those without oxide layer were prepared. The results showed that fatigue crack propagation rate (da/dN) of specimens with thermal oxide layer was faster than of specimens with native oxide layer and without an oxide layer in higher ΔK and was smaller in lower ΔK.

Strength and toughness trade-off optimization of nacre-like ceramic composites

Bio-inspired by abalone nacre, all ceramic brick-and-mortar composites with impressive mechanical properties have been recently manufactured. Albeit comprising only brittle constituents, extrinsic reinforcement mechanisms impart to these nacre-like ceramics high toughness and non-catastrophic crack propagation properties. While several models have been developed to understand the mechanical properties of natural and synthetic brick-and-mortar materials, they have always considered a ductile interface and focused mostly on intrinsic toughening mechanisms. Modeling so far has not captured the extrinsic toughening mechanisms responsible for the properties of nacre-like ceramics. Here we show that the Discrete Element Method (DEM) can account for reinforcement mechanisms such as microcracking and crack deflection, and quantitatively assess strength, initiation toughness and crack growth toughness. Two approaches are studied to enhance strength and toughness of nacre-like ceramics, either by reinforcing the interface globally (an increase of the interface strength) or locally (addition of nano-bridges). We combine the results to provide design guidelines for synthetic brick-and-mortar composites comprising only brittle constituents.

Microstructural Control of Fatigue Behaviour in a Novel α + β Titanium Alloy

The novel titanium alloy TIMETAL® 407 (Ti-407) has been developed as an alternative to Ti-6Al-4V (Ti-6-4), for applications that demand relatively high ductility and energy absorption. Demonstrating a combination of lower strength and greater ductility, the alloy introduces a variety of cost reduction opportunities, including improved machinability. Thermo-mechanical processing and its effects on microstructure and subsequent mechanical performance are characterised, including a detailed assessment of the fatigue and crack propagation properties. Demonstrating relatively strong behaviour under high-cycle fatigue loading, Ti-407 is nevertheless susceptible to time-dependent fatigue effects. Its sensitivity to dwell loading is quantified, and the associated deformation and fracture mechanisms responsible for controlling fatigue life are explored. The intimate relationship between thermo-mechanical processing, micro-texture and fatigue crack initiation through the generation of quasi-cleavage facets is highlighted. Consistent fatigue crack growth kinetics are demonstrated, independent of local microstructure.

Crack propagation and dynamic properties of coal under SHPB impact loading: Experimental investigation and numerical simulation

To investigate the crack propagation and dynamic mechanical properties of coal under high strain rate loading, 24 sets of Brazilian disk (BD) coal specimens with vertical and horizontal beddings were made, and tests were conducted by a split Hopkinson pressure bar (SHPB) system. Under different impact velocities, a high-frame and high-resolution camera was employed to capture the fracture process, and two high-dynamic strain gauges were used to record the stress pulse signals simultaneously. Using one-dimensional stress wave theory, the dynamic mechanical properties of coal with different bedding directions under different impact velocities were analyzed and discussed. Experimental results indicate that bedding directions not only have a major influence on dynamic mechanical properties such as dynamic tensile strength, strain rate and strain energy but also have a great influence on the crack propagation path. Then, based on the image processing technique and fractal method, cracks conforming to the fractal have been proven, and the results further illustrate that the fractal dimension of cracks on the coal surface increased in the fracture process under SHPB loading. Finally, a numerical model based on bond-based Peridynamic theory was proposed to simulate crack propagation and dynamic mechanical properties of coal under the SHPB test, and the displacement-strain-stress fields were also calculated, which further reveal the fracture mechanism and dynamic behavior of coal under different impact loading conditions.

Fatigue crack propagation in a copper/epoxy molding compound interface as impacted by mode-mixity

Microelectronic packages contain numerous bimaterial interfaces which influence both device design and reliability. The failure of these bimaterial interfaces have been observed to be a function of both the relative shear and tensile loads, otherwise referred to as “mode-mixity.” While the failure of such bimaterial interfaces has been the focus of much study, their performance under fatigue, in particular with respect to mode-mixity, is underexplored. Double cantilever beam tests for a copper /epoxy molding compound interface have been performed for several different mode-mixity conditions, both monotonically and cyclically. The resulting Paris’ laws are reported. In particular, the impact of mode-mixity on fatigue crack propagation is explored. The Paris’ law coefficients and exponents have been seen to be dependent on mode-mixity. The dependency of fatigue behavior on mode-mixity means that some of the properties of fatigue crack propagation can be determined from a bimaterial interfaces monotonic fracture behavior. Finally both numerical analysis in ANSYS and SEM surface characterization are performed to further the understanding of the mechanisms behind the observed trends in fatigue interfacial delamination propagation behavior as a function of mode-mixity.

Increase in micro-cracks beneath cleavage fracture surface in carbon steel ESSO specimens

Brittle cracks propagate rapidly, and subsequent brittle fractures in large-scale structures result in associated catastrophic damage. Recent studies have reported the generation of numerous micro-cracks during brittle crack propagation of low-carbon steel used in shipbuilding. As micro cracking induces energy dissipation by generating a new surface, it may thus affect crack propagation properties. However, the mechanism of micro cracking during rapid crack propagation is not yet properly understood. In this study, micro-cracks generated during rapid crack propagation were investigated for certain ESSO carbon steel specimens. Three types of carbon steel were used: high-carbon steel, low-carbon steel, and ultra-low-carbon steel. Numerous micro-cracks were observed underneath the main fracture surface, and their numbers increased with extension of the primary brittle crack. The total micro-crack lengths were evaluated with the stress intensity factor during crack propagation: a high stress intensity factor caused numerous micro-cracking over a large area, and the relationship between the stress intensity factor and total length of micro-cracks was similar, regardless of the amount of carbon in the steel. Furthermore, the influence of micro cracking on dissipation energy was analyzed using the Charpy impact test. A quenching treatment for low-carbon steel was applied to generate many martensite–austenite (M–A) constituents that induced numerous micro-cracks. Numerous micro-cracks were detected beneath the full cleavage fracture surface of the Charpy specimen with quenching treatment, and the Charpy absorbed energy was increased at −196 °C. This increase in micro-cracks might cause energy dissipation by generating a new surface during brittle crack propagation.

Experimental and numerical investigation of crack propagation and dynamic properties of rock in SHPB indirect tension test

To explore the dynamic mechanical properties and crack propagation law of rock under high strain rate impact loading, an experimental investigation with 12 sets of Brazilian disk (BD) rock specimens under Split Hopkinson Pressure Bar (SHPB) loading was undertaken. Unlike the previous researches that study the dynamic mechanical properties and crack propagation law individually, the relationship of them has also been investigated and discussed. More specifically, a dynamic resistance strain gauge and a high-frame camera were employed to simultaneously record stress wave data and rock destruction process videos under different impact velocities. Based on image processing technique, a new calculation method of crack propagation velocity was proposed and then the stress-strain characteristics and crack propagation features were analyzed. The experimental results indicate that: (1) Crack propagation rate of the crack along X-Y direction both rises with the impact velocity increase; (2) the strain of rock along the loading axis is much larger than the vertical direction; (3) there is a certain quantitative relationship between the stress-strain state and the crack area of the rock specimen after it has been damaged. Finally, a numerical model based on Peridynamic theory was developed to simulate crack propagation and dynamic constitutive relationship of rock materials with BD configuration in indirect tension test under SHPB loading, and further reveal the fracture behavior and mechanism of rock materials under high strain rate loading.

Influence of internal imperfections on the fatigue resistance of cast steel – testing methodology

Tensile fatigue specimen of G20Mn5 and G22NiMoCr5-6 were tested to quantify the influence of internal defects on the fatigue resistance of cast steel components. Defects with varying sizes, geometric shapes and distribution were enforced by influencing the solidification and recorded by computer tomography (CT). Besides the characteristics of the detected cavities, the surrounding fungoid microstructure is classified and evaluated. Later the specimens were tested under cyclic tension and S/N-curves are derived. These data form the basis for extensive numerical simulations of the damage process and the crack growth of every individual specimen. Both processes are affected by the local multiaxial stress states and have their origin in the inside of the specimen. For validation, knowledge of the crack initiation time and propagation properties are essential. Therefore, all specimens respectively the properties of the internal defects are monitored during testing with three different state-of-the-art non-destructive testing (NDT) methods. Background and application of these NDT techniques are described within this paper. Finally, fracture surface analyses show different failure modes and provide further information for model validation.

Relationship between dynamic fatigue crack propagation properties and viscoelasticity of natural rubber/silicone rubber composites.

In this paper, dynamic fatigue crack propagation properties of natural rubber/silicone rubber (NR/VMQ) composites are studied under constant tearing energy (G) input. Through dynamic fatigue crack growth testing, it is found that with the increase of VMQ fraction, NR/VMQ exhibits a lower crack growth rate (dc/dN). The viscoelastic parameters have been recorded in real-time during crack propagation, including the storage modulus E′, loss factor tan δ, and loss compliance modulus J′′, and their relationships with crack propagation behaviour have been established. The improved crack propagation resistance is attributed to the reduced J′′, resulting from the synergistic effect of increased E′ and decreased tan δ, and thus more energy dissipation occurred in the linear viscoelastic region in front of the crack tip, which consumed part of the energy for crack growth. Finally, good correlation between dc/dN and J′′ could be successfully established.

Influence of bitumen type on cracking resistance of asphalt mixtures used in pavement overlays

Cracking is one of the predominant distresses occurring in flexible pavements, especially in old pavements that were rehabilitated with an asphalt overlay. In such cases asphalt mixtures should be designed to ensure high resistance to reflective cracking because new asphalt layers are exposed to existing cracks of the old pavement. The nature of these cracks can be various (transverse, longitudinal as well as crazy cracking). One factor that minimizes this type of distress is the proper mix design process, which should involve selection of specific bitumen binder and mineral mix gradation. However, still there is no universally adopted laboratory test method that would allow to clearly assess resistance of asphalt mixtures to reflective cracking. This paper describes the usage of one of the devices developed to test asphalt mixtures in terms of such distress – Texas Overlay Tester. For this test, samples prepared in laboratory conditions (i.e. compacted with the use of Superpave Gyratory Compactor) as well as obtained in the field (by core drilling) can be used. The results are obtained not only quickly and easily, but also with sufficient repeatability. The described method characterizes both crack initiation and crack propagation properties of asphalt mixtures. In this work one type of mineral mixture was tested with 4 different types of bitumen (one neat bitumen, two ordinary polymer-modified and one polymer-modified with high polymer content). For selected cases extra additives (rubber and loose fibres) were also tested. In total, six asphalt mixtures were tested. A ranking of the used binders was created on the basis of the results in order to conclude which bitumen would ensure the best performance characteristics in terms of reflective cracking. The results have clearly shown that deliberate choice of the binder used in the asphalt mixture for the overlay will significantly improve its reflective cracking resistance or even fatigue resistance.

Influence of bitumen type on cracking resistance of asphalt mixtures used in pavement overlays

Cracking is one of the predominant distresses occurring in flexible pavements, especially in old pavements that were rehabilitated with an asphalt overlay. In such cases asphalt mixtures should be designed to ensure high resistance to reflective cracking because new asphalt layers are exposed to existing cracks of the old pavement. The nature of these cracks can be various (transverse, longitudinal as well as crazy cracking). One factor that minimizes this type of distress is the proper mix design process, which should involve selection of specific bitumen binder and mineral mix gradation. However, still there is no universally adopted laboratory test method that would allow to clearly assess resistance of asphalt mixtures to reflective cracking. This paper describes the usage of one of the devices developed to test asphalt mixtures in terms of such distress – Texas Overlay Tester. For this test, samples prepared in laboratory conditions (i.e. compacted with the use of Superpave Gyratory Compactor) as well as obtained in the field (by core drilling) can be used. The results are obtained not only quickly and easily, but also with sufficient repeatability. The described method characterizes both crack initiation and crack propagation properties of asphalt mixtures. In this work one type of mineral mixture was tested with 4 different types of bitumen (one neat bitumen, two ordinary polymer-modified and one polymer-modified with high polymer content). For selected cases extra additives (rubber and loose fibres) were also tested. In total, six asphalt mixtures were tested. A ranking of the used binders was created on the basis of the results in order to conclude which bitumen would ensure the best performance characteristics in terms of reflective cracking. The results have clearly shown that deliberate choice of the binder used in the asphalt mixture for the overlay will significantly improve its reflective cracking resistance or even fatigue resistance.

Fatigue crack propagation properties of copper nano-films with controlled surface oxide layer

Fatigue crack propagation properties of copper nano-films with controlled surface oxide layer

Crack propagation and mechanical properties of electrodeposited nickel with bimodal microstructures in the nanocrystalline and ultrafine grained regime

Abstract

Testing and evaluation for fatigue crack propagation of Ti-6Al-4V/ELI and 7050-T7452 alloys at high temperatures

This paper seeks to evaluate crack propagation properties and residual lives of metallic alloys subjected to fatigue loading at room and high temperatures. Fatigue crack growth tests were performed on Ti-6Al-4V/ELI and 7050-T7452 subjected to constant-amplitude and actual random-spectra loading at room temperature of about 25°C and at high temperatures of 250°C and 150°C to determine their crack growth properties and residual lives. The damage mode and mechanisms at high temperature were compared with those at room temperature on the basis of the results of fractographic analysis. Temperature-dependent residual lives under actual random-spectra load history were evaluated based on a modified accumulation damage rule accounting for the load interaction. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.