• Korean Journal of Metals and Materials
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• v.56 no.12
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• pp.845-853
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• 2018

To understand the effect of microstructural factors (i.e., the size of ${\alpha}$ phase, equiaxed vs bimodal structure) on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of mill-annealed Ti-6Al-4V (Ti64) alloy, three specimens of EQ (equiaxed)-8 (8 indicates the size of ${\alpha}$ grain), BM (bimodal)-8, and BM-16 were studied. The uniaxial HCF and FCP tests were conducted at an R ratio of 0.1 under sinusoidal fatigue loading. The microstructural influence (i.e., EQ vs BM) was not significant on the tensile properties of mill-annealed Ti64 alloy, and showed an increase in tensile strength and elongation with decreasing gauge thickness from 50 mm to 1.3 mm. The microstructure, on the other hand, affected the resistance to HCF substantially. It was found that the EQ structure in mill-annealed Ti64 has better resistance to HCF than the BM structure, as a result of different crack initiation mechanism. Unlike HCF behavior, the effect of microstructural features on the FCP behavior of mill-annealed Ti64 was not significant. Among the three specimens, BM-16 specimen showed the highest near-threshold ΔK value, probably because it had the greatest slip reversibility with large ${\alpha}$ grains. The effect of microstructural factors on the HCF and FCP behaviors of mill-annealed Ti64 alloy are discussed based on fractographic and micrographic observations.

• Advanced Composite Materials
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• v.16 no.1
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• pp.45-61
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• 2007

This paper describes a damage accumulation mechanism in cross-ply CFRP laminates $[0_2/90_2]_{2S}$ subjected to out-of-plane loading. Drop-weight impact and static indentation tests were carried out, and induced damage was observed by ultrasonic C-scan and an optical microscope. Both tests gave essentially the same results for damage modes, sizes, and load-deformation history. First, a crack occurred in the bottom $0^{\circ}$ layer accompanying some delamination along the crack caused by bending stress. Then, transverse cracks occurred in the middle $90^{\circ}$ layer with decreasing contact force between the specimen and the indenter. Measured local strains near the impact point showed that the stress state changed from a bending dominant state to an in-plane tensile dominant state. A cohesive interface element was used to simulate the propagation of multiple delaminations and transverse cracks under static indentation. Two types of analytical models are considered, one with multiple delaminations and the other with both multiple delaminations and transverse cracks. The damage obtained for the model with only multiple delaminations was quite different from that obtained from the experiment. However, the results obtained from the model with both delaminations and transverse cracks well explain the characteristics of the damage obtained in the experiment. The existence of the transverse cracks is essential to form the characteristic impact damage.

• Geotechnical Engineering
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• v.13 no.1
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• pp.159-168
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• 1997

In order to investigate the effect of parameters such as orientation and surface roughness of a reinforcing material as well as the water content of the clay matrix on the stress-strain and failure behavior of reinforced clay, uniaxial compression tests were performed on clay samples reinforced with a steel inclusion Test results showed that the increase or decrease in strength of reinforced clay samples was found to depend on the orientation of a steel inclusion as well as water content of clay samples. The possible weakening mechanism induced by reinforcement in clay samples was related to the development of cracks along the tips of interface between steel inclusion and clay matrix. A theoretical interpretation of failure behavior of reinforced clay was made by using fracture mechanics theory, and the experimental results were compared with the theoretical predictions. The predicted crack propagation direction obtained from fracture criteria for a material containing a closed crack with friction agreed reasonably well with the measured values obtained from tests.

• Composites Research
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• v.16 no.4
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• pp.44-50
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• 2003

The damage zone around a crack tip occurring before the fracture is a significant domain. which affects the toughening mechanism of materials. In this study. the growth process of damage zone in the vicinity of crack tip for rubber-modified epoxy resin is investigated using an acoustic emission(AE) analysis. The weight fractions of rubber(CTBN 1300$\times$B) in rubber-modified epoxy resin are 5 wt% and 15 wt%. The fracture toughness($K_{IC}$) and the fracture energy($G_{IC}$) were measured using 3 point bending single-edge notched specimens. The damage zone and rubber particles distributed around the crack tip were observed by a polarized optical microscope and an atomic force microscope(AFM). The damage zone around crack tip of rubber-modified epoxy resin was formed at 13 % loading and developed until 57 % loading of the fracture load. The crack initiated at 57 % loading grew repeatedly in the stick-slip propagation behavior. Based on time-frequency analysis, it was confirmed that AE signals with frequency bands of 0.15~0.20 MHz and 0.20~0.30 MHz were generated from cavitation and stable/unstable cracking inside the damage zone.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.231-237
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• 2000

A15052/AFRP laminates were developed principally to obtain a material with good fatigue strength, in which possible cracks would grow very slowly. Weight savings of more than 30% should be attainable in practice. Also, the crack bridging fibers could still was carry a significant part of the load over the crack, thus the COD and stress intensity factor was reduced at the crack tip. A15052/ AFRP laminates consists of three thin sheets of 5052-H34 aluminum alloy and two layers of [0] unidirectional aramid fiber prepreg. The cyclic-bending moment test was investigated based on applying the five kinds of bending moments. The size of the delamination zone produced between 5052-H34 aluminum alloy sheets and fiber-adhesive layers was measured from ultrasonic C-scan pictures taken around the fatigue crack. In addition, the relationship between the cyclic-bending moment and the delamination zone size was studied and the effect of fiber bridging mechanism was also considered.

• Journal of the Korean Geotechnical Society
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• v.22 no.11
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• pp.55-64
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• 2006

It is very Important to predict the damage zone of a rock mass induced by blasting for the excavation of an underground cavity such as a tunnel, as the damage zones incur mechanical and hydraulic instability of the rock mass potentially. Complicated blasting processes that can hinder the proper characterization of the damage zone can be effectively represented by two loading mechanisms. The first mechanism is the dynamic impulsive load-generating stress waves that radiate outwards immediately after detonation. This load creates a crushed annulus along with cracks around the blasthole. The second is the gas pressure that remains for an extended time after detonation. As the gas pressure reopens some arrested cracks and extends these, it contributes to the final structure of the damage zone induced by the blasting. This paper presents a simple method to evaluate the damage zone induced by gas pressure during rock blasting. The damage zone is characterized by analyzing crack propagations from the blasthole. To do this, a model of a blasthole with a number of radial cracks that are equal in length in a homogeneous infinite elastic plane is considered. In this model, crack propagation is simulated through the use of only two conditions: a crack propagation criterion and the mass conservation of the gas. The results show that the stress intensity factor of a crack decreases as the crack propagates from the blasthole, which determines the crack length. In addition, it was found that the blasthole pressure continues to decrease during crack propagation.

• Applied Microscopy
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• v.26 no.4
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• pp.447-456
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• 1996

Fracture surfaces of materials contain useful information ranging from crack path to the mechanism of fracture. Since limitation of electron transparency requires a sample in the form of thin foil for TEM observations, it is impossible to extract such information directly from the fracture surfaces. In this study, the method of surface replication from the ceramic fracture surface is employed to characterize the process of crack propagation in ceramic matrix composites using TEM analysis. The surface replica from the fracture surface in ceramic materials provides detailed surface morphology and more importantly, loosened particles on the fracture surface are collected. Electron diffraction and chemical composition analyses of these particles reveal crack path in the specimen. Furthermore, one can determine the mode of fracture by observing the fracture surface morphology from the image of replica. Two examples are given to illustrate the potential of the surface replication technique. In the first example, apparent toughness increase in $B_{4}C-Al$ composites at high strain rate is investigated by surface replication to elucidate the mechanism of fracture at different strain rates. The polytypes of SiC formed during the sintering of SiC-AlN composite and their effect on the fracture behavior of SiC-AlN composite are analyzed in the second example.

• Journal of the Korean Society for Precision Engineering
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• v.7 no.4
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• pp.130-139
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• 1990

It is well known that mechanisms of fracture and crack growth depend upon material characteristics such as fracture toughness, environmental condition, crack geometry and mechanical properties. It seems to be very important to investighate the effects of the above factors on the behavior of structural components which contain flaws for the detailed evaluation of their integrity. In this experimental research, fracture behaviors of SK-5 high carbon steel was investigated by using Acoustic Emission(AE) technique. Fracturing processes of materials were estimated through both the tension test with nominal specimens and the fracture test with compact tension specimens. The critical applied load which corresponds to the crack initiation and propagation is very improtant for the determination of yield strength of fracture toughness. The critical applied load($P_Q$) was determined through AE method and the source of AE signal was estimated by fractography analysis. The experimental results may contribute to the safety analyses and strength evaluation of structures.

• Geomechanics and Engineering
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• v.8 no.2
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• pp.237-255
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• 2015

From the related engineering principles, analytical solutions for horizontal crack initiation and propagation on a coal panel floor-underlying strata interface due to coal panel excavation are derived in this paper. Two important concepts, namely the critical panel width of horizontal crack initiation on the panel floor-underlying strata interface and the critical panel width of vertical fracture (crack) initiation in the panel floor, have been presented. The resulting analytical solution indicates that: (1) the first criterion can be used to express the condition under which horizontal plane cracks (on the panel floor-underlying strata interface or in the panel floor because of delamination) due to the mining induced vertical stress will initiate and propagate; (2) the second criterion can be used to express the condition under which vertical plane cracks (in the panel floor) due to the mining induced horizontal stress will initiate and propagate; (3) this orthogonal set of horizontal and vertical plane cracks, once formed, will provide the necessary weak network for the flow of gas to inrush into the panel. Two characteristic equations are given to quantitatively estimate both the critical panel width of vertical fracture initiation in the panel floor and the critical panel width of horizontal crack initiation on the interface between the panel floor and its underlying strata. The significance of this study is to provide not only some theoretical bases for understanding the fundamental mechanism of a longwall floor gas inrush problem but also a benchmark solution for verifying any numerical methods that are used to deal with this kind of gas inrush problem.

• Journal of the Korea institute for structural maintenance and inspection
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• v.10 no.5
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• pp.121-129
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• 2006

This study look into the mechanisms of growth and magnification of the cracks and delamination in the pre-cracked RC beams repaired with carbon fiber sheets. The experimental parameters were loading type, loading speed and crack. In the experiments, it was confirmed that a failure of beams began with development and propagation of the stepped delamination in the below the loading point due to the rapid change of shear force, but mechanisms of the failure were not influenced with loading type, loading speed and pre-cracks. Particularly, in the case of beams having the pre-cracks, growth of crack concentrated at the special crack below the loading point and led to failure of the beam by delamination due to magnification of crack.