In specimen V (Figure 8b), a 10-μm-long short crack was initiated at N/Nf = 16%. Therefore, it was inferred that the Nf was primarily the FCG lifetime, Ng, of the short surface cracks, including in the unnotched smooth specimen. This result, which was not affected by specimens P and V, was similar to those reported previously [18].
how to crack ishihara test result
The determination of the crack-opening level at R = 0.1 and da/dN as a function of ΔK and specimen thickness for the aluminum alloy 6061-T6 and for the steel S25C. The results will also be used in an assessment of the validity of a previously proposed relation based on the following CTOD considerations[8]:
In the surface-removal experiments, a fatigue crack was propagated under a constant stress-intensity factor range ΔK. After reaching the macroscopic level of fatigue crack closure K opmax, the specimen was taken from the fatigue testing machine and 0.5 mm was then removed from each specimen surface using an electric discharge machine (Tape Cut Model; FANUC Corporation, Chicago, IL). The specimen surfaces were then electropolished. In electrochemical polishing, the specimen was the anode and stainless steel was the cathode. Electropolishing was conducted in an electrolyte that was kept at 343 K (70 C). The electrolyte was composed of distilled water (250 cc), ethanol (380 cc), and phosphoric acid (400 cc). After surface removal, the fatigue crack propagation test was resumed and the K opmax level was determined anew.
The crack growth data were obtained either under constant ΔK, decreasing ΔK, or increasing ΔK test conditions in accord with the provisions of ASTM E 647-05.[15] The decreasing ΔK tests were conducted under the following conditions:
In a previous work[16] that involved the effect of an overload on the subsequent fatigue crack growth behavior, it was shown that the removal of 0.5 mm from each surface of a compact specimen immediately after the overload resulted in the elimination of much of the effect of the overload. It was, therefore, concluded that the retardation from an overload was largely caused by an enhanced level of surface-related PIFCC as the result of the overload. In the current study, we repeated this type of surface-removal experiment but without any overload involved.
A main conclusion reached as a result of this study is that PIFCC is a plane-stress, surface-related event, whereas RIFCC is a plane-strain, through-thickness event. Therefore, it is to be expected that as the thickness of the test specimen of 6061-T6 aluminum alloy (PIFCC) is increased, the rate of fatigue crack growth at a given ΔK level will increase. This trend has been observed already in the case of the delay effect associated with overloads for both aluminum and steel alloys, which is known to be caused by PIFCC.
In this study, it has been shown that PIFCC is a surface-related process associated with the crack tip plastic zone. The fact that the rates of fatigue crack propagation can be expressed as in Eqs. [6] and [7] suggests that the CTOD plays an important role in the crack growth process. All alloys will undergo both PIFCC as well as RIFCC. However, in any given case, one or the other will be dominant. For example, in the case of a 6.35-mm-thick compact tension specimen of a 9Cr-2Mo alloy, a back-face strain gauge detected only RIFCC. However, when a replication technique was used to determine the crack-opening level, it was found that the crack-opening level was higher than that obtained by the strain-gauge method.[25] As shown in Figure 2 for a similar alloy, this type of alloy at the 6.35-mm thickness level exhibits RIFCC. In this case, the volume of material involved in PIFCC is too small to influence the closure process. This trend is also observed in overload tests. It has been shown that the overload retardation effect is associated strongly with PIFCC, but as the specimen thickness is increased, the extent of retardation is decreased.[26] It has been shown also that two opening levels are associated with an overload test, the lower one of which occurs in the plane strain region of the specimen and the higher of which occurs in the plane stress region at the surface of the specimen.[27,28] It is reasonable to conclude that there are always two opening levels, even in constant amplitude loading. However, only if the volume of material involved in the surface opening process is sufficiently large, as in the case of the 6061 aluminum alloy, will PIFCC be evident.
It is well known that microcracks develop and grow in the tool due to the periodic thermal stress [1] that occurs during the intermittent cutting process. As a result, damage occurs in the tool, finally causing tool breakage. This phenomenon is called as the repeated thermal shock.
In this study, fatigue crack growth (FCG) behavior of cemented carbide was investigated by repeating thermal shock (RTS). The effect of WC particle size on FCG behavior of cemented carbide in RTS test was also investigated. Further, a rotating bending fatigue (RBF) test was conducted at room temperature to study the FCG behavior of the cemented carbide. Then, the difference between the FCG behavior in the RBF test and the FCG behavior in the RTS test was investigated.
Crack length of the short surface crack in the RTS tests was measured using the replica method [9] . The RTS tests were interrupted periodically. Then, to collect replicas of the specimen surface, specimen was loaded to 80% of the maximum thermal stress calculated by Equation (1). Then, acetyl cellulose film was pasted on the specimen surface to take the replicas of the specimen surface. Acetone was used as a solvent of the film. These replicas were then examined to measure the crack length with an optical microscope at magnifications of 100 200. Sometimes these replicas were observed with a scanning electron microscope (SEM) for a detailed observation.
Figure 4 shows the relationship between the rate of crack growth and the maximum stress intensity factor Kmax, which was obtained by the RTS test of the cemented carbide. In this figure, the data for the two different cemented carbides whose average WC grain sizes are 2.5 and 8.5 μm are plotted on the log-log paper. As seen from this figure, in the high Kmax region, a difference due to WC grain size is not clearly seen. However, in the low Kmax region, an effect of WC grain size can be observed. Specifically, the threshold value of the FCG for the cemented carbide with average WC grain size of 8.5 μm is larger than that of 2.5 μm.
that for the one with average WC grain size of 2.5μm. Regarding the effect of WC grain size on the relation, da/dN vs. Kmax, which was obtained under the RBF tests, similar results as in the present study were also observed in other study [12] .
It is very important to interpret the mechanical behavior of post-installed anchors in cracked concrete. There is, however, few testing method to investigate the influence of cracks in concrete on pull-out behavior in Japan. In European country, for instance, the ETAG method has been developed, and the ob-tained data was directly used in design of each post-installed anchor. The testing method, however, has disadvantage on its usage, such as use of large concrete substrate. The authors have proposed the simplified testing method for pull-out test with cracked concrete. This paper presents the experimental results of pull-out tests using various anchors, which were installed in cracked concrete with different crack width. The obtained results were compared with the results of ETAG method. The results obtained from both testing methods agree with each other, and proposed simplified testing methods was verified for evaluation of the performance of post-installed anchors in cracked concrete
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