Crack Opening Displacement Gauge Research Articles

Fatigue crack monitoring in train track steel structures using plastic optical fiber sensor

Plastic optical fiber (POF) sensors have shown excellent potential for damage detection and structural health monitoring in a variety of engineering structures. This paper discusses the feasibility of using POF sensors in conjunction with a signal-processing algorithm capable of detecting and monitoring fatigue-induced cracks in train track steel structures in real time. The POF sensor, which was modified from an existing design to increase the signal sensitivity, allows for accurate detection of a fatigue crack developed in a specimen, and was found to compare well to the reference acoustic emission (AE) sensors and crack opening displacement (COD) gauge attached to the specimen. The crack-detection technique, which relies on capturing the intensity variation of the POF sensor, was not susceptible to any signal fluctuations commonly associated with intensity-based optical fiber sensors. The results show that the technique has potential for use in detecting the initiation and propagation of specific segments of a structure vulnerable to cracking due to external cyclic loading, e.g. at welded joints in train tracks under train loads or offshore structures subject to wave loads. The POF sensor system is composed of inexpensive parts (LED light source, photodetectors, and data acquisition units) and can easily be installed to the host structure. To validate the proposed damage-detection technique, the instrumented specimens are subjected to cyclic loading in order to induce stable crack propagation in the specimen. A COD gauge and AE were used for the purpose of calibration and comparison. The results show remarkable resemblance in terms of crack initiation and propagation identification exhibited by all three types of sensors, highlighting the potential of the proposed sensor for crack initiation detection and subsequent monitoring of crack propagation.

제하 컴플라이언스법 및 직류전위차법을 이용한 Zr-2.5Nb 압력관 휘어진 CT 시편의 균열시작 평가

The direct current potential drop(DCPD) method and the unloading compliance(UC) method with a crack opening displacement gauge were applied simultaneously to the Zr-2.5Nb curved compact tension(CCT) specimens to determine which of the two methods can precisely determine the crack initiation point and hence the crack length for evaluation of their fracture toughness. The DCPD method detected the crack initiation at a smaller load-line displacement compared to the UC method. As a verification, a direct observation of the fracture surfaces on the curved compact tension specimens was made on the CCT specimens experiencing either 0.8 to 1.0㎜ load line displacement or various loads from 50% to 80 % of the maximum peak load, or P_(max). The DCPD method is concluded to be more precise in determining the crack initiation and fracture toughness, J in Zr-2.5Nb CCT specimens than the UC method.

Effect of second phase on the fatigue crack growth in AlSl 4340 steel

Increasing applications of AISI 4340 steel necessitate more precise characterization of its mechanical properties so that designers can better ensure failsafe behaviour in service. The fatigue fracture process of a component includes crack initiation and crack propagation. C r a c k initiation behaviour can be evaluated by studying low cycle fatigue and high cycle fatigue. The crack propagation life depends on fatigue crack growth rate and fracture toughness. Many researchers have conducted fracture toughness measurement [1-3], dynamic fracture propagation [4-6] and bending and torsion fatigue tests [7] in 4340 steel. It has been shown that the fracture toughness can be improved by using higher austenitizing temperature (1100-1200 °C) or through high temperature thermomechanical treatment [3]. However, little work has been done on the fatigue crack growth rate of 4340 steel [8]. The second phase plays an important role in determining the mechanical properties. The second phase hard carbide particles in a tool steel will develop stresses at the boundary of the matrix and the inclusion during external cycling loading [9] and reduce the low cycle fatigue properties. Zaczyk et al. [10] indicate that the absorbed energy during stable crack growth depends upon the amount the morphology of second phase particles in ductile steels. The resistance to shear instability and fracture toughness also relies on second phase particle dispersions [11]. Tomita and Okabayashi have proposed two heat treatments that can produce a dutile second phase in the martensitic matrix of 4340 steel [12-15]. The first process is isothermal heat treatment (IHT), which produces a lower bainite second phase, and the second is interrupt quench treatment (IQT), which produces a highly tempered martensite second phase. The existence of the ductile second phase can significantly improve the mechanical behaviour such as tensile properties, impact energy [12-14] and plane strain fracture toughness [15]. However, Tomita and Okabayashi did not address the fatigue properties. In the study reported here we investigated the effect of types and volume fraction of the second phase on the fatigue crack growth rate in AISI 4340 steel. The results are compared to those of conventional quenched and tempered 4340 steel. A commercial AISI 4340 steel was used in this study. The chemical composition was (wt%: C 0.39, Si 0.24, Mn 0.61, P 0.02, S 0.01, Ni 1.46, Cr 0.67, Mo 0.17). The martensitic transformation temperature Ms determined by a dilatometer was 330 °C. Based on the Ms temperature, three different heat treatments were carried out to produce ductile second phase in the matrix: isothermal heat treatment, interrupt quench treatment and direct quench treatment (DQT). Table I lists these heat treatment processes and specimen identification. The microstructures of specimens etched with a 3% nital solution were observed by optical microscopy. Fatigue crack growth tests in accordance with the ASTM E647 standard [16] were performed in a computerized MTS servo-controlled testing system. Compact tension specimens of thickness 5.4 mm and width 43.2 mm were used. A sinusoidal load with frequency 40 Hz and load ratio R = 0.1 was applied throughout the test. A crack opening displacement gauge was attached close to the notch of the specimen, and the growi~ag crack length was measured by the compliance method. Fatigue cycles and cyclic stress intensity factor at every moment of stressing could be recorded automatically. Two duplicate fatigue tests were performed for each specimen. The broken surfaces were studied by scanning electron microscopy (SEM) to understand

FATIGUE CRACK CLOSURE MEASUREMENTS ON 2024‐T3 SHEET SPECIMENS

Abstract A computer‐controlled procedure has been developed for automatic measurement of the crack opening stress Sop during fatigue tests. A crack opening displacement gauge (COD meter) is used to obtain digital data on the load versus COD curves. Three methods for deriving Sop from the data sets are compared (1) a slope method, (2) a tangent lines intersecting method, and (3) a tangent point method. The effect of the position of the COD meter with respect to the crack tip on Sop is studied in tests of 2024‐T3 specimens. Results of crack growth and Sop are presented for CA loading with an overload, and with an overload followed by an underload.

Prediction of delay cycles due to instant of single overload cycles

Crack closure experiments were performed on 6061-T6 aluminium alloy using a crack opening displacement gauge for various overload ratios (1.44, 1.67, 1.87 and 2.06). The single overload cycle is given at the beginning, after exhausting 25% of the constant amplitude loading (CAL) life and 50% of the CAL life of the specimen. On the basis of these experiments one expression is developed to predict delay cycles due to application of overload cycle. The overloads produce a beneficial effect when they are given early. Later overloads, although giving an increase in life, produce a response of lesser magnitude. The experimental results are plotted on a log-log graph of n d/n cal . versus overload ratio. The experimental data are fitted on a straight line. One power law is developed.

Investigation of yield strength and single cycle overload on crack closure

Crack closure experiments were performed on 6063-T6 and 6061-T6 aluminium alloys, using a crack opening displacement gauge, for various overload ratios (1·67, 1·88 and 2·06). On the basis of these experiments some relationships are developed. The delay period after application of a single overload increases with increase in the magnitude of overload. The retardation is decreased with increase in prestrain for the same overload ratio. After the overload cycle the increase in U values is less in prestrained material than in the as-received material. Crack growth rate also decreases after application of an overload cycle; it attains a constant amplitude crack growth rate and crack closure value after a certain number of cycles. For the same stress ratio, the delay period in the 6063-T6 alloy was found to be more than in the 6061-T6 alloy. The experimental results when plotted on log-log graph paper, show that N D N CAL vs overload ratio and crack length (when U = U min) vs overload ratio fit a straight line, from which the power laws are developed.

亀裂開口変位（ＣＯＤ）制御によるモードＩＩ形層間破壊靭性試験

End-notched flexure (ENF) test is commonly accepted to determine the mode II interlaminar fracture toughness of composite laminates. As the crack propagation in the ENF specimen is unstable under displacement-control, the critical energy release rate, GIIC, is determined at the onset of crack initiation but not during the crack propagation. In order to estimate the mode II fracture toughness from the initiation to the stable crack propagation, a new test method has been proposed in the present paper. A special COD gauge has been developed for the measurement of the crack opening displacement (COD) of ENF specimen and applied to the mode II interlaminar fracture toughness test. The crack initiation in the ENF specimen can be detected with higher accuracy from the load-COD diagram than the load-displacement dia-gram. Moreover, the crack propagates in the stable manner under the COD-control test, though it is unstable when the conventional displacement-control test is applied. Analytical compliance method has been successfully applied to the ENF test and the relation between fracture tough-ness, GIIC, and crack length, a, can be obtained automatically by using a computer aided test-ing system. Applicabilities of the COD-control test and the analytical compliance method to the ENF test are confirmed in the present paper.

Study on modified J-integral range and its correlation with fatigue crack growth

The research work reported in this paper is concerned with the studies on J-integral range ( ΔJ)/modified value of J-integral range ( ΔJ ∗ ) and to relate them to crack tip opening displacement (CTOD)/crack opening displacement (COD) and fatigue crack growth rate ( da dN ). Experiments were performed on 6063-T6 Al-alloy in both as-received and annealed states for different stress ratios ( R). CTOD and COD were measured using surface measurement technique by a small COD gauge, by mounting it near a fatigue crack tip at different positions behind the crack tip and at the crack tip respectively. It was found that ΔJ is a function of CTOD/COD. When conventional concept of J-integral is modified by introducing notional crack concept for considering the plasticity at the crack tip, the experimental results are found to come closer to the ΔJ ∗ values (found theoretically by notional crack concept). The fatigue crack propagation law though conventionally expressed as a function of stress intensity range ( ΔK) can also be expressed as a function of ΔJ ∗ . The experimental results for both states of the material (as-received and annealed) show that for large range of da/ dN, ΔJ ∗ is able to provide a satisfactory correlation in the form of power law and the crack growth rate equation is found to be independent of R.

Automated estimation of fatigue crack length and closure/opening stress

An experimental technique is proposed for the estimation of crack length as well as crack closure/opening stress during fatigue crack growth. A specially designed, single cantilever, crack opening displacement gauge is used to monitor these variables during fatigue crack propagation testing. The technique was experimentally validated through electronfractography.