Papers by Keyword: Crack Length

Abstract: The mode I interlaminar fracture toughness, G_IC, of unidirectional fiber-reinforced polymer matrix composite laminates was determined using the double cantilever beam (DCB) specimens. To ensure real-time correspondence with the growing crack length, load and displacement, the Digital Image Correlation (DIC) technology, a non-contact optical measurement technique, was selected for the fracture toughness tests in this paper. In addition, fracture toughness calculation programs were used to accelerate data processing.The results indicated that the DIC technology was reliable compared with the traditional technology (magnifying glass). The G_IC values obtained from all the three calculation methods (CC, MBT and MCC method) differed by no more than 3%. The SEM analysis showed that the crack propagation occurred along the fiber-matrix interface, resulting in plastic cracking and microcracks in the matrix. The observed intact fiber bundles indicated the matrix-dominated cracking in crack propagation, with localized fiber fracture at high stress and a small amount of fiber bridging during separation.

Abstract: The aluminum 5754 alloy is one of the widely used engineering materials in shipping, rivet making, tread plates and automotive industries. These engineering structures envisage variable loading conditions during their service. In addition to it, it is also experiencing seismic vibrations. Hence, the engineering components made from such aluminum alloy are susceptible to fatigue fracture. In the current study, the prediction of fatigue crack growth (FCG) in 5754 aluminum alloy was made using the exponential function. The beam specimen comes up with a cross-section of 25X25 mm², a span length of 300 mm with a mechanical notch length of 2.70 mm at the centre was subjected to four-point bending (FPB) employing hydraulic INSTRON 8800 tensile testing apparatus. The periodic loading condition deformed the material up to large plastic deformation. The applied load was further down the elasticity of the material. The experimental data provided the relation between crack length (a) to the number of cycles (N) to failure. The response surface methodology (RSM) and modified exponential equation were used to predict the FCG. In RSM, when “stress intensity factor (K)” and “number of the cycle (N)" were considered independent variables, the response (a) was optimum (maximum) as compared to when “stress intensity factor range (del K)” and “fatigue crack growth rate (da/dN)” were considered independent variables. Hence, for designing the aluminum 5754 alloys as engineering structures, it was the number of cycles which provides a safe design as compared to da/dN. The modified exponential equation using an exponential function predicted the FCG for aluminum 5754 alloy in the form of a beam specimen. The anticipated results agreed with experimental data as the prediction ratio was 1.20 and the % deviation was 3.7.

Abstract: In large-sized metal structures, various stress concentrators are often present, which affects the operation of the material. These are intermittent bonds, holes, welded joints, material defects, etc. As a result of overloads under the action of an external cyclic load on structures in the area of stress raisers, the cycle asymmetry, the level of maximum stresses and deformations increase. In this case, the determination of the limit values of the stress cycles can be performed using a diagram of the limit stress amplitudes. The paper presents an engineering method for calculating the limiting stress amplitudes and constructing Hay’s diagrams. It is based on the use of mathematical models of classical linear and structural-mechanical fracture mechanics. Analytically and by calculation, the validity of the method is shown, which consists in determining the endurance limits and limiting stress amplitudes under high-cycle loading in a wide range of variation of the cycle asymmetry coefficient for ferrite-pearlite steels with a yield strength of up to 400 MPa. Thus, a generalized calculation method has been developed for determining the endurance limits for high values of cycle asymmetry and cycle stresses. The error of the method is estimated in the area of low-cycle load. The influence of constant and average load in a cycle on the endurance limit has been investigated both for high-cycle and low-cycle loading. The proposed approach allows to construct Smith and Hay’s fatigue diagrams for the tensile region, taking into account the structural characteristics of the material and the error allowed for engineering calculations.

Abstract: In this study, a structural-deformation analysis of the process of crack development is performed, on the basis of which an engineering methodology is developed for assessing the endurance limit and resource of large-sized structures. A simple analytical dependence (correlation аналитическая зависимость) was obtained, which allows one to determine the critical size of macrocracks for ferritic-pearlitic steels without using the well-known Griffith formula. The results of calculating the cracks critical lengths of various steels depending on their yield strength are presented. The analytical dependence of the calculation of the fatigue limit for the most dangerous symmetric loading cycle according to the standard set of mechanical characteristics of ferrite-pearlite steel is presented. The obtained results make it possible to calculate the endurance limit of structural elements of marine equipment and other structures subject to cyclic loads

Abstract: In fatigue crack growth experiment and fracture toughness test, the crack length measurement is very important. Here, this paper proposes a new method for measuring crack length, namely in the case of a known load, according to the strain on the front end face of the three point bending specimen would be to get crack length.

Abstract: The safety of concrete structure can be evaluated by manual survey through the visible crack shown on structural surface, but the manual survey is costly and time-consuming. To improve this, a novel image-based crack length measurement is proposed. The method starts with getting time varying crack images by proposed inspection system. Then retrieve crack skeleton from crack images by pre-processing, and construct crack skeleton of tree structure. After getting crack trunk by pruning small edges from crack skeleton, the length of crack is figure out. This method can measure crack length automatically, which will make contribution to enhance the stability, durability and safety of concrete structure. The validity and accuracy of the proposed method was tested by an experiment using images of a real concrete surface.

Abstract: The finite element method was used to study the stress distribution of strip joints under different elastic modulus and crack length. The results show that with the elastic modulus increases, the crack tip of the peak stress also increases, which provide maximum load continue to expand on the joint crack is SY stress, while little impact to the continued crack expansion on joints is SXY stress. I-type crack perpendicular the surface Crack under the stress of characteristics; With the initial crack length increases, I-stress intensity factor present weak tendency to increase, II-stress intensity factor is still almost zero, so that even after a longer crack length, strip adhesive joints under load the fracture is still mainly based on I-type crack, which is also consistent with the strip load characteristics: external stress perpendicular to crack surface. Another I-type stress intensity factor slight increasing trend.

Abstract: The purpose of this study was making evident the overload effect in a spectre with constant amplitude cycles. The crack growth simulation was made on cracked specimen and was studied for four loading cases. Fatigue crack growth rate was calculated applying NASGRO equation and the crack growth retardation analyzed.

Abstract: The diverse studies on Al/GFRP laminates with the circular holes, therefore, have been carried out recently. The recent studies just focused on the behavior of the fatigue crack propagation and the delamination when the shape and the size of the notches were changed. Therefore, this study evaluated the location effect of the defects in the vicinity of the circular notch of the high strength monolithic aluminum and Al/GFRP laminates on the initiation life (Ni) of the fatigue cracks, the relationship between the crack length (a) and the fatigue life (N). In addition, the fatigue crack behavior of Al/GFRP laminates was studied when the fatigue loading and the interlaminar delamination took place at the same time during crack propagation. In conclusions, (1) for the monolithic aluminum, 10% of the failure life at θ2=30° was more increased than that at θ3=60°. (2) The crack length and the fatigue life behavior of Al/GFRP laminates according to the location of the artificial defect were different from those of the monolithic aluminum. Namely, the fatigue life of θ1=0° and θ2=30° were remarkably shorter than those of θ3=60° and θ4=90°.