Papers by Keyword: Mode II

Abstract: Mode I and Mode II fatigue crack growth on the equator of silicon nitride balls were tested under cyclic compressive loads. The mode I crack propagated in a straight direction along compressive axis. The angle of the mode II crack changed toward the direction perpendicular to tensile stress direction. The effect of mode I on cracks in differential mode II was strong after cycles.

Abstract: Carbon Fibre Reinforced Thermoplastics (CFRTP) are expected to be used in various fields for the point of their superior mechanical properties. CFRP laminates with continuous fibres tend to be damaged by microcracks in the layer and interlaminar delamination. Especially, it is necessary to evaluate the mode II delamination growth property, which is correlated with compression after impact (CAI) strength. It is reported that CF/Epoxy laminates with a thicker interlaminar resin layer show higher toughness. By applying an extra thick interlaminar resin layer to CFRTP in which thermoplastic resin with relatively higher fracture toughness is used for the matrix, CFRTP with higher interlaminar fracture toughness can be developed. In this study, the mode II delamination growth property of CFRTP laminates under static loading was evaluated for the specimens with various layer thicknesses of polyamide (PA) resin in the middle layer of the laminates. Their moldability and damage propagation properties were evaluated by three-point bending tests and end notched flexure (ENF) tests. CF/PA laminated composites with a thicker PA layer showed superior mode II delamination growth property under static loading since they had more ductile fracture due to a thicker PA layer.

Abstract: This paper deals with a numerical simulation of the interlaminar fracture toughness of woven carbon fibre reinforced polymer. Composite materials are increasingly used for their unique properties in many branches of engineering. They are also used for flexible components such as springs, couplings, etc. The strength of these parts must be determined not only in terms of their intralaminar properties but also in terms of their interlaminar properties. This paper provides a methodology for determining the main parameters for Mode II interlaminar fracture toughness using numerical simulation. End Notch Flexure (ENF) specimens were created for fitting fracture toughness parameters of the laminate according to ASTM standards. Three point bending ENF tests were carried out on a Zwick/Roell Z050 machine. The numerical simulation was created in Siemens Simcenter 12.0 using NX Nastran nonlinear solver. The results from the numerical simulation correspond to those from the experimental test with an accuracy of 4%.

Abstract: This paper is focused on the cracks growth in mode II of specimens made of cement paste with fly ash. Three prescriptions were made for testing in mode II, during the making air bubbles arose at the bottom of the specimen that were lubricated with oil. These little caverns can have an influence on the growth of the cracks in the fracture process zone. On the other side, specimens that were lubricated with a wax, no caverns were observed. There are complex mechanisms of crack growth in the fracture process zone, cavern or inertial elements have a fundamental effect on the success of the real tests in mode II. Specimens lubricated with wax have a 50 % higher successful running of the test than those, whose forms were lubricated with the oil.

Abstract: This paper is focused on testing in mode II. In this article there is presented the double-edge notched specimen based on the theory of the infinite plate to determine the critical factor K_II. There are three approaches to this testing here. Firstly, the real test documented by photos and measured data to determine the value of the critical stress intensity was carried out. Secondly, the real test evaluated by the digital image correlation and finally the numerical model in Adina was made. The success of the testing was lower than has been expected (only 24 %). All three prescriptions were used for all 55 specimens. The highest value of K_II is reached with the prescription without fly ash.

Abstract: This work is focused on experimental study of micromechanisms of mode II and mode III fatigue cracks in metallic materials in the near-threshold regime. The resistance to fatigue crack growth can be divided to an intrinsic component (ahead of the crack tip) and an extrinsic component (shielding, closure), which is significantly higher than the intrinsic one. Fracture surfaces from the Ti6Al4V alloy and pure zirconium were observed in three dimensions. Experiments were conducted using a special device for simultaneous crack loading in modes II and III. Additionally, pure mode II and pure mode III experiments were done using CTS and torsion specimens, respectively. At the beginning of all experiments, crack closure was eliminated due to precracks generated under cyclic compressive loading. A common mechanism of local mode II advances was observed in both modes II and III. The results were similar to those of pure titanium. The hcp metals exhibit a transition behaviour between materials with coplanar shear-mode crack propagation and materials with a high tendency to deflect to the opening mode I.

Abstract: In this paper, a new type of specimen, which can be easily made by rock core through geological exploration, of mode II rock crack has been designed and processed. The crack propagation forces F of the specimen are measured under different temperatures and different confining pressures. The crack extension patterns are observed. The 3D finite element model of the specimen is established. The stresses near crack tip are calculated to get the value of K_IIC when the crack initiation. The variation of K_IIC of the model II rock crack is obtained under different temperatures and confining pressures. The experiments and numerical results show that temperature and confining pressure have obviously influence on K_IIC, the value of K_IIC decreases with the temperature increasing, and increases with the confining pressure increasing. The results of the paper can provide an important reference for analyzing the stability of the rock cracks in underground engineering.

Abstract: FEM linear numerical analysis of stress distribution demonstrated that the model Zappalorto and Lazzarin for analysed V-hole ended notch, relatively correctly predicts the stress distribution at the tip of the notch, especially for small angles of opening. The maximum levels of hoop stress and a tangential for load Mode II strongly dependent on the radius of the notch. The stress reaches its maximum value at a certain distance from the tip of the notch, approximately equal to half of the hole circle radius. The most effective way to reduce the levels of maximum stress at the notch is to increase the radius. When changing radius of 0.5 mm to 2 mm can reduce the degree of stress concentration approx. 40%.

Abstract: The best way how to test fracture mode II and determinate the critical factor K_II of cement paste beams is investigated. Several methods are presented and the test based on a theory of the double-edge notched infinite plate is described in detail and will be carry out.

Abstract: Fatigue delamination of CF unidirectional laminates made of low temperature and vacuum bag cured prepreg, (out of autoclave-OOA), with MTM46 epoxy system was investigated under Mode II loading condition and the coefficients of the corresponding Paris equations were determined. The tests were carried out with the use of ENF beam specimens under displacement controlled conditions and at room temperature with the frequency, f=5Hz, and cycle parameter R=0.1. The crack length, a, was calculate based on the specimen compliance changes. For each specimen, for the entire range of load cycles, i.e. 1 nn_max, the a (n) relationship was approximated with the help of a single sigmoidal function and then the da/dn (n) relationship was determined. In addition, for the comparison purpose the 7-point approximation procedure recommended by ASTM E647 for the same purpose was applied. The differences in results produced by both the data reduction variants were presented for important variants stages and discussed.