The construction of a fracture resistance delta-R (or J-R) curve requires the appropriate measurement of crack-tip opening displacement (CTOD) as a function of crack extension. This can be made by different procedures following ASTM E1820, BS7448 or other standards and procedures (e.g., GTP-02, ESIS-P2, etc.) for the measurement of fracture toughness. However, all of these procedures require standard specimens, displacement gauges, and calibration curves to get intrinsic material properties. This paper deals with some analysis and aspects related to the measurement of fracture toughness by observing the surface of the specimen. Tests were performed using three-dimensional surface displacement measurements to determine the fracture parameters and the crack extension values. These tests can be conducted without using a crack mouth opening displacement-CMOD or load-line displacement gauge, because CMOD can be calculated by using the displacement of the surface points. The presented method offers a significant advantage for fracture toughness testing in cases where a clip gauge is not easy to use, for example, on structural components. Simple analysis of stereo-metrical surface displacements gives a load vs. crack opening displacement curve. Results show that the initiation of stable crack propagation can be easy estimated as the point of the curvećs deviation. It is possible to determine the deviation point if the crack opening displacement measurements are close to crack tip in the plastic zone area. The resistance curve, CTOD-R, is developed by the local measurement of crack opening displacement (COD) in rigid body area of specimen. COD values are used for the recalculation with the CMOD parameter as a remote crack opening displacement, according to the ASTM standard.
COBISS.SI-ID: 15050006
Many biological materials, such as bone, nacre, or certain deep-sea glass sponges, have a hierarchical structure that makes them stiff, tough, and damage tolerant. Different structural features contributing to these exceptional properties have been identified, but a common motif of these materials, the periodic arrangement of structural components with strongly varying stiffness, has not gained sufficient attention. Here we show that the periodicity of the material properties is one of the dominant reasons for the high fracture resistance of these structures and their tolerance to short cracks. If the composite architecture fulfills certain design rules, which are derived in this paper, the stiff structure becomes fracture resistant and, most of all, flaw tolerant. This architectural criterion inspired from nature provides useful guidelines for the design of defect-tolerant resistant man-made materials.
COBISS.SI-ID: 15200534
The effect of residual stress on the fatigue crack propagation was analysed for a loading regime close to threshold stress intensity factor range. Fatiguecrack propagation experiments were performed on single edge notched bending specimens machined from a welded plate. The residual stresses induced a variation in the crack propagation rate along the crack front. By varying the stress ratio and the stress intensity factor range different shapes of crack front can be realized. From the shape of the crack front and the variation of the crack front the resulting residual stresses and local stress intensity can be determined by means of finite element modeling. By using somesimplifications it is possible to estimate the limit values of the stress intensity factor induced by the residual stresses at selected regions.
COBISS.SI-ID: 13792534
The concept of [delta]J cyclic has been extended to fatigue initiation emanating from notch. The parameter is then named [delta]J[ro]. Validation of this parameter is made by fatigue tests made on Roman tile specimens made in X52 pipe steel. Here, fatigue initiation is detected by acoustic emission. It has been found that the fatigue initiation decreases after hydrogen absorption. This can be explained by interaction of hydrogen and plasticity as can be seen for tensileand fracture behaviour of X52 steel after introduction of hydrogen.
COBISS.SI-ID: 16206358
In the present investigation, the effect of both: rolling parameters (2 reduction rates and 3 cooling rates) and chemical elements such as: C, Mn, Nb,Ti, Mo, Ni, Cr, Cu and B, has been studied in relation to strength properties in low-carbon microalloyed steels with high niobium contents (up to 0,12 wt. % Nb). For this purpose, an experimental set-up was designed based on an intelligent design of experiments (DoE), resulting in 26 casts (laboratory casts).Acombination of metallography, Electron Back-Scattered Diffraction (EBSD) and tensile tests have been performed to study how processing parameters and chemical composition affect the strength. The results, where the proof stress, tensile strength, uniform and fracture elongations are the response variables, have been analysed statistically by means of multiple linear regression technique, leading to response equations. From the results, it was found that the effectiveness of niobium increasing the strength is reduced as carbon content increases. It makes better wedibility without pre-heating.
COBISS.SI-ID: 15654934