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
High strength steel grade 51CrV4 in thermo-mechanical treated condition is used as bending parabolic spring of heavy vehicles. Several investigations show that fatigue threshold for very high cycle fatigue depends on inclusion's size and material hardness. In order to determine allowed size of inclusions in spring's steel the Murakami's and Chapetti's model have been used. The stress loading limit regarding to inclusion size and applied stress has been determine for loading ratio R=-1 in form of S-N curves. Experimental results and prediction of S-N curve by model for given size of inclusion and R ratio show very good agreement. Pre-stressing and shot-peening causes higher compress stress magnitude and consequently change of loading ratio to more negative value and additionally extended life time of spring.
COBISS.SI-ID: 15078678
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