Ch08 Mechanical Failures

A B S T R A C T In this paper, results from the linear normalization (LN ) technique of Reese and Schwalbe for deriving J-crack resistance ( J±R) curves have been compared, related to J±Da ( J-integral±ductile crack growth) data points, to those obtained from traditional elastic compliance technique. Research results regarding a nuclear grade steel exhibiting a wide range of elastic±plastic fracture resistance agree quite well for both techniques until a certain level of toughness of the material. Below this critical level, LN produces inconsistent results for the sub-sized compact tension specimens (0.4T C[T]). The evidence suggests that the loss of applicability of the LN technique can be determined on the basis of the Z plastic factor (Z pl ) for the best linear correlation achieved for DP N ±Da (normalised load gradient±ductile crack growth) data.

A B S T R A C T In this paper, an assessment is made regarding the effects of J±R curve fitting and extrapolation methods in two J-integral criteria ± namely crack initiation, J i , and tearing instability, J 50 ± which were obtained through the linear normalization technique. Power-law, logarithmic and linear fits were concurrently applied to J±Da data derived from sub-sized compact tensile specimens machined from a nuclear grade steel and tested at 300 8C. Research results show that the logarithmic J±R fit is the most conservative approach within a broad range of elastic±plastic fracture resistance, compared to the conventional power-law fit. On the other hand, the linear fitting method provided the most non-conservative J-predictions. The values of J i and J 50 have been successfully correlated with the net energy absorbed during Charpy impact testing of the materials. b 0 original ligament length B G specimen gross-thickness BL blunting line C(T) compact tensile specimen EL exclusion line EPFM elastic±plastic fracture mechanics J J-integral J D deformation-J Ji crack initiation J J IC crack initiation J under plane-strain J±R J-crack resistance curve JRT fracture toughness specimen nomination J 50 Paris & Johnson's criterion for tearing instability LBB leak-before-break LN linear normalization NPV nuclear pressure vessel OL off-set line R correlation coefficient T specimen or plate thickness (in.) J 50 -Linear (LN) J 50 -Power J 50 -Logarith. J i -Linear (LN) J i -Power J i -Logarith.

1997

Almost all metallic materials manifest some plastic deformation in the region at the crack tip before catastrophic crack propagation. The general principle from which the Griffith theory [1] (see Section 3.1 of Chapter 3) is derived is not limited to materials that obey Hooke's law. The principle applies as well when dissipative mechanisms, such as plastic deformation, are present. Irwin and Orowan [2,3] showed that Griffith's principle can also be applied to materials that manifest ductile behavior, that is: (6.l) where U E is the stored energy, and Up is the energy consumed per unit thickness in plastic straining in the region at the crack tip. For ductile metals, where Up » Us' the expression for surface energy, Us' was omitted from Eq.

Elsevier eBooks, 2003

This is a 2D state of stress -only the independent stress components are named. A single stress component  z can exist on the z-axis and the state of stress is still called 2D and the following equations apply. To relate failure to this state of stress, three important stress indicators are derived: Principal stress, maximum shear stress, and VonMises stress.

The investigation is about the endurance limit of aluminum, brass and mild steel that has been done at the surface of the specimens on different surface roughness. The endurance limit of the specimens have been determined by testing under different loads on the fatigue testing machine and the life cycles of each specimens has been taken after crack occur on the specimen. Endurance limit is defined as the alternating stress that causes failures after some specified number of cycles. This study or investigation has the steps that is starting form the fabrication on different surface roughness, testing on the fatigue machine and gather all the data to compare the results.. The different surface roughness will give different life cycles. Then, comparison of the result needs to be done to get the best materials on different surface roughness to create a good choosing of materials in industry. Finally, studies of Endurance limit or fatigue strength can still be expanded and widened due to the other properties that can be tested such as curvature radii and elongation of the materials due to a break point.

Acta Materialia, 2016

This is the first of three overviews on failure of metals. Here, brittle and ductile failure under monotonic loadings are addressed within the context of the local approach to fracture. In this approach, focus is on linking microstructure, physical mechanisms and overall fracture properties. The part on brittle fracture focuses on cleavage and also covers intergranular fracture of ferritic steels. The analysis of cleavage concerns both BCC metals and HCP metals with emphasis laid on the former. After a recollection of the Beremin model, particular attention is given to multiple barrier extensions and the crossing of grain boundaries. The part on ductile fracture encompasses the two modes of failure by void coalescence or plastic instability. Although a universal theory of ductile fracture is still lacking, this part contains a comprehensive coverage of the topic balancing phenomenology and mechanisms on one hand and microstructure-based modeling and simulation on the other hand, with application examples provided.

International Journal of Plasticity, 2010

All damage and failure models, describing either the evolution of microvoids, the development of shear bands or local rupture, rely on the knowledge of the hardening function at large plastic strains which, then, becomes an essential prerequisite for any failure prediction.