Regions Of Strain Concentration Research Articles

Study on Fracture Assessment (3rd Report)

Japan Welding Engineering Society (WES) standard WES 2805 gives procedures for safety evaluation of defects at elastic-plastic condition in welded structures on the basis of the CTOD concept. The essential point of fracture strength assessment in WES 2805 is a relationship between CTOD value and local applied strain, so called “CTOD design curve”. It is important to confirm the applicability of local strain based CTOD design curve to the case of the structural stress concentration, because most of brittle fractures in real structures occur from the defects at the structural stress and strain concentrated region.In this series of studies on fracture strength assessment, an investigation on fracture test results for some kind of structure models with strain concentration was carried out, and modified WES CTOD design curve was proposed. Furthermore, wide plate tests with cracks emanating from a hole and stiffener model tests with a crack in the strain concentrated region were carried out, and applicability of the proposed CTOD design curve was investigated.In this study, wide plate tests with stress concentration using 2-dimensional finite element analysis were carried out, and it was shown that the relationship between CTOD and local strain was varied with the relative size of the plastically deformed region as compared with defect size. It the plastically deformed region size was lagerer than 4 times of crack length, modified WES CTOD design curve under-estimates CTOD value. It has been revealed that the proposed modified WES CTOD design curve is valid for fracture strength assessment of cracks in the stress concentrated region except above mentioned case.

An investigation into the fatigue crack growth behaviour of an IMI 829 titanium rotating disc material. (Dissertation): Hull, D.A. Diss. Abstr. Int. Mar. 1990 50, (9)

Low-cycle fatigue studies have been conducted on the near-alpha IMI 829 Ti alloy under constant-amplitude and variable-amplitude load- and strain-controlled loading conditions, and fundamental data have been generated relating to the effects of cyclic loading on elastic modulus, yield stress and plastic strain development. The results of these studies have been used to develop a finite element based damage accumulation model, which is capable of predicting both the stress and strain distributions in cyclically loaded components, and the time to failure (time to first crack) in critical regions of high stress and/or strain concentration. The model has been successfully validated using LCF tests conducted on notched specimens (Kt = 2.8 and 3.2) at ambient (20 °C) temperature.

Strain-based failure criteria for steel containments

The Containment Integrity Division of the Sandia National Laboratories (Sandia) has been conducting a program to evaluate the performance of containment buildings with internal pressure. Sandia has suggested that in the absence of leakage past penetrations, containment buildings will fail by rupturing after large plastic strains are developed up to ultimate strain of the material. This paper represents a portion of work conducted at Ames Laboratory for Sandia, the objective of which was to identify fabrication details that may affect the performance of a containment building. Construction drawings for nine steel containment buildings were surveyed, and several significant strain concentration regions were identified by using recommendations from Sandia and Section NE-3217 of the ASME Boiler and Pressure Vessel Code. These following regions were identified as: eccentricities in stiffener patterns around penetrations, eccentricities in containment shell middle surface, flat plate cover used on spare penetrations, containment base connection details, and containment heads. Examples of each of these regions were analyzed by the finite-element method, by simplified equations or both. In the case of middle surface eccentricities, the strains were found to be self-limiting. Even though flat plates have primary strains, they are typically designed so as not to control. Bolts in the base connection have primary strains and may control. The circumferential compressive strains introduced at the knuckle during buckling of the containment head grow as the pressure increases, but are somewhat restricted by the meridional tension. Finally, three analysis techniques and their associated failure criteria for the analysis of containment strength are introduced.

Assessment of cracks existing in the strain concentrated region from a viewpoint of brittle fracture

The significance of cracks existing in the strain concentrated region is discussed from a viewpoint of brittle fracture. In this study, the J-integral, of which the mathematical treatment is easier and stricter than that of COD (crack opening displacement) is considered as the fracture parameter. Also, averaged local strain, e ̄ , in the strain concentrated region without a crack is considered as the mechanical parameter which characterizes the state of the elastic-plastic region generated near the stress raiser. The relation between the J-integral of the cracked body and the local strain is investigated experimentally and theoretically with the aid of the elastic-plastic finite element analysis. Brittle fracture tests on the strain concentration models of the four kinds of the structural steel (SM41, SM50, HT80 and A508 C1.3) show that a newly proposed J design curve provides a good estimation for the strength of brittle fracture from a crack existing in the stress/strain concentrated region.

ＣＯＤ設計曲線の検討

The standard of Japan Welding Engineering Society WES 2805 gives the procedure for safety evaluation of defects in welded structures on the basis of the COD concept. The establishment of an appropriate relationship among applied load, defect size and COD value, so-called.“COD design curve”, is essential in the application of the COD concept to the safety evaluation of structures. The COD design curve adopted in WES 2805 takes the formδ=3.5eaWhere e: applied straina: equivalent through-thickness flaw size parameterIn practice, attention must be paid particularly to brittle fracture initiation from a part-through crack in a highly strain concentrated region. For practical use of the COD design curve, experimental verification must be made about the relevancy of the LEFM-based equivalency of flaw size parameter a for a general yielding condition, and also about the evaluation method of applied strain e for a localized highly strained region.In this study, bending tests using surface-cracked specimens were carried out, and it was shown that the determination procedure of parameter a currently used in WES 2805 is reasonable even in the elastic-plastic regime. Furthermore, from the experiments using tensile specimens, in each of which two corner cracks existed in stress concentrated region, a way of defining the applied strain e in the highly strained region with steep strain gradient in it and a new COD design curve were proposed.

Behavior of graphite/epoxy plates with holes under biaxial loading

An experimental investigation was conducted to study the behavior under biaxial tensile loading of quasiisotropic graphite/epoxy plates with circular holes and to determine the influence of hole diameter on failure. The specimens were 40 cm×40 cm (16 in.×16 in.) laminates of [0/±45/90] s layup. Four hole diameters, 2.54 cm (1.00 in.), 1.91 cm (0.75 in.), 1.27 cm (0.50 in.) and 0.64 cm (0.25 in.), were investigated. Deformations and strains were measured using strain gages and birefringent coatings. Equal biaxial loading was introduced by means fo four whiffle-tree grip linkages and controlled with a servohyraulic system. Initially, the circumferential strain is uniform around the boundary of the hole. Subsequently, with increasing load, regions of high strain concentration with nonlinear response develop at eight characteristic locations 22.5 deg off the fiber axes. Failure in the form of cracking and delamination initiates at these points. Maximum strains at failure on the hole boundary reach values up to twice the ultimate strain of the unnotched laminate. The effect of hole diameter on strength was described satisfactorily using an average biaxial-stress criterion. Good correlation was also obtained with theoretical predictions based on a tensor-polynomial failure criterion for the lamina and a progressive degradation model.

Effects of strain ageing on fatigue damage in low-carbon steel

Optical and electron microscopy were used in a study of the effects of dissolved carbon and of testtemperature (20–130°C) on the development of fatigue damage in a low-carbon steel. Strain ageing during fatigue increased the tendency for cyclic plastic strains to concentrate in active slip bands. At the surface of test-pieces quenched from 710°C and fatigued above 60°C, continuous slip activity and damage leading to crack-initiation were confined within narrow active bands. Below the surface the active bands formed straight channels, parallel to {112} and {110} slip planes in fatigue-hardened grains. Such channels, which were of very low dislocation density, evidently accommodated large-amplitude reversible dislocation movements. In quenched test-pieces, carbide precipitation occurred in the fatigue-aged matrix. Large-amplitude dislocation motion prevented precipitation in the active slip bands, but the edges and ends of the bands were sites of abnormal carbide growth. During prolonged cycling above about 60°C some precipitates grew large enough to bridge their associated slip bands, but they were too widely spaced to strengthen the band as a whole. It is concluded that active slip bands are not effectively strengthened by strain ageing during continuous cycling. Strain ageing strengthens the fatigue-hardened matrix and it must improve fatigue strength mainly by increasing the constraints imposed on regions of plastic strain concentration, particularly during stage I crack growth.

On strain ageing and the fatigue limit

Abstract Metallographic evidence, provided by a low-carbon steel that was strain-cycled under varying conditions of strain ageing, has shown that active fatigue slip-bands are kept narrow when strain ageing is rapid but, in general, dislocations within the active bands are not captured by solute-locking. It is concluded that, near room temperature, strain ageing must promote fatigue strength by strengthening the fatigue-hardened material that surrounds regions of strain concentration, so that the spread of fatigue damage is inhibited.