Martin Sperl

Evaluating the Integrity of Pressure Pipelines by Fracture Mechanics

Large engineering structures made with the use of sophisticated technology often include material defects and geometrical imperfections. These defects or imperfections do not exert their influence on the initial behaviour of structures designed in accordance with standard rules. Under the action of loading varying in time, however, they can reveal themselves in long-term operation by the initiation and growth of a fatigue crack from a defect root. Similarly, stress corrosion (SC) cracks can develop in a structure when there is an initial stress concentrator and the structure is exposed to both mechanical stress and a corrosion medium. A condition for the growth of a small fatigue crack is that the level of cyclic stress should be above the limit value given by barriers existing in a steel, and a condition for the growth of SC cracks is that the stress is greater than a certain limit value for a specific corrosion medium. It is important to pay due attention to the behaviour of cracks under various gas pipeline loading conditions in different environments, and to the influence of these conditions on the residual strength and life of the gas pipeline. The existence of a crack in the wall of a high-pressure gas pipeline mostly implies a shortened remaining period of reliable operation.

Creep damage index as a sensitive indicator of damage accumulation in thermoplastic laminates

This study aims to correlate a newly proposed local time-dependent indicator of the damage accumulation in a thermoplastic laminate matrix with the degradation of the mechanical properties of the material as a whole. The material tested here is a carbon fiber–reinforced plastic used in the aircraft industry. Isothermal fatigue transient tensile loading and three-point bending are used for full-scale tests. The results of fatigue pulsating tension loading of the laminate are presented together with the outcomes of nondestructive measurements of the residual stiffness of the laminate. Local elastic and time-dependent properties of the laminate matrix, used as nondestructive indicators of the degradation of the matrix caused by repeatedly applied loads, are compared with the full-scale outcomes. The results confirm that the microindentation technique applied to the laminate matrix can be a suitable method for assessing the influence of fatigue tensile loading on the basic and time-dependent properties of the laminate.

Fracture Toughness Testing for Improving the Safety of Gas Pipelines

For standard fracture mechanical tests flat specimens (principally CT or SENB) are required. When investigating fracture mechanical properties of thin – walled pipes this brings about a problem because it is necessary to straighten pipe bands. However, this operation causes internal stresses to be induced not only in the semi-product subjected to straightening but also in finished specimens. A question therefore arises to what extent are then the magnitudes of the fracture toughness determined representative for the actual cylindrical wall. To solve this problem fracture mechanics tests were caried out on flat (straightened) CT specimens as well as on curved CT specimens with the natural curvature. The R – curves as well as the resulting parameters of the fracture toughness, obtained for both types of CT specimens, were compared and it was concluded that the fracture toughness of the pipe material determined on straightened CT specimens was practically the same as that obtained on curved CT specimens.

Verification of the Trend of MPL Variation in Fatigue by Modern Methods

The paper presents experimental results concerning the variation of microplastic limit (MPL) during fatigue as determined by the measurement of changes in inductance L of specimens made from normalized low-C steel CSN 411375. Some indications have been obtained that MPL varies during fatigue with dislocation density. This was proved by nanoindentation, X-ray diffraction and transmision electron microscopy.

Local Viscoelastic Properties of a Thermoplastic/Carbon Laminate as an Indicator of Fatigue Damage

The paper is aimed at an assessment of influence of damage accumulation in a thermoplastic/carbon laminate during a tensile cycling loading on durability, Young modulus and both instant and time-dependent local mechanical characteristics of a thermoplastic matrix, analysed by means of instrumented microindentation. Standard mechanical data from three-point bending are compared with results of the local microindentation testing.

Utilizing electromagnetic properties to determine the micro-plastic limit of low-carbon steel CSN 411375

It is generally accepted that not all cyclic stresses below the fatigue limit are non-damaging. Particularly in spectrum loading, the inclusion of some cyclic stresses below the fatigue limit can reduce the fatigue lifeof acomponent. It is believed that the boundary between damaging and non-damaging stresses is the so-called micro-plastic limit (MPL) defined as a macro-stress at which dislocation pile-up stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress. This paper shows that MPL can be determined from changes in magnetic permeability during tensile loading. In our work, changes in magnetic permeability were measured indirectly - by measuring changes in electrical impedance (a.c. resistance and inductance). Measurements were performed on normalized low-C steel CSN 411375, and the microplastic limit was determined by evaluating the Δ R- σ and Δ L- σ records.