The Experts below are selected from a list of 12 Experts worldwide ranked by ideXlab platform

Theodore Nicholas - One of the best experts on this subject based on the ideXlab platform.

  • load history dependence of fatigue Crack growth thresholds for a ti alloy
    Engineering Fracture Mechanics, 1998
    Co-Authors: Y N Lenets, Theodore Nicholas
    Abstract:

    Abstract Fatigue Crack growth thresholds for Ti-alloy IMI 834 were determined at Kmax values equal to 6, 10 and 25 MPa m , using two different load patterns, both including an abrupt change in ΔK. Increasing ΔK applied to the initially Dormant Crack resulted in higher fatigue Crack growth thresholds as compared to the situation when an initially growing Crack was arrested by decreasing ΔK values. The discrepancy can be attributed to the Crack tip shielding associated with residual stresses in front of the Crack rather than with fracture surface contact behind the Crack tip. Practical merits of the two experimental techniques are discussed.

Herbst Mgj - One of the best experts on this subject based on the ideXlab platform.

  • Effect of Chloride on Environmentally Assisted Cracking of Low Alloy Steels in Oxygenated High Temperature Water
    2026
    Co-Authors: Herbst Mgj
    Abstract:

    The aim of this thesis was to derive a better understanding with regard to the effects of chloride on the general corrosion behaviour of low-alloy steels (LAS) in oxygenated high-temperature water (HTW) and to investigate the underlying mechanisms for Crack initiation and propaga-tion due to chloride assisted environmentally assisted Cracking (EAC). Therefore, systematic investigations on the effect of chloride on the EAC behaviour of LAS were performed to un-derstand and elucidate the underlying mechanisms.The overall thesis is divided into three parts focussing on the effect of chloride on:i) general corrosionii) Crack initiation, andiii) Crack growthof low-alloy steels in oxygenated high-temperature water.Studies on the effect of chloride on the general corrosion behaviour were performed by immer-sion tests that were evaluated using electrochemical monitoring techniques and different post-test investigation methods like SEM, ToF-SIMS, and others. From the performed investiga-tions it is concluded that the presence of small amounts of chloride in oxygenated HTW causes an incorporation of chloride into the oxide layer, a thinning of the oxide layer thickness, and pronounced pitting.The Crack initiation susceptibility of LAS was investigated using CERT tests. These tests showed an increased number of Crack initiation locations and a decrease of the elongation at fracture with increasing chloride concentrations.Crack growth rate tests clearly demonstrated that not the increase in the chloride concentration per se, but the conjoint occurrence of an active or Dormant Crack and increased chloride con-centration causes an increase in the observed Crack growth rates.For practical applications of LAS in oxygenated HTW this means that short term transients seem to be not harmful regarding component integrity, but long term increased chloride con-centrations should by prohibited since they cause increased general corrosion of LAS. Taking Crack initiation and Crack growth into consideration, the conjoint occurrence of increased chlo-ride concentrations and mechanical straining at stress levels above the yield strength should be avoided

Y N Lenets - One of the best experts on this subject based on the ideXlab platform.

  • load history dependence of fatigue Crack growth thresholds for a ti alloy
    Engineering Fracture Mechanics, 1998
    Co-Authors: Y N Lenets, Theodore Nicholas
    Abstract:

    Abstract Fatigue Crack growth thresholds for Ti-alloy IMI 834 were determined at Kmax values equal to 6, 10 and 25 MPa m , using two different load patterns, both including an abrupt change in ΔK. Increasing ΔK applied to the initially Dormant Crack resulted in higher fatigue Crack growth thresholds as compared to the situation when an initially growing Crack was arrested by decreasing ΔK values. The discrepancy can be attributed to the Crack tip shielding associated with residual stresses in front of the Crack rather than with fracture surface contact behind the Crack tip. Practical merits of the two experimental techniques are discussed.

Repplinger Christian - One of the best experts on this subject based on the ideXlab platform.

  • FATIGUE LIFE OPTIMIZATION OF A HIGH-PRESSURE LOADED ALUMINUM VALVE BODY AND LEAKAGE INVESTIGATION FOR ELASTOMERIC SEALS AT LOW TEMPERATURE
    University of Luxembourg ​​Luxembourg, 2021
    Co-Authors: Repplinger Christian
    Abstract:

    High-pressure hydrogen storage systems for fuel cell vehicles require a safe, compact, and light-weight design, especially the on-tank valves (OTV). These valves are directly connected to the high-pressure tank and manage the filling process of gaseous hydrogen up to a nominal pressure of 700 bar, the storage of the high-pressurized gas in the tank, and the supply of the fuel cell. The OTV enclosure implements several different technical devices, which are mostly sealed with radial elastomeric O-rings within one complex aluminum valve body. This work’s first part analyzes and optimizes the fatigue life of the aluminum alloy (EN AW 6061 T6) OTV-body by defining the optimum autofrettage pressure before identifying the most significant effects influencing the sealing behavior of high-pressure loaded elastomeric O-rings at low temperatures in the second part. Both parts are of great benefit for safe product development considering the challenging technical requirements, such as a maximum pressure of 1050 bar and a temperature range of +85 °C to -60 °C, needed for the application of the OTV in a fuel cell vehicle. The aluminum valve body includes several channels and bore intersections which are pressure-loaded in operation. This complex untreated valve geometry does not achieve the technical requirements of 150 000 pressure cycles without a failure. Determining the optimum autofrettage pressure for this complex aluminum valve body enables an improvement of the lifetime for this internally highly pressurized component. The autofrettage process induces residual compressive stress after the release of a single static overload pressure, leading to plastic deformation at the inner wall, whereas the outer pulsating operating pressure range. Due to the complex geometry of the aluminum valve body, a detailed elastic-plastic finite element analysis is used to determine the optimum autofrettage pressure. Three load steps are simulated in a non-linear way based on experimental stress-strain curves. The FKM-guideline is used to assess fatigue life and Crack initiation with detailed subsequent experimental verification. Even if small Cracks occur, residual compressive stresses prohibit Crack growth (non-propagating or Dormant Crack). This is analytically verified by fracture mechanical considerations (Crack closure effect) and is proven via internal fatigue pressure testing up to 500 000 load cycles. Crack propagation is analyzed by optical inspections with a microscope, computer tomography, and numerical determination. The autofrettage process intentionally induces residual compressive stresses. Relaxation of these residual stresses due to cyclic loading in service would endanger the effectiveness of autofrettage and could ultimately lead to unexpected fatigue failure. Therefore, strain-controlled experiments up to 500 000 load cycles and amending non-linear finite element simulations are done for the aluminum alloy EN AW 6061 T6 to study potential cyclic stress relaxation in four-point bending tests after controlled single static plasticization for residual stress generation. The elastomeric O-ring seals must ensure functionality at high-pressure and a wide range of temperatures. The elastomeric material`s performance is especially limited at low temperatures. Geometrical and material effects are analyzed and assessed by numerical simulations and experiments. An accurate material model is necessary to present the complex material behavior and its influences on the sealing behavior. Therefore, an elastomeric seal material modeling guideline is developed to present the most significant effects. The mechanical material behavior of elastomers depends on time, temperature, and pressure. A thermo-rheologically simple (TRS) visco-hyperelastic material model is defined with the time-temperature superposition principle (TTSP) and used for the finite element simulations. This material model is validated with several material tests. The numerical analyses are especially useful in highlighting the individual influences of machining tolerances, different thermal expansion coefficients, limited recovery, and stress relaxation of the elastomer. The appearance of compressibility or volume swelling and their impacts on the sealing behavior are also explained. Experimental leakage tests are done for several O-ring dimensions from -60 °C to +23 °C up to a maximum pressure of 970 bar. The effect of machining tolerances and the necessity of a back-up ring are analyzed in a first test session. A second test session compares a relatively thin O-ring to a thicker one and investigates the sealing behavior of different geometries of the gland and the back-up ring. These geometrical design optimizations lead to a clear improvement of the sealing behavior

Cook J. - One of the best experts on this subject based on the ideXlab platform.

  • Acoustic emission detection of fatigue Crack propagation in a power station steam chest environment
    Cranfield University, 1997
    Co-Authors: Cook J.
    Abstract:

    This thesis addresses the problem of detecting and positively identifying the approximately known acoustic emission signatures produced through fatigue Crack propagation in power station steam chests. This work includes extensive laboratory fatigue testing to produce and record signatures in specimens fabricated from the steam chest steel, on-site recording of the ambience noise levels from a fully operating oil-burning power station and develops and demonstrates the effectiveness of various signal processing techniques at extracting the signatures embedded in the noise. This noise is high amplitude, giving us a low signal to noise ratio, and is broad in the frequency domain, with both regular and irregular high-amplitude metallic noise transients that cover the entire frequency range of interest. It is therefore essential to use sophisticated signal post-processing techniques to detect and to identify the Crack signatures. The post-processing techniques developed and employed include time-frequency transformations, matched filters and signal expansion filters implemented in both in the time domain and in various two-dimensional time-frequency domains. From a performance comparison, both on the experimentally recorded data and on data digitally generated for the purpose, we determine the optimum signal processing method for our requirements and provide an assessment of the relative computational efficiencies. Generated for comparison are spurious but similar signatures characteristic of the power station steam chest environment; oxide crushing within an existing Dormant Crack and stress corrosion Cracking signatures produced by the same steel constantly loaded in a corrosive environment. It is demonstrated that there is sufficient distinction between these signatures and those produced by Crack propagation