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Schlagworte
- Ductile cast iron (4)
- Dynamic crack resistance curve (3)
- Key curve (2)
- 'Taffy' pressure tank failure (1)
- Aircraft crash (1)
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- Chlorine liquefaction process (1)
- Compliance ratio method (1)
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Eingeladener Vortrag
- nein (8)
The finite element method was used to simulate the thermo-mechanical behaviour of a flip chip under thermocyclic loading between -55°C (-67°F) and 125°C (257°F). In order to simulate realistically the inelastic material response of the solder bump, Chaboche's viscoplastic material model instead of a classical ORNL-theory was implemented into the finite element code ABAQUS and applied to a flip chip configuration. This so-called unified constitutive model is able to describe the Bauschinger effect as well as the creep/plasticity interaction. Herewith the influence of the substrate material on the inelastic deformation behaviour of a tin-lead solder bump was analysed. A comparison is given with respect to the influence of the two substrate materials namely Al<sub>2</sub>O<sub>3 </sub> and our self-developed Low Temperature Co-fired Ceramics (LTCC) on the inelastic response of the Sn60Pb40 eutectic solder material. It may be shown that the thermoelastic behaviour of the LTCC substrate significantly reduces the cyclic stresses in the bump compared to Al<sub>2</sub>O<sub>3</sub> as well as the cumulative inelastic strain which is responsible for the development of damage and hence flip chip failure
A two-dimensional analysis of the Charpy V-notch specimen subjected to impact loading, according to the standard DIN EN 10045-1, is carried out, using a transient explicit dynamic finite element program. An elastic-viscoplastic, temperature dependent, constitutive relation for a porous plastic solid based on the Gurson damage model is developed. Ductile fracture of the matrix material will be described by the nucleation and subsequent growth of voids to coalescence. An updated Lagrange–Jaumann formulation is employed accounting for large strain and rotation. The discretization is based on four-node plane strain solid elements with one Gauss point. The equations of motion are integrated numerically by an explicit integration algorithm utilising a lumped mass matrix. The predictions of the numerical analysis in terms of force deflection response, crack resistance behaviour and deformation energy absorbtion are compared with results from Charpy tests which were carried out according to the low-blow technique.
Dynamic fracture mechanics characteristics are required for design and safety proof of advanced
ductile cast iron (DCI) components. In this paper, the analytical compliance ratio
(CR) and numerical finite element (FE) key curve methods were investigated for dynamic Rcurve
testing. Two DCI materials, a purely ferritic and a ferritic one with 18 % of pearlite,
were tested at room temperature (RT) and -40 °C. The special focus is on the impact of the
materials damage and fracture behaviour on performance and limitations of the investigated
methods.
Systematic metallographic and fractographic microstructural analyses were performed accompanying
the test program. Summarizing systematics of the specific damage behaviour
and fracture mechanisms were developed taking microstructure, loading rate, specimen geometry
and test temperature into account.
There has been a change from ductile to brittle fracture mechanism observed at -40 °C with
ferritic DCI and even at RT with ferritic-pearlitic DCI. The ferritic-pearlitic DCI did not show Rcurve
behaviour at -40 °C and only single Juc values could be determined. Furthermore, with
ferritic-pearlitic DCI, random pearlite shares in the ligament caused the relation between
loading rate, force-deflection record and crack extension not to remain uniquely defined.
Therefore, the FE key curve method proofed not to be applicable since the FE model was
based on homogeneous isotropic material and ductile failure mode only. The analytical CR
key curve method worked well only with selected combinations of microstructure, specimen
geometry and temperature.
In summary it has to be concluded that a robust, accurate and sufficiently simple dynamic
key curve method that was suited for application in quality control of industrial relevant DCI
qualities cannot be provided currently.
Experimentelle und numerische ERmittlung dynamischer Risswiderstandskurven im Kerbschlagbiegeversuch
(2001)
Die bruchmechanische Beurteilung von Komponenten erfordert die Kenntnis von Risswiderstandskurven, die jedoch häufig aufgrund fehlenden Probenmaterials nicht zur Verfügung stehen. Vielmehr sind typische werkstofftechnische Kennwerte verfügbar, wie Streckgrenze, Zugfestig, Gleichmaßdehnung, Bruchdehnung sowie die Kerbschlagarbeit in der Hochlage und die laterale Breitung von Kerbschlagbiegeproben.
Das Materialmodell von Gurson beschreibt duktiles Risswachstum mit den Stadien Hohlraumentstehung, Hohlraumwachstum und Koaleszenz benachbarter Hohlräume bis zum makroskopischen Riss. Die Materialparameter des Gurson-Modells sind abhängig vom Werkstoff und der Temperatur, jedoch nicht von der Probengeometrie und Belastungsgeschwindigkeit, was eine Übertragung zwischen quasistatisch belasteten Bruchmechanikproben unterschiedlicher Geometrie und schlagartig belasteten Kerbschlagbiegeproben erlaubt.
Im vorliegenden Beitrag wird eine Methode zur Konstruktion von Risswiderstandskurven vorgestellt, die auf den grundlegenden Daten des Zugversuchs und der Kenntnis der Fließkurve basiert. Die Fließkurve bei dynamischer Belastung wird durch erprobte Vorgehensweisen ermittelt, die auf der Analyse der dann erhöhten Aktivierungsenergie von Versetzungen basieren. Die Parameter der duktilen Schädigung werden im wesentlichen durch die Simulation von Versuchen an gekerbten Rundzugproben und des Kerbschlagbiegeversuchs bzw. der Anpassung an die Kerbschlagarbeit in der Hochlage bestimmt.
Damit sind die Parameter der duktilen Schädigung bekannt, so dass durch die Simulation duktilen Rissfortschritts in Bruchmechanikproben die erforderlichen J-Risswiderstandskurven zur Verfügung gestellt werden können, was die Anwendung des klassischen J-Konzepts erlaubt. Wegen der Geometrieunabhängigkeit der ermittelten Materialparameter ist auch die direkte Anwendung des Gurson-Modells zur Sicherheitsanalyse angerissener Bauteile durchführbar.
Investigations on the breakdown of a heat recovery steam generator during the initial operation run
(2014)
Several years ago, in 2003, an industrial heat recovery steam generator in charge of generating process steam in a petrochemical refinery was installed and prepared for initial operation. The steam generator enclosed an evaporator section and a superheater section consisting mainly of bundles of tubes with the longest up to several metres in length. During initial operation test runs severe leakages in the evaporator and superheater modules became noticeable. The test runs were stopped and after disassembly, through-wall cracks in several tube bends were found. BAM was commissioned to carry out the investigations in order to find out the reasons for the failure of the tubes. During on-site inspection a number of relevant damaged components and parts were selected and taken away to the laboratories for detailed inspection. Planned analyses were to comprise metallographic as well as fractographic investigations, mechanical fatigue testing and experimental as well as finite element vibration analyses on specimens and components. Soon, the fracture mechanism was found to be mechanical fatigue due to the fact that the examined fracture surfaces showed the very characteristic beachmarks and colouring patterns. To identify the particular loading and time at which crack initiation and crack propagation took place, experimental and numerical vibration analyses of specific tubes as well as mechanical fatigue tests on tube bends were carried out. Thus it was possible to identify the eigenfrequencies of individual tubes, to estimate the dynamic response as well as the nominal stresses and, hence, experimentally characterise the in-service fatigue strength of the components. Mechanical and thermal comparative tests on tube bends were performed simulating the conditions during the initial test run in order to get crack surfaces comparable to that of the originally damaged components. Thereby it became obvious that the fatigue cracks were initiated by vibrations the tube bundles were exposed to during rail transport from the manufacturers site to the place of installation. Based on these results, the damaged components could be repaired or exchanged without modification of the construction, but the rules relating packaging and securing for shipping had to be revised.
This paper summarizes the investigation of a serious explosion, which shattered the chemical facility of a chlorine/alkaline plant in Northern Germany in 1981. A pressure tank made from HSB 50 S steel and used as storage of liquid residues from a chlorine liquefaction process violently ruptured and the explosion threw parts of the tank approximately 100 m through the air. The local county government ordered an investigation to determine the root cause of this incident.
The main objective of the investigation was to determine whether a nitrogen overpressure or a chemical reaction of residuals inside the so-called 'Taffy' receiver caused the explosion. A fracture mechanics based analyses evaluated the conditions the material was exposed to during the damage. Metallographic and mechanical tests confirmed that the material specifications were within the values specified by the guidelines but corrosion had reduced the wall thickness considerably.
Analytical calculations to determine the minimum required pressure for the rupture and to cause plastic deformation to the tank confirmed that the failure did not occur due to nitrogen overpressure and therefore, a chemical reaction must have occurred. A detailed chemical analysis confirmed that the explosive disintegration of methylnitrate, which is highly sensitive to heat and impact, and its halogenated derivatives likely caused the incident. Due to the design and operational mode of the plant, impurities could accumulate and form explosive compounds in the Taffy receivers without control.
Damage analyses on two heat exchanger units showed that in both cases inappropriate flow conditions of media caused very different failure mechanisms that resulted in irreparable damage. The first incident was the breakdown of an unalloyed steel condenser, which operated in a coal-fired power plant. A considerably high number of tubes successively leaked. Metallography identified lines of segregation in the microstructure of the tube walls, thus, giving evidence that both uniform corrosion and erosion corrosion caused by low-pressure wet steam were the root cause. The second incident was the breakdown of a recuperator made from chromium–nickel steel due to mechanical damage to tubes and baffle. This unit operated as part of a pilot plant to regain heat from the drying process of sewage sludge. It turned out that soiled vapour caused clogging of the cross-sectional area and therefore accelerating the flow velocity of the vapour. This inappropriate operating condition caused the tubes to oscillate so severely that they even banged together. Abrasive wear especially at the intersection through the holes of the baffle damaged the tubes and the whole unit irreparably.
The finite element method was used to simulate the thermo-mechanical behaviour of a solder bump configuration on Al2O3 substrate under thermo-cyclic loading between -55 °C and 125 °C. Chaboche's viscoplastic material model was used to describe the Bauschinger effect and the creep/plasticity interaction for the tin-lead solder material. The influence of two different viscoelastic underfill materials (Epoxy+50 % SiO2, Epoxy+70 % SiO2) on the inelastic deformation of the tin-lead solder was analyzed and compared. It may be shown that the thermally matched 'hard' underfiller (Epoxy+70 % SiO2) significantly reduces the accumulated inelastic strain in the bump compared to 'soft' underfiller or no underfill material. On the other hand, the maximum lateral normal stress will increase slightly.
Schwerpunkte sind die Untersuchungen zur Fortführung der Analyse des Einflusses der Mehrachsigkeit des Spannungszustandes auf das Risswiderstandsverhalten, um damit das lokale stabile Risswachstum in Bauteilen besser verstehen und sicher beurteilen zu können sowie die Weiterentwicklung des Verfahrens zur Bestimmung von Risswiderstands - Kennwerten aus dem Kerbschlagversuch.
The successive fatigue failure process, i.e. crack initiation and growth up to electrical failure, has been investigated for a flip chip configuration. A cracked and uncracked solder bump was analysed using an unified viscoplastic constitutive model (Chaboche). The electrical failure of a bump seems to be reached in general not before a deep crack has grown into the bump. In order to update the actual loading situation two different cracked structures were calculated.