Steel laminates with highly improved fracture toughness

Roland Kasberger

Research output: ThesisDoctoral Thesis

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Abstract

The aim of this thesis was to manufacture and test inhomogeneous steel-based laminates with a high resistance against fracture. These laminates exploit the material inhomogeneity effect as main toughening mechanism, which occurs independently to other toughening effects like delamination. This effect is caused by the presence of an inhomogeneity in Young’s modulus or yield stress in front of a propagating crack. The basic mechanism is a strong reduction in the crack driving force, when the crack grows perpendicularly from a soft or compliant material into a stronger or stiffer material. This is also known as the shielding effect of the material inhomogeneity. The hot press bonding process was used to manufacture steel-based multilayers with excellent interfacial strengths, using cold-working tool steels X210CrW12 or C45 as matrix and deep-drawing steel DC04 as soft interlayer(s). An empirical relation is given, which gives an estimate of the local thickness reduction and, therefore, the resulting composite geometry. Composites with varying number and thickness of the soft interlayer(s), as well as reference matrix specimen without interlayers were manufactured. The improvement in fracture resistance caused by soft interlayer(s) was determined in fracture mechanics experiments. The reference specimens, which contained a single matrix/matrix interface each, fractured catastrophically after reaching a critical J-integral for the matrix, J_c^M. In specimen with one or more interlayers crack propagation initiated at J_c^M as well, but the propagating crack arrested inside the interlayer. The load, measured in terms of the J-integral, had to be increased to cause final fracture at a critical J for the composite, J_c^ML. The ratio J_c^ML/J_c^M is a measure of the effectiveness of the interlayer(s) as a crack arrester. In X210CrW12/DC04 composites, the improvement is dependent on the interlayer thickness. For thin interlayers, J_c^ML/J_c^M is in the order of 3-5, with thick interlayers the improvement is even higher. Numerical calculations are able to predict J_c^ML/J_c^M, when thermal residual stresses and the material inhomogeneity effect are taken into account. In C45/DC04 composites, the arrested crack was rendered completely ineffective by the onset of debonding with further loading. The remaining specimen cross-section then behaves like tensile specimen and fails by reaching the plastic limit load. Compared to X210CrW12/DC04 composites, the improvement J_c^ML/J_c^M is much higher and almost a factor 300. The significant improvement of the fracture resistance by the material inhomogeneity effect was demonstrated in this thesis. An interesting feature observed in all fracture mechanics tests is the cleavage fracture mode of the DC04 interlayers. Cracks were successfully arrested despite the brittle fracture mode.
Translated title of the contributionStahl-Schichtverbunde mit stark verbesserter Bruchzähigkeit
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Buchmayr, Bruno, Assessor B (internal)
  • Kolednik, Otmar, Assessor A (internal)
Publication statusPublished - 2018

Bibliographical note

embargoed until null

Keywords

  • fracture mechanics
  • material inhomogeneity
  • steel
  • composite

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