PhD Position

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Eng_Hor_Blue_TECHNION

Crack-flaws interactions in brittle materials under dynamic loading

Supervisor: Professor Shmuel Osovski
Synopsis:

Brittle (or quasi-brittle) materials are frequently used for impact-loaded components. Dynamic crack growth in brittle materials, is largely dominated by the interaction of the growing crack with preexisting flaws, such as voids and micro-cracks, as well as the evolution of the later under the applied load and the interactions among them. The evolution of pre-existing flaws, as well as the creation of
new ones, was proposed to be responsible for a large portion of the excess energy dissipation in the dynamic case compared to quasi-static fracture. Micro-branching, which result from crack tip instabilities were also reported to act as an additional energy dissipating process in brittle fracture of fast growing cracks. In this regard, an open question remains: what is the effect of the initial population
of flaws, in terms of size and spatial distributions, on the dynamic fracture process? The goal of this research is to study the interaction of a dynamically loaded mode I crack with an existing distribution of flaws (damage). Using sub-surface laser engraving techniques, we will generate controlled distribution of voids and microcracks ahead of the initial notch in a dynamic mode I specimen. The specimen, recently developed by the TEHCNION-team, allows for the propagation of a mode I crack under a large range of loading rates and large distances (up to several cm). By measuring the roughness of the obtained fracture surfaces, we aim at identifying the correlation between the properties of the microstructural features (microvoids, microcracks) and the statistics of the fracture surface. The set of experimental results produced here will serve to understand the correlation between distribution of flaws and toughness as well as fracture surface roughness, thus providing a tool not only for calibrating brittle damage/fracture models but also for their validation and evaluation.

Research outputs:

A new experimental method to study the interactions between a growing crack in brittle media and the field of flaws spread ahead of it. The experimental observations are expected to yield a correlation between dynamic facture toughness, initials microstructure and fracture surface roughness. A reexamination of existing models for dynamic crack growth in brittle materials will be conducted, and their sensitivity will be evaluated based on the models ability to produce the right crack path and fracture surface roughness. Finally, specific flaws fields will be generated to demonstrate the ability of engineering a structure to direct a growing crack a way from regions which are desired to remain damage free.

Multidisciplinary/ intersectoral research approach:

The ESR will develop at the TECHNION an experimental campaign of unprecedented magnitude to show the interplay between the propagation of cracks in brittle media and the material flaws. Moreover she/he will conduct a secondment at the University of Lorraine where she/he will develop a model to describe the observed experimental results on the interaction between flaws and a dynamic crack.
Additionally the ESR will conduct a secondment in Spain, where he/she will visit the University Carlos III of Madrid and Airbus Aerospace & Defence. At the University Carlos III of Madrid she/he will implement the proposed damage model into a commercial Finite Element code (ABAQUS) for purposes of comparing the predictive power of the model with respect to the experimental data. In Airbus Aerospace & Defence she/he will identify, analyze and model (real) practical applications in which material defects cause the development of damage and subsequent failure of aerospace (brittle) structural materials.

Training activities:

The successful candidate will have access to the PhD program of the TECHNION as well as to the training activities organized within the OUTCOME consortium (see here for more information).

Benefits:

The successful candidate will be employed for 3 years and receive a financial package plus an additional mobility and family allowance according to the rules for Early Stage Researchers (ESRs) in an EU Marie Skłodowska-Curie Actions Innovative Training Networks (ITN):

  • Living allowance – 3380.57€ (per month).
  • Mobility allowance – 600€ (per month).
  • Family allowance – 500€ (per month – if applicable).

This amount is a gross contribution to the salary costs. Net salary will result from deducting all compulsory social security/direct taxes from the gross salary according to the law applicable to the agreement concluded with the ESR.

Additional information about the funding provided by the ITN  projects can be found here.

Key publications:

Ravi-Chandar K., Yang B. On the role of microcracks in the dynamic fracture of brittle materials. Journal of the Mechanics and Physics of Solids. 1997; 45: 535-563.
Rittel D., Maigre H. An investigation of dynamic crack initiation in PMMA. Mechanics of Materials. 1996; 23: 229-239.
Ravi-Chandar K., Knauus W.G. An experimental investigation into dynamic fracture: II. Microstructural aspects. International Journal of Fracture. 1984; 26: 65-80.
Osovski S., Srivastava A., Ponson L., Bouchaud E., Tvergaard V., Needleman A. 2014. Effect of loading rate on ductile fracture roughness and toughness. Journal of the Mechanics and Physics of Solids. 2014; 76: 20–46.
Osovski S., Srivastava A., Williams J. C., Needleman A. Grain boundary crack growth in metastable titanium β alloys. Acta Materialia. 2015; 82: 167-178.
Shukla A. Interaction of an explosively driven crack with a large flaw. ICF6, New Delhi (India) 1984.
McCauley J.W., Strassburger E., Patel P., Paliwal B., Ramesh K. T. Experimental Observations on Dynamic Response of Selected Transparent Armor Materials. Experimental Mechanics. 2013; 53:3-29.
Hu G., Liu J., Graham-Brady L., Ramesh K.T. 2015. A 3D mechanistic model for brittle materials containing evolving flaw distributions under dynamic multiaxial loading. Journal of the Mechanics and Physics of Solids. 2015; 78: 269-297.

Profile

We are looking for highly motivated early-stage researchers with the following profile:

  • Hands-on mentality, good organizational and communication skills.
  • Proactive attitude and ability to work both independently/autonomously and within a team.
  • Excellent communication skills in English.

In order to meet the specific requirements of the Marie Skłodowska-Curie funded PhDs, you must not have resided or carried out your main activity (work, studies, etc.) in Israel for more than 12 months in the last 3 years. You may be of any Nationality.

Required educational level:

Degree: Master degree or equivalent
Degree field: Engineering: civil, mechanical, aerospace
Degree: Master degree or equivalent
Degree field: Physics

Career stage:

Early stage researcher or 0-4 years (Post graduate).

Professional and/or research experience:

We will particularly consider those candidates with proven experience in technological and/or research activities. Publication/s in journals indexed in the Journal of Citation Reports will be especially welcomed.

Letter of motivation:

The candidates must provide a letter of motivation where they clearly state why, under their point of view, they should be enrolled in OUTCOME.

References:

At least, one recommendation letter from the scientist/s who mentored the candidate during her/his master studies is required. The letter must clearly expose the profile of the candidate with emphasis in the qualities which make her/him suitable for being recruited in OUTCOME. Additional recommendation letters from any other professor/professional will be most welcomed.

Specific qualifications:

Candidates should have a solid background in Solid Mechanics, Experimental Mechanics, Fracture Mechanics and Numerical Modeling.

Flexible working conditions:

We are committed to provide flexible hours and home working conditions for researchers having family obligations. The following web-site contains relevant information related to the EU equal opportunities policy.

Moreover, the web-site  facilitates geographic mobility by providing help to find a job for an accompanying partner.

The application period is closed.
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