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Student paper

Development of a material-specific multiaxial failure criterion for short-fiber-reinforced thermoplastics

Bachelor’s thesis or master’s thesis

Develop a multiaxial failure criterion for short-fiber-reinforced thermoplastics using novel injection molding and testing methods
Stuwi-Entwicklung eines werkstoffgerechten multiaxialen Versagenskriteriums für kurzfaserverstärkte Thermoplaste© IKV
A novel injection moulding tool enables the production of highly oriented tubular test specimens made from fibre-reinforced thermoplastics (top right). This allows material characterisation in tensile-compression-torsion testing (left) to be carried out in a manner appropriate to the material. The strain on the specimen surface is recorded using video extensometry (bottom right).

Topic of the work:

Short-fiber-reinforced thermoplastics are characterized by excellent processing properties. Fiber reinforcement can significantly increase mechanical properties such as stiffness and strength, resulting in great potential to increasingly replace metals as a construction material in structural applications. In these fields of application, combined stresses often occur, which change the relevant strength parameters. The theories established to describe strength were mainly derived in the 1970s and are based on the manufacturing and production processes available for this class of materials at the time. However, current investigations show that these approaches can only approximate the strength behavior under combined stresses. As a result, high safety factors are often applied when designing components, so that part of the material potential remains unused. Recent developments in the working group enable a material-specific experimental investigation of the strength behavior of short-fiber-reinforced thermoplastics under combined stresses and thus create the basis for improved design practice.

The work is related to this research project:

The work takes up an open scientific question that deals with the failure mechanisms of short-fiber-reinforced plastics under combined stresses. In particular, the varying fiber orientation over the component thickness currently makes it difficult to reliably predict the failure behavior. This results in either premature component failure or material-intensive oversizing. The aim of the work is to investigate the failure experimentally and to create the basis for improved dimensioning.

Objective:

Based on preliminary developments, a prototype of an injection mould was developed that will make it possible in future to achieve a homogeneous, highly oriented fiber orientation distribution in tube specimens close to series production. The prototype of a test method was also designed in line with this, with which any uniaxial and multiaxial stress states can be set. The aim of your work is to transfer these two new processes into an integrated manufacturing and testing concept. For this purpose, pipe test specimens are first produced by injection molding and the homogeneous, highly oriented fiber orientation is ensured by specific process adjustments. This is followed by uniaxial material testing for the targeted induction of normal and shear stresses using the new test method. The resulting stress state is checked and the test system is technically adapted if necessary. Finally, combined tests are carried out to determine the multiaxial strengths.

Your task:

Familiarization with the new injection moulding and testing process Familiarization with the new injection moulding and testing process
Production of tubular test specimens using the injection molding process Production of tubular test specimens while ensuring the high orientation of the fibers on the process side
Uniaxial testing of pipe specimens in the tension-compression-torsion test Uniaxial testing of pipe specimens in the tension-compression-torsion test
Evaluation and interpretation of test data Multiaxial testing with the new test concept and strain measurement in the test area using video extensometry
Evaluation and interpretation of the test data and identification of the fracture pattern

Your profile:

  • Natural science or engineering degree (e.g. general mechanical engineering, industrial engineering, materials science, applied polymer science)
  • Independent and quality-conscious way of working
  • Interest in experimental work and scientific data analysis
  • Ideally initial experience in the field of mechanical testing

These are your benefits:

  • Learn scientific work in a motivated team with intensive supervision
  • Participate in the co-development of a new testing methodology and support its transfer to near-series testing with high industrial relevance
  • Build on existing prior experience and expand it in a targeted manner in an experimental environment under intensive supervision
  • Learn the systematic collection, evaluation and interpretation of experimental measurement data

Has this vacancy piqued your interest? Then get in touch with me by e-mail or phone and send me your application documents.