TU Delft Laboratory for Mechanical Reliability
Research Projects: FATIGUE

Fatigue of short glassfibre reinforced Plastics

Jaap Horst

Objective

Goal of the research is to find a way to predict the fatigue behaviour of a part injection moulded from this material, especially in relation to the complex structure inside such a part, due to the orientation of the fibres, which can change from place to place in the injection moulded part.

Methods

Various methods have been used, which can be put into two classes: Experiments for the determination of the fatigue behaviour as related to the fibre orientation, in both crack growth [3,5] , as well as lifetime measurements [3,5,6,7].
To be able to understand the fatigue behaviour, experiments were executed to determine the fatigue fracture mechanism [6,8]. SEM fractography was done [8], also on cryogenically broken, fatigued specimens (to be published). Strength profiles after fatigueing have been made, using microtomed foils, of which the strength was determined [3,4,6,7,8]. Furthermore the fatigue lifetime experiments were monitored, giving information about the development of the Elastic Modulus, the creep and the energy dissipation during the experiment [4,5,6,8]

experimental

The fatigue experiments were carried out on a servo-hydraulic MTS 810 bench. The load frequency used was 1Hz, to avoid unacceptable temperature increase due to hysteretic heating and thermal failure of the specimen. Earlier experiments [5] showed the high sensitivity of the fatigue lifetime of this particular material to the test frequency, caused by hysteretic heating as a consequence of the high damping of the material. This is a consequence of the water absorption of the specimen, which lowers Tg to approximately experiment temperature. During the experiments the temperature at the surface of the specimens was measured using an infrared contactless thermometer. The minimum to maximum load ratio R was 0.1.

material

The material used was Polyamide 6 containing 30%wt. of glassfibres; Akulon K224- G6, provided by DSM, the Netherlands. Square plates of 100x100mm2 and 5.75mm thickness were injection moulded from this. The mould was injected through a line gate, to obtain a straight flow front. For fatigue and tensile experiments non standard dog-bone type specimens were milled from the plates.

Results and Conclusions

The fatigue behaviour is directly proportional to the tensile strength (UTS) of the specimen (with a certain fibre orientation), leading to a "Master curve" for fatigue [5,6].

Using SEM fractography, microductile and microbrittle areas of the fatigue fractured surface can be seen, the fatigue respectively the final fracture area of the fracture surface. On specimens that were fatigued first, but not until failure, and then fractured after emerging in liquid nitrogen, the plastic deformation resulting from fatigue, could be made visible. Depending on the degree of fatigue damage, voids at the fibre ends can be seen, or cracked areas (with lots of plastic deformation). In between the cracked areas however, brittle (cryogenically) fractured areas could be seen, where the crack walls were still connected.

Strength profiles showed the damage done by fatigueing, damage can be either general, over the entire thickness, or local, in certain layers of the specimen thickness. The location where damage develops is mainly dependent on the distribution of the layers through thickness [3,4,6,7,8].

The Creep Speed (Vc) correlates surprisingly well with lifetime (N), making it possible to predict the lifetime accurately. It is a good tool to deal with the scatter in those kinds of experiments where knowledge of the lifetime is needed before the specimen fails [5,6,8].

The fact that Vc correlates with N indicates that the variation in the lifetime of the specimens is determined by differences in the specimen that are of global character (aspects of the specimen as a whole), as creep is a global parameter. Consequently local variations (eg. scratches, notches or voids) have a small influence on lifetime.

The relationship between log Vc and log N holds for the entire load range from 0.8UTS down to 0.45UTS, not just for each load range apart. From this we can conclude that no change in failure mechanism takes place over the load range examined.

Failure Mechanism

The following failure mechanism in fatigue is presented [6,9], based on and explaining the experimental results;
  1. Initiation of damage at the fibre ends.
  2. Growth of this damage into voids, accompanied by debonding.
  3. The voids grow into microcracks, which may remain bridged by either drawn matrix material or unbroken fibres.
  4. The debonding relieves the constraint to which the matrix was subjected, which can therefore deform much more easily, forming bridges between the crack walls.
  5. The bridged crack grows, until a critical size is reached, and the specimen fails.

Literature

What you can find here is a list of all the literature concerning Composites that I have read, in text form. This list is very rude, contains the titles and authors. The list is searchable using the find option of your browser, thus making it possible for researchers to find from the list articles they might be interested in.

Publications

  1. Horst J.J.
    Literatuurstudie: Mechanische eigenschappen en vermoeiing van kort-glasvezelversterkte thermoplasten. Intern rapport LMB nr. K-272, TU Delft, February 1993.
  2. Horst J.J.
    Rapport vooronderzoek: Mechanische eigenschappen en vermoeiing van kort-glasvezelversterkte polyamide. Extern rapport LMB voor Akzo en DSM, TU Delft, April 1993.
  3. Horst J.J.
    Investigations report: Changes in Glassfibre reinforced Polyamide, due to fatigue loading. External report K298, TU Delft, December 1993.
  4. Horst J.J.
    Changes in glassfibre reinforced injection moulded polyamide due to fatigue loading. Conference papers of the 9th Int. conf, on Deformation, Yield and Fracture of Polymers. Churchill College, Cambridge, UK, 11-14 April 1994. paper p69.
  5. Horst J.J.
    Fatigue of fibre reinforced injection moulded Polyamide. Proceedings of the 10th Biennial European Conference on Fracture, ECF- 10, Berlin, 20-23 September 1994, pp. 1187-1192.
  6. Horst J.J.
    Determination of Fatigue Damage in short Glassfibre reinforced Polyamide, proceedings of the 3rd International Conference on Deformation and Fracture of Composites, 27-29 March 1995, Surrey, UK. pp. 473-482.
  7. Horst J.J.
    A method for detecting damage in Glassfibre reinforced Thermoplastics, using thin microtomed films, proceedings of the 7th International Conference on Mechanical Behaviour of Material, 28 May - 2 June 1995, the Hague, pp. 247-248.
  8. Horst J.J., Spoormaker J.L.
    Mechanisms of Fatigue in short Glassfibre reinforced Polyamide 6.
    Polymer Engineering and Science 36, No.22, November 1996, pp. 2718 - 2726.

  9. Horst J.J.
    Fatigue fracture mechanism of fibre reinforced injection moulded polyamide
    Localised Damage 96 conference of June 96 in Fukuoka, Japan
  10. Horst J.J., Spoormaker J.L.
    Fatigue fracture Mechanisms and Fractography of short Glassfibre reinforced Polyamide 6.
    Journal of Materials Science 32, 1997, pp. 3641-3651.

  11. Horst J.J., Salienko N.V., Spoormaker J.L.,
    Fibre-matrix debonding stress analysis for short fibre reinforced materials with matrix plasticity, finite element modeling and experimental verification.
    Composites part A 29, 1998, pp. 525-531

  12. Horst J.J.
    Influence of fibre orientation on fatigue of short glass fibre reinforced Polymide.
    Delft, 1997, ISBN 90-407-1532-7 / CIP.
    My dissertation, still a few copies are obtainable from the author.
(c) author of this page. Last modified on August 21, 2004.
Delft University of Technology
Faculty of Industrial Design Engineering
Laboratory for Mechanical Reliability