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Practical examples from various industries

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Media attack

for fiber-reinforced pump housings

Picture: left: Crack on the inner surface of the pump; right: Thin section in the cross-section of the thread IKV

Problem: Cracking of pump housings

The housing cover (PP-GF) of a circulation pump developed cracks after a short operating time of approx. 2 weeks, which became apparent due to the resulting leaks. The pump was used to pump alkaline media in the chemical industry (KOH up to 5% and water at 85°C). The pump was an established product that had already been used successfully in other areas. However, its previous use was limited to pumping hot water without the addition of other chemicals. With the help of suitable investigations, possible causes for the crack formation were to be determined.

Methodology: Microscopic examinations

  • Macroscopic and microscopic defect image documentation
  • Microscopic microstructure analysis in the area of cracks in the component
  • Scanning electron microscopic (SEM) examination of the component surface and the exposed crack surfaces, combined with energy-dispersive X-ray spectroscopy (EDX) to identify contained filling and reinforcing materials.

Result: Insufficient processing quality and media attack of the fibers

  • A pronounced weld line and numerous cavities were found in the area of the pump thread. The weld line coincided locally with the crack formation and media-induced material discoloration was found here.
  • The surface examination in the SEM no longer revealed any glass fibers in the damaged area. These were obviously dissolved and left numerous grooves in the polymer in which the fibers were previously embedded.

  • Picture left: Surface with dissolved fibers due to media attack; right: surface with fibers (component only tested with water at 85°C) IKV
  • A media attack on the glass fibers was also evident on the exposed crack surfaces. Core material areas without previous media contact, on the other hand, showed intact glass fibers. The media attack on the glass fibers could be attributed to the alkaline medium:
  • The main reason for the damage was therefore an inadequate definition of the pump’s requirements with regard to suitable pumpable media. The numerous cavities in the material also facilitated media absorption into the material, as did the inadequate mold filling on the component surface.
  • Images: damaged glass fibers under an electron microscope IKV

 

Fatigue

Determination of mechanical load limits

Fatigue of end-loaded series components IKV

Problem definition:

In order to characterise dynamically stressed plastics with regard to their fatigue properties, standardised Wöhler tests according to DIN are available on the one hand, but to date these are only intended for metallic materials and mean a long test time for plastics due to their strong temperature dependence. On the other hand, the load increase method offers a significant reduction in the number of test specimens required as well as a considerable time saving, but this method is not standardised. The two methods are presented in more detail below using an example. The aim of these tests is to determine the dynamic load limit at which the plastic can be loaded an infinite number of times without failure.

Methodology: Single-stage/multi-stage fatigue test

Figure 1: Schematic representation of a Hydropulser in a tensile test IKV
  • Characterisation of a PA 12 by means of a fatigue test on servo-hydraulically operated hydropulsers (Fig. 1).
  • In the fatigue test according to Wöhler (DIN 50100), test specimens are loaded at appropriately staggered load levels (see Figure 2, Part 1), the number of cycles at break for 5 to 10 test specimens is noted and plotted in the Wöhler diagram.
  • In the multi-stage fatigue test, also known as the load increase method, a test specimen successively passes through an increase in the medium and upper load after a defined number of load cycles, so that the load limit is displayed in the form of a critical stress/strain in the response signal, see Figure 2 Part 2.

    Figure 2: Single-stage and multi-stage vibration tests IKV

Result: Determination of critical load limits in the multi-stage vibration test

Figure 3 shows the result of a load increase test for the PA 12. The critical strain and the associated critical stress are derived from the strain/vibration cycle diagram using regression analysis. The pairs of values determined for the five test specimens examined are shown in Table 1.

Table 1: Critical stress derived from the multi-stage vibration test using regression analysis IKV

From the regression analysis, an average of 12 N/mm² is determined as the critical load limit. The load level before this is considered subcritical at 11.5 N/mm². This corresponds to around 24 % of the quasi-static tensile strength (48 N/mm²). Evaluation tests at 11.5 N/mm² in the fatigue test showed no premature sample failure up to 10 million load cycles.

Figure 3: Strain/vibration cycle diagram as a result of the load increase test IKV

Depending on the material, a load limit can thus be reliably determined in a shorter test time using the load increase method.

Investigation

of LED boards

Detachment of the silicone optics IKV

Problem: Detachments on LED boards

As part of a research project at IKV, LED circuit boards used in automotive headlights were provided with silicone optics using an LSR injection molding process. Despite constant process parameters, detachment of the silicone component in the area of the LED chip occurred on certain PCB batches, which meant that the desired light scattering was no longer achieved. In addition, cloudy streaks were sometimes observed in the silicone optics. Appropriate investigations should help to determine the cause of the detachments.

Methodology: Microscopic examinations

  • Macroscopic and microscopic defect image documentation
  • Laser confocal microscopic examination of conspicuous areas
  • Field emission scanning electron microscopic (FESEM) examination of cross-sections through the damaged area of affected lenses in combination with energy dispersive X-ray spectroscopy (EDX)

Result: Impurities and batch-dependent material fluctuations in the LED boards

In the area of the detachments, foreign particles were detected on exposed components both on the silicone optics and on the circuit boards. Fluid-like contamination was observed in the immediate vicinity of the LED chips. Cloudy streaks in the silicone optics obviously have their origin on the LED board due to the course of the streaks. Light microscopic examinations of other LED boards showed contamination on new parts as a result of inadequate transport protection.

Transverse micrograph through chip (FESEM) IKV

The streaking probably takes place during the injection molding process and is probably due to an interaction between the silicone compound and the embedding compound of the LED chips The EDX examination confirmed the use of different materials in the area of the LED chips. The detachments could be attributed to a batch in which components made of alternative materials were used. Germanium and silicon were used in the inconspicuous batch, while the batch with detachments was made of silicon and magnesium. With the help of these investigations, the causes of the defects could be eliminated immediately.