Untersuchung der transversalen Wärmeleitfähigkeit von CFK-Laminaten mithilfe der Transient-Hot-Bridge Technologie

Kurzfassung

As part of the project CarboTherm the Transient-Hot-Bridge (THB) method was used for the first time in order to measure the transverse thermal conductivity of fibrereinforced plastics. The sensors were laminated into the specimens. Measurements were also conducted on specimen-halves, in between which a sensor was clamped. The applicability of the THB-method on fibre-reinforced plastics was intended to be verified using the Guarded-Hot-Plate as well as the Laser-Flash method. Following this, the effects of different fibre volume fractions, laminate lay-ups, fibre types (glas-, PAN- and pitchfibres) as well as temperatures on the transverse thermal conductivity of fibre-reinforced plastics were to be investigated. During the first THB-measurements the measurment- and evaluation-software displayed the error message Bad Thermal Contact for almost all specimens. Experiences of the Physikalisch-Technische-Bundesanstalt, that provides the THB-equipment, suggested that a bad thermal contact between the sensors and the respective specimens were triggering the error message. Subsequently, the focus of this work drifted towards the assessment of causes and patching of the error message. Neither a digital-microscopical analysis on cross-sectional polishs of a specimen, with the sensor laminated within it’s lay-up, nor a quantification of the surface topography of the specimen-halves, in between which the sensor was clamped, provided any evidence for the presence of a bad thermal contact in the physical sense. An analysis of the source code of the THB-software revealed that a rigid bar was responsible for the reoccurring error message. Due to it’s constant ratio, with respect to the measurement time, the bar was misplaced for specimens with elevated values of thermal diffusivity and therefore falsely triggered the error message. The definition of the bar was improved whilst taking into account the thermal diffusivity of CFRPs. The modification of the source code was then verified on a reference material. By conducting a parameter study an ideal electric current intensity for measuring the thermal conductivity of the present CFRPs using the THB-method was determined. In addition, it was found that the measurement time has no impact on the result of the THB-measurement as long as the relevant range of measurement is covered. A comparison of the limited number of specimens suggests that the THB-method is indeed applicable on fibre-reinforced plastics. However, the value of the transverse thermal conductivity varies depending on whether the sensor is laminated into the specimen or clamped in between specimen-halves.