Reinhold W. Lang

Nanometer scale characterization of polymer films by atomic-force microscopy

Atomic-force microscopy was applied to perform a comprehensive surface roughness characterization of commercially available, highly transparent polymer films for transparent insulation applications. The morphological characterization included evaluation of the root-mean-square roughness, the lateral correlation length of roughness as well as the roughness exponent. In addition, high-resolution scans have been recorded yielding morphological information on a length scale of only a few ten nm. These measurements revealed the correlation between surface nanostructure and fabrication technique. For an impact-modified polymethylmethacrylate film with rubber inclusions phase images clearly uncovered the core-shell structure of these inclusions.

Optical properties of polymer films for transparent insulation

Commercially available, highly transparent polymer films for transparent insulation applications were investigated systematically as to their relevant optical properties in the solar and infrared wavelength range. The photometric characterisation in the solar range and the calculation of non-spectral, solar optical film properties using models for scatteringabsorbing media have shown, that the solar extinction is dominated by scattering occuring mainly at the surface. For various amorphous and semicrystalline films the root-mean-square surface roughness correlated well with the solar optical thickness. Regarding high infrared absorptance in the wavelength range of about 10 μm the carbon-oxygen single bond is highly effective for commercial materials with maximum service temperatures of about 100 °C. For 50 μm thick films of different polymer types with carbon-oxygen single bonds in the molecular structure a good correlation between the concentration of the functional corbon-oxygen group and the non-spectral, infrared optical thickness was found.

A correlation of creep and fatigue crack growth in high density poly(ethylene) at various temperatures

The creep crack growth (CCG) and fatigue crack growth (FCG) behaviour of two PE-HD pipe grades was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG-tests were performed under a sinusoidal load at a frequency of 1 Hz and R-ratios (Fmin/Fmax) of 0.1, 0.3 and 0.5; the test temperatures were 23 (only FCG), 60 and 80 °C. The results showed that FCG rates in PE-HD are caused by a combination of cyclic-induced and creep-induced damage, depending on the mean stress level. While for given values of KImax (FCG tests) and KI (CCG tests), respectively, at low test temperatures the cyclic component of the applied stress dominates crack growth rates with CCG rates (R = 1) being lower than the FCG rates (R < 1), at high test temperatures the creep component becomes increasingly important in affecting crack growth rates so that CCG rates even exceed FCG rates. The point of inversion from fatigue to creep dominated failure on the temperature scale apparently depends on molecular and morphological characteristics of the PE-HD type and occurs at around 80 °C for PE-HD 1 and around 60 °C for PE-HD 2 in this investigation.

Characterisation of the thermo‐oxidative degradation of polyethylene pipes by chromatographical, rheological and thermo‐analytical methods

In this work the influence of three different stabiliser systems on the stress rupture behaviour of high density polyethylene (PE-HD) pipes under constant internal pressure was examined at 60 and 80 °C with special consideration of the quasi-brittle failure by growth of a single crack in the failure regime preceding the global chemical degradation of the pipes. It could be proven that the pipes of the three formulations did not show global molecular and morphological differences in this failure regime and that the stabilisation was still intact. Therefore, the differences in failure times observed for the three formulations are believed to be a result of local ageing around the crack tip related to the combined influence of time, the elevated temperature, the presence of oxygen and water, and the high mechanical stresses in the immediate crack tip region.

Fracture of poly(vinylidene fluoride): a combined synchrotron and laboratory in-situ X-ray scattering study

Semi-crystalline polymers show a complex fracture mechanism, which is controlled by the micro-mechanisms associated with formation and breakdown of a plastic deformation region. Such regions develop at notches, cracks or other stress-raising defects. In the present paper, we use time-resolved synchrotron X-ray scattering techniques during the deformation process in poly(vinylidene fluoride) to study the plastic zone formation and fracture processes at different strain rates. This gives new insight into the micro-mechanisms of cavitation, lamellae separation and fibril formation in this particular material.

Crack growth in a pipe grade PVC material under static and cyclic loading conditions

The creep crack growth (CCG) and the fatigue crack growth (FCG) behavior of a commercial pipe grade PVC material was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG tests were performed under sinusoidal load control at a frequency of 5 Hz and at R-ratios (F min /F max ) of 0.1, 0.3 and 0.5; the test temperatures were 23 °C and 60 °C. The creep crack growth behavior (corresponding to R = 1) was studied at a test temperature of 60 °C. The results of the FCG tests revealed that fatigue crack propagation is primarily controlled by the cyclic component of the crack tip stress field rather than by the mean stress level. Comparing FCG and CCG data in terms of K Imax and K I , respectively, also confirmed the deteriorating effect of the fatigue loading on the crack growth resistance. Fracture surface investigations for both fatigue and static loading were performed to gain insight into the micromechanisms of crack advance.

Zelfo — An Engineering Material Fully Based on Renewable Resources

Abstract The development of Zelfo was based on a review on technologies for the production of „papier mǎche” in the second half of the 19th century. However, in contrast to „papier mǎche” Zelfo does not contain any additional bonding agent. Potential raw materials for the production of Zelfo include various plants and wastes with a high cellulose content (e.g., hemp, flax, waste paper). The first production step is a refining process, in which the raw materials are simultaneously hackled and ground and mixed with water. The result of this process is a microfiberpulp with a solid content of 1 to 15 m%, which is subsequently predried, cast or molded into a final shape and dried. Due to the possible variations in raw materials and in the production process, properties of Zelfo can be varied over a certain range. For example, values for density from 0.5 to 1.5 g/cm3, for tensile modulus from 1500 to 6550 MPa and for tensile strength from 7 to 55 MPa can be achieved. Thus, Zelfo is likely to compete with both conventional plastics and chipboard for certain applications.

Experimental Determination of Fatigue Crack Growth Behavior and Surface Strain Distribution of ‘Faint-Waist Pure Shear’ Specimens with Different Crack Tip Radii

Abstract The influence of the initial crack-tip radius on the fatigue behavior and the strain distribution in the vicinity of the blunted crack tip was determined experimentally using a servo-hydraulic testing machine and an optical full-field strain analysis method. Two different elastomer grades (SBR, EPDM) were selected for the experimental work. The strain analysis method used, based on the image correlation technique, was found to be an effective tool to determine strains, strain distributions and gradients near the crack tip for elastomeric materials. Different material behavior was observed in the two rubber types investigated. While the crack tip was regularly blunted (half circle shape) for EPDM and the strain gradient was low (less steep), the crack tip was sharp (less blunted) with a higher strain gradient for SBR. Furthermore the crack tip radius was found to be an important influencing factor on the initiation of crack growth, but not on the crack growth behavior after initiation. Based on th...

Determination of rate dependent fracture toughness of plastics using precracked Charpy specimens

Abstract To characterize the rate dependent fracture behavior of various engineering polymers, instrumented impact tests were performed with bending and tensile type specimens in the testing rate range of 10 −5 m/s up to 8 m/s. Load-time signals were recorded using an instrumented striker and a fixture equipped with a piezoelectric load cell and strain gages, respectively. Furthermore, the time-to-fracture was detected with different strain gage types applied to the specimen side surfaces in the vicinity of the crack tip. The data reduction to determine rate dependent fracture toughness values was carried out according to different procedures (conventional force based analysis and “dynamic key curve” method) taking specific local crack tip loading rates into account. In the quasi-brittle failure regime, good agreement was found between the fracture toughness values determined by force based and dynamic data reduction schemes.

Influence of Hot Chlorinated Water and Stabilizer Package on the Fatigue Crack Growth Resistance of Glass Fiber Reinforced Polyamide

To assess the potential use of polyamide (PA) for solar-thermal systems applications, the effect of water with varying chlorine content on the fatigue crack growth (FCG) resistance of two PA formulations differing in their stabilizer packages was investigated at 80 °C. A commercial PA containing 30 wt % glass fibers and a standard stabilization package (PA-0) was used as the reference material. For the other formulation, the reference material PA-0 was compounded with two additional stabilizers (PA-S1). Keeping the specimen geometry and initial loading conditions the same, the total number of cycles to ultimate specimen failure was found to be reduced with an increase in chlorine content for both materials. As to the effect of the chlorine content on crack growth kinetics, the most pronounced effect in enhancing the crack growth rates or decreasing the FCG resistance was determined between 0 ppm and 1 ppm chlorine content. When comparing the relative change of FCG resistance in chlorinated water (10 ppm) to the FCG resistance in non-chlorinated water (0 ppm), the additional stabilization in the material PA-S1 appears beneficial over the stabilization in the reference material PA-0.