Modeling the mode I fracture toughness of anisotropic low-density rigid PUR and PIR foams

Low-density foams have to possess a sufficient resistance to cracking in order to ensure the mechanical integrity of foam materials in service, even when not intended for load-bearing applications. In this study, mode I fracture toughness in the foam rise direction has been experimentally characterized for anisotropic rigid commercial polyurethane foams as well as for polyisocyanurate foams produced using polyols derived from rapeseed oil and filled with a montmorillonite nanoclay. Rectangular parallelepiped unit-cell based scaling relations expressing foam toughness via its relative density, cell dimensions, geometrical anisotropy, and the ultimate tensile stress of the base polymer have been employed for prediction of foam toughness. Assuming a brittle fracture of foam struts, a conservative estimate of toughness is obtained. It is demonstrated that considering the yielding of foam struts at the crack front as the criterion of crack extension provides a closer estimate of foam toughness.     

CNG und LNG aus biogenen Reststoffen – ein Konzept zur ressourcenschonenden Kraftstoffproduktion

A highly integrated process for the conversion of biogenic residues in combination with electricity from renewable sources to methane-based fuels was developed. Forest residues, straw and sewage sludge were identified as particularly relevant input feedstock. By combining gasification, high-temperature electrolysis and catalytic methanation, the biogenic carbon remains almost completely in the products. The matching temperature levels of the heat streams result in a very efficient process. The production costs are comparable to less resource-saving technologies and predictably lower than those of biomethane when plant capacities are increased.     

Evaluation of the effects of variations in chemical composition on the quality of Al-Si-Mg,Al-Cu,and Al-Zn-Mg cast aluminum alloys

The effect of slight variations in chemical composition on the quality of cast aluminum alloys from three different major alloy systems was evaluated. For the evaluation of the alloy quality, an index Q _D adjusted to damage tolerance requirements that are currently involved for the design of advanced lightweight structures is used. The quality index Q _D accounts for tensile strength and ductility as well as for material failure through yielding or fracture. For this investigation, experimental results obtained for variations in chemical composition of the alloy systems Al-Si-Mg, Al-Cu, and Al-Zn-Mg were exploited. In total, castings from 37 different batches from 10 aluminum alloys, varying in chemical composition, were evaluated. Quality characterization and alloy quality ranking were made by evaluating results of 512 tensile tests using the index Q _D as well as, for comparison, the quality index Q, which is currently used by the industry. The results obtained involving the index Q _D seem to be more realistic, from the viewpoint of damage tolerance design requirements.     

Effect of PVC on low-polar isobutylene polymerization in the presence of BCl3

Summary The low-polar polymerization of isobutylene (IB) in the presence of BCI₃ was carried out in CH₂Cl₂ ([IB]=7 mol/l) at -78, -20 and +20°C in the presence of vinyl chloride/2-chloropropene (VC/2CP) copolymer, representing PVC resin enriched in structures with chlorine atoms bonded to the tertiary carbons. The polymerization products consist of mixtures of polyisobutylenes (PIB) and grafted VC/2CP-g-PIB copolymer. Attention was focused on evaluation of the extent of the grafting reaction. VC/2CP-g-PIB was analyzed by GPC, ¹H-NMR and ¹³C-NMR spectroscopies and elemental analysis. The results obtained indicate that the rate of polymerization of IB decreases with increasing temperature but that the grafting efficiency increases. The experiments demonstrated that the VC/2CP copolymer induces strong grafting, compared with PVC, and that it might therefore be assumed that the C-Cl bonds with chlorine atoms bound to tertiary carbons directly initiate the polymerization of IB. The effect of the C-Cl bonds is discussed in terms of their catalytic and initiation activities.     

Wildkunde und Pathologie

Ohne Zusammenfassung     

A concept to optimize quality and cost in thermoplastic composite components applied to the production of helicopter canopies

A concept for the optimization of manufacturing processes of composite material components with regard to product’s quality and cost is introduced and applied for the case of thermoplastic composite helicopter canopies produced by ‘Cold’ Diaphragm Forming (CDF) process. The proposed methodology relies on the consideration of the processes thermal cycle as decisive for the component’s quality and cost. Quality and cost sensitivity analyses were made to derive material dependent Quality Functions (QFs) and process dependent Cost Estimation Relationships (CERs). QFs and CERs are exploited to derive iteratively the optimal thermal cycle. The processes thermal cycle is numerically simulated to allow for its virtual application on the material. To perform the optimization procedure a new software tool is developed. CDF heating system configuration along with the optimal thermal cycle for producing helicopter canopies were obtained. The results of the study were successfully exploited by EUROCOPTER to produce 1:3 scale prototypes.     

Degradation of Mode-I Fracture Toughness of CFRP Bonded Joints Due to Release Agent and Moisture Pre-Bond Contamination

An experimental investigation of the effects of pre-bond contamination on Mode-I fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints is presented in this paper. Two pre-bond contamination scenarios were considered; namely, the silicon-based release agent and moisture. The two contamination scenarios were realized in one of the composite substrates prior to bonding. The common characteristic of the two contamination scenarios is that they lead in the formation of defects in the form of weak bonds that cannot be detected by conventional non-destructive testing techniques. The contamination effects on Mode-I fracture toughness of the bonded joints were investigated by conducting mechanical tests on double-cantilever beam specimens and comparing the results with relative measurements taken from reference specimens. Prior to mechanical testing, the bonding quality of the specimens was tested using ultrasonic C-scan inspection. Both the release agent and moisture are found to significantly degrade the Mode-I fracture toughness of the joints. For the release agent, the effect was more significant for silicon concentrations over 5 at%; a complete lack of adhesion was observed for silicon concentrations over 7 at%. At low values of relative humidity, there was a small increase in Mode-I critical energy release rate while at larger values there is a decrease which reaches 26% for the higher relative humidity percentage. The results from the Non-Destructive Testing (NDT) tests verify the inability of conventional NDT to detect the defects resulting at the interface between the contaminated adherend's surface and the adhesive for both contamination scenarios.