Fatigue behavior of continuous-discontinuous sheet molding compounds

Sheet molding compound (SMC) composites have been well-established as nonstructural and semi-structural components in vehicles due to their high lightweight potential. Over the past decade, increasing need for further vehicle weight reduction has fueled endeavors to further improve mechanical properties of SMC and thereby to expand their suitability as structural components. One promising approach to achieve this goal is the combination of discontinuous glass fiber SMC with local continuous carbon fiber reinforcement, which is currently being investigated by the German-Canadian research training group GRK 2078. Such hybrid continuous-discontinuous composites enable the production of components with excellent mechanical properties while maintaining the advantages of the SMC process including geometric flexibility, short cycle times and cost efficiency. However, several factors prevent the exploitation of the composite’s full potential. Major obstacles for a safe and efficient application of continuous-discontinuous SMC are the lack of knowledge regarding their fatigue behavior and about uncharted effects of hybridization on damage behavior under cyclic loading conditions. This thesis describes a systematical analysis of the fatigue behavior of continuous-discontinuous SMC both under cyclic tensile and bending loads at different temperatures and frequencies. The hybrid composite, which consists of a discontinuous glass fiber SMC core and unidirectional carbon fiber SMC face plies, shows significantly higher fatigue resistance compared to discontinuous SMC without continuous reinforcement. The effect of hybridization is more pronounced under cyclic loading than under monotonic loading, which is a result of distinct damage mechanisms acting at distinct applied stresses. The mechanical behavior of continuous-discontinuous SMC is dominated by the continuous plies at high stresses and by the discontinuous ply at low stresses. The effect of hybridization is particularly distinctive under cyclic bending load on account of the composite’s sandwich-like structure. In addition, relative stiffness degradation of continuous-discontinuous SMC is less pronounced over a larger loading period compared to discontinuous SMC. While damage evolution within the discontinuous SMC ply remains largely unaffected by hybridization, the continuous SMC plies are enabled to withstand significantly higher cyclic loads than continuous SMC specimens that are not a part of a hybrid composite. When using a cross-ply instead of a unidirectional continuous reinforcement, early initiation of cracks in the 90° ply at comparatively low stresses leads to rapidly growing delaminations and large cracks in the discontinuous SMC ply. Consequently, the effect of hybridization under cyclic loading is less pronounced. While continuous-discontinuous SMC is largely insensitive to a change in frequency, enhanced temperature leads to early failure of the unidirectional ply on the compression loaded side, which results in a significant decrease of fatigue life

An Introduction to \u3cem\u3eSeshat: Global History Databank\u3c/em\u3e

This article introduces the Seshat: Global History Databank, its potential, and its methodology. Seshat is a databank containing vast amounts of quantitative data buttressed by qualitative nuance for a large sample of historical and archaeological polities. The sample is global in scope and covers the period from the Neolithic Revolution to the Industrial Revolution. Seshat allows scholars to capture dynamic processes and to test theories about the co-evolution (or not) of social scale and complexity, agriculture, warfare, religion, and any number of such Big Questions. Seshat is rapidly becoming a massive resource for innovative cross-cultural and cross-disciplinary research. Seshat is part of a growing trend to use comparative historical data on a large scale and contributes as such to a growing consilience between the humanities and social sciences. Seshat is underpinned by a robust and transparent workflow to ensure the ever growing dataset is of high quality

Widespread exploitation of the honeybee by early neolithic farmers

The pressures on honeybee (Apis mellifera) populations, resulting from threats by modern pesticides, parasites, predators and diseases, have raised awareness of the economic importance and critical role this insect plays in agricultural societies across the globe. However, the association of humans with A. mellifera predates post-industrial-revolution agriculture, as evidenced by the widespread presence of ancient Egyptian bee iconography dating to the Old Kingdom (approximately 2400 BC)(1). There are also indications of Stone Age people harvesting bee products; for example, honey hunting is interpreted from rock art(2) in a prehistoric Holocene context and a beeswax find in a pre-agriculturalist site(3). However, when and where the regular association of A. mellifera with agriculturalists emerged is unknown(4). One of the major products of A. mellifera is beeswax, which is composed of a complex suite of lipids including n-alkanes, n-alkanoic acids and fatty acyl wax esters. The composition is highly constant as it is determined genetically through the insect's biochemistry. Thus, the chemical 'fingerprint' of beeswax provides a reliable basis for detecting this commodity in organic residues preserved at archaeological sites, which we now use to trace the exploitation by humans of A. mellifera temporally and spatially. Here we present secure identifications of beeswax in lipid residues preserved in pottery vessels of Neolithic Old World farmers. The geographical range of bee product exploitation is traced in Neolithic Europe, the Near East and North Africa, providing the palaeoecological range of honeybees during prehistory. Temporally, we demonstrate that bee products were exploited continuously, and probably extensively in some regions, at least from the seventh millennium cal BC, likely fulfilling a variety of technological and cultural functions. The close association of A. mellifera with Neolithic farming communities dates to the early onset of agriculture and may provide evidence for the beginnings of a domestication process