A Comparison of Different Methods for the Detection of a Weak Adhesive/Adherend Interface in Bonded Joints

There are three main classes of defect which occur in adhesive joints: complete disbonds, voids or porosity in the adhesive layer, poor cohesion (ie a weak adhesive layer) and poor adhesion (ie a weak interface between the adhesive layer and one or both adherends). The detection of disbonds, voids and porosity generally presents few problems and significant progress has been made towards the development of techniques for monitoring the cohesive properties of the adhesive layer [1]. However, there is no satisfactory method for the detection of a weak interface between the adhesive and the adherend(s) and this remains one of the major challenges in NDE. It is the interlayer which is affected by the common problem of slight contamination due to, for example, grease on the adherend surfaces prior to bonding. The adhesive/adherend interface is particularly important in aluminium-aluminium joints in which an inappropriate interface structure can cause greatly enhanced susceptibility to environmental attack [2]. Inspection of the interlayer is difficult because it is frequently only of the order of 1µm thick, compared with an adhesive layer thickness of the order of 100 µm

Plant Epigenetic Mechanisms in Response to Biotic Stress

The environment changes faster than the ability of genetic recombination to generate natural genetic diversity. In this context, epigenetic regulation of gene expression has the potential to provide organisms with an alternative mechanism for phenotypic variation by controlling the extent of plasticity that can be achieved in response to environmental changes. There is now substantial evidence suggesting roles for epigenetic regulation of several different aspects of the plant response to biotic stress. At the basic level of gene expression, posttranscriptional gene silencing mediated by small RNAs and chromatin remodelling controlling transcriptional gene silencing are essential for the induced resistance responses activated during pest and pathogen attack. Beyond this, there is also evidence that histone modifications and DNA methylation are associated with immune memory, or defence priming, such as systemic acquired resistance (SAR). In addition, recent evidence indicates that epigenetic modifications can also generate longer-term defence priming responses that can be inherited across generations. In this chapter, we will discuss the roles of epigenetics in these different modes of biotic stress resistance, and suggest ways in which we may in the future be able to exploit epigenetic systems for crop protection