Dynamic crushing characteristics of spruce wood under large deformations
An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson’s formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
On the fracture morphology in wood
SummaryFailure forms caused by axial ultimate compression stress in three softwood and nine hardwood species and in model specimens made of wood, paper and plastics are described. Three categories of failure forms are distinguished: 1. Wood characteristic failure forms are connected with the general anisotropic structure of the wood. 2. Failure forms specific to the species are modifications of the first category arising from predominant anatomical structures. 3. Modifications of the failure forms are also induced experimentally. The interdependence among the anatomical structure, strength characteristics and failure forms of the wood specimens are examined by statistical methods. The structure cipher introduced in this paper, as a numerical characteristic of the anatomical features of the wood species, is seen to be the most important influencing factor as regards the intensity and pattern of the fracture, followed in second and third place by the geometry of the specimen and its specific gravity. Specimen volume and other factors are shown to have only a marginal influence on the fracture morphology.
Ultrathin Wafer Pre-Assembly and Assembly Process Technologies: A Review
Ultrathin silicon wafer technology is reviewed in terms of the semiconductor applications, critical challenges, and wafer pre-assembly and assembly process technologies and their underlying mechanisms. Mechanical backgrinding has been the standard process for wafer thinning in the semiconductor industry owing to its low cost and productivity. As the thickness requirement of wafers is reduced to below 100 μm, many challenges are being faced due to wafer/die bow, mechanical strength, wafer handling, total thickness variation (TTV), dicing, and packaging assembly. Various ultrathin wafer processing and assembly technologies have been developed to address these challenges. These include wafer carrier systems to handle ultrathin wafers; backgrinding subsurface damage and surface roughness reduction, and post-grinding treatment to increase wafer/die strength; improved wafer carrier flatness and backgrinding auto-TTV control to improve TTV; wafer dicing technologies to reduce die sidewall damage to increase die strength; and assembly methods for die pick-up, die transfer, die attachment, and wire bonding. Where applicable, current process issues and limitations, and future work needed are highlighted.