How short-range adhesion slows down crack closure and contact formation.

While viscoelastic, adhesive contact rupture of simple indenters is well studied, contact formation has received much less attention. Here, we present simulations of the formation of contact between various power law indenters and an adhesive, viscoelastic foundation. For all investigated indenters, we find that the macroscopic relaxation time τ scales approximately with 1/ρ1.8, where ρ is the range of adhesion. The prolongation of contact formation with Tabor parameter is rationalized by the increased dissipation that short-range adhesion causes on a moving crack.

Transverse and normal interfacial stiffness of solids with randomly rough surfaces.

Using a theoretical approach and computer simulations, we calculate the normal stiffness K(perpendicular) and the transverse stiffness K(parallel) of the interface between two contacting isotropic solids with randomly rough surfaces and Poisson ratio ν. The theoretical predictions for K(perpendicular) agree well with the simulations. Moreover, the theoretical result for the ratio K(perpendicular)/K(parallel) is (2 - ν)/(2 - 2ν), as predicted by Mindlin for a single circular contact region. Finally, we compare the theory to experimental ultrasonic data.

Microstructural insight into pedestrian pelvic fracture as assessed by high-resolution computed tomography.

Pelvic and femoral neck bone surface strains were recorded in five full-body human cadaver vehicle-pedestrian impacts. Impacts were performed at 40 km/h using automotive front ends constructed to represent those used in previously reported finite element simulations. While experimental kinematics and bone strains closely matched model predictions, observed pelvic fractures did not consistently agree with the model, and could not be solely explained by vehicle geometry. In an attempt to reconcile injury outcome with factors apart from vehicle design, a proxy measure of subject skeletal health was assessed by high-resolution quantitative computed tomography (HRqCT) of the femoral neck. The incidence of hip/pelvis fracture was found to be consistent with low volumetric bone mineral density and low trabecular bone density. This finding lends quantitative support to the notion that healthy trabecular architecture is crucial in withstanding non-physiological impact loads. Furthermore, it is recommended that injury criteria used to assess vehicle safety with regard to pedestrians consider the increased susceptibility of elderly victims to pelvic fracture.

Simple microscopic theory of Amontons's laws for static friction.

A microscopic theory for the ubiquitous phenomenon of static friction is presented. Interactions between two surfaces are modeled by an energy penalty that increases exponentially with the degree of surface overlap. The resulting static friction is proportional to load, in accordance with Amontons's laws. However, the friction coefficient between bare surfaces vanishes as the area of individual contacts grows, except in the rare case of commensurate surfaces. An area independent friction coefficient is obtained for any surface geometry when an adsorbed layer of mobile atoms is introduced between the surfaces. The predictions from our simple analytic model are confirmed by detailed molecular dynamics simulations.

Biomechanical assessment of soft tissue cervical spine disorders and expert opinion in low speed collisions.

The multidisciplinary research of injury mechanisms and injury prevention requires the assessment of the technical and biomechanical circumstances of a collision; moreover, the causality assessment in the individual cases is facilitated by taking these aspects into account. In fact, only specially trained engineers and biomechanical experts are in a position to evaluate these relevant basic facts. In many crucial court cases, important technical factors such as collision angle, structural stiffness, extent of intrusion and the vehicle's velocity change are often ignored. The purely medical causality assessment is often based only on a coincidence of time of the 'accident' and the onset of the disorders. Unfortunately, statements about the 'accident speed' or the nebulous 'accident energy' are often made by clinicians with neither a proper collision documentation nor the necessary biomechanical and technical background. In order to overcome shortcomings of injury causality assessment as well as the terminology associated with soft tissue cervical spine injuries, a subdivision of the term 'accident severity' into four classes is proposed. Consequently, an 'accident severity assessment' can only be performed by a collaboration of four corresponding classes of experts, i.e. the engineer (dynamic loading of the vehicle), the biomechanical expert (biomechanical loading of the occupant), the physician (clinically diagnosable injuries), and eventually the psychiatrist (subjective sequelae individually experienced by the victim).

Comparison of car seats in low speed rear-end impacts using the BioRID dummy and the new neck injury criterion (NIC).

Long-term whiplash associated disorders (WAD) 1-3 sustained in low velocity rear-end impacts is the most common disability injury in Sweden. Therefore, to determine neck injury mechanisms and develop methods to measure neck-injury related parameters are of importance for current crash-safety research. A new neck injury criterion (NIC) has previously been proposed and evaluated by means of dummy, human and mathematical rear-impact simulations. So far, the criterion appears to be sensitive to the major car and collision related risk factors for injuries with long-term consequences. To further evaluate the applicability of NIC, four seats were tested according to a recently proposed sled-test procedure. 'Good' as well as 'bad' seats were chosen on the basis of a recently presented disability risk ranking list. The dummy used in the current tests was the Biofidelic Rear Impact Dummy (BioRID). The results of this study showed that NICmax values were generally related to the real-world risk of long-term WAD 1-3. Furthermore, these results suggested that NICmax calculated from sled tests using the BioRID dummy can be used for evaluating the neck injury risk of different car seats.

Time-resolved flow cytometry for the measurement of lanthanide chelate fluorescence: II. Instrument design and experimental results.

A time-resolved flow cytometer capable of measuring a luminescence with a decay time in the range of 10 microseconds to 2 ms, typical for some lanthanide chelates, is presented. The instrument permits acquisition of conventional light scatter and prompt fluorescence signals as well as detection of slowly decaying luminescence by a photon counting unit for a selectable time period of 1 microsecond to 1 ms. During photon counting, the laser beam is turned off by an acoustooptic deflector. The design of a flow chamber with an average geometrical light collection efficiency of 35% over a distance of 1.7 mm is presented and analyzed by ray tracing. A pulse processing system featuring digital integration of the conventional signals and a transputer system for the acquisition and the transfer of the measured parameter values to a host computer is described. Instrument function is verified with lyophilized human lymphocytes stained for the CD8 antigen with dye-loaded liposomes. Quantitation of cell-associated europium chelate fluorescence, displaying a decay time of 1.6 ms, is demonstrated. Elimination of fast decaying background emission generated by DNA-associated ethidium bromide is shown. The background generated by instrument components in the time-gated measurement channel is characterized, and measures for its complete elimination are discussed.