Towards Quantitative Wall Shear Stress Measurements: Calibration of Liquid Crystals

The wall shear stress distribution on an aerodynamically loaded component is of both practical and fundamental importance. Significant examples are the improvement of the performance of a vehicle, e.g. drag reduction, and more basic problems, such as the characterization of surface flows, e.g. with respect to flow control. The liquid crystal technique represents a promising diagnostics for measuring wall shear stress magnitude and orientation. In contrast to most other techniques, the direct measurement of two-dimensional wall shear stress distributions is straightforward. In order to establish quantitative measurements of wall shear stress using the liquid crystal technique, an in-depth understanding of the influencing parameters is required. For their investigation, a novel generic flat plate test section was designed. The experiments are performed in such a way that a turbulent boundary layer is triggered at a corresponding Reynolds number within the test section. Due to the generic test case, precise and well-known flow boundary conditions can be established, which in turn are validated by probe measurements. Velocity and temperature profiles are recorded with high spatial resolution using a miniaturized combined Pitot-thermocouple probe. Furthermore, the operational range of the new test rig is presented. Preliminary wall shear stress measurements confirm the well-defined flow conditions in the test section and the potential of the measurement technique.

Extending R with C++: A Brief Introduction to Rcpp

R has always provided an application programming interface (API) for extensions. Based on the C language, it uses a number of macros and other low-level constructs to exchange data structures between the R process and any dynamically-loaded component modules authors added to it. With the introduction of the Rcpp package, and its later refinements, this process has become considerably easier yet also more robust. By now, Rcpp has become the most popular extension mechanism for R. This article introduces Rcpp, and illustrates with several examples how the Rcpp Attributes mechanism in particular eases the transition of objects between R and C++ code.

Extending R with C++: A Brief Introduction to Rcpp

R has always provided an application programming interface (API) for extensions. Based on the C language, it uses a number of macros and other low-level constructs to exchange data structures between the R process and any dynamically-loaded component modules authors added to it. With the introduction of the Rcpp package, and its later refinements, this process has become considerably easier yet also more robust. By now, Rcpp has become the most popular extension mechanism for R. This article introduces Rcpp, and illustrates with several examples how the Rcpp Attributes mechanism in particular eases the transition of objects between R and C++ code.

Fatigue Behavior of Weld Repaired AISI 4130 Aeronautic Steel Used in Critical Flight Safety Structures

Since the 1950s, fatigue is the most important project and operational consideration for both civil and military aircrafts. For some aircraft models the most loaded component is one that supports the motor: the "Motor Cradle". Because they are considered critical to the flight safety the aeronautic standards are extremely rigorous in manufacturing them by imposing a "zero index of defects" on the final weld quality (Safe Life), which is 100% inspected by Non-Destructive Testing/NDT. This study has as objective to evaluate the effects of up to four successive TIG welding repairs on the axial fatigue strength of an AISI 4130 steel. Tests were conducted on hot-rolled steel plate specimens, 0.89 mm thick, with load ratio R = 0.1, constant amplitude, at 20 Hz frequency and in room temperature, in accordance with ASTM E466 Standard. The results were related to microhardness and microstructural and geometric changes resulting fromwelding cycles.

Probabilistic Simulation for Combined Cyclic Fatigue in Composites

The combined cyclic fatigue is computationally simulated by a judicious combination of three independent computational modules: composite mechanics, a multi-factor equation module (MFIM) and probabilistic algorithm (FPI). The inputs to each module are constituent material properties, processing variables, loading and environmental conditions and probabilistic variables. The composite fatigue is simulated by considering an inplane loaded-component in a structure which is subjected to fatigue because of cyclic loading. Typical results show that the low probability fatigue cycle is about 50 percent of the corresponding static value.

Simulation of Stress-Fracture in Human Vertebral Body due to Extreme Weight Lifting

Lumbar vertebrae are a heavily loaded component of human body. They are subjected to repetitive loading in daily activities. However, limited information on failure mechanism of lumbar vertebrae are available to date. Thus, the need to develop an analytical model to predict stress-fracture characteristics of vertebral body. A linear elastic fracture mechanics approach has been considered and a mathematical model has been proposed so that the predictions can be made more easily related to the occurrence of injury. Study reveals that for a person weighing 1334 N and lifting a weight of 345 kg during squat exercise causes a vertebral stress-fracture at 12 repetitive standing lifting. While same load at lowest position yields a stress-fracture at less than 3 lifting. Numerical study shows that for change of position from standing to lowest position resultant compressive force acting on spine increases by two times whereas the possibility of stress-fracture increases by five times. Similarly at dead lift exercise, lifting 325 kg from standing to lowest position increases resultant compressive forces on vertebrae by 2.5 times. However, stress-fracture ratio increases by six times. Study reveals that for a person weighing 800 N (height = 1.8 m) and lifting a weight of 900 N, vertebrae can be subjected to stress-fracture by three cyclic lifting. Rate of injury is dependent on flexion angle i.e. as flexion angle increases, so does rate of injury.

Fracture Mechanics and Non-Destructive Testing for Structural Integrity Assessment

Fracture mechanics parameters can be applied for the analysis of failures of structures, and also for prevention of failures when defects in a structure are detected and defined. The approach is presented through stages: detection of defects, stress-strain analysis of loaded component, characterization of material properties required for structural integrity assessment and application of convenient procedure. In this way the decision about next use of defective component can be made (to continue the operation, increased care by inspection, exclusion the component from next service, with eventual repair, if possible). Special attention is paid to the most popular testing procedures for crack resistance parameters.

Non Invasive Measurement of Strain and Temperature by Neutron Diffraction

Two methods have been developed to determine temperature non-invasively within engineering components by neutron diffraction. The integrated intensity of a diffraction line depends on temperature through the Debye-Waller factor. The angular position of the line, in the absence of an applied load, depends on temperature through the thermal expansion coefficient. Temperature may thus be determined by accurate relative intensity measurements with respect to a reference temperature and, alternatively, by accurate measurement of the interplanar spacings. It was also shown to be feasible to measure the strain response to an applied load at elevated temperatures. Measurements were made on Waspalloy and the Ti alloy AMS 4928. For Waspalloy, for example, the thermal expansion at zero stress gave the average temperature with a precision of ± 4 K and agreed with thermocouple measurements to within 5 K on average. The intensity data suggests that temperature can be measured with a precision of ± 10 K in a loaded component.

A method for determining the mechanical characteristics of orthotic knee joints

All orthotic equipment must operate at the highest possible standards of safety and, should structural failure of a loaded component occur, there must be a minimal possibility of injury to the user. Because of the lack of definitive data on the in-service loading of lower limb orthoses it is not possible to base a test procedure on “real” loading conditions. In this paper a method of destructive testing, based on the assumption that the predominant loading consists of bending about the medio-lateral and anterior-posterior axes, is described. This method makes it possible to measure the bending strength of a knee joint side member assembly and to define the brittleness of the failure. It is suggested that the latter definition makes it possible to predict the potential safety of a particular knee joint should in-service failure occur. Some laboratory failures are described and recommendations, based on the test programme, are made for new joint designs.