Karl-Gustaf Sundin

High strain-rate tensile testing and viscoplastic parameter identification using microscopic high-speed photography

A combined experimental/numerical method for determination of constitutive parameters in high strain-rate material models is presented. Impact loading, using moderate projectile velocities in combination with small specimens (sub mm) facilitate tensional strain rates in the order of 104–105 s−1. Loading force is measured from one-dimensional wave propagation in a rod using strain gauges and deformation is monitored with a high-speed camera equipped with a microscope lens. A sequence of digital photographs is taken during the impact loading and the plastic deformation history of the specimen is quantified from the photographic record. Estimation of material parameters is performed through so called inverse modelling in which results from repeated FE-simulations are compared with experimental results and a best choice of constitutive parameters is extracted through an iterative optimisation procedure using the simplex method. Results are presented from a preliminary tension test of a mild steel (A533B) at a strain rate well over 104 s−1. The sensitivity of the evaluated material parameters to errors in measured quantities is studied. The method, especially the optical technique for measurement of deformation will be further developed.

Analysis of elastic waves in non-uniform rods from two-point strain measurement

Abstract A method is established which permits evaluation of the histories of normal force and particle velocity at an arbitrary cross-section E of a non-uniform linearly-elastic rod from measured strain histories at two different cross-sections, A and B . Other quantities at E , such as stress, strain, displacement, acceleration and power transmission, can be determined in terms of the evaluated normal force and particle velocity. The method is an extension of that developed by Lundberg and Henchoz for uniform rods. Theoretical results, based on one-dimensional elastic wave theory, are provided for the general case as well as for cases of piecewise constant characteristic impedance. Experimental tests with different configurations of non-uniform rods and strain gauges show that the normal force histories determined at E with the present method (from measured strains at A and B ) agree well with those obtained through direct strain measurement at E .

Identification of lumped parameter automotive crash models for bumper system development

During the design and development process of bumper systems for the automotive industry, information about the future car model is limited. Normally, iterative finite element (FE) analyses of different crash loading tests are used to find an appropriate bumper system to the coming car model. Because of the lack of information, only a rough model of the car is normally utilised in the FE simulations. This leads to uncertainties in the bumper design since the dynamic response of the car is dependent on the load case and the properties of the actual bumper system. This paper presents a method for identification of lumped parameter models based on results from crash tests of a Volvo S40. The ability to predict the measured results for models with different number of degrees of freedom (DOF) is investigated. Also, a validation of the model together with an FE mesh of the bumper system is presented. The results clearly show that a linear mass spring damper model with 2 DOF can be used to predict the response from the measurements in case of symmetric loading. Further increase of the number of DOF only causes small or no improvements of the agreement between the predicted and measured crash response.

Experimental study of a percussive process for rock fragmentation

Abstract The efficiency of a percussive process for fragmentation of rock and similar materials has been studied experimentally. The percussive system comprised a cylindrical hammer and a cylindrical bit with the same characteristic impedance. The bit was terminated with a wedge. In front of the wedge there was a heavy block of concrete. The length of the bit and the initial gap between bit and concrete were varied systematically. The force versis penetration relationship and the work of fragmentation were determined in each test using a new technique based on measurement of strains at two cross-sections of the bit. Each test was simulated individually using a previously developed one-dimensional model. The results of simulations and experimental tests were found to agree well.

A method for determination of in-hole dynamic force-penetration data from two-point strain measurement on a percussive drill rod

The aim of the present investigation is to establish a testing method suited for determination of in-hole dynamic force-penetration data for non-uniform standard percussive drill rod-bit configurat ...

Modified split Hopkinson pressure bar method for determination of the dilatation-pressure relationship of lubricants used in elastohydrodynamic lubrication

Abstract In theoretical calculations of film thickness, pressure distribution and friction in an elastohydrodynamically lubricated (EHL) conjunction it is necessary to model the physical/ mechanical behaviour of the lubricant. It is important to know, for example, the dilatation-pressure or the density-pressure relationship. In this paper a modified split Hopkinson pressure bar system for determination of the compressibility of oil is presented. It makes it possible to test oils under conditions similar to those found in real EHL contacts: loading duration in the range of 100–300 μs and pressures of almost 2 GPa. An empirical model has been suggested for mathematical description of the dilatation-pressure relation of the specific oils. A naphthenic mineral oil and a synthetic oil, 5P4E, have been tested under adiabatic conditions and at pressures up to 1.5 and 1.9 GPa respectively. The adiabatic results have been recalculated to isothermal conditions for comparison.

Experimental determination of mechanical impedance through strain measurement on a conical rod

Abstract An impedance gauge based on measurement of strains at two different cross-sections of a conical gauge rod is tested. The gauge rod is in contact with the object at its narrow end while its wide end is driven by a harmonic vibrator. The length of the gauge rod is 260 mm and its end diameters are 9·8 mm and 20·1 mm. For cylindrical test objects a fair agreement is generally obtained between experimental and theoretical point impedances in the frequency range 50 Hz to 5 kHz. Further improvements in accuracy are expected to be feasible.

Correlation of vehicle crash model parameters to car properties in low-speed collisions: a design of experiments approach

In the current study, a methodology for relating model parameters in a one dimensional Mass Spring Damper (MSD) model to global properties of a car, e.g. axial stiffness, bending stiffness and mass, is presented. It is shown that these three vehicle properties affect the vehicles crash performance in low-speed collision tests used for industrial verification of bumper system performance. Based on information of the properties for a vehicle under development, parameters in the MSD model can be adjusted to give the correct boundary conditions for a finite element (FE) crash simulation with a candidate bumper design. In the FE simulations, the MSD model is then coupled to the FE mesh of candidate bumper design to find a bumper that meets the crash performance requirements of a car under development. The methodology is based on Design of Experiments (DOE) and FE simulations on a public domain model of a Ford Taurus. The knowledge gained from this study gives a valuable tool to use in design and development of bumper systems for the automotive industry.

Performance test of a mechanical impedance gauge based on strain measurement on a rod

An impedance gauge based on measurement of strains at two cross sections of a slender rod has been tested on practical objects. Sensitive semiconductor strain gauges are used and the impact force n ...

Mechanical impedance gauge based on measurement of strains on an impacted rod

Abstract An impedance gauge based on measurement of strains at two cross-sections of a slender rod is studied. The gauge rod is in contact with the object at one end while it is impacted at the other end. The impedance is evaluated from the two strain signals by means of a two-channel FFT-analyzer and a desk-top computer. Gauge prototypes with cylindrical and conical geometries are tested in the frequency range 50 Hz to 5 kHz for cylindrical objects with known theoretical point impedances. Satisfactory agreement between experimental and theoretical impedances is obtained for both prototypes.