Strength estimation of self-piercing rivets using lower bound limit load analysis

Abstract

This paper summarises the authors' work on strength and failure mode estimation of self-piercing rivets (SPRs) for automotive applications. First, the static cross tension strength of an SPR joint is estimated using a lower bound limit load based strength estimator. Failure mode associated with the predicted failure strength can also be identified. It is shown that the cross tension strength of an SPR joint depends on the material and gage combinations, rivet design, die design and riveting direction. The analytical rivet strength estimator is then validated by experimental rivet strength measurements and failure mode observations from nine SPR joint populations with various material and gage combinations. Next, the estimator is used to optimise rivet strength. Two illustrative examples are presented in which rivet strength is improved by changing rivet length and riveting direction from the original manufacturing parameters.     

Instrumentation and simulation of industrial steel wire rod cooling line

Abstract

A coupled thermal and metallurgical mathematical model has been developed to simulate the Stelmor process for controlled cooling of steel wire rod. The model predicts the through section microstructure, which is subsequently used to estimate the mechanical properties, in terms of ferrite fraction, ferrite grain size, and interlamellar pearlite spacing. Plant trials were conducted to evaluate heat transfer coefficients for the water and forced air cooling stages of the process in terms of the process parameters. To enable the development of a control model, the effect of individual process parameters, including their extreme settings, on rod temperature distribution was studied. An inverse modelling approach has been used to optimise the value of the heat transfer coefficient. The model results were compared with plant trials for a range of rod diameters, rolling speeds, number of active water boxes, and the operation ratio of the Stelmor fans. This validation was performed for three carbon–manganese steel grades (0.07–0.67%C) with rod diameters of 5.5–14.0 mm. A good agreement between the predicted and measured thermal behaviour of the rod was achieved.     

Effect of prior austenite characteristics on mechanical properties of thermomechanically processed multiphase TRIP assisted steels

Abstract

To develop any multiphase transformation induced plasticity (TRIP) assisted steel through thermomechanical processing routes, the effects of materials and process variables are to be thoroughly identified. Accordingly, the present work was programmed to study the influence of the state of prior austenite on the subsequent transformations and the related final phase combinations. In this framework, using a low carbon TRIP assisted multiphase steel, the compression specimens were subjected to different thermomechanical processing schedules to generate different prior austenite characteristics. The results indicated that the characteristics of martensite/austenite (M/A) microcomponent in the final microstructure were dictated by the state of prior austenite. In fact, increasing prior austenite grain size led to large decrease in the amount of M/A phase and this, in turn, resulted to higher strength without sacrificing the ductility. It was also found that static recrystallisation of austenite brings about more percentage of M/A phase compared with dynamic recrystallisation.     

Fracture micromechanics of static subcritical growth and coalescence of microcracks in sintered Fe–1·5Cr–0·2Mo–0·7C steel

Abstract

Nucleation of microcracks, their growth and coalescence are analysed in powder metallurgy (PM). Fe–1·5Cr–0·2Mo–0·7C steel by fractography allied to surface replica microscopy – at several stress levels as the maximum tensile stress in three-point bend specimens was raised to 99·6% of the transverse rupture strength TRS of 1397 MPa. The fatigue limit in this material is ～240 MPa, at which stress level no microcracks were detected in static loading. Numerous microcracks, ranging in size from <5 to ～20 μm, however, were nucleated above ～800 MPa, i.e. beyond the yield strength of ～620 MPa. With increasing stress, some microcracks became dormant, whilst others grew subcritically, stress step-wise, to some 400 μm. Of particular importance are observations of the coalescence of two and three of such microcrack systems to produce a critical, propagating crack. The then estimated stress intensity factor K _a, could reach K _1C, independently estimated to be ～36 MPa m^1/2. Microcrack coalescence was associated with easy paths for crack growth, principally prior particle boundaries linking pores. Ways of making subcritical crack growth more difficult and hence improving both static and dynamic mechanical properties, are considered.     

Influence of static magnetic fields on the protective ability of phosphate coatings on aluminium alloys

Abstract

Phosphate coatings on aluminium are used to improve the corrosion resistance of aluminium before the latter is painted. Phosphate coatings deposited in phosphate solution treated with a magnetic field prior to deposition can take on new properties. A magnetic field can either improve or deteriorate the properties of the phosphate coating formed, depending on the composition of aluminium.     

2—PERMANENT-PRESS EFFECTS IN WOOL: PART V: METHODS OF STABILIZING SET

Shrink-resist wool set in the normal way can lose wanted set or acquire unwanted set during machine-washing and tumble-drying because the molecular-bond rearrangements responsible for setting can still occur during the laundering cycle. The set can be stabilized to withstand such laundering by any of the following methods:

1. the inhibition of thiol-disulphide interchange by:

   1. removing ionized thiol groups, or

   2. converting disulphide bonds to non-interchangeable links, such as —S—, —SCH₂S—;

2. the introduction of new cross-links; or

3. denaturation to form structures that are slow to rearrange during washing.

The stability of set is enhanced by a combination of the methods because these operate by different mechanisms. Prevention of thiol-disulphide interchange appears to be more important during washing, whereas the presence of cross-links appears to be more important to aid recovery during tumble-drying.     

Error Analysis and Effective Adjustment of the Walking-Ready Posture for a Biped Humanoid Robot

A walking control algorithm is generally a mixture of various controllers; it depends on the characteristics of the target system. Simply adopting one part of another researcher's algorithm does not guarantee an improvement in walking performance. However, this paper proposes an effective algorithm that can be easily adopted to other biped humanoid robots; the algorithm enhances the walking performance and stability of the robot merely by adjusting the walking-ready posture. The walking performance of biped humanoid robots is easily affected by an unsuitable walking-ready posture in terms of accuracy and repeatability. More specifically, low accuracy for the walking-ready posture may cause a large difference between an actual biped robot and its mathematical model, and the low repeatability may disturb the evaluation of the performances of balance controllers. Therefore, this paper first discusses the factors that detrimentally affect bipedal walking performance and their phenomena in the walking-ready posture. The necessary conditions for an ideal walking-ready posture are then defined based on static equilibrium and a suitable adjustment algorithm is proposed. Finally, the effectiveness of the algorithm is verified through dynamic computer simulations.     

Recrystallisation and dilatometric behaviour of low carbon and ultralow carbon steels

Abstract

During continuous heating of a cold rolled low carbon steel, a dilatometric anomaly has been detected before the well known ferrite to austenite transformation. The detailed study of the processes occurring during the heating and comparison with the case of an ultralow carbon steel showed that this anomaly is related to the change in dislocation density during recrystallisation.     

42—PERMANENT-PRESS EFFECTS IN WOOL. PART XIII: DELAYED-CURE CROSS-LINKING

Experiments are described in which cross-linking agents containing N-hydroxymethyl, N-methoxymethyl, or aziridinyl groups were tested for their effectiveness in stabilizing set in chlorinated wool by a delayed-cure procedure that involved the application of the cross-linking agent and sodium bisulphite to fabric, followed by setting and curing. Premature cross-linking, leading to unstable set, occurred with all the reagents tested except hexamethoxymethylmelamine.     

Thermophysical properties of slags for process control

Abstract

Thermophysical properties have been shown to be important in a number of industrial processes. Unfortunately, for a number of properties there are scant data available and/or the data that are available are of dubious quality. In the Thermo-Physical Property Group at the National Physical Laboratory (NPL) the aim has been to develop measurement techniques capable of working in the difficult environments that are typical of industrial processes. This paper presents current developments at NPL in techniques to measure the thermal diffusivity of liquid slags using a laser flash and insights into the copper smelting process using interfacial tension techniques.