Design and application of experimental methods for steel sheet shearing

Shearing is the process where sheet metal is mechanically cut between two tools. Various shearing technologies are commonly used in the sheet metal industry, for example, in cut to length lines, slitting lines, end cropping etc. Shearing has speed and cost advantages over competing cutting methods like laser and plasma cutting, but involves large forces on the equipment and large strains in the sheet material. The constant development of sheet metals toward higher strength and formability leads to increased forces on the shearing equipment and tools. Shearing of new sheet materials imply new suitable shearing parameters. Investigations of the shearing parameters through live tests in the production are expensive and separate experiments are time consuming and requires specialized equipment. Studies involving a large number of parameters and coupled effects are therefore preferably performed by finite element based simulations. Accurate experimental data is still a prerequisite to validate such simulations. There is, however, a shortage of accurate experimental data to validate such simulations. In industrial shearing processes, measured forces are always larger than the actual forces acting on the sheet, due to friction losses. Shearing also generates a force that attempts to separate the two tools with changed shearing conditions through increased clearance between the tools as result. Tool clearance is also the most common shearing parameter to adjust, depending on material grade and sheet thickness, to moderate the required force and to control the final sheared edge geometry. In this work, an experimental procedure that provides a stable tool clearance together with accurate measurements of tool forces and tool displacements, was designed, built and evaluated. Important shearing parameters and demands on the experimental set-up were identified in a sensitivity analysis performed with finite element simulations under the assumption of plane strain. With respect to large tool clearance stability and accurate force measurements, a symmetric experiment with two simultaneous shears and internal balancing of forces attempting to separate the tools was constructed. Steel sheets of different strength levels were sheared using the above mentioned experimental set-up, with various tool clearances, sheet clamping and rake angles. Results showed that tool penetration before fracture decreased with increased material strength. When one side of the sheet was left unclamped and free to move, the required shearing force decreased but instead the force attempting to separate the two tools increased. Further, the maximum shearing force decreased and the rollover increased with increased tool clearance. Digital image correlation was applied to measure strains on the sheet surface. The obtained strain fields, together with a material model, were used to compute the stress state in the sheet. A comparison, up to crack initiation, of these experimental results with corresponding results from finite element simulations in three dimensions and at a plane strain approximation showed that effective strains on the surface are representative also for the bulk material. A simple model was successfully applied to calculate the tool forces in shearing with angled tools from forces measured with parallel tools. These results suggest that, with respect to tool forces, a plane strain approximation is valid also at angled tools, at least for small rake angles. In general terms, this study provide a stable symmetric experimental set-up with internal balancing of lateral forces, for accurate measurements of tool forces, tool displacements, and sheet deformations, to study the effects of important shearing parameters. The results give further insight to the strain and stress conditions at crack initiation during shearing, and can also be used to validate models of the shearing process.

Health and quality of care from older peoples' and formal caregivers' perspective

Aim: The overall aim of this thesis was to gain a deeper understanding of older people's view of health and care while dependent on community care. Furthermore to describe and compare formal caregivers' perceptions of quality of care, working conditions, competence, general health, and factors associated with quality of care from the caregivers' perspective. Method: Qualitative interviews were conducted with 19 older people in community care who were asked to describe what health and ill health((I), good and bad care meant for them (II). Data were analyzed using content analysis (I) and a phenomenological analysis (II). The formal caregivers; 70 nursing assistants (NAs) 163 enrolled nurses (ENs) and 198 registered nurses (RNs), answered a questionnaire consisting of five instruments: quality of care from the patient's perspective modified to formal caregivers, creative climate questionnaire, stress of conscience, health index, sense of coherence and items on education and competence (III). Statistical analyses were performed containing descriptive statistics, and comparisons between the occupational groups were made using Kruskal-Wallis ANOVA, Mann-Whitney U-test and Pearson's Chi-square test (III). Pearson's  product moment correlation analysis and multiple regression analysis were performed studying the associations between organizational climate, stress of conscience, competence, general health and sense of coherence with quality of care (IV). Results: The older people's health and well-being were related to their own ability to adapt to and compensate for their disabilities and was described as negative and positive poles of autonomy vs. dependence, togetherness vs. being an onlooker, security vs. insecurity and tranquility vs. disturbance (I).  The meaning of good care (II) was that the formal caregivers respected the older people as unique individuals, having the opportunity to live their lives as usual and receiving a safe and secure care. Good care could be experienced when the formal caregivers had adequate knowledge and competence in caring for older people, adequate time and continuity in the care organization (II). Formal caregivers reported higher perceived quality of care in the dimensions medical-technical competence and physical-technical conditions than in identity-oriented approach and socio-cultural atmosphere (III). In the organizational climate three of the dimensions were close to the value of a creative climate and in seven near a stagnant climate. The formal caregivers reported low rate of stress of conscience. The RNs reported to a higher degree than the NAs/ENs a need to gain more knowledge, but the NAs and the ENs more often received training during working hours. The RNs reported lower emotional well-being than the NAs/ENs (III). The formal caregivers' occupation, organizational climate and stress of conscience were associated with perceived quality of care (IV). Implications: The formal caregivers should have an awareness of the importance of kindness and respect, supporting the older people to retain control over their lives. The nursing managers should employ highly competent and adequate numbers of skilled formal caregivers, organize formal caregivers having round the clock continuity. Improvements of organizational climate and stress of conscience are of importance for good quality of care.

Experimental and computational investigation of the roll forming process

One of the first questions to consider when designing a new roll forming line is the number of forming steps required to produce a profile. The number depends on material properties, the cross-section geometry and tolerance requirements, but the tool designer also wants to minimize the number of forming steps in order to reduce the investment costs for the customer. There are several computer aided engineering systems on the market that can assist the tool designing process. These include more or less simple formulas to predict deformation during forming as well as the number of forming steps. In recent years it has also become possible to use finite element analysis for the design of roll forming processes. The objective of the work presented in this thesis was to answer the following question: How should the roll forming process be designed for complex geometries and/or high strength steels? The work approach included both literature studies as well as experimental and modelling work. The experimental part gave direct insight into the process and was also used to develop and validate models of the process. Starting with simple geometries and standard steels the work progressed to more complex profiles of variable depth and width, made of high strength steels. The results obtained are published in seven papers appended to this thesis. In the first study (see paper 1) a finite element model for investigating the roll forming of a U-profile was built. It was used to investigate the effect on longitudinal peak membrane strain and deformation length when yield strength increases, see paper 2 and 3. The simulations showed that the peak strain decreases whereas the deformation length increases when the yield strength increases. The studies described in paper 4 and 5 measured roll load, roll torque, springback and strain history during the U-profile forming process. The measurement results were used to validate the finite element model in paper 1. The results presented in paper 6 shows that the formability of stainless steel (e.g. AISI 301), that in the cold rolled condition has a large martensite fraction, can be substantially increased by heating the bending zone. The heated area will then become austenitic and ductile before the roll forming. Thanks to the phenomenon of strain induced martensite formation, the steel will regain the martensite content and its strength during the subsequent plastic straining. Finally, a new tooling concept for profiles with variable cross-sections is presented in paper 7. The overall conclusions of the present work are that today, it is possible to successfully develop profiles of complex geometries (3D roll forming) in high strength steels and that finite element simulation can be a useful tool in the design of the roll forming process.