Development of a damage-based criterion for ductile fracture prediction in sheet metal forming

In the micro-bending process of sheet metals, springback caused by the considerable elastic recovery during unloading has a significant effect on the dimension precision of micro-bent sheet metal parts. In design of micro-bent parts and bending process, this phenomenon needs to be considered. However, the traditional well-established knowledge for analysis of macro-scaled sheet metal bending processes may not be very suitable and efficient in analysis of micro-scaled sheet metal bending due to the geometry and microstructure size effects in the process. To explore the unloading springback of sheet metals after micro-bending, the experimental studies on the interactive influence of a few significant parameters including sheet thickness, grain size, and punch radius on the springback in micro U-bending process of copper alloy sheets were conducted. An appropriate constitutive model based on the surface layer model, which is widely used in micro-scaled forming processes, is proposed to predict the amount of springback and the relationship among these parameters is further established. The research shows that the springback angle generally increases with the decreasing sheet thickness, and for different punch radii, the springback angle has different variation trends for the sheet metals with different grain sizes. The developed analysis method is verified via the comparison of the calculated stress–strain curves with the tensile test results of three kinds of copper alloy sheets. The findings on how the size effects affect the springback behaviors of in micro-bending process are validated by the experimental results, finite element simulation, and the comparison in-between.

Analysis of the Size Effect on Springback Behavior in Micro-Scaled U-Bending Process of Sheet Metals†

In a world where the concept of just-in-time has been lifted outside of the manu-facturing world and onto the market, the increasing demand of luxury wares like cars is accompanied only by tougher industrial conditions and no room for error. Loss of productivity and decrease in efficiency can be devastating to any manufac-turing company in today’s market climate. One of the largest causes of these is-sues is production downtime due to machine breakdown or unexpected failure. 
To prevent such events from happening, Volvo Cars has decided to investigate the possibility of using vibration analysis for predictive maintenance and condition monitoring of presses used to manufacture sheet-metal car body components. 

Although the concept of using vibration analysis for predictive maintenance is not new and the positive effects have been established, uses in complex machinery with intermittent cycles as mechanical presses have at the time of this thesis not been widely studied. This thesis aims to research if vibration analysis is a useable tool for predictive maintenance in mechanical presses and what information it might provide. 

Vibration analysis is a wide field of technology and there are endless tools and methods to perform analysis. This thesis mainly focuses on the possibilities in using Power Spectral Density (PSD), Fast Fourier Transform (FFT) and statistical parameters like Root Mean Square (RMS) and Kurtosis. These are used in many applications and there are many companies that are familiar with implementing these tools, making them suitable to research. 

After a thorough investigation of the use of PSD, FFT, RMS, and Kurtosis on the main components of the press crown and driving system, it is possible to conclude that vibration analysis is useable when using predictive maintenance on a press. Some configuration of PSD, FFT, and RMS gives the most benefits in detecting and analysing faults and issues. However, more data must be collected to establish when alarms must be issued to the maintenance crew and to build thorough models that provide enough information for making informed diagnosis and decisions based on vibration analysis.

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Vibration analysis for condition monitoring of mechanical presses

Pressing of rectangular saggers with an elastic medium

To provide high precision of punching, particularly in the case of thin workpieces, the invention proposes a punching tool (1) which is to be inserted between the ram (15) and the bolster (16) of a press, has a cutting punch (6) and a die (9), the tool (1) consisting of a top part (2) and a bottom part (4) which can be moved towards one another and are connected to one another at one end via an intermediate piece (3), and, approximately opposite the intermediate piece (3), the cutting punch (6) being fixed to the top part (2) and the die (9) to the bottom part (4), and an opening force acting between the top part (2) and the bottom part (4).

Punching tool for insertion between the ram and the bolster of a press

A press 1 comprises an upper part 2 with a slide 8 for a stamping tool 13, and a lower part 3 with a platen 6 for a counterpart tool 7. The up-and-down movement of the slide 8 is guided by columns 14. Metal components such as cans or can lids are fed from a device 4. A wheel 12 adjusts a V-belt 10 from a cone 11 to a flywheel 9. Heating cartridges 15 bring the upper part 2 up to operating temperature before the press 1 is brought into operation so as to reduce registration error.

Lang Dr Klaus E

Papers

Sheet metal stamping

Stanzautomat und verfahren zur vermeidung eines justierfehlers bei einem stanzautomaten