Indication of cracking during pressing of advanced thin plates using an AE-based monitoring system

Summary, in English

The acoustic emission that is detectable in the forming of complex thin part

plates contains information about the quality of the pressed plates. The information can

be used as process control and to filter out advanced thin plates with defects such as

cracks.

The article describes the results obtained in a try-out pressing process in special

developed punch-die pairs with different pressing depths in a press at Alfa Laval in

Lund. At the trials two module testing tools called SE30 and SE25 were used to create a

different degree of formability by different surface enlargement in the same complex

original design. Used sheet materials were stainless steel and two qualities of titanium.

The used measuring system was based on a PC with AE-sensor of model 8152 from

Kistler.

The results of experiments show that it is possible to detect and distinguish the

plates that fracture during the forming operation. For a large number of plates detectable

signals from areas close to cracks with only local necking were also obtained. This local

necking cannot or can hardly be detected by light-test or light detecting equipment.

The left figure shows a metal sheet

with fractures and the right figure

describes measured acoustic

emission as a function of time and

the number of the details. In this

example the first four plates are

fractured and the last five plates are

without any fractures. There are several differences between the two types of signals of

acoustic emission. The signal that indicates cracking of the plate differs by a sudden

high acoustic emission and also a slightly lower level before the crack formation is

started. Before the fracture occurs the sheet metal is stretched out and the lower level of

acoustic emission also indicates less movement of the sheet metal just before the time of

fracture. The purpose of the conducted experiment was to find out whether the selected approach

was appropriate. The obtained result shows unambiguously that the chosen approach is

pertinent and that an AE-based monitoring system can be used as a process control

system to ensure the quality of production of advanced thin plates in stainless steel and

titanium. Information can be saved from the main forming operation and can be used to

see trends both long and short term.