High Frequency Mechanical Impact (HFMI), also known as Ultrasonic Impact Treatment (UIT ), is a high-frequency weld impact treatment designed to improve the fatigue resistance of welded structures.In most industrial applications this process is also known as ultrasonic peening (UP).
It is a cold mechanical treatment that involves striking the weld toe with a needle to create an enlargement of its radius and to introduce residual compressive stresses.
In general, the basic UP system shown could be used for treatment of weld toe or welds and larger surface areas if necessary.
Freely Movable Strikers
The UP equipment is based on known from the 40’s of last century technical solutions of using working heads with freely movable strikers for hammer peening. At that time and later, a number of different tools based on using freely movable strikers were developed for impact treatment of materials and welded elements by using pneumatic and ultrasonic equipment. The more effective impact treatment is provided when the strikers are not connected to the tip of actuator but could move freely between the actuator and the treated material. The tools for impact treatment of materials and welded elements with the freely movable strikers that are mounted in a holder are shown. In the case of so-called intermediate element-striker(s) a force of only 30 - 50 N is required for treatment of materials.
Sectional view through tools with freely movable strikers for surface impact treatment.
It shows a standard set of easy replaceable working heads with freely movable strikers for different applications of UP.
A set of interchangeable working heads for UP
During the ultrasonic treatment, the striker oscillates in the small gap between the end of the ultrasonic transducer and the treated specimen, impacting the treated area. This kind of high frequency movements/impacts in combination with high frequency oscillations induced in the treated material is typically called the ultrasonic impact.
Technology and Equipment for Ultrasonic Peening
The ultrasonic transducer oscillates at a high frequency, with 20-30 kHz being typical. The ultrasonic transducer may be based on either piezoelectric or magnetostrictive technology. Whichever technology is used, the output end of the transducer will oscillate, typically with amplitude of 20 – 40 mm. During the oscillations, the transducer tip will impact the striker(s) at different stages in the oscillation cycle. The striker(s) will, in turn, impact the treated surface. The impact results in plastic deformation of the surface layers of the material. These impacts, repeated hundreds to thousands of times per second, in combination with high frequency oscillation induced in the treated material result in a number of beneficial effects of UP.
The UP is an effective way for relieving of harmful tensile residual stresses and introducing of beneficial compressive residual stresses in surface layers of parts and welded elements.
In the fatigue improvement, the beneficial effect is achieved mainly by introducing of the compressive residual stresses into surface layers of metals and alloys, decrease in stress concentration in weld toe zones and the enhancement of the mechanical properties of the surface layer of the material.
Industrial Applications of UP
The UP could be effectively applied for fatigue life improvement during manufacturing, rehabilitation and repair of welded elements and structures. The UP technology and equipment were successfully applied in different industrial projects for rehabilitation and weld repair of parts and welded elements. The areas/industries where the UP was applied successfully include: Railway and Highway Bridges, Construction Equipment, Shipbuilding, Mining, Automotive and Aerospace.