Mechanical Fatigue

Mechanical fatigue is a phenomenon of localized cumulative damage that leads to a final sudden breakage or to a complete loss of functionality of mechanical components. The first article on the subject dates back to the publication of Wilhelm Albert in 1837, where the concept of "repeated stress" on the chains used in coal mines was introduced. In the following 20 years, the phenomenon was observed more deeply and carefully. Poncelet introduced the concept of "fatigue", Rankine introduced the stress intensity factor, and Wohler began the first systematic studies on fatigue. Talk about "fatigue limit" of materials started in the first years of the 20th century. In those years, the only possible approach was a "stress-life" type that is an approach that does not analyze the mechanism of damage accumulation but only the resistance to cyclic stresses up to the final event of breakage. Today, their understanding of the phenomenon is much more extensive. Modern approaches are much more accurate than those of the past. These approaches go from the "strain-life" type, which relates the degree of local strain to fatigue life, and continue on to the whole world of fracture mechanics in its various formulations, from the simplest to the most complex, which take into account how fatigue cracks nucleate and propagate. Modern and powerful computer simulations allow them to make high precision estimates of the stresses to which a component is subject but nevertheless, they are not yet fully able to predict with absolute certainty the actual duration of a component subject to fatigue. The more complex the technological history of a component, the more difficult the prediction of its resistance to fatigue. An example of this is related to residual stresses. Each phase of the production cycle, whether you want it or not, introduces a "quota" of residual stresses. Thermochemical treatments and mechanical processing in their entirety introduce residual stresses of tension or compression depending on the case. Residual stresses are extremely important for the fatigue life of a mechanical component. "Beneficial" shot peening residual stresses overlap and combine with pre-existing ones introduced by manufacturing processes, giving the overall characteristics of fatigue strength of the component. The importance of an accurate knowledge of shot peening residual stresses as well as the global ones is clear and obvious. Whatever the origin, residual stresses can be measured by appropriate investigative techniques.

X Ray Diffraction

In the case of shot peening, the most suitable technique is that of X-ray diffraction. It consists in projecting an X-ray beam onto the surface of the material to be analyzed. The X-ray beam interacts with the atoms of the crystal lattice which in turn emit, among other things, X-ray beams coherent (same wavelength and same phase) with the incident beam. From the analysis of the incident and the diffracted rays, through appropriate algorithms, it is possible to trace back the interatomic distance of the crystal lattice and, consequently, the residual stress of the measured configuration. The advantage of this technique is the extreme sensitivity on the surface and the capacity to measure very high residual stress gradients. A second technique that can be used is the blind hole drilling method with a strain gauge rosette. It consists in glueing a strain gauge rosette on the surface to be measured and drilling a hole in the center thereof. The material removed from the hole causes a relaxation and a rebalancing of the surrounding material. The strain gauge detects deformations induced by the rebalancing. Through appropriate algorithms, these deformations are consequently transduced into residual stresses. The advantage of this technique is its capacity to explore elevated depths up to 2 mm, while on the contrary, it is not very sensitive in the first surface layers. Depending on the case, Peen Service applies the most appropriate technique or, whenever possible, combines them appropriately to obtain the advantages of both.

Continuous Research

From what has been said so far, when trying to understand the mechanisms of mechanical fatigue, it becomes necessary to keep in mind all aspects of the technological processes involved, from the metallurgical to the design and up to the final application. For this reason,   Peen Service is constantly engaged in research and works in synergy with its clients in order to find solutions to their problems. Nowadays, with all the information, literature and tools available, one might think it is possible to control all relevant parameters to obtain a perfect fatigue life estimate. The reality is sometimes more complex than what can be simulated. Human contribution and experience are a fundamental part of the design process and with shot peening, everything is amplified by the fact that the process interacts with both the metallurgical and the design process aspects up to those concerning final functionality. It happens that the root causes of failures are spread over several aspects often not easy to discern. For this reason, more than 15 years ago, Peen Service gave life to a research program in collaboration with three Italian universities (Politecnico of Milan, Engineering Faculties at Trento and Bologna), dedicated to wide basic research and to the dissemination of the shot peening design approach. It seems strange to speak of design referring to a process and not to a product, but the potential performance can be very high only if the shot peening parameters are optimized on the basis of the specific case, through a conceptual framework similar to a real part design. In order to understand what should be the appropriate approach, Peen Service holds courses and seminars in universities and technical institutes.

An Application

One example concerns an elastic metallic joint for an aeronautical application. The component was made … of two highly deformable welded flanges. The parts to be treated were titanium discs only a few tenths of a millimeter thick. Conventional shot peening would have caused unacceptable distortions of the component. In cooperation with the customer, a measurement campaign of the shot peening residual stress was activated with the dual purpose of optimizing the peening stress profile and, at the same time, ensuring the fundamental geometric characteristics. The optimized result was subsequently validated by means of experimental fatigue tests.

22 Shot Peening Machines

Peen Service has always had a strategic vision based on quality. Shot peening in particular is influenced by the technology used and, in order to obtain the highest quality levels, advanced compressed air machines able to guarantee the control of the process parameters within the tightest tolerances are necessary. Peen Service has twenty-two fully automated compressed air machines. The quality and the level of performance of the shot peening can be obtained and ensured only by the simultaneous presence of a good design and a rigorous control of the production process. For this reason, there are detailed technical specifications, such as, for example, the ones related to the aerospace industry, which evolve over time in order to adapt to technical progress. The respect of these specifications is one of the keys to a good result. Last but not least, are the preparation and qualification of technical staff. Peen Service operators are constantly trained through courses and practical training throughout their entire career.

One Step Beyond

It is not possible to become a shot-peener all of a sudden; experience and judgment are absolutely irreplaceable values acquirable only in the field. The logic that in the past characterized the customer-supplier relationship in which the supplier blindly followed the customer’s instructions, is replaced in Peen Service by a broader view in which the supplier can provide his know-how for a synergetic technological growth.

For Information:
Peen Service s.r.l.
Via Augusto Pollastri 7
40138 Bologna, Italy
Tel. +39.051.533550
Fax +39.051.531307
E-mail: m.bandini@peenservice.it
www.peenservice.it