Berthold Scholtes

Bio: Berthold Scholtes is an academic researcher from University of Kassel. The author has contributed to research in topics: Residual stress & Shot peening. The author has an hindex of 21, co-authored 146 publications receiving 2019 citations. Previous affiliations of Berthold Scholtes include Karlsruhe Institute of Technology.

On the influence of mechanical surface treatments—deep rolling and laser shock peening—on the fatigue behavior of Ti–6Al–4V at ambient and elevated temperatures

Abstract: It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly-stressed metallic components. Deep rolling (DR) is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of DR on the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) behavior of a Ti–6Al–4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near-surface microstructures. Preliminary results on laser shock peened Ti–6Al–4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ∼450 °C, i.e. at a homologous temperature of ∼0.4 T/T m (where T m is the melting temperature). Based on cyclic deformation and stress/life ( S / N ) fatigue behavior, together with the X-ray diffraction and in situ transmission electron microscopy (TEM) observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti–6Al–4V at such higher temperatures, despite the almost complete relaxation of the near-surface residual stresses. In the absence of such stresses, it is shown that the near-surface microstructures, which in Ti–6Al–4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.

Laser Peening without Coating as a Surface Enhancement Technology

Abstract: Laser peening without coating (LPwC) is an innovative surface enhancement technology, which imparts compressive residual stress without any surface preparations. Materials were peened in aqueous environment with laser pulses of about 100 mJ from a Q-switched and frequency-doubled Nd:YAG laser. Surface roughness of the materials somewhat increased due to ablative interaction. Compressive residual stress nearly equal to the yield strength of the materials appeared at the surface after LPwC in spite of the possible heat effect by direct laser irradiation to the materials. The depth of the compression reaches 1 mm or more from the peened surface. High-cycle fatigue properties were evaluated through rotating-bending or push-pull type testing for an austenitic stainless steel (SUS316L), a titanium alloy (Ti-6Al-4V) and a cast aluminum alloy (AC4CH). LPwC significantly prolonged the fatigue lives despite the increase in surface roughness. Accelerating stress corrosion cracking (SCC) tests showed that LPwC completely eliminated SCC susceptibility of sensitized austenitic stainless steels, nickel-based alloys and their weld metals. LPwC has been utilized to prevent SCC in Japanese nuclear power reactors since 1999.

Effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on Residual Stress State and Fatigue Strength of AISI 304

Abstract: The effects of a new mechanical surface treatment method, called ultrasonic nanocrystal surface modification (UNSM), on near-surface microstructures and residual stress states as well as on the fatigue behavior of an austenitic steel AISI 304 are investigated and discussed. The results are compared with consequences of other mechanical surface treatment methods such as deep rolling or shot peening.

Mechanical and thermal stability of mechanically induced near-surface nanostructures

Abstract: Mechanical surface treatments, such as deep rolling, shot peening, hammering, etc., can significantly improve the fatigue behaviour of metallic materials owing to near-surface nanocrystallisation, strain hardening and compressive residual stresses. In this paper, we investigate the stability of near-surface microstructures of deep rolled austenitic stainless steel AISI 304 and turbine blade alloy Ti–6Al–4V during high temperature fatigue (up to 600 °C) by transmission electron microscopy and X-ray diffraction. The investigated nanocrystalline regions are stable during short time annealing and unstable during long time annealing at 600 °C. Isothermal fatigue in the low cycle fatigue regime at high stress amplitudes does not alter the nanocrystalline region up to 600 °C.

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Residual stress. Part 2 – Nature and origins

Abstract: Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be detrimental when it reduces the tolerance of the material to an externally applied force, as is the case with welded joints. On the other hand, it can be exploited to design materials or components which are resistant to damage, toughened glass being a good example. This paper, the second part of a two part overview, the first part having been devoted to measurement techniques, examines the nature and origins of residual stresses across a range of scales. This extends from the long range residual stress fields in engineering components and welded structures, through the interphase stresses present in composites and coatings, to the microscale interactions of phase transformations with local stresses.