Fatigue Limit Improvement and Rendering Defects Harmless by Needle Peening for High Tensile Steel Welded Joint

Abstract

The effects of needle peening (NP) on the bending fatigue limit of a high tensile steel (HTS) HT780 (JIS-SHY685)-welded joint containing an artificial semicircular slit on the weld toe were investigated. Three-point bending fatigue tests were conducted at a stress ratio of R = 0.05 for NP-treated welded specimens with and without a slit. The fatigue limits of all specimens increased by 9-200% due to the NP treatment. Furthermore, NP-treated specimens with slit depths of a = 1.0 mm exhibited high fatigue limits that were equal to those of NP-treated specimens without a slit. Therefore, a semicircular slit of less than a = 1.0 mm could be rendered harmless through NP treatment. This result indicates that the reliability of HTS-welded joints can be significantly improved via NP for surface defects with depths that are less than 1 mm, which are not detected through non-destructive inspection (NDI). Therefore, the problem regarding the reliability of HTS-welded joints that restricts the industrial utilization of HTS can be solved by performing both NDI and NP. The dominant factor that contributed to the improvement of the fatigue limit and increase in the acceptable defect size was the introduction of large and deep compressive residual stress with non-propagating cracks. could restrain crack opening and render the crack harmless [9][10][11][12][13][14][15][16]. For example, Takahashi et al. [9] reported that the fatigue limit of a spring steel JIS-SUP9A specimen, which contained crack-like semicircular slits with a depth of 0.2 mm that decreased the fatigue limit by 50%, could be rendered harmless through shot peening. Furthermore, Houjou et al. [13] reported that the acceptable semicircular slit depths that could be rendered harmless after NP were 1.0 and 1.2 mm for stainless steel JIS-SUS316 and low-carbon steel JIS-SM490A welded joints, respectively. These results indicate that surface defects that are smaller than the detection limit of NDI, which was reported to be approximately 1-mm deep using a typical eddy current flaw detection method [17], can be rendered harmless by NP.If the surface defects below the detection limit of NDI in a HTS-welded joint could be rendered harmless by NP, the reliability of the welded joint could be significantly improved and the industrial use of HTS would be expanded. Therefore, the present study involved clarifying the surface defect size in a HTS-welded joint that can be rendered harmless by NP.Specifically, fatigue tests were conducted for both NP-treated and untreated HTS-welded specimens that contained crack-like semicircular slits at the weld toe. The fatigue limit of metals changes depending on residual stress, hardness, metal microstructure and stress concentrations. Therefore, the changes of these factors after NP were investigated to better understand the fatigue test results. Furthermore, the presence of non-propagating cracks was investigated on NP-treated welded specimens with a semicircular slit tested under the fatigue limit to ascertain whether an acceptable defect...

Effects of laser peening on the fatigue strength and defect tolerance of aluminum alloy

“…Takahashi et al clarified that the fatigue strength of spring steel specimens with artificial surface defects could be improved with SP by the same level as that attained in the cases of defect‐free steel specimens subjected to the same SP . Similar effects of SP and needle peening have been reported in various other materials for the rendering of harmless surface defects . Specifically, Takahashi et al clarified that semicircular slits with depths of 0.1 mm in the aluminum alloy A7075‐T651 could be rendered harmless by performing SP .…”

“…14,15 Similar effects of SP and needle peening have been reported in various other materials for the rendering of harmless surface defects. [16][17][18] Specifically, Takahashi et al clarified that semicircular slits with depths of 0.1 mm in the aluminum alloy A7075-T651 could be rendered harmless by performing SP. 19 Several studies have shown that the propagation of fatigue cracks in aluminum alloys is delayed by applying LP at the tip of these cracks.…”

Fracture, Fatigue, and Structural Integrity of Metallic Materials

“…Leitner et al [5] analyze how overloads induce fatigue crack growth retardation on EA47 steel round bars containing semi-elliptical cracks, comparing the experimental results to the predictions provided by a modified NASGRO equation. Finally, Fueki et al [6] evaluates the fatigue limit improvement caused by needle peening in high tensile (strength) steel HT780. All these contributions cover different sectors, such as aerospace, railway, bridge design, or energy generation, among others.…”

Improving Fatigue Limit and Rendering Defects Harmless through Laser Peening in Additive-Manufactured Maraging Steel

“…Therefore, if the surface defects can be rendered harmless in terms of the fatigue limit through peening, the reliability of additive-manufactured metals can be improved, which can contribute to the increased industrial usage of additive-manufactured parts. Surface defects can be rendered harmless through various types of peening on conventionally manufactured metals [22][23][24][25]. Takahashi et al reported that a semicircular surface defect with a less than 0.2 mm depth could be rendered harmless by applying SP to spring steel [22].…”

“…Takahashi et al reported that a semicircular surface defect with a less than 0.2 mm depth could be rendered harmless by applying SP to spring steel [22]. Fueki et al clarified that a semicircular surface defect with a depth of less than 1 mm could be rendered harmless by applying needle peening to high-tensile steel-welded joints [23]. Takahashi et al compared the maximum defect size that can be rendered harmless by SP and cavitation peening (CP) in 7075 aluminum alloy.…”