Laser peening (LP) is a well-established technique for introducing compressive residual stress (RS) near the surface of metal components to improve their high-cycle fatigue properties. In this study, butt-welded joint specimens of SBHS500 steel with angular distortion were prepared and treated with LP. X-ray diffraction showed that the maximum compressive RS over 400 MPa was introduced near the surface by applying LP with an irradiated pulse energy of 7.5 mJ, a spot size of 0.42 mm and a pulse density of 800 pulses/mm2. The effect reached a depth of approximately 0.15 mm from the surface. The specimens were subjected to a uniaxial fatigue test with a stress ratio of 0.1 together with specimens without LP. The results showed that the fatigue life was prolonged by LP. However, in the stress range of 300 MPa, the detrimental effect of angular distortion exceeded the beneficial effect of LP, and the fatigue life was not extended by LP.

Laser peening (LP) is a well-established technique for introducing compressive residual stress (RS) near the surface of metal components, to improve their high-cycle fatigue properties. The authors have developed a compact LP device with a thumb-sized Nd:YAG microchip laser mounted on a collaborative robot arm. The device was applied to 9-mm-thick HT780 high-strength steel plate samples with irradiated pulse energies of 7.5−8.0 mJ, spot sizes of 0.42−0.58 mm and pulse densities of 100−1,600 pulses/mm2. X-ray diffraction showed that the maximum compressive RS was over 500 MPa near the surface, and the LP effect reached a depth of approximately 0.1 mm from the surface. Butt-welded HT780 samples were laser-peened with a pulse energy of 7.7 mJ, spot size of 0.49 mm and pulse density of 800 pulses/mm2. Then, the samples were subjected to a uniaxial fatigue test with a stress ratio of 0.1. The results showed that the fatigue strength at 107 cycles was improved by at least 50 MPa, comparable to the improvement attained by LP in a previous study with a pulse energy of 200 mJ from a conventional Nd:YAG laser.

Laser peening can introduce compressive residual stresses on the surface of various materials, thereby effectively prolonging their fatigue lives. In this study, the effects of laser peening with pulse energies of 20 mJ and 70 mJ on residual stress and fatigue life were investigated on two types of HT780 box-welded specimens with different sizes, considering the realization of a portable laser peening system equipped with a small laser device with low pulse energy. As a result of such low pulse energy laser peening, it was found that the depth of compressive residual stress becomes shallower compared to the current laser peening with a pulse energy of 200 mJ, while the residual stress on the surface remains at the same level. Bending fatigue tests were conducted with the stress ranges at 100 MPa and 150 MPa, which revealed that the fatigue life of the specimens with low pulse energy laser peening was at least 50 times and 8 times longer than that of the as-welded specimens, respectively. Comparing the fatigue test results of different size specimens, it is clear that laser peening with low pulse energy is effective in extending the fatigue life of HT780 box-welded specimen regardless of its size.

Various peening techniques have been used to improve the fatigue strength of steel structures. Among them, base metal impact hammer peening shows significant improvement in fatigue strength in ordinary steel, but the effect on high-strength steel has not been sufficiently studied. Accordingly, this study applied base material hammer impact peening to test specimens of 780 MPa grade high-strength steel (HT780) and 490 MPa grade ordinary steel (SM490), and the residual stress was measured and simulated. The experimental results clarified that a large compressive residual stress was introduced into the inner part of the plate thickness near the indentation in the high-strength steel, although the range of introduction of residual stress was equivalent in both the ordinary steel and high-strength steel.

Laser peening introduces compressive residual stresses on the surfaces of various materials and is effective in enhancing fatigue strength. Using a small microchip laser, with energies of 5, 10, and 15 mJ, the authors applied laser peening to the base material of an HT780 high-strength steel, and confirmed compressive residual stresses in the near-surface layer. Laser peening with a pulse energy of 15 mJ was then applied to fatigue samples of an HT780 butt-welded joint. It was confirmed that laser peening with the microchip laser prolonged the fatigue life of the welded joint samples to the same level as in previous studies with a conventional laser.

Due to the increase of the amount of traffic and the weight of trucks, the fatigue cracks are occurring at the welding joints of steel bridges. This report shows the properties on the surface and depth direction of residual stress of welded joint specimens which were treated by hummer peening in order to study the effect of the hummer peening to the high-strength (HT780) and the conventional (SM490) steel. It was also clarified that the fatigue life of the high-strength steel welded joint specimens which were treated by hummer peening were more than twice as long as the untreated them.

Laser peening can introduce compressive residual stress on the surface of various materials and, therefore, is effective in enhancing the fatigue strength. This study clarifies whether the laser peening with lower pulse energy comparing to the preceding studies generates compressive residual stress, and whether such stress would account for prolonged fatigue life in the welded zone of high-strength steel HT780. As a result, laser peening condition, which can generate large compressive residual stress on the base metal surface, were selected under the pulse energy of 6, 10 and 20mJ, respectively. With the reduction of the pulse energy, it was observed that the depth of the compressive residual stress tended to decrease. Fatigue lives of the toe of the butt welded joints pretreated by laser peening with the selected conditions were prolonged to the same level of fatigue lives with the pulse energy of 200mJ.

Recently, response control dampers are used to improve seismic response of high-rise building. In general, it is required to arrange the dampers efficiently on the plane of high-rise building structure. However, it is not easy to obtain the optimal placement of the dampers, because the damper’s performance depends on the dynamic behavior. Therefore, in this paper, a method to obtain an optimal placement of the response control dampers on the plane of high-rise building structure is proposed. In the present method, first, the dampers are placed on all possible places in the frame structure, and then the dampers are gradually removed by Evolutionary Structural Optimization (ESO) method. The accumulated damping energy of the damper is used to determine the removal order. The effectiveness of the present method is verified by comparison with general placement plans.

Fatigue life extension of materials and structures have great impact on any engineering field, and then have been extensively researched up to the present. Among several life extension techniques for metals and its structures, the laser peening (LP) with nano-second laser and/or femto-second laser have been highlighted, since the both of the laser peening techniques can generate deep compressive stress and material hardening fields. However, the difference between these processes and the optimum condition on the fatigue life extension are not well understood since their are conducted under extremely high strain-rate conditions. In this work, the effects of laser peening processes incorporating both nano-second laser and femto-second laser on the predicted mechanical properties are studied by using a FEM models created with ABAQUS Dynamic schime. A Johnson-Cook material model considering temperature and strain-rate ef-fects is adopted in the analysis, and a mechanism of residual stress, material haedening and deformation generation in the models and difference of the predicted results between both LP processes is discussed in detail.

Laser peening can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. This study targets 780 MPa grade high-strength steel (HT780) in order to clarify whether laser peening generates compressive residual stress on the surface of HT780, and whether such stress would account for prolonged fatigue life in the welded zones of HT780. As a result, large and deep compressive residual stress was generated on the base metal surface and at the boxing toe of HT780 under the peening conditions employed for 490 MPa grade steel. The smaller the applied stress range, the greater was the improvement of the fatigue life of the high-strength steel boxing toe by laser peening.

Laser peening can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. In this study, we conducted laser peening in air with a water film formed by a nozzle and examined whether the distribution of residual stress along the thickness was different from that achieved laser peening in water in our previous studies. We also assessed the resultant residual stress and fatigue life when the pulse energy was reduced for developing a simple method to conduct laser peening on large structures. Compressive residual stress equivalent to that observed after laser peening in water was obtained in nozzle-type laser peening in terms of magnitude and depth. With the reduction of pulse energy, it was observed that the depth of the compressive residual stress tended to decrease significantly and the fatigue life also tended to reduce. The results indicate that the depth of the compressive residual stress has a considerable effect on the fatigue life of welded structures as well as the magnitude of the surface residual stress.

The purpose of this study is to clarity the influence of beam-end details on the fracture and deformation capacity of beam-to-column connections made of H-SA700B high-strength steel. This paper presents the results of full-scale tests of site-welded beam-to-column connections with four types of the beam-end details: straight, cutout type horizontal haunch, weld type horizontal haunch and drilled flange. Based on the test results, both of the two types of horizontal haunch beam-to-column connections can avoid brittle fracture and have larger deformation capacity compare to the straight beam-to-column connection. However, the drilled flange beam-to-column connection cannot avoid brittle fracture.

In this study, the relationship between the heat cycle and the Charpy absorbed energy is defined by simulating various heat cycles by performing the synthetic HAZ test using steel with a relatively high Charpy absorbed energy and steel with a distinctly low Charpy absorbed energy. On the basis of this relationship, the authors have studied the effects of the steel toughness on the Charpy absorbed energy of weld HAZs inputted the several type of welding heat cycle. As the results, the Charpy absorbed energy of the CGHAZ dropped in the case of single-pass welding, but the Charpy absorbed energy increased considerably when the maximum temperature decreased, irrespective of the steel toughness. The Charpy absorbed energy of the weld interface in multi-layer welding was greatly susceptible to the achieved temperature after the maximum achieved temperature, and the range of the temperature after the maximum temperature was achieved in which the Charpy absorbed energy was recovered was found to be different depending on the steel toughness.

Laser peening without coating (LPwC) can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. This study used butt welded structural steel joints to investigate changes in the residual stress and the hardness near the toe of the welded zone and to examine the major factor causing the improved fatigue strength due to LPwC. It is concluded that the generation of compressive residual stress by LPwC is the major factor improving the fatigue strength, because the reduction in compressive residual stress due to stress relief annealing decreased the fatigue strength to the same level as that of butt welded joints without LPwC.

Purpose Laser peening without coating (LPwC) is an innovative surface enhancement technology for introducing compressive residual stress in metallic materials. The purpose of this study is to examine the characteristic at the laser-peened welded zone and the fatigue lives of the welding joints. Design/methodology/approach LPwC conditions for 490MPa grades of structural steels were selected. By using the conditions, the characteristic at the laser-peened welded zone, residual stresses, hardness and roughness of welding toes were examined. Moreover, the fatigue lives of the toes of box-welded joints and butt welded joints pre-treated by LPwC were compared to the fatigue lives of those that were not pre-treated by LPwC. Findings The main results are: LPwC conditions for 490MPa grade steels were established; residual stresses, Vickers hardness and roughness at the laser-peened welded zone were revealed; and LPwC can dramatically extend the fatigue life of welded joint. Originality/value The effects of LPwC on structural steels, which are widely used in bridge members, have not been well clarified; the effect of LPwC on welded zones in these structures is particularly unclear. If LPwC can be carried out such that compressive residual stress is imparted on structural steels and the welded zones in the bridge members, the fatigue lives of bridge members will be greatly increased. The paper fills some of these gaps. © 2011 Emerald Group Publishing Limited. All rights reserved.

The purpose of this study is to clarity the influence of the beam-end details on the fracture and deformation capacity of beam-to-column joints using SA44O. This paper presents the results of full-scale tests of welded beam-to-column joints using SA440 with some type of the beam-end details( method of drilled flange, non-scallop method and scallop method) on site and shop.Based on the test results, non-scallop method and the method of drilled flange have large deformation capacity and deformation capacity of site welded beam-to-column joints with the method of drilled flange improved over two times lager than common site welded beam-to-column joints.

Laser peening is an innovative surface enhancement technology to introduce a compressive residual stress on metallic materials. In this paper, residual stress, Vickers hardness and surface profile of four grades of structural steel were examined to clarify the applicability of laser peening. The results obtained are summarized as follows. 1) Laser peening conditions, which can generate large and deep compressive residual stress, were established for 400 to 780MPa grade steels. 2) Compressive residual stress becomes deeper with the increase of material strength, whereas the hardened layer becomes shallower in the same laser peening conditions. 3) The surface got dented by 10 to 30μm from the initial surface and the surface roughness (Ra) became 3.5 to 13μm after laser peening.

The weld defect of the welded beam-to-column joint in steel structure causes the brittle fracture and has possibilities to reduce the deformation capacity. It is considered that the effect of the weld defect on the deformation capacity from the difference of between shop and site welding beam-to-column joints is different. Therefore it is important to understand this difference. In this paper, full-scale welding test was done it were shop and site welding beam-to-column joints. Then, the effect of the deformation capacity of test specimens caused by the difference of the position of welding defects between two joint methods was studied. As the results, the deterioration ot deformation capacity caused by the welding delect was low using shop welding beam-to-column joints. and high using site welding beam-to-column joints.

It is well known that the strain rate affects tensile behavior for example, it affects the tensile strength and fracture elongation of metals. When examining the tensile behavior of steel, it is important to pay attention to the heat-affected zone (HAZ) as well as the base metal. The effects of the strain rate on the tensile strength were investigated by using specimens with stress concentrations in particular, the tensile strength of SM400, SM490, and HT780 were obtained from HAZs by performing tensile tests at strain rates of 0.085%/s, 85%/s, and 1.000%/s. The HAZs were simulated by using a thermal/mechanical simulator. The main results are summarized as follows: 1) Regardless of the grade of steel, the tensile strength of specimens that underwent ductile fracture increase at high strain rates, whereas the tensile strength of specimens that underwent brittle fracture did not increase. 2) Regardless of the grade of steel and regardless of whether welding heat was supplied or not, the rate of increase in the tensile strength as a result of the high strain rate can be predicted from the tensile strength, which can be determined by static testing.

For brittle fractures in welding connections, it is important to pay attention to toughness decrease in weld heat affected zone (HAZ). In this paper, relationship between Charpy absorbed energy of simulated HAZs of “low toughness steel” and welded conditions were investigated. As the results, welding heat input did not affect the absorbed energy of simulated CGHAZ, but the absorbed energy became very large by decrease in maximum temperature in case of single-run welding. And heat welded conditions, by that Charpy absorbed energy became large in the CGHAZ in case of multi-layer welding, were investigated.

High-speed tensile tests on Zr-based bulk metallic glass were carried out to determine the effect of strain rate on the tensile behavior of metallic glasses. In these tests, two strain rates, 1.7 x 10(0)/s and 2.3 x 10(1)/s, were selected, and the results were compared to those of the static test (1.8 x 10(-5)/s). Zr(48)Cu(36)Al(8)Ag(8) bulk metallic glass was chosen for this study. The diameter, gauge length, and overall length of the bulk specimen were 6 mm, 30 mm, and 60 mm, respectively. Regardless of the strain rate, all the specimens fractured in the elastic range. Sparking phenomena were observed at the fracture points. The tensile strength of the Zr-based bulk metallic glass becomes large; in particular, it is approximately four times the corresponding value for the crystallized specimen for all the strain rates. The tensile strength and fracture elongation of metallic glass tend to become small with an increase in the strain rate. Young's moduli were almost in the same range for all strain rates. Drastic changes in the tensile behavior of Zr-based metallic glass caused by an increase in the strain rate were not observed.

It is well known that strain rate affect tensile behavior, such as tensile strength and fracture elongation, of metal. In this paper, effect of strain rate on tensile property (tensile strength and elongation after fracture) of strain aged SM400, SM490 and HT780 are investigated. Tensile tests were performed under strain rate of 0.085, 85 and 1000%/s. Main results are summarized as follows. 1) Regardless of aging condition, tensile strength became large by the effect of high strain rate. Increase rate of tensile stress became large with rising in strain rate. 2) Elongation after fracture also became large by the effect of high strain rate. Increase rate of elongation became large with rising in strain rate. However the increase rates of elongation of aged steels were small compare to those of virgin steels. 3) The increase rate of tensile stress by effect of the high strain rate became small with increasing tensile strength under static test. Regardless of the aging condition, the increase rate of tensile stress by the effect of high strain rate can be assumed only by the tensile strength under static test.

This paper presents the result of full-scale test of site welding beam-to-column joints with the method of drilled flange and three dimentional elastic-plastic finite element analysis. Based on the test result, the method of drilled flanges hase large deformation capacity as compared with common site welding beam-to-column joints. In stress ratio is over 1.05, deformation capacity of the specimen which was used the method of drilled flange improved over two times larger than that wasn't used the method of drilled flange. Based on the analysis result, in case that the hole position is too near to column face, the method of drilled flange can't decrease stress concentration of the scallop part. So we should plan hole position is 0.75 times as long as flange width.

The authors have applied laser peening without coating (LPwC) to metallic materials. Compressive residual stress nearly equal to the yield strength of the materials was imparted on the surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC had a significant effect to prevent the SCC initiation of sensitized materials of SUS304, Alloy 600 and the weld metal, Alloy 182. Push-pull type fatigue testing demonstrated that LPwC drastically enhanced the fatigue strength of fillet-welded rib-plates of SM490A.

In the thermoelastic stress analysis, stress distribution is measured by lock-in infrared thermography, which correlates temperature change due to the thermoelastic effect with reference loading signal. Loading signal from external source, such as load-cell, strain gage or displacement gage, is usually employed as a reference signal in the conventional lock-in technique. In this study, a self-reference lock-in infrared thermography was newly developed, in which a reference signal was constructed by using the same sequential data on thermoelastic temperature change. Temperature change in a region of interest was correlated with that in a remote area for reference signal construction. The lock-in algorithm based on the least squares method was employed for signal processing under random loading. It enabled us to measure the distribution of relative intensity of applied stress under random loading without using any external loading signal. Proposed self-reference lock-in thermography was applied for crack identification based on the detection of significant thermoelastic temperature change due to the singular stress field in the vicinity of crack tips. It was found that significant temperature change was observed at the crack tip in the self-reference lock-in thermal image, demonstrating the feasibility of the proposed technique

In The Great Hanshin-Awaji Earthquake Disaster, brittle fractures with plastic strain were observed in beam-column connections of steel building frames. It is considered that the mechanical properties of weld metal, especially the ductility of weld metal, have a substantial effect on the fracture. In this paper we describe bi-axial loading test results using cruciform butt specimens under monotonic loading. The specimens are modeled on a cruciform joint by taking out the part of the beam-flange to column-flange connection and welded by two types of welding consumable. So we use two series of specimen that have different ductility. The purpose of this paper is to examine the effect of the ductility of weld metal on fractures under monotonic loading at room temperature and -40 °C. The high strain rates during the earthquake are substituted for static tests at -40 °C. The main results under monotonic loading are summarized as follows. 1) The fracture-surface appearance mainly depends on the specimen temperature and hardly depends on the ductility of the weld metal. 2) In the case of the ductile weld metal specimen, it presented a brittle fracture-surface at -40 °C, but the elongation is almost equal to that at room temperature which presented a ductile fracture-surface. 3) In the case of the brittle weld metal specimen, the elongation at -40 °C fell to less than 1/2 of that at room temperature, mainly because the brittle fracture occurred after the plastic deformation. 4) In the case of the brittle weld metal specimen, the elongation in the bi-axial tests tends to become large with specimen temperature rising. 5) In the range of this experiment, only the absolute values of reduction of area in tensile test correspond to the elongation capacity in the bi-axial loading test using cruciform butt specimens.

Laser peening was applied to A7075-T73 aluminum alloy and HT780 high-strength steel with low energy pulses of 2.5 mJ to 20 mJ which is attainable by ultra-compact handheld lasers. Then, we measured surface residual stresses of both materials by x-ray diffraction using a cos α method and confirmed that compressive residual stresses were built on the surface. Depth distribution of residual stress was evaluated by alternately repeating the x-ray diffraction and electrolytic polishing. Effect of laser peening on the residual stress penetrated into a depth of about 0.3 mm to 0.4 mm for the HT780. Welded joints of HT780 were laser-peened with low energy pulses of 10 mJ and 20 mJ and followed by uni-axial fatigue loading with a stress ratio of 0.1. Resulting fatigue strengths and lives were comparable to those of the HT780 welded joint peened with pulse energy of 200 mJ. It was clear through these experiments that laser peening could introduce compressive residual stresses on the surface of the tested materials and improve the fatigue properties of HT780 even if low energy pulses were applied. Such results would lead to downsizing the device, reducing the cost and expanding the application of laser peening.

衝突や爆発などの衝撃作用を受ける建築物の挙動の中で、人的被害の直接要因となりうる非構造材破壊や、部分的な構造要素損傷が建物の大部分の破壊を誘発するような連鎖崩壊に着目し、動画解析を用いて、こうした破壊現象の進行過程を定量的に検証することを研究目標とした。主たる研究成果として、多くの建物のファサードの一般的外装材料である板ガラスを扱い、飛来物の衝突実験を実施して、その衝撃破壊挙動を高速カメラで記録し、ガラス破片の飛散過程を動画解析により定量的に評価した。また、実験結果に関するFEMシミュレーションを行い、高速カメラで記録した破壊挙動の解析による再現が十分可能であることを確認した。

レーザパルスを鋼材表面に当てて強化することにより、ガラスのような脆い破壊を防止できるか否かを基礎的実験により検討した。その結果、研究で用いた鋼材,試験片形状およびパラメータの範囲では、破壊までの変形を大きくする効果期待できる場合もあるが、材料が脆く温度も低いという非常に厳しい条件では逆に脆い破壊を早期に生じさせてしまう場合もあることが明らかとなった。

地震による構造物の被害は最大加速度よりも最大速度と良い相関があることが従来から言われており, 最大速度100cm/s以上が大被害地震の目安との報告もある. しかし, この領域の載荷速度は従来の構造実験では再現することが困難だとされていた. 本課題では, 最高載荷速度120cm/s, 最大荷重2000kNの超高速衝撃構造性能評価システムを用いて高力ボルト摩擦接合の引張試験を実施した. その結果, 載荷速度が摩擦接合の各種限界状態や変形性能に及ぼす影響について明らかにした