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A method of fatigue life prediction of spot welded joint under multi-axial loads has been developed by fatigue life estimation working groups in the committee on fatigue strength and structural reliability of JSAE. This method is based on the concept of nominal structural stress ( σ ns) proposed by Radaj and Rupp, and improved so that D value is not involved in stress calculation. The result of fatigue life estimation of actual vehicle with nominal structural stress which was calculated through newly developed method had very good correlation with the result of multi-axial loads fatigue test carried out with test piece including high strength steel.

Eight kinds of the spot-welded test specimens were studied to obtain the basic fatigue data on steel sheets for car bodies. Analytical methods such as the strain amplitude at outside and inside plates close to the nugget, the maximum nominal stress and Kθmax criteria of fracture mechanics were used. The authors propose that the ratio of strains at two points around the nugget can provide evaluation of the bending load and estimation of the fatigue strength.

As a demand for vehicles of higher functionality grows, automakers and material suppliers are devoting increasing efforts to develop technologies for greater safety, lighter weight, higher corrosion resistance, and enhanced quietness. The resin-sandwiched vibration damping steel sheet (VDSS), developed as a highly functional material for reducing vehicle vibration and noise, has been used for oil pans1) and compartment partitions2). First applied for a structural dash panel of the new Mazda 929, a Zn-Ni electroplated VDSS which allows direct electric welding has contributed to greater weight reduction as well as improved quietness.

The influence of tensile strength on fatigue strength and the effect of strengthening mechanism on fatigue notch factor were investigated into conventional mild steels, HSLA steels, DP steels and TRIP steels. The grade of studied steels was altered from 440MPa to 780MPa. Not only smooth fatigue specimens with side surface ground and smooth fatigue specimens with laser-cut side surface but also fatigue specimens with a pierced hole were prepared for each of steel sheets. Fatigue tests were conducted in an axial load method. These experiments made it clear that the fatigue limits of smooth specimen increase along the tensile strength approximately independent of strengthening mechanism but those of notched specimen do not necessarily increase along the tensile strength. Namely, fatigue limits of DP steels and TRIP steels with notch increase in proportion to tensile strength although those of HSLA steels with notch do not increase.

The Strategic Alliance for Steel Fuel Tanks (SASFT), an international group of steel producers and manufacturing companies, recently completed a major corrosion study of various steel ‘systems’ for automobile fuel tanks. The ten steel systems included low carbon steels (either pre-painted or post-painted with protective coatings) and stainless steels. The 2-year corrosion test program included testing in salt solutions to simulate road environments for the exterior of a fuel tank. Special test specimens were designed to represent a manufactured tank. The external tests used were the Neutral Salt Spray test (ASTM B117) with exposures up to 2000 hours and the Cyclic Corrosion test (SAE J2334) with exposures up to 120 and 160 cycles to represent vehicle lives of 15 years and 20 years, respectively. Additionally, the resistance to an aggressive ethanol-containing fuel (internal tank corrosion) was assessed by using uniquely designed drawn cups of the various steel systems.

In this paper the fatigue life of welded steel sheet structures is predicted by using FE-Fatigue, which is one of fatigue analysis software tools on the market, and these predicted results are evaluated by reference to corresponding experimental results. Also, we try to predict these structures by using two fatigue life prediction theories established by the JSAE fatigue and reliability committee to compare prediction results. It was confirmed that spot welds fatigue life predictions agree qualitatively with corresponding experimental results and arc welds fatigue life predictions are in good agreement with corresponding experimental results in cases where the SN curve database is modified appropriately.

A metal support for use in an automotive catalyst is exposed to the severe heat cycle brought about by the intermittent flow of a high temperature exhaust gas. Accordingly, the metal support must have high beat resistance(ex. oxidation resistance) and a rigid structure. Therefore, 20% chromium-5% aluminum ferritic stainless steel(containing small quantities of rare earth metals and titanium) is used as a highly beat resistant honeycomb foil in addition to a highly mechanical durable brazing honeycomb structure. This study examined the durability of a metal honeycomb installed in a gasoline engine. Both an engine bench durability test of a manifold converter type metal support which is connected directly to the exhaust manifold of the gasoline engine and a vehicle durability test of an under the floor type metal support were carried out to evaluate oxidation damage of the metal honeycomb as well as its mechanical durability.

Carburizing heat treatment is one of the automobile component manufacturing steps, which consumes a large amount of energy. Raising the carburizing temperature can shorten the carburizing time and save the energy, but involves the risks of grain coarsening and attendant property deterioration. The authors have clarified the precipitation behavior of aluminum nitride (A1N) in the automobile gear manufacturing process and the optimum precipitation of A1N in as-rolled steel bars to prevent the grain coarsening. Through the application of the controlled rolling technique to ensure the optimum precipitation of A1N in continuously cast steel of uniform chemical composition, the authors have substantially saved energy while maintaining high quality, and developed a high-temperature carburizing steel expected to minimize and stabilize quenching strains.

The main operations for the manufacturing of auto parts are the cutting of the flange and the stamping. In order to perform accurate fatigue calculation it is necessary to have the material properties for each point of the structure. Usually, only the fatigue curve obtained on the flat sheet with polished edges is used because it represents the basic metal behaviour. The real edge quality decreases the fatigue limit while the hardening induced by the stamping increases it. To take these effects into account allows a better fatigue calculation of the structural part.

The application of tailored blanks to autobody parts has progressed because of its numerous advantages. The forming of tailored blanks has, however, a lot of technical problems. Among the problems, weld-line movement and formability deterioration are the most significant ones in case of deep drawing. The weld-line movement and formability change were examined experimentally as a function of weld-line location in square cup deep drawing. The weld-line movement of tailored blank consists of two sources. One is the geometrical reason, and the other is due to the hardening of weld bead. The formability of tailored blank is inferior to that of an original blank by the existence of hardened weld region. The mode of fracture changes from wall breakage to a fracture adjacent to punch radius when the weld-line was close to the punch corner.

New 440MPa class high-strength steel, which had high r-value(1.6) and elongation(38%), was applied to outer-panel for the first time in the world. In this development FEM simulation was carried out to clarify the necessary steel properties, and the production conditions in strip mill were established. 10-kg weight reduction was realized by using this steel.

In order to clarify the effect of design and materials of the hem as well as the climatic factors on perforation corrosion of the automobile doors, field car and laboratory investigation has been carried out Field car investigation revealed that corrosion of the hem can be minimized by using two side galvanized steel plus adhesives. The ratio of wet/dry environment was evaluated in laboratory on hemmed sample, and it was found that the design of the hem in conjunction with the various wet/dry ratio affected the corrosion rate differently.

We have taken notice of Zn-Fe alloy electroplating with an eye to developing new corrosion-resistant steel sheets for automotive use with both cosmetic corrosion resistance and perforating corrosion resistance, and as a result of investigations into its paintability and corrosion resistance over the whole range of its compositions, we have come to a conclusion that steel sheets with two-layer Zn-Fe alloy electroplating that consists of a thin upper layer with a 75 to 85% Fe content and a lower layer with a 10 to 20% Fe content is the best choice.

The newly developed sheet steel lightly coated with an organic composite is as follows. Zn-Ni alloy plated sheet steel with a coating weight of 30 g/m2 and average Ni concentration of 11.5 ∼12.0 % is chromated through electrolysis. The coating weight of chromate film is 50 ∼90 mg/m2 in Cr. Furthermore, emulsified olefin-acrylic acid copolymer resin mixed with colloidal silica of particle size 7 ∼8 nm applied to a thickness of 1.0 ∼1.8 μm. Olefin-acrylic acid copolymer resin and colloidal silica are mixed at the rate of 100 and 30 (parts by weight). It maintains excellent corrosion resistance even after forming, C-ED paint corrosion resistance and paint adhesion. Furthermore, it has excellent perforation resistance. The product has excellent weldability and is well suited to continuous forming, too.

Aluminum alloy sheets are used for automotive body-panels, but their small Young's modulus results in inferior shape-fixability than conventionally-applied steel sheets with similar strengths. Smaller radius of curvature, indicating better shape-fixability, is found at the center of a panel press-formed with higher blank holder force (BHF). Higher force can be applied for press-forming of alloy sheets with larger strain-hardening exponent (n value) induced by an increased addition of Mg. Recently-developed 5000 series alloy sheets containing 5.5 pct Mg and 0.3 pct Cu have an elongation over 33 pct at an ultimate tensile strength of 270 MPa and can be press-formed with better shape-fixability.

To give the wear-resistance to titanium intake valves by simple oxidation treatment, oxidation condition and microstructure of Ti-6Al-4V bars were studied. The wear test using a valve simulator shows that the wear of the face oxidized at 820°C for 1 and 4h in air is superior to that of ferrous valves. The best micro-structure of Ti-6Al-4V bar is an acicular structure with the prior β grain size of 30 to 60 μ m in average, which prevents distortion during the oxidation treatment and has excellent mechanical properties.

The application of high-strength steel sheets to car bodies is expanding to improve the crashworthiness and achieve weight reduction [1, 2]. Conversely, in recent years, the occurrence of liquid metal embrittlement (LME) cracks has been discussed in resistance spot welding using a Zn-based coated high-strength steel [3-5]. This study examined the factors causing LME cracks and identified the locations of LME cracks found in resistance spot welds using a Zn-coated high-strength steel sheet. Furthermore, through an analytical approach using a scanning electron microscopy (SEM) and transmission electron microscopy (TEM), for a joint with an LME crack, it was found that (1) grain boundary fracture occurred at LME crack portion and its fracture surface was covered with Zn, (2) Zn penetrated into prior-austenite grain boundaries near the LME crack, and (3) Zn concentration decreased toward the tip of the Zn-penetrated site.