Mark E. Barkey

Effect of fatigue damage on the dynamic response frequency of spot-welded joints

Abstract The effect of fatigue damage on the dynamic natural frequency response was studied over the entire fatigue life process for tensile-shear spot-welded joints. The results of the experimental study showed that the changes to the specimens’ natural frequency varied non-linearly with the cycle ratio or damage fraction, and that relatively large changes in the natural frequency could be measured near the end of the fatigue life.

Failure modes of single resistance spot welded joints subjected to combined fatigue loading

Combined loading fatigue tests were conducted on spot welded coupons of a galvanized sheet steel for proportional, constant amplitude loading conditions. The factors that influence the failure modes of the coupons are described and photographs of typical failure modes are presented in addition to the fatigue life results of the tests. The fatigue crack initiation sites for all of the tests were at the edge of the spot weld nugget at the sheet interface. The applied load amplitude and nugget diameter had the most significant effect on the final fatigue life of the coupons. The main factors that determine the final failure mode of the fatigue specimens are the ratio of shear and normal force (i.e. the applied loading angle), the applied mean load, and nugget diameter. The observations are related to common spot weld fatigue damage parameters.

Fatigue life estimation of spot welded joints using an interpolation/extrapolation technique

A numerical interpolation/extrapolation technique is presented that can utilize the fatigue life and strength databases of spot welded joints that have been developed by previous researchers. These databases typically included multiple independent variables, such as specimen dimension and type, material property, R-ratio, and welding condition, such as are often encountered in a typical design of experiments approach to testing. In this study, Rules interpolation technique is modified and a new extrapolation scheme is adopted. The test data for multiaxial ultimate strength of spot welded joints under combinations of tensile and shear force are used to demonstrate the suitability of the interpolation technique in multivariable databases, and the technique is also applied to the calculation of total fatigue life of spot welded joints based on Swellams experimental data. The results show that the calculations and test values are reasonably matched. This approach is also compared the methods of Swellam and Sheppard.

Stresses in bent copper tubing: Application to fatigue and stress-corrosion cracking failure mechanisms

The residual and applied stresses in u-bent copper tubing are addressed in the context of both cyclic fatigue and stress-corrosion cracking. Failures as a result of fatigue and stress corrosion cracking in u-bent copper tubing have been observed to initiate at nonintuitive locations when only the applied stresses on the component are considered. This paper presents both qualitative classical and quantitative finite-element stress analysis results for the forming of u-bends. The resulting residual stress distributions are compared to fracture patterns generated by both fatigue and stress-corrosion cracking mechanisms.

Metal Fatigue Analysis Handbook

The local strain–life method was developed in the late 1950s and was shown to be more effective for low- and midcycle fatigue life prediction than the stress-life approach for the fatigue analysis of components. This approach is also preferred when the load history is irregular, where the mean stress and the load sequence effects are thought to be of importance. This method also provides a rational approach to differentiate the high cycle fatigue and the low cycle fatigue regimes as well as to include the local notch plasticity and mean stress effect on fatigue life.

Stress-Based Uniaxial Fatigue Analysis Using Methods Described in FKM-Guideline

The process of prevention of failure from structural fatigue is a process that should take place during the early development and design phases of a structure. In the ground vehicle industry, for example, the durability specifications of a new product are directly interweaved with the desired performance characteristics, materials selection, manufacturing methods, and safety characteristics of the vehicle. In the field of fatigue and durability analysis of materials, three main techniques have emerged: nominal stress-based analysis, local strain-based analysis, and fracture mechanics analysis. Each of these methods has their own strengths and domain of applicability—for example, if an initial crack or flaw size is known to exist in a structure, a fracture mechanics approach can give a meaningful estimate of the number of cycles it takes to propagate the initial flaw to failure. The development of the local strain-based fatigue analysis approach has been used to great success in the automotive industry, particularly for the analysis of measured strain time histories gathered during proving ground testing or customer usage. However, the strain life approach is dependent on specific material properties data and the ability to measure (or calculate) a local strain history. Historically, the stress-based fatigue analysis approach was developed first—and is sometimes considered an “old” approach—but the stress-based fatigue analysis methods have been continued to be developed. The major strengths of this approach include the ability to give both quantitative and qualitative estimates of fatigue life with minimal estimates on stress levels and material properties, thus making the stress-based approach very relevant in the early design phase of structures where uncertainties regarding material selection, manufacturing processes, and final design specifications may cause numerous design iterations. This article explains the FKM-Guideline approach to stress-based uniaxial fatigue analysis. The Forschungskuratorium Maschinenbau (FKM) was developed in 1994 in Germany and has since continued to be updated. The guideline was developed for the use of the mechanical engineering community involved in the design of machine components, welded joints, and related areas. It is our desire to make the failure prevention and design community aware of these guidelines through a thorough explanation of the method and the application of the method to detailed examples.

Strain-Controlled Low-Cycle Fatigue Properties of Extruded 6061-T6 Aluminum Alloy

One of the most commonly extruded aluminum alloys is the 6061 alloy largely because of its good formability and high specific strength (Ref 1). The ability of the 6061 aluminum alloy to be extruded or otherwise formed into complex geometries at relatively low cost is the reason for its widespread use in aerospace, construction, transportation, and many other industries. In fact, 6061 aluminum alloy has been produced in various forms including plate, extrusion, foil, sheets, pipes, forgings, and structural forms (Ref 2). Since most load bearing components fail due to cyclic loading, analytical prediction and finite element modeling of fatigue damage of such a prevalent alloy is critical for safe designs. A review of literature regarding fatigue in 6061 aluminum alloys reveals most experimental studies characterized fatigue behavior based on load-control cyclic tests. However, for applications where low-cycle fatigue (LCF) is dominant with variable-amplitude histories and sequence effects, strain-controlled fatigue tests better characterize the fatigue behavior compared to stress-controlled tests (Ref 3). However, only a few data sets on strain-controlled fatigue of 6061 aluminum alloy exist and are not easily located, with the one data set published in an obscure journal (Ref 4), and the other published in a conference proceeding (Ref 5). The purpose of this paper is to provide readily available LCF properties of an extruded 6061-T6 extruded aluminum alloy and to validate the results of Ref 5. Furthermore, the cyclic stress-strain behavior, including initial and stabilized hysteresis loops, is not reported in literature and thus will be presented here. The material used in this study was an extruded 6061-T6 aluminum alloy. Taber Extrusion, LLC (Russellville, AR) provided the extruded panels. The nominal composition in atomic weight of the 6061 aluminum alloy is given in Table 1 (Ref 6). For comparison purposes, Table 2 lists the monotonic properties of the extruded 6061-T6 aluminum alloy. Cylindrical specimens were machined from the extruded alloy so that the loading axis was parallel to the extrusion direction (ED). The fatigue specimens were designed following ASTM Standard E.606 specifications (Ref 7) with a nominal 27-mm gage length and diameter of 6.35 mm. Prior to testing, all specimens were hand ground in the loading directionwith 800-grit silicon carbide paper to remove residual stresses and any machining marks. Fully reversed cyclic (R = 1) tests were performed using a servo-hydraulic MTS load frame at room temperature with relative humidity level of 45%. The fatigue specimens were tested to failure under strain-controlled conditions at 5 Hz frequency at the following strain amplitudes: 0.002, 0.003, 0.004, 0.005, 0.006, and 0.007. The specimen tested at 0.002 strain amplitude was fatigued at 5 Hz to 20,000 cycles, and then the test was stopped and resumed in load control at 30 Hz to failure. A fatigue-rated MTS extensometer with a 25.4-mm length gage was used to directly measure the axial strain induced in the gage length. Final failure of the specimen was defined as a 50% drop in peak load during the test, as recommended by ASTM Standard E.606 (Ref 7). Figure 1 shows the results of the strain-controlled fatigue tests conducted on the extruded 6061-T6 aluminum alloy. Note that the 0.002 amplitude shown in Fig. 1 did not fail but is plotted here as a run-out. Also shown in Fig. 1 is the strain-life equation (Ref 3), including the elastic and plastic strain amplitudes, where the total strain-life is the summation of the elastic and plastic strain components. In strain-controlled cyclic deformation, the elastic strain amplitude is more dominant in small strains or longer lives, and the plastic strain amplitude is particularly dominant in large strains or short lives (Ref 3). The total fatigue life is shown in Eq 1

A Microstructure and Microhardness Characterization of a Friction Plug Weld in Friction Stir Welded 2195 Al-Li

In this study, extruded 2195-T8 plugs that were friction welded into the friction stir weld of 2195-T8 base metal plates were examined. This study characterizes the resulting microstructure and microhardness of the plug weld interfaces with the friction stir welded material and base metal in an effort to identify the extent of the thermal, thermomechanical, and mechanical effects introduced by the friction plug welding process. A zone of recrystallized material was observed around the plug weld circumference. The thickness of the recrystallized layer was measured to be 30–122 μm. The hardness measured near the plug weld interface was found to be 110–130 HK100g , or approximately 35% less than the base metal hardness. By characterizing the hardness of these zones, insight can be gathered into the transformation of material from the friction plug welding process around the fusion zone, and areas that may control the fatigue behavior of the joint.

Fatigue Damage and Dynamic Natural Frequency Response of Spot-Welded Joints

The changes of dynamic frequency response, commonly used to determine the dynamic characteristic of built-up structures, were studied over the entire fatigue failure process for tensile-shear spot-welded joints. The results of an experimental study showed that the natural frequency varies non-linearly with the fatigue damage fraction. This behavior was modeled using finite element analysis of a progressively growing crack, initiating at the joining surface, then progressing to the outside surface of the specimen, and finally extending from the spot weld nugget. The- relationship between dynamic frequency response and crack propagation may be applied to study effect of aging (high mileage) in NVH quality.

Fatigue failures of austenitic stainless steel orthopedic fixation devices

Failures of four different 300-series austenitic stainless steel biomedical fixation implants were examined. The device fractures were observed optically, and their surfaces were examined by scanning electron microscopy. Fractography identified fatigue to be the failure mode for all four of the implants. In every instance, the fatigue cracks initiated from the attachment screw holes at the reduced cross sections of the implants. Two fixation implant designs were analyzed using finite-element modeling. This analysis confirmed the presence of severe stress concentrations adjacent to the attachment screw holes, the fatigue crack initiation sites. Conclusions were reached regarding the design of these types of implant fixation devices, particularly the location of the attachment screw holes. The use of austenitic stainless steel for these biomedical implant devices is also addressed. Recommendations to improve the fixation implant design are suggested, and the potential benefits of the substitution of titanium or a titanium alloy for the stainless steel are discussed.