JPS6310378B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for predicting fatigue life and fatigue crack occurrence positions that are effective as design standard data for spot welded joints.

Here, "fatigue life" includes various quantities related to so-called fatigue strength, such as fatigue limit, fatigue time strength, and fatigue time life. Various quantities related to predicted fatigue strength, especially load range (algebraic difference between maximum and minimum load of repeated loading)
and the corresponding time or number of repetitions until fatigue failure occurs. Note that in the case of random loads, the same applies to the case of repeated loads as specified above.

Despite the fact that there is a strong need for a method for predicting the fatigue life and fatigue crack initiation location of spot welded joints in spot welded structures during operation or implementation testing from the standpoint of safety design of spot welded structures. However, no practical method has been developed yet. This is because the stress concentration near the spot weld is high due to the special nature of the joint structure, resulting in a complex non-uniform stress field.

However, experimentally, as shown in Figures 1 to 3, fatigue cracks have been detected outside the electrode indentation 3 of the spot weld in a single spot welded joint between two metal plates 1 and 2. A strain gauge 5 is attached to a position near the nugget (molten part) 4 in the direction of the load axis where this occurs, and various quantities related to fatigue strength, such as the load range and the corresponding number of repetitions, are determined from the strain output of the strain gauge 5. Alternatively, methods of predicting fatigue life by detecting time, etc. have been considered.

However, this method cannot be used unless the direction of the load axis is known, and it requires a valid location for pasting the strain gauge. Therefore, this method can be used at multiple points where the principal stress direction is often unknown. This method was difficult to apply to spot welded structures and remained at the stage of laboratory investigation.

The present invention was developed in view of the current situation, and is useful not only for laboratory studies but also for determining the fatigue life and fatigue crack occurrence positions of spot welded joints in multi-point spot welded structures during operation or mounting tests. The purpose is to provide a method that can predict extremely accurately in a short time.

A feature of the technical means of the method of the present invention taken to achieve the above object is that a strain gauge is attached to the surface of the electrode indentation of the spot weld.

That is, the present invention affixes a plurality of strain gauges radially arranged at equal intervals on the electrode indentation surface of a spot weld, processes the strain output obtained from the strain gauges, and calculates the results related to fatigue strength. This system is designed to obtain various information, and has the advantage of being able to obtain the information directly and accurately. In particular, the electrode indentation part where the strain gauge is pasted is a joint part that is integrated by the nugget (molten part), and has higher rigidity than other parts, and is susceptible to local bending deformation that occurs on the front surface of the nugget. It is possible to obtain a strain output related to the stress directly acting on the spot weld. Therefore, the obtained strain output is based on the location where the strain gauge is attached (electrode indentation surface).
There is a one-to-one correspondence with the amount related to the stress of
It is possible to obtain a load range-strain output diagram with good linearity over a wide load range, and as a result, fatigue life can be predicted quickly and accurately.

Furthermore, according to the method of the present invention, by appropriately selecting the pattern of strain gauges, the direction of principal stress acting on each spot weld can be easily and accurately detected. (always occurs in the principal stress direction)
can be predicted at the same time. in this way,
One of the features of the present invention is that the fatigue crack position can be predicted while a fatigue load is acting on the spot welded joint, and this provides valuable data for the rational safety design of spot welded structures. can be provided and is extremely effective. Furthermore, from the strain output information that changes with the initiation and propagation of fatigue cracks, it is possible to compare the so-called fatigue crack behavior, such as the timing of initial fatigue crack initiation and propagation rate, which was impossible to measure in previous studies. It can be detected in a short time and non-destructively.

Furthermore, according to the present invention, since the strain gauge is attached to the surface of the electrode indentation, the spot weld is located at the end of the member, or the member is bent in the vicinity of the spot weld. In any case, strain gauges can always be attached and are not restricted by the shape of the structure.

Hereinafter, embodiments of the present invention will be described based on the drawings from FIG. 4 onwards. FIG. 4 shows an example of a strain gauge pattern for carrying out the method of the present invention. The strain gauge 5 consists of eight pieces, and the electrode indentation 3
They are radially arranged and pasted at equal intervals on almost the same circle on the surface. The larger the number of strain gauges 5, the more accurately the principal stress direction can be detected, which is effective. In the figure, 6 is a gauge terminal and 7 is a lead wire.

In order to explain the principle of the present invention, FIG. 5 shows the strain obtained by the eight strain gauges 5 shown in FIG. 4 when a repeated oscillating load is applied to the single spot welded joint shown in FIG. 1. This is a model-like illustration of the strain output.

FIG. 5 4-0 shows an example of a repeated load waveform, in which the vertical axis shows the magnitude L of the repeated load, and the horizontal axis shows the time t. In the figure, Lmax indicates the maximum load of repeated loads, Lmin indicates the minimum load of repeated loads, and ΔL indicates the load range of repeated loads.

FIG. 5 4-1 to 4-8 show the strain output waveforms of gauge Nos. 1 to 8 of the strain gauge 5 shown in FIG.
The graphs are respectively shown in correspondence with the time t of the 0 repeated load waveform. In this case, since the load direction is known in advance, the gauge No. 1 is the one that corresponds to the load axis direction, and the gauges numbered clockwise are No. 2, No. 3, ..., No. 8. It is attached. In Figure 5 4-1 to 4-8, vertical axis strain ε=
The line 0 indicates the value when the strain output at the minimum load Lmin of the repeated load shown in FIG. 5 4-0 is set to zero. When such signal processing is performed, if the strain amplitude is above the line ε = 0, the repetitive load and the waveform are in phase (Fig. 5-4-1, 2,
5, 8), and conversely, if it is below the line of ε = 0, the phase is reversed (Fig. 5-4-3, 4, 6,
7) can be easily determined. In addition, this example is based on the minimum load of repeated load.
The results are shown in the case of complete unilateral swing with Lmin=0.

The strain output waveforms obtained from the strain gauges 5 shown in Figure 4 have specific correlations in phase and amplitude range with respect to repeated loads, as shown in Figures 4-1 to 4-8, respectively. are doing. Although not shown here, each strain output contains a unique DC component, and the magnitude of this DC component is one of the effective amounts of information when predicting fatigue life. It can be. In this embodiment, in order to characterize the phase relationship between each strain output and repeated load and the size of the strain amplitude range, a model diagram is shown in which a part of the DC component is cut out.

In the case of this example, the relationship between the phase and strain amplitude range between the strain outputs obtained from each strain gauge is the relationship between the three gauges No. 1, No. 2, and No. 2 in front of the load axis. 8, and the rear gauge No. 5 are in phase with the repeated load, and all other gauge outputs are in opposite phase. The strain amplitude is maximum at gauge No. 1 located in front of the load axis. Next, the in-phase pair of gauges No. 2 and 8, the opposite-phase pair of gauges No. 3 and 7, the opposite-phase pair of gauges No. 4 and 6, and the gauge
The order is No.5. By processing the relationship between these strain outputs, the maximum load axis direction and repeated load range can be detected, so that fatigue life and fatigue crack positions can be predicted. That is, the maximum load axial direction can be easily determined because it is in phase with the repeated load and has the maximum strain amplitude. Therefore,
The fatigue crack position can be easily predicted since it is in the direction of the maximum load axis.

The repeated load range necessary to predict the fatigue life is calculated separately in advance from a repeated load range-strain output diagram (reference diagram, this diagram is a gauge in the maximum load axial direction), as shown in Figure 6. No.
This corresponds to the ∠-ε diagram of 1. ) and the strain output obtained in the above-mentioned direction. Based on the repeated load range obtained in this way, the fatigue life can be predicted from the LN diagram (reference diagram) obtained in advance as shown in FIG.

The reference line diagrams in Figures 6 and 7 are for fatigue tests under various repeated loads on single-point spot weld fatigue test pieces in which strain gauges with the same pattern as in Figure 4 were affixed to the electrode indentation surface in the same manner. It can be created by organizing the obtained strain output in the repeated load range.

Note that even when other general repeated loads (for example, random loads) are applied, the fatigue life and the fatigue crack occurrence position can be predicted in the same manner as described above.

FIG. 8 shows an example in which strain gauges 5 are formed in a radial pattern, with gauge terminals at each tip.
T ₁ ...T _o is provided. However, the gauge terminal T ₁ at the start end and the gauge terminal T _o at the end are separated. In this case, the state of strain output in all directions can be measured in more detail by sequentially measuring resistance changes between two terminals.

[Brief explanation of the drawing]

Fig. 1 is a plan view of a single spot welded joint test piece, Fig. 2 is a cross-sectional view taken along the line A-A, Fig. 3 is an explanatory diagram of the conventional method, and Fig. 4 is a diagram for carrying out the method of the present invention. A plan view showing how the strain gauges are installed.
Figure 5 is a model diagram of the strain output by the strain gauge shown in Figure 4, Figures 6 and 7 are model diagrams of the reference line diagram, and Figure 8 is a plan view showing another pattern of the strain gauge. . 1, 2... Metal plate (test piece), 3... Electrode impression,
4... Nugget (molten part), 5... Strain gauge,
6... Gauge terminal, 7... Lead wire.

Claims (1)

[Claims] 1. A plurality of strain gauges are radially arranged and pasted at equal intervals on the electrode indentation surface of a spot weld, and strain outputs obtained from the strain gauges in multiple directions are processed to form a spot weld. A method for predicting the fatigue life and fatigue crack initiation location of welded joints. 2. Predicting fatigue crack life and fatigue crack occurrence position according to claim 1, wherein the strain gauge affixed to the surface of the electrode indentation is formed in a radial pattern, and the strain gauge is provided with a large number of gauge terminals. how to.