Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-07-06
2001-02-13
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06186011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to procedures for developing a resultant force based spot weld failure criterion for predicting multi-axial loaded spot weld failure.
2. Discussion of Prior Art
Spot welding is widely used in joining sheet metal members in the manufacture of automotive bodies because such welding can be extremely fast. Spot welding is the simplest form of resistance welding in which electrodes apply some pressure when squeezingly contacting the sheet metal assembly while low voltage current, accompanied by sufficient amperage, is passed between the electrodes to locally raise the sheet metal to a melting and welding temperature.
The strength and stiffness of the spot weld joints can vary considerably depending on the direction, complexity and cyclic nature of the joint loading. The unreliability of current methods to predict the failure mode of a specific spot welded joint in automotive bodies has misled designers to inaccurately design spot weld structures out of an abundance of precaution. With the imposition of stricter fuel economy standards for automotive vehicles, the spot weld body structures have come under severe scrutiny to allow for weight reduction.
Existing structural analysis techniques for spot welded structures consider only two types of loadings or failure modes at any one time that may act on a structure; such method lacks the increased accuracy and reliability that will assist in structural safety and weight reduction without jeopardizing risk of failure over the useful life of the structure. Often, different failure modes act on the welded structure at the same time, including a variety of modes selected from shear tension, in-plane rotation, coach-peel, and normal or pull tension. These failure modes need to be considered not only by themselves, but in combination for predicting accurate conditions for combined failure of the spot welded structure.
SUMMARY OF THE INVENTION
What is needed is an improved method technique for analyzing spot welded structures that enhance accuracy of predicting failure modes at specific load type, load cycle, and sheet metal thickness.
It is also an object of this invention to practice the method technique herein with computerized test simulation software to provide a computerized model of spot weld structures into which measured strength and stiffness data is referenced for analyzing any intended design.
It is another object of this invention to practice such method technique by use of novel test metal coupons for unitary of mixed failure modes; such coupons, when tested to failure, should provide increased accuracy for strength and stiffness data needed by the method technique herein.
In a first aspect, the invention meeting the above objects, is a method of analyzing spot welded sheet metal structures subjected to forces that promote one or more basic failure modes of either shear tension, in-plane rotation, coach peel, normal tension, or any combination of such basic modes, the method comprising: (i) providing spot welded sheet metal test coupons for analysis of the selected failure mode for the structure to be analyzed and subjecting such test coupons to progressively increasing forces that eventually achieve failure in the selected failure mode to thereby generate measured load and displacement test data; (ii) analyzing the data from the basic modes to derive and extract spot weld strength and stiffness information and then surface fitting such extracted data to create a database; (iii) establishing a computer math model for simulating the spot weld failure modes of the structure and populating the database thereinto to enable simulated physical tests of the structure; (iv) creating a cumulative-damage failure criterion for the spot welded structure that ratios (a) the resultant strength information from the populated math model for selected combinations of load and selected combinations of time and structure thickness, and (b) the measured strength information for such combinations of load derived from the test data; and (v) solving the criterion to determine whether it indicates failure for the combination of resultant strength and other values selected.
The invention, in a second aspect, is a structural test coupon for measuring characteristics of a spot welded structure when tested to failure in a selected mode, the coupon comprising: first and second parts, each part having a panel mated to the other and structurally joined by spot weld at such mating; each part has an integral force pulling panel which is effective to apply one or more of shear tension, in-plane rotation, coach-peel, or normal tension.
REFERENCES:
patent: 3880535 (1975-04-01), Durham et al.
patent: 4027529 (1977-06-01), Olsen
patent: 5602341 (1997-02-01), Lee et al.
patent: 5785463 (1998-07-01), Eaton et al.
Stewart William Steven
Wung Pey Min
Ford Global Technologies Inc.
Noori Max
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