DD255403A1 - METHOD FOR DETERMINING CORRIGATED RIP RESISTANCE CURVES OF DUCTILE MATERIALS - Google Patents

Abstract

The invention relates to a method for the determination of corrected crack resistance curves of ductile materials, wherein the respective value of the J integral is determined from the energy consumed in the material testing operation in the test body using torn test bodies according to a known curve method. The invention is applicable in cases where dissipative energy fractions due to crack growth during material testing can corrupt the fracture mechanical J integral values determined from the absorbed energy. The inventive method is characterized in that first the squared crack resistance curve is determined, then the tangent to the squared crack resistance curve is determined at the point of the square of the critical J integral value and then the corrected crack resistance curve is found by etching the tangent.

Description

For this 2 pages drawings

Field of application of the invention

The invention relates to a method for the determination of corrected crack resistance curves of ductile materials, which is determined using torn test specimens according to a known Einkurvenverfahren the respective value of the J integral from the energy consumed in Werkstoffprüfvorgang in the specimen energy, and the intersection of the crack resistance curve with the Rißabstumpfungsgeraden the critical J integral value J | _C defines.

In particular, the invention is applicable in cases where dissipative energy fractions due to crack growth during material testing can distort the fracture mechanical J integral values determined from the absorbed energy.

Characteristic of the known state of the art

It is known to use crack resistance curves to characterize the material resistance to the growth of cracks in ductile material components.

These describe the change in the J-integral breaking parameter with increasing crack length, the J integral being a measure of the potential energy turnover at the crack tip due to the elongation of the crack. If this breaking characteristic exceeds a material-specific critical value J _{! C} , the crack is lengthened.

Experimentally, the different values of the J integral are determined according to the so-called pseudocompliance method.

For this purpose, for several identical test specimens with different crack lengths, the areas under the load load application point shift curves are plotted against the corresponding crack lengths. The negative slope of this curve for a given crack length divided by the thickness of the flat test specimen gives the respective J integral value (see K.-H.

Schwalbe "fracture mechanics of metallic materials", C. Hanser-Verlag 1980, p.310-311, and H. Blumenauer, G. Pusch "technical fracture mechanics", Dt. Verlag für Grundstoffindustrie 1981, pp. 119-120).

Although this known method provides largely accurate values of the J integral for determining the crack resistance curve.

A disadvantage, however, is that to determine a single J integral value several experimental procedures, d. H. several specimens are required.

To avoid this disadvantage, it is already known to use so-called Einkurvenverfahren. In each case, a J-integral value is determined approximately from a single load-displacement curve (see Schwalbe, loc. Cit., Pp. 312 et seq., Blumenauer, loc. Cit., Pp. 120/121).

However, these incursion techniques neglect dissipative energy fractions due to crack growth during the testing process, which are not available for the actual broadening of the crack. As a result, J integral values result, in particular for large values of crack length increase, which considerably overestimate the fracture toughness of ductile materials and thus the strength of component parts.

In US Standard ASTM E 813-81 for determining the critical J integral Jic for crack growth, a correction method is described in order to reduce said errors. However, this method is complicated and hardly practicable and not suitable for non-instrumented test methods.

In our own investigations, it was found that the above-mentioned dissipative energy proportions are proportional to the product of J-integral and breaking modulus. These dissipative energy components are equated with stable crack growth material-specific energy that compensates for the excess energy during the crack growth process.

It can be deduced from this that the square of the energy consumed for the separation of the material per crack extension shows a linear behavior as a function of the crack length increase.

Object of the invention

It is an object of the invention to provide a method for determining crack resistance curves according to a single-curve method, wherein, taking into account the above-mentioned knowledge, largely accurate J integral values can be determined on the basis of simple and practical method steps.

Explanation of the essence of the invention

The invention is based on the object, a method for the determination of corrected crack resistance curves of ductile materials, which is determined using torn test specimens according to a known Einkurvenverfahren the respective value of the J integral from the energy consumed in Werkstoffprüfvorgang in the specimen energy, and the intersection of the Rißwideistandskurve the Rißabstumpfungsstraight defines the critical J integral value J _{! C} , taking into account dissipative energy components due to crack growth during the Prüfyorgangs with good approximation. According to the invention, this object is achieved in that the squared crack resistance curve is first determined pointwise, then the tangent to the squared crack resistance curve is determined at the point of the square of the critical J integral value and then the corrected crack resistance curve can be determined by etching the tangent. In cases in which a tangent with no or negative slope would result at the squared point of the critical J integral value, it is expedient that the tangent be replaced by a compensation straight line (with positive slope), which is based on measured values in the region above critical J integral value is adjusted.

embodiment

The invention will be explained in more detail using an exemplary embodiment. In the accompanying drawing show:

Fig. 1: a conventionally determined and corrected by the inventive method crack resistance curve, Fig. 2: the squared crack resistance curve and a tangent applied at the critical point.

The conventionally determined crack resistance curve J shown in FIG. 1 was obtained from the data of an instrumented impact test. For this purpose, area A under the load load application point shift curve for different hammer energies was determined on unilaterally scribed, identical 3-point bend samples, and the associated crack extension a, averaged over the thickness B of the test object, was measured. The respective value of the J integral results (see Blumenauer, supra, pp. 120-121) from the relation

J = 2A

B (Wa ₀ ) - (1)

The size W - a _o corresponds to the residual ligament width of the test specimen with the initial tear length a ₀ .

Corresponding relationship (1) measured / calculated values for the respective J integral can be seen from FIG. 1 (example: steel HS 60-3 Ni at 0 C). Also apparent is the intersection S of the crack resistance curve J with the crack truncation straight line RAG, which is a pseudo-crack extension due to plastic deformation of the crack tip to the critical J integral value J | _{C describes} the actual crack extension at point a _c .

The method according to the invention for determining a corrected crack resistance curve is now carried out such that in a first method step the crack resistance curve J is squared pointwise, so that the corresponding square curve J ² (FIG. 2) is obtained.

This curve J ² has in the intersection S corresponding point S 1 normally a positive slope. At this point S 1, the tangent T to the squared curve J ^{2 is} determined in the second method step. In the following step this tangent T is square root, so that the corrected Rißwiderstandskurve J _corr is determined (s. Fig. 1).

That is, for the corrected crack resistance curve Jk _O rr, the relationship

Λογγ ^;

(J ² , c + C _T ^ a) ^{1 /!} (2)

written (C _T = slope of the tangent T).

If the position of the point S T is only approximately known for reasons of measurement accuracy, the tangent T is replaced by a straight line A (not shown), which is adapted to measured values in the estimated range above the point S 1 according to known methods of compensation calculation.

Claims (2)

1. A method for the determination of corrected crack resistance curves of ductile materials, which is determined using torn test specimens according to a known Einkurvenverfahren the respective value of the J integral of the energy consumed in the material testing process in the specimen energy, and the intersection of the crack resistance curve with the Rißabstumpfungsgeraden the critical J Integral value Jic defines, characterized in that the squared crack resistance curve (J ² ) is first determined pointwise, then the tangent (T) to the squared crack resistance curve (J ² ) is determined at the point (J ² ic) from the square of the critical J integral value and then the corrected crack resistance curve Ukorr) can be determined by etching the tangent (T). 2. The method according to claim 1, characterized in that the tangent (T) by a balancing line (A) is replaced, which is adapted to measured values in the range above the critical J integral value (J ² ic). "