Hole-Drilling 방법을 통한 잔류 피로 측정 Hole-Drilling 방법을 통한 잔류 피로 측정 | HBM

MTS3000 - 스트레인 게이지를 이용해 잔류 응력을 자동으로 측정하기 위한 시스템 입니다.

부품의 강한 작용은 시각적인 신호가 나타나지 않는 이러한 부품에 존재하는 잔류 응력에 의해 영향을 받습니다. 그러므로, 이 목적은 부품안에서 기계적인 스트레스를 결정하는 것입니다. 홀 드릴링 방식으로 잔류 응력을 결정할 때, 직경 1.6mm의 작은 구멍이 제품안으로 뚫리고 스트레인 게이지는 스트레인 결과를 계측하는데 이용됩니다.

SINT Technology은 필요한 앰프와 이런 프로세스가 불편없이 필요한 도구들을 제공하도록 하는 MTS3000 시스템 등 모두 제공합니다. 이 시스템은 300,000 rpm으로 드릴링하도록 허락된 스테핑 모토를 사용합니다. 제품안으로의 뚫는 구멍에 대한 단계적인 드릴링 때문에 일어나는 스트레인 변화는 이런 프로세스를 위해 특별하게 설계된 스트레인 게이지 로제트에 의해 발견될 것입니다.

신호 프로세스는 디지털화 되어 수행합니다. 시스템 컨트롤 기능에 더해, 소프트웨어 패키지는 계측된 스트레인으로부터 기계적인 스트레스를 컴퓨터로 처리되도록 하는 4개의 서로 다른 평가 알고리듬들을 포함합니다. 전체적인 계측 프로세스는 PC로 컨트롤이 할 수 있습니다. 이것은 최적 재현성 뿐만 아니라 높은 계측 신뢰도가 가능합니다.

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Back Calculation Method

The reprocessing software allows the residual stresses in the material to be calculated from the measured strains.

The choice of back calculation method is very important in producing the most accurate representation of the real stress state. Many researchers have contributed, and continue to contribute, to the extensive literature describing the hole drilling method.

Currently, four different back calculation methods exist in the reprocessing software: the Uniform Stress Method according to ASTM E837-13 standard, the Non-Uniform Method according to ASTM E837-01 standard, the Schwarz- Kockelmann’s Method and the Integral Method.

Uniform Stress Method [Standard ASTM E 837-01]

This method, described in the ASTM E 837-01 standard, is based on the assumption that stresses do not vary with distance from the specimen’s surface. For this reason, the method does not consider spatial resolution. Nevertheless, when measured residual stresses are close to uniform stress field, this is the best method to choose, because it is the least sensitive to the effects of test errors.

It provides also a fast estimate of the average residual stress level into the specimen; that’s why this type of calculation is universally used and accepted.

Non-Uniform Stress Method [Standard ASTM E 837-13]

This method, described in the ASTM E 837-13 standard, introduces the computation of non-uniform stresses. Calculation steps and depth are fixed by this standard, and the calculation process refers to the Integral Method (see below for further details) with Tikhonov regularization to reduce the random errors in the calculated stresses.

The ASTM E 837-13 is the only complete standard on residual stresses available at world level.

Schwarz-Kockelmann’s method

Kockelmann’s method is based on the theory that there is a correlation function between the strain derivative and the stress distribution, expressed as a function of the hole’s depth. The bond is formed by a pair of coefficients (Kx and Ky), calculated on a simulation model, that relate stress and strain.

From these stress values it is possible to calculate the principal stresses and angle by using Mohr’s Circle.

Integral Method

This method, proposed by G. S. Schajer, provides a separate residual stress analysis at every hole-drilling depth increment. In this method, the contributions to the total measured strain relaxations of the stresses at all depths are considered simultaneously giving a higher spatial resolution than the other methods.

To simplify the problem of residual stress evaluation, Schajer proposed that the stress field could be described by means of step-wise functions whose value is constant through the partial hole depths. Using this hypothesis, Schajer established the numerical coefficients that are used for the calculation. The maximum depth that the method can be used for is 0.5 times the mean radius of the strain rosette used for the test.

The integral method should be chosen when residual stresses are expected to vary significantly with depth; however, it also has the highest sensitivity to test errors.

Flyer Residual Stress Measurements