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기술정보 기술설명

기술정보

Residual Stress – Direct Stress Measuring Method

Overview
Residual stress is the internal stress remaining within a material even in the absence of external forces, which significantly affects the material's strength and fatigue life. Traditional residual stress measurement methods, such as X-ray diffraction, neutron diffraction, and cutting methods using strain gauges, have limitations when applied to complex shapes or small components.
Residual Stress Measurement Using Instrumented Indentation Test (IIT)
Instrumented Indentation Test (IIT) provides a non-destructive method for directly measuring residual stress near the surface, applicable to a wide range of materials and shapes, from nano scale to large structures. Unlike other methods, IIT does not require strain gauges, making it easier to install, and it allows for rapid and flexible measurements on-site, offering greater efficiency than traditional methods.
Residual Stress Measurement Using Instrumented Indentation Test (IIT)
시험방법 매개변수 측정두께 측정부위 (공간분해능) 요소 시험기간 단위 문제점
비파괴적 XRD 격상지수 0.002 ~ 0.5mm 0.01 ~ 0.5mm² 면내 응력 시간
  • · 결정의 소재만 측정 가능
  • · 결정립 크기/조직효과
Synchrotron 0.1 ~ 100mm 0.001 ~ 1mm² 면내 응력 시간
Neutron 1 ~ 100mm 0.5mm² 모든 응력 시간
Ultrasonic 파동속도 1 ~ 150mm 1mm² 면내 응력 시간
  • · 미세조직 효과
  • · 분위기 효과(온도)
파괴적 Hole drilling 표면 변형률 0.2 ~ 2mm 0.5mm² 면내 응력 시간
  • · 낮은 공간분해능
  • · 기계공중 소성
RIng-core 5 ~20mm 시간
Deep-HD 1 ~ mm 5mm² 면내 응력 일
Sectioning 5mm² 수직 응력 일
Sitting 0.5mm² 수직 응력 시간
Contour 0.01~1mm² 수직 응력 일
  • · 표준 측정의 불확실성
  • · 기계공중 소성
계장화 압입시험(IIT)을 통한 잔류응력 측정
시험유형 시험방법 매개변수 측정부위 (공간분해능) 시험부위 요소 시험기간 단위 장점
준파괴적 계장화 압입시험 압입하중
(변형측정 X)		 0.0001 ~ mm 0.0000001 ~ 1mm² 면내 응력 분
  • · 빠르고 용이한 시험법(<30sec/point)
  • · 표면 근접 측정
  • · 직접적인 응력 측정
  • · 높은 현장 적용성
  • · 멀티스케일 시험 평가
  • · 미세조직의 영향을 받지 않음
Basic Principle of Residual Stress Measurement
Figure 1 illustrates the effect of residual stress on the load-displacement curve. When compressive residual stress is present, a higher load is required, while tensile residual stress requires a lower load. In IIT, residual stress is quantitatively evaluated by comparing the load-displacement curves for cases with and without residual stress.
Figure 1. Basic Principle of Residual Stress Measurement
Figure 1. Basic Principle of Residual Stress Measurement
In Figure 1, L0 represents the load at the hS point under no stress, and LS represents the load at the hS point under stress conditions. The difference (∆L) between these values is related to surface residual stress and is addressed as a standard in ISO TS 19096.
Advantages of the Instrumented Indentation Test (IIT)점
Direct Stress Measurement:

IIT directly measures surface stress by analyzing the indentation load-depth curve.

Consistent Accuracy

The effects of microstructure and environmental factors are minimal, and the procedure is simple, ensuring consistent results.

Convenient Stress Difference Analysis

Since a stress-free state is not required, it is easy to evaluate stress changes by comparing indentation load-displacement curves.

Non-Destructive Evaluation

Residual stress can be measured without damaging the material, preserving its structure after testing.

Wide Applicability

IIT is highly versatile, applicable to metals, amorphous materials, polymers, and other materials, making it widely used in industrial applications.

Comparison of IIT with Other Methods
To ensure the accuracy of stress measurements obtained through IIT, a validation methodology was implemented, comparing IIT results with a predetermined benchmark stress level in the same material. A specimen made of SA-508 Gr. 1A was subjected to controlled bending within its elastic deformation range to induce stress, which was accurately measured using strain gauges, as shown in Figure 2. IIT was then conducted using AIS equipment along the specimen's central line, allowing for a direct comparison between the measured stress and the benchmark stress during the application of elastic bending stress.
Figure 2. Illustration of the experimental setup using a 4-point bending jig to apply stress to a specimen of SA-508Gr.1A.
Figure 2. Illustration of the experimental setup using a 4-point bending jig to apply stress to a specimen of SA-508Gr.1A.
Figure 3. Comparative analysis of IIT-measured stress values with known elastic bending stress. (a) SA-508 Gr.1A (b) STS304 (c) SCM415
Figure 3. Comparative analysis of IIT-measured stress values with known elastic bending stress. (a) SA-508 Gr.1A (b) STS304 (c) SCM415

Figure 3(a) shows the close correlation between the measured and expected stress levels, exhibiting minimal deviation of less than50 MPa across the entire stress range. Furthermore, Figure 3(b) and (c) present additional materials tests (STS 304 and SCM 415), where IIT consistently aligns with the known bending stress, emphasizing the technique's reliability and precision. /p>

Figure 4 shows a comparison of residual stress measurements on welded specimens using IIT and the saw-cut method, where similar trends were observed between the two methods. The stress was removed from the specimen after cutting, as indicated by the red color.

Figure 4. Comparison of Residual Stress Measured using IIT and theSaw-Cut Method
Figure 4. Comparison of Residual Stress Measured using IIT and theSaw-Cut Method
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