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Using Digital Image Correlation to Improve Stress-Corrosion Cracking Evaluation by NACE TM0177-B

Picture for Using Digital Image Correlation to Improve Stress-Corrosion Cracking Evaluation by NACE TM0177-B
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NACE TM0177 Method B is a standard method for evaluating stress-corrosion cracking resistance. Here, Digital Image Correlation determined strain distributions over the surface of bend samples made from corrosion-resistant alloys.

Product Number: 51317--8992-SG
ISBN: 8992 2017 CP
Author: Brandi Clark
Publication Date: 2017
Industries: Science of Corrosion , Corrosion Monitoring and Control
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NACE TM0177 Method B is a standard method for evaluating stress-corrosion cracking (SCC) resistance. To conduct the test a beam of material is loaded into a three-point bend fixture and exposed to the specified test solution then inspected for cracking (failure) at predetermined time intervals. The three-point bending equation is used to calculate peak stress in the center; however the bend equation is only valid for elastic deformation. In actuality the specimens have a pair of holes drilled at the centerline to act as stress concentrators resulting in stresses that are both inelastic and non-uniform near the holes. As a result the reported critical stress (Sc) obtained from Method B is a “pseudo-stress” rather than a true stress. In this study Digital Image Correlation (DIC) was used to determine the actual strain distributions over the surface of the bend samples and compare the corresponding peak stress to the pseudo-stress calculated using Method B. For example in Type 316 stainless steel at 0.5 mm deflection the peak stress determined by DIC was over 320 MPa but the calculated pseudo-stress was only 190 MPa. A series of corrosion-resistant alloys (Type 304 316 2205 420 and 440 stainless steels) and two pipeline steels (API 5L X65 and X80) were included in the study. In this paper we will report measured strain distributions for the various materials and discuss the implications of these results for SCC testing using Method B.

Key Words: bent-beam test, corrosion resistant alloy (CRA), critical stress, digital image correlation (DIC), pipeline steel, stress corrosion cracking (SCC), TM0177

 

NACE TM0177 Method B is a standard method for evaluating stress-corrosion cracking (SCC) resistance. To conduct the test a beam of material is loaded into a three-point bend fixture and exposed to the specified test solution then inspected for cracking (failure) at predetermined time intervals. The three-point bending equation is used to calculate peak stress in the center; however the bend equation is only valid for elastic deformation. In actuality the specimens have a pair of holes drilled at the centerline to act as stress concentrators resulting in stresses that are both inelastic and non-uniform near the holes. As a result the reported critical stress (Sc) obtained from Method B is a “pseudo-stress” rather than a true stress. In this study Digital Image Correlation (DIC) was used to determine the actual strain distributions over the surface of the bend samples and compare the corresponding peak stress to the pseudo-stress calculated using Method B. For example in Type 316 stainless steel at 0.5 mm deflection the peak stress determined by DIC was over 320 MPa but the calculated pseudo-stress was only 190 MPa. A series of corrosion-resistant alloys (Type 304 316 2205 420 and 440 stainless steels) and two pipeline steels (API 5L X65 and X80) were included in the study. In this paper we will report measured strain distributions for the various materials and discuss the implications of these results for SCC testing using Method B.

Key Words: bent-beam test, corrosion resistant alloy (CRA), critical stress, digital image correlation (DIC), pipeline steel, stress corrosion cracking (SCC), TM0177

 

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