What is Bend testing?
Bend testing is the most commonly, economical and simple qualitative mechanical testing used to check the material ductility and soundness. The test is more commonly applied as QC tool to butt/ groove weld joints (For procedure or performance qualification) compare to materials. The test is simple in operation, require least equipment setup and operator skills. The test is usually carried out using Universal testing machine but simple setup can be made on the shop floor very easily.
Bend Test Procedure (Guided Bend Test)
Bend tests are generally performed on a universal testing machine using a 3 or 4 point bend fixture. The bend test uses a coupon that is bent in three-point bending to a specified angle (defined by the die radius). The In Guided bend test the test specimen is wrapped around a former of specified diameter. Guided bend test is the most common test method used for welding procedure qualification (PQR) and welder performance qualification specifications. The diameter of the former is related to the material thickness and the angle of bend and can be 90, 120, or 180º. The outside of the bend is extensively plastically deformed (in tension), so that any defects in, or embrittlement of, the material will be revealed by the premature failure of the coupon. Standards generally indicate a defect of 3mm or more is cause for rejection.
Below approximately 12mm material thickness, transverse specimens are usually tested, with the root or face of the weld in tension. Material over 12mm is normally tested using the side bend test that tests the full section thickness.
The bend test is being used in welder performance tests to substitute the NDT. We normally use RT or UT for welder qualification to test the weld soundness but we can also opt for Bend testing to substitute these NDT test as permitted in ASME Section IX .We can do root bend & face bend test according to the details given in ASME Section IX, Table QW-452.1(a). We can choose either NDT method or mechanical testing as permitted by the code. There is restriction on although on some welding processes and welder can only be qualified by bend test and radiography and ultrasonic will not be acceptable. (For example in GMAW with Short-Circuiting as required in ASME Section IX and AWS D1.1).
Two main types of bend test i.e. 3 point bending and guided bend test are shown in below figure.
Type of Test Specimen for Bend Test
The Below diagram shows the types of Bend test:
- Face bend: When the load is applied on the root side so that the weld face will be in the tension.
- Root Bend: Here load is applied on the face side so that the weld root will be in the tension.
- Side bend: The load is applied on any of the sides of the specimen cut transverse to the welding direction.
- Longitudinal Bend: The longitudinal bend also has two types: a) face longitudinal and b) root longitudinal depending on the direction of the load applied similar to face and root bend as shown in the below diagrams.
Face, Root, Transverse & Longitudinal Bend Specimens dimensions
The dimensions of bend test coupon taken from plate butt weld and pipe butt weld are given below.
In Figure a, b, c below, the dimensions of Face bend and Root transverse bend and Longitudinal bend specimen are shown. The thickness, width and length of bend specimen is based on the test coupon thickness as shown in below figure. The generally used width of bend test is 38 mm and length of the specimen is minimum 150 mm as per ASME Section 9.
Side Bend Specimens dimensions
1.When weld deposit t is less than coupon thickness T, side-bend specimen thickness may be ‘t’.
2. When coupon thickness T equals or exceeds 1-1/ 2 in. (38 mm), use one of the following:
(a) Cut specimen into multiple test specimens of thickness y of approximately equal dimensions 3/ 4 in. to 1-1/ 2 in. (19 mm to 38 mm).
y = tested specimen thickness when multiple specimens are taken from one coupon.
(b)The specimen may be bent at full width. See requirements on jig width in QW-466.1.
Bend Testing Standards
1. ASTM E290 – 14 Standard Test Methods for Bend Testing of Material for Ductility
2. ASTM E190 – 14 Standard Test Method for Guided Bend Test for Ductility of Welds
3. ISO 7438:2016 Metallic materials — Bend test
Quality Testing and Acceptance criteria of Bend test results per ASME Section IX
ASME Section IX is having same acceptance criteria for bend test acceptance for (PQR) procedure qualification and (WPQ) welder performance qualification, so the maximum permitted imperfections during the test is same either we qualifying a welder or a WPS. As per ASME Section IX following rules are apply for bend test result acceptance:
1. No discontinuity shall be more than 1/8 inch (3 mm) in the length (in any direction) must be noticed in the convex surface (Outer side of the bend specimen after bending) after the test.
2. Any resultant crack & other discontinuity which are less than 1/8 inch (3mm) shall be acceptable. In case of crack that are more than 1/8 inch (3 mm) but initiated from the edge of the test coupon, is acceptable. The reason is that the edges are subjected to stress concentration due to the test & should not be a reason for rejection.
As bend test is a mandatory test requirements as per ASME Section IX for PQR qualification, the result of bend tests must be recorded in the procedure qualification record (PQR). Same rule apply, if the bend test is used for welder qualification in lieu of NDT.