STRAIN TESTING RIG AND METHOD OF EVALUATING STRAIN CHARACTERISTICS OF SPECIMEN
A strain testing rig for an elongate specimen has a first grip for the first end portion of the specimen, a second grip for the second end portion of the specimen, and a drive mechanism for moving the first grip away from the second grip to stretch the specimen lengthwise. The first grip and the second grip are configured to maintain a grip on the specimen as the cross section of the specimen decreases during stretching. Each of the first grip and the second grip can include a tapered dog, a press for compressing the respective end portion of the specimen, and a clamp for clamping the respective end portion of the specimen in a direction perpendicular to the action of the clamp. A method of testing strain characteristics can involve using digital image correlation to separately measure strain on each of the at least three independent strain zones.
1 . A method of testing strain characteristics of a specimen, the method comprising:
stretching the specimen such that such that strain on each of at least three independent strain zones of the specimen is substantially independent of strain on the remainder of the specimen;
using digital image correlation to separately measure strain on each of the at least three independent strain zones.
2 . The method of claim 1 , further comprising applying a pattern to a surface of each of each of the independent strain zones.
3 . The method of claim 2 , wherein the pattern comprises a pattern of tracking dots.
4 . The method of claim 1 , wherein the said using digital image correlation comprises capturing images of each of the independent strain zones before said stretching; during said stretching; and after said stretching and separately calculating strain in each of the independent strain zones based on the captured images.
5 . The method of claim 1 wherein stretching the specimen comprises contacting the specimen with at least three movable restraints engaging the specimen at respective lines of contact spaced apart along a longitudinal axis of the specimen, each restraint forming one of the strain zones.
6 . The method of claim 5 wherein stretching the specimen further comprises moving the restraints to stretch independent strain zones so that the length of each strain zone increases.
7 . The method of claim 6 wherein stretching the specimen comprises increasing the length of the specimen in each strain zone by about the same amount.
8 . The method of claim 7 wherein stretching the specimen further includes bending the specimen in each strain zone, a greater portion of a total deformation of each strain zone is caused by stretching rather than bending.
9 . The method of claim 5 wherein contacting the specimen with at least three movable restraints comprises actuating a cylinder to drive movement of the restraints.
10 . The method of claim 9 further comprising monitoring with a sensor a linear stroke of the cylinder.
11 . The method of claim 9 further comprising monitoring the force being applied by the cylinder to the restraints.
12 . The method of claim 1 further comprising imparting a substantially uniform force on each of the strain zones during stretching.