Know Your Material Well

Accurate prediction of frequently encountered material fracture during the forming process is vital in everyday applications. An insight into the methods to identify the material that help successfully deal with the fracture phenomena.

Material at room temperature is largely dependent on its pre-work including mechanical and metallurgical treatment. Therefore, the material properties are different from situation to situation even though their chemical compositions are the same. It should be emphasized that the material properties, especially the strain-hardening effects have a profound influence on the predictions of cold metal forming processes and that it is sometimes needed to reveal the material properties by users, which are unique due to their specific pre-works including drawing or heat treatment.

Identification of material at the room temperature

Material properties, including the true stress-strain curves, are known to be indispensable to process design engineers in the field, as the quality of simulation results directly depends on the accuracy of the material properties involved. A true stress-strain curve needed for metal-forming process simulation is affected by the various conditions of manufacturing history, metallurgical treatments, and the chemical composition in the material as well. Metal-forming simulation engineers are hence in desperate need of the true stress-strain curves that reflect in a fruitful manner the peculiar conditions of the materials at hand. It is, however, rather a demanding thing to obtain such material properties from the experiments, while very restrictive amount of information about true stress-strain curves can be found in the published literatures. A lot of simulation engineers may helplessly stick to the material properties supplied by the software companies, which tend to be given in a quite limited way and are sometimes unverified.

Validity of the conventional methods

Most of the conventional material identification methods applicable to the tensile, compression and twisting tests do yield true stress-strain relations only valid for the range of strains less than 0.5. However, the maximum strain often exceeds 1.0 in bulk metal forming, such as in forging, extrusion, and rolling. It further reaches 3.0 sometimes in the multi-stage automatic cold forging, the so-called cold former forging easily seen for the product of fasteners, as annealing cannot be applied between stages.

In a tensile test, the true strain reaches its maximum value at the smallest cross-section in the necked region, and it may exceed 1.5 just before a ductile material fractures. Therefore, one should be able to obtain the flow stress of materials at a large strain if finite element methods are used to predict the localized deformation behavior during the tensile test. AFDEX/MAT, one of AFDEX modules, uses this fact to obtain the flow stress at the large level of strain, based on an iterative error-reducing scheme from the localized deformation behavior in the necked region.

Engineering and true stress-strain curves of SCM435 and PHTS.

Deformation history of the tensile tests of SCM435 and PHTS.

AFDEX/MAT module

AFDEX/MAT is now applied to two materials - PHTS and SCM435 - which show quite different behaviors in terms of strain hardening. PHTS is a pre-heat-treated steel of ESW105 and SCM435 is a typical strain-hardening material, and their resulting plastic flow stress curves are shown in Figure 1. With the flow stress information, the related tensile tests are simulated, and the predictions are compared to the experiments in Figure 1, indicating that the flow stresses accurately reflect actual phenomena occurring in tensile test. The maximum error is less than 0.3 percent. The tensile test to obtain the flow stress is then conducted according to ASTM E8. The initial yield stress of PHTS is 2.8 times larger than that of SCM435, while difference in the yield stress at the facture point between the two turns out to be small enough to be neglected. This behavior is the result of both the high initial yield stress and negligible strain-hardening performance of PHTS, as seen in Figure 1. The deformation history of the tensile tests of SCM435 and PHTS is shown in Figure 2, which shows distinct difference between the two materials. PHTS specimen starts to neck in the early stroke and its necking region is quite insignificant compared with SCM435, resulting in a relatively small elongation before the fracture point.

History of fracture formation by the Brozzo et al. damage model.

Comparison of tensile load from the initial point to the final fracture point.

Fracture prediction of a tensile test

Fracture prediction of a tensile test is now conducted using a crack propagation scheme of predicting fracture and Brozzo et al. damage model. Critical damage value of the material tested was obtained from comparison of the experiments with predictions just before the fracture point in its tensile test.

Figure 3 shows the history of the fracture formation, indicating that the early crack propagates horizontally up to two thirds of the radius of the material. The crack growth is then finalized in the inclined direction. Figure 4 shows the tensile load–elongation curve, with emphasis on the fracture region. It should be emphasized that the slope predicted by AFDEX is very stiff and closer to the experiments, when compared to other predictions which can be found from the available literature.

Prediction of chevron defect occurring in forward extrusion

A typical forward cold extrusion process is chosen to reveal the mechanics of chevron crack. The radius and height of the material is 6.995 mm and 28.7 mm, respectively. The critical damage is assumed to be 0.20, which is quite minor compared to the critical damage values of normal low or medium carbon steels. The small critical damage value is adopted to invoke the central bursting defects on purpose because they do not occur when the standard commercial materials for the structural parts are used.

To determine the effect of the die conical angle, a central bursting defect formation is simulated for four different die conical angles under the fixed conditions R.A. = 25% and µ = 0.03. The predictions are shown in Figure 5.

Compared to the predictions found from related literatures, AFDEX provides more realistic shape predictions for central bursting defects (i.e. an obtuse V-shape, which is quite typical). It can also be noted that the maximum normalized diameter predicted by AFDEX is of quite a larger value compared to the other research works. Note that a crack propagation approach is hired in AFDEX, while most other researchers adopt a
simpler approach of the element deletion approach.